Two-Step Adsorption of PtCl₆^2– Complexes at a Charged Langmuir Monolayer: Role of Hydration and Ion Correlations

Abstract

Anion exchange at positively charged interfaces plays an important role in a variety of physical and chemical processes. However, the molecular scale details of these processes, especially with heavy and large anionic complexes, are not well-understood. We studied the adsorption of PtCl6 2anionic complexes to floating DPTAP monolayers in the presence of excess Clas a function of the bulk chlorometalate concentration. In situ x-ray scattering and fluorescence measurements, which are element and depth sensitive, show that the chlorometalate ions only adsorb in the diffuse layer at lower concentrations, while they adsorb predominantly in the Stern layer at higher concentrations. The response of DPTAP molecules to the adsorbed ions is determined independently by grazing incidence x-ray diffraction, and supports this picture. Molecular dynamics simulations further elucidate the nanoscale structure of the interfacial complexes. The results suggest that ion hydration and ion-ion correlations play a key role in the competitive adsorption process. TOC GRAPHICSKEYWORDS Air/water interface, anion recognition, Hofmeister effects, specific ion effects, solvent extraction.

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

“…14,[42][43][44][45] However, PtCl6 2anions both adsorb tightly to the DPTAP interface and are strongly hydrated. 27 The present work demonstrates that PtCl6 2retains part of its hydration shell after adsorption and can keep the second hydration shell oriented (WWHB-Up), a behavior that is showing that larger multivalent anions behave significantly differently than their lighter Hofmeister anions.…”

“…previous surface x-ray study on the same system shows that the area per DPTAP molecule (at a constant surface pressure) decreases with increasing [PtCl6 2-]. 27 Therefore, the addition of PtCl6 2-should not increase the amount of water molecules between the lipid tails. In addition, as it is ATR-FTIR measurements provide further evidence that the AIWHB band arises from water solvating PtCl6 2anions.…”

“…x-ray scattering techniques. 27 Those measurements suggest that the DPTAP monolayer structure is stable under these conditions for the duration of the experiments. The SFG measurements of the CH region for each sample in this study (not shown) support this expectation.…”

“…It is known that the thermochemical radii of ions plays an important role in their interfacial interactions, 23 and a recent study demonstrated qualitative differences in the adsorption behavior of heavy and light anions at a charged interface. 27 However, the surface x-ray scattering techniques used in that study did not provide any information about the interfacial water structure. The current work leverages the well-established ability of VSFG to study interfacial water-ion interactions in numerous systems 4,16,[21][22][28][29][30] (including Langmuir monolayers on electrolyte solutions - Figure 1), which allows for facile control of the interactions between organic functional groups and ions.…”

“…Water Solvation Sub-ensemble Analysis. The water solvation sub-ensemble analysis has been performed on the trajectories of previously reported classical molecular dynamics (MD) simulations, 27 which used the GROMACS package (version 4.5.5) 31 and the CHARMM 36 force field. [32][33][34] GROMACS is a generic molecular dynamics (MD) simulation package for molecular structure at the nanometer resolution.…”