(257h) Effects of Lattice Spacing on Water Films: Implications for Ice Nucleation

Atmospheric ice affects the microphysics of clouds, which in turn has an effect on the weather and climate. As liquid-to-solid phase transitions on mineral surfaces are the primary pathways for ice formation in the atmosphere, our overall goal is to elucidate the mechanisms through which surfaces affect these transitions and develop predictive abilities to correlate surface properties to the thermodynamics of the phase transitions. In this work, we use microsecond long molecular dynamics (MD) simulations at 230K to study the structure, and dynamics of water near kaolinite-like surfaces. Kaolinite is the most abundant mineral dust in the atmosphere. We specifically investigate the effect of lattice spacing (decreasing and increasing kaolinite’s lattice spacing by 10% and 20%) on water structure in water films of varying thicknesses (1, 2 and 5 monolayers (ML)).  We study the effect of lattice spacing and film thickness on possible ice nucleation by looking at the density, molecular orientation, hydrogen bonding and the tetrahedrality of water arrangement. Our simulations indicate significant changes in water structure near the kaolinite-like surfaces. For example, we find that there appears to be a critical film thickness that allows the surface structuring effects to propagate out further from the surface without interference from the vapor-liquid interface. The water at the surface of the kaolinite exhibits two different orientations.  These orientations are characterized by angles of ~50° and ~60° between water dipole and surface normal. Our tetrahedral order parameter indicates that no ice nucleation has occurred over the time scales of our simulations. Ice nucleation is a rare event and requires long waiting times. It is possible that longer simulations will result in ice nucleation in our systems. Our results show promise in the increased possibility of ice nucleation for decreased spacing and large film thicknesses. This study will help us ascertain the properties important to promote ice nucleation. The insights gained also have implications in designing materials that can prevent ice nucleation in applications such as power-lines, car windshields, and computer chips.