CONTROL OF SOLVATION ENTROPY ON THE ELECTROSTATIC ADSORPTION OF CATIONS ONTO SILICA

Silica surfaces possess negative charge over the pH range of most natural waters and the extent of this charge development is proportional to the concentration and type of cations in solution. The most abundant cations in natural waters, Na+, K+, Ca2+, and Mg2+ are generally agreed to promote negative charge accumulation by cation-specific electrostatic adsorption and their solvation properties are believed to play a central role. Despite the importance of these interactions to inorganic and biological systems, long-standing discrepancies continue between studies reporting the most basic surface-specificity trends.

The origin of cation specificity trends on colloidal silica is examined by conducting especially designed surface charge titration experiments. With an internally consistent data set for seven Group IA and IIA cations, we test the hypothesis that solvent structure-breaking or -making impacts of cations having electrostatic interactions with surface silanols increase the entropy of the local system to an extent that negates the small positive enthalpy of adsorption.

These data are evaluated using the Triple Layer surface complexation model to determine the adsorption constant and inner layer capacitance associated with each type of cation. The resulting parameters reveal that cation-silica electrostatic interactions are described by a simple thermodynamic approach that focuses on cation solvation entropy as the primary independent variable. The energy differences are described by a construct that emphasizes the changes in solvent structure and order involved in bringing the hydrated cation to the interface. We also find that the IIA cations on silica exhibit reverse lyotropic behavior like that reported for other oxides and findings in this study suggest the concept of lyotropy is rooted in the solvent-structuring ability of cations at the interface to modify the entropy of the local system. Cation specificity and charge development on silica surfaces are, hence, reflected largely through differences in the inner layer capacitance. The adsorption constant is a secondary parameter for these electrostatic interactions. Entropy is the primary energy term that determines differences in these electrostatic cation-surface interactions.