Desorption potential

Under these conditions rather low limiting cnrrents arise that are independent of potential np to the desorption potential of the organic snbstance. This effect can be explained in terms of the difficulties encountered by the reactant metal ions when, in penetrating from the bulk solution to the electrode surface, they cross the adsorbed layer. 

Electroneutral substances that are less polar than the solvent and also those that exhibit a tendency to interact chemically with the electrode surface, e.g. substances containing sulphur (thiourea, etc.), are adsorbed on the electrode. During adsorption, solvent molecules in the compact layer are replaced by molecules of the adsorbed substance, called surface-active substance (surfactant).t The effect of adsorption on the individual electrocapillary terms can best be expressed in terms of the difference of these quantities for the original (base) electrolyte and for the same electrolyte in the presence of surfactants. Figure 4.7 schematically depicts this dependence for the interfacial tension, surface electrode charge and differential capacity and also the dependence of the surface excess on the potential. It can be seen that, at sufficiently positive or negative potentials, the surfactant is completely desorbed from the electrode. The strong electric field leads to replacement of the less polar particles of the surface-active substance by polar solvent molecules. The desorption potentials are characterized by sharp peaks on the differential capacity curves. 

The measured desorption capacities are too poor to design a separation process based on these materials. Further investigations could be made adding co-solvents to CO2 to achieve a higher desorption potential. 

The distribution coefficient is a measure of the sorption/desorption potential and characterizes flic tendency of an organic compound to be sorbed to the aquifer matrix. The... 

At desorption potentials the adsorption-desorption (tensammetric) peaks appear on the C-E curves, resulting from sudden changes of the surface charges and/or surface coverage within a narrow potential range. 

The reductive desorption has received particular attention for the following reasons. First, we can calculate the number of thiols adsorbed on the electrode surface from the desorption charge, even if the SAM does not have an electroactive functional group. Second, the adsorption states of the thiol can also be characterized from the desorption potential, which has been found to be strongly dependent on the identity of the thiol, packing state, surface crystallinity of the substrate, and several other factors. The first application... 

The mpe was determined for the reductive desorption of alkanethiol SAMs of various chain lengths from hexanethiol (C6SH)toC18SHina0.1 M KOH/ethanol solution. It linearly increases with the chain length with a slope close to the mass of CH2, as expected for a one-electron process. However, the mpe is much smaller (by 148 g mol ) than the molar mass of the desorbed alkanethiol. This difference is attributed to the simultaneous adsorption of cation species from solution, which is supported by the observed cation dependence of mpe and by the fact that the pzc of a bare gold electrode is more positive than the desorption potential. 

From a detailed comparison of the peak areas and the STM images, the minimum size of the domain that behaves as a two-dimensional bulk phase is about 15 nm or 50 thiolate molecules in the case of binary SAM of HDT and MPA [17]. A domain that consists of less than 50 molecules has a desorption potential in the middle of the two peaks corresponding to the desorption of phase-separated two types of domains. 

Of the two types of domains in a phase-separated binary SAM, the domains having less negative desorption potential can be selectively removed from the surface by holding the electrode potential at a value between the two reduction potentials. 

Fig. 11 Fluorescence excitation spectra (p-polarized incident light) at the adsorption potential ( ads) and at the desorption potential ( des) for single, double and triple touch layers of 12-AS adsorbed onto Au(l 11)...

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