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 al kanethiols self-assembled monolayers on gold substrates. These were prepare d using end-methylated (undecyl mercaptan) and end-hydroxylated (11-mercapt o-1-undecanol) compounds of the same length to avoid physical roughness of the resulting monolayer. The characterization of the surface composition wa s achieved by X-ray photoelectron spectroscopy measurement, showing that th e surface composition is statistically not different from the composition o f the grafting solution. The critical surface tension gamma(c) varies linea rly within the range 20.3-39.3 mN/m as the hydroxylation of the surface inc reases. Contact angle hysteresis is maximum on the 50% hydroxylated surface . The relaxation of the shape of a squalane drop to its equilibrium configu ration is well described by the molecular-kinetic theory of wetting, attest ing that the energy dissipation during the spreading phenomenon is mainly c ontrolled by the friction of the Liquid molecules on the solid substrate. V ersus the fraction of methylated/hydroxylated compounds on the surface, we observe a wetting transition for squalane. Below this wetting transition, t he friction coefficient xi between the squalane and the substrate is a line ar combination of the friction coefficients measured on the corresponding p ure substrates; above the wetting transition, the friction remains quasi-co nstant.