Calorimetric acid-base titrations of aqueous goethite and rutile suspensions

Calorimetric acid-base titrations of aqueous goethite and rutile colloidal suspensions revealed that, for both solids, proton desorption enthalpies were largely reversible (with respect to sign) over the pH range 4-10 and no ionic strength dependence (0.01-0.1 M NaNO₃) was apparent. Near the apparent zero point of charge (zpc) for the oxides (8.0 ± 0.2 for goethite, 6.0 ± 0.2 for rutile) the enthalpy vs. pH curves change slope. Weighted average enthalpies for all titrations were ±29.6 and ±14.7 kJ/mol at pH 4.5 and ±37.1 and ±30.0 kJ/mol at pH 9.5 for goethite and rutile, respectively. These data, in conjunction with acid-base modeling results, allowed trends in the thermodynamic functions (free energy, enthalpy, entropy) to be interpreted. Differences in these functions between the oxides are thought to be due, in part, to their ability to structure H₂O near the interface. Rutile is believed to structure H₂O more strongly with the result that counterion association enthalpies are less exothermic than those for goethite at similar ionic strength and pH values.