Authors: Viktor Rozsa
Source: Knowledge@UChicago (University of Chicago) (article, 2020-01-01)
Field: Chemistry — Physical and Theoretical Chemistry

First Principles Studies of Aqueous Systems at Ambient and Extreme Conditions Water is the quintessential hydrogen-bonded system, exhibiting exotic and anomalous properties with application in fields varying from biophysics to planetary science to molecular engineering for energy technologies. In this dissertation, we examine aqueous solutions of solvated ions in bulk water and "extreme conditions" corresponding to nanoconfinement and elevated pressure/temperature. We apply first principles molecular dynamics simulations to analyze both structural alterations to water as well as more subtle and intrinsically quantum mechanical quantities, including Raman and infrared spectra, ionic conductivity, and molecular polarizabilities. Understanding how water is modified by ions is crucial to the solvation properties involved in biophysical processes as well as a range of energy applications. While the effect of ions on nearby water molecules is relatively well understood, alterations to water beyond the first solvation shell due to ion presence remain controversial. We present a systematic first-principles molecular dynamics study of alkali cations in water. Our simulations support the view that the water structure is only modified locally by the presence of cations. We found that molecular polarizabilities are fingerprints of hydrogen bonding modifications, which occur at most up to the second solvation shell for all cations in bulk water. In addition, we apply our simulations of solvated K+ to isotopic fractionation of potassium. Perturbations to aqueous solutions by confining media have been the focus of numerous studies, finding application in a wide range of green technologies. Yet several open questions remain, including the extent to which nanoconfinement modifies the structural and dielectric properties of water, and the additional effect of solvated ions. We carried out a first-principles molecular dynamics study of pure water and LiCl and KCl solutions under confinement within carbon nanotubes (CNT) of small diameter (1.1-1.5 nm). Under confinement we found that the overall value of the molecular polarizability of water molecules near the surface is determined by the balance of two effects, that vary in CNT of different radii: the presence of broken hydrogen bonds at the surface leads to a decrease of the polarizabilities of water molecules, while the interaction with the CNT enhances polarizabilities. Interestingly the reductions of dipole moments of interfacial water molecules under confinement is instead driven only by changes in the water structure and not by interfacial interactions. The behavior of water at extreme conditions plays a critical role in planetary science, yet remains poorly understood. We carried out a first principles investigation of water at high temperature, between 11 and 20 GPa. Our results are consistent with the recent estimates of the water melting line below 1000 K, and show that on the 1000 K isotherm the liquid is rapidly dissociating and recombining through a multi-molecular mechanism. We found that short-lived ionic species act as charge carriers giving rise to an ionic conductivity that at 11 and 20 GPa is six and seven orders of magnitude larger, respectively, than at ambient conditions. Conductivity calculations were performed entirely from first principles, with no a-priori assumptions on the nature of charge carriers. Our computed Raman spectra, which are in excellent agreement with experiment, show no distinctive signatures of the hydronium and hydroxide ions present in our simulations. Instead, we found that infrared spectra are sensitive probes of molecular dissociation, exhibiting a broad band below the OH stretching mode ascribable to vibrations of complex ions. Given that real geochemical fluids are mixtures of water and various impurities, we also studied the properties of ion solvation at high pressure and temperature. We employ first principles molecular dynamics to study the structure, diffusion, and vibrational properties of the solutions. We found that the solvation of ions at these conditions brings no new vibrational signatures in Raman and infrared spectra of the liquid. We also computed the changes induced by ions to the dielectric constant of the liquid water, and found that pure water is an upper bound on the dielectric constant of the solutions. We computed the ionic conductivity at the same conditions, finding a moderate increase in the conductivity of water due to the ions, despite no significant changes to global molecular dissociation. Our results indicate that the effect of monovalent ions over a broad range of size on the structure of water at these conditions is minimal. Aqueous solution ; Environmental science ; Chemistry Various Chemistry Research Topics ; Spectroscopy and Quantum Chemical Studies