Alkali metal cations’ relative affinity to the carboxylic acid moiety is pH dependent, and typically remain hydrated.

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)

No abstract

Vibrational sum frequency spectroscopy has been used to study the molecular properties upon compression of a highly charged arachidic acid Langmuir monolayer, which displays a first-order phase transition plateau in the surface pressure–molecular area (π–A) isotherm. By targeting vibrational modes from the carboxylic acid headgroup, alkyl chain, and interfacial water molecules, information regarding the surface charge, surface potential, type of ion pair formed, and conformational order of the monolayer could be extracted. The monolayer was found to be fully charged before reaching the two-dimensional phase-transition plateau, where partial reprotonation, as well as the formation of COO– Na+ contact ion pairs, started to take place. After the transition, three headgroup species, mainly hydrated COO–, COOH, and COO– Na+ contact ion pairs, could be identified and their proportions quantified. Comparison with theoretical models shows that predictions from the Gouy–Chapman model are only adequate for the lowest surface charge densities (<−0.1 C/m2). In contrast, a modified Poisson–Boltzmann model that accounts for finite size of the cation captures many of the experimental observables, including the partial reprotonation and surface potential changes upon compression. The experimental results provide a quantitative molecular insight that could be used to test potential extensions to the theory.