A molecular dynamics study of water and aqueous solutions in confinements of differing surface roughness and hydroaffinity

• Main Author: Harrach, Michael
• Format: Others
• Language: German; en
• Published: 2015
• Online Access: https://tuprints.ulb.tu-darmstadt.de/4688/1/Dissertation_Michael_F_Harrach_neue_Adressen.pdf; Harrach, Michael <http://tuprints.ulb.tu-darmstadt.de/view/person/Harrach=3AMichael=3A=3A.html> (2015): A molecular dynamics study of water and aqueous solutions in confinements of differing surface roughness and hydroaffinity.Darmstadt, Technische Universität, [Ph.D. Thesis]

Due to its abundance and its relevance for life, water is one of, if not the, most important liquid(s). It has various anomalies and an extremely rich phase diagram. Not only are water, and correspondingly aqueous solutions, everywhere, but often they are found in confined conditions. Examples range from the biological, e.g. the cells in our own bodies, to technical applications of microfluidics and, of course, geological concerns. There are many areas concerning the behavior of water that are still not completely understood. In many such cases molecular dynamics simulations can play an important role, since they not only allow us to experience a resolution on the molecular scale, but also the possibility to actualize gedanken experiments which are not accessible to current experimental methods - or if accessible, at the very least, highly expensive. Of course, it is of utmost importance concerning the insights from such simulations to be aware of their limitations, and to keep the influence of the inevitable simplifications any molecular dynamics model will necessitate in mind. It is necessary to distinguish between effects from geometrical parameters, finite-size effects, the characteristics of different water models, and their interaction potentials. The aim of this study is to investigate the behavior of pure water and aqueous solutions, namely isobutyric acid and water, in confinement, with special regard given to the roughness and structure of the confining interfaces. This is done by first considering hydrophobic and hydrophilic confining walls, with a planar geometry, and systematically varying wall roughness while keeping the same average potential, as well as observing the effects on different water models, namely TIP3P, TIP4P and TIP5P. We find that in the hydrophobic regime the smooth wall generally represents a usable abstraction of the atomically rough walls, while in the hydrophilic regime there are pronounced differences in structure and dynamics between all stages of wall roughness. For a small lattice constant, however, the smooth and the atomically rough wall still share a number of structural and dynamical similarities, although the degree of conformity depends on the water model. Out of the three water models, TIP5P water shows the largest degree of tetrahedral ordering and is often the one that is least perturbed by the presence of the wall. Further, the behavior of SPC/E water in cylindrical confinement is investigated. In this case confinement refers to amorphous silica pores and amorphous ice pores as well as completely smooth pores, where the potential felt at a given distance from the pore wall is an averaged atomic potential. As compared to rough walls, smooth walls induce stronger distortions of water structure for both silica and ice confinements. On the other hand, unlike the smooth pores, the rough pores strongly slow down water dynamics at the pore wall. The slowdown vanishes when reducing the atomic charges in the wall, i.e. when varying the hydroaffinity, while keeping the surface topology intact, indicating that this is not a geometric effect, but rather due to the fact that the wall atoms provide a static energy landscape along the surface, e.g. fixed anchor-points for hydrogen bonds, to which the water molecules need to adapt, blocking channels for structural rearrangement. In the smooth pores, water dynamics can be faster than in the bulk liquid, not only at the pore wall, but also in the pore center. Changes in the tetrahedral order rather than in the local density are identified as the main cause for this change of the dynamical behavior in the center of smooth pores. Lastly, molecular dynamics simulations of an isobutyric acid (iBA) and TIP3P water mixture in a cylindrical silica nanopore are performed for a number of different weight percentages. The general behavior is only weakly dependent on the mixture ratio and contrary to expectations the iBA features prominently at the pore wall, despite the hydrophilic nature of the pore. Increasing temperature triggers more thorough mixing of the components which, coupled with other results, suggests an energetic cause for the accretion of iBA at the pore boundary.