What makes the solubilization of water in reversed micelles exothermic or endothermic? A titration calorimetry investigation

Thermal effects of the solubilization of water in reversed micelles based on sodium (or potassium, ammonium) bis(2-ethylhexyl)phosphate (1a, 1b, 1c), potassium 2-ethylhexyl mono(2-ethylhexyl)phosphate (2a), and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) have been investigated by titration calorimetry. It was found that the overall reaction is exothermic for the reversed micelles of 1a, 1b, and 2a, while it is endothermic for the reversed micelle of 1c. The influences of Ni2+ and urea on the molar enthalpy (ΔHm) of the solubilization of water have also been studied. The ΔHm value is reduced significantly by the presence of 0.1 M Ni2+, while it increases again on going from 0.1 to 0.2 M Ni2+. The ΔHm value decreases with increasing urea concentration in the reversed micelles of both 2a and AOT. Using a method in which the total molar enthalpy [ΔHm(T)] is correlated to the molar ratio of water to surfactant (R), the molar enthalpies of bound (ΔHB) and free (ΔHF) water, and the maximum number of water molecules that can bond to a surfactant molecule (NB) have been obtained. Both the qualitative and quantitative results reveal that the following three thermal effects are involved in the solubilization of water in reversed micelles: (1) an endothermic effect due to the breaking of the hydrogen-bonded network in bulk water (effect 1); (2) an exothermic effect arising from the interactions of water with the counterion and other head groups of the surfactant (effect 2) and (3) an endothermic effect due to the expansion or the dismantling of the quasi-lattice between the counterion and the anion of the surfactant (effect 3). For the AOT-based reversed micelle, effects 1–3 cannot totally explain its overall endothermicity. Thus, some special reasons have also been proposed to account for the "‘unusual’' thermal behaviour of the AOT system.