pH-dependent wettability of carboxyphenyl films grafted to glassy carbon

Surfaces than can switch their properties in response to external stimuli are of fundamental as well as technological interest. A prerequisite for successful switching in thin surface layers is sufficient free volume in the layer to allow molecular motions or reactions. Multilayer films grafted from aryldiazonium salts have a loosely packed structure and are good candidates for preparation of switchable surfaces. In this work, the pH-dependent wettability of carboxyphenyl films on glassy carbon surfaces is examined using water contact angle measurements. The film structure is manipulated by exposing freshly grafted films to solvents of different polarity; this influences the wettability differences observed between low- and high-pH measurements. The order of measurement of contact angles (from low pH to high, or vice versa) also influences the pH-dependent wettability. The results are consistent with film reorganization, including the formation of dimeric hydrogen-bonded structures, in response to the polarity and pH of the surrounding medium.