Cosolvent Effect on Goethite Surface Protonation

摘要

Extensive research has been conducted on the surface acidity of hydrous metal oxides in aqueous solutions. However, little information is available on mineral surface acidities in mixed solvent solutions. The present study examined goethite surface protonation in acetone-water and methanol-water mixtures with potentiometric titrimetry. Surface acidity constants, K_+~(int) = [XOH_2~+]e~(FΨ/RT)/[XOH][H~+] and K_-~(int) = [H~+][XO~-]e~(-FΨ/RT)/[XOH], were calculated using constant capacitance and diffuse-layer models. The number of apparent surface sites increased with increasing cosolvent content. Linear log K_+~(int) vs 1/(dielectric constant ε) was observed. Simple electrostatic correction using a modified Born equation adequately predicted changes of K_+~(int) from water to mixed solvents. Changes in K_-~(int) were less tractable, however, showing nonlinearity with 1/ε.