Compressibility and Dielectric Relaxation of Mixtures of Water with Monohydroxy Alcohols

Effective Debye relaxation models for binary solutions of polar liquids at terahertz frequencies

There are many effective medium models that accurately describe the dielectric properties of mixtures. However, these models assume that the components are non-interacting. This assumption is not valid for solutions of polar liquids, resulting in significant deviations between the measured and theoretically predicted values of the complex index of refraction of the mixtures. We present three effective medium theories by expanding the well-known Debye relaxation model for solutions of polar liquids in the terahertz (THz) regime. The new effective medium models proposed in this paper predict the individual relaxation Debye parameters based on the cooperative motion dynamics and self-associative properties of each mixture, and therefore explain the deviation of the dielectric functions of the solutions from the traditional effective medium models. These models are verified through reflection measurements of four alcohol-water solutions acquired through THz time-domain spectroscopy (THz-TDS). Compared to the current mixed medium models, the new effective Debye theorem predicts the dielectric properties of polar solutions more accurately and has the potential to explain inter-species mixing schemes and interactions.

Comparative analysis of ethanol dynamics in aqueous and non-aqueous solutions

In this study, we compare the results for vibrational, reorientational and hydrogen bond dynamics of ethanol in water and in hexane across the whole concentration range. Water and hexane are both commonly used as solvents, but so far, it has been unclear to what extent they modify the solute dynamics. Ethanol is chosen as the solute because it is an aliphatic molecule that is miscibile with both solvents. It is known that ethanol forms micelle-like domains in water and cyclic clusters resembling loops in hexane. This structural micro-heterogeneity is well known both in experiments and in simulations. The main question we raise here is: is there a signature of micro-heterogeneity in the dynamical quantities of ethanol? We focus on quantities such as the vibrational spectra, the reorientational correlation functions, the self-diffusion coefficients, the ethanol-ethanol hydrogen bond correlation functions and the corresponding hydrogen bond histograms. For the first time ever, we compute the van Hove functions to reveal the dynamical variations of spatial correlations in these systems. All these results complement each other and provide a unifying dynamical description of ethanol in binary mixtures.

Simple corrections for the static dielectric constant of liquid mixtures from model force fields

Pair-wise additive force fields provide fairly accurate predictions, through classical molecular simulations, for a wide range of structural, thermodynamic, and dynamical properties of many materials. However, one key property that has not been well captured is the static dielectric constant, which characterizes the response of a system to an applied electric field and is important in determining the screening of electrostatic interactions through a system. A simple correction has been found to provide a relatively robust method to improve the estimate of the static dielectric constant from molecular simulations for a broad range of compounds. This approach accounts for the electronic contribution to molecular polarizability and assumes that the charges that couple a molecule to an applied electric field are proportional to the effective force field charges. In this work, we examine how this correction performs for systems at different temperatures and for binary mixtures. Using a value for the electronic polarizability, based on the experimental index of refraction, and a charge scaling factor, determined at a single temperature, we find that the static dielectric constant can be predicted remarkably well, in comparison to the experimentally measured values. This provides good evidence that the effective charges that appear in pair-wise additive force fields developed to reproduce the potential energy surface of a system are not the same as those that determine the static dielectric constant; however, they can be captured in a relatively simple manner, which is dependent on the particular force field.