Ion-exchange membrane

Abstract

A method for electroseparation includes introducing water molecules into a first nanochannel at a membrane matrix. The first nanochannel has a nanochannel wall that defines a first nanochannel lumen that extends through the membrane matrix from a nanochannel inlet to a nanochannel outlet. The first nanochannel has one or more ions present at the nanochannel wall. The method further includes creating a bonded water chain within the first nanochannel lumen using the introduced water and the one or more ions to provide a flow path blockage within the first nanochannel lumen; and transporting the bonded water chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen.

Description

ION-EXCHANGE MEMBRANERELATED APPLICATION

[0001] This disclosure claims priority to US provisional patent application number 63 / 491,563, filed on March 22, 2023, and claims priority to US provisional patent application number 63 / 520,961, filed on August 22, 2023. The entire contents of each of these provisional patent applications is hereby incorporated by reference.TECHNICAL FIELD

[0002] This disclosure generally relates to ion-exchange membranes as well as related devices, systems, and methods.BACKGROUND

[0003] An ion-exchange membrane can be a semi-permeable membrane that transports certain ions while blocking other ions or neutral molecules.

[0004] Ion exchange membranes can be used, for example, in electrodialysis. In such an application, an ion-exchange membrane can be employed in an aqueous, or mixed-aqueous, media including one or more salts, which may be acidic, neutral, or basic. Other examples of application include, for instance, batteries, fuel cells, and electrolysis cells.SUMMARY

[0005] In general, various embodiments relating to ion-exchange membranes as well as devices, systems, and methods related to ion-exchange membranes are disclosed herein.

[0006] Typical ion-exchange membranes can have fixed charges and act primarily by Donnan exclusion. Specifically, ions of opposite charge to the fixed charges penetrate the membrane and maintain bulk electroneutrality while remaining mobile. These ions can be referred to as counter-ions. On the other hand, ions of the same charge as the fixed charges cam be referred to as co-ions and are generally excluded from the membrane by the Donnan potential.

[0007] Ion-exchange membranes can include a matrix constructed of a tight gel with continuous, often branching and interconnected, channels through the membrane. These ion-exchange membranes rely on narrowness, length, and / or tortuosity of the channels to restrict water movement through the membrane. When such ion-exchange membranes are used for salt-splitting electrodialysis to produce acids and bases from salts, a number of limitations associated with such ion-exchange membranes can make them less efficient, particularly as the acid concentration increases. Among these is leakage of hydrogen ions down their electrochemical gradient through the anion-exchange membrane toward the cathode. Leakage of hydrogen ions is thought to be particularly difficult to manage because of the small size of the hydrogen ions, the ability of the hydrogen ions to move quickly through electrically neutral chains of hydrogen-bonded water molecules by jumping to adjacent water oxygens, and the sufficiently small size of the hydrogen ions enhancing their movement by quantummechanical tunneling through local energy barriers. This enhanced conduction of hydrogen ions is termed the Grotthuss mechanism. Anion exchange membranes also commonly exhibit leakage of free acid (e.g., HC1) down it concentration gradient by a mechanism which is not w ell understood. Osmotic transport of water through the gel matrix can also limit performance by diluting the product. A further limitation of conventional anion exchange membranes is that in high-pH aqueous environments the quaternary ammonium groups which provide the fixed positive charges can be subject to chemical degradation due to reaction with or catalysis by hydroxide ions. The degradation mechanisms can include Hoffman elimination and direct nucleophilic substitution.

[0008] In addition, leakage of anions through the cation-exchange membrane can also be a drag on efficiency. Again, effective movement of hydroxide ions by hydrogen-ion jumps through chains of hydrogen-bonded water molecules can contribute to leakage, and, again, hydrogen ion tunneling can contribute to inefficiency in this case as well.

[0009] Transport of hydrogen ions along a chain of hydrogen-bonded water molecules, and the opposite transport of hydroxide ions along the same chain, can be stoichiometrically and mechanistically equivalent processes. These are discussed here as either transport of hydrogen ions or hydroxide ions to emphasize that in each case the desired product is leaking from the intended location, and also because it is plausible that in each case the ion in higher concentration is more likely to initiate a transport event.

[0010] The behavior of ions (anions or cations) in confined spaces is not w ell understood. In bulk solution ions are generally hydrated (surrounded by water molecules which are bonded, strongly or weakly, to the ion). For the w ater molecules directly adjacent to the ions one of two type of bonding are usually involved. Coordinate bonds involve adjacent water donatingan electron pair to an empty valence position on a cation. These are generally stronger for smaller and more highly charged cations. For anions, hydrogen bonds are involved. These involve sharing of a proton (hydrogen ion) between electron pairs on a donor (in this case water) and an acceptor (the anion). Generally the proton remains closer to the donor but because of quantum mechanical effects it can also have a partial association with the acceptor. Hydrogen bonds are commonly considered to be due to dipole-dipole interaction but some of the strongest bond, may be considered quasi-covalent. Since water has two hydrogen donating sites and two hydrogen accepting sites, hydrogen-bonded chains of water are commonly branched. In confined spaces there is evidence of that ordered structures such as stacked squares and hexagons can occur, having hydrogen bonding both within a level and along the stack. The influence of ions on such structures of ions is not clear. The individual bonds involved in the hydration of large univalent anions such as chloride and bromide are relatively weak. In free solution, anions of a given size and charge are generally more weakly hydrated than cations, so it is surprising that experiments have shown uncharged nanotubes to be selective for cations over anions of similar diameter, when the nanotube diameter is too small to permit entry by the hydrated ions.

[0011] Embodiments disclosed herein can provide a high-performance electroseparation membrane for water, environmental, chemical, and energy applications. Such embodiments can be useful, for example, in one illustrative application by applying electrodialysis via an electroseparation membrane embodiment disclosed herein to desalinate brackish water cost effectively. For instance, an electroseparation membrane embodiment disclosed herein (e.g., FIG. 1) can be configured for bipolar membrane electrodialysis to recover useful chemicals and water supply from what has otherwise been considered w aste brine and, thereby, provide a cost effective and environmentally friendly solution by extracting such useful elements from what in the prior art has been considered a w aste and disposed of byproduct.

[0012] Embodiments disclosed herein can be configured to help prevent, or reduce, hydrogen ion leakage through anion-exchange membranes. Likewise, this same principle can be applied to limiting hydroxide movement through cation-exchange membranes. And, embodiments of ion-exchange membranes described herein could also have reduced or minimal leakage of larger co-ions and likewise reduced or minimal leakage of w ater. Some such embodiments can include membrane having a plurality of transverse nanochannels that each define a flow paths across the membrane. These nanochannels can be of approximately uniform width or diameter, and, in some such examples, these uniform diameternanochannels can be unbranched (e.g., a generally linear pathway through the membrane with one inlet and one outlet). The unbranched nanochannels may be discrete or clustered. Each such nanochannel may conveniently be the internal channel (also referred to as the lumen) of a nanotube, or may be formed in other ways, for instance by track etching or by dissolution of a nanofiber. These nanochannels may have charged groups on or in their walls or in a second layer separated from the lumen by a thin dielectric, semiconducting, or conductive layer.

[0013] The embodiments, and associated teachings, set forth herein can provide a number of useful advantages. As one example, embodiments, and associated teachings, set forth herein can provide a nanochannel that is configured to receive anions or cations and use these received anions or cations within the nanochannel to facilitate blockage of Grotthuss conduction / proton jumping, which, for instance, can help to facilitate production of higher concentration acids or bases. In one particular such example, a nanotube can be configured to receive sodium or chloride ions within a channel of the nanotube, and the channel of the nanotube can have a cross-sectional area just large enough to receive a sodium or chloride ion and then use the received sodium or chloride ion to block passage of other molecules past the sodium or chloride ions within the channel. This could further include using one or more bonding valences of another molecule (e.g., H2O and / or H?O+) within the channel and with the sodium or chloride ion within the channel to further restrict a parasitic flow path within the nanochannel. For instance, in this particular example, a chloride ion (Cl') within the channel of the nanotube can have a hydrogen bond with each of a water molecule (H2O) and a hydronium molecule (H3O ) without permitting a hydrogen bond between the water molecule (H2O) and a hydronium molecule (FLO ). thereby restricting Grotthuss conduction. Likewise, such bonding can restrict water movement to match movement of the sodium or chloride ion, setting a quasi-stoichiometric relationship and thus the composition of the product stream. As another example, embodiments, and associated teachings, set forth herein can include, when the nanochannel is formed by a nanotube, a charged polymer wrap at the external surface of the nanotube forming the nanochannel through which anions or cations are received and conveyed to thereby establish a charged layer at the nanotube, separated from the nanotube channel only by the thin dielectric nanotube wall, while maintaining the geometric configuration of the nanochannel, described elsewhere herein, that is configured reduce leakage. In embodiments where such a charged polymer wrap is included, this can help to control the concentration of an acid or base output via the nanotube. As a furtherexample, embodiments, and associated teachings, set forth herein can include a proton selective membrane based on a polymer wrapped nanotube, for instance, as part of a watersplitting membrane apparatus for chemical production, or for use in electrolysis, batteries, or fuel cells.

[0014] One embodiment includes an anion-exchange membrane. This anion-exchange membrane can include one or more nanotubes. The one or more nanotubes can include one or more walls defining a lumen, and these one or more walls can have fixed positive charges or mobile positive charges. To help restrict, or reduce, leakage of hydrogen ions through the anion-exchange membrane, the lumen of the nanotube can be of sufficient diameter to permit free movement of the counterions while, at the same time, restricting movement of the counterions to be in a generally single file arrangement, and the lumen of the nanotube can be sufficiently restrictive so that continuous chains of hydrogen-bonded water molecules are interrupted by each anion. As a result, such an anion-exchange membrane embodiment can help to restrict, or reduce, leakage of ions (e.g., hydrogen ions).

[0015] Another embodiment includes a cation-exchange membrane. This cation-exchange membrane can include one or more nanotubes. The one or more nanotubes can include one or more walls defining a lumen, and these one or more walls can have fixed negative changes or mobile negative charges. To help restrict, or reduce, leakage of hydroxide ions through the cation-exchange membrane, the lumen of the nanotube can be of sufficient diameter to permit free movement of the counterions while, at the same time, restricting movement of the counterions to be in a generally single file arrangement, and the lumen of the nanotube can be sufficiently restrictive so that continuous chains of hydroxide-bonded water molecules are interrupted by each cation. As a result, such a cation-exchange membrane embodiment can help to restrict, or reduce, leakage of ions (e.g., hydroxide ions).

