Enhanced ion pairing of calcium, zinc, and lanthanum with acetate in silica nanopores

The well-established intrinsic properties and reactivity of molecules in aqueous environments are influenced when confined in pores, cages, channels, or slits. Nanoconfined chemical systems are common in the environment, such as in soils and sedimentary rocks. In this study, the impact of nanoscale confinement on the ion pairing involving Ca²⁺, Zn²⁺, and La³⁺ with CH₃COO⁻ anions was investigated. The concentrations of free (uncomplexed) CH₃COO⁻ and the Mⁿ⁺–CH₃COO⁻ species (acetate complexed with a cation, M = Ca²⁺, Zn²⁺, or La³⁺) were quantified using Raman spectroscopy performed on bulk aqueous solutions and on the same solutions confined within 4 nm amorphous silica nanopores. The calculated equilibrium constants increased by 0.09, 1.70, and 2.5 units for Ca²⁺–, Zn²⁺–, and La³⁺–CH₃COO⁻, respectively, when compared to analogous constants measured for bulk solutions. These shifts in the equilibrium constants correspond to the free energy of formation of contact ion pairs in confinement becoming more favorable by 0.11, 0.77, and 0.40 kJ⸳mol⁻¹ compared to bulk solution phase. In support of the experimental findings, molecular dynamics and density functional theory calculations predict the stabilization of contact ion pairs in confined systems compared to bulk aqueous environment. The observed increase in the equilibrium constants of contact ion pairing is attributed to the lower desolvation cost in the silica nanopore compared to unconfined aqueous solution.