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 2 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 Ca 2+ 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. File list (2) download file view on ChemRxiv Rashwan_Manuscript.pdf (1.77 MiB) download file view on ChemRxiv Rashwan_SupportingInformation.pdf (1.07 MiB)

The importance of specifically adsorbed ions for electrokinetic phenomena: Bridging the gap between experiments and MD simulations

“…In Fig. 5(a), we compare the ζ-potential at various concentrations between our EOF simulations, the SC experiments of van der Lorenz et al, 26 our results for the different f -values and the simulation data from Biriukov et al 36 are in reasonable agreement with the experimental results from van der Heyden et al 9 and Rashwan et al 60 Notably, there is a considerable difference between ζ-potentials found by van der Heyden et al 9 and Rashwan et al 60 at 100 mmol/l. Comparable differences are found between values obtained from different electrokinetic techniques.…”

“…Heyden et al9 and Rashwan et al,60 the Atomic Force Microscopy (AFM) experiments from Siretanu et al,8 and previous MD results from EOF and SC simulations from Lorenz et al26 and EOF simulations from Biriukov et al36 Compared to the results from…”

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.…”

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

“…[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 (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.…”