Size and Origins of Long-Range Orientational Water Correlations in Dilute Aqueous Salt Solutions

The Role of Polarizability in Isoelectronic Ions: The Case of Pseudohalides

Specific ion effects are widespread and have been studied for over a century, yet they remain poorly understood. Terms like "kosmotropes" and "chaotropes" are convenient rules of thumb but the frequent reversal of the Hofmeister series implies their limitations. Polarizability is often used to classify ions, with kosmotropes considered low in polarizability and chaotropes high. However, for polyatomic ions, this framework becomes misleading. The anisotropic nature of polarizability in polyatomic ions plays a decisive role in shaping their behavior. In this work, we study pseudohalides (KOCN, KSCN, and KSeCN) aqueous solutions to explore these effects. We evaluate properties of these anions through experimental measurements of conductivity, density, viscosity, infrared spectra, and polarizability. Our results demonstrate that, even for linear isoelectronic polyatomic ions, the anisotropy of polarizability governs their hydration behavior.

The heterogeneity of aqueous solutions: the current situation in the context of experiment and theory

The advancement of experimental methods has provided new information about the structure and structural fluctuations of water. Despite the appearance of numerous models, which aim to describe a wide range of thermodynamic and electrical characteristics of water, there is a deficit in systemic understanding of structuring in aqueous solutions. A particular challenge is the fact that even pure water is a heterogeneous, multicomponent system composed of molecular and supramolecular structures. The possibility of the existence of such structures and their nature are of fundamental importance for various fields of science. However, great difficulties arise in modeling relatively large supramolecular structures (e.g. extended hydration shells), where the bonds between molecules are characterized by low energy. Generally, such structures may be non-equilibrium but relatively long-lived. Evidently, the short times of water microstructure exchanges do not mean short lifetimes of macrostructures, just as the instability of individual parts does not mean the instability of the entire structure. To explain this paradox, we review the data from experimental and theoretical research. Today, only some of the experimental results on the lifetime of water structures have been confirmed by modeling, so there is not a complete theoretical picture of the structure of water yet. We propose a new hierarchical water macrostructure model to resolve the issue of the stability of water structures. In this model, the structure of water is presented as consisting of many hierarchically related levels (the stratification model). The stratification mechanism is associated with symmetry breaking at the formation of the next level, even with minimal changes in the properties of the previous level. Such a hierarchical relationship can determine the unique physico-chemical properties of water systems and, in the future, provide a complete description of them.

How Thick is the Air-Water Interface?─A Direct Experimental Measurement of the Decay Length of the Interfacial Structural Anisotropy

The air-water interface is a highly prevalent phase boundary impacting many natural and artificial processes. The significance of this interface arises from the unique properties of water molecules within the interfacial region, with a crucial parameter being the thickness of its structural anisotropy, or "healing depth". This quantity has been extensively assessed by various simulations which have converged to a prediction of a remarkably short length of ∼6 Å. Despite the absence of any direct experimental measurement of this quantity, this predicted value has surprisingly become widely accepted as fact. Using an advancement in nonlinear vibrational spectroscopy, we provide the first measurement of this thickness and, indeed, find it to be ∼6-8 Å, finally confirming the prior predictions. Lastly, by combining the experimental results with depth-dependent second-order spectra calculated from ab initio parametrized molecular dynamics simulations, which are also in excellent agreement with this experimental result, we shed light on this surprisingly short correlation length of molecular orientations at the interface.

Long-Range Orientational Organization of Dipolar and Steric Liquids

Long-range orientational correlations in liquids have received recent renewed interest, in particular for the neat water case. These long-range orientational correlations, exceeding several tens of nanometers, originate from the presence of the strong permanent water dipolar moment. However, the exact dependence with the dipolar moment and the role of other local forces like steric hindrance has never been addressed. In this work, we experimentally measure long-range correlations for a set of liquids differing by their molecular weight and dipolar moment, in order to reveal the origin of their long-range organization. Hence, we show that the dipolar moment of a solvent molecule is not the unique feature determining the orientational correlation. Steric hindrance significantly helps to structure the liquids as well. In order to quantify these long-range correlations, we also derive theoretically the polarization resolved second harmonic scattering intensity as a function of the rotational invariants describing the dipolar and octupolar interaction.

