SEPARATING ANIONIC AND CATIONIC ORGANIC COMPOUNDS, AND A PREPARATION METHOD THEREOF

Technical Field of the Invention

The present invention relates to a composite membrane in nanofiber form developed to be used in separating cationic compounds from solutions containing both cationic and anionic organic compounds, and a method for preparing the composite membrane.

State of the Art (Prior Art)

It is essential to use membranes with selectivity for selectively removing some undesired molecules from the medium in filtration processes. In some formulations, it is desired for a solution not to contain any organic compound other than the active substance that may be in an anionic form since organic compounds of cationic form that may potentially be present in the solution may impair the performance of an active substance in anionic form through electrostatic interactions. On the other hand, using organic compounds in anionic and cationic forms simultaneously is inevitable during the initial preparation stages of such formulations. In such cases, organic compounds charged oppositely compared to the active substance need to be removed from the medium before obtaining the final product formulation. Although selective filtration processes utilize membranes of various materials in film, nanofiber, and aerogel forms, membrane materials in nanofiber form prepared by the electro-spinning method have begun to draw attention in recent years. However, a membrane material that features the desired high selectivity (affinity) and adsorption performance have yet to be achieved in the studies conducted in the state of the art.

The state of the art comprises various membrane materials used for separating certain substances from one another through filtration. The patent application numbered EP0600347A2 discloses the use of an adsorption material modified with polynuclear metal hydroxides for the selective elimination of inorganic phosphate from protein-containing body fluids. Another patent application numbered CN105363357A in the state of the art discloses a poly(vinyl alcohol) membrane prepared by cross-linking poly(vinyl alcohol) and a diene compound, wherein said hydrophilic membrane demonstrates good flux performance for water. However, the membrane material disclosed in the patent application numbered CN105363357A does not perform a selective separation for ionic compounds in an aqueous solution. The patent application numbered JP2006345876A discloses a separation membrane with selective permeability prepared by using a polysulfone-based polymer and polyvinylpyrrolidone for filtering bovine blood, wherein the albumin sieving coefficient of said membrane at a flow rate of 20 mL/min after 15 minutes was determined to be 0.01-0.1.

The patent document numbered RU2663831C1 in the state of the art relates to highly selective polyimide membranes prepared for gases. The patent application numbered CN108421422A discloses a nanofiltration composite membrane used for selective ion separation. Said membrane comprises a substrate supporting layer, a middle porous layer, and an ultrathin separating layer. The separating layer in said membrane is composed of oxidized graphene and polyamide. In the state of the art, a process for preparing membranes from open-ended nanotubes embedded in a polymeric matrix in order to perform a separation based on the pore size was performed in patent documents numbered US20090321355A1 and MX2011000140A, and accordingly, the selective passage of molecular species with suitable molecular sizes was achieved. In the patent application numbered US20140209533A1, multilayer sintered membranes with different sizes of micro and nanopores were prepared from metals and inorganic compounds (ceramic, silicate, clay, zeolite, phosphate, etc.), and were used for separating water from alcohol-water mixtures. The patent application numbered W02016129801A1 discloses a polymeric membrane structure comprising membrane protein. The membrane disclosed in said patent application is concluded to be used efficiently for water purification processes in filtration devices since the membrane prepared in said patent application only allows the passage of water molecules.

In the state of the art, selective filtration processes are carried out by using membranes with various pore sizes in the selective filtration of molecules with various sizes. Again, in the state of the art, composite membranes are also used for selective filtration processes by creating membranes with a plurality of layers having different compositions.

In the state of the art, some of the selective filtration materials that allow the passage of water molecules only perform separation by not letting other molecules through. In the applications in the state of the art, several different types of membrane materials have been produced through various methods in order to selectively separate a specific gas from a gas mixture, to selectively separate water from alcohol- water mixtures, to selectively separate phosphate ion from protein-containing body fluids, to selectively separate albumin protein from the blood, and to selectively separate specific molecules from a molecule mixture with various molecule sizes. However, filtration materials available in the state of the art do not allow for selectively separating cationic organic compounds from an aqueous mixture solution comprising cationic and anionic organic compounds.

