Interactions between charged surfaces in aqueous solutions, widespread in soft matter and biology, are very complex and, despite many efforts, their full explanation remains challenging. We support the idea that, in contrast to extremely small separations ($d\le 2\mathrm{nm}$), where many effects, prominently those linked to the structure of liquid water, interfere, electrostatics alone rules over larger distances ($d\ge 5\mathrm{nm}$) at low ionic strength. We set up specially designed surface force apparatus (SFA) experiments to measure the elastic compressibility modulus of a stack of charged membranes with monovalent counterions, directly and with high precision. We demonstrate that electrostatics alone, if implemented beyond Poisson-Boltzmann theory, fully accounts for the data, nonelectrostatic contributions playing at best a minor role.

• Received 17 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.268302