GB2274409A - Thin film mosaic composite membranes - Google Patents

Abstract

Semipermeable composite membranes which comprise on a microporous support a thin polymer film of mosaic structure whose isolated anionic and cationic areas, preferably in form of stripes, extend to the thickness of the film. The mosaic structure can be achieved by e.g. masking/demasking techniques with photoresist materials combined with chemical modification steps in order to prepare the positively and negatively charged areas. These membranes have good permeability for electrolytes, such as salts of mono- or polyvalent inorganic acids, while retaining low moleculas weight organic solutes.

Description

Thin Film Mosaic Composite Membranes.

The present invention relates to semipermeable thin film composite membranes, wherein the thin film contains a macroscopic distribution of mosaic forming anionic and cationic charges (sites), optimally in the form of stripes, resting on a microporous support, such as an asymmetric ultrafiltration support. These membranes have good permeability for electrolytes, such as salts of mono- or polyvalent inorganic acids, while retaining low molecular weight organic solutes. While stripes are the preferred form of the configuration of charges in the thin film, other configurations such as circular, oblong, diamond, or trisagonal are also possible as long as the dimensions and charge density meet the requirements for good mosaic performance.

The separation of mono-, di- or polyvalent salts, such as sodium chloride, sodium sulfate or sodium triphosphate from low molecular weight (MW less than 1000) organic compounds in (aqueous) solutions, via membranes is an important industrial separaration problem which has not been economically solved.

Membranes have been shown to offer an economical solution to many separation problems because of their ability to concentrate without a phase change, and to separate different solutes.

The traditional membrane process of reverse osmosis (RO) rejects all salts and organics.

The relatively newer membranes of selective RO (also termed nanofiltration) cannot efficiently achieve the above separations even though they are designed to pass salt and retain the organic solutes. The mode of separation in selective RO is based on size, solubility and electrostatic discrimination, and the proper choice of materials has not been found to give effective transport of multivalent ions (e.g. sulfates), while maintaining high rejection to low molecular weight organic solutes.

However, membrane structures containing separated macroscopic domains (0.05 to 100 microns) of anionic and cationic ion exchange materials connecting the opposite faces of the rejecting layer (called a charged mosaic membrane) have a built-in salt transport mechanism. They have been postulated and shown to give separation between organic solutes and salts. Under a pressure gradient these membranes preferentially transport ion across the corresponding oppositely charged domains of the mosaic, while effectively retaining the organic solute (H. Kowatoh et al., Macromolecules 21, 625-628, 1988).

Mosaic membranes which were designed in the past for piezodialysis processes have also been shown to give high water flux, while, at the same time, giving a permeate enriched in salt (F.B. Leitz, J. Shore, Office of Saline Water, Res. Dev. Program Report No. 775, 1972).

Mosaic membranes are membranes with a macroscopic distribution of cationic and anionic sites. Typically, though not exclusively, they are arranged as particles, such as cationic andlor anionic particles distibuted in a neutral matrix, or particles of one charge distributed in a matrix of the other charge. In this case, particles may be defined as regular or irregular - approaching such shapes as spheres, multi-sided, fibers, cones and others.

The different approaches in achieving the structure of mosaic membranes comprise such methods as the introduction of preformed particles in a matrix via resin suspension in a casting solution of the matrix, block or random copolymerizations, or phase separation in a common solvent (material incompatibility).

Mosaic membranes have not yet become commercially important in separation processes because of the difficulty to produce them on large scale reproducibly.

Of all the above approaches, one good method of manufacturing mosaic membranes is based on the in-situ phase separation of the two phases from a casting dope made in a common solvent. Main difficulty in this approach is connected to the need to simultaneously control a large number of casting variables, which will lead to a formation of an asymmetric mosaic membrane. As a result of this difficulty, so far only few casting dopes were found that produce well functioning mosaics. It is relatively difficult to make pinhole-free, dense membranes with small pores in the anionic and cationic domains.

Most of the mosaic RO membranes do not have high enough rejection to low (e.g. less than 400 MW) molecular weight solutes, or if they have high enough rejection, their water fluxes are too low.

