Previous thermodynamics studies of alkali cation (M⁺) exchange and adsorption at interfaces from this group (among others) have predominantely been conducted by exchanging equal concentration solutions containing one M⁺ cation for another, and vice versa. These experiments highlighted the role that hydration properties play in governing the enthalpic sign of exchange reactions. An underlying caveat of these experiments, namely those conducted under a flow regime, is that they introduce a steep concentration gradient that could potentially influence the local equilibrium between the cations and the surface.
Herein, we examine the thermodynamics of Li, Na, K, Rb, and Cs exchange at the quartz-aqueous interface from variable solution compositions using operando in situ flow microcalorimetry. For instance, solutions are adjusted to incrementally increase the presence of other cations, going from one containing 100% Li⁺ exchanging with 100% M⁺, 100% Li⁺ exchanging with 80% Li⁺ + 20% M⁺, to finally 100% Li⁺ exchanging with 50% Li + 50% M⁺.
The resulting calorimetric signals obtained from mixed composition solutions differed from those reported for isomolar solution exchange and exhibited a complex landscape of sequential endothermic and exothermic reactions, particularly for the exchange of the larger Rb and Cs cations. The results still point to the overwhleming influence of hydration and dehydration energies on the net sign of the reaction enthalpy, but suggest different underlying kinetics possibly driven by differing site (SiOH vs. SiO^-) selectivities. A discussion of the implication of these results on our undersanding of cation exchange as a basis for contaminant transport, mineral dissolution and precipitation, etc. will be included.

Nicholas Allen

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