Molecular dynamics simulations of concentration fluctuations in binary mixtures

Abstract

This thesis studies the glassy slowdown in binary mixtures. We investigate the role of concentration fluctuations for complex dynamical behavior in the glassy regime and the influence of confinement on the structure and dynamics of mixtures. To study these topics, we use molecular dynamics simulations, which is a versatile tool to gain detailed atomistic information. We model two different binary mixtures; one realistic ethylene glycol-water mixture and one where the dynamical contrast can be tuned while the molecular structure is identical. Ethylene glycol and water are miscible in an equimolar ratio over a wide range of temperatures. The temperature-dependent investigation shows that the mixture behaves essentially like a common glass former, yielding similar time scales of structural relaxation and a single glass transition temperature of both species. The study of this mixture in a realistic silica confinement reveals micro-phase separation of the mixture into layers, which depend on the properties of the matrix and the liquid-liquid interactions. We observe that dynamics are slowed down close to the wall. However, the temperature dependence is reduced in confinement, which leads to faster dynamics at low temperatures compared to structural relaxation in the bulk mixture. The model mixture is composed of two water-like molecules with different molecular polarity and is prone to demixing upon cooling. Close to the spinodal, we observe concentration fluctuations in the mixture, which are stabilized by the glassy slowdown of molecular dynamics when the temperature range of vitrification coincides with the spinodal. The model mixture resembles many aspects of dynamically asymmetric binary glass formers, namely a decoupling of the time scales of the structural relaxation of the two components and anomalous shapes of correlation functions. The simultaneous evaluation of local concentration fluctuations and dynamical correlation functions shows that the molecular dynamics of the mixture are strongly affected by concentration fluctuations. Thus, we are able to rationalize the puzzling dynamical observations with the occurrence of concentration fluctuations when the phase separation is approached. Studies of the model mixture in neutral confinement reveal dynamical slowdown and spatially decoupled dynamics even in the absence of micro-phase separation. Using a nanostructured pore, we observe induced demixing and an effective shift of the critical temperature. Concentration gradients are reflected in the structural relaxation in the form of shifted time scales and non-linear diffusion due to the accompanying diffusion barriers.