Surprisingly, it was found that thin films of stripes of alternating positive and negative areas on a preferably asymmetric support give good performing mosaic membranes which combine good salt passage with good rejection to low molcular weight organic solutes, such as glucose or sucrose. In addition to stripes, other patterns of regular or irregular shapes may be used for manufacturing mosaic membranes.

It is, therefore, the principal object of the present invention to provide new asymmetric semipermeable mosaic composite membranes.

Other objects of the present invention are processes for the preparation (manufacture) of the inventive membranes, as well as their use in processes for separating organic, low molecular weight solutes from inorganic salts of mono- or polyvalent inorganic acids.

These and other objects of the present invention will become apparent from the following detailed description.

The present invention accordingly provides in its main aspect a semipermeable composite membrane which comprises a microporous support coated with a thin polymer film of mosaic structure whose isolated anionic and cationic areas extend to the thickness of the film.

Composite membranes are in general characterized by a thin dense upper layer (usually less than 10 and preferably less than 5.0 microns thick), which is the selective barrier extending continuously from a thicker (10 to 100 microns) microporous structure.

Polymeric materials for the microporous support can be chosen from materials which are film formers andlor can be preferably cast into asymmetric membranes. Such materials can be chosen from optionally cross-linked polyolefines, polysulfones, polyether sulfones, polyether ketones, polyether-ester ketones, polyether imides, polyphenylene oxides, polyphenylene sulfides, polyamides, polyimides, polyamide-imides, polycarbonates, polyacrylonitriles, polyethers, polybenzimidazoles, polytetrafluoro ethylenes, polyvinylidene fluorides, cellulosics, and mixtures thereof, or ceramics, such as alumina.

Preferred polymeric materials for the microporous support are polyolefines, such as polypropylenes, polyacrylonitriles, polyamides or polysulfones. Of particular usefulness are cross-linked polyacrylonitriles as described in US-A-5 028 337 and US-A4 906 379.

Polyolefines, such as polypropylene are also useful as they are solvent resistanteven when they are not cross-linked. Also polyether ketones and polytetrafluoro ethylenes are solvent resistant.

The optional cross-linking step for these polymeric materials can be carried out with at least difunctional compounds capable of reacting with functional groups, such as e.g.

amino. hydroxy or carboxylic acid groups found on the polymer backbone or as end groups. These at least difunctional compounds contain at least two functional groups selected from multiple unsaturated bonds, or epoxide, aziridine, aldehyde, isocyanate, isothiocyanate, hydroxyl, amino, carboxylic acid, carboxylic anhydride, carboxylic halide, halotriazinyl and halopyrimidyl moieties.

Suitable (charged) polymers for preparing the thin polymer film can be inorganic and preferably organic polymers that are charged or can become charged by chemical modifications when forming the isolated anionic and cationic areas whithin the thin film.

The term "charge" means that fixed negative (anionic) or fixed positive (cationic) charges or both, are present in the polymers.

Preferably, the thin polymer film is obtainable from halomethylated polyphenylene oxides, polyether sulfones, polysulfones or polystyrenes, each quaternated with tertiary amines; sulfonated andlor carboxylated polystyrenes, polysulfones or polyether sulfones; or homo- or copolymers on the basis of polydialkyl siloxanes, containing groups that can be charged with amino or halogen compounds.

Suitable species are e.g. halomethylated polyphenylene oxides, polysulfones or polyether sulfones, quaternated with tertiary amines; or polymers on the basis of polydialkyl siloxanes, such as polydimethyl siloxanes containing in addition groups that can be charged with amino or halogen compounds, e.g. vinyl methyl, (acyloxypropyl)methyl, (aminopropyl)methyl, (chloromethylphenethyl)-methyl, chloropropyl(methyl), (epoxycyclohexylethyl)methyl, or (mercaptopropyl)methyl pendants attached to the silicon atoms, as homo- or copolymers with polydimethyl siloxanes.

Especially preferred are halo(bromo)methylated 2,6-polyphenylene oxides, polysulfones or polyethersulfones, each quaternated with tertiary amines.

