Water-water correlations in electrolyte solutions probed by hyper-Rayleigh scattering

Charge Gradients around Dendritic Voids Cause Nanoscale Inhomogeneities in Liquid Water

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.

Multistep Micellization of Standard Surfactants Evidenced by Second Harmonic Scattering

Processes involving in solution a reduced number of molecules are difficult to identify and characterize. Here, we show that micellization of standard surfactants, namely sodium dodecyl sulfate and trimethyl tetradecyl ammonium bromide, two nonefficient compounds for quadratic nonlinear optics, can be investigated by second harmonic scattering (SHS). In particular, the formation of aggregates at concentrations smaller than the critical micellar concentration is evidenced through a nonmonotonic behavior of the SHS intensity as a function of the surfactant concentration. A simple model based on chemical equilibria between monomers and micelles is proposed to account for the experimental observations. Signature of long-range molecular orientation correlation is revealed by polarization resolved experiments and is discussed regarding micellization and charge-induced effects.

Dielectric response with short-ranged electrostatics

The dielectric nature of polar liquids underpins much of their ability to act as useful solvents, but its description is complicated by the long-ranged nature of dipolar interactions. This is particularly pronounced under the periodic boundary conditions commonly used in molecular simulations. In this article, the dielectric properties of a water model whose intermolecular electrostatic interactions are entirely short-ranged are investigated. This is done within the framework of local molecular-field theory (LMFT), which provides a well-controlled mean-field treatment of long-ranged electrostatics. This short-ranged model gives a remarkably good performance on a number of counts, and its apparent shortcomings are readily accounted for. These results not only lend support to LMFT as an approach for understanding solvation behavior, but also are relevant to those developing interaction potentials based on local descriptions of liquid structure.

Forces between zinc sulphide surfaces; amplification of the hydrophobic attraction by surface charge

Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).

Comment on "Water-water correlations in electrolyte solutions probed by hyper-Rayleigh scattering" [J. Chem. Phys. 147, 214505 (2017)]

The work by Shelton [J. Chem. Phys. 147, 214505 (2017)] discussed and interpreted differences with a previous study by Chen et al. [Sci. Adv. 2, e1501891 (2016)] regarding the influence of electrolytes on the structure of water. It is argued by Shelton [J. Chem. Phys. 147, 214505 (2017)] that impurities and hyper-Raman scattering contributions are the reasons for differences in the measured second harmonic intensity between the above two studies. Here, we show that these proposed effects are not relevant and discuss the influence of pulse parameters, focusing on pulse duration, since these two sets of experiments are performed with substantially different pulse durations, 100 ns and 190 fs, respectively. We show that inelastic higher-order effects play a role in the experiment with 100 ns laser pulses (the probed structure is that of the electrolyte solution that is modified by a laser pulse), while in the experiment with 190 fs laser pulses, only the elastic second-order response is measured (probing the unperturbed water structure).

Response to "Comment on 'Water-water correlations in electrolyte solutions probed by hyper-Rayleigh scattering'" [J. Chem. Phys. 149, 167101 (2018)]

The Comment proposes that the discrepancy between two second harmonic scattering (SHS) experiments for D₂O electrolyte solutions with 100 ns and 190 fs laser pulses is due to unexplained processes for 100 ns pulses that give non-quadratic power dependence for the second harmonic signal. However, the different power dependence of the second harmonic scattering signals measured with 100 ns and 190 fs laser pulses is due to changes in laser beam propagation and focal intensity caused by the combined effect of thermal defocusing and Kerr lens self-focusing. Non-quadratic power dependence does not explain the discrepancy in the second harmonic scattering results.

What Does Second-Harmonic Scattering Measure in Diluted Electrolytes?

We derive a theoretical expression of the second harmonic scattering signal in diluted electrolytes compared with bulk water. We show that the enhancement of the signal with respect to pure water observed recently for electrolytes at very low dilution in the micromolar range is a mere manifestation of the Debye screening that makes the infinite-range dipole-dipole solvent correlations in 1/ r³ disappear as soon as the ionic concentration becomes finite. In q space, this translates into a correlation function having a well known singular behavior around q = 0, which drives the observed ionic effects. We find that the signal is independent of the ion-induced long-range behavior of the function ⟨cos ϕ( r)⟩ that has been recently discussed. We find also that the enhancement depends on the experimental geometry and occurs only for in-plane polarization detection, as observed experimentally. On the contrary, the measured isotope effect between light and heavy water cannot be fully explained.

The spatial range of protein hydration

Proteins interact with their aqueous surroundings, thereby modifying the physical properties of the solvent. The extent of this perturbation has been investigated by numerous methods in the past half-century, but a consensus has still not emerged regarding the spatial range of the perturbation. To a large extent, the disparate views found in the current literature can be traced to the lack of a rigorous definition of the perturbation range. Stating that a particular solvent property differs from its bulk value at a certain distance from the protein is not particularly helpful since such findings depend on the sensitivity and precision of the technique used to probe the system. What is needed is a well-defined decay length, an intrinsic property of the protein in a dilute aqueous solution, that specifies the length scale on which a given physical property approaches its bulk-water value. Based on molecular dynamics simulations of four small globular proteins, we present such an analysis of the structural and dynamic properties of the hydrogen-bonded solvent network. The results demonstrate unequivocally that the solvent perturbation is short-ranged, with all investigated properties having exponential decay lengths of less than one hydration shell. The short range of the perturbation is a consequence of the high energy density of bulk water, rendering this solvent highly resistant to structural perturbations. The electric field from the protein, which under certain conditions can be long-ranged, induces a weak alignment of water dipoles, which, however, is merely the linear dielectric response of bulk water and, therefore, should not be thought of as a structural perturbation. By decomposing the first hydration shell into polarity-based subsets, we find that the hydration structure of the nonpolar parts of the protein surface is similar to that of small nonpolar solutes. For all four examined proteins, the mean number of water-water hydrogen bonds in the nonpolar subset is within 1% of the value in bulk water, suggesting that the fragmentation and topography of the nonpolar protein-water interface has evolved to minimize the propensity for protein aggregation by reducing the unfavorable free energy of hydrophobic hydration.