The influence of surface functional groups of organic crystals on triboelectrification

Triboelectric charging is a ubiquitous phenomenon with both historical significance in the discovery of electricity and contemporary importance in applications like particulate processing and small-scale power generation through ambient energy harvesting. However, despite its long history, the fundamental mechanisms underlying triboelectric charging remain poorly understood. This complexity arises from the sensitivity of the phenomenon to a multitude of physical and environmental factors, making the interpretation of experimental results challenging. Recent advances, particularly the development of triboelectric nanogenerators, have led to a growing consensus that the electron transfer mechanism dominates in triboelectric charging, although this perspective is not universally accepted among researchers. First principles calculations have demonstrated their ability to probe the underlying mechanisms for charge transfer and this work has been applied to gain insight into various triboelectric charging phenomena observed in experiments. This thesis employs Density Functional Theory (DFT) to gain insights into various aspects of triboelectric charging observed in experimental settings. Firstly, the influence of surface properties, crystal facets, and the presence of surface contaminants on various electronic structure properties relevant to triboelectric charging is investigated. This analysis underscores the substantial impact that surface conditions can have on charge transfer phenomena. Secondly, the work function, a key parameter for modeling triboelectric charging, is predicted for different facets of pharmaceutical crystals using DFT. These calculations predict variations in work function, depending on the crystal facet and the presence of surface water. Finally, the charging characteristics of individual Active Pharmaceutical Ingredient (API) molecules are investigated through a theoretical "slab-molecule" approach. To elucidate the mechanisms of charge transfer, charge density difference, density of states and Hirschfeld charge analysis were employed. This thesis aims to highlight, through the application of first principles calculations, the substantial influence that surface chemistry and contamination can exert on triboelectric charging. Additionally, it seeks to introduce innovative methods for gaining insights into the triboelectric charging properties of individual molecules.