The ion enrichment behavior due to surface-induced phase separation and the concomitant phase transition of electrolyte solutions between a liquid and a solid confined within nanopores of porous silicon is examined using concentrated aqueous solutions. We performed open-circuit potential measurements and differential scanning calorimetry (DSC) while varying the concentration of aqueous tetraethylammonium chloride (TEACl) solution. Open-circuit potential measurements revealed that the local OH- concentration within the nanopores increases as the bulk TEACl concentration increases. DSC measurements indicated that TEA+ cations are enriched within the nanopores and an extremely high concentration of TEA+ remarkably increases the local OH- concentration. This increase in the local pH should realize the selective precipitation of metal hydroxides within the nanopores. However, such precipitation was not observed in our investigations using aqueous solutions containing zinc cations. The experimental results suggest that ionic species within the nanopores of porous silicon are more stable than those in a bulk solution due to the formation of ion pairs with enhanced stability as well as kinetic factors that increase the activation energy for precipitation.
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