Computer simulation studies of micro-heterogeneous liquid mixtures

Liquid mixtures are part of our everyday lives and are important in numerous chemical research and industrial applications. While often apparently featureless, uniform and uninteresting, kept in bottles and containers in the chemical laboratory, their molecular-level structure and physical properties can be completely the opposite, and are continually attracting scientific interest. Strong repulsive and attractive interactions between the molecules can create most intriguing local arrangements, molecular aggregates and complexes, whose spatial organisation is often difficult to characterise, or rationalise. Moreover, the same liquid components can behave completely differently depending on the mixing ratio, strongly affecting macroscopic properties, e.g. mass density, viscosity or melting/boiling points. To gain insight into the complex world of binary liquid mixtures, deep eutectic solvents and ionic liquids, a combination of experimental and theoretical, or computational, studies is necessary. In this thesis, the focus is on understanding how the microscopic molecular organization in organic solvent mixtures affects the physico-chemical and solvating properties. While in all of the presented studies the systems are investigated both experimentally and computationally, the main focus of my thesis is on computational modelling, primarily based on molecular mechanics. In this approach, the system of interest is modelled by interacting particles representing either single atoms of groups of atoms, and their interactions and movements are described by classical mechanics. This approximate method allows for the modelling of large molecular systems, e.g. liquid mixtures with structural heterogeneities on the nanometer length scale and microsecond time scale. Specifically, my work has been devoted to the following problems: (1) Explaining how large anions in mixed liquid solvents can be apparently preferentially solvated by the less polar components, described in Chapter 3. (2) Explaining excess thermodynamics properties for relatively simple organic solvent liquid mixtures, described in Chapter 4, and extending the investigation approach to more complex “pseudo-binary” mixtures of DES with water or methanol, where it has not been applied before, described in Chapter 6. (3) Developing coarse-grained models of charged liquid solvent, to be able to verify the long-range ordering effects connected to solvent micro-segregation, described in Chapter 5. The fundamental concepts and background are provided in Chapter 1 & 2.