EFFECT OF COUNTERION COMPETITION ON MICELLAR GROWTH HORIZONS FOR CETYLTRIMETHYLAMMONIUM MICELLAR SURFACES - ELECTROSTATICS AND SPECIFIC BINDING

Counterion-mediated micellar growth for cetyltrimethylammonium 2,6-dic hlorobenzoate alone and together with cetyltrimethylammonium chloride has been investigated using small-angle scattering techniques. Variati ons in counterion composition at constant ionic strength cause large c hanges in micelle size. The selectivity coefficient for adsorption of the aromatic counterion over chloride ions at CTA(+) surfaces has been determined. The effects of the two counterions on micellar surface po tentials, together with H-1 NMR chemical shift data, allow inferences to be drawn about the penetration of the aromatic counterion into the head group region of the micelles. The aromatic ion increases the surf actant-packing parameter by both increasing the average volume per sur factant monomer, a cosurfactant-like effect, and decreasing the area p er head group by screening electrostatic repulsions. Counterions such as chloride, which show only surface adsorption, affect only area and are much less effective at driving micellar growth. Some comparisons a re also made with literature data for CTABr.