Charge Gradients around Dendritic Voids Cause Nanoscale Inhomogeneities in Liquid Water

Abstract

Water is the matrix of life and serves as a solvent for numerous physical and chemical processes. The origins of the nature of inhomogeneities that exist in liquid water and the time scales over which they occur 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 nonuniform liquid. The coherent fs-ESHS intensity was interpreted, using molecular dynamics simulations, as arising from charge density fluctuations with 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.

Birth of the Hydrated Electron via Charge-Transfer-to-Solvent Excitation of Aqueous Iodide

“…These microstate-dependent fluctuations are illustrated using transition dipole moments obtained from time-dependent (TD-)­DFT calculations of the initial CTTS state, as shown in Figure S2. This is consistent with an electron that is initially “trap-seeking”, meaning that inherent (and fluxional) voids in the structure of liquid water , manifest as variability in the location of the lowest-energy CTTS excitation.…”

Predicting the Geometry of Core–Shell Structures: How a Shape Changes with Constant Added Thickness

“…24,25 In solute physics, the shape of the cavity a solute occupies within a solvent is vital to understanding the solute−solvent interactions involved. 9,10,12,26 This is especially true for proteins, where the shape is both extremely complex and vital to their function. 17,19 The first solvation shell around the particles or macromolecules itself may also be of great interest.…”

Liquid Water: When Hyperpolarizability Fluctuations Boost and Reshape the Second Harmonic Scattering Intensities

“…We notably reproduce for the first time the depolarization ratio of pure liquid water without any adjustable parameters. To go beyond the present work, challenges remain, such as integrating orientational correlations at long distances, , improving the precision of the electronic response calculations (e.g., including several molecules), ,,, and comparing QM/MM calculations to experimental results in terms of absolute intensities of different phases . Our QM/MM approach will now be developed to study the SHS signal of other solvents or aqueous solutions.…”

“…10,15−17 In particular, for water, the impact of orientational correlations on the second harmonic intensity has been investigated, underlining the importance of both shortand long-range structuration, 18−23 or density heterogeneities appearing transiently in liquid water. 24,25 The effects induced by interfaces or nano-objects have also been explored. 26−29 However, a quantitative interpretation of all contributions is still missing.…”

“…Among other experimental techniques, such as nuclear magnetic resonance, IR-spectroscopies, and X-ray or neutron scattering, second harmonic scattering (SHS) is a method of choice to investigate the molecular structure of liquids. The contribution of orientational correlations in SHS intensities has been known for decades − and has recently found renewed interest in probing the molecular structure of various liquids such as organic solvents, ionic liquids, surfactant solutions, , and salted or pure water. ,− In particular, for water, the impact of orientational correlations on the second harmonic intensity has been investigated, underlining the importance of both short- and long-range structuration, − or density heterogeneities appearing transiently in liquid water. , The effects induced by interfaces or nano-objects have also been explored. − However, a quantitative interpretation of all contributions is still missing. The frameworks developed to investigate orientational correlations from SHS data ,, are often based on the assumption that all the molecules of the system have the same second harmonic response, i.e., the same first hyperpolarizability tensor β.…”