Counterion effects on colloid stability of cationic vesicles and bilayer-covered polystyrene microspheres

The major factor responsible for the very low colloid stability (W) of dioctadecyldimethylammonium (DODA) chloride, bromide, or acetate vesicles in the presence of monovalent salt is identified. Large DODAAc vesicles prepared in 0.6 M D-glucose remain stable over the entire range of NaAc concentrations tested (0-150 mM NaAc). The pH measured for this highly concentrated D-glucose solution is 5.1-5.3. Because acetic acid is a neutral and small molecule that can freely permeate the DODAAc vesicle, upon addition of an acidic NaAc plus D-glucose solution to the vesicle outside, acetic acid forms and reaches the vesicle interior acting as an acetate carrier; acetate binds to the inner vesicle surface and eliminates any charge asymmetry between the inner and outer surface. Counterions such as chloride or bromide do not cross the membrane to reach the inside. Thereby asymmetry of the charge distribution generated upon external addition of NaCl or NaBr, respectively, to DODACl or DODABr vesicles cannot be relaxed, causing the hydrophobic defects that decrease W. For DODA vesicles and bilayer-covered polystyrene microspheres of similar sizes, W is systematically higher for the covered particles. For the large bilayer-covered particles, salt-induced aggregation is reversible upon salt removal, whereas for the small ones it is not. From bromide to acetate, counterion size and hydration increases and so does W. DODAAc vesicles at low pH and covered particles are the most stable dispersions. DODAAc from vesicles adsorbs with high affinity onto sulfate latexes. An adsorption maximum at intermediate supernatant concentrations typically points out a competition between vesicle/vesicle adhesion and vesicle/latex interaction as DODAAc concentration increases in the supernatant.