Contact Angle Stability: Reorganization of Monolayer Surfaces?

Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH₃ groups at their air/monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (percent OH). The effect of different storage environments and temperature on the changing contact angles is discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize, over a time dependent on the chain length, by surface reorganization and the adsorption of contaminants.