A Theoretical Study of the Role of Interfacial Water and Triboelectric Charging in Insulators

In electrophotographic printing, the adsorption of water on critical surfaces has an overwhelming but poorly understood effect on the triboelectric charging. In this work, Molecular Mechanics was used to find the most probable water adsorption sites for isolated PMMA, poly(methyl methacrylate), and isolated silica models, and for intermolecular complexes between PMMA and silica which are responsible for triboelectric charge transfer. Density functional theory (DFT) was used calculate the local energy minima, the adsorption of water molecules on different surface functional groups, and the analysis of the frontier orbitals, to quantitatively compare the energy gap differences for electronic charge transfer, and thus the effect of hydration site on PMMA and silica on electronic charge transfer. Four surface water adsorption modes that affect electron charge transfer between PMMA and silica were found: water adsorbed on the PMMA carbonyl, water adsorbed at the active site of the LUMO on silica, water as a spacer between PMMA and silica, and water as a charge carrier dissipating charge. This work provides a detailed mechanistic understanding of the effect of water on the surface electronic structure of amorphous silica and PMMA that are critical to electrophotographic applications, and no doubt to other applications where the surface electronic structure is relevant.