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Explicit Solvent Effects on Macromolecular Interactions From a Solvent-Augmented Contact Value Theorem

The Derjaguin-Landau-Verywey-Overbeek (DLVO) theory has been a remarkably accurate framework for the characterization of macromolecular stability in water solvent. In view of its solvent-implicit nature neglecting the electrostatics of water molecules with non-negligible charge structure and concentration, the precision of the DLVO formalism is somewhat puzzling. In order to shed light on this issue, we derive from our earlier explicit solvent formalism [S. Buyukdagli el al., Phys. Rev. E, 2013, 87, 063201] a solvent-augmented contact value theorem and assess the contribution of solvent molecules to the interaction of charged membranes. We find that in the case of hydrophobic membranes with fixed charges embedded in the membrane surface, the nearly exact cancellation of various explicit solvent effects of substantially large magnitude but opposite sign keeps the intermembrane pressure significantly close to the double layer force of the DLVO theory. Then, in the case of hydrophilic surface charge groups within the aqueous region, due to the spatial separation of the membrane substrate from the location of the fixed charges where the non-local dielectric response of the structured solvent is sharply localized, the interfacial field energy and the contact charge densities remain unaffected by the explicit solvent. As a result, the hydration of the lipid head groups suppresses the signature of the solvent molecules from the membrane interaction force.