In many real-world commercial applications, such as electrophotographic printing, the adsorption of water on critical surfaces has an overwhelming but poorly understood effect on the desired materials performance. This work is focussed to better understand the effect of water on the triboelectric charge transfer between surfaces in xerography.

Molecular mechanics was used to find the most probably water adsorption sites for isolated polymethylmethacryate (PMMA) 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 the hydration site on PMMA and silica on electronic charge transfer.

Four ways were found for surface water adsorption to affect the electronic charge transfer between PMMA and silica: (1) adsorption of a water molecule on the carbonyl group of PMMA decreases the energy level of the highest occupied molecular orbital (HOMO) on PMMA, thus making electron charge transfer more difficult because of the transfer barrier increase; (2) a water molecule acts as part of the electron accepter when adsorbed at the active site of the lowest unoccupied molecular orbital (LUMO) on silica, which lowers the LUMO and thus lowers the charge transfer barrier, but at the same time creates a charge dissipation pathway as a part of the charge transfer is now to water, rather than to silica; (3) a water molecule acts as a spacer between PMMA and silica, which increases the energy gap for charge transfer and thus decreases the charge transfer; and (4) water molecules can act as a charge carrier, dissipating electrons by accepting charge when adsorbed on silica and then donating back charge when adsorbed on PMMA.

This work provides a detailed mechanistic understanding of the effects 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.