Investigating the thermodynamic causes behind the anomalously large shifts in pKa values of benzoic acid-modified graphite and glassy carbon surfaces

The difference between the values of 4-carboxyphenyl groups, covalently attached to either graphite (BAcarbon) or glassy carbon (BA-GC) surfaces, and benzoic acid in soln. is explored using potentiometric titrn. and cyclic voltammetry. In soln., benzoic acid has a pKa of 4.20 at 25°. However, the obsd. pKa value on the graphitic surfaces shows significant deviations, with BAcarbon exhibiting a large shift to higher pKa values (pKa = 6.45) in contrast to BA-GC, which is shifted to lower pKa values (pKa = 3.25). Potentiometric titrns. at temps. between 25° and 50° allowed us to det. the surface pKa of these materials at each temp. studied and hence to det. the enthalpy, entropy, and Gibbs' energy changes assocd. with the ionization of the carboxylic acid groups. It was found that the enthalpic contribution is negligible and that the changes in surface pKa values are entropically controlled. This suggests that solvent ordering/disordering around the interface strongly influences the obsd. pKa value, which then reflects the relative hydrophobicity/hydrophilicity of the different graphitic surfaces. [on SciFinder(R)]