Standard free energy of transfer of ionic surfactants from water to water + acetone mixtures from vapour pressure measurements

Abstract

The standard free energy of transfer, ΔG^°_t of sodium decylsulphate (SDS) and of decyltrimethyl-ammonium bromide (DTMABr) from pure water to mixtures of water and acetone (AC) have been determined by accurate vapour pressure measurements of dilute solutions at 298.15 K. Precise conductance measurements were also made for the two ionic surfactants in water and in water + AC mixtures in the same solute concentration range. ΔG^°_t was negative, went through a minimum and increased in the AC rich mixtures for both ionic surfactants. These results are qualitatively similar to those for tetrabutylammonium bromide which forms no micelles in the same solvents; they are also consistent with the results inferred from critical micelle concentration (c.m.c.) data on dodecyltrimethylammonium bromide in the same binary solvent system. Using an extrathermodynamic approach for assignment of ΔG^°_t single ion values, it is shown that these experimental findings can be accounted for solely by the opposing behaviour of anion and cation towards the solvent molecules. Comparison of ΔG^°_t for SDS and DTMABr (which are almost equal over the whole AC concentration range) indicates that the contribution of the OSO^–₃ group is of the same sign (positive from water to water + AC mixtures) and virtually equal to that of the bromide ion.