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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≤2  nm), where many effects, prominently those linked to the structure of liquid water, interfere, electrostatics alone rules over larger distances (d≥5  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.

The effect of inserting a neutral water-soluble adsorbing polymer on the flexibility of amphiphilic bilayers in a lamellar phase is investigated. The L α system is a stack of charged undulating bilayers composed of sodium dodecyl sulfate (SDS) and octanol separated by aqueous solutions of polyethylene glycol (PEG). The mean bending elastic modulus κ is determined from the quadrupole splittings in the solid state NMR spectra of the perdeuterated octanol chains embedded in the membranes that undergo collective fluctuations. Parameters for describing the membrane behavior (bilayer thickness, elastic compressibility modulus, order parameter) are obtained by supplementing the NMR data with complementary experiments (x-ray scattering), NMR spectral simulations, and theoretical considerations. A fairly complete picture of the membrane rigidity emerges for any location in the lamellar phase thanks to a broad sweep of the lamellar domain by systematically varying the membrane fraction along dilution lines as well as the polymer composition. The most remarkable result is the difference between dilute and semi-dilute regimes. In the dilute PEG solution, no (or slight positive shift) polymer contribution to the rigidity curvature of the layered system is noted within the experimental resolution (≤0.3 k B T) and κ remains around 2.7 k B T. In contrast, the membrane rigidity increases steadily upon polymer addition once the crossover concentration c p * is exceeded, amounting to a 60% increase in κ at polymer concentration 2.5 c p * in the aqueous interlayers. These results are discussed with regard to the theoretical expectation of membrane rigidification upon irreversible polymer adsorption. [ABSTRACT FROM AUTHOR]

Using the Surface Force Apparatus (SFA) technique, direct force-distance measurements between mica surfaces immersed in a polymer-doped lamellar phase are presented. In the aqueous layers separating the charged and undulating bilayers of sodium dodecyl sulfate (SDS)/octanol, water is replaced by a polyethylene glycol aqueous solution (PEG, Mw [ABSTRACT FROM AUTHOR]