Surface Induced Frustration of Inherent Dipolar Order in Nanoconfined Water

Abstract

Surface effects could play a dominant role in modifying the natural liquid order. In some cases, the effects of the surface interactions can propagate inwards, and even can interfere with a similar propagation from opposite surfaces. This can be particularly evident in liquid water under nano-confinement. The large dipolar cross-correlations among distinct molecules that give rise to the unusually large dielectric constant of water (and in turn owe their origin to the extended hydrogen bond (HB) network) can get perturbed by surfaces. The perturbation can propagate inwards and then interfere with the one from the opposite surface if confinement is only a few layers wide. This can give rise to short-to-intermediate range solvent-mediated interaction between two surfaces. Here we study the effects of such interactions on the dielectric constant of nano-confined liquids, not just water but also ordering at protein surfaces. The surfaces work at two levels: (i) induce orientational realignment, and (ii) alter the cross-correlations between water molecules. Molecular dynamics simulations and statistical analyses are used to address these aspects in confinement of slit pores, nano tube/cylinder, and nano sphere. In addition, we consider the hydration layers of multiple proteins with vastly different structural features. These studies give us a measure of the extent or the length scale of cross-correlations between dipole moments of water molecules. We find an interesting orientational arrangement in the protein hydration layers, giving rise to long-range molecular cross-correlations. To decouple the effect of HB from the effect of geometry, we additionally study acetonitrile under nanoconfinement. Importantly, while a protein's interior is characterized by a small dielectric constant, the dipole moment of a peptide bond is large, and thus susceptible to fluctuations in water.