An aminated resin material, its preparation and application

Abstract

The present invention relates to an aminated resin material and its preparation and application. The aminated resin uses anisotropic microspheres (Janus) as the matrix, and then atomic transfer radical polymerization (ATRP) technology is used to graft polymer brushes on the surface of the microspheres to increase the density of functional groups on the microsphere surface. Finally, amino groups are introduced onto the microsphere surface through an epoxy-amine ring-opening reaction to obtain the aminated resin, which is used as a metal ion adsorption material. The present invention has the characteristics of simple equipment, pure products, monodispersity, and uniform pore size. The preparation of the aminated resin in the present invention has the advantages of controllable operation and simple method.

Description

Technical Field

[0001] The present invention relates to an aminated resin material and its preparation and application. Specifically, an anisotropic (Janus) microsphere is prepared by seed emulsion polymerization using glycidyl methacrylate as a monomer and pentaerythritol tetraacrylate as a crosslinking agent. Then, an atom transfer radical polymerization technique (ATRP) is used to graft a polymer brush on the surface of the microsphere to increase the functional group density on the surface of the microsphere. Finally, an amine group is introduced onto the surface of the microsphere through an epoxy-amine ring-opening reaction to prepare an aminated resin as a metal ion adsorption material. Background Art

[0002] The term "Janus" comes from the ancient Roman god Janus with two faces. It is a type of colloidal material with non-centrosymmetric characteristics in morphology, composition, and properties. It was proposed by the French scientist De Gennes when he won the Nobel Prize in 1991. Naming it Janus is an apt portrayal of asymmetric particles (Literature 1. Gennes PG. Soft matter. Science. 1992, 256(5056): 495 - 497). In the past few decades, anisotropic microparticles with controllable morphology and small size distribution, including raspberry-like, bowl-like, snowman-like, and open hollow shapes, etc., have attracted research interest due to their applications in fields such as sensors, self-assembly construction, drug delivery and controlled release, solid surfactants, catalysts, etc. (Literature 2. Biswas A, Nagaraja AT, and McShane MJ. Fabrication of nanocapsule carriers from multilayer-coated vaterite calcium carbonate nanoparticles. Acs Appl Mater Inter. 2014; 6(23): 21193 - 21201). Many methods have been developed to synthesize anisotropic microparticles, such as emulsion polymerization, seed swelling method, suspension polymerization, self-assembly hetero-condensation method, etc. Among them, emulsion polymerization has become the most basic means for synthesizing anisotropic particles due to its simplicity, large-scale production, and controllability. Janus materials have a clearly divided two-component and two-structure, and special functions can be compounded on both sides of Janus materials, thus obtaining more applications (Literature 3. Zhou Wanrong, etc. Preparation and functional application of Janus particles, Progress in Chemistry, 2018, 30, 1601 - 1614.). In addition, the rich epoxy groups of glycidyl methacrylate can bridge between biomacromolecules / drug molecules and carriers, which is of great significance in biology and biomedicine. Asymmetric polystyrene / glycidyl methacrylate Janus particles have potential applications in drug delivery and antibacterial materials.

[0003] With the development of human industrial activities, heavy metal pollution has spread globally. Since heavy metal ions are harmful to organisms, the removal of heavy metal ions from industrial wastewater has attracted much attention. Copper plays a crucial role in animal metabolism (Reference 4. Yin K, Wang QN, Lv M, et. al . Microorganism remediation strategies towards heavy metals. Chem Eng J. 2019, 360: 1553 - 1563). However, excessive intake of copper can cause serious toxicological problems, such as vomiting, cramps, convulsions, and even death. Many methods have been developed, such as chemical precipitation, membrane filtration, ion exchange, mixed solvent extraction, and adsorption. Adsorption has become one of the commonly used methods in wastewater treatment due to its simple operation. The surface functional groups of the adsorbent have a great influence on the adsorption capacity, which is one of the important characteristics in the evaluation of adsorbents. Adsorbents with carboxyl groups remove adsorbents through ion exchange, while those containing nitrogen, including amine, hydrazine, thioamide, and imidazoline groups, can not only chelate cationic metal ions but also adsorb anionic adsorbates through electrostatic interactions (Reference 5. Chen Y, He M, Wang C, et. al . A novel polyvinyltetrazole - grafted resin with high capacity for adsorption of Pb(ii), Cu(ii) and Cr(iii) ions from aqueous solutions. J Mater Chem A. 2014, 2: 2). It is worth noting that amine groups have been found to belong to the most effective group for the removal of pollutants from aqueous solutions. In addition, an increase in the density of functional groups on the adsorbent surface may increase the adsorption capacity. Summary of the Invention

