Physical chemical studies of short-chain lecithin homologues. 3. Phase separation and light scattering studies on aqueous dioctanoyllecithin solutions

Some phase separation phenomena in aqueous dioctanoyllecithin solutions including the effects of NaCl and Lil on the phase diagram are reported. At low electrolyte concentrations (below 0.2 M) both salts cause the upper consolute temperatures (u.c.t) to decrease, probably due to a decrease of electrostatic attraction between the lipid molecules. At higher salt concentrations the effect of Lil continues in the same direction (salting-in) but NaCl leads to an increase of the u.c.t. (salting-out).Micellar weight determinations could be performed at room temperature in homogeneous lecithin solutions containing 0.2 M Lil. An attempt was made to interpret the light scattering data with the help of the open association model (equal association constants for aggregation steps beyond a certain step) and the Flory-Huegins type of thermodynamic nonideality. The angular dependence of the light scattering points to very large and extended micelles. The radii of gyration are approximately proportional to the square root of the micellar weights.