Adsorption equilibria at interface separating electrolyte solution and phosphatidylcholine-stearylamine liposome membrane

The effect of the pH of an electrolyte solution on the electric surface charge of the liposome membrane was studied. The membrane of vesicles contained egg phosphatidylcholine (PC) with different proportions of stearylamine (ST). The surface charge density of the membrane was determined as a function of pH from electrophoretic mobility measurements. A six equilibria model describing the solution ions adsorption on the PC-ST liposome membrane surface was presented in this paper. The knowledge of the association constants of the -PO(−) and -N(+)(CH3)3 groups of PC with H+, OH−, Na+, Cl− ions: KA1H, KB1OH, KA1Na, KB1Cl, that had been presented earlier, allowed to determine the association constants of the -N(+)H3 group of ST with OH− and Cl− ions: KB2OH, KB2Cl. The proposed model has been proved to be correct by comparing the resulting theoretic charge variation curves of the PC-ST liposomal membrane with the experimental data.