Effect of pH on the interaction between zwitterions and titanium oxide

The isoelectric points (IEPs) of two zwitterions, glycine and both-terminals-terminated poly(ethylene glycol) (NH(2)-PEG-COOH), were determined from the titration curves, and the thicknesses of zwitterion layers immobilized on titanium (Ti) with immersion and electrodeposition at various pH based on IEPs were evaluated with ellipsometry to investigate the effect of pH and the immobilization technique on the interactions between the zwitterions and the Ti surface. From the titration curves, pK(1), pK(2), and the IEP of glycine were determined as 2.8, 8.9, and 5.9, respectively, and pK(1), pK(2), and the IEP of NH(2)-PEG-COOH were determined as 2.1, 11.7, and 6.9, respectively. At a certain specific pH, (+)H(3)N-CH(2)-COO(-) or (+)H(3)N-PEG-COO(-) was formed by hydrolysis of glycine or NH(2)-PEG-COOH. In addition, the Ti surface was negatively charged at this pH. As a result, for immersion, the electrostatic reactivity between terminal groups of zwitterions and hydroxyl groups on the Ti surface was the highest and the thickness of the immobilized layer was significantly the largest at pH 12. For electrodeposition, glycine, with its lower molecular weight, was more easily attracted to the Ti surface than NH(2)-PEG-COOH, which has a higher molecular weight, while the thickness of the immobilized layer was the greatest at pH 12 in both zwitterions.