Miscibility and nonideality of mixing of heptanol and octylsulfinylethanol in the adsorbed film and micelle

The surface tension of the aqueous solution of a mixture of 1-heptanol and 2-(octylsulfinyl)ethanol (OSE) was measured as a function of the total molality and composition of the mixture at 298.15 K under atmospheric pressure. The total surface density and the compositions of the adsorbed film and micelle were evaluated by applying thermodynamic equations to the surface tension, and then the phase diagrams of adsorption and micelle formation were drawn. Heptanol and OSE molecules are miscible with each other in the adsorbed film and micelle. A negative azeotropy of adsorption (i.e., the phase diagram of adsorption with a minimum) was observed. The occurrence of the azeotropy was attributed to the similarity of the surface activities of heptanol and OSE, (in contrast, the chemical structures of the molecules are different from each other). The nonideality of mixing in the adsorbed film was quantitatively shown by the negative values of the excess Gibbs free energy and area of adsorption calculated from the activity coefficient in the adsorbed film. The nonideality was ascribed to the fact that the interaction between heptanol and OSE molecules in the adsorbed film is larger than those between the same component molecules in the pure films. The activity coefficient and excess chemical potential of OSE indicated that heptanol and OSE molecules mixed almost ideally in the micelle. A large discrepancy in composition was observed between the adsorbed film and micelle coexisting at the critical micelle concentration. This discrepancy is attributed to the difference both in size and property of the headgroups of heptanol and OSE molecules and in the geometric shape between the adsorbed film and micelle.