Potential-Dependent Wetting of Aqueous Solutions on Self-Assembled Monolayers Formed from 15-(Ferrocenylcarbonyl)pentadecanethiol on Gold

Aqueous solutions of 0.1 M NaClOr (pH 1.5, 10 mM KHzPOn) show potential-dependent wetting on self-assembled monolayers (SAMs) formed from 15-(ferrocenylcarbonyl)pentadecanethiol (FcCO(CH2)lbSH; Fc = [45-CbHs]Fehl5-CsHsl) adsorbed on Au surfaces. Contact angles (d) decreased from 71o to 43o (A cos d = -0.40) when the electrical potential of the SAM was increased from 0.3 to 0.5 V (vs a Ag wire reference electrode) and then increased from 43o to 58o when the potential of the SAM was returned to 0.2 V. Repeated cycling between these values of the potential leads to a progressively decreasing response, as the Fc groups were destroyed by side reactions. Contact angles of aqueous solutions on SAMs formed from CH3(CHz)pSH decrease by only 2o (from -115o to 113o, A cos d = 0.05) over the same range of potentials (Sontag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992,8, 2560-2566). The contrast between the wettability of SAMs terminated with Fc and CH3 groups suggests that potential-dependent wetting of the former is caused primarily by the electrochemical oxidation of the electrically neutral, surface-confined Fc to the more polar and plausibly more wettable Fc+ cation. Linear sweep cyclic voltammetric measurements upport this hypothesis. Surfaces of gold patterned with SAMs formed from FcCO(CHz)rsSH and CHg(CH2)IsSH were used to construct a micrometer-scale "gate" that controls the flow of liquid down a surface.