  We have studied the electrostatic screening effect of NaCl solutions on the
interactions between anionic lipid bilayers in the fluid lamellar phase using a
Poisson-Boltzmann based mean-field approach with constant charge and constant
potential limiting charge regulation boundary conditions. The full DLVO
potential, including the electrostatic, hydration and van der Waals
interactions, was coupled to thermal bending fluctuations of the membranes via
a variational Gaussian Ansatz. This allowed us to analyze the coupling between
the osmotic pressure and the fluctuation amplitudes and compare them both
simultaneously with the measured dependence on the bilayer separation,
determined by the small-angle X-ray scattering experiments. High-structural
resolution analysis of the scattering data revealed no significant changes of
membrane structure as a function of salt concentration. Parsimonious
description of our results is consistent with the constant charge limit of the
general charge regulation phenomenology, with fully dissociated lipid charge
groups, together with a four-fold reduction of the membranes' bending rigidity
upon increasing NaCl concentration.