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.

Nanometer-Scale Correlations in Aqueous Salt Solutions

The intermediate-range (1-4 nm) correlation of cations, anions, and water in aqueous alkaline earth salt solutions is measured using synchrotron X-ray diffraction. We differentiate from the entire solution structure factor, S_X(Q), a separate region at low Q (<1.5 Å^-1) containing local diffraction maxima (prepeaks) that indicate nanometer-scale oscillatory behavior. These features are quantitatively reproduced with classical molecular dynamics simulations. At high concentrations, the prepeaks emerge from correlations arising from the existence of small quasi close-packed lattice-like structures comprised of cation hydration spheres. We also analyze the concentration dependence of the prepeak and discuss the overall results in light of the rich literature dealing with intermediate-range correlations underlying universal phenomena in concentrated electrolytes.

Hydration interactions beyond the first solvation shell in aqueous phenolate solution

We investigate the orientational dynamics of water molecules solvating phenolate ions using ultrafast vibrational spectroscopy and density functional theory-based molecular dynamics simulations.

Specific Ion and Concentration Effects in Acetate Solutions with Na+ , K+ and Cs

How salt ions affect solutes and the water beyond the solvation shell is not well understood. Molecular dynamics simulations of alkali-acetate solutions were analysed here in order to examine if, and how, different cations and solute concentrations affect the water structure and the interactions between water and acetates. The results revealed that water structure is perturbed to more than 1 nm away from the acetates and that this effect is more pronounced in physiological than in molar electrolyte concentrations. Analysis of simulations of a soluble protein revealed that the water orientation is perturbed to at least 1.5 nm from the protein structure. Furthermore, modifications to the orientation of water around carboxylate side chains were shown to depend on the local environment on the protein surface, and could extend to well over 1 nm, which may have an effect on protein dynamics during MD simulations in small water boxes.

Structure and Dynamics of the Hydration Shell: Spatially Decomposed Time Correlation Approach

Molecular simulations provide insight into solvation structures and dynamics with unparalleled spatial and temporal resolution. Here, we take advantage of this fact and develop a set of generally applicable computational tools for a detailed analysis of the hydration shell around an ionic or molecular solute. These tools allow us to quantify and visualize orientationally resolved radial distribution functions as well as distance-resolved orientational time-correlation functions of water molecules surrounding the solute. Such a detailed view of the hydration shells allows us to unravel important structural and dynamical features, which are not accessible when employing standard analysis techniques.

Automated detection of many-particle solvation states for accurate characterizations of diffusion kinetics

Discrete-space kinetic models, i.e., Markov state models, have emerged as powerful tools for reducing the complexity of trajectories generated from molecular dynamics simulations. These models require configuration-space representations that accurately characterize the relevant dynamics. Well-established, low-dimensional order parameters for constructing this representation have led to widespread application of Markov state models to study conformational dynamics in biomolecular systems. On the contrary, applications to characterize single-molecule diffusion processes have been scarce and typically employ system-specific, higher-dimensional order parameters to characterize the local solvation state of the molecule. In this work, we propose an automated method for generating a coarse configuration-space representation, using generic features of the solvation structure-the coordination numbers about each particle. To overcome the inherent noisy behavior of these low-dimensional observables, we treat the features as indicators of an underlying, latent Markov process. The resulting hidden Markov models filter the trajectories of each feature into the most likely latent solvation state at each time step. The filtered trajectories are then used to construct a configuration-space discretization, which accurately describes the diffusion kinetics. The method is validated on a standard model for glassy liquids, where particle jumps between local cages determine the diffusion properties of the system. Not only do the resulting models provide quantitatively accurate characterizations of the diffusion constant, but they also reveal a mechanistic description of diffusive jumps, quantifying the heterogeneity of local diffusion.

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.