Consequently, the inadequacy of available solutions necessitated making an improvement in the relevant technical field. The present invention, eliminating the disadvantages in the state of the art, relates to a composite membrane in nanofiber form developed to be used as a filtration material for selectively separating cationic organic compounds from solutions comprising cationic and anionic organic compounds, and a method for preparing said composite membrane.

Brief Description and Objects of the Invention

The present invention, to eliminate the disadvantages in the state of the art, relates to a composite membrane in nanofiber from developed to be used as a filtration material for selectively separating cationic organic compounds from solutions comprising cationic and anionic organic compounds, and a method for preparing said composite membrane. The inventive composite membrane has a water-insoluble, flexible, and pliable (bendable) structure. An object of the present invention is to ensure that the inventive composite membrane may be used as a filter for selectively separating a cationic organic compound from a solution comprising the cationic and anionic organic compound.

Another object of the present invention is to render the surface of membrane functional such that said membrane surface can differentiate cationic and anionic molecules, thereby enabling the membrane to selectively retain cationic molecules on the surface.

Yet another object of the present invention is to selectively separate a cationic organic compound from an aqueous solution comprising cationic and anionic organic compounds. The cationic organic compound within the filtered organic substance mixture selectively adheres to the inventive nanofiber composite membrane used as a filter.

Respective examinations performed with the inventive composite membrane in nanofiber form indicate that more than 97% of the cationic organic substance from the solution comprising equimolar cationic and anionic organic compound adhere to the composite membrane used as the filtration material, while more than 99% of the anionic organic substance in the equimolar mixture passes through the filter by being strained.

Yet another object of the present invention is to ensure that cationic organic substances may be recovered through the distillation of the extract after the organic compounds on the inventive nanofiber composite membrane are extracted via a suitable solvent by filtration, as well as to ensure that the anionic organic compounds may be recovered through the distillation of the aqueous solution in the filtrate thereof.

The inventive nanofiber composite membrane comprises cross- linked fibers and is water-insoluble. Thus, the inventive composite membrane may readily be used in the filtration of the aqueous solutions.

The flexible and pliable (bendable) structure of the inventive composite membrane, along with the fact that it does not suffer from any deformation due to being bent or plied allows the inventive nanofiber composite membrane to be readily used as a filter.

DESCRIPTION OF THE FIGURES

FIGURE 1 illustrates the water-insoluble fractions of cross- linked and non-cross-linked composite membranes in nanofiber form. FIGURE 2 illustrates the maximum tensile stress values of cross-linked and non-cross-linked composite membranes in nanofiber form.

FIGURE 3 illustrates the maximum tensile strain values of cross-linked and non-cross-linked composite membranes in the nanofiber form.

FIGURE 4 illustrates the Young's modulus values of cross- linked and non-cross-linked composite membranes in nanofiber form.

FIGURE 5 illustrates the SEM images of fibers of composite membranes at various magnification rates. ((A, a) Non-Cross-Linked Composite Membrane Fiber, (B, b) Cross-Linked Composite Membrane Fiber, (C, c) Cross-Linked Composite Membrane Fiber kept in water for 24 hours).

FIGURE 6 illustrates the change of the BY2 concentration as a function of time for the adsorption of the cationic organic compound (of BY2) on the cross- linked composite membrane material at 25 °C. FIGURE 7 illustrates the change of the AB74 concentration as a function of time for the adsorption of the anionic organic compound (of AB74) on the cross-linked composite membrane material at 25 °C.

FIGURE 8 illustrates the change of the adsorbed BY2 amount per gram composite membrane material (of q _t) as a function of time for the adsorption of cationic organic compound (of BY2).

