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A classical non-polarizable force field for the common halide (F−, Cl−, Br−, and I−) and alkali (Li+, Na+, K+, and Cs+) ions in SPC/E water is presented. This is an extension of the force field developed by Loche et al. for Na+, K+, Cl−, and Br− (JPCB 125, 8581–8587, 2021): in the present work, we additionally optimize Lennard-Jones parameters for Li+, I−, Cs+, and F− ions. Li+ and F− are particularly challenging ions to model due to their small size. The force field is optimized with respect to experimental solvation free energies and activity coefficients, which are the necessary and sufficient quantities to accurately reproduce the electrolyte thermodynamics. Good agreement with experimental reference data is achieved for a wide range of concentrations (up to 4 mol/l). We find that standard Lorentz–Berthelot combination rules are sufficient for all ions except F−, for which modified combination rules are necessary. With the optimized parameters, we show that, although the force field is only optimized based on thermodynamic properties, structural properties are reproduced quantitatively, while ion diffusion coefficients are in qualitative agreement with experimental values. [ABSTRACT FROM AUTHOR]

We compare the dielectric spectra of aqueous MgSO4 and Na2SO4 solutions calculated from classical molecular dynamics simulations with experimental data, using an optimized thermodynamically consistent sulfate force field. Both the concentration-dependent shift of the static dielectric constant and the spectral shape match the experimental results very well for Na2SO4 solutions. For MgSO4 solutions, the simulations qualitatively reproduce the experimental observation of a slow mode, the origin of which we trace back to the ion-pair relaxation contribution via spectral decomposition. The radial distribution functions show that Mg2+ and SO 4 2 − ions form extensive water-separated—and thus strongly dipolar—ion pairs, the orientational relaxation of which provides a simple physical explanation for the prominent slow dielectric mode in MgSO4 solutions. Remarkably, the Mg2+– SO 4 2 − ion-pair relaxation extends all the way into the THz range, which we rationalize by the vibrational relaxation of tightly bound water-separated ion pairs. Thus, the relaxation of divalent ion pairs can give rise to widely separated orientational and vibrational spectroscopic features. [ABSTRACT FROM AUTHOR]

We study the ion density, shear viscosity and electroosmotic mobility of an aqueous monovalent electrolyte at a charged solid surface using molecular dynamics simulations. Upon increasing the surface charge density, ions are displaced first from the diffuse layer to the outer Helmholtz layer, increasing its viscosity, and subsequently to the hydrodynamically stagnant inner Helmholtz layer. The ion redistribution causes both charge inversion and reversal of the electroosmotic mobility. Because of the surface-charge dependent interfacial hydrodynamic properties, however, the charge density of mobility reversal differs from the charge density of charge inversion, depending on the salt concentration and the chemical details of the ions and the surface. Mobility reversal cannot be described by an effective slip boundary condition alone – the spatial dependence of the viscosity is essential. [ABSTRACT FROM AUTHOR]

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The surface tension of the air-water interface increases upon addition of inorganic salts, implying a negative surface excess of ionic species. Most acids, however, induce a decrease in surface tension, indicating a positive surface excess of hydrated protons. In combination with the apparent negative charge at pure air-water interfaces derived from electrokinetic experiments, this experimental observation has been a source of intense debate since the mid-19th century. Herein, we calculate surface tensions and ionic surface propensities at air-water interfaces from classical, thermodynamically consistent molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions show outstanding quantitative agreement with experiment. Of the studied ions, only H3O+ adsorbs to the air-water interface. The adsorption is explained by the deep potential well caused by the orientation of the H3O+ dipole in the interfacial electric field, which is confirmed by ab initio simulations. [ABSTRACT FROM AUTHOR]

For ion channel gating, the appearance of two distinct conformational states and the discrete transitions between them are essential, and therefore of crucial importance to all living organisms. We show that the physical interplay between two structural elements that are commonly present in bacterial mechanosensitive channels-namely, a charged vestibule and a hydrophobic constriction-creates two distinct conformational states, open and closed, as well as the gating between them. We solve the nonequilibrium Stokes-Poisson-Nemst-Planck equations, extended to include a molecular potential of mean force, and show that a first order transition between the closed and open states arises naturally from the diffusioosmotic stress caused by the ions and the water inside the channel and the elastic restoring force from the membrane. [ABSTRACT FROM AUTHOR]

