Theoretical Studies on Dynamics in Complex Systems: Binary Mixtures, Electrolyte Solutions, Nanoconfined Water, Glass Transition Phenomena, and Boltzmann’s H-function

Abstract

The thesis reports theoretical and computer simulation studies of the structure and dynamics in several complex systems, namely, aqueous binary mixtures, electrolyte solutions, nanoconfined water, and glass-forming liquids of anisotropic molecules. A common thing in the study of various systems mentioned above is the exploration of nonequilibrium relaxation phenomena in interacting fluids. Based on the systems and phenomena of interest, the thesis has been divided into five major parts: (i) Study of Transport Properties and Relaxation Phenomena of Aqueous Binary Mixtures, (ii) Unravelling Anomalous Concentration-Dependent Viscosity in Electrolyte Solutions, (iii) Exploring Transport Properties in Nanoconfined Water, (iv) Study of Glassy Dynamics in a Liquid of Anisotropic Molecules and (v) Investigation of Nonequilibrium Relaxation Phenomena with Boltzmann’s H-function. In our studies, we employ molecular dynamics simulations and theoretical analyses to unravel the intricacies of complex liquids and glasses. In the first part, we investigate the microscopic origin of non-ideality in viscosity and transport properties of aqueous binary mixtures. We find that the non-ideality can be explained through the formation of quasi-stable extended structures. Detailed analyses, including mode coupling theory (MCT) and inherent structure (IS) analysis, provide valuable insights into the anomalous composition dependence of viscosity. In the second part, the investigation extends to composition-dependent anomalies in the viscosity of aqueous electrolyte solutions, highlighting the crucial role of cross-correlations, often overlooked in previous studies. In the third part of the thesis, the research delves into nanoconfined water, revealing disparate static and dynamic correlation lengths and their implications. Frequency dependent viscosity provides further understanding of the structural aspects. In the fourth part of the thesis, we investigate the glass transition phenomena by introducing a novel class of binary mixtures of molecules that interact with each other through anisotropic (angle-dependent) interactions. This model system exhibits distinct thermodynamic and dynamic features of glass transition. The splitting of the rotational and translational spectrum reveals the dynamic heterogeneity in the system. These resemble the Johari-Goldstein bifurcation, well-known in glass transition literature. Sudden large changes in the diffusion coefficient and rotational correlation times in mesoscopic domains indicate first-order transitions between low and high-mobility domains. In the fifth part of the thesis, the study analyses non-equilibrium relaxation, assessing sensitivity to interaction potential and dimensionality using Boltzmann’s H-function. The evaluation of the H-function for molecules with orientational degrees of freedom showcases its potential in understanding translational and rotational contributions to entropy, emphasizing translation-rotation coupling as a function of molecular shape. This comprehensive exploration makes a substantial contribution to condensed matter science, providing valuable insights and paving the way for further progress.