[0004] The present invention relates to an aminated resin material and its preparation and application. Specifically, Janus microspheres are prepared by seed emulsion polymerization using GMA as a monomer and PETA as a crosslinking agent. Then, atom transfer radical polymerization (ATRP) technology is used to graft polymer brushes on the surface of the microspheres to increase the functional group density on the surface of the microspheres. Finally, an amino group is introduced onto the surface of the microspheres through an epoxy-amine ring-opening reaction to prepare an aminated resin as a metal ion adsorption material.

[0005] The specific preparation process of the aminated resin includes the following steps:

[0006] (1) Preparation of polystyrene seed microspheres by dispersion polymerization: 0.2 - 0.3 g of azobisisobutyronitrile (AIBN), 1 - 2 g of polyvinylpyrrolidone (PVP), 90 - 100 mL of ethanol, and 5 - 10 mL of water are added to a 250 mL single-neck round-bottom flask and ultrasonicated for 20 - 30 min until the solids are completely dissolved. 20 - 30 mL of styrene is added thereto, and at 60 - 80 °C, with a rotation speed of 100 - 200 rpm, mechanical stirring is carried out for 20 - 24 h. After the reaction ends, it is washed 3 - 4 times with 100 - 200 mL of ethanol, and the product is stored in a 0.2% - 0.5% (w%) sodium dodecyl sulfate (SDS) solution.

[0007] (2) Preparation of Janus particles by seed emulsion polymerization: 0.1 - 0.4 mL of GMA, 30 - 120 mg of PETA, 0 - 0.42 mL of toluene, 0 - 0.42 mL of cyclohexanol, and 20 - 40 mg of DMPA are ultrasonically emulsified and then mixed with 70 - 150 mg of styrene seeds, and swollen at room temperature at 100 - 300 rpm for 16 - 24 h, and then exposed to light for reaction for 3 - 8 h. The product is washed alternately with water and ethanol 2 - 4 times and vacuum dried at 60 - 70 °C for 6 - 12 h to obtain semi-raspberry-shaped Janus particles.

[0008] (3) Synthesis of a macromolecular initiator: 200 - 600 mg of Janus microspheres are taken and dispersed in 8 - 12 mL of H2SO4 (0.1 - 0.2 mol / L) solution and reacted for 12 - 36 h (40 - 80 °C). The product is washed with deionized water until neutral and vacuum dried. The dried microspheres are accurately weighed and dissolved in 8 - 12 mL of dichloromethane, magnetically stirred in an ice bath for 20 - 40 min, then 0.2 - 0.6 mL of triethylamine is added and stirred for 8 - 12 min, 0.3 - 0.5 mL of 2-bromoisobutyryl bromide and 3 - 5 mg of 4-dimethylaminopyridine are added dropwise, and the reaction is carried out for 1 - 3 h, and then transferred to react at 20 - 30 °C for 12 - 36 h. Subsequently, it is washed 3 - 5 times with dichloromethane, deionized water, and ethanol respectively, and then vacuum dried at 50 - 60 °C for 10 - 12 h to obtain the product.

[0009] (4) Grafting GMA using ATRP technology: Accurately weigh 100 - 300 mg of the macromolecular initiator, 10 - 20 mg of 2,2'-bipyridine, and 0.1 - 0.2 mL of GMA, and disperse them in a three-necked flask containing 8 - 12 mL of a mixed solution of ethanol and deionized water. Connect the flask to the ATRP device, add 5 - 8 mg of CuBr, and then purge with nitrogen to remove oxygen. Carry out the polymerization reaction under the protection of nitrogen at 50 - 60 °C for 10 - 12 h. After the reaction, wash the product 3 - 5 times with ethanol, deionized water, and an aqueous solution of 0.1 - 0.2 mol / L disodium ethylenediaminetetraacetate. Vacuum dry the intermediate product. Obtain the product Brush@Janus microspheres.

