Water distillation membrane and method of use thereof

Abstract

The present disclosure relates to a membrane for water distillation, and a system comprising the membrane. The membrane comprises a porous material comprising cellulosic fibers in which a long chain fatty acid is grafted to one or more hydroxyl groups of the cellulosic fibers. During distillation, the contact angle between water and the membrane is greater than 90 degrees.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 63 / 349,738, filed January 18, 2023, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to water distillation membranes prepared by acylation of hydroxylated porous materials with long-chain fatty acids, and methods of desalinating aqueous solutions using these membranes. Background Art

[0004] A variety of filtration technologies are widely used across various industries, such as water desalination. One of these technologies, membrane separation, offers the opportunity to provide efficient and sustainable production systems. Therefore, new membrane separation technologies are urgently needed.

[0005] For example, in the area of ​​water desalination and purification, water shortages are increasingly affecting both developing and developed countries. Two methods currently widely used for this purpose are distillation and reverse osmosis. A disadvantage of distillation is its high energy consumption. A disadvantage of reverse osmosis is the need for expensive, non-biological membranes. Another disadvantage of reverse osmosis is that the purified water still contains a large amount of dissolved salts, which limits its long-term application for drinking water and irrigation, particularly due to the accumulation of residual salts over time. Furthermore, both processes are expensive: when applied on a large scale, the final cost per cubic meter of water can be very similar for both methods.

[0006] Membrane water distillation is another approach in this field. These membranes must act as a 100% waterproof barrier to aqueous solutions while being highly permeable to gases, allowing the vapor phase to efficiently pass through the membrane pores. While different membrane configurations exist, the distillation process is primarily driven by partial vapor pressure differences, typically caused by temperature differences. To minimize the distance water molecules must travel in the vapor phase, achieving maximum distillation efficiency, the membranes must be as thin as possible. Because the temperature difference can be very small and does not need to reach the boiling point of water at atmospheric pressure, the permeation of gases through the membrane requires very little energy. Some systems can operate using small solar panels, resulting in a very low carbon footprint. This technology can also be used to filter and remove non-volatile contaminants from water.

[0007] However, despite these advantages, water membrane distillation is currently only used on a very small scale in the industry, mainly due to membrane fouling and the high cost of replacing the membranes. Water membrane distillation therefore requires an expensive pre-purification step to eliminate impurities that may reduce the efficiency of the membrane and lead to its premature replacement. Another problem with water membrane distillation is related to the possibility of solid salt deposition due to the evaporation of water. These solid salt deposits can contaminate the membrane and thus seriously hinder the distillation process. Therefore, their concentration needs to be kept below the crystallization threshold, resulting in large amounts of toxic brine (this is also observed in reverse osmosis). Therefore, distillation membranes are more suitable for the purification of low-salinity water, while their use for high-salinity water such as seawater is limited. Current water distillation membranes are usually composed of microporous polytetrafluoroethylene (PTFE) membrane materials (such as ) or polypropylene, which are not bio-sourced, expensive, and mostly non-recyclable.

[0008] Therefore, there is a pressing need for a low-cost, bio-sourced material that can achieve the same barrier properties as existing solutions, but reduces the need for pretreatment and the cost of the membrane and the overall distillation process. This would enable, but is not limited to, large-scale water desalination in centralized or distributed configurations with a very low carbon footprint and low energy consumption. Furthermore, the membrane distillation process could also allow for the production of solid salt as a byproduct for use by the chemical industry.

[0009] Furthermore, distillation membranes can be used to recover elements such as gold, uranium, and lithium. Lithium has become particularly important over the past few decades as lithium-ion batteries have become a crucial energy storage source for automotive and electronic applications. Beyond batteries, lithium is also used in glass and ceramics, chemicals and pharmaceuticals, metallurgy, and lubricants. Consequently, the demand for improved recovery processes and equipment for these applications continues to grow.

[0010] The present inventors have discovered a method for preparing water distillation membranes that is low-cost, efficient, and environmentally friendly. Summary of the Invention

[0011] The present disclosure relates to water distillation membranes that can be used to desalinate aqueous solutions, methods of desalinating aqueous solutions using the membranes, and systems comprising the membranes.

