Terahertz and dielectric spectral calculations of aqueous CaCl2 solutions using polarizable models: Intra- and inter-species contributions involving permanent and induced dipoles and ion current

Abstract

Intermolecular interactions, structure, and dynamics of aqueous CaCl2 solutions are investigated through calculations of the terahertz (THz) and dielectric spectra from molecular dynamics simulations using explicit polarizable models for both water and the ions. Calculations are performed for three different concentrations of CaCl2 in water at room temperature. For each system, the difference absorption spectrum shows several features in the THz region. We dissected the difference absorption spectrum further into ion–ion, water–water, and ion–water components. The ion–ion contribution is further dissected into cation–cation, anion–anion, and cation–anion self and cross correlation contributions. The ion–water terms are further separated into cation–water and anion–water correlations. These dissections provide detailed insights into ion–ion and ion–water correlations and dynamical behavior of the hydrated ions. We further separated the cation–water and anion–water contributions into permanent dipole–permanent dipole, permanent dipole–induced dipole, and induced dipole–induced dipole correlation components, which reveal the nature and magnitude of these interactions contributing to the ion–water THz spectrum. Our calculations of the anisotropy of induced dipole moments reveal that the presence of Ca2+ ions in the vicinity of Cl− ions increases the anisotropy of the induced dipole moments of the anions. The present study also reveals a significant presence of contact ion pairs in the CaCl2 solutions, especially at higher concentrations. We also explored the influence of heterogeneity of water hydrogen bonds around Ca2+ ions on THz spectral features of hydration shell water. We also calculated the low-frequency dielectric spectrum, including the static dielectric constant of each solution, which accounts for both dipole–dipole and current–dipole contributions.