[0010] (5) Preparation of aminated resin by epoxy-amine ring-opening: Put the dried Brush@Janus microspheres into a round-bottom flask containing 6 - 8 mL of THF and 10 - 12 mL of ethylenediamine. Stir the reaction mixture at 100 - 150 rpm and 10 - 25 °C for 10 - 12 h. After the reaction is completed, thoroughly wash the aminated resin with acetone and deionized water and dry it at room temperature.

[0011] The raw materials for preparing the aminated resin involved in the present invention are cheap and easily available. Using ATRP technology, it has a large adsorption capacity for metal ions.

[0012] The present invention uses anisotropic microspheres (Janus) as the matrix, then adopts atom transfer radical polymerization technology (ATRP) to graft polymer brushes on the surface of the microspheres to increase the functional group density on the surface of the microspheres, and finally introduces amino groups onto the surface of the microspheres through epoxy-amine ring-opening reaction to obtain an aminated resin as a metal ion adsorption material.

[0013] The present invention has the characteristics of simple equipment, pure products, monodispersity, and uniform pore size. The preparation of the aminated resin in the present invention has the advantages of controllable operation and simple method. Brief Description of the Drawings

[0014] Figure 1 . Schematic diagram of the preparation of raspberry-like aminated resin in the examples.

[0015] Figure 2 . Helium ion electron microscope images of raspberry-like microspheres in the examples. (a) is the seed microspheres, and (b) is the Janus microspheres.

[0016] Figure 3 . Infrared characterization diagrams of the materials in the examples. (a) Janus microspheres, (b) macromolecular initiator, (c) Brush@Janus microspheres, (d) aminated resin.

[0017] Figure 4. XPS spectra of the aminated resin before and after adsorbing copper ions in Example 2. (a) Before adsorption, (b) full XPS spectrum after adsorption; (c) high-resolution XPS spectrum of N 1s before adsorption (d) high-resolution XPS spectrum of Cu 2p after adsorption.

[0018] Figure 5 . (a) Adsorption isotherm of copper ions at room temperature in Example 1, (b) fitting curve of Langmuir equation.

[0019] Figure 6 . (a) Kinetic adsorption curve of copper ions at room temperature in Example 2, (b) fitting curve of the second-order kinetic equation for the adsorption process of copper ions by the aminated resin. Detailed implementation manners

[0020] Preparation of the aminated resin in Example 1

[0021] Preparation of styrene seeds: Weigh 1.5 g of polyvinylpyrrolidone (PVP) and 0.2 g of 2,2'-azobisisobutyronitrile (AIBN) in a round-bottom flask, add 20 mL of styrene, 95 mL of anhydrous ethanol (chromatographically pure), and 5 mL of water, and ultrasonicate until the solids are completely dissolved. Then place the round-bottom flask in an oil bath at 70 °C and react at 90 rpm for 6 h. After the reaction, centrifuge the product and wash it three times with anhydrous ethanol. The washed product is dispersed in a 0.2% SDS solution by mass fraction to obtain a styrene seed solution with a density of 0.3 g / mL.

[0022] Preparation of Janus microspheres as the matrix: In a 25 mL small beaker under light-shielding treatment, add the organic phase: 0.21 mL of functional monomer GMA, 0.07 g of cross-linking agent PETA, and 0.66 mL of porogen cyclohexanol. Weigh 40 mg of photoinitiator DMPA and dissolve it in the organic phase. Add 8.4 mL of aqueous phase PVA (5%)~SDS (0.2%) to the organic phase to obtain a mixed solution with oil-water two-phase stratification. Emulsify the mixed system in a cell disruptor until the solution becomes milky white and there are no oil droplets on the surface. Mix the emulsion with 110 mg of styrene seed solution, transfer the mixed system into a quartz conical flask, and swell it at 200 rpm in an oscillating incubator at room temperature for 24 h, then expose it to light and react for 6 h, with an ultraviolet light intensity of 6 μW / cm 2 . Centrifuge to collect the product, and wash it three times alternately with water and ethanol to remove the unreacted organic phase and aqueous phase, and dry it at 60 °C.

