STUDIES ON THE MEMBRANE PROPERTIES OF INORGANIC GEL EXHIBITING ION EXCHANGE BEHAVIOUR

Abstract

With the growing utility of membranes in biological, bio-chemical and analytical research and with their great demand in industrial and technological processes, basic studies on various aspects of membrane phenomenon has assumed greater dimen sions in recent years. The pioneering work of Donnan on the physico-chemical aspects of the diffusion of ions through semi permeable membranes separated by solutions of different activities provided great impetus for further work in this fascinating yet highly complex field of research. Donnan*s work was followed by the brilliant researches of chemists, bio-chemists and physio logists such as Teorell , Meyer , Marshall4, Schlogl5 Sollner6, Staverman , Overbeek , Pungor , Barrer °, Scatchard11, Kobatake12, 13 Lakshminarayanaiah etc., who individually as well as collecti vely, contributed handsomely to the development of this branch of knowledge. No doubt several definitions for the term membrane have been put forward from time to time, a precise definition has yet to come. The elaborate definition of Sollner (loc.cit) *A membrane is a phase or structure interposed between two phases or compartments which obstructs or completely prevents gross movement between the latter, but permits passage with various degrees of restriction, of one or several species of particles from the one to the other or between the two adjacent phases or compartments and which thereby acting as a physico-chemical 2 machine transforms with various degrees of efficiency accord ing to its nature and the nature and composition of the two adjacent phases or compartments, the free energy of the adja cent phases or compartments or energy applied from the outside to the latter, into other forms of energy', or the tersely worded definition of Lakshminarayanaiah (loc.cit) 'A phase, usually netrogeneous, acting as a barrier to the molecular and ionic species present in the liquid and/or vapours contacting the two surfaces (the term netrogeneous used to indicate the internal physical structure and external physico-chemical per formance' are good enough to comprehend the meaning and signi ficance of this simple term. Broadly speaking membrane can oe put under two cate gories: natural (biomembrane such as that of lipids with polar groups oriented towards the two aqueous phase - extracellular and intracellular-phases of the cell) and the artificial memb ranes (i) site free [(both quartz or paraffin) solid and liquid (benzene or silicon) ]. (ii) with sites [inorganic and organic ion exchangers with fixed sites and liquid ion exchangers having mobile sites ]. This classification, based on structure,however, does not carry us too far since classification based on the method of preparation, will be more meaningful if fundamental studies are the main concern of the investigator. The synthetic membranes (having the pre-requisite of good chemical and mechanical stability) can be subdivided into two types :(i) homogenous membranes (ii) hetrogerjpus memb ranes. 3 Homogeneous membranes : These membranes are coherent ion exchanger gels in the shape of discs, ribbons etc. 2heir structure is that of usual ion exchange resins. Examples of this type are polymethacrylic acid (PMA) , phenolsulfonic acid (PSA), polystyrene 14-15 sulphonic acid (PSSA), and cellulose esters. Orderwise they are obtained by copolymerization of methacrylic acid and ethy lene glycol dimethylacrylate, polymerisation of phenolsulfonic acid with formaldeyde and sulfonation of polystyrene and using a formulation consisting of cellulose acetate 23 percent, dioxane 65.5 percent, anhydrous Mg (C104) 1.2 percent and 16 distilled water 10.4 percent . These membranes have found much use in fundamental transport studies and for applications in some industrial opera tions such as the treatment of brakish waters, saline water conversion etc. Hetrogeneous membranes : These membranes furnish a much greater variety for fundamental and applied studies than the homogeneous ones. They consist of colloidal ion-exchanger particles coated or embedded in an inert substrate or binder (polystyrene , polyethylene , silicone resins19, wax20, Teflon21 etc.). Anecessary require ment for such membranes is that the ion exchanger particles should remain in contact with one another, rather than be separated by the inert material. The volume percentage of ion 4 exchange material should be made as high as is compatible with the required mechanical strength. Most hetrogeneous membranes form 50 - 75 percent ion exchange material. In contrast to nonreinforced membranes which are obtained by casting techniques from solutions of membrane forming substances, reinforced membranes have found wide appli cation. They are prepared by impregnating the cloth by a monomer solution ( or a partially polymerized gel) followed by complete polymerization, subsequently the membrane so ob tained is made ionogenic by sulfonation, chloromethylation etc. 22 For example, a permselective anion exchange membrane was prepared using 2-polyvinyl chloride fabric which was covered with a copolymer of 2-methyl-5-vinyl pyridine, styrene, and DVB. The supported membrane thus obtained was treated success ively with methyl iodide, a solution of m-phenyl-nediamine and acidic aqueous formaldehyde solution. In a similar fashion cation exchange membrane 5 was obtained from a piece of Orion cloth soaked at 5° with liquid resin made from a mixture of 200 g PSA and 70 gm of 32 percent HCHO cooled at 15 0. Artificial hetrogeneous membranes are also prepared by applying pastes or polymeric materials to existing sheets of films. Polyethylene sheets have been used in a number of processes. A cation exchanger from this material has been obtained by simply soaking it in chlorosulfonic acid and subse quently washing it with NaOH solution and water ~ . Slightly unusual type of polymer membranes containing colloidal metal have been reported27: cellophane film was immersed for 1 min. 5 in a solution prepared from 20 g AgffO,, 200 ml HgO, 50 ml 10 percent HaOH, and 6 5 ml 38 percent HH,. Next it was immer sed for 3 seconds in a solution of 500 ml 40 percent HCHO. Finally it was immersed in 1500 ml H20 to give a film contain ing black colloidal Ag. Next it was placed in a solution con taining' sodium styrene sulfonate (200 parts), H20 (600 parts), glycidyl methcrylate (300 parts), dioxane (100 parts), and (NH4) 320g ( 5 g). The solution was made acidic (pH = 5) by adding acetic acid. The mixture was heated for 15 min. at 85°C to form a purplish black film. Metal tungstate and metal ferrocyanide ' membranes have been prepared by depositing thorium tungstate or metal ferrocynide in the pores of parch ment thimble by the method of double decomposition. Membranes have also been prepared by photochemical treatment. Powerful ionizing radiations have been used to bring about the polymerisation of monomers incorporated into various materials . Extraordinarily rapid development has taken place in the search of membranes which could serve as models for natu ral membranes. Since living' membranes (bimolecular leaflets of lipid) are 75-100 A° thick, it was difficult to realize arti ficial membranes from polymeric materials of this thickness. Several lipid permeable membranes of this thickness have been prepared from materials such as collodion , thiolated gelatin"52, phosphorylated55 fatty alcohols55'54, butanol liquid55(liquid membranes), phospholipids , proteins , etc.