FIGURE 9 illustrates the adsorption isotherm for the adsorption of cationic organic compound (of BY2) on the cross-linked composite membrane material in the water at 25 °C (The straight line indicates the Langmuir isotherm).

FIGURE 10 illustrates the absorption spectra of the solution comprising only 1.5xl0 ⁵ mol.L ¹ Acid Blue 74 (AB74), of the solution comprising only 1.5xl0 ⁵ mol.L ¹ Basic Yellow 2 (BY2), of the binary equimolar mixture solution comprising 1.5xl0 ⁵ mol.L ¹ Acid Blue 74 and Basic Yellow 2 (both empirical and theoretical), and the absorption spectra of the filtrate obtained as a result of the vacuum filtration assay.

Detailed Description of the Invention

The present invention relates to a composite membrane in cross- linked nanofiber form developed to be used as a filter for selectively separating cationic organic compounds from solutions comprising cationic and anionic organic compounds. The inventive composite membrane in nanofiber form comprises at least two polymers and/or oligomers of natural and/or synthetic characteristics, at least one solvent, and at least one cross-linking material.

The present invention may comprise, as a polymer and/or oligomer; cellulose acetate, poly(lactic acid), polystyrene, polyethylene terephthalate), poly(vinyl alcohol), polysulfone, polycaprolactone, poly (vinylidene fluoride), polyurethane, zein, collagen, gelatin, elastin, keratin, alginate, guar gum, xanthan gum, chitosan, chitosan oligosaccharide lactate, cellulose, pectin, alpha-glucan, beta-glucan, dextran, xylene, kappa-carrageenan, lambda-carrageenan, iota carrageenan, N,N',N"-triacetylchitotriose, pullulan, alpha-cyclodextrin, beta-cyclodextrin, gamma-cyclodextrin, agar, chitin, locust bean gum, hyaluronic acid, poly (vinyl butyral-co-vinyl alcohol-co-vinyl acetate), polyethylene glycol) methyl ether methacrylate, poly(vinyl acetate), polyethylene glycol) methyl ether, polyethylene glycol) diacrylate, poly(butylene succinate-co-terephthalate ), polyethylene succinate), poly(4- styrene sulfonic acid), poly(vinyl sulfonic acid), poly(4- styrene sulfonic acid-co-maleic acid), poly(ethylene-co- acrylic acid), poly-D-glutamic acid, poly(ethylene-co-glycidyl methacrylate), poM (methyl vinyl ether-alt-maleic acid), poly (methacrylic acid), poly (acrylic acid), poly(methyl methacrylate-co-methacrylic acid), poly (2-acrylamido-2- methyl-l-propanesulfonic acid), poly (acrylamide-co-diallyl ammonium chloride), poly(acrylic acid-co-maleic acid), poly (allylamine hydrochloride), poly(diallyl dimethylammonium chloride), poly(vinyl chloride-co-acrylic acid), poly(vinyl phosphonic acid), poly(styrene-alt-maleic acid), poly(styrene- co-methacrylic acid), poly[(isobutylene-alt-maleic acid, ammonium salt)-co-(isobutylene-alt-maleic anhydrate)], poly (acrylamide-co-acrylic acid), poly(tert-butyl acrylate- co-ethyl acrylate-co-methacrylic acid), poly(acrylamide-co- acrylic acid), poly(ethylene-co-methacrylic acid), poly(N- isopropylacrylamide-co-methacrylic acid), poly(2- propylacrylic acid), poly (2-ethylacrylic acid), poly(N- isopropylacrylamide-co-acrylic acid), poly (styrene)-block- poly(acrylic acid), poly(acrylonitrile) and/or poly(ethylene oxide) and/or one of the derivatives thereof or any combination thereof.

The present invention may comprise at least one of acetonitrile, chloroform, N,N-dimethylformamide, toluene, benzene, water, dichloromethane, 1,2-dichloroethane, methyl alcohol, ethyl alcohol, n-propanol, n-butanol, ethyl acetate, tetrahydrofuran, formic acid, acetic acid, trifluoroacetic acid, and methyl ethyl ketone solvent or any combinations thereof as a solvent for mixture/mixtures comprising of polymer and/or oligomer combinations.

