Counterion effects on properties of cationic vesicles

The effect of counterion nature and concentration on phase transition, bila yer structure, vesicle size, vesicle internal volume per mole of amphiphile , and surface potential is evaluated for cationic vesicles composed of dioc tadecyldimethylammonium (DODA) acetate, chloride, or bromide. Over a range of ionic strengths (0-5 mM monovalent salt), no interdigitation was detecte d in the bilayer structure for the three DODA counterions. The preferential type of aggregate formed from self-assembly of DODA salts is a large vesic le composed of a single traditional and noninterdigitated bilayer. Vesicle size and zeta-potentials were inversely related, i.e., an increase in zeta- potential was accompanied by a decrease in vesicle size. The largest zeta-p otentials and smallest sizes were obtained for bilayer vesicles of DODA ace tate which have the largest and more hydrated counterion. The effect of ion ic strength (0-5 mM NaAc, NaCl, or NaBr as monovalent salt) was a slight de crease followed by a significant increase in vesicle size as a function of salt concentration. The results for counterion effects on vesicle size agre e with predictions from the self-assembly model by Israelachvili and co-wor kers.