Forces that control pH-dependent aggregation of nonionic glycolipid vesicles

clarify forces that control pH-dependent aggregation of nonionic glycolipid vesicles, we have measured forces between layers of a nonionic glycolipid, 1,3 -di- O-phytanyl-2-O-(beta-(D)-maltotriosyl)glycerol, Mal(3)(Phyt)(2), in two different pH conditions, pH 4 and pH,9.8. Mal(3)(Phyt)(2) vesicles rapidly aggregate at pH4, but the vesicle dispersion is stable at pH 9.8 (see: J. Colloid Interface Sci. 2000, 223, 235.). At pH 4, the lipid layers were attracted by van der Waals forces at long distances, whereas at short distances they were repelled by "hydration" forces. Practically no double-layer force was observed at pH 4. At pH 9.8, on the other hand, the double-layer force associated mainly with "adsorption" of OH- ions at the lipid surface was superposed on the core,interactions, the van der Waals forces, and the hydration forces. The force data indicate that the changes in pH, although affecting the double-layer force, have a negligible influence on the range of the hydration forces and the magnitude of the adhesion forces between the Mal(3)(Phyt)(2) layers. It is therefore the double-layer force that controls the pH-dependent stability of the Mal(3)(Phyt)(2) vesicle dispersion. Commonly assumed hydration-based mechanisms such as an ion-induced interfacial dehydration and/or an ion-induced decrease in hydration forces do not appear relevant to the present case.