[0016] One embodiment includes a method for electroseparation. This method embodiment includes the steps of: introducing water molecules into a first nanochannel embedded in a membrane matrix to form a first membrane, where the first nanochannel has a nanochannel wall that defines a first nanochannel lumen that extends through the membrane matrix from a nanochannel inlet to a nanochannel outlet, and the first nanochannel has one or more positive charges present at, or adjacent to, the nanochannel wall. This method embodiment further includes the steps of: creating a bonded water chain within the first nanochannel lumen using the introduced water and the one or more ions to provide a flow path blockage within the first nanochannel lumen; and transporting the bonded water chain within the first nanochannellumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen.

[0017] In a further embodiment of this method, the created bonded water chain defines a bonded water chain width, the first nanochannel lumen defines a first nanochannel lumen width large enough to transport the created bonded water chain though the first nanochannel lumen, and the first nanochannel lumen width is small enough to block or severely restrict penetration of water molecules between the ion and the nanotube wall, except those waters which are bonded to the ion. As one example, creating the bonded water chain within the first nanochannel lumen can include creating a plurlaity of bonded water chains within the first nanochannel lumen, and the first nanochannel lumen width can be small enough to transport the plurality of bonded w ater chains within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen via the single-file transport orientation. In one specific such example, the one or more ions include one or more hydrogen bond acceptor negative charge ions. For instance, the one or more hydrogen bond acceptor negative charge ions can serve to limit a number of possible orientations of hydrogen-bonded water molecules to a first subset of hydrogen-bonded water molecule orientations, and a combination of the first nanochannel lumen width and the first subset of hydrogen-bonded water molecule orientations can act to restrict the water making up the plurality of hydrogen-bonded w ater chains w ithin the within the first nanochannel lumen to move in approximate synchrony with the anions maintaining the flow path blockage within the first nanochannel lumen via the single-file transport orientation, until it reaches the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen.

[0018] In one such example, the one or more hydrogen bond acceptor negative charge ions can include at least one chloride ion, and this at least one chloride ion can form a hydrogen bond with each of a first water molecule and a first hydronium molecule within the first nanochannel lumen without forming a hydrogen bond between the first water molecule and the first hydronium molecule. As another example, when the one or more hydrogen bond acceptor negative charge ions is within the first nanochannel lumen, the first nanochannel lumen is sufficiently small to prevent or severely restrict migration of an introduced w ater molecule past the one or more hydrogen bond acceptor negative charge ions within the first nanochannel lumen.

[0019] In one instance, the one or more hydrogen bond acceptor negative charge ions can include one or more chloride ions, and the first membrane is placed in an electrodialysis stack in apposition to the cation-exchange face of a conventional bipolar ion exchange membrane. In such an instance, the method can further include introducing the water molecules into the first nanochannel at the nanochannel inlet, and further include transporting the chloride ions and the water constituting the bonded water chains within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen, so that the combined transport of chloride through the nanochannels of the first membrane and transport of hydrogen ion by the bipolar membrane causes the tw o streams to combine to form hydrochloric acid and the flow path blockage within the first nanochannel prevents the hydrochloric acid from being unduly diluted by water transport and further protects it from being neutralized by parasitic transport of hydroxide ion.

[0020] In one such embodiment, the method can still further include: introducing water molecules into a second nanochannel in a second membrane matrix forming a second membrane, where the second nanochannel is different than the first nanochannel, the second nanochannel has a second nanochannel wall that defines a second nanochannel lumen that extends through the second membrane matrix from a second nanochannel inlet to a second nanochannel outlet, and the second nanochannel has one or more negative charges present at the second nanochannel wall; creating a second bonded water chain within the second nanochannel lumen, using the introduced water and the one or more cations, to provide a flow path blockage within the second nanochannel lumen; and transporting the second bonded water chain within the second nanochannel lumen to the second nanochannel outlet while maintaining the flow path blockage within the second nanochannel lumen.

[0021] In one instance, the one or more cations can include one or more sodium ions, and the first membrane is placed in an electrodialysis stack in apposition to the anion-exchange face of a conventional bipolar ion exchange membrane In such an instance, the method can further include introducing the water molecules into the first nanochannel at the nanochannel inlet, and further include transporting the sodium ions and the water constituting the bonded water chains within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen, so that the combined transport of sodium through the nanochannels of the first membrane and transport of hydroxide ion by the bipolar membrane causes the two streams to combine to form sodium hydroxide and the flow path blockage within the first nanochannel prevents the sodium hydroxide from being undulydiluted by water transport and further protects it from being neutralized by parasitic transport of hydrogen ion.

[0022] In a further embodiment of this method, creating the flow path blockage within the first nanochannel lumen can act to block proton jumping within the first nanochannel lumen and thereby act to increase a concentration of an output at the nanochannel outlet. For example, this method embodiment can further include: transporting the bonded water chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen so that when the first membrane is placed in apposition to a conventional bipolar membrane, and a suitable voltage is applied to the stack (see, e.g., FIG. 1) the combined output streams from the first membrane and the bipolar membrane constitute a concentration of hydrochloric acid that is greater than a concentration of hydrochloric acid when the flow path blockage is not present. As another example, this method embodiment can further include: transporting the bonded water chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen so that when the first membrane is placed in apposition to a conventional bipolar membrane, and a suitable voltage is applied to the stack (see, e.g., FIG. 1) the combined output streams from the first membrane and the bipolar membrane constitute a concentration of sodium hydroxide that is greater than a concentration of sodium hydroxide when the flow path blockage is not present. The conventional bipolar membrane specified above may be replaced, as it would be in the end sections of a bipolar-membrane electrodialysis stack (see figure 1), by electrodes producing hydrogen ions and hydroxide ions respectively, or by a water-splitting membrane of other than conventional design if such a membrane is developed.

[0023] In a further embodiment of this method, the first nanochannel can define a uniform width extending through the membrane matrix from the nanochannel inlet to the nanochannel outlet. For example, the first nanochannel can define a linear pathway though the membrane matrix from the nanochannel inlet to the nanochannel outlet.

[0024] Another embodiment includes an anion-exchange membrane. This anion-exchange membrane can include a membrane matrix and a plurality of nanochannels each extending through the membrane matrix. Each of the plurlaity of nanochannels has a nanochannel wall that defines a nanochannel lumen that extends through the membrane matrix from a nanochannel inlet to a nanochannel outlet. The nanochannel w all has at least one or more cationic groups fixed within or adjacent to the wall and isolated from the external solution bynon-conductive and effectively water-impermeable polymeric material, which may include the membrane matrix and / or a layer or layers specifically associated with the nanochannel. Preferably there are multiple cationic groups distributed approximately uniformly along the length of the nanotube, with the spacing comparable to or a small multiple of the nanotube diameter. The nanochannel wall is configured to reduce leakage of hydrogen ions by permitting a hydrogen bond between one or more water molecules introduced into the nanochannel lumen and the at least one anion within the channel. And the nanochannel wall is configured such that the anion or anions within the lumen and the associated hydrogen- bonded water molecules provide a flow path blockage within the nanochannel lumen that reduces leakage of hydroxide ions from the nanochannel lumen.

[0025] In a further embodiment of this anion-exchange membrane embodiment, the at least one ion includes at least one chloride ion. The nanochannel wall is configured to permit formation of hydrogen bond(s) between the chloride ion and each of a first water molecule and a first hydronium molecule within the first nanochannel lumen while preventing a hydrogen bond between the first water molecule and the first hydronium molecule. And the nanochannel wall defines a width of the nanochannel lumen that provides the flow path blockage within the nanochannel lumen using the hydrogen bond between the chloride ion and each of the first water molecule and the first hydronium molecule within the first nanochannel lumen.

[0026] Another embodiment includes a cation-exchange membrane. This cation-exchange membrane embodiment includes a membrane matrix and a plurality of nanochannels each extending through the membrane matrix. Each of the plurlaity of nanochannels has a nanochannel wall that defines a nanochannel lumen that extends through the membrane matrix from a nanochannel inlet to a nanochannel outlet. The nanochannel wall has at least one or more anionic groups fixed within or adjacent to the wall and isolated from the external solution by non-conductive and effectively water-impermeable polymeric material, which may include the membrane matrix and / or a layer or layers specifically associated with the nanochannel. Preferably there are multiple anionic groups distributed uniformly along the length of the nanotube, with the spacing comparable to or a small multiple of the nanotube diameter. The nanochannel wall is configured to reduce leakage of hydroxide ions by permitting a coordinate bond between one or more water molecules introduced into the nanochannel lumen and the at least one cation within the channel, but only in an approximately axial orientation within the nanochannel. And the nanochannel wall isconfigured such that the bond between the one or more water molecules and the at least one ion provide a flow path blockage within the nanochannel lumen that reduces leakage of hydroxide ions from the nanochannel lumen.

[0027] In a further embodiment of this cation-exchange membrane, the nanochannel wall is configured to permit formation of a coordinate bond between a cation within the nanochannel lumen and each of a first water molecule and a first hydroxide ion within the first nanochannel lumen without forming a hydrogen bond between the first water molecule and the first hydroxide molecule. And the nanochannel wall defines a width of the nanochannel lumen that enables the flow path blockage within the nanochannel lumen using the cation and its bonded waters.

[0028] Another embodiment includes suppression of surging behavior in electrically driven ion transport. In this embodiment, when dissolved ions are transferred through uncharged nanotubes under the influence of an electrical potential spanning the nanotubes, a surging behavior has been observed wherein the transported ion induces a bulk flow of the aqueous electrolyte through the nanotube lumen which includes both cations and ions. If this behavior occurred appreciably in a membrane separation process it would degrade the performance to the point of uselessness. By fixing permanent charges in close proximity to the nanochannel, spread out along its length, this surging behavior is suppressed: the fixed charges are isolated from the bulk solution and so for electroneutrality attract counterions into the nanochannel lumen. Any surge of water or neutral electrolyte into the lumen would displace the mobile counterions within the lumen, resulting in a charge separation and a restraining force opposing the surge.

