Effect of salts on the formation of C8-lecithin micelles in aqueous solution

The effect of divalent and trivalent salts (CaCl2, CaBr2, MgCl2, MgBr2, LaCl3, CeCl3, La(NO3)(3), and Ce(NO3)(3)) on the micelle formation in Cs-lecithin solutions was investigated using the techniques of static and dynamic light scattering. The critical micelle concentration (cmc), mean hydrodynamic radius (Rh), gyration radius (R-g), and weight-average molecular weight of the micelles were measured as functions of salt identity and concentration, amphiphile concentration, and temperature. It was found that the micelles in solutions of magnesium are less likely to form and less stable; their standard enthalpy is less exothermic as the ionic strength increases. On the contrary, the micelles in solutions of calcium and trivalent salts form easily, and are more stable; their standard enthalpy is also more exothermic as the ionic strength increases. Based on our model of the Gibb's free energy for the salt-added solutions, we obtained the following formula for the effect of salts on cmc: In(cmc)' = In(cmc) + k(1) I-1/2 + k(2)I, where (cmc)' and (cmc) are the critical micelle concentrations in salt-added and salt-free solutions, respectively, I is the ionic strength, and k(1) and k(2) are the salt effect parameters. The agreement between the formula and the experimental data for all the systems under study shows that the formula is more satisfactory than those suggested previously by other authors in describing the effect of salts on the cmc in the micellar solutions of not only zwitterionic but also nonionic surfactants. (C) 2001 Academic Press.