Lawrence Berkeley National Laboratory

Aqueous films on mineral surfaces control the physical, chemical, and biological transport processes in the atmosphere, soil, and rocks. Despite the importance of thin films for various research and engineering fields, there are still unanswered questions regarding the roles of the different forces affecting the nature of water films. One of these, the focus of this study, is the development of abnormally thick water films on quartz surfaces. In this study, we developed a density-functional-theory-based model to describe the time-dependent evolution of water films and identify the governing forces responsible for thickening films. We simulated the diffusion of water vapor from ambient air toward mineral surfaces and the formation and thickening of water films at various relative humidity values. Our model predicts an abnormal water film thickness on a hydroxylated quartz surface compared to a surface free of hydroxylation, which explains experimental observations. We further used the model to understand the key interaction forces at different stages of water film formation and thickening. Our model suggests that the attractive hydrogen bonding and van der Waals forces initiate a seed layer of water, and the electrostatic forces, generated by the hydroxylated and thus charged surface, lead to the thickening of water films. This generalizable model can provide insights into the peculiarities of water film development on various mineral surfaces.