[0029] The details of one or more examples are set forth in the accompanying description below. Other features, objects, and advantages will be apparent from the description.BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The following draw ings are illustrative of particular embodiments of the present invention and, therefore, do not limit the scope of the invention. The drawings are intended for use in conjunction with the explanations in the following description. Embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. The features illustrated in the drawings are not necessarily to scale, though embodiments within the scope of the present invention caninclude one or more of the illustrated features (e.g., each of the illustrated features) at the scale shown.

[0031] FIG. 1 is a schematic diagram of a bipolar-membrane electrodialysis apparatus in a salt-splitting configuration. This illustrated bipolar-membrane electrodialysis cells in the saltsplitting configuration are shown making hydrochloric acid and sodium hydroxide from a sodium chloride brine. Two cells of a stack are show n for the illustrated example at FIG. 1. Other embodiments can include multiple cells arranged in series.

[0032] FIG. 2 is a diagram illustrating water and hydrogen ion leakage paths in a small section of a conventional an ion exchange membrane. FIG. 2 is not drawn to scale. More specifically, FIG. 2 shows four strands of the polymer matrix, with positive charges (typically quaternary ammonium groups) fixed to the polymer backbone. Crosslinks are not shown and the charged groups and counterions are shown at less than full density for clarity7. FIG. 2 shows two leakage paths. Osmotically driven transport of w ater is show n with a dashed arrow. Water is transported down its chemical potential gradient, which is normally from regions of low to high solute concentration. Hydrogen ion transport by the Grotthuss mechanism is shown by the short dot-dash arrows. Hydrogen ions jump from one water molecule or hydronium ion to an adjacent water or hydroxide ion to which it is hydrogen bonded. In this illustration one water molecule has just transferred a hydrogen ion to an adjacent water molecule, producing one hydroxide and one hydronium ion. Ensuing hydrogen transfers, shown by the dot-dash arrows, result in delivery of hydroxide ion to the positive side of the membrane (top of figure), and hydronium ion to the negative side: The net result is that a hydronium ion is effectively transferred from the positive site of the membrane to the negative side in spite of the Dorman potential preventing it from entering the membrane interior. In this conceptual, schematic figure at FIG. 2, water molecules are spaced widely for purpose of illustrative clarity. The individual hydrogen transfer steps would be much shorter than shown, occurring between immediately adjacent water molecules having a center-to-center distance between the oxygen atoms on the order of 0.3 nanometers.

[0033] FIGS. 3A and 3B illustrate an exemplar}7geometric configuration of a carbon nanotube and the resulting ion arrangement, within a channel of the carbon nanotube, that can be imparted by the geometric configuration of a carbon nanotube. For the example of FIGS. 3 A and 3B is illustrated for a hard-sphere, hard-shell application, and exemplary dimensions are shown for the carbon nanotube and its defined channel as well as the illustrated ions and their relative arrangement, though other dimensions are within the scope of the presentdisclosure. The examples at FIGS. 3A and 3B shows such dimensions in A (Angstrom units) where 10 A = 1 nm.

[0034] In particular, FIG. 3A shows a geometric construction to illustrate a diameter of a carbon nanotube that is configured such that a chloride ion will interrupt hydrogen bond chains among water molecules and hydronium ions (in another example hydroxide ions can be substituted for the hydronium ions in the same illustrated carbon nanotube). This simplified model assumes ions and water molecules are generally hard spheres having ionic radii (C1-, H3O+) or van der Waals (H2O) radii as noted in FIG. 3A and assumes that the nanotube is a generally hard shell. The hydronium ion and / or the water molecule could approach the chloride more closely if they were hydrogen bonded to it, but in that case they would not be oriented to make a hydrogen bond with each other. The nanotube diameter is defined, in this example, as the diameter of the cylinder passing through the centers of the carbon atoms, and the nanotube walls are taken to be 3.4 A thick, ith the wall midpoint coinciding with said cylinder. The centers of the water molecules are separated by 4 A, whereas the preferred lengths for hydrogen bonds can be 2.7-3.0 A. A hydrogen bond of 4 A length will be relatively weak, and the tunneling barrier will be relatively wide. Hydrogen bonding to the chloride, or to another water on the same side of the chloride ion, can be preferred. The chloride ion is shown adjacent to the nanochannel wall, to illustrate effective blocking of water paths by chloride, even in its most disadvantageous position. In normal operation the chloride is free to move laterally and will mostly be located in a more central position. It should be understood that this illustrated construction at FIG. 3A is one illustrative example, for instance, where ions and / or water molecules in the nanotube channel are not strictly hard spheres and the nanotube w all is not strictly a hard shell, the illustrative dimensions can vary7from that shown as a function of the different in the hardness of the sphere and hardness of the nanotube w all shell (e.g., for ions ± 2.5% the dimensions shown here; ± 5% the dimensions shown here; ± 7.5% the dimensions shown here) (e.g., for nanotube ± 5% the dimensions shown here; ± 10% the dimensions shown here; ± 20% the dimensions shown here; ± 30% the dimensions shown here). Furthermore, the electron density of the nanotube wall (e.g., the n orbitals that establish the van der Waals surface) may not be centered on the cylinder of carbon atoms, especially in smaller diameter nanotubes, due to re-hybridization to accommodate the curvature of the wall. The electron distribution may be further modified when the nanotube is surrounded by a charge-bearing polymerwrapping, to a degree depending on the density and the proximity to the wall of the charged groups.

[0035] FIG. 3B shows the same nanotube, but with the nanotube wall details omitted for clarity, as well as same ions and water molecule as in the nanotube channel as shown and described in reference to FIG. 3A, but FIG. 3B includes illustration of hydrogen positions and bond angles. This illustration at FIG. 3B demonstrates that if either the water or the hydronium ion is hydrogen bonded to the chloride, that water and / or hydronium ion’s other hydrogens are not in position to form a hydrogen bond bypassing the chloride. In this case as illustrated, the chloride not only interrupts but terminates the chain of water / hydronium hydrogen bonds. FIG. 3B illustrates the hydronium bond angle as 113°, and in other embodiments within the scope of the present disclosure can be within a range from 100°- 120°, within a range from 105 -115, or within a range from 107°-l 13°.

[0036] As FIGS. 3A and 3B illustrate, as applied to the exemplary application using a chloride ion within the nanotube lumen, a single fde orientation can be created within the nanotube lumen. For example, by introducing water molecules into the nanotube lumen with the nanotube wall including the chloride ion, a hydrated (e.g., fully hydrated) chloride ion can be formed within the nanotube channel, and this hydrated (e.g., fully hydrated) chloride ion can act to provide a flow path blockage that can reduce leakage and increase output efficiency of the nanotube lumen. For instance, using the chloride ion and introducing water molecules into the nanotube lumen, a hydrogen-boned water chain can be formed from one or more water molecules (e.g., one or more adjacent water molecules, such as three adjacent water molecules), one or more hydronium ions (e.g., one hydrated H+), and the chloride ion. FIGS. 3A and 3B demonstrate illustratively how a hydrogen-bonded water chain cannot travel past the chloride ion within the nanotube channel. Because the chloride ion is a type of hydrogen bond acceptor, the chloride ion can act to restrict possible orientations of hydrogen- bonded water. As such, in various embodiments, it can be both the width of the nanochannel lumen and the orientation of the hydrogen-boned water chain that act to create the flow path blockage.

[0037] In one specific example, a membrane can include a first nanochannel that is configured to be selective for hydrogen ions and a second nanochannel that is configured to be selective for monovalent anions. A hydrogen ion nanochannel and an anion-selective nanochannel at the membrane can form an acid-permeable membrane. A hydroxide ion nanochannel and a cation-selective nanochannel at the membrane can form a base-permeablemembrane. Both of the acid-permeable membrane and the base-permeable membrane can be included in a common stack to recover separate products streams of acid and base, respectively.

[0038] FIGS. 4A-4G illustrate exemplary embodiments of wrapped nanotubes at side elevational views. The exemplary illustrations at FIGS. 4A-4G are not necessarily shown to scale, and are intended to provide schematic, conceptual illustrations of various wrappings that can be applied to a nanotube in accordance with various embodiments disclosed herein.

[0039] FIG. 4A shows a nanotube wrapped by a linear polymer. The polymer chain can be longer than required to wrap the nanotube and may extend beyond either or both ends of the nanotube.

[0040] FIG. 4B shows a nanotube wrapped by a linear polymer, similar to FIG. 4A in that the linear polymer chain can be longer than required to wrap the nanotube, but different from that of FIG. 4A in that the liner polymer chain has been tethered to one end portion of the nanotube by a side-chain linkage at that end portion of the nanotube. The portion of the linear polymer extending beyond the tethered end of the nanotube may be of the same or different composition than the portion wrapping the nanotube.

[0041] FIG. 4C shows a nanotube wrapped by two linear polymer molecules, with one such linear polymer molecule tethered to one end portion of the nantotube by a side-chain linkage at that end portion of the nanotube and the other such linear polymer molecule tethered to other, opposite end portion of the nanotube by a side-chain linkage at that end portion of the nanotube. As illustrated at FIG. 4C, there can be a gap between the two linear polymer molecule chains where the nanotube is not wrapped, and, in this illustrated embodiment, that gap is present at generally a longitudinal central region of the nanotube.

[0042] FIG. 4D shows a nanotube wrapped by two linear polymer molecules with one such linear polymer molecule tethered to one end portion of the nantotube by a side-chain linkage at that end portion of the nanotube and the other such linear polymer molecule tethered to other, opposite end portion of the nanotube by a side-chain linkage at that end portion of the nanotube. As illustrated at FIG. 4D, there can be a region of overlap where one but not both of the linear polymer molecule chains can bind to the nanotube, thus leaving the other of the two linear polymer molecule chains to have a freely hanging end portion, such as at a generally longitudinal central region of the nanotube in the illustrated embodiment.

[0043] FIG. 4E shows a nanotube wrapped by two linear polymer molecules with one such linear polymer molecule tethered to one end portion of the nantotube by a side-chain linkage at that end portion of the nanotube and the other such linear polymer molecule tethered to other, opposite end portion of the nanotube by a side-chain linkage at that end portion of the nanotube. The tethered polymer chains, for instance of FIG. 4E, can be chosen to leave a gap near the longitudinal midpoint of the nanotube of sufficient size to accommodate several shorter polymer molecules at that gap between the polymer chains. These shorter molecules can mostly fill the gap but do not necessarily bind in register with each other. Though not shown here, in another embodiment there can also be regions of overlap resulting in free hanging end(s), such as in FIG. 4D.