[0012] In various embodiments, the present disclosure relates to membranes for desalination of water, membranes for purifying liquids (such as water), and membranes for recovering components from liquids (such as water) (e.g., for recovering lithium, etc.). The membrane comprises: at least one hydroxylated porous substrate comprising one or more long-chain fatty acids grafted to one or more hydroxyl groups of the substrate, wherein the contact angle θ between water and the substrate is greater than 90°, for example, greater than about 120°, or greater than about 150°. The membrane may comprise more than one substrate, for example, at least two substrates, at least three substrates, etc., wherein the substrates may be the same or different. The membrane and / or substrate may be hydrophobic, for example, having a water pressure threshold for water repellency of at least 5 cm H2O, or superhydrophobic (having a contact angle approaching 180°). The hydroxylated substrate is solid and may, for example, be selected from a porous, flexible or rigid substrate. In various embodiments, the substrate comprises, consists essentially of, or consists of a cellulosic material or fiber, which is optionally cross-linked. In various embodiments, the fatty acid can be selected from C6-C50 fatty acids, preferably C8-C50 fatty acids, more preferably C14-C50 fatty acids, and most preferably C18-C50 fatty acids. As non-limiting examples, the fatty acid can be selected from behenic acid, palmitic acid, stearic acid, or a combination of two or more of the above.

[0013] In a further embodiment, the present disclosure relates to a method for desalinating an aqueous solution, the method comprising evaporating the liquid through a membrane according to the present disclosure. For example, the method can be a method for desalinating water, a method for purifying a liquid (such as water), or a method for recovering a component from a liquid (such as water), for example, a method for recovering lithium from an aqueous solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1A is an image showing the effect of salt water evaporating from the front of a four-ply tissue made of cellulose grafted with long-chain fatty acids.

[0015] Figure 1B is another image showing the effect of salt water evaporating from the reverse side of a four-ply tissue made of cellulose grafted with long-chain fatty acids.

[0016] Figure 2 This image compares a distilled water droplet and a saturated brine droplet. The higher contact angle observed for the saturated brine droplet indicates that the membrane's hydrophobicity improves with increasing salt concentration. This is due to the salting-out effect of dissolved salts and specifically explains why the water in the brine can be completely evaporated, leaving only the solid salt without leakage through the membrane. DETAILED DESCRIPTION

[0017] The present disclosure relates to a water distillation membrane prepared by acylation of a porous hydroxylated solid substrate with a long-chain fatty acid, a method for desalination of an aqueous solution using the membrane, and a desalination system comprising the membrane.

[0018] In various embodiments, useful hydroxylated solid substrates include solid materials containing reactive hydroxyl groups (-OH). The hydroxylated material is preferably as porous as possible to maximize the flow rate of gas through the membrane; the material may contain reactive hydroxyl groups inherently or may be treated to impart reactive hydroxyl groups. The solid material may be rigid or flexible and capable of at least partially extending in a plane. For example, the porous solid material containing reactive hydroxyl groups may be a cellulosic material, such as paper, tissue paper, cardboard, or fabric.

[0019] By way of non-limiting example, the hydroxylated solid substrate can be entirely or predominantly cellulosic, such as paper, cardboard, or any material consisting primarily or entirely of cellulosic materials or fibers.

[0020] For example, the substrate may comprise one or more cellulose sheets formed partially or entirely of cross-linked cellulose fibers interconnected by hydrogen bonds and covalent bonds formed by at least one set of cross-linking atoms. A useful, but non-limiting, set of cross-linking atoms is a derivative of 1-chloro-2,3-propylene oxide. A cross-linked cellulose substrate can offer additional advantages for membrane distillation, such as allowing the grafted long-chain fatty acids to have lower rotational freedom. When the rotational freedom is minimized, contact angles are observed to be higher and more stable over time. This can, for example, facilitate long-term barrier performance against highly concentrated salt water.

