Probing the emergence of electrostatic attraction between like-charged phospholipid membranes

Counterintuitive electrostatic attraction between similarly charged macroions arises when multivalent counterions are present. Surprisingly, recent experiments have demonstrated that monovalent counterions alone can induce attraction among fully charged phospholipid membranes [1]. This phenomenon has been attributed to the low dielectric constant of interfacial water. To deepen our understanding of the interplay between hydration and electrostatic forces, we examined the interaction between an odd number of phospholipid layers deposited on a silica substrate. Specifically, at the air/solid interface, we controlled osmotic pressure by adjusting air relative humidity. Using the D17 neutron reflectometer at ILL (Grenoble, France) and ellipsometry, we measured the interlayer spacing for membranes with varying charged surface density. From this data, we deduced the disjoining pressure between the layers. Our findings reveal a continuous transition between a repulsive mean-field regime and an attractive strong coupling regime. This work represents the first experimental measurement of a crossover between weak and strong coupling, shedding light on the emergence of electrostatic attraction in water-confined layers.