CALORIMETRIC STUDIES ON THE STATE OF WATER IN REVERSED MICELLES OF SODIUM BIS(2-ETHYLHEXYL)SULFOSUCCINATE IN VARIOUS SOLVENTS

The state of water in reversed micelles was studied by measuring the enthalpies of water in sodium bis(2-ethylhexyl)sulfosuccinate, aerosol OT (AOT), reversed micelle solutions with a conduction type of microcalorimeter. The molar enthalpy changes of water were obtained by mixing AOT solutions of various R values with anhydrous AOT solutions, where R means the molar ratio of water to surfactant AOT. An enthalpy curve against R was drawn by adding consecutively the observed enthalpy changes from high to low R. The curve showed a maximum point, and the R values at the point in chloroform and benzene were smaller than those in isooctane and cyclohexane, depending on the molecular structure of the solvent. The enthalpy values were in the order of chloroform > benzene > isooctane > cyclohexane in overall R. These curves were interpreted not in terms of the so-called two-state model, but in terms of a three-state model based on water partitioned between three states. The effects of respective solvents on the enthalpy curves were discussed in connection with the interaction of polar groups of AOT reversed micelles with the solvent molecules.