Klameth, Felix (2015)
From Brownian motion to supercooled water in confinements - a molecular dynamics simulation study.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

In biological cells, water is confined by macromolecules. Additionally, huge amounts of water are confined in minerals in the lower Earth mantle, so, water in narrow confinements is important in technology and nature. Therefore, it is relevant to examine how the surfaces affect the properties of water, e.g., to understand the role of water in biological processes. The aim of the present study is to investigate supercooled water in confinements using molecular dynamics simulations. For this purpose, structure and dynamics are ascertained for the water model SPC/E in silica pores and amorphous ice pores. Water appears to be a simple liquid, but it shows many anomalies which are not understood. Popular approaches propose a liquid-liquid phase transition to explain the anomalies. This phase transition is located at supercooled temperatures. However, water starts to crystallize at low temperatures, thus, experimentalists employ narrow confinements to prevent the crystallization. As those systems are very complex, the goal of this study is to systematically vary the surface, the confinement geometry, the size of the confinements, and the density of confined water to distinguish between finite-size, density, and surface effects. It is found that the surface strongly affects the structure of water. Water in silica confinements exhibits an induced disorder, in contrast to the amorphous ice confinements, where the structure of water is not changed compared to that of the bulk liquid. Despite this difference, it turns out that using these pores yields a similar change of the relaxation mechanism of water molecules near the wall, accompanied by a tremendous slowdown of dynamics. Furthermore, comparing the sizes of the confinements suggests that the recently proposed phase transition of water in silica confinements, observed in molecular dynamics simulations, can be presumably attributed to a finite-size effect. The amorphous ice pores are additionally used to obtain information on intrinsic length scales of water. Recently proposed theories which attempt to elucidate the still unresolved issue about the dynamic behavior of supercooled liquids, like the random first-order transition theory (RFOT) or the elastically non-linear Langevin equation (ECNLE) approach, attribute the non-Arrhenius temperature dependence of dynamics of supercooled liquids to an increasing intrinsic length. In this study, two length scales are obtained. Both increase as a linear function of inverse temperature and both show a simple relation to the structural correlation times of the bulk liquid, implying fragile behavior. However, it is not clear which of the length scales dominates. This point is crucial for the RFOT theory, thus, the ECNLE approach was used. It was possible to describe the tremendous slowdown of dynamics within the amorphous ice pores by using the vibrational short time dynamics on a picosecond timescale. Furthermore, it was possible to transfer the findings from the pore system to the bulk and obtain a detailed understanding of the mechanism of motion, e.g., explaining the Stokes-Einstein breakdown. Thus, a striking relation between short time dynamics and structural correlation times up to several hundred nanoseconds is observed. The presented approach appears to be not only restricted to the specific water model used for this study, therefore, this might be of great interest for the understanding of supercooled liquids in general. Next, a specific motion of water, the $\pi$-flip, is found for molecules near the walls. An idea was proposed to quantify this effect for the bulk system, so, correlation times and the activation energy ($E_A = 0.44$ eV) of this process can be estimated. Owing to the Arrhenius-like temperature dependence, it is argued that this process should become relevant at temperatures near the glass transition temperature of water. Finally, internal protein motion is ascertained. Subdiffusive behavior is observed, thus, a complex mathematical treatment to account for this motion is applied. This approach uses power-law waiting time distributions to describe motion in a harmonic potential. The calculations give a good approximation of dynamics.

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