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 C(8)E(5). The C(8)E(5) micelles are modeled as hard sphere s interacting via a temperature-dependent Yukawa attraction and the sa lt ions are modeled as positively and negatively charged hard spheres interacting via a Coulombic potential. The excess thermodynamic proper ties due to the Coulombic and Yukawa potentials are evaluated using th e analytical solutions to the Ornstein-Zernike equation obtained for t he 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 conso lute boundary for a salt-free C(8)E(5) micelle-water solution. Ion-sol vent interactions are indirectly accounted for by using previously det ermined adjusted values for the cation size and the dielectric constan t of the medium, We evaluate theoretical coexistence curves for the C( 8)E(5) micelle-salt-water mixtures in the temperature-micelle volume f raction and temperature-salt molarity planes, We calculate the changes in the lower critical solution temperature (LCST) for the C(8)E(5) mi celle-salt-water mixture as a function of salt concentration for the s alts NaF, NaCl, NaBr, NaI, and Na2SO4 and compare the trends seen with experiments. When ion-solvent interactions are indirectly accounted f or, the theory correctly predicts the salting-out trends exhibited by NaF, NaCl, and NaBr. For the 1:2 salt (Na2SO4), charge effects resulti ng from the higher charge on the ions play a more important role in sa lting-out than ion-solvent interactions do, The theory, however, canno t predict the salting-in phenomena exhibited by NaI, thus indicating t hat salting-in is the result of variations in the intermicellar attrac tion as a function of the salt type and salt concentration, The theore tical results also indicate that excluded-volume forces resulting from the different sizes of the salt ions cannot alone account for the sal ting-in and salting-out phenomena seen in aqueous nonionic micellar so lutions. (C) 1996 Academic Press, Inc.