[0023] Synthesis of Macromolecular Initiator: 400 mg of Janus microspheres were taken and dispersed in 10 mL of H2SO4 (0.1 mol / L) solution and reacted for 24 h (60 °C). The product was washed with deionized water until neutral and dried under vacuum. 350 mg of the dried microspheres were accurately weighed and dissolved in dichloromethane (10 mL). Under ice bath, it was magnetically stirred for 30 min, then triethylamine (0.4 mL) was added and stirred for 10 min. 2-bromoisobutyryl bromide (0.32 mL) and 4-dimethylaminopyridine (3.04 mg) were added dropwise and reacted for 2 h, and then transferred to react at 25 °C for 24 h. Subsequently, it was washed 5 times with dichloromethane, deionized water, and ethanol in sequence, and then dried under vacuum at 60 °C for 12 h to obtain the product. Grafting GMA by ATRP technology: 300 mg of macromolecular initiator, 15.48 mg of 2,2'-bipyridine, and 0.3 mL of GMA were accurately weighed and dispersed in a three-necked flask containing 10 mL of a mixed solution of ethanol and deionized water (4 / 1, v / v). The flask was connected to the ATRP device, and after adding 7.5 mg of CuBr, the system was purged with nitrogen to remove oxygen. The polymerization reaction was carried out under the protection of nitrogen at 60 °C for 12 h. After the reaction, the product was washed 5 times with ethanol, deionized water, and 0.1 mol / L disodium ethylenediaminetetraacetate aqueous solution in sequence. The intermediate product was dried under vacuum. The product Brush@Janus microspheres was obtained.

[0024] Preparation of Aminated Resin by Ring-Opening of Epoxy-Amine: 400 mg of dried Brush@Janus microspheres were put into a round-bottom flask containing 6 mL of THF and 12 mL of ethylenediamine. The reaction mixture was stirred at 150 rpm and 25 °C for 12 h. After the reaction was completed, the aminated resin was thoroughly washed with acetone and deionized water in sequence and dried at room temperature.

[0025] Adsorption Experiment: First, 10 mg of aminated resin materials were weighed separately into 10 mL centrifuge tubes, and 10 mL of copper ion solutions with concentrations of 1 - 10 mmol / L were added. Then the centrifuge tubes were placed on a constant temperature shaker and shaken overnight at 150 rpm for 24 h. Finally, the solution was filtered with a filter membrane with a diameter of 0.24 μm, and the filtrate was collected. After the filtrate was centrifuged and filtered through the membrane, 200 μL of chromogenic reagent (sodium diethyldithiocarbamate), 1 mL of stabilizer (starch), and 100 μL of buffer (NH4Cl - NH3 . H2O) were added and diluted to 50 times the original volume to detect the absorbance.

[0026] Material Synthesis and Characterization:

[0027] Figure 1Schematic diagram for the preparation of aminated resin in the examples. First, we used the seed emulsion polymerization method to prepare Janus microspheres with glycidyl methacrylate as the monomer and pentaerythritol tetraacrylate as the crosslinking agent. Then, we grafted polymer brushes on the surface of the microspheres using ATRP technology to increase the functional group density on the surface of the microspheres. Finally, we introduced amino groups onto the surface of the microspheres through an epoxy-amine ring-opening reaction to prepare aminated resin as a metal ion adsorbent material.

[0028] Figure 2 Helium ion electron micrographs of the seed microspheres and Janus microspheres in the examples. Before the reaction, the seed microspheres were spherical with a particle size of 2 μm; after the reaction, the shape was semi-raspberry-like microspheres with a particle size of 3 - 4 μm. Figure 3 Infrared characterization diagrams of Janus microspheres, macroinitiator, Brush@Janus microspheres, and aminated resin. There are two peaks at 757 cm -1 and 906 cm -1 , which are attributed to the characteristic absorption of epoxy groups in Janus microspheres (symmetric and asymmetric stretching vibrations of C-O) ( Figure 3 a). After the epoxy ring-opening, a characteristic absorption peak of hydroxyl groups appears at 3463 cm -1 , and the absorption peak of ester carbonyl groups at 1733 cm -1 is enhanced, indicating the successful introduction of 2-bromoisobutyryl bromide ( Figure 3 b). In the spectrum of Brush@Janus microspheres, the peaks at 757 cm -1 and 906 cm -1 correspond to the stretching vibrations of epoxy groups, which are formed by grafting GMA with the macroinitiator, indicating the successful introduction of epoxy groups in GMA onto the surface of Janus microspheres ( Figure 3 c). The aminated resin is synthesized by the epoxy-amine ring-opening reaction of the epoxy groups on the surface of Brush@Janus microspheres with ethylenediamine. Therefore, the peak of the epoxy group almost disappears in the spectrum of the aminated resin, but a relatively wide and strong peak appears at 3370 cm -1 , which is the stretching vibration peak of amino groups ( Figure 3 d).

