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 solution is explored using potentiometric titration and cyclic voltammetry. In solution, benzoic acid has a pKa of 4.20 at 25 degrees C. However, the observed 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 titrations at temperatures between 25 and 50 degrees C allowed us to determine the surface pKa of these materials at each temperature studied and hence to determine the enthalpy, entropy, and Gibbs' energy changes associated 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 observed pKa value, which then reflects the relative hydrophobicity/hydrophilicity of the different graphitic surfaces.

Figure 4a,b and the values in Tables 1-3 were incorrect as published. The corrected figures, values in tables and conclusion should be as follows:In conclusion, enthalphic contribution was neglected by mistake, but it cannot be neglected, and the different surface pK a values were found to be controlled by both the enthalpic and entropic terms. The ΔH°values for BAcarbon powder and BA-GC are different in sign; both ΔS°values are appreciably different from the values in bulk solution (Tables 1-3). The lateral interactions on the carbon surface and/or hydrogen bond formation are responsible for the enthalpic contribution, whereas the entropic contribution attributed to the ordering of solvent molecules at the interface between the carbon substrate and the solution is likely responsible for the variation in the observed pK a values, reflecting the different hydrophobicity/ hydrophilicity of the graphite and glassy carbon surfaces.