Potential-dependent adsorption equilibrium

Sometimes, the amount of species adsorbed from a solution onto an adsorbent, e.g. activated carbon, depends on the eiectrical potential applied to the adsorbent the amount adsorbed at equilibrium depends on the potential applied. This equilibrium phenomenon of potential dependent adsorption could provide a basis for potentialswing adsorption. Bisinger and Keiier (1990) have utilized such a phenomenon (see Zabasajja and Savinell (1989) for electrosorption of alcohols on graphite surfaces and references to earlier smdies) to develop a potential-swing adsorption process. They have experimentally studied... 

When the electrode interface is in the state of Fermi level pinning, however, the potential of the compact layer changes with the electrode potential hence, the equilibrium of the adsorption-desorption of protons on semiconductor electrodes depends on the electrode potential in the same way as that on metal electrodes. 

It was assumed that the equilibrium follows a Langmuir isotherm with competitive adsorption of ions (the coverages of which are potential dependent), then the rate of reduction was formulated as... 

Poskus and Agafonovas [483] have applied radioactive Tl-204 to study its UPD on a polycrystalline gold electrode in alkaline solutions. The potential dependence of the equilibrium surface concentration obtained from the radiometric method has been compared to that calculated from CV. Surface concentration of Tl decreased monotonically as the potential was changed from the more positive Nern-stian values. This dependence exhibited a minimum without reaching zero. At more positive potentials (with respect to the minimum), adsorption of T1+ induced by specifically adsorbed hydroxyl anions occurred. 

For the flat band potential situation, i.e. at E = Epb and for eAtpsc = e(Es — Eb) = 0, one obtains an appropriate relation for the Fermi level of the oxide Ep,ox in dependence of pb and the potential drop in the Helmholtz layer Es — Eso (Eq. (23)). The potential drop Aadsorption equilibrium at the oxide surface, i.e. from its isoelectric point. The flat band potential Epb may be determined by extrapolation of the potential dependence of the photocurrent as will be shown in Fig. 40 of Section 6.2 for passivating CU2O on Cu. With these data the positions of the energy bands of Fig. 39 have been determined, however with the assumption of an energy difference of the Fermi level from the conduction or the valence band of 0.25 eV, respectively. For the anodic oxides of Cu, the position of the bands has been determined independently by UPS measurements (Section 6.2). 

The adsorption capacitance is related to the potential dependence of the equilibrium surface excesses ... 

Potential dependent halide adsorption, including order-disorder phenomena, is well known [134, 246, 247], and it is reasonable to expect that the breakdown of the PEG-C1 blocking layer might also be potential dependent. In a similar fashion, the composition and structure of thiols and disulfides adsorbed on gold from simple electrolytes have been shown to be potential dependent [282]. In the present example, it is also possible that the approach of SPS to the electrode surfaces is screened by complexation with the potential dependent concentration of Cu+ that is generated at the electrode. Importantly, the equilibrium Cu+ concentration in the additive-free system (i.e. Equations 2.1 and 2.2) is of the order of 400 (tmol L 1 and Cu+ is known to form complexes with all the additives under consideration [239, 279, 280, 283-285]. Furthermore, the equilibrium Cu+ concentration decreases exponentially with potential, that is, 60 mV per decade of concentration [283-285]. Thus, the increasing rate of SPS adsorption with... 

In order to know the potential dependence of the adenine adsorption, AR/Ro at adsorption equilibrium was obtained from the R/Ro versus t measurement by a potential step method with the solutions containing different concentrations of adenine. The potential was first set at —0.1 V,... 

The preceding sections pertain to equilibrium conditions of electrosorption of a reactant which adsorbs without a simultaneous charge transfer. Under the latter conditions, or in the case of intermediates of an electrode reaction adsorbed on the electrode, adsorption isotherms contain additional potential dependent terms. 

