Calorimetric Evidence in Support of the Nanopore Inner Sphere Enhancement Theory on Cation Adsorption

The nanopore inner sphere enhancement (NISE) theory provides a new theoretical model of cation adsorption within confining nanopore channels. Inside nanopore channels, hydrated ions can dehydrate and more easily adsorb via an inner sphere mechanism. Adsorption data showed that in certain nanopores, weakly hydrated monovalent cations adsorbed more strongly than divalent cations, which tend to be strongly hydrated. Flow adsorption calorimetry is a valuable tool for directly measuring the heats of the ion exchange process and was used to measure the heats of Na and Ca exchange on three zeolite minerals: zeolite Y, mordenite, and ZSM-5. The data collected showed equal and reversible exchange reactions on mordenite but a strong endothermic Na adsorption and weak exothermic Ca adsorption on ZSM-5. On zeolite Y, the calorimetric signal was below the instrument detection limit of 5 to 7.5 mV. These differences coincide with the adsorption mechanisms and relative competitiveness predicted by the NISE theory for these two ions on the three zeolites studied. These data elucidate an exchange reaction where Ca is outcompeted by Na, which is often considered to be a weak background electrolyte.