The inventive composite membranes can be prepared by various methods comprising in general a series of steps, such as (a) casting a polymer on a microporous support to form a thin polymer film, cross-linking it, and optionally subjecting it to a further chemical modification, (b) partially masking the cross-linked and optionally chemically modified polymer film, (c) charging the non-masked (non-protected) areas of the polymer film by a chemical modification, (d) demasking the polymer film, and (e) charging the demasked areas of the polymer film by a chemical modification step to generate charges opposite to those generated in step (c).

The masking technique can be achieved e.g. by photoresist materials known from and used in the photolithography and based on either positive prints or negative printing.

Photoresists are photosensitive materials which change their solubility after exposure to light. They are typically novolac resins which have diazo naphthoquinone (DANQ) sensitizers attached to the polymer backbone or add to the resist formulation (W. M.

Moreau, Semiconductor Lithography: Principles, Practices and Materials. Plenum Press, New York, 1988).

DANQ sensitizers act as inhibitors to decrease the solubility of the photoresist in basic aqueous solutions. Irridation of films through a mask causes the DANQ to form a carbene which then under goes a Wolff-rearrangement in the presence of water to form a base-soluble indenecarboxylic acid photo product. During development, the dissolution (which is the preferred measure) of the photoresist in the irradiated areas result in a positive image. Any photolithographic design may be used as long as the unexposed areas are sufficiently protected with the photoresist polymer from the chemical reactions introducing chemical functions into the exposed areas.

In addition to novolac resins other photo resist systems are e.g.: - Poly(t-butyloxycarbonyloxystyrene) (PBOCST), which undergoes thermolysis with heat to liberate two gaseous products, isobutene and carbon dioxide, and leaves behind the phenolic polymer poly(p-hydroxy styrene) (PHOST). The thermolytic reaction is catalyzed by acid. Hence one can cast films of solutions of PBOCST with an acid generator, for example arylsulfonium or iodonium salts which are photoactive acid generators (PAG). In addition, the resist results in the decomposition of the onium salt (PAG), liberating an acid species, that upon subsequent baking catalyzes cleavage of the protecting group to generate the phenolic polymer. Aqueous base developers selectively remove the irradiated regions affording high resolution. Alternatively, organic solvent developers may be used to give high quality negative tone images.

- Other materials besides PBOCST may be based on poly(4-t-butoxycarbonyloxystyrene sulfone) and nitrobenzyl ester PAG materials, e.g. 2,6-dinitrobenzyl tosylate.

- Acid-catalyzed depolymerization of poly-phthalaldehyde (PPA) to provide a self developing positive resist. The aromatic PPA is highly soluble in common solvents and can be cast as isotropic homogeneous film. The acetal backbone of PPA is labile towards acid and therefore catalytically cleaved by reaction with a photochemically generated acid. Thus PPA, containing a small amount of sulfonium salt acid and irradiated, provides positive relief images.

Other methods, such as mechanical screening or adsorption of particles which block an area and are then removed, may also be used. The sole requirement being the generation of a pattern ef exposed and unexposed areas. The size of these areas being in the order of 0.1 to 50 microns, and preferably of 0.5 to 5 microns. Thus, the shape or size of the different areas is not critical. They may be stripes or regular patterns, such as circles, hexagons, pentagons, or irregular shapes.

The particles are removed by rinsing the membranes with a solvent which dissolves or swells the paticles, but not the membrane, e.g. cyclohexane.

An alternative method to generating mosaic structures by a masking technique other than photolithography is the adsorption onto the surface of the thin film of particles (dots) of a polymer which protect the underlying surface from chemical reactions, and which particles can be subsequently removed, and charging by certain chemical reactions can be done over these exposed areas. The criteria for the particles being that they can be adsorbed and desorbed when needed and that they are chemically resistant against reactions, that are performed on the exposed areas.

In one preferred approach, particles of polyvinyl chloride (PVC) are used. In this method, PVC particles of about 1 to 50 microns, preferably 1 to 5 microns, are suspended in water and coated onto the surface of the thin film. The particles of PVC adhere after a short contact with a suitable organic solvent for PVC, such as acetone or toluene. The solvent partially dissolves and adheres the PVC particles to the thin film. The masking pattern is dependent on the particle size distribution, concentration or extent of coverage, which in turn is a function of particle size and number of particles in the solution which coats the film. Preferably half of the film surface is masked.

