The equilibrium and structure of lecithin-cholate mixed micelles

Abstract

The nature of the equilibrium: mixed micelles-intermicellar solution, has been determined for the system Lecithin-Sodium Cholate. The composition of the intermicellar solution has been estimated after light scattering data, and the results confirmed by other methods. The composition and size of the mixed micelles have then been evaluated. Provided that the mixed micelles are in equilibrium with a solution containing at least the CMC of cholate, their size was shown to diminish progressively as the ratio Lecithin/Cholate present in the mixed micelles decreased. At the same time the concentration of the free cholate dissolved in the intermicellar solution, in equilibrium with the mixed micelles, was shown to increase continuously as the size of the mixed micelles decreased. The thermodynamic equilibrium of these systems has been treated in a simple manner. The existence of stable mixed micelles in equilibrium with a free cholate concentration below the CMC was seen to correspond to a metastable state, and in the presence of small amounts of salt such systems immediately coagulated. Finally it is shown that the proposed model satisfactorily accounted for the variation in size of the mixed micelles with the micellar Lecithin/Cholate ratio.