STUDIES ON SURFACE ACTIVE AGENTS

Abstract

In view of widespread applications of surfactants, the study of their fundamental properties is of importance. The survey of literature shows that many properties of surfactants in solution have be°n extensively investigated. This thesis deals with investi gations on some aspects of surfactant solutions not yet investigated in detail. The investigations carried out pertain to counterion association studies in aqueous solution of sodium dodecyl sulphate, preparation and properties of some new cobalt complex soaps and solubilization by nonionic surfactants in nonaqueous media. The results of these investigations are briefly summarised as under: (1.) Counterion association studies of sodium dodecyl sulphate(SDS) micelles in presence of additives: The ionic micelles of SDS have sufficient number of sodium ions associated on their surface due to high charge density. Any factor that would affect micellar charge density is expected to change counterion association significantly. Therefore, counterion association of 3DS micelles in presence of both soluble and insoluble additives has been investigated. The additives include propanol, butanol, pentanol, hexanol; Tween 20,40,60 and 80; lauric acid, lauryl alcohol and Span 20,40,60 and 80. These studies have been carried out by the direct measurement of the counterion, i.e. Na concentration using a new membrane electrode system - chromium ferricyanide exchanger membrane - which is not poisoned by SDS anions when pretreated with it. The following cell has (ii) been set up to determine sodium ion activity. External reference electrode (SCE) NH.ND, salt bridge Test solution Membrane O.lM Nacl internal solution ; Internal • reference ' electrode J <SCE) i i The potential of the cell was determined for different concentrations of SDS taken as test solution. In order to find the concentration of Na from the potential data, the membrane was calibrated using different concentration of Nacl in place of test solution. Since the membrane potential is a function of sodium ion activity, different calibration plots under the same experimental conditions, were operative while measuring potentials of SDS solutions, were prepared and the data so obtained was used to determine the Na concentration in SDS solutions. From free sodium ion concentration, the degree of micellar dissociation( a ) has been determined by using Botre's equation. The results indicate that a increases with increase in alcohol and Tween's concentration and the order of effectiveness in increasing micellar dissociation is hexanol > pentanol > butanol > propanol for alcohols and Tween 80 > 60 > 40 > 20 for Tweens. These results indicate that additives with higher hydrophobic content are transferred more into the micellar phase from the bulk and thus cause separation of charge which consequently increases micellar dissociation. (iii) Further a number of golubilizates viz., lauric acid, lauryl alcohol; Span 20,40,60 and 80 have been dissolved in 0.025M SDS solutions and the release of counterions has been studied as a function of solubilizate concentration. The results indicate that lauric acid is more effective than lauryl alcohol and for Spans the order Js Span 80 >60 > 40 > 20. These trends are explained. on the basis of head group interaction with micelle exterior. (2) Solubility and micellar behaviour of some new cobalt complex soaps in various solvents: Malik, Jain and Coworkers from this laboratory have found that a number of metal soaps such as Ag, Cd, Cr and Co soaps are only sparingly soluble in pure organic solvents. The low solubility of metal soaps makes it difficult for them to be used as additives in lubricating formulations. If metal soaps of higher solubility can be prepared, they might turn out to be better substitutes in many applications. It is with this aim they began to synthesize new metal soaps by the interaction of metal complex cations with sodium/potassium soaps and found that cobalt hexammine soaps (Malik, Jain and Siddiqui, international Conf., Budapest, 1975) are more soluble than ordinary cobalt soaps. These studies do not clearly bring out the role of complex cation on the solubility, as the hydrophobic content between cobalt hexammine soap and ordinary cobalt soap is different. To study the role of size and charge of the cation i.e., the role of polar interaction between the head groups of the soap molecule, two series of new cobalt complex soaps viz., cobalt chloroaquo tetrammine and dichloro( iv) tetrammine laurate, rmyristate, palmitate and stearate have been synthesized by the interaction of corresponding cobalt complex and sodium soaps. The resulting cobalt complex soaps have been characterised by c hemic a 1 analysis. The solubility of these soaps in-large number of solvents has been determined by using Co as radiotracer and they have b-en found much more soluble than ordinary cobalt soaps in organic solvents. The comparison of the solubility data as a function of size and charge of the cationic part of ths soap molecule point out that polar interactions play dominant role in determining the solubility of a metal soap. The effect of temperature on the solubility of these soaps in benzene, m-xylene and toluene ;has also been investigated. The solubility was found to increase enormously abcve certain temperature, characteristic of each soap. This data was used to calculate their critical micelle concentration (c.m.c.) and critical solution temperature (C.S.T. ). The c.m.c,of the soaps increases as the carbon chain length is increased from laurate to stearate. (3) ^4g_e_llization of cobalt complex soaps in alcohols, alcoholbenzene mixtures and thermodynamics of mice11ization: The micellization behaviour of these cobalt complex soaps in propanol, butanol and mixtures of alcohols with benzene has been investigated using conductance measurements. They have been found to undergo micellization in these solvents. From the data, c.m.c.of the soaps has been determined and an attempt has been made to correlate the variation in c.m.c. of various soaps with hydrocarbon chain length and dielectric constant of the (v) medium. The results show that the micelles formed are of reverse type. Further, the effect of temperature on the c.m.c. of these soaps in butanol has been investigated and an attempt has been made to calculate thermodynamic parameters of the micellization process. The results obtained are qualitative in nature in view of several assumptions involved in these calculations. Nevertheless, the useful qualitative information obtained is that micellization in butanol is enthalpy directed process and the contribution to micellization per -CH2 group is rather very small. 4. solubilization of silver stearate by nonionic surfactant in non aqueous media: The solubilization of silver stearate in Span 20,40,60 and 80 solutions in benzene, cycJohexane, carbon tetrachloride, toluene, chloroform, acetone,DMF, nitrobenzene and benzene mixtures with DMF and nitrobenzene has been investigated using Ag as a radio tracer. The results indicate that the amount of silver stearate dissolved increases tremendously above certain concentration of the Spans. This observation leads to the conclusion that silver soap molecule is solubilized by span micelles apparently through mixed micelle formation. The data, therefore, could be used to determine the c.m.c.of the 'Spans. The variation in c.m.c. of various Spans has been correlated with the dielectric constant of the medium and hydrocarbon chain-length of the Spans. The results indicate that as the dielectric constant of the solvent increases, the c.m.c. is decreased and as the carbon chain-length of Span (vi) increases, the c.m.c. is decreased. This decrease in c.m.c is more pronounced in solvents of high dielectric constant as compared to solvents of low dielectric constant. Further, in solvents of • high dielectric constant, at higher concentration of Spans, it seems that micelles are acquiring a different shape which are not as efficient in solubilizing silver stearate as the aggregates at lower concentration.