[0021] By acylation of the hydroxylated solid substrate with a long-chain fatty acid, the substrate is rendered substantially impermeable to liquid aqueous salt solutions while not affecting its permeability to gases. In various embodiments, the fatty acid useful as the acylating agent is selected from fatty acids having 6 to 50 carbon atoms, such as fatty acids having 8 to 50 carbon atoms, 14 to 50 carbon atoms, or 18 to 50 carbon atoms; or fatty acids having 6 to 40 carbon atoms, 8 to 40 carbon atoms, 14 to 40 carbon atoms, or 18 to 40 carbon atoms; or fatty acids having 12 to 30 carbon atoms, 16 to 28 carbon atoms, or 18 to 24 carbon atoms. For example, the fatty acid can have a C6, C7, C8, C9, C10, C11, C12, C13, C14, C15, C16, C17, C18, C19, C20, C21, C22, C23, C24, C25, C26, C27, C28, C29, C30, C31, C32, C33, C34, C35, C36, C37, C38, C39, C40, C41, C42, C43, C44, C45, C46, ​​C47, C48, C49 or C50 hydrocarbon chain, or can have a hydrocarbon chain with a range of carbon atoms with any of the aforementioned values ​​as upper and lower limits. By way of non-limiting example only, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, melissic acid, linoleic acid, oleic acid, palmitoleic acid, arachidonic acid, docosahexaenoic acid, or a mixture of two or more thereof may be used. In one embodiment, the fatty acid comprises, consists essentially of, or is composed of behenic acid, palmitic acid, stearic acid, or a combination of two or more thereof. In certain embodiments, the fatty acid used comprises, consists essentially of, or is composed of fatty acids of plant and / or animal origin.

[0022] In the acylation reaction, a long-chain fatty acid reagent reacts with the reactive hydroxyl groups on the solid substrate to form an ester group between the substrate and the hydrocarbon chain of the fatty acid as follows:

[0023] M-OH+R-CO-Cl→MO-CO-R+HCl

[0024] in:

[0025] M-OH = hydroxylated solid substrate;

[0026] R = hydrocarbon chain, such as C5-C49; a and

[0027] R-CO-Cl = long chain fatty acid chloride.

[0028] The acylation process can be carried out by any known means. For example, the methods described in WO 2022 / 033698, WO 2022 / 117926 or WO 2023 / 233202 can be used, which are incorporated herein by reference in their entirety.

[0029] As an example only, the porous hydroxylated solid substrate can be treated with fatty acid chloride, for example, by contacting the substrate with fatty acid chloride, applying fatty acid chloride to the substrate, or distributing fatty acid chloride on the substrate. The hydroxylated solid substrate can be treated with fatty acid chloride in any way, for example, using a distribution device with a coating surface that can deposit fatty acid chloride at least on the surface of the substrate and optionally deposit it into the thickness of the substrate. The fatty acid chloride can be present in a composition comprising other components (such as solvents, additives, auxiliary agents, etc.), or it can be the only component that the substrate contacts during the acylation process. Once the porous hydroxylated solid substrate is treated with fatty acid chloride, the treated substrate is heated to a temperature (called acylation temperature) that is lower than the vaporization temperature of at least one fatty acid chloride on the substrate, thereby causing the hydroxylated substrate to be acylated by the reaction between the gaseous fatty acid chloride and at least one reactive hydroxyl group of the substrate.

[0030] The hydroxylated solid substrate may be grafted with fatty acids at a grafting rate ranging from about 0.01% w / w to about 1% w / w.

[0031] Specific heating devices, such as, but not limited to, air ovens, may be used to allow the fatty acid chlorination reagent to reach equilibrium between its liquid and gaseous states.The acylation temperature may vary, but in certain embodiments may be from about 140°C to about 300°C.

[0032] The treated substrate can be maintained at the acylation temperature for any desired period of time to allow the reaction process to proceed. For example, the treated substrate can be heated for a period of time that can range from about 0.1 seconds to several seconds.

[0033] The membrane comprising the substrate according to the present disclosure has unexpected and favorable advantages in being used for distillation applications. For example, the substrate has a high degree of hydrophobicity, and even in certain embodiments is super-hydrophobic, but still has permeability to gas (including water vapor). In addition, the contact angle θ of the substrate according to the present disclosure and water is greater than 90 °, for example, greater than or equal to 100 °, greater than or equal to 110 °, greater than or equal to 120 °, greater than or equal to 130 °, greater than or equal to 140 °, or greater than or equal to 150 °, and the durability of the contact angle can be maintained. Moreover, the contact angle θ increases with the increase of the fatty acid carbon chain length. Therefore, the increased hydrophobicity and increased contact angle θ allow the manufacture of membranes with higher porosity according to the present disclosure, which is very suitable for filtration applications such as desalination, water purification, lithium recovery, etc.

[0034] Furthermore, the cellulose-based substrates according to the present disclosure, unlike typical membranes, can maintain recyclability and compostability and are highly cost-effective. The cellulose-based membranes according to the present disclosure also have the additional advantage of easily recovering collected solids during the filtration process because the cellulose material can be burned without releasing harmful substances into the environment.