[0044] FIG. 4F shows multiple poly mer chains wrapping a single nanotube, with the multiple polymer chains wrapping the nanotube being parallel helices relative to one another. An opposite side portion of the nanotube not seen at FIG. 4F can be similar to or the same as the side portion of the nanotube shown at FIG. 4F.

[0045] FIG. 4G shows multiple polymer chains wrapping a single nanotube, with the multiple polymer chains wrapping the nanotube being tethered to an end portion of the nanotube by linkage of the polymer end group rather than by a sidechain so that the polymer chains make short loops as they bend back to reach the nanotube.

[0046] FIGS. 5A-5G show various embodiments of nanotubes. FIG. 5A shows an unmodified nanotube, and FIGS. 5B-5G show constructions of charged and reactive polymer layers formed by the micelle-swelling method. FIG. 5B shows both ends of the nanotube modified to form a ring of positively charged groups, most conveniently quaternary ammonium groups, which can be accomplished, for instance, by activating the tube-end carboxyls with thionyl chloride followed by reaction with cholamine. FIG. 5C shows a modified nanotube incorporated in a mixed core-shell micelle with a quaternary ammonium detergent, and the charged ends of the nanotube can restrict the detergent to the sides of the nanotube. FIG. 5D shows a modified nanotube with monomers and cross-linkers infiltrated into the micelle, with some of the monomers bearing additional reactive groups x to enable subsequent modification, for instance, the monomer mixture may consist of styrene, chloromethyl styrene to enable subsequent modification, and divinylbenzene as a crosslinker. FIG. 5E shows a modified nanotube with monomers reacted in situ with suitable catalysts to form a cross-linked polymer, thereby forming a reactive shell surrounding the nanotube. FIG. 5F shows a modified nanotube where the reactive sites have been converted to positivelycharged quaternary ammonium groups, for instance, by reaction of chloromethyl groups on the polymer with a trialkylamine, and the detergent layer has been removed by any of several known methods, for instance, by filtration followed by washing since this polymer modification may render the nanotubes dispersible without detergent. FIG. 5G shows a modified nanotube having an optional second layer of polymer constructed overlying a first layer containing reactive groups to bond with the polymer matrix during subsequent membrane assembly, and, as with FIG. 5F, the detergent layer can be removed by any of several known methods.

[0047] FIGS. 6A-6C show- schematic illustrations of electric potential within and surrounding a core-shell micelle as the charged layer surrounding the micelle is produced. Plots at FIGS. 6A-6C are shown passing through the center of the micelle, near the length midpoint w here the micelle is effectively cylindrical. These plots are schematic and not to scale. Slopes and inflection points shown here were obtained by inspection employing the one-dimensional form of the Poisson equation as a first approximation.

[0048] FIG. 6A shows electric potential due to the charged detergent micelle alone. The position of the nanotube within the micelle is shown but it is uncharged. The electric potential rises with increasing slope as the position approaches the micelle and it then levels off when passing the locus of positive charges on the surface.

[0049] FIG. 6B show s electric potential with mobile ions in the nanotube lumen, but before construction of the charged polymer shell around the nanotube exterior. Anions have entered the nanotube under the influence of the positive electrical potential. An equivalent number of anions leave the diffuse layer surrounding the micelle to maintain bulk electro-neutrality. The electric potential rises with increasing slope as the position approaches the micelle, but now turns sharply downward as the position passes the micelle surface. The slope turns positive again as we pass the anions within the nanotube, forming a round-bottomed vee as shown at the plot of FIG. 6B. The electric potential within the micelle can increase with distance from the center. Chloride formed from reaction of chloromethyl groups in the polymer shell immediately surrounding the nanotube migrates along this potential gradient toward the micelle surface.

[0050] FIG. 6C, after the positively charged shell around the nanotube has been completed, shows anions remaining in the nanotube lumen to balance the charge of the polymer shell, and anions retained in the to the diffuse laver around the nanotube to maintain bulkelectroneutrality. As shown at the plot of FIG. 6C, the electric potential again climbs with increasing slope toward the micelle surface, then initially levels off crossing the shell of positive charges at the micelle surface, and then turns downward upon crossing the positively-charged shell surrounding the nanotube but then turns upward again passing the negatively charged ions in the nanotube interior. To whatever extent positive charges of the micelle increase the anion loading of the nanotube beyond that which is balanced by the charged polymer shell, the potential within the micelle can be intermediate between what is shown at the plots for FIGS. 6B and 6C.

[0051] FIG. 7 is a schematic diagram of a cross-section of the membrane with nanochannel elements. The illustrated nanochannels can derive their functional specificity from their internal diameter and their associated fixed charges. Four different specificity -types of nanochannel elements are shown: (i) anion channels sized to pass hydrated chloride ions, having positive charge shells; (li) cation channels sized to pass partly -hydrated sodium ions, having negative charge shells, (iii) hydrogen (hydronium) ion channels sized to pass water and hydronium ions, having negative charge shells, and (iv) hydroxide ion channels, sized to pass water and hydroxide ions, having positive charge shells. Charge and ion distributions are diagrammatic and qualitative, and water molecules (including those hydrating or interspersed with the ions) are not shown in the diagram at FIG. 7.DETAILED DESCRIPTION

[0052] The following detailed description is exemplary in nature and is not intended to limit the scope, applicability , or configuration of the invention in any way. Rather, the following description provides some practical illustrations for implementing embodiments of the present invention. Examples of constructions, materials, and / or dimensions are provided for selected elements. Those skilled in the art will recognize that many of the noted examples have a variety of suitable alternatives.

[0053] Embodiments of ion-exchange membranes are disclosed herein. Ion-exchange membrane embodiments disclosed herein can be used, for instance, in an acid-concentrating or acid-generating electrodialysis stack application. As one example, ion-exchange membranes within the scope of the present disclosure can help to reduce or eliminate leakage of ions through the membrane. Notably, this can increase the efficiency of the ion-exchange membrane embodiments within the scope of the present disclosure, for instance resulting inincreased acid or base concentrations produced in electrodialysis (e.g., bipolar membrane electrodialysis).

[0054] Ion-exchange membrane embodiments can include an anion-exchange membrane and a cation-exchange membrane and a proton-selective membrane. Such ion-exchange membrane embodiments can include one or more nanotubes.

[0055] As one example, one embodiment includes an anion-exchange membrane. This anion- exchange membrane can include one or more nanotubes. The one or more nanotubes can include one or more walls defining a lumen, and these one or more walls can have fixed positive charges or mobile positive charges. To help restrict, or reduce, leakage of hydrogen ions through the anion-exchange membrane, the lumen of the nanotube can be of sufficient diameter to permit free movement of the counterions while, at the same time, restricting movement of the counterions to be in a generally single file arrangement, and the lumen of the nanotube can be sufficiently restrictive so that continuous chains of hydrogen-bonded water molecules are interrupted by each anion. As a result, such an anion-exchange membrane embodiment can help to restrict, or reduce, leakage of ions (e.g., hydrogen ions).

[0056] As another example, a second embodiment includes a cation-exchange membrane. This cation-exchange membrane can include one or more nanotubes. The one or more nanotubes can include one or more walls defining a lumen, and these one or more walls can have fixed negative changes or mobile negative charges. To help restrict, or reduce, leakage of hydroxide ions through the cation-exchange membrane, the lumen of the nanotube can be of sufficient diameter to permit free movement of the counterions while, at the same time, restricting movement of the counterions to be in a generally single file arrangement, and the lumen of the nanotube can be sufficiently restrictive so that continuous chains of hydrogen- bonded water molecules are interrupted by each cation. As a result, such a cation-exchange membrane embodiment can help to restrict, or reduce, leakage of ions (e.g., hydroxide ions).

[0057] When the counterion sufficiently occludes the lumen of the nanotube to interrupt any chains of hydrogen-bonded water, it also will significantly reduce or prevent leakage of water past the ion.

[0058] In each of these embodiments, nanotubes can be prepared from a variety of suitable materials including, for example, carbon, boron nitride, alumina, titania, and various perskovites. The resulting materials may be electrical conductors, semiconductors, or insulators. Among the available materials, carbon and boron nitride nanotubes, in someinstance, can be particularly economical. Carbon nanotubes may be semiconducting or may be metallic conductors, depending on the chirality of the individual tube. Either sort, or a mixture or the two, may be employed in various embodiments. Boron nitride nanotubes are insulators. For certain applications, semiconducting or insulating nanotubes are preferred to metallic nanotubes. For example, except as otherw ise noted, diameters of nanotubes, such as carbon or boron nitride nanotubes, can refer to the diameter of the cylinder passing through the centers of the constituent atoms (e.g., the constituent carbon or boron and nitrogen atoms), as is common usage of the term diameter in the art. Internal or external diameters can refer to the respective van der Waals diameters of the tube, that is to say the cylinder of closest approach for a small molecule.

[0059] Of the variety of suitable nanotube materials for the nanotube wall, carbon, for example, can be useful because it readily forms nanotubes of appropriate diameter, because trigonally bonded carbon does not have strong localized Lewis base activity (compared for instance with a bridging oxygen on an alumina surface), and because the delocalized structure of the pi-electron bonding network facing the interior of the nanotube (which mediates the effect of any net charge), is expected to decrease the activation energy for migration of an ion in the lumen relative to interaction with a fixed charge site, thus lowering resistance to transport. Carbon nanotubes of appropriate diameter are commercially available or can be produced by known processes. Anion-permeable membranes constructed in this way may be employed in high pH environments including base production, alkaline electrolyzers, and alkaline. When the nanochannels are constructed from carbon nanotubes having quaternary ammonium groups immobilized in a polymer shell immediately external to the carbon layer forming the nanotube, the carbon shell protects the quaternary ammonium group from reactions catalyzed by or involving hydroxide ion including Hoffman elimination and direct nuclephilic substitution.

[0060] In some embodiments, fixed or mobile charges can be added to an exterior surface of the nanotube wall, particularly where the nanotube wall is relatively thin, such as a singlewalled carbon nanotube. The charged shell may surround substantially the entire length of the nanotube or only part of the length of the nanotube. However w hen the membrane is being used to provide a substantial concentrating effect or to control the delivered concentration, substantially uniform coverage may be preferable. A further polymer layer may be applied over a first polymer layer to aid in assembly of the membrane. This second polymer layermay include charged groups to aid positioning the nanotube and / or reactive groups to couple with the membrane matrix.

