Standard enthalpies of transfer of ionic surfactants from water to water + t-butyl alcohol mixtures at 298.16 K

Abstract

The standard enthalpy of solution of two ionic surfactants, sodium decylsulphate and trimethyldecyl-ammonium bromide, as well as those of tetramethylammonium bromide and methylsodium sulphate, have been determined in water + t-butyl alcohol (Bu^tOH) mixtures at 25 °C. The maximum endothermic effect observed for the surfactants at 0.05 mole fraction of Bu^tOH is the largest ever measured in these mixtures at 25 °C [ΔH^°_t(max)= 76.6 kJ mol^–1] for sodium decylsulphate. It is observed that in water + Bu^tOH mixtures the effect is larger for a rod-like ion than for the bulky tetrabutylammonium ion. The reverse is true in water + acetone mixtures. These results are discussed in terms of the structures of the respective binary solvents. The relevance of these results to the opposite variations in the critical micelle concentration of the surfactants in water on the addition of acetone (increasing) or Bu^tOH (decreasing) is also considered. Finally the contribution of the n-nonyl hydrocarbon chain of each ionic surfactant to the standard enthalpy of transfer is calculated. The hydrocarbon chain attached to the cationic head group is more endothermic than the attached to the anionic head group over the whole range of water + Bu^tOH and water + acetone mixtures.