[0035] Furthermore, because the membranes according to the present disclosure comprise a material having high gas permeability but little or no capillary suction capacity, water vapor is allowed to permeate driven by local vapor pressure differentials. Such local vapor pressure differentials can be very small and therefore may require very little energy input, so that systems comprising membranes according to the present disclosure can, for example, be solar-powered, such as using photovoltaic panels or solar thermal collectors.

[0036] Thus, membranes and systems according to the present disclosure provide opportunities for desalination applications that are more efficient, less expensive, and more environmentally friendly than traditional filtration systems, and can be used on either small or large scales.

[0037] Furthermore, because they can provide distilled water at low cost almost anywhere, the membranes and systems can be used in water electrolysis, particularly in acidic systems where it is necessary to prevent catalyst fouling.

[0038] In a particularly preferred embodiment, the method according to the present disclosure is a method for desalinating water using a membrane or system as described herein. In another preferred embodiment, the method according to the present disclosure is a method for purifying a liquid (e.g., water) using a membrane or system as described herein. In yet another preferred embodiment, the method according to the present disclosure is a method for recovering a component (e.g., lithium) from a liquid (e.g., water) using a membrane or system as described herein.

[0039] Having described various embodiments of the present invention in detail, it will be apparent that modifications and variations may be made to these embodiments without departing from the scope of the present disclosure as defined in the appended claims. Furthermore, it should be understood that while the examples illustrate one embodiment of the present disclosure, they are not intended to limit the various aspects described herein. It should be understood that all definitions provided herein apply solely to the present disclosure.

[0040] As used herein, the terms "comprising," "having," and "including" (or their verb forms "comprise," "have," and "include") are used in their open, non-limiting sense.

[0041] In this application, unless expressly stated otherwise, the use of the singular includes the plural. The terms "a," "an," "the," and "at least one" are understood to encompass both the singular and the plural unless the context clearly dictates otherwise. "One or more" and "at least one" are interchangeable and specifically include individual components as well as mixtures / combinations.

[0042] The term "and / or" should be understood to include a conjunction and / or an optional one of them.

[0043] As used herein, the phrases "and mixtures thereof," "a mixture thereof," "and combinations thereof," "a combination thereof," "or mixtures thereof," "or mixtures thereof," "or combinations thereof," "or a combination thereof," and the like are used interchangeably to refer to a list of ingredients immediately preceding the phrase, e.g., "A, B, C, D, or mixtures thereof" means that A, B, C, D, A+B, A+B+C, A+D, A+C+D, etc. may be selected without limiting variations thereof. Thus, these ingredients may be used alone or in any combination.

[0044] For the purpose of this disclosure, it should be noted that in order to provide a more concise description, some quantitative expressions herein are not followed by the term "about". It should be understood that, regardless of whether the term "about" is explicitly used, each numerical value given herein is intended to refer to the actual value given, and also to an approximate value that can be reasonably inferred based on a person of ordinary skill in the art, including approximate errors due to experimental and / or measurement conditions.

[0045] All ranges and values ​​given herein are intended to include subranges and subvalues ​​with any disclosed points as endpoints, and all endpoints are intended to be inclusive unless an exception is explicitly stated. Thus, a range of "1% to 10%, e.g., 2% to 8%, e.g., 3% to 5%" is intended to encompass ranges such as "1% to 8%, "1% to 5%, "2% to 10%," and so forth. All numbers, values, ranges, etc. are intended to be modified by the term "about," even if not explicitly stated, unless otherwise explicitly stated. Similarly, a range of "about 1% to 10%" is intended to modify the 1% and 10% at both ends by "about." As used herein, the term "about" means a difference of up to ±10% from the stated value, e.g., ±9%, ±8%, ±7%, ±6%, ±5%, ±4%, ±3%, ±2%, or ±1%. Similarly, the endpoints of all ranges are understood to be disclosed individually; for example, a range of 1:2 to 2:1 is considered to disclose ratios of 1:2 and 2:1.

[0046] As used herein, if a component is described as being present "up to" a certain amount, this means that the component is actually present in the composition, ie, it is present in an amount greater than 0%.

[0047] Unless otherwise specified, all amounts and ratios described herein are given based on the total weight of the composition. Unless otherwise specified, all percentages herein are percentages by weight of active material.