[0061] In such embodiments where fixed or mobile charges are added to the exterior surface of the nanotube wall, the nanotube wall where such charges are added can be conductive or nonconductive. If the wall is nonconductive it can act as the dielectric of a capacitor, transmitting electrostatic forces from the charged on the exterior to charged or polar molecules in the lumen. When the exterior charges are fixed, the charge distribution seen by a particle in the nanotube lumen can reflect the distribution of the exterior charges.

[0062] If the nanotube wall is conductive, the charge carriers in the wall can be mobile, and their distribution can be influenced by both the distribution of external surface charges and by their mutual interaction with other mobile charges both in the wall and in the lumen of the nanotube. Where the nanotube wall is conductive, conduction in the wall will transmit the electrostatic potential longitudinally, and, in such case, the modification of the nanotube wall can be nonuniform, for instance it may be concentrated at one or both ends of the nanotube. When conductive carbon nanotubes are employed it is preferred to fully embed the nanotube in a nonconductive matrix, and chemically modify the tube ends, to preclude electrochemical reactions with the surrounding solution.

[0063] Selectivity for anions or for cations may be imparted or enhanced by providing a region of net charge adjacent to and outside the loading-end opening of the nanotube to enrich the desired ion by Donnan effects, for instance, by coupling and sealing an open end of the nanotube to one face of a conventional ion exchange membrane, or by providing a shell of charged groups in close proximity to the nanotube exterior. Direct chemical modification of the nanotube wall may be undesirable because it could induce kinks and irregularities which may impede smooth fluid flow through the nanotube. However modification of end carbons does not appear to be detrimental to smooth fluid flow.

[0064] In an anion-exchange membrane embodiment, one or more nanotubes of the anion- exchange membrane can be prepared with a length equal to or greater than a thickness of the membrane, with an internal diameter of the nanotube (e.g., defined between opposite internal surfaces of the wall defining the lumen) greater than twice the ionic radius of the anion to be transported, but, in some embodiments, preferably not exceeding twice the sum of the anion ionic radius and that of the hydronium ion. The nanotubes can be prepared having fixed or mobile charges. With fixed charges, the charge spacing can permit some control over theconcentration of ions achievable. Comparably to a conventional ion exchange resin or membrane, the ion distribution will tend toward both bulk and local electroneutrality, so if the charges on the nanotube are uniformly spaced, the oppositely charged mobile ions in the lumen will tend toward the same spacing. Each ion will be associated with a small number of tightly bound water molecules. For embodiments of carbon nanotubes of inside diameter 0.6- 0.8 nm, the required charge spacing along the length may be estimated as the sum of the hydrated radius of the ion to be transported and 0.26-0.35 nm for each water molecule to be transported in excess of the water of hydration. While there may be continuous hydrogen- bonded chains of water molecules between the ions, in the case of a nanotube transporting anions most of these will terminate at the anion (itself a hydrogen-bond acceptor) or within rings of water between the ions. Close to the anion, the rotational freedom of the water is limited so that proton transfer reactions tend to reverse rather than propagate. Furthermore, since there are multiple ions spaced along the nanotube, any net leakage or slippage of water could require each water molecule get past multiple pinch points where the larger cation or anion effectively blocks the lumen of the nanotube. So both proton transport and osmotic water leakage through the membrane can be substantially prevented or blocked.

[0065] Due to the potential sensitivity to the anion diameter, a particular membrane may be best suited for a particular anion and a different separation (e.g., to produce sulfuric rather than hydrochloric acid) may require a different membrane at the ion-exchange membrane. Conversely, nanotube diameters and charge densities could be used to accommodate two anions of similar but not identical diameter (e.g., chlorine and bromide) with reduced efficiency.

[0066] Likewise, a similar approach may be applied to cation-exchange membrane embodiments.

[0067] An useful approach to produce an proton-selective channel is to make use of Grotthuss conduction. Transport of hydronium (protonated water) ions across the membrane in one direction can be electrically equivalent to transport of hydroxide across the membrane in the opposite direction. Stoichiometrically, movement of either hydroxide or hydronium ions results in transport of water, both resulting from the ions themselves and from movement of water dragged by the ions. Transport of protons across the membrane by the Grotthuss mechanism of jumping from one water molecule to another does not entail the net transport of water. To encourage Grotthuss transport, the channel should be of sufficiently small diameter to exclude cations and anions other than hydronium and hydroxide. This can includeananotube diameter less than approximately 0.78 nm, more preferably less than 0.75 nm, and possibly as small as approximately 0.7 nm. This nanotube does not require surface charges. It may have charged groups affixed to the ends but does not require them for function.

[0068] To construct a membrane of various ion-exchange membrane embodiments, a parallel array of suitable nanotubes, such as those described elsewhere herein, can be embedded in a ridged or resilient polymer matrix forming the membrane. In another example, the array of nanotubes can be bonded directly to each other in such a way that any gaps are reduced or eliminated (e.g., sealed), but the lumens defined by the respective nanotubes can remain unobstructed. In some embodiments, these lumens can be protected by masking (e.g., at the inlet and / or outlet of the nanotube) or prior introduction of a protective capping agent to the nanotube lumen. After formation of the membrane, the protective capping agent, when used, can be removed by chemical, thermal, electrical, or mechanical means as appropriate.

[0069] Composite membranes including nanotubes and one or more additional membrane layers, where the additional layer or layers provide added strength and support, or where both the nanotubes and the one or more additional layers contribute to the selectivity of the membrane in addition to their mechanical roles, are also within the scope of this disclosure.

[0070] In one such embodiment, one or more single walled carbon nanotubes of approximately uniform length can be utilized. For the one or more single walled carbon nanotubes, an aspect ratio of 200: 1 to 2000: 1 can be used. Longer tubes may be used provided they are not unsuitably prone to bending and tangling, and tubes of lower aspect ratios may be used if they are suitable to the separation desired and are of sufficient length to give a mechanically robust membrane. Also, double-walled nanotubes can be utilized provided the inner tube is sufficiently firmly fixed within the outer tube that it does not compromise the strength or performance of the membrane by migration or dis-association during fabrication or in the application contemplated. For many applications, it can be preferred that the nanotubes are of approximately uniform internal diameter.

[0071] The one or more carbon nanotubes can be induced to form a parallel array. This parallel array can be stabilized by adding, and polymerizing, a polymerizable and / or crosslinkable mixture which can act as an adhesive and sealant thereby bonding the nanotubes together and closing off flow paths between the nanotubes without capping or otherwise closing off or restricting the lumens (the internal cylindrical channels) of the nanotubes.

[0072] Carbon nanotubes after initial purification often have carboxylic acid end-groups, for instance as a consequence of oxidative uncapping and oxidizing acid treatment to remove metallic impurities. These carboxylic acid end-groups can provide convenient attachment points for modifying groups which may then be used to aid in membrane assembly, direct further modifications, or modulate the transport selectivity of the nanotube. For example, one common chemical route to end modification is to activate the end-groups by conversion to acid chlondes by reaction with thionyl chloride. The acid chloride end groups may then be reacted with nucleophilic groups such as amines or alcohols to produce amide or ester linkages, respectively. This approach may be used to couple charged groups to the nanotube as a means to enhance transport of counter ions and helping to exclude co-ions. Suitable precursors include cholamine or taurine to add positive or negative charges, respectively. Higher homologs, hydrophilic spacer groups, and multiply charged analogues may also be used. Some or all of these may contain additional reactive groups to serve as points of attachment for wrapping polymers.

[0073] Other coupling chemistries may be needed if stability in the presence of strong acid or base solutions is necessary. Suitable approaches will be apparent to those skilled in the arts of synthetic organic chemistry, polymer modification, or carbon chemi st ry.

[0074] The presence of a ring of charged groups at the ends of the nanotube may be used to restrict the coverage of the nanotube by micelle-forming amphiphiles or reactive polymers or monomers in subsequent modification and membrane assembly steps. Groups coupled to the end of the nanotube may also be used to position wrapping polymers or to direct and attach the ends or nanotubes to a supporting membrane, as described further below.

[0075] Nanotubes can be wrapped with organic polymers. These polymers may be functionalized with particular chemical groups according to the requirements of the membrane structure and the separation the membrane is intended to perform. Particular functionalizations may include charged groups to establish ionic selectivity and affinity, cross-linkable groups to lock the w rapping in place, and reactive groups to bond with the membrane matrix. Functionalization may occur before or after the polymer is wrapped around the nanotube or may occur stepwise. Functional groups may be introduced into the polymer during synthesis, optionally as co-monomers. Such functional groups may be further modified before or after wrapping the nanotube and may be modified in a stepwise fashion. For instance, the polymer may be polystyrene-co-chloromethylstyrene, and the chloromethyl group may be modified to produce cationic sites by reaction with a suitabletrialkylamine. Optionally a portion of the chloromethyl sites may be reacted with a different modifier having a suitably nuclephilic group, simultaneously with or in advance of the amine modification, to introduce cross-linkers which may be photochemically active or otherwise reactive. Reactive groups may also be introduced by, for instance, photochemical or free- radical routes which can couple the desired functionality' to the polymer backbone or side chains. Polymer wrapping is known to be facilitated by pi-electron interactions with the nanotube wall. The pi electrons of the polymer may be in (without limitation) aromatic or carbonyl groups, vinyl groups, or dienes. Examples of simple and versatile polymers which can wrap nanotubes include without limitation polystyrene and derivatives, polyvinylpyrollidone and derivatives, and poly methyl methacry late and derivatives, and polyethyleneimine. The polymer chosen for a particular application should be chosen for chemical resistance under the anticipated conditions of use. Polymer wrapping may occur by displacement of surfactants from surfactant-dispersed nanotubes or else polymers may be used as the primary' dispersing agent, usually with the aid of sonication. These methods are widely known in the art.

[0076] The ends of the nanotube may first be modified with a reactive group that localizes the polymer to the end before, or as. it wraps around the nanotube. In some cases, the polymer can bear a reactive group to couple with the reactive group on the nanotube; these can, for example, be a thiol and a maleimide or another pair of groups chosen for quick and selective reaction to give stable products. In this instance, the length of the polymer can be chosen to leave an unreacted tail extending beyond the reactive group and thus beyond each end of the nanotube.

