Effect of Oil Droplet Coexistence upon Potential-Dependent Phase Change of Surfactant Adlayer on Gold Electrode: Interplay of Hexadecane Droplets and Dodecyl Sulfate Adlayer
2018
Morooka, Tetsuro | Sagara, Takamasa
When a small immiscible oil droplet is placed on an electrode surface in a surfactant aqueous solution, a drastic decrease in the interfacial differential capacitance of the electrode is observed. To clarify this behavior at a molecular level, interplay of an n-hexadecane (HD) droplet with a dodecyl sulfate anion (DS–) adlayer on an Au electrode was described using the results of the measurements of cyclic voltammograms, potential-dependent contact angles, and attenuated total reflection surface-enhanced infrared reflection absorption spectra (ATR-SEIRAS). When DS– alone adsorbs on a Au(1 1 1) electrode, it forms an adlayer of a hemi-micellar structure, which changes to an interdigitated bilayer at positive potentials. When HD alone is present on the electrode surface as droplets, it does not spread as an oil liquid film, but an ordered monolayer of HD is formed around the droplets as found in the present ATR-SEIRAS study. When coexisting, 1 μL droplet of HD showed a stronger tendency to spread on the Au(1 1 1) electrode surface than when HD alone is present, because DS– adsorption took place at both HD/water and electrode/water interfaces. When the electrode surface with many HD microdroplets (ϕ <50 μm) was immersed in Na-DS (SDS) solution, we found that the formation of a mixed adlayer consisting of HD and DS– significantly lowered the interfacial differential capacitance down to 5 μF cm–². ATR-SEIRAS revealed that the alkyl phase of this mixed adlayer was more liquid-like than the DS– adlayer alone and more solid-like than the HD droplet alone. At positive potentials, the interdigitated bilayer of DS– was formed regardless of whether HD droplets were present on the electrode surface. At very positive potentials, a HD droplet can spread over the DS– adlayer.
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