Sulfate Adsorption at the Buried Fluorite–Solution Interface Revealed by Vibrational Sum Frequency Generation Spectroscopy

Understanding the structure and energetics of adsorbed ions at buried mineral/solution interfaces has great importance to the geochemical and atmospheric chemistry communities. Vibrational spectroscopy is a powerful tool for the study of mineral/solution interfaces as these techniques can be applied in situ, are sensitive to surface structures, and are generally nondestructive. The use of vibrational sum frequency generation spectroscopy (VSFG), which is inherently interface-specific, is applied here to study the adsorption of sulfate at the buried fluorite (CaF2)/Na2SO4 solution surface at pH 7 and 298 K in the presence of an aqueous background electrolyte, NaCl. The use of VSFG allowed for the resolution of adsorbed sulfate complexes from sulfate molecules which reside in the interfacial electric double layer yet remain fully solvated. The sulfate anion is found to adsorb with a bidentate inner-sphere structure at the fluorite surface with an average surface free energy of adsorption of −31 ± 3 kJ/mol for pH 7 solutions at 298 K.