In the stripe mosaic approach, the mosaic pattern of the positively and negatively charged stripes is chemically etched through the thickness of the thin film where the said pattern has been formed by the photoresist material.

In particular, there are different procedures (procedures (1) to (3) mentioned hereinafter) for preparing the inventive membranes and introducing the charged groups into the polymeric, membrane-forming film which comprise: (1) (a) casting a halomethylated polyphenylene oxide on a microporous support and cross-linking it with an aliphatic or aromatic diamine.

(b) partially masking the cross-linked polymer film with a photoresist mask, (c) forming negative charges in the non-masked areas of the polymer film by reaction with a sulfonating agent, followed by hydrolysis, (d) demasking the polymer film, and (e) forming positive charges in the demasked areas by quaternization with a trialkyl amine; ; More particularly a thin film of a reactive polymer, such as a halomethylated polyphenylene oxide, or halomethylated polysulfone or polyether ketone, wherein halo(gen) is chloro, iodo and preferably bromo, is formed by coating an asymmetric or symmetric, solvent stable support, such as of polypropylenes, cross-linked polyacrylonitriles, polyamides or polyether-ketones, or of ceramics such as alumina, and then cross-linking the film to protect it against organic solvents, which are used in various stages forming the mosaic structure.

The amount of halogen, e.g. of bromine in the bromomethylated polyphenylene oxide should be in the range of about 3.0 to 6.0 meqlg, and preferably between 4.0 and 6.0 meqig. The cross-linking with e.g. di- or polyamines should consume about 1 to 2meqlg only.

The said cross-linking can be achieved for example by reacting the halomethyl groups with aromatic or aliphatic di- or polyamines, such as the phenylene diamines, preferably m-phenylene diamine.

The thin cross-linked film is then masked, and sulfonated with a solution of a sulfonating reagent in a non-reactive solvent for about 0.5 to 10 minutes on the unmasked areas to give negative domains after hydrolysis. Suitable sulfonating agents e.g.are chlorosulfonic acid, which is preferred, sulfur dioxide and sulfur trioxide as well as their complexes, formed by the condensation of a tertiary amine (pyridine), sulfur trioxide or sulfur trioxide gas (dioxane), and hydrochloric acid.

After removing the mask, the film is quaternized on the remaining halomethylated groups with a lower(C1-C4)-trialkyl amine, such as triethyl amine, to form positively charged areas.

A typical result is an about 10 micron (range 0.1 to 10, preferably 0.1 to 5 microns) thick film (membrane) on the microporous polypropylene support with stripes of about 50 microns width (range about 0.1 to 100, preferably 0.1 to 10, and most preferred 0.5 to 5 microns) of alternative sulfonated and quaternary ammonium groups.

The factors controlling performance require a charging through the thickness of the thin film. The problem is that if the film is relatively thick, their long reaction times cause lateral diffusion of the reactants and mixing of both positive and negative groups. Thus, going to thinner films ( < 5 microns), preferably below 2 microns or even below 1.0 micron gives sharp patterns of anionic and cationic areas, and allows for thinner stripes.

(2) (a) casting a halomethylated polyphenylens oxide on a microporous support, cross-linking it with an aliphatic or aromatic diamine, and reacting the obtained polymer film with ammonia, (b) partially masking the cross-linked and further reacted polymer film with a photoresist mask, (c) forming negative charges in the non-masked areas of the polymer film by diazotization, hydrolysis, chlorosulfonation and again hydrolysis, (d) demasking the polymer film, and (e) forming poitive charges in the demasked areas by quaternization with an alkylating agent; In this second procedure, the thin film of the cross-linked halomethylated polymer is aminated with e.g. ammonia. This amination step can be carried out in the presence of a swelling agent dissolved in water. Useful agents are e.g. dioxane, tetrahydro furan, dimethyl formamide, or N-methyl pyrrolidone.Without these water-soluble swelling agents, the reaction rates are too low, in general, because the aminination agents do not diffuse efficiently into the thin film (membrane). A suitable solution for the amination step is a 25% solution of aqueous ammonia (ammonium hydroxide) in dioxane 50/50 V/V, with the amination being carried out at room temperature.

