Attractive Interaction between Fully Charged Lipid Bilayers in a Strongly-Confined Geometry.

We investigate the interaction between highly charged lipid bilayers in presence of monovalent counterions. Neutron and X-ray reflectivity experiments show that the water layer between like-charged bilayers is thinner than for zwitterionic lipids, demonstrating the existence of counter-intuitive electrostatic attractive interaction between them. This can be explained by correlations between counterions beyond the classical Poisson-Boltzmann theory of electrostatics, in the Strong Coupling limit. Our results show the limit of the strong coupling continuous theory in highly-confined geometry and are in agreement with a decrease of the water dielectric constant.