Electrochemistry and contact angles of an ionic liquid sessile droplet on films of monolayer-protected au nanoparticles

An unwalled electrochemical cell formed by a sessile droplet of 1-butyl-3-methylimidazolium hexafluorophosphate resting on electroactive surfacesself-assembled alkanethiolate monolayers, films of Au nanoparticles and of ferrocene cast from evaporated solution dropletsis used to investigate the voltammetry and surface energy of these electrode/ionic liquid interfaces. Auxiliary and quasi-reference 25-mu m-diameter Pt wire electrodes are inserted into the top of the sessile droplet. The cyclic voltammetry of a film of hexanethiolate-protected Au-140 nanoparticles shows that they undergo quantized double layer charging at the ionic liquid interface, accompanied by insertion of hexafluorophosphate anions into the nanoparticle film. The contact angle of the ionic liquid droplet resting on the nanoparticle film is observed to decrease when the gold electrode potential is controlled to oxidize the interface to the Au-140(1+) state. The increase in surface charge and the alteration in chemical composition of the oxidized Au-140 film surface combine to cause a decrease in surface tension at the interface and the ionic liquid to become more wetting of it. Analogous behavior is seen with ferrocene films. In both cases, the changes in contact angles are not reversed upon returning potentials to reducing values. Reductive charging of the Au-140 surface does not occur; the imidazolium cation does not insert. Films of Au-38 nanoparticles do not show voltammetric currents for oxidation of the surface to Au-38(1+) and, correspondingly, no changes in contact angle.