The model predicts Oi.arf occupies the overwhelming number of sites and the number of empty positions is small for a sufficiently large range of potentials around the equilibrium at 1.2 V. The empty sites show a reasonable dependence of the O2 concentration. Below 0.5 V adsorption is decreasing exponentially and the electron reaction rate is saturating accordingly (see Figure 8.3). 

Parallel events in the field of in situ IR spectroscopy (for a review of sulfate IR studies, see Ref. 26) resulted in a coupled shift to molecular level bi-sulfate anion was recognized in sulfate adlayer even in solutions with predominating sulfate. It was com-pletey new situation, when chemical equilibrium is affected by adsoibate-surface interactioa In usual terms of solution equilibria, the effect corresponds to increase of pKa from its bulk value (ca. 2) to 3.3-4.7 (pKa is potential-dependent). To agree this situation with bulk thermodynamics, one should simply use electrochemical potential instead of chemical. The phenomena of adsorption-induced protonation is relative to UPD, when adsoibate-surface interaction shifts redox equihbria. In more molecular terms, the species determined as bi-sidfate ions are probably interfacial ion pairs, i.e., the phenomenon can be considered as coadsorptioa This situation is screened in purely thermodynamic analysis, as excess surface protonation is hidden in Gibbs adsorptions of sulfate and H. However it becomes important for any further model consideration, as it can affect lateral interactions and the order in the adlayer. The excess adsorption-induced protonation of various anions is a very attractive field. In particular it is the only chance to explain why multicharged oxoanions can form complete mono-layers on platinum. 

Flildebrandt and Stockburger found that there are two conformational states of cyt. c upon adsorption on a silver electrode [38-40]. Cytochrome c adsorbed on the silver electrode at negative potentials (<0.04 V) exhibits the state I (cyt. C ) conformation and that adsorbed at positive electrode potentials (>0.04 V) exhibits the state II (cyt. cn) conformation. The structure of the RR spectrum of cyt. c in the solution is fully maintained in the state I, which must be the same for the whole cyt. c molecules. In the state II, the spin state of the heme iron is in the mixed 5cHS and 6c LS conformation. The conformational states I and II are at potential-dependent equilibrium. The most stable species of ferro-cyt. c is the reduced form of state I (cyt. ci +) in the electrode potential range, more negative than 0.2 V and that of ferri-cyt. c is the oxidized form of state II (cyt. cn +) in the electrode potential range more positive... 

The shape of the C-E curves measured by the a.c. method and, in particular, the height of the peaks depend on the frequency, since at sufficiently high frequencies the establishment of the adsorption equilibrium lags behind the change in the potential [3]. 

For the calculation of o one proceeds from the thermodynamic condition for the sorption equilibrium. The adsorption equilibrium is characterised, as mentioned earlier, by the condition that the chemical potential jjL(T,p,x) for the substance dissolved in water and for the substance dissolved in the plastic geomembrane are equal. The dependence of the chemical potential of the dissolved substance on the concentration can formally be written as ... 

Chapter 2 provides a simple formula for calculating the basic forces or potentials for adsorption. Thus, one can compare the adsorption potentials of two different molecules on the same site, or that of the same molecule on two different sites. The calculation of pore size distribution from a single adsorption isotherm is shown in Chapter 4. The effects of pore size and shape on adsorption are discussed in both Chapters 2 and 4. Chapter 3 aims to provide rules for sorbent selection. Sorbent selection is a complex problem because it also depends on the adsorption cycle and the form of sorbent (e.g., granules, powder, or monolith) that are to be used. The attributes sought in a sorbent are capacity, selectivity, regenerability, kinetics, and cost. Hence, Chapter 3 also includes a summary of equilibrium isotherms, diffusion steps, and cyclic processes. Simple sorbent selection criteria are also presented. 

However, Km is depended on interfacial tension as well as surface potential and cannot be used as a measure of the formation of bimolecular enzyme-substrate complex. The apparent Km value of pancreatic lipase for triglycerides strongly depends on physical properties of the interface, and, respectively on adsorbed state of the lipase. Sometimes Km is determined as a reverse value of the constant of adsorption equilibrium Km = 1/K. 

---