Emersion process

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

These measurements have verified that the work function of an electrode, emersed with the double layer intact, depends only on the electrode potential and not on the electrode material or the state of the electrode (oxidized or covered with submonolayer amounts of a metal) [20]. Work function measurements on emersed electrodes do not serve the same purpose as in surface science investigations of the solid vacuum interface. At the electrochemical interface, any change of the work function by adsorption is compensated by a rearrangement of the electrochemical double layer in order to keep the applied potential i.e. overall work function, constant. Work function measurements, however, could well be used as a probe for the quality of the emersion process. Provided the accuracy of the measurement is good enough, a combination of electrochemical and UPS measurements may lead to a determination of the components of equation (4). 

As was discussed above, it is essential to determine the effect, if any, that the emersion process has on the double layer. To do this, Wilhelm and colleagues have performed the definitive type of blank experiment. CO was adsorbed onto the Pt working electrode from sulphuric acid electrolyte. After adsorption, the CO-saturated solution was replaced with pure electrolyte. The potential of the electrode was then ramped in order to oxidise off the adsorbate, as C02, and the voltammogram so obtained is shown in Figure 2.118(a). The experiment was then repeated CO was adsorbed as before, but the electrode was emersed and transferred into the UHV chamber, before being re-immersed and the potential ramp applied. The voltammogram so... 

Emersion of an electrode from electrolyte with its double layer intact is now a widely accepted phenomenon and technique. Not only is it a phenomenon which deserves careful consideration and study, but also a process which opens up a new set of experimental methods to the study of the electrochemical double layer. Electrode emersion involves the careful removal of an electrode from electrolyte under potentiostatic control, usually hydrophobically 11-5). When fairly concentrated electrolyte parts ("unzips") from the electrode surface during hydrophobic emersion, the double layer remains essentially intact on the electrode surface and no electrolyte outside the double layer remains. This phenctnenon is not due to the presence of organics or other impurities as seme have suggested. The emersion process works well with rigorously clean electrode surfaces (5). 

Double layer emersion continues to allow new ways of studying the electrochemical interphase. In some cases at least, the outer potential of the emersed electrode is nearly equal to the inner potential of the electrolyte. There is an intimate relation between the work function of emersed electrodes and absolute half-cell potentials. Emersion into UHV offers special insight into the emersion process and into double layer structure, partly because absolute work functions can be determined and are found to track the emersion potential with at most a constant shift. The data clearly call for answers to questions involving the most basic aspects of double layer theory, such as the role water plays in the structure and the change in of the electrode surface as the electrode goes frcm vacuum or air to solution. 

It has long been realized that the heat of adsorption can be calculated more accurately from determinations of heats of immersion than from equilibrium vapor pressures of adsorbates. Harkins and Boyd (8) and Jura and Harkins (10) have discussed the emersion process and have developed an expression for the enthalpy of desorption that is the negative of the one above. That the immersion process is equivalent to the process we are discussing can readily be shown with the aid of the following two-step process ... 

CHANGE OF MENISCUS SHAPE DURING IMMERSION AND EMERSION PROCESSES... 

Problem 2. A different problem appears when the material is set in place on the road and we want to avoid displacement of the binder by water. It is a matter of protecting the structure, if, for example, the laying is followed by rain. This is equivalent to the emersion process and the force of displacement is then the emersion adhesion tension. 

The interpretation of XPS data is not always straightforward as is exemplified by different conclusions drawn by different investigators for the same electrode reaction. These discrepancies can be overcome if certain standards for electrode preparation, emersion and transfer processes are developed. The effects of the relative complexity of the emersed electrochemical interface on XPS and UPS data analysis in terms of (electro)chemical shifts and work function changes have to be considered. 

Emersion has been shown to result in the retention of the double layer structure i.e, the structure including the outer Helmholtz layer. Thus, the electric double layer is characterised by the electrode potential, the surface charge on the metal and the chemical composition of the double layer itself. Surface resistivity measurements have shown that the surface charge is retained on emersion. In addition, the potential of the emersed electrode, , can be determined in the form of its work function, , since and represent the same quantity the electrochemical potential of the electrons in the metal. Figure 2.116 is from the work of Kotz et al. (1986) and shows the work function of a gold electrode emersed at various potentials from a perchloric acid solution the work function was determined from UVPES measurements. The linear plot, and the unit slope, are clear evidence that the potential drop across the double layer is retained before and after emersion. The chemical composition of the double layer can also be determined, using AES, and is consistent with the expected solvent and electrolyte. In practice, the double layer collapses unless (i) potentiostatic control is maintained up to the instant of emersion and (ii) no faradaic processes, such as 02 reduction, are allowed to occur after emersion. 

Therefore it is very important to complete the data obtained by (photo) electrochemical techniques with surface sensitive spectroscopic measurements. One promising possibility of gaining microscopic information on interfacial processes is the use of UHV surface science techniques. However due to the analysis requirements emersion of the samples from the electrolyte and transfer into UHV is necessary. During this procedure the semiconductor interface may change drastically. Alternatively the basic chemical and physical interactions of electrolyte components may be studied by adsorbing redox components on defined semiconductor surfaces thus simulating semiconductor/electrolyte junctions. 

When a polymer surface that has been kept in liquid water is emerged from liquid water and kept in dry condition, the reverse process to what is described in the water immersion occurs. This phenomenon is identical to what is described in the dry air situation above (Sec. 3) except that the starting point or the reference state is different. However, when this process is viewed as the recovery process of the decay of hydrophobicity, which took place when the sample was immersed in water the first time, there are significant differences in both the rate of change and the extent of recovery. This discrepancy is due to the fact that these processes are different processes, i.e., the driving force for the surface configuration change in the immersion and that in the emersion are completely different, and these processes are not a part of a reversible process. 

The influence of water immersion has little effect on very hydrophobic polymers consequently, the effect of emersion is nearly identical to the case without water immersion. The influence of water immersion becomes progressively greater with the increase of hydrophilicity. With highly hydrophilic polymers, the surface of water-immersed polymer is completely different from that of dry polymer, and consequently the emersion becomes drying process of wet polymers. 

Figure 26.9 A typical Wilhelmy force loop of a relatively hydrophobic surface the force loop is composed of three cycles each consisting of one immersion (advancing) and one emersion (receding) process. The sample plate is polycarbonate (PC) modified with TMS plasma.

Figure 26.10, and (d e) in Figure 26.11] until the force in the direction of the wet surface exceeds the adhesive tension of emersion, Te, at the polymer surface/water interface. After exceeding the adhesive tension, the three-phase contact line starts to move toward the prewetted surface, seen as (D E) in Figure 26.9 and (C D) in Figure 26.12. The adhesive tension in the receding process is usually less than that in the advancing process unless the surface was completely wetted by the first immersion (i.e., zero contact angle). When the complete wetting occurs on the first immersion, the first emersion line retraces the first immersion line, such is the case with O2 plasma-cleaned glass.

---