I have investigated the Au electrode/solution interface in the coexistence of n-hexadecane (HD) and dodecyl sulfate anion (DS−). In addition to the voltammetric and potential-dependent contact angle () measurements at a Au(1 1 1) single crystal electrode, in situ ATR-SEIRAS measurements were carried out at a Au thin film electrode fabricated by electroless deposition. I have revealed new potential-dependent behavior of a mixture of both HD and DS− on Au electrode at a molecular view. They form a new mixed adlayer on the electrode surface around pzc, not removing from the surface by forming micelles.
Although ionic surfactant-alkane conjugation has been found at a gas/liquid interface [46, 47], the findings in this work at an electrode/solution interface are unprecedented.
In the potential region-I, where formation of DS− hemi-micelle adlayer takes place, differential capacitance was lowered to < 5 F cm−2 under coexistence of HD microdroplets deposited by Procedure A. This result indicates the formation of a new structure of interactive HD + DS− adlayer at the electrode/water interface. In situ SEIRAS experiments shows formation of disordered, liquid-like adlayer consisting of HD and DS− in the potential region-I. When HD coexists, the formation of hemi-micelles of DS− never takes place in the
113 potential region-I, but the formation of the interdigitated bilayer composed only of DS− does at more positive Es than the potential of _ peak. When HD exists on the electrode surface alone, HD forms its microdroplets and surrounding ordered monolayer. In the measurements under potential control, the shape change of the droplet was affected by the adsorption of DS− at HD/water interface and the formation of adlayer at the electrode/water interface around the HD droplet.
In summary, a mixed adlayer of HD and DS− significantly decreases the electrode capacitance at the potential region-I, and at more positive potentials, the formation of interdigitated bilayer still takes place with coexistence of HD on same Au electrode surface.
Also, HD droplet can spread over the adlayer of DS−.
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118
Chapter 7
Potential-Dependent Contact Angle Change of Fluoro Liquid Droplet on Gold Electrode
ABSTRACT. In this chapter, the electrowetting of a hydrofluoroester solvent of a low adhesion energy, Novec 7100™, was examined as a liquid droplet on a Au(111) electrode.
No hysteresis was observed for Novec 7100 in a cosE curve obtained by potential step measurement in contrast to conventional organic solvent. The value of of Novec 7100 droplet increased over pzc in contrast of HD which does not change the shape over pzc.
These results show that the adhesion energy of a liquid droplet was found to play an important role on the electrowetting behavior. When with Br adsorption, the maximum potential of cos-E curve of Novec 7100 was shifted to the negative potential in line with the electrocapillary curve with specific adsorption. In addition, to evaluate the adhesion energy of HD droplet, the E curve obtained from M–E curve was compared to the cos–E curve in positive potential step measurements. It was concluded that the difference of S/W over 6 mN/m needs to exceed the adhesion energy of HD droplet to Au(111) surface.
119 7.1. Introduction
The focus on this chapter is to study the effect of adhesion energy on the electrowetting.
In Chapter 5, I studied the electrowetting of HD liquid droplet on Au(1 1 1) electrode. The contact angle () of a HD droplet changes as a function of the electrode potential described by an electrocapillary equation. Specific adsorption of Br anion as the atomic level surface structure change on the electrode could be a driving force of the electrowetting. The surface charge density of the electrode drastically changes by the specific adsorption. As shown in Fig. 5-1-c and Fig. 5-5, there are plateau regions which do not occurred change observed for HD droplet on Au(1 1 1) electrode surface. These regions will be related to the adhesion and stiction energy between the liquid droplet and the electrode surface [1]. It is known that the surface pressure of water thin layer causes the difference between the experimental value observed by the measurement and the theoretical value of Hg surface pressure [2, 3].
I used a hydrofluoroether solvent (Novec 7100™) as a liquid droplet to perform the electrowetting on Au(1 1 1) electrode. When using Novec 7100 with low adhesion energy, the -E curve observed will have narrower plateau region and show increasing at the positive potential region.