In-plane distribution in mixtures of cationic and anionic surfactants

Mixtures of cationic and anionic surfactants, also called "catanionic'' mixtures, self-assemble into aggregates which show a rich variety of morphologies in the nanometre to micron range controlled by the molar ratio between surfactants. At the molecular scale, little is known about the local distribution of surfactants in the bilayer. Here, we determine the in-plane distribution of the cationic and anionic surfactants in catanionic bilayers using neutron scattering and the contrast matching technique. The coexistence of two different in-plane orders is experimentally demonstrated: both surfactants share a common two-dimensional hexagonal lattice with a long-range order, but the distribution of alternate (+) and (-) charged head groups shows only a two-dimensional liquid-like local order. Comparing experiments and Monte-Carlo simulations, we establish that this lateral liquid order is attenuated by the counterions of the bilayers, but is less than expected in a mean-field approach. This demonstrates that electrostatic interactions participate in, but do not completely determine, the local distribution of surfactants of opposite charge in the bilayer.