Spontaneous spreading of liquid droplets on mixed alkanethiol monolayers:: Dynamics of wetting and wetting transition

We report and analyze the static and dynamic wetting properties of mixed alkanethiols self-assembled monolayers on gold substrates. These were prepared using end-methylated (undecyl mercaptan) and end-hydroxylated (11-mercapto-1-undecanol) compounds of the same length to avoid physical roughness of the resulting monolayer. The characterization of the surface composition was achieved by X-ray photoelectron spectroscopy measurement, showing that the surface composition is statistically not different from the composition of the grafting solution. The critical surface tension gamma(c) varies linearly within the range 20.3-39.3 mN/m as the hydroxylation of the surface increases. Contact angle hysteresis is maximum on the 50% hydroxylated surface. The relaxation of the shape of a squalane drop to its equilibrium configuration is well described by the molecular-kinetic theory of wetting, attesting that the energy dissipation during the spreading phenomenon is mainly controlled by the friction of the Liquid molecules on the solid substrate. Versus the fraction of methylated/hydroxylated compounds on the surface, we observe a wetting transition for squalane. Below this wetting transition, the friction coefficient xi between the squalane and the substrate is a linear combination of the friction coefficients measured on the corresponding pure substrates; above the wetting transition, the friction remains quasi-constant.