The present invention, in order to prevent the inventive composite membrane in nanofiber form from dissolving in water, as a cross-linking agent, comprises at least one of; tripolyphosphate sodium, potassium, calcium or aluminum salt, 11-maleimidoundecanoic acid, glycerol ethoxylate, succinic acid, genipin, hexa (ethylene glycol) dithiol, tetraethyl orthosilicate, citric acid, glycerol ethoxylate-co-propoxylate triol, maleic acid, glutaraldehyde, glyoxal, pentaerythritol ethoxylate, 1,4-phenylendiacryloyl chloride, p- divinylbenzene, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, trimethoxypropane ethoxylate, formaldehyde, 1,2,3,4-butanetetracarboxylic acid, hydroxyethyl methacrylate, methacrylamide, methacrylic acid, acrylic acid, polyethylene glycol)-dimethactylate, acrylamide, hydroxyethyl acrylate, di(4-hydroxyl benzophenone) dodecanedioate, itaconic acid, Arcofix NEC and/or epichlorohydrin and/or at least one of the derivatives thereof or any combination thereof and if necessary, as a photoinitiator, benzophenone, 2-hydroxy-l-[4-(2- hydroxyethoxy)phenyl]-2-methyl-l-propanone or 2-hydroxy-4 (2-hydroxyethoxy)-2-methyl-phenyl and a UV lamp with 365 nm of wavelength.

The inventive composite membrane comprises cross-linked nanofibers and has a water-insoluble, flexible, and pliable (bendable) structure. The inventive composite membrane is used as a filter for selectively separating cationic organic compounds from a solution comprising cationic and anionic organic compounds together.

The inventive method for preparing a composite membrane in cross-linked nanofiber form to be used for selectively separating cationic organic compounds from solutions comprising cationic and anionic organic compounds comprises the process steps of; i. Completely dissolving a polymer mass in a range between 0.3-35 g inside a suitable solvent or a solvent mixture having a volume between 10-125 ml by mixing it inside a container at a suitable temperature in a range between 20-100°C for a duration between 10-360 minutes and ensuring that the solution is at the room temperature, ii. Weighing other polymer(s) and/or oligomer (s) in analytical balance such that the total mass ratio is in a range between 10-500% with respect to the polymer mass inside the first container and transferring them into a second container, and dissolving other polymer(s) and/or oligomer(s) contained inside the second container inside a suitable solvent or a solvent mixture by mixing them at a suitable temperature in a range between 20-100°C for a duration of 10-360 minutes and cooling it down to the room temperature if necessary, iii. Adding a suitable cross-linking agent in an amount between 0.05-20 g to the solution inside the second container and stirring it inside a magnetic stirrer for a period of 10-120 minutes, iv. Adding the solution inside the second container to the polymer solution inside the first container and mixing them for a period between 1-10 hours at room temperature until a homogeneous solution is obtained and ensuring that the pH value thereof is in a range between 1 and 7, v. Transferring the prepared solution to an injector and feeding the solution to an electrode attached thereto such that the flow rate is in a range between 0.1-10 mL/hour, vi. Maintaining the distance between the injector-type tip and the collector in a range between 5-25 cm and obtaining the inventive composite membrane in nanofiber form by means of the electro-spinning method by applying a voltage in a range between 10-30 kV, vii. Drying the prepared composite membrane in nanofiber form at a temperature range between 20-40°C, viii. Ionically or covalently cross-linking the fibers of the composite membrane via one of the photochemical, chemical, or thermal methods, ix. Obtaining the composite membrane by cutting the cross- linked composite fibers as desired.

