Understanding Water Structure in an Ion-Pair Solvation Shell in the Vicinity of a Water/Membrane Interface

The anomalous properties of interfacial water at the surface of a lipid membrane and their implications on nearby chemical processes are well recognized. However, we have found that ion pairing thermodynamics may not be significantly affected by interfacial water in a classical, nonpolarizable force field. To trace the root cause of such a counterintuitive finding, we performed atomistic molecular dynamics simulations to explore the impact of polarizable interactions and characterize the hydration structure of a sodium chloride (NaCl) ion pair at the surface of a model lipid membrane and in a bulk phase. Our study reveals that the effect of the aqueous interface on the first solvation shell of the ion pair and thus on the ion pairing thermodynamics becomes more pronounced in the polarizable model, and that the free energy profile along the interionic distance cannot capture the difference in the degree of solvent participation in ion pairing at the water/membrane interface. This study also forms the basis for the future design of a reaction coordinate that takes the behavior of the interfacial water into account.