Density depletion and enhanced fluctuations in water near hydrophobic solutes: identifying the underlying physics

TL;DR

This paper explores the physical origins of density depletion and increased fluctuations in water near hydrophobic solutes, linking these phenomena to a critical drying transition and developing a scaling theory validated by simulations.

Contribution

It introduces a scaling theory connecting water behavior near hydrophobes to a critical drying transition, supported by density functional theory and Monte Carlo simulations.

Findings

Density depletion and fluctuations are linked to critical drying phenomena.

The scaling theory accurately predicts density and fluctuation profiles.

Simulation results support the theoretical framework.

Abstract

We investigate the origin of the density depletion and enhanced density fluctuations that occur in water in the vicinity of an extended hydrophobic solute. We argue that both phenomena are remnants of the critical drying surface phase transition that occurs at liquid-vapor coexistence in the macroscopic planar limit, ie. as the solute radius $R_s\to\infty$. Focusing on the density profile $\rho(r)$ and a sensitive spatial measure of fluctuations, the local compressibility profile $\chi(r)$, we develop a scaling theory which expresses the extent of the density depletion and enhancement in compressibility in terms of $R_s$, the strength of solute-water attraction $\varepsilon_s$, and the deviation from liquid-vapor coexistence $\delta\mu$. Testing the predictions against results of classical density functional theory for a simple solvent and Grand Canonical Monte Carlo simulations of a popular water model, we find that the theory provides a firm physical basis for understanding how water behaves at a hydrophobe.