Counterion binding and regulation of interactions between charged bilayers

Interactions between didodecyldimethylammonium (DDA) bilayers were measured using osmotic stress in the presence of bromide and chloride as mixed electrolyte. DDA bromide exhibits a discontinuity in its force curve, corresponding to a coexistence region in the DDAB/water phase diagram. whereas DDA chloride has a continuously repulsive interaction profile. Mixed electrolyte systems, interacting at constant electrolyte chemical potential, also display demixing. The selective binding of bromide and chloride in lamellar phases has been measured as a function of interlamellar separation and compared with selective binding to the air/solution interface. A site-binding model was used to interpret the selective binding and osmotic stress results and suggests that these bilayers interact at constant potential.