In an embodiment of the present invention, the inventive method for preparing a composite membrane in cross-linked nanofiber form to be used for selectively separating cationic organic compounds from solutions comprising cationic and anionic organic compounds comprises the process steps of; i. Completely dissolving a polymer mass in a range between 2-25 g inside a suitable solvent or a solvent mixture having a volume between 20-100 mL by mixing it inside a container at a suitable temperature between 25-85°C for a period of 3 hours and cooling it down to the room temperature if necessary, ii. Weighing other polymer(s) and/or oligomer (s) in analytical balance such that the total mass ratio is in a range between 35-350% with respect to the polymer mass inside the first container and transferring them into a second container, and dissolving other polymer (s) and/or oligomer(s) contained inside the second container inside a suitable solvent or a solvent mixture by mixing them at a suitable temperature in a range between 25-45°C for a period of 1 hour and cooling it down to the room temperature if necessary, iii. Adding a suitable cross-linking agent to the solution inside the second container with respect to the polymer mass inside the first container such that the mass ratio thereof is in a range between 5-35% and stirring the solution again in a magnetic stirrer for 30 minutes, iv. Adding the solution inside the second container to the polymer solution inside the first container and mixing them for a period of 4.5 hours at room temperature until a homogeneous solution is obtained and ensuring that the pH value thereof is in a range between 1 and 7, v. Transferring the prepared solution to an injector and feeding the solution to an electrode attached thereto such that the flow rate is in a range between 0.3-5 mL/hour, vi. Maintaining the distance between the injector-type tip and the collector in a range between 7-22 cm and obtaining the inventive composite membrane in nanofiber form by means of the electro-spinning method by applying a voltage in a range between 13-25 kV, vii. Drying the prepared composite membrane in nanofiber form at a temperature of 40°C, viii. Ionically or covalently cross-linking the fibers of the composite membrane via one of the photochemical, chemical, or thermal methods, ix. Obtaining the composite membrane by cutting the cross- linked composite fibers as desired.

In the present invention, samples of 0.0100 g were weighed, and said samples were individually placed into beakers containing 50 mL of water at a temperature of 25°C in order to determine water solubility of both the cross-linked composite membrane in nanofiber form and the non-cross-linked composite membrane in nanofiber form. After 24 hours, samples were removed from beakers and dried at 40°C by using a vacuum drying oven until a constant mass value is obtained. Subsequently, the water solubility of said samples was determined by using the initial and final mass values of the samples. Water solubility analysis results of the inventive composite membrane are illustrated in Figure 1.

Non-cross-linked composite membrane sample dissolved in water completely briefly after it was released into the water. The cross-linked composite membrane, however, showed no water solubility indications.

2 x 13 cm rectangular samples were cut from both cross-linked and non-cross-linked samples in order to test the mechanical properties of the inventive composite membrane. The samples were placed into a desiccator cabinet with 52% of relative humidity, wherein the cabinet was located inside an incubator at 25°C, and in which a glass container filled with saturated magnesium nitrate hexahydrate (Mg(NO ₃ ) ₂ .6H ₂ O) solution was provided, and were conditioned therein for a period of 7 days. Samples conditioned for a period of 7 days at 25 °C and 52% of relative humidity were placed into a mechanical testing device and tested for mechanical strength at a velocity of 50 mm/sec under a 500 N load cell. Maximum tensile stress, maximum tensile strain, and Young's modulus values of samples were determined. Figures 2, 3, and 4 respectively illustrate the maximum tensile stress, maximum tensile strain, and Young's modulus values obtained for composite membranes subjected to the cross-linking process and for composite membranes that were not subjected to the cross-linking process.As illustrated in Figures 2, 3, and 4, maximum tensile stress and Young's modulus values of the composite membrane in nanofiber form increase significantly after the cross-linking process. The mechanical strength of the composite membrane was improved by means of the cross-linking process. Moreover, it is observed that the cross-linking process induces a decrease in the maximum tensile strain value of the composite membrane in the nanofiber form. Even though the cross-linking process induces a decrease in the flexibility of the composite membrane, the cross-linked composite membrane maintains a certain degree of flexibility before reaching the breaking point.