We calculate the electro-osmotic mobility and surfaceconductivityat a solid–liquid interface from a modified Poisson–Boltzmannequation, including spatial variations of the dielectric functionand the viscosity that where extracted previously from molecular dynamicssimulations of aqueous interfaces. The low-dielectric region directlyat the interface leads to a substantially reduced surface capacitance.At the same time, ions accumulate into a highly condensed interfaciallayer, leading to the well-known saturation of the electro-osmoticmobility at large surface charge density regardless of the hydrodynamicboundary conditions. The experimentally well-established apparentexcess surface conductivity follows from our model for all hydrodynamicboundary conditions without additional assumptions. Our theory fitsmultiple published sets of experimental data on hydrophilic and hydrophobicsurfaces with striking accuracy, using the nonelectrostatic ion–surfaceinteraction as the only fitting parameter. [ABSTRACT FROM AUTHOR]

We derive the theoretical framework to calculate thedielectricresponse tensor and determine its components for water adjacent tohydrophilic and hydrophobic surfaces using molecular dynamics simulations.For the nonpolarizable water model used, linear response theory isfound to be applicable up to an external perpendicular field strengthof ∼2 V/nm, which is well beyond the experimental dielectricbreakdown threshold. The dipole contribution dominates the dielectricresponse parallel to the interface, whereas for the perpendicularcomponent it is essential to keep the quadrupole and octupole terms.Including the space-dependent dielectric function in a mean-fielddescription of the ion distribution at a single charged interface,we reproduce experimental values of the interfacial capacitance. Atthe same time, the dielectric function decreases the electrostaticpart of the disjoining pressure between two charged surfaces, unlikepreviously thought. The difference in interfacial polarizability betweenhydrophilic and hydrophobic surfaces can be quantized in terms ofthe dielectric dividing surface. Using the dielectric dividing surfaceand the Gibbs dividing surface positions to estimate the free energyof a single ion close to an interface, ion-specific adsorption effectsare found to be more pronounced at hydrophobic surfaces than at hydrophilicsurfaces, in agreement with experimental trends. [ABSTRACT FROM AUTHOR]

The framework for deriving tensorial interfacial dielectric profiles from bound charge distributions is established and applied to molecular dynamics simulations of water at hydrophobic and hydrophilic surfaces. In conjunction with a modified Poisson-Boltzmann equation, the trend of experimental double-layer capacitances is well reproduced. We show that the apparent Stern layer can be understood in terms of the dielectric profile of pure water. [ABSTRACT FROM AUTHOR]

The article focuses on the dynamic energy flow in the hydrogen bond network of liquid water by a pump-probe experiment. It mentions that in living organisms, the same property makes water a superb thermal buffer for the function of biochemical reactions; and to elucidate the molecular mechanism of the energy dissipation in water, the time scale of energy dissipation and the strength of intermolecular interactions should be determined by experiments.

Luft‐Wasser‐Grenzfläche D. J. Bonthuis et al. berechnen in der Zuschrift auf S. 16058 Oberflächenspannungen und Ionenaffinitäten zur Luft‐Wasser‐Oberfläche durch Moleküldynamiksimulationen. [ABSTRACT FROM AUTHOR]

Air–Water Interface In their Communication on page 15846; ff., D. J. Bonthuis et al. describe the calculation of surface tensions and ionic surface propensities at air–water interfaces by molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Nonlinear field dependence of electrophoresis in high fields has been investigated theoretically, yet experimental studies have failed to reach consensus on the effect. In this Letter, we present a systematic study on the nonlinear electrophoresis of highly charged submicron particles in applied electric fields of up to several kV/cm. First, the particles are characterized in the low-field regime at different salt concentrations and the surface charge density is estimated. Subsequently, we use microfluidic channels and video tracking to systematically characterize the nonlinear response over a range of field strengths. Using velocity measurements on the single particle level, we prove that nonlinear effects are present at electric fields and surface charge densities that are accessible in practical conditions. Finally, we show that nonlinear behavior leads to unexpected particle trapping in channels. [ABSTRACT FROM AUTHOR]