Nonideal mixing of decyl methyl sulfoxide and decyldimethylphosphine oxidein adsorbed films and micelles

The surface tension of aqueous solution of decyl methyl sulfoxide (DeMS) an d decyldimethylphosphine oxide (DePO) mixture was measured as a function of the total concentration of the surfactants and the composition in the surf actants. The compositions of the adsobed film and micelle were evaluated by applying thermodynamic equations to the surface tension and the phase diag rams of adsorption and micelle formation were drawn. The DeMS and DePO mixt ure showed a negative deviation from the ideal mixing in adsorbed film and a positive deviation in micelles, The activity coefficients of surfactant i n the adsorbed film and micelle were calculated from the phase diagrams and the nonideality of mixing in the adsorbed film and micelle was clarified i n terms of the excess thermodynamic quantities of adsorption and micelle fo rmation. Judging from the negative values of the excess Gibbs free energy a nd area of adsorption and a positive one of the excess Gibbs free energy of micelle formation, it was concluded that the difference in head group betw een DeMS and DePO molecules causes a favorable packing in the adsorbed film and an unfavorable one in the micelle, (C) 1998 Academic Press.