[0029] Material Application

[0030] To investigate the adsorption performance of the aminated resin, we separately weighed 10 mg of the aminated resin material into 10 mL centrifuge tubes and added 10 mL of copper ion solutions with concentrations of 1 - 10 mmol / L. Then, the centrifuge tubes were placed on a constant temperature shaker and shaken overnight for 24 h at 150 rpm. Finally, the solution was filtered through a filter membrane with a diameter of 0.24 μm, and the filtrate was collected. After centrifuging and filtering the filtrate, 200 μL of a color reagent (sodium diethyldithiocarbamate), 1 mL of a stabilizer (starch), and 100 μL of a buffer (NH4Cl - NH3 . H2O) were added, and it was diluted to 50 times the original volume to detect the absorbance. The results are as Figure 5 shown. When the copper ion concentration is less than 7 mmol / L, the adsorption capacity of the material increases rapidly with the increase in concentration; then, when the copper ion concentration is greater than 8 mmol / L, the adsorption isotherm reaches a plateau; when it continues to increase to 9 mmol / L, there is no obvious increase in the adsorption amount of the material. The maximum adsorption amount of copper ions under the experimental conditions was calculated to be 349.6 mg / g. The copper ion adsorption data of the aminated resin were simulated using the Langmuir model, which was in good agreement with the Langmuir model, indicating that the adsorption of copper ions by the aminated resin conforms to the monolayer adsorption mechanism.

[0031] Example 2

[0032] The preparation process of the aminated resin was the same as that in Example 1.

[0033] Adsorption experiment: First, 10 mg of the material was dispersed in 10 mL (8 mmol / L) of a copper ion solution, and then the concentration of copper ions in the remaining solution was detected every 1 h. Finally, a kinetic adsorption curve was plotted.

[0034] Material synthesis and characterization

[0035] Figure 4 This is the XPS characterization of the aminated resin before and after adsorbing copper ions. To study the elemental composition and chemical states of some elements on the surface of the aminated resin, C1s, O1s, and N1s scanning spectra were obtained using XPS ( Figure 4 a). The full spectrum of the aminated resin confirmed the presence of nitrogen atoms in the resin. In the high-resolution N1s spectrum of the aminated resin ( Figure 4 c), C - N and N - H could be identified at 399.4 eV and 401.0 eV respectively, indicating that ethylenediamine was successfully grafted onto the surface of Janus. Compared with the aminated resin, copper elements were present in the adsorbed material, and the content of copper elements was 5.71%, while there were no copper elements in the aminated resin before adsorption. The aminated resin and the adsorbed material could also be proven by the high-resolution XPS spectrum scanning results ( Figure 4 b). There were no characteristic peaks of any copper ions in the spectrum of the aminated resin;Figure 4 In the spectrum of the material after adsorption, there are two characteristic peaks of copper ions with binding energies at Cu 2p3 / 2 of 932.5 eV and Cu 2p1 / 2 of 952.2 eV, both belonging to the peaks of Cu 2p, further indicating the presence of copper ions. The above results show that there is an interaction between the material and metal ions.

[0036] Material application

[0037] To investigate the adsorption kinetic process of the aminated resin, first disperse 10 mg of the material in 10 mL (8 mmol / L) of copper ion solution, then detect the concentration of copper ions in the remaining solution every 1 h, and finally plot the kinetic adsorption curve. The results are as Figure 6 shown. In the first 120 min of adsorption, the adsorption capacity of the material has been showing a rapid increasing trend; when the adsorption time is greater than 240 min, the change in the adsorption capacity of the material is not obvious; when the adsorption time is greater than 480 min, the adsorption sites of the material are almost completely occupied by metal ions, and the adsorption capacity remains unchanged, that is, the adsorption equilibrium is reached. The adsorption of copper ions by the aminated resin conforms to the pseudo-second-order kinetic model.