[0077] In some embodiments, polymer encasement and / or polymer modification to add charge can be utilized. For example, one or more layers of polymer may be added in situ surrounding the nanotube by the micelle sw elling technique. In this case, nanotubes are mixed with or dispersed into a solution of an anionic or cationic detergent, so that the detergent forms a micelle surrounding the sides of the nanotube. It can be preferred to use a cationic detergent if the tube is to function as an anion exchanger, or to use an anionic detergent if the tube is to function as a cation exchanger. Optionally, the tube end may first be modified with charged groups, anionic for a cation exchanger, cationic for an anion exchanger, forming a ring of charge around the end opening of the nanotube. Detergents having a single long alkyl chain of 10-16 carbons or more preferably 12-14 carbons are suitable. The cationic detergent should preferably be of the quaternary ammonium typehaving three methyl groups and the longer alkyl chain attached directly to the nitrogen. Under these circumstances and in the absence of substantial excess detergent, the micelle may not cover the end openings of the nanotube.

[0078] The micelle swelling technique is based on the observation that styrenic and some acrylic monomers will partition into the interior of the micelle and will polymerize therein under the action of a suitable catalyst system, such as sodium persulfate with TEMED, added to the aqueous phase. Furthermore, the polymer chains are formed or migrate to a position adjacent to the nanotube wall and form an adsorbed polymer shell. This shell may be further stabilized by inclusion of suitable crosslinking monomers in the polymerization mix.

[0079] For applications described herein, styrenic monomers can be preferred. The mixture may include styrene, chloromethylstyrene, and divinylbenzene (e.g., a few moles percent divinylbenzene). The concentration of chloromethylstyrene in the mix, by moles percent, can control the density of ionic sites that will be produced in the next stage. Or to put it another way the number of chloromethylstyrenes per unit nanotube length determines the spacing of the charges on the final modified nanotube. Other monomers or polymer systems may be employed if they behave similarly in forming a polymer shell around the nanotube, if the polymer has suitable chemical resistance for the application, and if they permit assembly into a membrane of adequate strength and resiliency for the application.3

[0080] The polystyrene layer may be chemically activated while still encapsulated in the micelle. For an anion exchanger a trialkylamine and particularly trimethylamine can be suitable. For a cation exchanger, the chloromethylstyrene is first reacted with hydrogen sulfide (e.g., added as sodium sulfide to a mixture buffered at or below neutrality), then the resulting thiol is oxidized with a suitable peroxide, giving a sulfonic acid.

[0081] Optionally, a further layer of polystyrene may be formed over the first layer and its charged groups. Either or both layers may be formulated to contain reactive groups (for instance vinylic groups) to react with the subsequent layer. The outer layer will be adjacent to the membrane matrix in the finished membrane and crosslinking between the layers will strengthen the adhesion.

[0082] A polymerizable and / or cross-linkable mixture can then added so that it spreads through the nanotube layer, between the tubes, and wets the reactive polymer coating when present. In some cases, the components of this mixture can be chosen to be too big to penetrate the lumens of the nanotubes. This mixture can be polymerized in place (e.g., byphotochemical action or by a thermally activated catalyst) to form an impermeable matrix surrounding but not closing off the nanotubes. This matrix forms a barrier closing off any potential flow paths other than the nanotube lumens.

[0083] The polymerizable and / or cross-linkable mixture can be chosen so that it wets the nanotubes with the reactive polymer coating the nanotubes. Its viscosity can be sufficiently low such that it readily penetrates and fills the open spaces in the layer of nanotubes.

[0084] The polymerizable and / or cross-linkable mixture can be selected so that the resulting polymer is resilient rather than brittle and is configured for adhesion to the nanotubes. And, the polymerizable and / or cross-linkable mixture can be selected so as to be mechanically strong and chemically resistant under anticipated conditions of use. For instance, the polymerizable and / or cross-linkable mixture may contain styrene, alkyl styrenes, divinylbenzene, and / or low-molecular weight polybutadiene. Other mixtures may be employed depending of the mechanical properties and chemical resistance required of the final product.

[0085] Wetting and adhesion can be estimated by use of Hansen solubility parameters and other methods known to those skilled in the art.

[0086] The parallel array of nanotubes called for above may be assembled or produced by any of several ways. Some of these ways are better suited for particular applications than others as will be appreciated from the following teachings.

[0087] Nanotubes may be grown by chemical vapor deposition on, and perpendicular to, a flat supporting surface as ordered arrays. These subsequently may be processed to open the closed ends or to remove metal catalysts (depending on the grow th direction), and / or to modify the end carbons without removing them from the surface, before or after they have been incorporated and fixed in a membrane matrix.

[0088] Nanotubes (modified as described herein) may be mixed with a polymerizible and / or cross-linkable mixture and spread on an immiscible fluid surface which may commonly be water or brine. The spread surface film is then compressed with a moving barrier and as the available area per nanotube decreases the nanotubes turn upright. The polymerizible and / or cross-linkable mixture flows between the upright tubes by capillarity and stabilizes the array. The arrangement can then be locked in by polymerization initiated chemically or photochemically.

[0089] In a commercial process, nanotube membranes can be assembled by a roll-to-roll process in which the nanotubes are uprighted by an electrostatic field.

[0090] Nanotubes can be assembled into arrays by electrostatic attraction between oppositely- charged groups attached to the nanotubes.

[0091] Electrostatic erection and alignment of nanotubes has been described. In one specific embodiment, two charged polymers can be selected, with one such charged polymer being a polyanion and the other such charged polymer being a poly cation. These polymers can be selected to have a propensity- to coil around and coat the nanotube and can oftentimes be generally linear.

[0092] Then the solution or suspension of nanotubes can be divided into two parts, one part to be coated with the polyanion and one part with the poly cation. The polymer may coat the entire length of the nanotube. Alternatively, after the two sublots of nanotubes are coated with the respective polymers, they can be recombined under conditions of sufficiently7high ionic strength that the nanotubes of opposite charge adhere by electrostatic attraction, but the initial adsorption can be sufficiently loose that the nanotubes can rearrange for maximum overlap and to eliminate any gaps or loops. The ionic strength may then be progressively reduced so that the aggregates form progressively7larger aligned aggregates. These may then be harvested and spread on either a flat surface or a liquid interface, infiltrated with a polymerizable and / or cross-linkable mixture, and consolidated into a membrane by polymerization.

[0093] Optionally, one or both polymers may extend beyond the end of the nanotube and may be secured in position by reaction with a modified end group of the nanotube, as described previously herein. Ordinarily it will be preferable that only one of the charged polymers (anionic or cationic) extend appreciably beyond the ends of their respective nanotube, while the other polymer ends roughly flush with the end of its nanotube. As an alternative to a charged extension, a chemically reactive polymer chosen to react with groups on a base membrane can be coupled to the charged polymer or to the ends of the nanotubes of one charge.

[0094] A composite membrane may be constructed using the polymer-wrapped nanotubes described previosuly herein with a suitable base membrane to provide structural alignment. This membrane may be simply an ion-permeable support, or it may impart selectivity-.

[0095] Nanotubes of appropriate dimensions can be used to give size selectivity to an electrodialysis membrane or to control back-migration of co-ions, particularly H+ or OH-, as described elsewhere herein. For instance, an electro spun anion exchange membrane with a reasonably flat surface and having uniformly distributed positive charge may be used as a base layer to construct a composite membrane.

[0096] In one approach, the positively charged anion-exchange membrane can be spread flat in the bottom of a shallow pan or tank. Nanotubes can be prepared with a layer of positive charges adjacent to the nanotube wall, with an insulating layer over them. Polyanion extensions are coupled to one end of the nanotube. The nanotubes can be dispersed in liquid to make a dilute solution or dispersion and applied in a layer of solution, for instance several millimeters thick above the base membrane. The membrane and nanotube suspension can be gently agitated. The nanotubes can bind to the base membrane, and this process can continue until the membrane is coated to the desired density. Then the nanotubes can be sealed to together and to the base membrane with a polymerizable and / or cross-linkable mixture.

[0097] A similar process would be used for a negatively charged cation-exchange membrane, with the charges reversed from the above description.

[0098] In each of the noted examples using charged polymer extensions to tether the nanotubes to the base membrane, it can be preferred to keep the ionic strength sufficiently high that the complexes can rearrange to maximize contact. That is. the nanotubes are not fixed in their first-contact position but can slip and rearrange until they approach their position of maximum stability. It may be found desirable in particular cases to gradually increase or decrease the ionic strength.

[0099] Once the base membrane is fully coated, the overlying fluid can be drained and the structure can be rinsed to remove excess salt. Water can then be removed by rinsing with an alcohol or other suitable solvent, evaporation, or both steps in succession.

[0100] In many cases it can be desirable that the polymerizable and / or cross-linkable mixture not penetrate the base membrane since that could block the membrane's pores and cap the adjacent nanotube ends. To this end. the base membrane can be rewetted with water prior to application of the polymerizable and / or cross-linkable mixture. In favorable cases this rewetting can fill the nanotubes with water by capillary action and aid in excluding the polymerizable and / or cross-linkable mixture from the nanotube interior.

[0101] In some examples, the base membranes can be modified before assembly to have reactive groups to bond with the polymerizable and / or cross-linkable mixture

[0102] In another version, a suspension of nanotubes can be prepared in a polymerizable and / or cross-linkable mixture. The nanotubes optionally can be coated with a reactive polymer before preparation of the suspension. The ends of the nanotubes can be chemically modified to render them hydrophilic or to aid in further reactive steps.

[0103] As noted previously, the reactive components of the polymerizable and / or crosslinkable mixture can be chosen to be too big to penetrate the lumens of the nanotubes. The mixture can include an inert, volatile diluent, for example n-heptane, which can be added before the other components of the polymerizable and / or cross-linkable mixture and which can be permitted to enter the lumens of the nanotubes to aid in excluding the reactive components of the mixture.

[0104] The composition of the polymerizable and / or cross-linkable mixture can be chosen so that it wets the nanotubes (and, when present, the reactive polymer coating the nanotubes). The viscosity of the polymerizable and / or cross-linkable mixture can be sufficiently low that it readily penetrates and fills the open spaces in the layer of nanotubes. The polymerizable and / or cross-linkable mixture can further be selected so that the resulting polymer is resilient rather than brittle and has good adhesion to the nanotubes and such that the resulting polymer can be chemically resistant under anticipated conditions of use. It can also be preferably selected so that the nanotube suspension is less dense than the aqueous medium employed in the next step, with or without the volatile diluent.

