Activity coefficients of CaCl2 and MgCl2 in the presence of dipalmitoylphosphatidylcholine-phosphatidylinositol vesicles in aqueous media

Mean activity coefficients of CaCl2 and MgCl2 salts have been measured in the presence of phospholipid vesicles, by use of the EMF method with ion-exchange membrane electrodes, as a novel procedure for studying the direct interactions between electrolytes and phospholipid vesicles. Mixed lipid sonicated vesicles have been prepared from dipalmitoylphosphatidylcholine (DPPC)-phosphatidylinositol (PI) mixtures, covering a range of composition. The CaCl2 and MgCl2 concentration range studied was 0.01-0.001 mol l(-1). For both salts studied, the activity coefficients decrease linearly as the vesicle concentration in the suspension increases while the concentration of salt was kept constant. Maintaining a constant vesicle concentration, the decrease in the activity coefficients is more pronounced the more dilute the concentration of the salts. Quantitative differences were observed in the behaviour of both salts which are explained on the basis of a higher affinity of the Ca2+, compared with that of Mg2+ ions, to the vesicle surface. Using a simple model for the vesicle-salt systems, the number of cations specifically bound to the vesicles has been calculated. In view of the results it can be concluded that estimation of the activity coefficients of the salts is a useful method for the study of the interactions taking place in vesicle-salt systems.