Structure effect on nonideal mixing of alkyl methyl sulfoxide and alkyldimethylphosphine oxide in adsorbed film and micelle

The surface tension of aqueous solutions of the octylmethylsulfoxide (OMS)- decyldimethylphosphine oxide (DePO) and decylmethylsulfoxide (DeMS)-octyldi methylphosphine oxide (OPO) mixtures was measured as a function of the tota l molality of the surfactants and the composition in the surfactants, Compo sitions in the adsorbed film and micelle were thermodynamically evaluated f rom the total molality vs composition and the critical micelle concentratio n (CMC) vs compositon curves, respectively. The phase diagrams of adsorptio n and micelle formation were drawn and compared with those of the DeMS-DePO mixture previously studied. A large gap in composition was observed betwee n the adsorbed film and the micelle coexisting at the CMC and ascribed to t he differences in geometric structure between them and the constituent mole cules. Negative deviation from ideal mixings in adsorbed film and micelle w as observed for the OMS-DePO mixture, whereas a positive one was observed f or the DeMS-OPO, The structure effect on the nonideal mixings was clarified by using the excess thermodynamic functions of adsorption and micelle form ation calculated from the activity coefficients in the adsorbed film and mi celle. (C) 1999 Academic Press.