Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface

TL;DR

This study uncovers how heavy anionic complexes like PtCl6^2- alter water structure at charged interfaces, revealing unique hydration behaviors and implications for ion-specific effects in various systems.

Contribution

It provides the first detailed experimental and molecular dynamics analysis of heavy ion complexes at interfaces, highlighting their distinct hydration and adsorption mechanisms.

Findings

Heavy PtCl6^2- complexes partially retain hydration spheres at interfaces.

Three distinct water molecule types identified around complexes based on orientation and bonding.

Unique interfacial water structure linked to peculiar adsorption behavior of heavy anionic complexes.

Abstract

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.