[0105] This nanotube suspension can then be spread on a layer of water or an aqueous solution of suitable density in a rectangular mold having four fixed walls and one movable barrier extending generally the full width of the mold below and above the water level but not fully reaching the bottom. Initially this movable barrier can be adjacent to one of the fixed sides or end walls. After the nanotube suspension has formed a generally uniform layer and the diluent, if any, has partly or mostly evaporated, the barrier can be moved gradually to compress the nanotube-containing layer. As the layer is compressed, the nanotubes can orient themselves to occupy less surface area and eventually be packed in an upright array, with the polymerizable and / or cross-linkable mixture in the interstices between the nanotubes. Optionally this reorientation can be aided by light sonic or ultrasonic oscillation. The amount of polymerizable and / or cross-linkable mixture and the degree of compression can be chosencarefully such that the polymerizable and / or cross-linkable mixture does not flood over or under the ends of the nanotubes in this condition.

[0106] This mixture can be polymerized in place (e g., by photochemical action) to form an impermeable barrier surrounding but not closing off the nanotubes and coupling them together to form a membrane.

[0107] In addition, methods of producing hydrochloric acid are within the scope of the present disclosure. For example, such methods can include embodiments of a method for producing concentrated hydrochloric acid, for instance, with reduced energy7consumption.

[0108] Hydrochloric acid of relatively high purity can be produced by electrolysis of concentrated salt solutions (e.g., NaCl) under known conditions to produce H2 and CI2, as well as NaOH, where the H2 and CI2 are recombined by combustion to produce HC1 gas. The HC1 gas can be subsequently absorbed into water to produce aqueous hydrochloric acid.

[0109] An alternative process, bipolar membrane electrodialysis ("BMED"). can produce dilute hydrochloric acid and sodium hydroxide in a more energy efficient manner by water splitting, with a lower per-cell voltage drop than electrolysis. However the maximum concentration currently obtainable with conventional membranes is about 2 moles per liter, and reaching that level requires operating in conditions yielding low current efficiency.

[0110] The limiting factors of BMED are generally threefold: dilution of the product by osmotic water flow, high voltage drop in the water-splitting region of the bipolar membrane, and, perhaps most significantly, a lack of selectivity of conventional anion exchange and cation exchange membranes against flows of co-ions H+ and OH-.

[0111] Notably, by utilizing unconventional anion exchange membranes, and in many cases also unconventional cation exchange membranes, with enhanced selectivity7against co-ion flow, and in many cases also increased resistance to osmotic flow, production of higher concentration acid and base can be achieved. Thus, as a more specific example, higher concentration hydrochloric acid can be produced by utilizing an anion exchange membrane, and in some cases additionally a cation exchange membrane, with enhanced selectivity7against co-ion flow, and in some cases such anion and / or cation exchange membranes with increased resistance to osmotic flow. In addition to producing a higher concentration hydrochloric acid as compared to BMED and conventional electrolysis, utilizing such anion and / or cation exchange membranes with enhanced selectivity against co-ion flow and / or increased resistance to osmotic flow can reduce energy consumption associated withhydrochloric acid production. As such, utilizing such anion and / or cation exchange membranes with enhanced selectivity against co-ion flow and / or increased resistance to osmotic flow can not only yield higher concentration hydrochloric acid production but also do so in a way that is more energy efficient as compared to BMED and conventional electrolysis.

[0112] Salt-splitting electrodialysis using unconventional membranes in this disclosure can improve the economic performance of conventional hydrochloric acid plants. Although production of hydrochloric acid at commercial concentration of approximately 36-37% w / w (36-37% of the solution’s weight is hydrochloric acid) can be desirable, a substantial improvement in process economics is possible at a more modest increase. For example, improved process economics can be achieved, for instance, by producing hydrochloric acid at 9-10% w / w, or, in some cases, at a higher concentration of about 12% w / w or, in some further cases, at a higher concentration of about 18% w / w or higher. In such cases, the intermediate concentration acid can optionally be used, instead of water, to absorb conventionally produced gaseous hydrochloric acid. The intermediate-concentration hydrochloric acid can also be used in brine preparation, such as for ion exchange regeneration and pH control. In addition to energy savings that this approach could provide, it can also provide a convenient and inexpensive means to increase the capacity of an existing chlor-alkali plant producing HC1 by adding an electrodialysis unit in parallel.

[0113] In addition to production of new hydrochloric acid using bipolar membrane electrodialysis, the reduced-leakage membrane electrodialysis can help to reduce costs of hydrochloric acid recovery from process / waste streams as well as provide a more efficient means for controlling pH. Moreover, while the present disclosure describes embodiments of hydrochloric acid production, embodiments disclosed herein for electrodialysis with reduced co-ion leakage can also apply to production and concentration of other types of acids and bases as well.

[0114] A process referred to as diffusion dialysis can be utilized to recover mineral acids from etching or picking baths employed in metal treatment. This process can use the known and otherwise undesirable permeability of conventional ion exchange membranes to hydrogen ions to separate free mineral acids from metal salts. The acid anion passes through the membrane as a conventional counterion, while the hydrogen ion leak maintains electroneutrality. The larger metal ions are blocked by the membrane. An analogous processis sometimes used to recover sodium hydroxide. However, because of their reliance on co-ion leakage, these processes are only employed with streams of high acid or base concentration.

[0115] Consistent with techniques disclosed herein, by incorporating hydrogen-selective nanochannels in an anion exchange membrane, and similarly by incorporating hydroxideselective nanochannels in a cation-exchange membrane, and placing these two membranes to form opposite sides of a channel, acid and base can be recovered as separate streams from a brine feedstock. For instance, if the feed is a sodium chloride brine, the flux of chloride ions through the anion-exchange nanochannels creates a membrane potential (negative on the product side) which drives the hydrogen-ion flux through the hydrogen-selective nanochannels in the same membrane, yet sodium is restricted from entering these channels by its diameter. Similarly the flux of sodium through the cation-exchange nanochannels creates a membrane potential (positive on the product side) which drives the flux of hydroxide through the hydroxide-selective nanochannels in that membrane. Catalytic groups may be affixed to the membranes to increase the rate of water dissociation. Catalytic groups may include, for instance, hydrous metal oxides with fast exchange kinetics, or organic molecules bearing amine or carboxylate functionalities, alone or in combination. When the nanochannel walls shield the fixed charged groups from degradation by the aqueous medium and its acidic and basic constituents, the system may be designed with temperature-resistant polymers to permit high temperature operation, thereby facilitating water dissociation and mass transfer.Accordingly, for instance, the operating temperature may be made > 80 C; or > 95 C; or > HO C. Suitable polymers for operation at these temperatures are known in the art.

[0116] The membrane stack can conveniently be ordered with two anion-exchange membranes, followed by two cation-exchange membranes, except with single membranes adjacent to the ends of the stack. The channels between the membranes can, in one preferred embodiment, be long relative to their width to facilitate counterflow operation and be oriented vertically with the brine feed in upflow between the anion-exchange and cationexchange membranes and the acid and base products flowing downward between the aa and cc pairs respectively. The channels can, in one preferred embodiment, be relatively thin in the direction perpendicular to the membranes and can, optionally, include one or more screens or other mixing devices (e.g., within the flow path defined by any one or more channels) to aid in mass transfer.

[0117] FIG. 7 illustrates a schematic diagram (e.g., not to scale) of a diffusion dialysis system using mixed-nanochannel membranes as disclosed elsewhere herein to recover sodiumhydroxide and hydrochloric acid from a stream of NaCl brine. The spent brine may optionally be further fractionated, for instance, by reverse osmosis or electrodialysis, to supply part of the water requirement and return reconcentrated NaCl feedstock to the separation.

[0118] Various non-limiting exemplary embodiments have been described. It will be appreciated that suitable alternatives are possible without departing from the scope of the examples described herein. These and other examples are within the scope of the claims.

Claims

What is claimed is:

1. A method for electroseparation comprising the steps of: introducing water molecules into a first nanochannel, the first nanochannel embedded in a membrane matrix to form a first membrane, the first nanochannel having a nanochannel wall that defines a first nanochannel lumen that extends through the membrane matrix from a nanochannel inlet to a nanochannel outlet, the first nanochannel having one or more positive charges present at, or adjacent to, the nanochannel wall; creating a bonded water chain within the first nanochannel lumen using the introduced water and one or more ions to provide a flow path blockage within the first nanochannel lumen; and transporting the bonded water chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen.

2. The method of claim 1, w herein the created bonded water chain defines a bonded w ater chain width, wherein the first nanochannel lumen defines a first nanochannel lumen width large enough to transport the created bonded water chain though the first nanochannel lumen, and wherein the first nanochannel lumen width is small enough to block passage of introduced water molecules that are unbonded to one or more ions within the first nanochannel lumen.

3. The method of claim 2, wherein creating the bonded water chain within the first nanochannel lumen comprises creating a plurlaity of bonded water chains within the first nanochannel lumen, and wherein the first nanochannel lumen width is small enough to transport a single-file transport orientation of the plurality of bonded water chains w ithin the first nanochannel lumen to transport the plurlaity of bonded water chains within the first nanochannel lumen to the nanochannel outlet while maintaining the flow" path blockage within the first nanochannel lumen via the single-file transport orientation.

4. The method of claim 3, wherein the one or more ions comprise one or more hydrogen bond acceptor negative charge ions.

5. The method of claim 4, wherein the one or more hydrogen bond acceptor negative charge ions limit a number of possible orientations of hydrogen-bonded water molecules to a first subset of hydrogen- bonded water molecule orientations, and wherein a combination of the first nanochannel lumen width and the first subset of hydrogen-bonded water molecule orientations act to restrict the water at the plurality of hydrogen-bonded water chains within the the first nanochannel lumen to move in approximate synchrony with the anions to maintain the flow path blockage within the first nanochannel lumen via the single-file transport orientation until the single-file transport orientation reaches the nanochannel outlet.

6. The method of claim 5, wherein the one or more hydrogen bond acceptor negative charge ions comprise at least one chloride ion, wherein the at least one chloride ion forms a hydrogen bond with each of a first water molecule and a first hydronium molecule within the first nanochannel lumen without forming a hydrogen bond between the first water molecule and the first hydronium molecule.

7. The method of claim 5, wherein, when the one or more hydrogen bond acceptor negative charge ions is within the first nanochannel lumen, the first nanochannel lumen is sufficiently small to restrict migration of an introduced w ater molecule past the one or more hydrogen bond acceptor negative charge ions within the first nanochannel lumen.

