Dielectric saturation in water as quantitative measure of formation of well-defined hydration shells of ions at various temperatures and pressures. Vapor–liquid equilibrium case

Cluster Formation Induced by Local Dielectric Saturation in Restricted Primitive Model Electrolytes

Experiments using the Surface Force Apparatus (SFA) have found anomalously long-ranged charge-charge underscreening in concentrated salt solutions. Meanwhile, theory and simulations have suggested ion clustering to be a possible origin of this behavior. The popular Restricted Primitive Model of electrolyte solutions, in which the solvent is represented by a uniform relative dielectric constant, εr, is unable to resolve the anomalous underscreening seen in experiments. In this work, we modify the Restricted Primitive Model to account for local dielectric saturation within the ion hydration shell. The dielectric "constant" in our model locally decreases from the bulk value to a lower saturated value at the ionic surface. The parameters for the model are deduced so that typical salt solubilities are obtained. Our simulations for both bulk and slit geometries show that our model displays strong cluster formation and these give rise to long-ranged density correlations between charged surfaces, at distances similar to what has been observed in SFA measurements. An electrolyte model wherein the dielectric constant remains uniform does not display similar clusters, even with εr equal to the low saturated value at ion contact. Hence, the observed behaviors are not simply due to an enhanced Coulomb interaction. In the latter case, cluster growth is counteracted by long-ranged repulsions between like-charged ions within clusters; this is an effect that is considerably reduced when the dielectric response drop is local. Our results imply that long-ranged interactions in these systems are mainly due to cluster-cluster correlations, rather than large electrostatic screening lengths.

Thermodynamic Analysis of Capillary and Electric Field Effects on Liquid-Vapor Equilibrium: A Study on the Water-Ethanol Mixture

This study investigates phase equilibrium manipulation in nonideal mixtures through a combined capillary and external electric field approach. Utilizing thermodynamic principles, an expression is established for estimating the equilibrium liquid mole fraction in a confined system subjected to a localized electric field within a capillary that is filled with a liquid phase in equilibrium with its vapor counterpart. Applied to a water-ethanol system, the model suggests large shifts in the equilibrium liquid mole fraction of water due to the electric field and capillary effects. These findings reveal that while the capillary's influence remains negligible for radii exceeding 10 nm, capillaries of smaller dimensions, when exposed to electric fields of around 300 MV/m, can amplify the equilibrium liquid water mole fraction by up to 55%. This suggests the potential for phase equilibrium control through larger capillaries and lower electric fields, while intriguing complexities arise at very small radii.

Mineralization of phosphorylated cellulose: crucial role of surface structure and monovalent ions for optimizing calcium content

Cellulose can be phosphorylated to produce organic matrices with highly adsorptive properties for, e.g., biocompatible materials for biomedical applications. We focus on the effects of phosphorylation of surfaces of crystalline nanocellulose and, in particular, on the competitive adsorption of mono- and divalent cations (Na+ and Ca2+) typically contained in mineralizing salt mixtures using all-atom molecular dynamics (MD) simulations. Phosphorylation was applied at 12% and 25% both in water and CaCl2 solutions. Our main result shows that Na+ and Ca2+ cations are concentrated in different interfacial layers with Na+ ions penetrating much closer to the surface. This behavior cannot be described by the Poisson-Boltzmann theory or implicit solvent simulations. Our analysis shows that the physical origin of this observation is due to a balance between the electrostatic interactions and hydration free energy associated with the ions. Adsorption levels of the different ions also respond differently to changes in the degree of phosphorylation. We show that the number of adsorbed Na+ ions per phosphate group increases whereas the number of adsorbed Ca2+ ions decreases with an increasing degree of phosphorylation (or when the number of binding sites increases). The decrease in the number of adsorbed Ca2+ ions can be explained by an increasing "charge-charge" repulsion between the Ca2+ ions attracted by the charged surface. Importantly, our results demonstrate the existence of an optimum degree of phosphorylation in terms of adsorbed Ca2+ ions and can be used as a guideline in materials design, for example, when choosing the cellulose matrix or with other similarly structured biomolecular and polymer surfaces.

Ultraviolet charge-transfer-to-solvent spectroscopy of halide and hydroxide ions in subcritical and supercritical water

Exploring charge-transfer-to-solvent excitation of aqueous halide anions by vacuum ultraviolet spectroscopy – new insights up to 380 °C.