Terahertz and Dielectric Spectroscopy of Aqueous MgCl2 Solutions: Self- and Cross-Correlation Contributions of Permanent and Induced Dipoles Using Polarizable Models

Abstract

We have investigated the terahertz (THz) and dielectric spectra of aqueous MgCl₂ solutions through molecular dynamics simulations using the AMOEBA polarizable force field for both water and ions. We calculated the differenceabsorption spectra for three different concentrations of MgCl₂ by subtracting the pure water contribution from the total absorption spectra of the solutions. The difference THz spectra of the solutions are divided into ion-ion, water-water, and ion-water components, with further resolution into self- and cross-correlations. Through these dissections, we gained insights into spectral features arising from ion-ion and ion-water correlations and also from changes in water-water correlations caused by ions. The contributions of permanent and induced dipole auto- and cross-correlations provide further insights into the nature of low-frequency vibrational modes involving cation-water and anion-water interactions. Our results reveal that the hydrogen bond stretch motion of water is suppressed by ions, whereas an additional contribution comes from the chloride ion-water hydrogen bond stretch motion. The low-frequency modes in the ion-water part are found to be mainly dominated by cation-water interactions. Furthermore, the results of the low-frequency dielectric spectra show that the relaxation of the collective dipole of water is slowed significantly in the presence of MgCl₂ salts, primarily due to the strong local electric fields produced by the divalent Mg²⁺ ions in the solutions.