  Silver iodide is one of the most potent inorganic ice nucleating particles
known, a feature generally attributed to the excellent lattice match between
its basal Ag-(0001) and I-(000-1) surfaces, and ice. This crystal termination,
however, is a type-III polar surface, and its surface energy therefore diverges
with crystal size unless a polarity compensation mechanism prevails. In this
simulation study, we investigate to what extent the surrounding aqueous
environment is able to provide such polarity compensation. On its own, we find
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. The simulations we
perform utilize recent advances in simulation methodology in which appropriate
electric and electric displacement fields are imposed. A useful by-product of
this study is the direct comparison to the commonly used Yeh-Berkowitz method
that this enables. Here we find that naive application of the latter leads to
physically unreasonable results, and greatly influences the structure of water
in the contact layer. We therefore expect these results to be of general
importance to those studying polar/charged surfaces in aqueous environments.
