PHYSICO-CHEMICAL STUDIES ON THE INTERACTION OF SURFACE ACTIVE AGENTS WITH ISOPOLY ACIDS AND COBALT AMMINE

Abstract

Micellization in non-aqueous solvents has not been extensively investigated. This phenomenon varies very differently from that observed in the aqueous medium since the number of parameters involved in controlling the c.m.c, size and shape of micelles are quite large. The first important point of difference is that the micelle structure is reversed (polar head groups instead of the hydrocarbon part merge into the core of the micelle). Other dissimilarities are that the micelles do not possess a significant net charge; micellization does not involve large decrease in interfacial energy; dipole-dipole interaction between the head groups in the micelle centre; irregular trend in both values of c.m.c. and micellar size; effect of solubilized impurities like water etc. Micellization in aqueous systems means loss of hydrocarbon/ water interfacial energy and loss of Water structure around the hydrocarbon chain. Opposing factors are electro static repulsion between charged polar groups (in the case of ionized surfactants) and translational energy, In order to transform monomers into micelles work has to be done against repulsion and in reducing the translational freedom. Although both these factors bring about increase in free energy, the overall decrease in energy due to loss of hydrocarbon/water interfacial energy and v/ater structure outweighs this increase with the result that stable micelles are formed. In other words, the negative change m entropy due to freedom loss (disorder to order transition) is counter balanced by the positive entropy change due to loss of water structure. An overall increase in entropy thus takes place, thereby inducing conditions favourable to micellization. The micelle formation in non-aqueous solvents is largely affected by the nature of the polar part of the surfactant . It appears that the largest energy changes arise from dipole-dipole interactions between head groups and to some extent hydrogen bond formation between them. The head groups thus provide the necessary energy for micellization (solvation effects for micellization are not considered since these effects, which obviously should be more dominant in aqueous systems, are not well understood even in them). On the other hand, the hydrocarbon part does not affect micellization significantly. However, the effect of hydrocarbon part on micelle formation has been recognized, as evinced by the decrease in the aggregation number of micelle with increase of carbon chain length of quarternary amines and fatty acid zinc soaps'2'. Besides these two factors, viz. interaction part betv/een head groups and hydrocarbon/and length of hydrocarbon chain, (3) another factor to which Kitsathora •and • Kon-Nov hac recently drawn attention to, is the interaction betv/een the hydrocarbon part and the solvent molecules. Considering the entropic and enthalpic parameters part for the interaction of hydrocarbon/with solvent molecules for an oilsoluble surfactant system, they derived a relationship betv/een equilibrium aggregation number \ and carbon chain number N of the surfactant: ) = 1.iL / o N Existence of spherical micelles implies that there are strong attractive forces between the polar heads, v/hich in turn, provide an effective shielding of the polar core from the solvent. Interfacial effects between polar head and solvents become effective when the micelles are laminar or rod-shaped. 3 Several factors control the size and shape of micelles in non aqueous solvents. The polar core is most effectively shielded from interaction by the solvent if the micelles are spherical. Change from spherical to laminar or rod-shaped micelles would imply interfacial changes at the edges due to polar head-solvent interaction. It is well established that ionic surfactants form larger micelles than non-ionic ones. On similar considerations one can expect that more polar is the monomer, the larger will be the micelle formed. (4) The presence of small amounts of water has a significant effect on the nature of soap solutions in non-aqueous solvents. In the case of aryl stearate soaps in benzene,a large decrease in viscosity takes place on the addition of water. & change in micelle shape from elongated to spherical was suggested to account for the viscosity change. The v/eight average molecular weight of Aersol OT in toluene v/as also affected by the presence of water. Initially a small number of large aggregates (5) were formed, which split up as the solution agedx ', Generally speaking the size of the micelle is very much affected by the change of solvent. Lecithin which gives large micelles in (6) water (64,000 monomers) gives only 70-73 monomer aggregates (7) in benzenev '. This surface active compound is freely soluble in a large number of solvents and can be taken up as a typical compound for studying the effect of solvents