First Principal Study of the Hydrogen Bond and Molecular Dynamics Study of Confined Water

Abstract

Water is a unique and mysterious substance that existed ordinarily around us. It has many unusual properties that does not have convincing scientific interpretation. Although, water structure and its hydrogen bonding between its molecules determine most of the enigma, many explanations of the nature of hydrogen bonding are still controversial. Water is central to many studies and it also has huge important to our daily life; this is because water covers 70% of the earth, but in many areas clean water is as precious as gold. An economical and fast method is so much needed to desalination the sea water. Beyond those applications, fundamental research is as important as application itself in living systems and biology. The first research topic in this thesis was to explain the two optical peaks (28 and 37 meV) observed from neutron scattering results by using first-principles methods. In Chapter 3, neutron results are reproduced firstly. Then by pressurising the Ice Ic and Ice VIII in ab initio simulations, two sets of results are studied thoroughly. Force constant matrices are extracted and calculated. Results show that a previous assumption of two types of hydrogen bond may be explained by one hydrogen bond and one O-O interaction. Analysis also shows O-O interaction is only triggered when there are hydrogen bonds forming. These results are by supported all the types of ice phases studied in the chapter 4. In chapter 4, Raman spectrum of ice Ic phase is also carried out to compare with ab-initio results where a q-vector accumulation method in the BZ was developed, which explains the missing peaks in the Raman spectra compared to the ab initio simulations. The second topic was to study the flow of water through confined spaces (graphene nano-channels) by using a classic molecular dynamics (MD) method. The simulation based on the structural provide by the paper and the simulations were based on a few classic water–water potentials given in the literature. By analysing the simulation results, it was found that the fast-flowing water has a transition point when the confined channel height is equal to about 7 layers of graphene (2.72nm). This transition is explained by the changing of the water distribution in the channel from an ordered layer structure to amorphous-like structure of water. A square and pentagon shaped water structure has been observed. Also, comments are applied on the use of the Particle Mesh Ewald summation (PME). PME were not implemented in my simulation due to the confined nature of my systems. The results are consistent with experimental results.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jichen Li (Supervisor) & Henggui Zhang (Supervisor)