Claims

1. A method for preparing an aminated resin, characterized in that: Synthesis of a macromolecular initiator: Take 200 - 600 mg of anisotropic microspheres Janus, disperse them in 8 - 12 mL of 0.1 - 0.2 mol / L H2SO4 solution, and react at 40 - 80 °C for 12 - 36 h; Wash the product until neutral and dry it under vacuum; Dissolve the dried microspheres in 8 - 12 mL of dichloromethane, stir magnetically in an ice bath for 20 - 40 min, then add 0.2 - 0.6 mL of triethylamine and stir for 8 - 12 min, dropwise add 0.3 - 0.5 mL of 2-bromoisobutyryl bromide and 3 - 5 mg of 4-dimethylaminopyridine, react for 1 - 3 h, and then transfer to react at 20 - 30 °C for 12 - 36 h; Subsequently, wash with dichloromethane, deionized water, and ethanol 3 - 5 times each, and then dry under vacuum at 50 - 60 °C for 10 - 12 h to obtain the macromolecular initiator; Grafting glycidyl methacrylate GMA using atom transfer radical polymerization technology ATRP: Accurately weigh 100 - 300 mg of the macromolecular initiator, 10 - 20 mg of 2,2'-bipyridine, 0.1 - 0.2 mL of GMA, disperse them in a container containing 8 - 12 mL of a mixed solution of ethanol and deionized water, connect the flask to the ATRP device, add 5 - 8 mg of CuBr and purge with nitrogen to remove oxygen; Carry out the polymerization reaction under the protection of nitrogen, maintain the reaction at 50 - 60 °C for 10 - 12 h; After the reaction is completed, wash the product 3 - 5 times successively with ethanol, deionized water, and 0.1 - 0.2 mol / L disodium ethylenediaminetetraacetate aqueous solution; Vacuum dry the intermediate product; Obtain the product Brush@Janus microspheres; Preparation of aminated resin by epoxy-amine ring-opening: Put 200 - 600 mg of dried Brush@Janus microspheres into a round-bottom flask containing 6 - 8 mL of THF and 10 - 12 mL of ethylenediamine; Stir the reaction mixture at 100 - 150 rpm and 10 - 25 °C for 10 - 12 h; After the reaction is completed, wash the aminated resin successively with acetone and deionized water, and dry it at room temperature to obtain the aminated resin.

2. The method according to claim 1, wherein: The anisotropic microspheres Janus are in the shape of semi-raspberry and have a particle size of 2 - 4 μm.

3. According to the method described in claim 1, characterized in that: The preparation process of the anisotropic microspheres Janus is as follows: Using polystyrene microspheres as seeds, glycidyl methacrylate GMA as a monomer, pentaerythritol tetraacrylate PETA as a cross-linking agent, cyclohexanol and toluene as porogens, and benzil dimethyl ether DMPA as a photoinitiator, prepare the anisotropic microspheres Janus by means of seed emulsion polymerization, and the polystyrene microspheres are prepared by dispersion polymerization.

4. According to the method described in claim 1, characterized in that: The volume ratio of ethanol to deionized water in the mixed solution of ethanol and deionized water is 2 / 1 - 4 / 1.

5. The method according to claim 1, wherein: The preparation process of the anisotropic microspheres Janus is operated according to the following steps, Under light - shielding conditions, 0.1 - 0.4 mL of GMA, 30 - 120 mg of pentaerythritol tetraacrylate (PETA), 0 - 0.42 mL of toluene, 0 - 0.42 mL of cyclohexanol, and toluene and cyclohexanol cannot be 0 at the same time, and their volume sum is 0.2 - 0.5 mL. After ultrasonic emulsification of 20 - 40 mg of benzoyl dimethyl ether (DMPA), it is mixed with 70 - 150 mg of polystyrene microsphere seeds, swollen at 100 - 300 rpm at room temperature for 16 - 24 h, and then subjected to exposure reaction for 3 - 8 h, with an ultraviolet light intensity of 6 - 8 μW / cm 2 ; The product is washed alternately with water and ethanol 2 - 4 times and vacuum - dried at 60 - 70 °C for 6 - 12 h to obtain anisotropic microspheres Janus.

6. An aminated resin prepared by the method described in any one of claims 1 - 5.

7. Use of the aminated resin according to claim 6 as a metal ion adsorbent material, characterized in that: The described aminated resin is applied to the adsorption of metal copper ions in solution.

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