Self-Assembled Monolayers: Effects of Surface Nanostructure on Wetting
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The determination of solid–vapor (γsv) and solid–liquid (γsl) interfacial tensions is of importance in a wide range of problems in pure and applied sciences.
Because of the difficulties involved in measuring directly the surface tension involving a solid phase, indirect approaches are called for. Several independent approaches have been used to estimate solid surface tensions, including direct force measurements, contact angles, capillary penetration into columns of particle powder, sedimentation of particles, solidification front interaction with particles, film flotation, gradient theory, the Lifshitz theory of van der Waals forces, and the theory of molecular interactions. Among these methods, contact angle measurements are believed to be the simplest.
We report low-rate dynamic contact angle data of various liquids on self-assembled monolayers (SAMs) of octadecanethiol on annealed and nonannealed gold. It was found that the interpretation of solid surface tensions using contact angle data on less well-prepared polycrystalline nonannealed gold surfaces can be misleading. Our findings were supported by reflectance infrared (IR) spectra and atomic force microscopy (AFM) data indicating that surface nanostructure, defects, and polycrystallinity can be important factors for a systematic study of wettability on SAMs in terms of surface energetics. We found that the contact angle and adhesion patterns of various liquids on SAMs of octadecanethiol adsorbed onto annealed gold substrates are consistent with recent experimental data for the relatively thick polymer-coated surfaces. The variation of surface structure in terms of surface energetics can be estimated only when a fundamental understanding of contact angles and surface tensions is known.