Determination of surface potential in liposomes.

The ability of the fluorescent pH indicator 4-heptadecylumbelliferone to detect the electrical potential at the surface of negatively charged liposomes, was investigated. The vesicles were prepared from mixtures of egg lecithin and dicetyl phosphate at different molar ratios in NaCl solutions of various concentrations. It has been found that the dependence of the experimental surface potential in the proportion of charged lipid in the vesicles and on the salt concentration in the aqueous phase, was very similar to the predictions of the Gouy-Chapman equation as calculated by assuming a reasonable value for the mean molecular area of the lamellar lipids. In view of the good correlation obtained between the experimental and theoretical results, it is concluded that 43-heptadecylumbelliferone is quantitatively sensitive to changes in double-layer potential at the surface of lipid vesicles.