8. The method of claim 4, wherein the one or more hydrogen bond acceptor negative charge ions comprise one or more chloride ions.

9. The method of claim 8, wherein the water molecules are introduced into the first nanochannel at the nanochannel inlet, and wherein transporting the bonded w ater chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow- path blockage within the first nanochannel lumen causes the nanochannel outlet to output hydrochloric acid.

10. The method of claim 9, wherein the first membrane is placed in an electrodialysis stack in apposition to a cation-exchange face of a bipolar ion exchange membrane, and further comprising: introducing water molecules into a second nanochannel embedded in a second membrane matrix, the second nanochannel being different than the first nanochannel, the second nanochannel having a second nanochannel wall that defines a second nanochannel lumen that extends through the second membrane matrix from a second nanochannel inlet to a second nanochannel outlet, the second nanochannel having one or more negative charges present at, or adjacent to, the second nanochannel wall; creating a second bonded water chain within the second nanochannel lumen, using the introduced water and the one or more cations, to provide a flow path blockage within the second nanochannel lumen; and transporting the second bonded water chain within the second nanochannel lumen to the second nanochannel outlet while maintaining the flow path blockage within the second nanochannel lumen.

11. The method of claim 10, wherein the one or more cations comprise one or more sodium ions, wherein the water molecules are introduced into the second nanochannel at the second nanochannel inlet, and wherein transporting the second bonded water chain within the second nanochannel lumen to the second nanochannel outlet while maintaining the flow path blockage within the second nanochannel lumen causes the nanochannel outlet to output sodium hydroxide.

12. The method of claim 1, wherein creating the flow path blockage within the first nanochannel lumen acts to block proton jumping within the first nanochannel lumen and thereby acts to increase a concentration of an output at the nanochannel outlet.

13. The method of claim 12, further comprising: as a result of transporting the bonded water chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen such that when the first membrane is placed in apposition to a bipolar membrane to form a stack and voltage is applied to the stack a combined output stream fromthe first membrane and the bipolar membrane have a concentration of hydrochloric acid that is greater than a concentration of hydrochloric acid when the flow path blockage is not present.

14. The method of claim 12, further comprising: as a result of transporting the bonded water chain within the first nanochannel lumen to the nanochannel outlet while maintaining the flow path blockage within the first nanochannel lumen such that when the first membrane is placed in apposition to a bipolar membrane to form a stack and voltage is applied to the stack a combined output stream from the first membrane and the bipolar membrane have a concentration of sodium hydroxide that is greater than a concentration of sodium hydroxide when the flow path blockage is not present.

15. The method of claim 1, wherein the first nanochannel defines a uniform width extending through the membrane matrix from the nanochannel inlet to the nanochannel outlet.

16. The method of claim 15, wherein the first nanochannel defines a linear pathway though the membrane matrix from the nanochannel inlet to the nanochannel outlet.

17. An anion-exchange membrane comprising: a membrane matrix; and a plurality of nanochannels each extending through the membrane matrix, each of the plurlaity of nanochannels having a nanochannel wall that defines a nanochannel lumen that extends through a membrane formed by the membrane matrix from a nanochannel inlet to a nanochannel outlet, the nanochannel wall including at least one cationic group fixed within or adjacent to the nanochannel wall and isolated from an external solution by a non-conductive and effectively water-impermeable polymeric material at the membrane, wherein the nanochannel wall is configured to reduce leakage of hydrogen ions by inducing a hydrogen bond between one or more water molecules introduced into the nanochannel lumen and the at least one anion within the nanochannel lumen, and wherein the nanochannel wall is configured such that the at least one anion within the nanochannel lumen and the associated hydrogen-bonded water molecules provide a flow path blockage within the nanochannel lumen that reduces leakage of hydroxide ions from the nanochannel lumen.

18. The anion-exchange membrane of claim 17, wherein the at least one ion comprise at least one chloride ion, wherein the nanochannel wall is configured to induce formation of hydrogen bond(s) between the chloride ion and each of a first water molecule and a first hydronium molecule within the first nanochannel lumen while preventing a hydrogen bond between the first water molecule and the first hydronium molecule, and wherein the nanochannel wall defines a width of the nanochannel lumen that provides the flow path blockage within the nanochannel lumen using the hydrogen bond between the chloride ion and each of the first water molecule and the first hydronium molecule within the first nanochannel lumen.

19. A cation-exchange membrane comprising: a membrane matrix; and a plurality' of nanochannels each extending through the membrane matrix, each of the plurlaity of nanochannels having a nanochannel wall that defines a nanochannel lumen that extends through the membrane matrix from a nanochannel inlet to a nanochannel outlet, the nanochannel wall including at least one anionic group fixed within or adjacent to the nanochannel wall and isolated from an external solution by a non-conductive and effectively water-impermeable polymeric material at the membrane, wherein the nanochannel wall is configured to reduce leakage of hydroxide ions by inducing a coordinate bond between one or more water molecules introduced into the nanochannel lumen and the at least one cation within the nanochannel lumen, and wherein the nanochannel wall is configured such that the coordinate bond between the one or more water molecules and the at least one cation provide a flow path blockage within the nanochannel lumen that reduces leakage of hydroxide ions from the nanochannel lumen.

20. The cation-exchange membrane of claim 19, wherein the at least one ion comprise at least one hydroxide ion, wherein the nanochannel wall is configured to induce formation of the coordinate bond between the cation within the nanochannel lumen and each of a first water molecule and a first hydroxide molecule within the first nanochannel lumen without forming a hydrogen bond between the first water molecule and the first hydroxide molecule, and wherein the nanochannel wall defines a width of the nanochannel lumen that provides the flow path blockage within the nanochannel lumen using the hydrogen bondbetween the cation and each of the first water molecule and the first hydroxide molecule within the first nanochannel lumen.

21. A method of suppressing surging behavior in electrically driven ion transport, the method comprising the steps of: imparting an electrical potential to span at least one nanotube, the at least one nanotube defining at least one nanochannel that has a nanochannel length, the nanochannel length including a fixed permanent charge at, or adjacent to, the nanochannel; transporting one or more dissolved ions in an aqueous solution through the at least one nanochannel while the electrical potential is imparted to span the at least one nanotube; and while transporting the one or more dissolved ions in the aqueous solution through the at least one nanochannel, attracting one or more counterions from the fixed permanent charge at, or adjacent to, the nanochannel and into the nanochannel lumen to cause a charge separation at the aqueous solution within the at least one nanochannel.

22. A method of forming a membrane comprising the steps of: floating a plurality of nanotube suspensions and a polymerizable mixture on a surface of water contained in a mold; compressing the plurality of nanotube suspensions and the polymerizable mixture at the surface of the water to cause the plurlaity of nanotube suspensions to orient in an upright array with the polymerizable mixture at interstices between the plurality of upright nanotube suspensions; and after compressing, applying photopolymerization to the plurality of upright nanotube suspensions.

23. A method of producing hydrochloric acid or hydrobromic acid together with sodium hydroxide or potassium hydroxide, the method comprising the steps of: providing one or more brines that contain one or more salts selected from the group consisting of: sodium chloride, sodium bromide, potassium chloride, and potassium bromide; using one or more cation-exchange membranes and one or more anion-exchange membranes to produce a product acid stream and a product base stream each having a concentration equal to or greater than 3 moles per liter while consuming energy that is less than energy consumed for production of a same quantity of the product acid stream and asame quantity of the product base stream by conventional electrolysis with production of gaseous HC1 or HBr and absorption by water.

24. The method of claim 23, wherein using one or more cation-exchange membranes and one or more anion-exchange membranes to produce the product acid stream and the product base stream comprises using one or more cation-exchange membranes and one or more anion- exchange membranes to produce the product acid stream and the product base stream each having a concentration greater than 4 moles per liter while consuming energy that is less than energy consumed for production of the same quantity of the product acid stream and the same quantity of the product base stream by conventional electrolysis with production of gaseous HC1 or HBr and absorption by water.

25. The method of claim 24, wherein using one or more cation-exchange membranes and one or more anion-exchange membranes to produce the product acid stream and the product base stream comprises using one or more cation-exchange membranes and one or more anion- exchange membranes to produce the product acid stream and the product base stream each having a concentration greater than 5.5 moles per liter while consuming energy that is less than energy' consumed for production of the same quantity of the product acid stream and the same quantity of the product base stream by conventional electrolysis with production of gaseous HC1 or HBr and absorption by water.

26. A method of increasing the capacity of a conventional electrolysis plant producing hydrochloric acid or hydrobromic acid by producing dilute hydrochloric acid by salt-splitting electrodialysis and using this dilute acid, instead of water, to feed the countercunent absorber used to recover the gaseous HC1 or HBr.

27. The method of claim 26, wherein a concentration of the dilute acid is at least 3 moles per liter.

28. A method of making an ion exchange membrane comprising the steps of: making one or more ty pes of particles where each of the one or more types of particles includes one or more nanochannels having a specified internal diameter and a specified density of charges in or adjacent to the nanochannel wall;assembling the one or more types of particles into a monolayer with open ends of the one or more nanochannels exposed at two faces; and using a chemical process to fix the two faces in place.

29. The method of claim 28, wherein the chemical process comprises irradiation.

30. The method of claim 28. wherein each nanochannel-containing particle has a further non- conductive layer or layers over a layer of charges, and wherein an outer nonconductive layer is configured to aid assembly of the nanochannel particles into the monolayer.

31. The method of claim 30, wherein the outer nonconductive layer covers the layer of charges and is hydrophobic, wherein ends of nanochannel walls are hydrophilic, and wherein a membrane is assembled by mixing nanochannel-containing particles with a non-water- miscible, polymerizable or cross-linkable mixture and spreading a resulting mixture of the nanochannel -containing particles with the non-water-miscible, polymerizable or crosslinkable mixture on water, a water-rich mixture, or on a surface wetted with water or a waterrich mixture.

32. The method of claim 31, wherein the outer nonconductive layer further comprises reactive groups to bond with the polymerizable or cross-linkable mixture.

33. The method of claim 28, wherein two or more classes of nanochannels are included in a predetermined proportion to permit passage of different ions.

34. The method of claim 28, wherein another, different set of nanochannels is included, wherein the another, different set of nanochannels is configured to transport water and exclude both cations and anions.

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