Charge gradients around dendritic voids cause nanoscale inhomogeneities in liquid water

TL;DR

This study uses femtosecond elastic second harmonic scattering and molecular dynamics simulations to demonstrate that liquid water exhibits nanoscale charge density fluctuations around voids, indicating inhomogeneity at the molecular level.

Contribution

It provides experimental and simulation evidence that liquid water is non-uniform at the nanoscale, challenging the view of water as a homogeneous liquid.

Findings

Water shows charge density fluctuations around transient voids.

Void number increases with temperature above room temperature.

Nanoscale inhomogeneities are not necessarily linked to the two-state model of water.

Abstract

Water is the matrix of life and is generally considered a homogeneous, uniform, liquid. Recent experiments report that water is instead a two-state liquid. However, subsequently, these findings were contested. The structure of water and whether we should think of it as uniform therefore remains an open question. Here, we report femtosecond elastic second harmonic scattering (fs-ESHS) of liquid water in comparison to an isotropic liquid (CCl4) and show that water is indeed a non-uniform liquid, The coherent fs-ESHS intensity was interpreted, using molecular dynamics simulations, as arising from charge density fluctuations and consequentially enhanced nanoscale polarizabilities around transient voids having an average lifetime of 300 fs. Although voids were also present in CCl4, they were not characterized by hydrogen bond defects and did not show strong polarizability fluctuations, leading to fs-ESHS of an isotropic liquid. The voids increased in number at higher temperatures above room temperature, in agreement with the fs-ESHS results. The measured nanoscale-femtosecond inhomogeneities therefore do not necessarily relate to the proposed two state model of water, but instead underscore the elusive nature of liquid water, and undoubtedly have an impact on any type of transition that occurs in liquid water.