Real-time observation of cation exchange kinetics and dynamics at the muscovite-water interface

Sang Soo Lee,Kathryn L Nagy,Neil C Sturchio,Paul Fenter

doi:10.1038/ncomms15826

Abstract

Ion exchange at charged solid–liquid interfaces is central to a broad range of chemical and transport phenomena. Real-time observations of adsorption/desorption at the molecular-scale elucidate exchange reaction pathways. Here we report temporal variation in the distribution of Rb+ species at the muscovite (001)–water interface during exchange with Na+. Time-resolved resonant anomalous X-ray reflectivity measurements at 25 °C reveal that Rb+ desorption occurs over several tens of seconds during which thermodynamically stable inner-sphere Rb+ slowly transforms to a less stable outer-sphere Rb+. In contrast, Rb+ adsorption is about twice as fast, proceeding from Rb+ in the bulk solution to the stable inner-sphere species. The Arrhenius plot of the adsorption/desorption rate constants measured from 9 to 55 °C shows that the pre-exponential factor for desorption is significantly smaller than that for adsorption, indicating that this reduced attempt frequency of cation detachment largely explains the slow cation exchange processes at the interface.

Highlights

Ion exchange at charged solid–liquid interfaces is central to a broad range of chemical and transport phenomena
Charged solid–liquid interfaces are primary sites for a wide array of chemical reactions including ion adsorption and molecular uptake[1,2,3,4,5], catalysis[6,7,8,9,10] and energy storage[11,12,13,14]. Fundamental understanding of these processes depends on the ability to see changes in interfacial structure during reactions within the electrical double layer (EDL)[15,16,17,18,19]
Time-dependent changes in the interfacial structure during Rb þ and Na þ exchange were probed by using in situ timeresolved X-ray reflectivity (TXR)

Summary

Introduction

Ion exchange at charged solid–liquid interfaces is central to a broad range of chemical and transport phenomena. Recent work[20,21,22] demonstrated that the EDL structure at solid–water interfaces can be complex, having coexisting ions in different hydration states including inner-sphere (IS) complexes that adsorb at the surface in a partially solvated state and outer-sphere (OS) complexes that adsorb in a fully solvated state (Fig. 1a) Distinguishing these adsorbed species requires determination of the interfacial structure at the molecular scale. Adsorption of simple alkali cations and associated changes of interfacial hydration at the muscovite (001)–water interface have been studied extensively using various approaches including surface force apparatus[27,28,29,30], atomic force microscopy[31,32], in situ X-ray surface scattering[22,33,34,35,36] and computational simulations[34,37,38]. Na þ was interpreted to result from a larger energy cost for partial dehydration of more strongly hydrated Na þ compared to Rb þ

Methods

Results

Conclusion