Structural and dynamic properties of the solid-liquid interfaces studied by Molecular Dynamics simulations

Abstract

The solid-liquid interface represents systems of great and diverse technological importance. The interfacial phenomena at solid-liquid interfaces play an important role in a wide range of biological, chemical and industrial processes, such as heterogeneous catalysis, environmental remediation, waste disposal, biomineralization, and others. In order to understand these processes, it is important to study solid-liquid interfaces at the atomic level.

Nowadays, these interfaces are mainly explored at the atomic scale with AFM, providing detailed insights into the liquid structure. Since it was discovered, AFM experienced numerous technical improvements that led to the development of 2D and 3D force mapping technique. Although these new techniques allow us to visualize hydration layers at solid-liquid interfaces with molecular resolution, the biggest flaw is with regards to the interpretation of the results due to the very complex nature of the experiments themselves. Hence, the connection between the measured signal and physical processes usually requires additional analysis tools.

In order to provide a better understanding of the processes being investigated by AFM, in this work classical molecular dynamics techniques (MD) are employed. As part of this work, we used MD simulations to support and explain AFM images obtained at highly reactive surface steps. Up until now, AFM imaging of the heterogeneous step edges was accompanied by many difficulties and as such, has not been obtained. In this thesis, we represented the first obtained 3D AFM topographic images of the hydration structures at heterogeneous edges and provided additional understanding of the atomic structure of the hydration layers and the processes at such edges by performing MD simulations. The distribution of charge in the solid-liquid interfaces plays an essential role in a wide range of processes in biology, geology, and technology. It was also noticed that AFM measurements in electrolytic solutions resulted in improved atomic-scale image stability and resolution in respect to the AFM experiments performed in pure water. Hence, in this thesis we demonstrated the mechanism of ion influences on the interface structures by performing MD simulations at hydrophilic solid-liquid interfaces such as muscovite mica and calcite in high molar solutions (~5 M), also expanding the understanding of AFM measurements in high molar solutions.

The last topic included in this thesis refers to the theoretical investigation of the origin of the hydration layers. We showed that ordering at the interface is mainly the result the attractive interactions only, although there are some indications that confinement alone (in the absence of the attractive interactions) can also be a source of layering.