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Structure of the Silica/Divalent Electrolyte Interface: Molecular Insight into Charge Inversion with Increasing pH

preprint
submitted on 20.07.2020 and posted on 21.07.2020 by Mokhtar Rashwan, Benjamin Rehl, Adrien Sthoer, Akemi Darlington, Md. Shafiul Azam, Hongbo Zeng, Qingxia Liu, Eric Tyrode, Julianne Gibbs
The molecular origin of overcharging at mineral oxide surfaces remains a cause of contention within the geochemistry, physics, and colloidal chemistry communities owing to competing “chemical” vs “physical” interpretations. Here, we combine vibrational sum frequency spectroscopy and streaming potential measurements to obtain molecular and macroscopic insights into the pH-dependent interactions of calcium ions with a fused silica surface. In 100 mM CaCl2 electrolyte, we observe evidence of charge neutralization at pH~10.5, as deducted from a minimum in the interfacial water signal. Concurrently, adsorption of calcium hydroxide cations is inferred from the appearance of a spectral feature at ~3610 cm-1. However, the interfacial water signal increases at higher pH, while adsorbed calcium hydroxide appears to remain constant, indicating that overcharging results from hydrated Ca2+ ions present within the Stern layer. These findings suggest that both specific adsorption of hydrolyzed ions and ion-ion correlations of hydrated ions govern silica overcharging with increasing pH.

History

Email Address of Submitting Author

julianne.gibbs@ualberta.ca

Institution

University of Alberta

Country

Canada

ORCID For Submitting Author

0000-0001-5819-2306

Declaration of Conflict of Interest

There are no conflicts to declare.

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