The membrane is then masked, and the amino groups in the non-masked areas are hydrolyzed via diazotization with sodium nitrite to hydroxyl groups. This area is then chlorosulfonated and hydrolyzed to a negatively charged stripe. The mask is removed and the remaining amino groups of the adjacent stripes are quaternized with an alkylating agent, such as dimethyl carbonate or preferably methyl iodide, forming positively charged stripes. The quaternization step with e.g. methyl iodide can be performed from an aqueous/alcoholic (water/ethanol 50/50) solution at an alkaline pH (pH 10) for 30 to 180 minutes at temperatures in the range of 15 to 200C (room temperature) to about 60 C.

The nature of the quaternating agents is not limited, although the introduction of alkyl groups should not significantly increase the hydrophobicity and thus a decrease of water flux.

This method allows for the possiblity of achieving two, neatly charged areas provided that the yields of the reaction with ammonia, and of the diazotization are high.

(3) (a) casting a halomethylated polyphenylene oxide on a microporous support, cross-linking it with an aliphatic or aromatic diamine, and reacting the obtained polymer film with chlorosulfonic acid, (b) partially masking the cross-linked and further reacted polymer film with a photoresist mask, (c) forming negative charges in the non-masked areas of the polymer film by hydrolysis of the chlorosulfonic groups, (d) demasking the polymer film, and (e) forming positve charges in the demasked areas by reacting the remaining chlorosulfonic acid groups with an N,N-disubstituted alkylene diamine, followed by quaternization with an alkylating agent.

In this third method, the thin film of a cross-linked halomethylated polymer, e.g.

bromomethylated polyphenylene oxide, crosslinked with m-phenylene diamine, is reacted with chlorosulfonic acid to form a uniform chlorosulfonated thin film. This film is then masked and anionic stripes are formed in the non-masked areas by hydrolysis of the chlorosulfonic groups.

The positive stripes are obtained after removing the mask and reacting the remaining chlorosulfonic groups with an amino alkyl tertiary amine, such as alkylene(Q-C5)diamines, with one amino disubstituted by di-alkyl of 1 to 4 carbon atoms each, e.g. dimethylamino propylamine, followed by quaternization of the tertiary amino group with an alkylating agent, such as methyl iodide.

The electrical resistance through the mosaic film of the inventive thin film mosaic composite membranes should be about less than 0.4/Ohm cm.

The electro-osmotic coefficient through each charged area is as a rule greater than 15.

To achieve these electrochemical properties of the thin films, the ion-exchange capacity of both the cationic and anionic areas should be greater than 1.1 meq/g (milliequivalents/gram). The upper limit is set by the need to prevent swelling in water which will lower the density. An upper limit in most cases is 10 meq/g.

The inventive membranes are useful for separating organic compounds of low molecular weight from aqueous inorganic salts containing solutions. The corresponding method for separating these compounds from said aqueous media, which comprises disposing the solutions on one side of the inventive composite membrane and filtering them through the membrane by applying a hydraulic pressure against said solutions and said membrane being greater than the osmotic pressure of said solution, is a further object of the present invention.

The molecular weight range of the organic compounds to be separated (cut-off level of the inventive membranes) may be less than about 1000, preferably between about 150 and 700.

The inorganic salts present in the solutions, which are subjected to the membrane treatment, are preferably alkali metal salts of mono- or polyvalent inorganic acids, such as alkali metal halides, sulfates or phosphates, e.g. sodium chloride, sodium sulfate and sodium triphosphate.

The inventive membranes are very suitable for membrane separating processes, especially RO-processes. They can be prepared and used as flat in plate and frame devices or spiral wound elements, hollow fibers or tubular membranes in corresponding devices, such as modules. They have superior rejection to organic compounds of low molecular weight, good flux properties, low or almost no rejection to salts, superior flexibilty, and good resistance to chemical and/or biological degradation.