on micellar size. The dielectric constant of the solvent controls the size of the micelle. As the dielectric constant is decreased, there is less interfacial energy between the monomer and the solvent. Usually solvents of dielectric constant below 30 give reversed micelles ^) due to strong interaction between head groups as well as due to the repulsive forces between the solvent and the head groups. Thermodynamics of Micellization The understanding of the thermodynamics of micellization is of much theoretical and practical importance. The process of micellization involves the reversible aggregation of N molecules of the amphiphile to form a micelle. N - m *= M For larger values of N, the standard free energy of micellization /\ G has been related ' to the c.m.c. as foUows: Ag° = RT In c.m.c. Many investigators * have further assumed that if the variation of N with temperature is negligible the enthalpy of formation A>H° can be obtained by using the following relation: 2 AH° = - RT 6 In c.m.c. ST Recently Birdi has related AG° to both the c.m.c. and aggregation -i P o number N. AG° = RT In c.m.c. + RT/N InN Shape and size of micelles Solutions of heavy metal soaps have proved to be the most useful systems for ascertaining the micellar behaviour of soaps in non-aqueous solvents. In this context the data obtained with benzene as the solvent are worth mentioning. It has been reported that magnesium, zinc or aluminium dioleates give small micelles containing upto three monomers*18*. In contrast to that, disoaps, ^RC00)2 AlOH, in benzene gavemicelles containing 500 and 1000 monomers(u1 9)'. Metal salts of phenyl stearates, ethyl hexyl sebacate, dinonyl naphthalene sulphonates 5 and phenyl stearates in benzene have been studied by Singleterry and co- workers*4, 20, 2l\ The aggregation number was approximately two for dinonylnaphthalene sulphonates with ten different cations in benzene, so that the micelles were little affected by the nature of cations or by moisture*7' 2l* . The usual methods of determining the c.m.c, viz., conductance and surface tension, have been little applied in non-aqueous solutions for obvious reasons. Many other methods, viz., viscosity, osmotic pressure, solubility, ebullioscopic measurements etc. have been successfully employed to determine the c.m.c. in non-aqueous media. Some very important information on this aspect has also been obtained from solubilization, light scattering and fluorescent depolarization. The latter technique has been used to demonstrate that sodium and barium dinonyl naphthalene sulphonatsin benzene have c.m.c. 's as low as 10"6 - 10"7 moles/litre*22\ Uses of suTfactants Surfactants are of great importance'2^' in detergency in the textile industry, biological actions, emulsification, in cosmetic preparations, metal and mineral technology, etc., due to their distinctive properties: i) Their moderate maximum concentration of molecularly dispersed species, ii) Surface and interfacial tension. iii) Dispersion in very dilute solutions, due to the adsorption and orientation of molecules at the interface. 6 iv) Micelle formation above a certain concentration, and v) Solubilization of water insoluble substances by micelles. A very large proportion of the total surfactant production is used in washing of fabrics and textile materials. The surfactants are also used in cleaning hard surfaces, metal surfaces, glass, ceramics, non-metallic inorganic surfaces., paint surfaces, plastics, linoleum^ ' etc. In textile and dry cleaning indust ries, they are used in several textile processing operations, such as dyeing, wetting, emulsification, (25) dispersion and other similar gross effects . In the dyeing of textiles, surfactants are used as dispersing agents, levelling agents, fixing agents and stripping agents^ '. Apart from detersive applications, the use of surfactants in many fields of medical and pharmaceutical practice has increased appreciably within the last few years. A large number of surfactants have found striking application as germicides and among the quartemary ammonium (27) salts, many of the most powerful modern bactericides have been found Anionic surfactants such as lauryl sulphate and nonionic surfactants such as Tweens and Spans have been widely adopted for preparing skin lotions in which the effective medicament is suspended cr emulsified. In metal and mineral technology surfactants are used in flotation, (28) electroplating and surface finishing of metals . In building and construction industries too, the importance of surfactants is being realised. Specific areas in which surfactants have brought about major changes include the preparation and use of asphalt bonding materials, concrete, soil stabilization etc' '. 7 Besides the above listed applications, surfactants are used in agriculture, leather industry, preparation of synthetic rubbers, polymers, plastics, paints, in petroleum and chemical processing industry and in fire fighting.