Computational Studies of Hydrophobic Force Law, Dynamics in Model Asymmetric Binary Mixtures, and Contribution to One and Two Dimensional Infrared Spectroscopy

Abstract

Depending on the system and phenomena of interest, the thesis has been partitioned into three major parts. The first part is on the theoretical infrared spectroscopic studies and vibrational phase relaxation of supercritical water across the Widom line. The second part contains the calculations of strength and range of hydrophobic force between two graphene-like surfaces in aqueous solutions. The third part deals with the dynamics in model asymmetric binary mixtures. The first part of the thesis consists of four chapters which deal with theoretical and computer simulation studies of one and two-dimensional infrared spectroscopy, and vibrational phase relaxation of water in the supercritical regime. The evolution of one-dimensional infrared (1D-IR) spectrum is explored in supercritical water (SCW) region by varying the density across the Widom line just above the critical temperature. The infrared lineshape shows a crossover from Lorentzian to Gaussian as the Widom line is approached. The vibrational phase relaxation rate (often referred to as Raman line width) of water and nitrogen is also studied near their respective critical temperatures. Both display anomalous behavior in the form of a sharp rise in the relaxation rate as the critical point is approached. The enhanced heterogeneity of SCW near critical density is captured faithfully by two dimensional (2D-IR) spectra. The timescale of about 100 fs for the heterogeneity is obtained from the loss of ellipticity of the 2D-IR spectrum. The second part of the thesis contains five chapters which deal with the calculations of strength and range of hydrophobic force law in water and different aqueous co-solvents. The separation distance dependence of the hydrophobic force is examined by systematically varying the distance (d) between two graphene-like hydrophobic surfaces in water. The hydrophobic force shows bi-exponential distance dependence. The hydrophobic force exhibits a distance mediated crossover from a liquid-like to a gas-like behavior at around d ~ 12 Å. Importantly, this study reveals that the primary cause of hydrophobic attraction is the disruption of the orientational correlations of liquid water. Furthermore, the confined water is found to form an ice-like ordered structure at high pressure (10,000 atm) and room temperature, in agreement with the experimental study of Algara-Siller et al. (Nature 519 (7544), 443 (2015)). It is also found that the correlation length of the hydrophobic force law increases upon lowering the temperature of the system. In this part, the effects of amphiphilic co-solvents (e.g. ethanol and DMSO) on the attractive force between two graphene-like hydrophobic surfaces are thoroughly explored by varying both the distance and the co-solvent concentrations. The hydrophobic force exhibits a strong dependence on cosolvent composition. Part 3 consists of two chapters and focuses on the dynamics in model asymmetric molecules. In this part, a new model system is introduced to study systematically the role of molecular shape in the transport properties of dense liquids (i.e. by varying temperature over a wide range at a fixed pressure). Importantly, for this model binary mixtures, both slip and stick hydrodynamic prediction breaks down in a major fashion, for both prolates and oblates and particularly so for rotation. Moreover, prolates and oblates themselves display different dynamical features in the mean square displacement and in orientational correlation functions.