These membranes are especially useful for recovering organic compounds of low molecular weight from chemical reaction mixtures (solutions) or from waste water. These compounds can then be reused or disposed of.

The separation effect (the rejection) of the inventive membranes can be measured as follows: The solutions containing the organic solute and the salt, e.g. a sulfate, are introduced into a stainless steel cylinder containing a circular membrane, and using nitrogen, subjected to pressure of 40 bars. The solution is stirred magnetically. 150 ml of the solution (to be tested), which contains the substance to be tested in the concentration C1 (g of substance per g of solution), are used. The liquid which collect on the outlet side of the membrane is examined to determine its content (concentration) C2 of the substance to be tested; 3 samples each of 5 ml being taken from the start of the experiment. In general, the amount which flows through the membrane and the composition of the 3 samples are constant.

The rejection can be calculated from the values obtained, using the equation: C1-C2 R=----------------- . 100 (%) C1 The amount of the material passed through the membrane per surface area and time unit is found to be: F=VS-lrl (V = volume, S = membrane surface area, t = time) F is approximately expressed in m3/m2.d, i.e. cubic meters per square meter surface area of the membrane and per day, or in Vm2.h, i.e. liters per square meter surface area of the membrane and per hour.

In addition to the measurements on flat membranes, measurements on tubular membranes 60 cm long and with an outer diameter of 1.4 cm are also carried out. For this purpose, these tubular membranes are placed in a perforated tube made of stainless steel.

The whole is placed in a tube made of polycarbonate. The outflow from the membrane is between this outer polycarbonate tube and the steel tube. The liquid is added as a stream of the solution in turbulent or laminar flow unde pressure. The flow rate is kept constant at 10 to 15 liters per minute. The rejection (R) and the flux (F) are calculated in the same way as for the flat membranes.

In the following examples parts and percentages are by weight, if not otherwise indicated.

The temperatures are in degrees centigrade.

Example 1 A thin polymer membrane of bromomethylated polyphenylene oxide (PPOBr) (5.0 meq Br/g) is formed by dip coating a 10% aqueous solution containing also 1.0 meq/g of metaphenylene diamine, with a coating speed of 2 cm/min on a microporous polypropylene (Celgard - registered trademark) support. The resultant film thickness is 1.2 microns. The cross-linked PPOBr membrane is aminated with ammonium hydroxide 25%/dioxane (50/50) for 209 hours, and then masked with a photoresist mask. The amino groups in the non-masked areas are hydrolyzed via diazotization with an aqueous solution containing 2% of sodium nitrite and 1 N hydrochloric acid for half an hour and then placed in an aqueous solution of 0.05 M of sodium hydroxide for 2 hours.

The so treated areas are then chlorosulfonated with a solution of 2.5% of chlorosulfonic acid in n-hexane for 1.5 minutes, and hydrolyzed to form negative charges.

The mask is then removed with a photoresist remover solution comprising xylene/toluene and base and the remaining amino groups of the adjacent areas are quaternized with an aqueous solution containing 0.5% of methyl iodide at a pH of 10 for 150 minutes at 400C.

The resulting membrane has 99.5% rejection to DNS (dinitrostilbene sulfonic acid), 13% to sodium sulfate, and a flux of 160 l/m2d.

Example 2 Example 1 is repeated with the same membrane mounted on a solvent stable polyacrylonitrile (instead of a polypropylene) support.

The membrane shows the following results: 98% rejection to DNS, 10% rejection to sodium sulfate, and a flux of 300 l/m2d.

Example 3 A thin film of a 1% aqueous solution of PPOBr (5 meq Br/g) containing 1 meq of m-phenylene diamine is coated on a cross-linked polyacrylonitrile asymmetric membrane support giving a dry film of 0.2 to 0.4 microns.

The membrane is aminated with ammonia and then masked with an aqueous suspension of polyvinylchloride (PVC) of 5 to 20 microns in diameter by coating it on the surface of the coated asymmetric polyacrylonitrile membrane. The particles of PVC adhere to the surface after a short contact with toluene. The amino groups in the non-masked areas are hydrolyzed via diazotization with sodium nitrite to hydroxyl groups as described in Example 1.

