Electric field inside a “Rossky cavity” in uniformly polarized water

Abstract

Electric field produced inside a solute by a uniformly polarized liquid is strongly affected by dipolar polarization of the liquid at the interface. We show, by numerical simulations, that the electric "cavity" field inside a hydrated non-polar solute does not follow the predictions of standard Maxwell's electrostatics of dielectrics. Instead, the field inside the solute tends, with increasing solute size, to the limit predicted by the Lorentz virtual cavity. The standard paradigm fails because of its reliance on the surface charge density at the dielectric interface determined by the boundary conditions of the Maxwell dielectric. The interface of a polar liquid instead carries a preferential in-plane orientation of the surface dipoles thus producing virtually no surface charge. The resulting boundary conditions for electrostatic problems differ from the traditional recipes, affecting the microscopic and macroscopic fields based on them. We show that relatively small differences in cavity fields propagate into significant differences in the dielectric constant of an ideal mixture. The slope of the dielectric increment of the mixture versus the solute concentration depends strongly on which polarization scenario at the interface is realized. A much steeper slope found in the case of Lorentz interfacial polarization also implies a higher free energy penalty for polarizing such mixtures.

Electrostatics of liquid interfaces

“…An approach consistent with the picture of surface polarization dominating the electrostatic response of a liquid dielectric 17,22,34 is proposed here. It reformulates the boundary value electrostatic problem in terms of the surface charge density and the corresponding surface charge susceptibility.…”

“…4 This procedure presents some clear conceptual difficulties, but, from the practical perspective, has also run into problems when applied to molecular-size objects 22,43 and to nanometer-scale liquid interfaces. 17,34 The deviations from the expected behavior are not limited to quantitative disagreements in calculated electrostatic energies, but reach the level of qualitative differences. The scaling of the liquid polar response to an ion placed in the center of a void 17 shows a cross-over from the expected ∝ a −1 scaling (Born model) to ∝ a −(4−6) scaling with increasing void's radius a (Fig.…”

Affine-response model of molecular solvation of ions: Accurate predictions of asymmetric charging free energies

“…Standard implicit-solvent models usually assume that the potential in the solute is zero if the solute were completely uncharged, and therefore do not include this electrostatic contribution to the PMF or solvation free energy. 28 In the simplest possible affine-response model, the static potential is non-zero, but assumed constant throughout the solute. Earlier studies have shown that the static potential is remarkably constant even in real proteins with complex (non-spherical) geometries.…”

Biomolecular electrostatics and solvation: a computational perspective

“…Finally, the polarization of molecular solvents is also closely linked to variations in local density (Ashbaugh & Truskett, 2001; Beglov & Roux, 1996; Beglov & Roux, 1997; Dzubiella & Hansen, 2004; Dzubiella et al, 2006a; Dzubiella et al, 2006b; Paliwal et al, 2006); e.g., the presence of cavities or other solutes. In particular, the introduction of a cavity or uncharged solute into a polar solvent such as water can create significant interfacial polarization, often resulting in a positive potential inside the cavity (Ashbaugh, 2009; Cerutti et al, 2007; Harder & Roux, 2008; Martin et al, 2011). …”