The Effects of Salts on the Lower Consolute Boundary of a Nonionic Micellar Solution

A molecular model based in statistical thermodynamics is used to study salt effects on the lower consolute boundary of the aqueous non-ionic surfactant C8E5. The C8E5 micelles are modeled as hard spheres interacting via a temperature-dependent Yukawa attraction and the salt ions are modeled as positively and negatively charged hard spheres interacting via a Coulombic potential. The excess thermodynamic properties due to the Coulombic and Yukawa potentials are evaluated using the analytical solutions to the Ornstein-Zernike equation obtained for the mean spherical approximation closure. The Yukawa parameters for the micelle-micelle attractions are determined by fitting the theoretical phase diagram for a pure Yukawa fluid to the experimental lower consolute boundary for a salt-free C8E5 micelle-water solution. Ion-solvent interactions are indirectly accounted for by using previously determined adjusted values for the cation size and the dielectric constant of the medium. We evaluate theoretical coexistence curves for the C8E5 micelle-salt-water mixtures in the temperature-micelle volume fraction and temperature-salt molarity planes. We calculate the changes in the lower critical solution temperature (LCST) for the C8E5 micelle-salt-water mixture as a function of salt concentration for the salts NaF, NaCl, NaBr, NaI, and Na2SO4 and compare the trends seen with experiments. When ion-solvent interactions are indirectly accounted for, the theory correctly predicts the salting-out trends exhibited by NaF, NaCl, and NaBr. For the 1:2 salt (Na2SO4), charge effects resulting from the higher charge on the ions play a more important role in salting-out than ion-solvent interactions do. The theory, however, cannot predict the salting-in phenomena exhibited by NaI, thus indicating that salting-in is the result of variations in the intermicellar attraction as a function of the salt type and salt concentration. The theoretical results also indicate that excluded-volume forces resulting from the different sizes of the salt ions cannot alone account for the salting-in and salting-out phenomena seen in aqueous nonionic micellar solutions.