Electronic Structure Sensitivity to Hydration in Smectite Clays Unveiled by Near-Ambient Pressure X-ray Photoelectron Spectroscopy

A Boucly and F Rochet and H Tissot and Q Arnoux and JJ Gallet and F Bournel and S Carniato and E Dubois and L Michot and V Marry, JOURNAL OF PHYSICAL CHEMISTRY C, 128, 17112-17123 (2024).

DOI: 10.1021/acs.jpcc.4c04072

Utilizing near-ambient pressure X-ray photoelectron spectroscopy (NAP- XPS) under pressures up to 10 mbar, this study investigates the hydration behaviors of two swelling (negatively charged) smectite clays, specifically hydroxyhectorite and hydroxysaponite, under the influence of counterions Cs+, Na+, and Sr2+, which differ in size and affinity for water. NAP-XPS reveals differences in the electronic structures of hydroxyhectorite and hydroxysaponite that are related to the location of cation substitution within the negatively charged phyllosilicate layers. Importantly, this study provides atomic-scale insights into the interactions between water and these clays, focusing on changes in the counterion core-level binding energies due to hydration. We discuss the fact that the binding energy shift due to hydration results from the combination of two components: a physical component related to a change in the electrostatic potential (calculated after molecular dynamics simulation) and a chemical component related to a change in charge transfer between ligands and counterions due to the replacement of phyllosilicate oxygen atoms with water molecules (as suggested by quantum chemistry calculations). These insights-critical for understanding the electronic structure of swelling smectites-open promising avenues for future research into the hydration mechanisms of other nanostructured minerals using NAP-XPS.

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