title: On the glass transition of bulk and confined polyamorphic liquids: A molecular dynamics simulations study
Creators: name: Horstmann, Robin
Publisher: name: UNSPECIFIED
Year: 2023

description: Supercooled liquids and the glass transition are not satisfactorily understood to date. The temperature dependence of dynamical properties eludes theoretical prediction. No model can be successfully applied to all liquids. One liquid is particularly complex in its supercooled regime - water. This seemingly simple liquid exhibits the most anomalies of any neat liquid, and most of these are thought to be related to the existence of two distinguishable liquid phases with different density in the supercooled regime, i.e., water exhibits polyamorphism. However, most of the relevant temperature range lies in the so-called no-man's land, a region of the phase diagram in which bulk water rapidly crystallizes and which is therefore experimentally inaccessible to the bulk liquid. Therefore, experimental studies often exploit the fact that crystallization of water is suppressed in nanoscopic confinements or water mixtures. The present work deals with both areas of research, water's polyamorphism and dynamics of supercooled liquids, confined and mixed, with the use of molecular dynamics simulations. They allow for detailed analysis and systematic variation of the liquid and enable easy supercooling. Partial charges of the TIP4P/2005 and SPC/E water models were scaled which led to strong shifts of dynamics in temperature. These were reconciled by using the high-temperature activation energy as the relevant energy scale as long as structural properties were the same. For the TIP4P/2005 model and a set of reduced charges, isochore crossing in the phase diagram confirmed the existence of a liquid-liquid critical point (LLCP) in the supercooled regime at positive or negative pressures, depending on the molecular polarity. The two-structure equation of state (TSEOS) formalism was used to describe the data and determine the location of the LLCP. In addition, reduction of the partial charges accelerated dynamics at the LLCP and simulations with elongated boxes in the double metastable regime allowed for the coexistence of high-density (HDL) and low-density (LDL) liquid phases and the determination of their dynamics as a function of temperature. The results are in agreement with observations from isochoric and isobaric simulations and translational motion was observed for all state points. It was found that the temperature dependence of the dynamics at a constant fraction of the low-density state (LDS) is Arrhenius-like. Thus, the presumed fragile-to-strong transition (FST) of water is not caused by a transition from fragile HDL to strong LDL but by the fast transition between these liquid states when the system is cooled through the Widom line at constant pressure. This is consistent with experimental observations slightly above water's glass transition temperature Tg and reinforces the question of whether HDL or LDL on their own exhibit an FST. Models for the temperature dependence of reactive mixtures were tested but were unable to describe simulation results at the lowest studied temperatures. A family of functional forms for the temperature dependence of dynamical properties of supercooled liquids was derived. These functions allow their description over the entire temperature range from the boiling point to the glass transition and with or without an FST. The second-order functions predict a high and low-temperature Arrhenius regime connected by an intermediate fragile regime. Knowledge of the path in the phase diagram of charge-scaled water-like systems, whether they cross the Widom line at increased charges or not, allowed for more rigorous testing of these functional forms. They are sensitive to deviations from Vogel-Fulcher-Tammann (VFT) behavior and apply well to data from charge-scaled water and silica simulations, which have a pronounced FST, as well as to real liquids. The possibility that supercooled liquids in general have a low-temperature Arrhenius regime and the characteristics of such FSTs were discussed. Simulations of charge-scaled water models in chemically neutral pores were performed and static and dynamic length scales associated with changes of water's structure and dynamics near the pore wall were extracted. These correlation lengths were used to test theories of the glass transition and discussed in the context of water's two phases. Signs of crossing the Widom line could not be found in the temperature dependence of the correlation lengths within the moderately supercooled temperature range. The slowdown at the pore wall relative to the pore center was characterized using two empirical functions for additional activation energies caused by the liquid-confinement interface. Furthermore, the potential energy landscape (PEL) imprinted on the liquid was quantified using a novel approach based on Boltzmann statistics and predicted and measured mobility gradients are in agreement. Lastly, the origin of slow solvent processes observed in dielectric spectroscopy studies of dynamically asymmetric binary mixtures was determined in simulations. For mixtures of picoline and poly-methylmethacrylate and of water and polylysine, fractions of slow solvent molecules were not found. Instead, the PEL imprinted by the slow polymer molecules causes preferred locations and orientations for the solvent molecules. A mechanism was proposed in which the solvent molecules exchange fast compared to the relaxation of the polymer molecules but have correlated orientations. This causes long-lived cross correlations that can be misinterpreted as slow solvent contributions in coherent measurements. Other sources of cross correlations were quantified and the dependency on measured molecular property and correlation function were discussed. The dynamical heterogeneity of solvent dynamics was traced back to the variation of the local solvent concentration and it is broad but unimodal. The same observations, slowly decaying cross correlations and absence of self correlation on these time scales, were made for other binary mixtures, suggesting that these effects are relevant to a wide range of systems.