[0048] As used herein, the term "film" is intended to refer to a single substrate or multiple substrates that are typically, but not necessarily, in physical contact with one another. For example, a single layer of a fatty acid-treated hydroxylated cellulose material may constitute a film, or a film may include multiple layers of fatty acid-treated hydroxylated cellulose material, such as a double-layer, triple-layer, etc. tissue. However, it should be understood that if a film comprises more than one substrate, the substrates may be the same or different, and a film may also comprise one or more substrates that have been fatty acid-treated according to the present disclosure and one or more substrates that have not been fatty acid-treated according to the present disclosure.

[0049] As used herein, the term "reactive hydroxyl (-OH)" refers to accessible hydroxyl groups that are capable of reacting with long-chain fatty acids under the gaseous conditions described herein. These hydroxyl groups can be located on the surface of the material or within the thickness of the material and, unless otherwise expressly stated, should not be limited in their location.

[0050] As used herein, the term "hydrophobic" and variations thereof means that the pressure required to cause a liquid to penetrate the membrane by capillary action is at least about 5 cm of water column height, which is calculated according to Jurin's law: P = -2(γ)*cos(θ) / d, where P is the pressure required to drive water through the membrane by capillary action, γ is the surface tension of the liquid permeating the membrane, θ is the acute contact angle between the droplet and the membrane, and d is the pore size of the membrane.

[0051] From Jurin's law, it can be easily deduced that a higher contact angle allows a larger pore size to obtain the same critical pressure.

[0052] The term "superhydrophobic" refers to a situation where the contact angle approaches 180°. At such a high contact angle, a water droplet no longer adheres to the substrate surface but is able to roll freely on the surface.

[0053] The following examples are provided to illustrate one embodiment of the present disclosure and are not intended to be limiting. It will be apparent to those skilled in the art that various modifications and variations may be made to the present embodiment without departing from the spirit or scope of the present invention.

[0054] Example

[0055] The following examples are intended to be non-limiting and illustrative only.

[0056] Example 1

[0057] A 20 cm x 20 cm membrane, comprising four layers of a porous cellulose fiber substrate (lightweight tissue), was treated with 0.1% w / w stearic acid chloride. The acylation reaction was then completed by heating in an air-circulating oven at 180°C for a few seconds, after which the substrate was cooled at room temperature, resulting in a hydrophobic cellulose fiber membrane.

[0058] After the substrate has returned to room temperature, a bag is formed, salt water is added, and the bag is sealed. The membrane containing salt water is placed in an air-circulating oven at 80°C until all water has evaporated through the membrane. The membrane is then removed from the oven and the bag is opened.

[0059] Figure 1A The film after the bag is opened is shown. It can be seen that when the water evaporates, the salt remains. In addition, it is observed that except for the substrate layer in contact with water (substrate 1, Figure 1A ), the remaining three substrates in the membrane (substrates 2-4) remain completely intact. This can be seen in Figure 1B , which shows substrate 4 at the end of the experiment.

[0060] Thus, this example demonstrates that the treated cellulose membrane can be used to effectively distill salt water, wherein the membrane allows water vapor to permeate but retains the salt crystals. This is also confirmed by the fact that the remaining three layers of the substrate remain intact.

[0061] Example 2

[0062] A membrane with a size of 20 cm×20 cm comprises four layers of substrates made of porous cellulose fibers, the materials of which are similar to those used in Example 1 and are treated in the same manner as in Example 1 to obtain a hydrophobic cellulose fiber membrane.

[0063] An apparatus was prepared to investigate the distillation capacity of treated cellulose fiber membranes for brine as follows. Two commercially available stackable plastic boxes (10 cm x 10 cm x 5 cm high) with their openings facing upward were placed one above the other to form a 2 cm deep liquid reservoir in the lower box. Fifty (50) holes with a diameter of 5 mm were drilled in the bottom of the upper box to allow gas exchange between the two boxes. The bottom of the drilled holes in the upper box was covered with a plastic mesh support having a size of 10 cm x 10 cm and a pore size of 1 mm. The hydrophobic cellulose fiber membrane was then placed on the plastic mesh so that the membrane formed a water reservoir in the upper box with the excess membrane edge extending upward along both sides of the box. Brine with a salinity of 35 g / L was added to the water reservoir until the depth reached 2 cm.

[0064] Two thermometer probes were then attached to the device, one in contact with the saline solution in the upper box, and the second in contact with the bottom of the lower box. The device was then placed on a cold pad and heated under an infrared lamp. The cold pad maintained the contents of the lower box at a temperature close to 0°C, while the infrared lamp heated the saline solution in the upper box to above room temperature.

