Stabilization of AgI’s polar surfaces by the aqueous environment, and its implications for ice formation – Dr Stephen Cox, University of Cambridge

May 1, 2019 @ 2:15 pm – 3:15 pm
Department of Chemistry
Unilever lecture theatre
Lisa Masters

Silver iodide is probably the best inorganic ice nucleating particle known, a feature generally attributed to the excellent lattice match between its basal crystal faces and ice. However, the polar nature of this crystal termination means the surface energy diverges with crystal size unless a polarity compensation mechanism prevails. In this simulation study, the extent to which the surrounding aqueous environment is able to provide such polarity compensation is investigated. On its own, it is found that pure water is unable to stabilize the AgI crystal in a physically reasonable manner, and that mobile charge carriers such as dissolved ions, are essential. In other words, proximate dissolved ions must be considered an integral part of the heterogeneous ice formation mechanism.

This work relies on recent advances in simulation methodology in which appropriate electric and electric displacement fields are imposed (“finite field methods”). A useful by-product of this study is the direct comparison to the commonly used Yeh-Berkowitz method that this enables. Here it is found that naive application of the latter leads to physically unreasonable results, and greatly influences the structure of water in the contact layer. This work should therefore be of general importance to those studying polar/charged surfaces in aqueous environments. I will also highlight recent work that uses the finite field methods to understand the response of bulk electrolyte solutions to imposed electric and electric displacement fields.