These areas are then chlorosulfonated with a 1.5% solution of chlorosulfonic acid in n-hexane for 0.66 minutes, and hydrolyzed to form the negativly charged parts of the membrane.

After removing of the PVC particles by immersing the the membrane in cyclohexane the remaining amino groups are quaternized with a 0.5% solution of mehtyl iodide in ethanol/water (50/50) at a pH of 10 for 150 minutes at 400C.

The resultant membrane has a rejection of 97% to DNS, 0 to 20% to sodium sulfate, and a flux of 600 Vm2d.

Claims (31)

Claims:

1. A semipermeable composite membrane which comprises a microporous support coated with a thin polymer film of mosaic structure whose isolated anionic and cationic areas extend to the thickness of the film.

2. A composite membrane according to claim 1, which comprises a microporous support of optionally cross-linked polyolefines, polysulfones, polyether sulfones, polyether ketones, polyether-ester ketones, polyether imides, polyphenylene oxides, polyphenylene sulfides, polyamides, polyimides, polyamide-imides, polycarbonates, polyacrylonitriles, polyethers, polybenzimidazoles, polytetrafluoro ethylenes, polyvinylidene fluorides, cellulosics, and mixtures thereof, or ceramics.

3. A composite membrane according to claim 1 or 2, whwerein the microporous support is an asymmetric ultrafiltration support.

4. A composite membrane according to any one of claims 1 to 3, wherein the thin polymer film is obtainable from halomethylated polyphenylene oxides, polyether sulfones, polysulfones or polystyrenes, each quaternated with tertiary amines; sulfonated and/or carboxylated polystyrenes, polysulfones or polyether sulfones; or homo- or copolymers on the basis of polydimethyl siloxanes, containing groups that can be charged with amino or halogen compounds.

5. A composite membrane according to claim 4, wherein the polymers are further chemically modified when forming the isolated anionic and cationic areas.

6. A composite membrane according to claim 4 or 5, wherein the polymers are halomethylated 2,6-polyphenylene oxides, polysulfones or polyethersulfones, quaternated with tertiary amines.

7. A composite membrane according to any one of claims 1 to 6 in flat or tubular form.

8. A process for the preparation of the semipermeable composite membrane according to claim 1, which comprises (a) casting a polymer on a microporous support to form a thin polymer film, cross-linking it, and optionally subjecting it to a further chemical modification, (b) partially masking the cross-linked and optionally chemically modified polymer film, (c) charging the non-masked (non-protected) areas of the polymer film by a chemical modification, (d) demasking the polymer film, and (e) charging the demasked areas of the polymer film by a chemical modification step to generate charges opposite to those generated in step (c).

9. A process according to claim 7, wherein the microporous support is of optionally cross-linked polyolefines, polysulfones, polyether sulfones, polyether ketones, polyether-ester ketones, polyether imides, polyphenylene oxides, polyphenylene sulfides, polyamides, polyimides, polyamide-imides, polycarbonates, polyacrylonitriles, polyethers, polybenzimidazoles, polytetrafluoro ethylenes, polyvinylidene fluorides, cellulosics, and mixtures thereof.

10. A process according to claim 8 or 9, wherein the microporous support is an asymmetric ultraflitration support.

11. A process according to any one of claims 8 to 10, wherein the optionally charged polymer is a halomethylated polyphenylene oxide, a polyether sulfone, a polysulfone or a polystyrene, each quaternated with tertiary amines; a sulfonated and/or carboxylated polystyrene, a polysulfone or a polyether sulfone; or a homo- or copolymer on the basis of polydimethyl siloxanes, containing groups that can be charged with amino or halogen compounds.

12. A process according to any one of claims 8 to 11, wherein cross-linking is carried out with at least difunctional compounds capable of reacting with the functional groups of the polymer, preferably amino, hydroxy or carboxylic acid groups.

13. A process according to claim 12, wherein the at least difunctional compounds contain at least two functional groups selected from multiple unsaturated bonds, or epoxide, aziridine, aldehyde, isocyanate, isothiocyanate, hydroxyl, amino, carboxylic acid, carboxylic anhydride, carboxylic halide, halotriazinyl and halopyrimidyl moieties.

