INTERACTIONS BETWEEN CHARGED SPHERIC VESICLES

The low colloidal stability of synthetic amphiphile vesicles in the presence of monovalent salt is further investigated in order to clarify the nature of the additional attractive interaction previously described. Two working hypotheses are tested: the possibility of adhesion at the secondary minimum and the occurrence of hydrophobic defects in the bilayers. Reversibility (R) of aggregation upon dilution of added salt and uptake of a hydrophobic dye are determined over a range of NaCl concentration (0-150 mM). For large dioctadecyldimethylammonium chloride vesicles at 50 mM NaCl, R goes through a minimum and dye uptake goes through a maximum. Consistently, a nonreversible process, i.e. vesicle fusion, was reported to occur upon addition of 50 mM NaCl. From 60 to 150 mM NaCl, R increases and dye uptake decreases, showing that aggregation at the secondary minimum becomes important as a determinant of colloidal instability over a higher [NaCl] range. For large dihexadecyl phosphate vesicles, aggregation is basically irreversible upon salt dilution.