Heterogeneous ice nucleation is a crucial phenomenon in various fields of fundamental and applied science. We investigate the effect of surface cations on freezing of water on muscovite mica. Mica is unique in that the exposed ion on its surface can be readily and easily exchanged without affecting other properties such as surface roughness. We investigate freezing on natural (K+) mica and mica in which we have exchanged K+ for Al3+, Mg2+, Ca2+, and Sr2+. We find that liquid water freezes at higher temperatures when ions of higher valency are present on the surface, thus exposing more of the underlying silica layer. Our data also show that the size of the ion affects the characteristic freezing temperature. Using molecular dynamics simulations, we investigate the effects that the ion valency and exposed silica layer have on the behavior of water on the surface. The results indicate that multivalent cations enhance the probability of forming large clusters of hydrogen bonded water molecules that are anchored by the hydration shells of the cations. These clusters also have a large fraction of free water that can reorient to take ice-like configurations, which are promoted by the regions on mica devoid of the ions. Thus, these clusters could serve as seedbeds for ice nuclei. The combined experimental and simulation studies shed new light on the influence of surface ions on heterogeneous ice nucleation.
Bibliographical noteFunding Information:
The authors acknowledge financial support from the National Science Foundation (AGS #1541944 and AGS #1541998). M.L. acknowledges support from the National Science Foundation (AGS #1823334 and #2001490). P.H. was funded through the Summer Undergraduate Research with Faculty program at the College of Charleston. J.Z. and N.N.L. contributed equally to this article. Support from Dr. Timothy Leftwich in the Applied Chemical and Morphological Analysis Laboratory at Michigan Tech is appreciated. We thank the anonymous reviewers. Their constructive comments played a significant role in refining our presentation of results and formulation of the hypothesis.
© 2020 American Chemical Society.