Microscopic Investigations of Dynamics of Linear Diatomics and Ions in Water and Aqueous Binary Mixtures

Abstract

This thesis explores the structural and dynamical aspects of small, charged and neutral, solute molecules in both pure water and in aqueous binary mixtures, with emphasis on water-ethanol binary mixture. We employ both extensive computer simulations and sophisticated theoretical analyses. Throughout the thesis, the focus has been on a microscopic level understanding of many of the unique features that these systems exhibit. Based on the system of interest, we divide the thesis into three major parts. In the first part, we deal with the dynamics of two linear neutral diatomic molecules, namely carbon monoxide (CO) and nitric oxide (NO), in water. We find a significant result that the translational motion of these small diatomics is strongly coupled to their rotational motion, which in turn are coupled to the complex motions of the surrounding water. We examine the validity of the hydrodynamic predictions by modeling these solutes as prolate ellipsoids. For the translational diffusion, the predicted values agree well with the hydrodynamic boundary value predictions. In rotational diffusion, the slip boundary values are found to be in good agreement with the simulation results. We also develop a mode-coupling theory to understand the complexity of translation-rotation coupling. We devote part two to understand the structure and dynamics of ions in water. We find that the charged linear diatomic cyanide (CN-) ion exhibits certain differences, while the neutral diatomic CO and NO show similar features. We observe that the water molecules around CN- are more structured than those around the neutral solutes. Thus, the translational diffusion of the ion becomes slow compared to that of neutral molecules. We also find that the rotational motion of the ion is slower than the neutral solutes. In the third part, we study the same solutes in aqueous binary mixtures. We explore the translational and rotational dynamics of three probes, CO, NO, and CN- at different compositions of water-ethanol binary mixture. We find multiple anomalous results such as (i) faster rotational motion of CO and NO than CN- in the binary mixture. However, for CO and NO, the fastest rotation is in pure ethanol; for CN-, the rotation is slowest in pure ethanol. (ii) The larger translational diffusion of the neutral molecule in pure ethanol than in water but the reverse for the ion. (iii) A pronounced anomaly in the composition dependence of translational-rotational dynamics at low ethanol composition, and (iv) a reentrant type behavior in the viscosity dependence of orientational relaxation. For the first time, we implement a calculation of the rotational binary friction following the sophisticated kinetic theory scheme of Evans and co-workers. We develop a detailed mode-coupling theory and suggest that such an approach if completely implemented, can provide a more reliable description than the hydrodynamic predictions. In another work, we study the rigid monovalent cations Li+, Na+, K+, and Cs+ at various compositions of water-ethanol mixtures. For the first time, we report the diffusion constants of these cations at various compositions of the water-ethanol mixture. Though the diffusion of cations becomes slow with the increasing composition, at a particular composition, the diffusivity of ions increases with the increase in its size. We also observe that the coordination number of water in the first solvation shell of all the four cations decreases with the increase in ethanol composition, with the highest being in pure water and the coordination number of ethanol increases with the increase in ethanol compositions with a maximum in pure ethanol. We could establish a novel finding of ion-induced segregation in the mixture. The ions are found to induce a microheterogeneity where water molecules preferentially solvate them while the ethyl groups of ethanol occupy the intervening space. The last part of the thesis includes the future directions and research prospects derived from this thesis. We also discuss a preliminary study on zinc and lithium ions in water, considering their application to the organic framework of batteries.