INTRODUCTION A molecular model based in statistical thermodynamics is used to study salt effects on the lower consolute boundary of the aqueous Aqueous solutions containing polyoxyethylene surfactants non-ionic surfactant C 8 E 5 . The C 8 E 5 micelles are modeled as hard of the form CH 3 (CH 2 ) i01 O (CH 2 CH 2 O) j H (abbreviated as spheres interacting via a temperature-dependent Yukawa at-C i E j ) separate into micelle-rich and micelle-poor liquid traction and the salt ions are modeled as positively and negatively phases when heated above the cloud point temperature at charged hard spheres interacting via a Coulombic potential. The concentrations greater than the critical micellar concentraexcess thermodynamic properties due to the Coulombic and Yution. A plot of the cloud point temperature versus the concenkawa potentials are evaluated using the analytical solutions to trations of the two phases is U-shaped, which means that the Ornstein-Zernike equation obtained for the mean spherical the phase envelope is a lower consolute boundary. Addition approximation closure. The Yukawa parameters for the micellemicelle attractions are determined by fitting the theoretical phase of salts to aqueous surfactant solutions has a profound effect diagram for a pure Yukawa fluid to the experimental lower consoon the lower consolute boundary. For example, Fig. 1 [plotlute boundary for a salt-free C 8 E 5 micelle-water solution. Ionted using the data from Weckstro Β¨m and Zulauf (1)] shows solvent interactions are indirectly accounted for by using prethat the addition of NaCl and NaBr ''salts out'' or depresses viously determined adjusted values for the cation size and the the lower consolute boundary of the C 8 E 5 -water mixture dielectric constant of the medium. We evaluate theoretical coexiswhereas the addition of NaI ''salts in'' or elevates the lower tence curves for the C 8 E 5 micelle-salt-water mixtures in the temconsolute boundary. In this paper we use the integral equaperature-micelle volume fraction and temperature-salt molarity tion theory of statistical thermodynamics to model the effect planes. We calculate the changes in the lower critical solution of various salts on the lower consolute boundary of aqueous temperature (LCST) for the C 8 E 5 micelle-salt-water mixture as solutions containing the nonionic surfactant, C 8 E 5 . a function of salt concentration for the salts NaF, NaCl, NaBr, It has been known since the early 1950's that salts have NaI, and Na 2 SO 4 and compare the trends seen with experiments.
When ion-solvent interactions are indirectly accounted for, the a significant effect on the cloud point temperatures of nontheory correctly predicts the salting-out trends exhibited by NaF, ionic surfactant solutions. One of the earliest papers to de-NaCl, and NaBr. For the 1:2 salt (Na 2 SO 4 ), charge effects resulting scribe this effect is that of Doscher et al. (2). They measured from the higher charge on the ions play a more important role in the turbidity and viscosities of Triton X-100, a surfactant salting-out than ion-solvent interactions do. The theory, however, with a polyoxyethylene head group and an alkyl phenol tail, cannot predict the salting-in phenomena exhibited by NaI, thus as a function of added salt and concluded that certain salts, indicating that salting-in is the result of variations in the intermisuch as NaCl, salt out the surfactant whereas other salts, cellar attraction as a function of the salt type and salt concentrasuch as CaCl 2 , salt in the surfactant. tion. The theoretical results also indicate that excluded-volume Numerous investigators have measured the salting-out and forces resulting from the different sizes of the salt ions cannot salting-in effects of various salts on aqueous nonionic surfacalone account for the salting-in and salting-out phenomena seen tant solutions in an effort to explain this phenomenon. in aqueous nonionic micellar solutions.