Effect of bromide adsorption on electrowetting of Au electrode with hexadecane

- Contact angle (θ) change of an n-hexadecane droplet was tracked in water.
- Br− adsorption lowered the Au(1 1 1) electrode/water (S/W) interfacial tension.
- Br− adsorption raised θ of the droplets of 1 μL and < ca. 33 pL.
- A plot of θ vs. surface charge enabled us to pinpoint Br− desorption potential.

Electrowetting of a Au/aqueous solution interface with hexadecane (HD) was largely affected by specific adsorption of Br− on the Au surface. The adsorption distinctly changed the potential dependence of the contact angle (θ) of HD droplets on a Au(1 1 1) electrode surface in line with electrocapillary relationship. Measurements of θ as a macroscopic observable, being sensitive to the atomic level change of the electrode surface such as surface reconstruction and Br− adsorption, allowed us to monitor the state of the Au/aqueous solution interface as demonstrated by pinpointing the Br− desorption potential. The amplitude of potential-controlled reshaping of a HD 1.0 μL droplet was enhanced by specific adsorption of Br−. The specific adsorption also affected HD microdroplets (<50 μm diameter ≈ 33 pL volume) in the same way as 1.0 μL droplets as revealed by in situ electrochemical fluorescence imaging measurements. Overall, ionic adsorption provides us with opportunities of fine control of the dynamics of oil droplet in an electrode potential range narrower than 1.5 V.

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