After gold plating the non-cross-linked composite membrane sample, cross-linked composite membrane sample, and cross- linked composite membrane sample soaked in water for 24 hours, scanning electron microscope (SEM) images of each composite membrane were taken for the surface analysis of the inventive composite membrane. Figure 5 illustrates the SEM images recorded at different magnifications for composite membranes. An examination of the SEM images shows that the cross-linking process applied to the composite membrane caused no changes in the morphology of the fibers. Furthermore, SEM images of the cross-linked composite membrane soaked in water for 24 hours showed that fibers intertwined and slightly swollen. SEM measurements indicated that the fiber diameter was 226 ± 57 nm for the non-cross-linked composite membrane, 227 ± 65 nm for the cross-linked composite membrane, and 290 ± 57 nm for the cross-linked composite membrane soaked in water for 24 hours. These results show that the cross-linking process causes no changes in the average fiber diameter values of the composite membrane. Furthermore, only a 28% increase is observed in the fiber diameter of the cross-linked composite membrane although the cross-linked composite membrane was soaked in water for 24 hours.

Nitrogen adsorption/desorption isotherms were obtained at -196 °C in order to calculate the specific surface area values of the cross-linked and non-cross-linked composite membrane materials prepared by means of the inventive method. The multipoint Brunauer-Emmett-Teller (BET) specific surface area value was calculated as 19.0 m ² /g for the non-cross-linked composite membrane material, and 16.3 m ² /g for the cross-linked composite membrane material. A slight decrease was observed in the specific surface area value of the composite membrane material as a result of the cross-linking process implemented to ensure that the composite membrane is water-insoluble.

Cationic Basic Yellow-2 (BY2) dyestuff and anionic Acid Blue- 74 (AB74) dyestuff were used as model organic compounds in order to determine the performance features of the inventive composite membrane. Initially, adsorption kinetics assays were carried out in order to determine the adsorption balance period for each dyestuff-membrane system in adsorption assays. In adsorption kinetics assays, a dyestuff solution with 25 mL 2.0xl0 ⁵ mol/L concentration was filled to an Erlenmeyer flask and began stirring at 25°C and 150 rpm in a temperature- controlled stirrer after adding 0.0030 g of cross-linked composite membrane thereto. Samples at specific volumes were taken from the mixture at certain time intervals and the obtained samples were placed into a temperature-controlled UV- VIS spectrophotometer, and subsequently, the absorbance value of each solution was measured at the maximum absorption wavelength of the dyestuff solution. Subsequently, dyestuff concentration was calculated from the absorbance value by using the calibration graph for the corresponding dyestuff. Figures 6 and 7 illustrate the dyestuff concentration changes during the adsorption of BY2 and AB74 as a function of time, respectively, in the presence of the initial dyestuff concentration of 2xl0 ⁵ mol/L in water over the cross-linked composite membrane material.

As illustrated in Figure 7, the concentration of cationic BY2 dyestuff remaining in the aqueous solution decreases over time during the adsorption process. The initial decrease amount is significantly higher. This shows that the cationic BY2 dyestuff is quite well adsorbed on the cross-linked composite membrane material. The concentration of anionic AB74 dyestuff in an aqueous solution did not decrease during the adsorption process of 1440 minutes. In other words, anionic AB74 dyestuff is not adsorbed by the cross-linked composite membrane. Therefore, while the cross-linked composite membrane shows a high affinity towards the cationic organic compound, its affinity towards the anionic organic compound is significantly lower.

Since the cross-linked composite membrane adsorbed only the BY2 dyestuff, the quantity of dyestuff (BY2) adsorbed per gram of composite membrane (q _t ) was determined as a function of time, and the obtained results are illustrated in Figure 8. The consistency of adsorption kinetics data with pseudo-first order and pseudo-second order kinetics equations was analyzed, and parameter values and correlation coefficients for each kinetic model were calculated. Respective analyses showed that the empirical adsorption kinetics data for the adsorption of BY2 pigment by the cross-linked composite membrane material are highly consistent with the pseudo-second order kinetics model (r ² > 0.998). The results of the adsorption kinetics assay showed that the anionic AB74 dyestuff did not attach to the composite membrane material. Therefore, an adsorption isotherm study was conducted only for the BY2 pigment.

