  Ion hydration and interfacial water play crucial roles in numerous phenomena
ranging from biological to industrial systems. Although biologically relevant
(and mostly smaller) ions have been studied extensively in this context, very
little experimental data exist about molecular scale behavior of heavy ions and
their complexes at interfaces, especially under technologically significant
conditions. It has recently been shown that PtCl62- complexes adsorb at
positively charged interfaces in a two-step process that cannot fit into
well-known empirical trends, such as Hofmeister series. Here, a combined
vibrational sum frequency generation and molecular dynamics study reveals that
a unique interfacial water structure is connected to this peculiar adsorption
behavior. A novel sub-ensemble analysis of MD simulation results show that
after adsorption, PtCl62- complexes partially retain their first and second
hydration spheres, and it is possible to identify three different types of
water molecules around them based on their orientational structures and
hydrogen bonding strengths. These results have important implications for
relating interfacial water structure and hydration enthalpy to the general
understanding of specific ion effects. This in turn influences interpretation
of heavy metal ion distribution across and reactivity within, liquid
interfaces.
