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

Abstract

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 CaCl<sub>2</sub> 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<sup>-1</sup>. However, the interfacial water signal increases at higher pH, while adsorbed calcium hydroxide appears to remain constant, indicating that overcharging results from hydrated Ca<sup>2+</sup> 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.

Divalent Ion-Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface

“…The resonant vibrational SFG experiments attributed these trends to displacement of the hydration layer above the silica surface by ions retaining their centrosymmetric hydration shell. 55 These trends were attributed to close association of Ca 2+ to the interface, 55,[57][58][59][60][61][62] a finding that would be consistent with the non-resonant c (2) estimates reported here (as well as the largest reduction in interfacial potential by CaCl2 relative to NaCl).…”

“…[47][48][49] We therefore find experimental evidence for a significant change in interfacial structure with increasing surface coverage for some of the ions we surveyed, consistent with reports by others for mica:water [50][51][52][53] and silica:water [47][48][49]54 interfaces. Previous studies of divalent cations, specifically magnesium and calcium, by Gibbs and co-workers, 55,56 have shown that low concentrations of these salts (0.033mM) attenuate the vibrational sum frequency generation Ma and Geiger 10 (SFG)-resonant water signal in comparison to NaCl at similar concentrations albeit at a higher pH than the conditions studied here. The resonant vibrational SFG experiments attributed these trends to displacement of the hydration layer above the silica surface by ions retaining their centrosymmetric hydration shell.…”

“…This outcome is surprising given the precedent for relatively large changes in the hydration structure at the interface. 45,55 Simulations have shown, for example, a highly ordered first solvation shell for divalent cations such as Mg 2+ vs a fairly labile hydration structure for Na + . 67 Furthermore, Na + is predicted to form direct contact ion pairs with silanol groups, whereas Mg 2+ is proposed to not bind directly to silanol groups at the surface, but rather form a hydrogen bonded complex through its tightly bonded hydration sphere.…”

La3+ and Y3+ interactions with the carboxylic acid moiety at the liquid/vapour interface: identification of binding complexes, charge reversal, and detection limits.

“…The relatively sharp feature observed at ~3670 cm -1 provides additional support for the charge inversion at high YCl3 concentrations. The high-frequency band is located in a similar frequency range as that observed for the OH stretches of Si-OH, Al-OH, and Ca-OH at the aqueous interface with silica, [43,79] alumina, [80] and calcium fluoride, [81] respectively. Thereby, it is assigned to the OH stretch of an yttrium hydroxide cation (i.e.…”

Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

“…In summary, we determined the effect of ionic strength on the SFG spectra of the surfacebound waters at the silica/water interface through deconvolution of the spectra into surface-bound water, or (2) , and diffuse layer water, or (3) 3 , components. In lieu of a calculated surface potential for the analysis, we obtained the zeta potential from streaming current measurements, which has been related to both SFG [77][78] and SHG, [79][80][81] but not used to deconvolute the (2) and (3) contributions. With the reported phase-sensitive SFG 32 and SHG 31 measurements of the silica/water interface as reference, we applied the maximum entropy method to our measured SFG intensities to obtain the complex SFG spectra.…”