0.0030 g of composite membrane material was individually added to each solution having different dyestuff concentrations at 25 mL in each Erlenmeyer flask in the adsorption isotherm study carried out at 25 °C for the BY2 dyestuff and these Erlenmeyer flasks were stirred at 150 rpm for 24 hours in a temperature- controlled stirrer. Nanofiber materials were removed from the Erlenmeyer flasks by means of a pair of forceps at the end of the 24-hour period and the absorbance values of dyestuff solutions were measured in the maximum adsorption wavelength (Amaximum) of the BY2 dyestuff. The measured absorbance values were converted into concentration values via the calibration chart. Subsequently, the equilibrium adsorption amount of dyestuff adsorbed per gram of composite membrane (q _e )versus equilibrium dyestuff concentration (C _e ) graph was plotted and the adsorption isotherm graph for the BY2 dyestuff was obtained. Figure 9 illustrates the adsorption isotherm curve obtained for the adsorption of BY2 dyestuff at 25 °C by the cross-linked composite membrane. The consistency of the obtained adsorption isotherm data with the Langmuir and Freundlich models were tested and the results of correlation coefficients indicated that the Langmuir model provides a better representation of the empirical isotherm data than the Freundlich model. The maximum adsorption capacity value calculated through the Langmuir model was 2.55xl0 ³ mol/g for the adsorption of the BY2 dyestuff by the cross-linked composite membrane material. Empirical results obtained as a result of the adsorption assays showed that the cross-linked composite membrane material features a higher affinity towards the cationic dyestuff adsorption and no affinity towards the anionic dyestuff. Vacuum filtration method was employed in order to determine that which one of the dyestuffs is adsorbed better by the cross-linked composite membrane and which one of the dyestuffs is allowed to pass through the cross-linked composite membrane from an aqueous dyestuff mixture solution containing both the cationic BY2 dyestuff and the anionic AB74 dyestuff. For this particular assay, a piece of cross-linked composite membrane was cut suitable in size for the vacuum filtration apparatus and placed onto the filtration apparatus. The equimolar mixture solution containing 50 mL 1.5xl0 ^-5 mol/L of BY2 and AB74 prepared inside a volumetric flask was discharged from the vacuum filtration apparatus and filtrated through the cross- linked composite membrane via a vacuum pump. Figure 10 illustrates both empirical and the theoretical absorption spectra of the dyestuff solutions comprising 1.5xl0 ^-5 mol/L AB74 only, the solution comprising 1.5xl0 ^-5 mol/L BY2 only, and the binary equimolar mixture solution comprising 1.5xl0 ^-5 mol/L AB74 and BY2 dyestuffs, as well as the absorption spectrum of the dyestuff filtrate obtained as a result of the vacuum filtration assay.

Dyestuffs concentrations remaining in the filtrate portion were determined by the analysis procedure of binary mixtures through the spectrophotometric method. The fact that more than 97% of the BY2 dyestuff in the equimolar pigment mixture solution of 1.5xl0 ⁵ mol/L remained on the cross-linked composite membrane and that more than 99% of the AB74 dyestuff passed to the filtrate as a result of the filtration assay show that the cross-linked composite membrane selectively separated and retained the cationic dyestuff and that the affinity of the cross-linked composite membrane towards the cationic dyestuff is significantly high. This particular result proves the fact that the inventive cross-linked composite membrane has selectively separated and retained the cationic dyestuff. The results obtained from the respective assays and studies show that the inventive cross-linked composite membrane may be used as a filter for selectively separating cationic dyestuffs from a cationic-anionic dyestuff mixture solution.

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