[0065] After 30 minutes, water condensation was observed in the lower box, indicating that the heated brine in the upper box's reservoir was distilling into the cooler lower box. Thermal equilibrium was observed, with the lower box's temperature approximately 5°C and the upper box's temperature approximately 70°C. Four hours later, the two boxes were disconnected, and the amount of condensed water in the lower box was measured to be 50 mL.

[0066] The condensate was then evaluated to determine its salt content. Tasting the condensate revealed no detectable saltiness, and subsequent evaporation of the condensate revealed only minimal residual solids. Thus, this example demonstrates that an apparatus comprising a treated cellulose fiber membrane and a system including such a membrane can effectively distill salt water at temperatures below the boiling point of water.

[0067] The above examples demonstrate that water desalination methods using membranes according to the present disclosure have advantages over existing methods, such as reverse osmosis, which desalinates water but does not collect the salt, but instead discharges it back into the ocean.

Claims

1. A method for desalting an aqueous solution, the method comprising: Water is evaporated from a solution through a membrane comprising: At least one hydroxylated porous substrate comprising one or more long chain fatty acids grafted to one or more hydroxyl groups of the substrate, wherein the contact angle θ between water and the substrate is greater than 90°.

2. The method of claim 1, wherein the hydroxylated substrate is flexible or rigid.

3. A method according to any preceding claim, wherein the hydroxylated substrate comprises cellulose, and the cellulose is optionally cross-linked.

4. A method according to any preceding claim, wherein the hydroxylated substrate consists essentially of cellulose, and the cellulose is optionally cross-linked.

5. A method according to any preceding claim, wherein the hydroxylated substrate consists of cellulose, and the cellulose is optionally cross-linked.

6. A method according to any preceding claim, wherein the long chain fatty acid is selected from C6-C50 fatty acids, preferably C8-C50 fatty acids, more preferably C14-C50 fatty acids, most preferably C18-C50 fatty acids.

7. The method according to any preceding claim, wherein the long-chain fatty acid is selected from behenic acid, palmitic acid, stearic acid, or a combination of two or more thereof.

8. The method according to any preceding claim, wherein the contact angle θ is greater than or equal to 100°, preferably greater than or equal to 110°, more preferably greater than or equal to 120°, even more preferably greater than or equal to 130°, still more preferably greater than or equal to 140°, and most preferably greater than or equal to 150°.

9. A method for desalinating or purifying water, the method comprising: Water is distilled through a membrane comprising: At least one hydroxylated substrate comprising one or more long chain fatty acids grafted to one or more hydroxyl groups of the substrate, wherein the contact angle θ between water and the substrate is greater than 90°.

10. The method of claim 9, wherein the hydroxylated substrate is porous.

11. The method according to any one of claims 1 to 10, wherein the water filtration pressure threshold of the membrane is at least 5 cm H2O.

12. A system for distilling a liquid, the system comprising at least one membrane, the membrane comprising: At least one hydroxylated porous substrate comprising one or more long chain fatty acids grafted to one or more hydroxyl groups of the substrate, wherein the contact angle θ between water and the substrate is greater than 90°.

13. The system of claim 12, wherein the hydroxylated substrate comprises cellulose, and the cellulose is optionally cross-linked.

14. The system of claim 12 or 13, wherein the hydroxylated substrate consists essentially of cellulose, and the cellulose is optionally cross-linked.

15. The system of any one of claims 12 to 14, wherein the hydroxylated substrate is composed of cellulose, and the cellulose is optionally cross-linked.

16. The system according to any one of claims 12 to 15, wherein the long chain fatty acids are selected from C6-C50 fatty acids, preferably C8-C50 fatty acids, more preferably C14-C50 fatty acids, and most preferably C18-C50 fatty acids.

17. The system according to any one of claims 12 to 16, wherein the long-chain fatty acid is selected from behenic acid, palmitic acid, stearic acid, or a combination of two or more thereof.

18. The system according to any one of claims 12 to 17, wherein the contact angle θ is greater than or equal to 100°, preferably greater than or equal to 110°, more preferably greater than or equal to 120°, even more preferably greater than or equal to 130°, still more preferably greater than or equal to 140°, and most preferably greater than or equal to 150°.

19. The system according to any one of claims 12 to 17, which is a system for desalinating water.

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