The determination of binding site density and association constants for monovalent cation adsorption onto liposomes made from mixtures of zwitterionic and charged lipids.

The Gouy-Chapman-Stern theory predicts that the shape (but not the magnitude) of the surface potential dependence on the electrolyte concentration mainly reflects the screening of the surface charge but not cation adsorption. So this dependence does not allow to determine two parameters of the theory (the surface density of binding sites (S) and the association constant (K)) simultaneously. Therefore the fitting procedure for the determination of S and K was suggested as a test of cation adsorption for the surface or zeta potential measurements at a fixed electrolyte concentration but with a variable ratio of charged and neutral components in the lipid mixture. This procedure was applied to the electrophoretic measurements made by the method of photon correlation spectroscopy in the suspensions of PS/PC or CL/PC liposomes in the monovalent electrolytes. For KCl and NaCl electrolytes it only leads to the different association constants (about 0.2 and 0.8 M-1, respectively) corresponding with data from the literature but to the same value of the surface charge density (about -16 microC/cm2) which is smaller than usually postulated for PS membranes. The fitting of zeta potentials measured in tetraalkylammonium salts shows a small cation adsorption but a large surface charge density (about -21 microC/cm2).