Calorimetric study of alkali and alkaline-earth cation adsorption and exchange at the quartz-solution interface

Cations in natural solutions significantly impact interfacial processes, particularly dissolution and surface charge measurements for quartz and silica, which are amongst the most naturally abundant and technologically important solids. Thermodynamic parameters for cation-specific interfacial reactions have heretofore been mostly derived instead of directly measured experimentally. This work investigates the energetics of adsorption and exchange reactions of alkali metal (M⁺) and alkaline earth (M²⁺) cations with the quartz surface by flow adsorption microcalorimetry, in tandem with in-situ pH measurements. The magnitudes of the heats of adsorption and exchange were found to increase along the Hofmeister series i.e., Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺ and Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺, and exhibited strong correlations to bulk cation hydration enthalpies (ΔH_hyd). These results suggest inner-sphere adsorption for all studied cations and highlight the role ΔH_hyd plays in rationalizing these reactions and controlling their net overall enthalpy. pH measurements demonstrate that quartz surface charge will vary depending on the cation present, as is well known for amorphous forms of silica. Along with calorimetric signals, pH data revealed kinetic differences between the adsorption and desorption reactions of M⁺ and M²⁺, and individual cations within each group.