Effect of the head group of surfactant on the miscibility of sodium chloride and surfactant in an adsorbed film and micelle

The thermodynamic treatment of a surfactant mixture was applied to the mixt ure of sodium chloride, NaCl, with octyl methyl sulfoxide (OMS) and that wi th decyldimethylphosphine oxide (DePO), The surface tension of aqueous solu tions of the mixtures was measured as a function of the total concentration and the composition of the mixtures at 298.15 K, The total surface densiti es of the mixtures and the composition of the adsorbed films and micelles w ere evaluated by applying thermodynamic equations to the expeimental result s. It was found that the adsorbed film and micelle are almost composed of t he surfactant and there is slight attractive interaction between the ions o f NaCl and the head groups of OMS and DePO molecules in the adsorbed films and micelles, A difference in the miscibility of NaCl and surfactant was ob served between the OMS and DePO systems and attributed to the difference in the hydration of the head group between OMS and DePO molecules. The compar ison of these results with those of the mixtures of NaCl with tetraethylene glycol monooctyl ether (C8E4) and dodecylammonium chloride (DAC) indicated that the small difference in the miscibility in an adsorbed film and micel le among these nonionic surfactant systems arises from the difference in hy dration and structure of the head groups and the large one between the noni onic surfactant and DAC systems results from electrostatic interactions bet ween dodecylammonium and sodium ions, (C) 2001 Academic Press.