CONTACT-ANGLE TITRATIONS OF CYSTEAMINE SELF-ASSEMBLED MONOLAYERS ON GOLD - A SENSITIVE TOOL FOR MONITORING THE SURFACE MODIFICATION PROCESS

The self-assembled monolayers(SAMs), which were prepared by spontaneou s adsorption of cysteamine on gold surface, were characterized using c ontact-angle titrations. Contact-angle titration data acquired for the cysteamine SAMs exhibited a smooth transition in contact angle betwee n plateau regions at low and high pH value. The surface pK(b) was esti mated to be 1.8+/-0.2, much lower than that in solution. The electroac tive azobenzene group was anchored to the monolayer via amide surface reaction. Thus the formed SAMs were further subjected to exchange in d ilute pentanethiol solution. At each stage of modification, the surfac e was monitored by contact-angle titration and cyclic voltammetry. It was found that the titration curves obviously shifted to the higher co ntact-angle direction with the increase of surface hydrophobicity. The results are in good agreement with the cyclic voltammetric measuremen ts, demonstrating that the contact-angle titration is a useful techniq ue for monitoring the step by step surface reactions.