Probing interfacial water structure induced by charge reversal and hydrophobicity of silica surface in the presence of divalent heavy metal ions using sum frequency generation spectroscopy

Foad Raji; Cuong V Nguyen; Ngoc N Nguyen; Tuan A H Nguyen; Anh V Nguyen

doi:10.1016/j.jcis.2023.05.125

Probing interfacial water structure induced by charge reversal and hydrophobicity of silica surface in the presence of divalent heavy metal ions using sum frequency generation spectroscopy

J Colloid Interface Sci. 2023 Oct:647:152-162. doi: 10.1016/j.jcis.2023.05.125. Epub 2023 May 21.

Authors

Foad Raji¹, Cuong V Nguyen¹, Ngoc N Nguyen¹, Tuan A H Nguyen², Anh V Nguyen³

Affiliations

¹ School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia.
² Sustainable Minerals Institute, The University of Queensland, QLD 4072, Australia.
³ School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia. Electronic address: anh.nguyen@eng.uq.edu.au.

PMID: 37247479
DOI: 10.1016/j.jcis.2023.05.125

Abstract

Hypothesis: Adsorption of divalent heavy metal ions (DHMIs) at the mineral-water interfaces changes interfacial chemical species and charges, interfacial water structure, Stern (SL), and diffuse (DL) layers. These molecular changes can be detected by probing changing orientation and hydrogen-bond network of interfacial water molecules in response to changing local charges and hydrophobicity.

Experiments: Sum-frequency generation (SFG) spectroscopy was used to probe changes in vibrational resonances of interfacial OH vs. DHMI concentration and pH. SFG spectra were deconvoluted using the measured surface potential and maximum entropy method in conjunction with the electrical double-layer theory for the SL and DL structures and correlated by hydrophobicity.

Findings: Three surface charge reversals (CRs) were detected at low (CR1), medium (CR2), and high (CR3) pHs. Unlike CR1, SFG signals were minimized at CR2 and CR3 for DHMIs-silica systems highlighting considerable alterations in the structure of interfacial waters due to the inner-sphere sorption of metal hydroxo complexes. SFG results showed "hydrophobic-like" stretching modes at > 3600 cm^-1 for Pb-, Cu-, and Zn-treated silica. However, contact angle measurements revealed the hydrophobization of silica only in the presence of Pb(II), as confirmed by an in-depth SFG analysis of the hydrogen-bond network of the interfacial water molecules in the SL.

Keywords: Charge reversal; Heavy metal ions; Hydroxo complexes; SFG; Silica.