Studies on the hydration of Aerosol OT and the formation of reversed micelles in decane by a calorimetry and ab initio calculations

Relative apparent molar enthalpies, Lϕ, of the mixed solute, [{sodium bis(2-ethylhexyl) sulfosuccinate}, (Aerosol OT; AOT) + rH2O] in decane, and Lϕ of water in the solution of (AOT + H2O + decane) at a molality, m, of AOT (m = 0.15 mol · kg−1), were determined at T = 298.15 K; where r is the amount-ratio of water to AOT. The relative partial molar enthalpies for the mixed solutes and H2O were calculated from the observed Lϕ. As the molality of the mixed solute increased the partial molar enthalpies of the mixed solute, L(AOT+rH2O),ϕL(AOT+rH2O),ϕ decreased sharply for the systems with r > 1, and then, reached constant value at m larger than 0.02 mol · kg−1 because aggregates or reversed micelles were formed with the help of water. Endothermic enthalpies associated with the formation of aggregates were observed for the system with r = 0. The relative partial molar enthalpy of water, LH2OLH2O showed a sharp peak at r = 0.6 due to the stable complex of (3AOT + H2O). The value of LH2OLH2O for r > 0.6 decreased with increasing r, whereas a shoulder was found in the region of 3 < r < 6. The state of water in this region was attributed to the secondary hydration-shell surrounding Na+ that was proposed by Frank–Wen and Samoilov. Ab initio calculations were carried out to investigate the successive hydration of Aerosol OT and the association energies as well as the configurations of hydrated AOT were correlated to the calorimetric results. The spectra of FT-IR were also measured for (AOT + rH2O) solved in decane and cyclohexane at m = 0.15 mol · kg−1 with r for 0 to 20 and the hydration of the polar group of AOT was discussed.