Evaporation dynamics of microdroplets on self-assembled monolayers of dialkyl disulfides.

We present a study of the static wettability and evaporation dynamics of sessile microdroplets of water on self-assembled monolayers (SAMs) prepared with unsymmetric dialkyl disulfides CH(3)-(CH(2))(11+m)-S-S-(CH(2))(11)-OH (m = 0, +/- 2, +/- 4, +/- 6) on gold-covered mica. The advancing and receding contact angles decrease linearly with increasing hydrophilicity of the SAM. The latter was changed either via the molar ratio or via the chain length of the hydroxyl-terminated alkyl chains in the monolayer. In contrast to SAMs made of thiols, the contact angle hysteresis was 10 degrees for all disulfides, irrespective of their chain lengths. During evaporation of single droplets, a transition from pinning to constant contact angle mode was observed. The transition time between the modes increases with the surface hydrophilicity, leading to longer pinning. This way, the time for complete droplet evaporation decreases by approximately 30% owing to the fact that during pinning the overall droplet area stays large for a longer time. For single droplets the measured total evaporation times agree well with the calculated ones, showing the validity of the standard evaporation model for both evaporation modes. In contrast to the results for single droplets, many droplets with different initial volumes show a power-law dependence on the total evaporation time with an exponent different from 1.5 as expected from the standard model. For disulfides with m not equal 0, the exponent is in the range of 1.40-1.47 increasing with the surface hydrophilicity. For the SAMs with m = 0 the exponent increases up to 1.61 for the most hydrophilic surface. We explain this deviation from the standard evaporation model with the presence of a liquid precursor film around the droplet, which either enhances or decelerates evaporation. Our results suggest that SAMs of dialkyl disulfides offer the possibility to tune the wettability of gold surfaces in a more controlled way than thiols do.