Electric double layer models are typically used to describe the structure of charged interfaces in contact with aqueous electrolyte, but the origin of certain phenomena like overcharging remains a source of debate. The combination of the intrinsic mineral reactivity, the pH, the ionic strength and identity all play critical roles in the resulting electric double layer structure. Here we utilize a combination of techniques to determine the impact of divalent ions and varying pH on mineral oxide interfaces like silica. By combining the surface-specific measurement vibrational sum frequency generation with streaming potential or current measurements we are able to correlate the relationship between the interfacial water structure and the zeta potential. We show that overcharging of the silica electric double layer in the presence of 100 mM calcium occurs at ~pH 10.5, which correlates with the minimum in the interfacial water structure. Moreover, we see evidence of hydrolyzed calcium ions as the EDL approaches the isoelectric point. At higher pH where a positive zeta potential is observed, the interfacial concentration of this species remains constant providing some insight into the mechanism of overcharging. I will also discuss the behavior of the interface in the presence of different divalent ions as well as the addition of other mineral oxides to the aqueous mixture.

Monitoring the pH-dependent interfacial water structure at mineral oxide/aqueous interfaces in the presence of divalent ions

ACS Spring 2022

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