14. A process according to claim 8, wherein the further chemical modification in step (a) is carried out with compounds containing acid or amine functions.

15. A process according to any one of Claims 8 to 14, wherein in step (b) half of the area of the film surface is masked.

16. A process according to claim 15, wherein the masked area is stripes, regular patterns or of irregular shape.

17. A process according to claim 15 or 16, wherein the area is masked with micron-wide polymeric stripes(photo-resists) or dots.

18. A process according to claim 16 or 17, wherein the size of the masked area is of the order of 0.1 to 50 microns, preferably of 0.5 to 5 microns.

19. A process according to any one of claims 15 to 18, wherein masking is made by polymeric material, preferably of polyvinylchlorides.

20. A process according to claim 8, wherein in charging steps (c) and (e) the non-masked or demasked areas of the polymeric film are modified by one or a series of chemical reaction steps using mono- or polyfunctional compounds generating ionic charges.

21. A process acording to claim 20, wherein the mono- or polyfunctional compounds are acids, bases, or diazotizing and alkylating agents, or a combination of them.

22. A process according to claim 20 or 21, wherein steps (c) and (e) comprises hydrolytic steps or alkylating of tertiary nitrogen atoms.

23. A process according to any one of Claims 8 to 22, which comprises (a) casting a halomethylated polyphenylene oxide on a microporous support, cross-linking it with an aliphatic or aromatic diamine, and reacting the obtained polymer film with chlorosulfonic acid, (b) partially masking the cross-linked and further reacted polymer film with a photoresist mask, (c) forming negative charges in the non-masked areas of the polymer film by hydrolysis of the chlorosulfonic groups, (d) demasking the polymer film, and (e) forming positve charges in the demasked areas by reacting the remaining chlorosulfonic acid groups with an N,N-disubstituted alkylene diamine, followed by quaternization with an alkylating agent.

24. A process according to any one of claims 8 to 22, which comprises (a) casting a halomethylated polyphenylene oxide on a microporous support and cross-linking it with an aliphatic or aromatic diamine.

(b) partially masking the cross-linked polymer film with a photoresist mask, (c) forming negative charges in the non-masked areas of the polymer film by reaction with a sulfonating agent, followed by hydrolysis, (d) demasking the polymer film, and (e) forming positive charges in the demasked areas by quaternization with a trialkyl amine.

25. A process according to any one of claims 8 to 22, which comprises (a) casting a halomethylated polyphenylene oxide on a microporous support, cross-linking it with an aliphatic or aromatic diamine, and reacting the obtained polymer film with ammonia, (b) partially masking the cross-linked and further reacted polymer film with a photoresist mask, (c) forming negative charges in the non-masked areas of the polymer film by diazotization, hydrolysis, chlorosulfonation and again hydrolysis, (d) demasking the polymer film, and (e) forming positive charges in the demasked areas by quaternization with an alkylating agent.

26. A process according to any one of claims 23 to 25, wherein the phenylenediamine in step (a) is m-phenylenediamine.

27. A process according to claim 23 or 25, wherein the alkylating agent in step (e) is a C1-C4-alkyl halide.

28. A process according to claim 24, wherein the trialkylamine in step (e) is a C1-C4-trialkylamine.

29. Use of the composite membranes according to any one of claims 1 to 7 or obtainable according to the process for the preparation of any one of claims 8 to 28 for the separation of low molecular compounds from inorganic salts, preferably salts of polyvalent inorganic acids.

30. A method for separating organic compounds of low molecular weight from aqueous, inorganic salt containing solutions, which comprises disposing the solutions on one side of the composite membranes according to any one of claims 1 to 7 or obtainable according to the process for the preparation of any one of claims 8 to 28, and filtering them through the membranes by applying a hydraulic pressure against said solutions and said membranes being greater than the osmotic pressure of said solutions.

31. A method according to claim 30, wherein the molecular weight of the organic compounds is less than about 1000 and preferably between 150 and 700.