THE EFFECT OF SALT IDENTITY AND CONCENTRATION ON LIQUID LIQUID-PHASE SEPARATION IN AQUEOUS MICELLAR SOLUTIONS OF C-8-LECITHIN

We report measurements of the location and shape of the coexistence curve for binary liquid-liquid phase separation in aqueous micellar solutions of C8 -lecithin, as a function of the concentrations of KI, NaI, LiI, KCl, NaCl, and LiCl. The salts containing iodide dramatically lower the critical temperature for phase separation Tc as the salt concentration is increased. The chloride salts, however, first lower and then raise Tc as the salt concentration is increased. The identity of the added cations used has a comparatively minor effect on Tc. Potassium ions are more effective in changing Tc than sodium ions, which in turn are more effective than lithium ions. We have used our data to deduce the dependence on salt identity and concentration of the parameters C and Δμ, which reflect the strength of intermicellar interactions and the extent of micellar growth, respectively, within a Gibbs free energy model for micellar solutions. The theoretical coexistence curves calculated using the deduced parameters agree well with our experimental findings. © 1990 American Institute of Physics.