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

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 PtCl6 2complexes 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, PtCl6 2complexes 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.

Ion-specific clustering of metal–amphiphile complexes in rare earth separations

“…The (de)hydration of anionic complexes are known to play important roles in the SX process. 58,59 This interpretation resonates with our recent interfacial work showing that NO3ions have relatively less impact on interfacial water structure at air/aqueous interface occupied by quaternary amine surfactants. 50 Any species transferred from the aqueous to the organic phase has to be sufficiently dehydrated.…”

Ion-Specific Clustering of Metal-Amphiphile Complexes in Rare Earth Separations

“…14,56,57 TOMA complexes are known to play important roles in the SX process. 58,59 This interpretation resonates with our recent interfacial work showing that NO3ions have relatively less impact on interfacial water structure at air/aqueous interface occupied by quaternary amine surfactants. 50 Any species transferred from the aqueous to the organic phase has to be sufficiently dehydrated.…”

Ion-Specific Clustering of Metal-Amphiphile Complexes in Rare Earth Separations

“…The (de)hydration of anionic complexes are known to play important roles in the SX process. 57,58 This interpretation resonates with our recent interfacial work showing that NO3ions have relatively less impact on interfacial water structure at air/aqueous interface occupied by quaternary amine surfactants. 49 Any species transferred from the aqueous to the organic phase has to be sufficiently dehydrated.…”