ACTIVE CONTROL OF WETTING USING APPLIED ELECTRICAL POTENTIALS AND SELF-ASSEMBLED MONOLAYERS

We have used applied electrical potentials, in combination with self-a ssembled monolayers (SAMs) of alkanethiolates supported on gold films, to control the wettability of a surface over wide ranges. A surface c an be transformed from nonwetting to wetting, or the reverse, with tim e constants of seconds. The method is based on a competition between r eductive electrochemical desorption of a hydrophobic SAM and its re-fo rmation from alkanethiol in solution. Self-assembled monolayers formed from either CH3(CH2)(15)-SH or CH3(CH2)(2)SH were hydrophobic (80 deg rees < theta < 110 degrees, theta = contact angle) toward aqueous solu tions of electrolyte at neutral potentials but became hydrophilic (the ta similar to 10 degrees) at reducing potentials (E < -1.3 V vs Ag wir e): Contact angles of aqueous solutions containing CH3(CH2)(2)SH retur ned to their initial values (theta similar to 80 degrees) after the re ducing potentials were removed. Because the change in wettability was dependent on the structure of the organic molecule in the monolayer, i t was possible to prepare patterned SAMs in which certain regions were transformed from hydrophobic to hydrophilic by changing potential, wh ile other regions were inert.