Electrode hydrophilic

The capacity of the metal phase (CM) and the potential drop in the thin metal surface layer have been discussed by Amokrane and Badiali,122,348 as well as by Damaskin et a/.349 353 The value of was found to increase in the order Ga < In(Ga) < Tl(Ga) Hg if it was assumed that the capacity of a solvent monolayer C = const. The negative value of the surface charge density <7, at which the Cs,ff curve has a maximum, decreases in the order Ga > In(Ga) > Hg, i.e., as the hydrophilicity of the electrode decreases. 

Parsons-Zobel plots have been constructed for all the systems at a = 0 in the range 0.02 < c < 0.2 M. These plots were linear, with the value of the slope very close to unity. The values of C, obtained by extrapolation of the C-1, C plots to C a = 0 were in good agreement with those calculated from the C,E curves for the 0.1 M NaC104 + DMF system according to the GCSG model.358 The value of Q increases in the sequence of electrodes Hg < Tl(Ga) < In(Ga) < Ga as the hydrophilicity of the electrode surface rises. 

Good agreement between C(- and the dipole moment of the solvent (H20) molecules (i.e., by the hydrophilicity of metals) established by Trasatti25,31 was found and the reasons for this phenomenon were explained 428 The Valette and Hamelin data150 251 387-391 are in agreement with the data from quantum-chemical calculations of water adsorption at metal clusters 436-439 where for fee metals it was found that the electrode-H20 interaction increases as the interfacial density of atoms decreases. 

The temperature dependence of the electrical double-layer parameters has been determined for real393,398 as well as quasi-perfect Ag planes.382,394 For quasi-perfect Ag electrodes, the value of 3 ffa0/9rhas been found to be higher for Ag(100) than for Ag(lll), and so it was concluded that Ag(lll) is more hydrophilic than Ag(100). For real surfaces,382,385,386 dEff=0/BT increases in the order (110) < (100) <(111). The same order of planes has been observed for Au 446-448 BEa /BT linearly increases as AX (interfacial parameter) decreases, i.e., as the hydrophilicity of Ag and Au electrodes decreases.15 32 393 397 398 446 48... 

The AX sequence (111) < (100) < (110) in Table 28 has been questioned by Valette,389,399 who proposed (110) < (100) <(111). He also suggested252 the sequence Ag < Au < Cu rather than Au < Ag < Cu. It happens that these two different pictures have been obtained using the same experimental values of E0wq. In particular, data for exactly the same electrodes of Ag are used to arrive at different conclusions. It is clear that the controversy issues from a different concept of selection of values. Trasatti410 has discussed this point at length and has proven that Valette s hydrophilicity series for Cu, Ag, and Au is based on an inadequate choice of work function values. 

The puzzling point is that both sets of electrodes give the same values of Ea=o. Thus, on the basis of Effm0 vs. 0 correlations, the same conclusions should be reached. However, Popov et al. do not discuss AX values they arrive at their hydrophilicity scale on the basis of other parameters, which will be considered later on. If the situation is true, the two sets of electrodes can give different AX sequences only if different 0 values are involved. However, this would mean that surfaces of a given metal can have the same (7-0 but a different (P.This ambiguous situation has been pointed out by Trasatti32 in a recent paper and calls for further study. 

Most electrode materials are hydrophilic and readily wetted by aqueous solutions. Two methods are used to create and maintain an optimum gas/solution ratio in the electrode. The first method employs a certain excess gas pressure in the gas space. This causes the liquid to be displaced from the wider pores in finer pores the liquid continues to be retained by capillary forces. The second method employs partial wetproofing of tfie electrode by the introduction of hydrophobic materials (e.g., fine PTFE particles). Tfien the electrolyte will penetrate only those pores in the hydrophilic electrode material where the concentration of hydrophobic particles is low. 

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

Emersion resulting in substantial amounts of electrolyte remaining on the (hydrophilic) electrode is much more commonly observed. When the solvent evaporates from the surface, the electrolyte is left behind as small crystallites. These can distort LEED and RHEED patterns and, more importantly, can render quantitative evaluation of the surface cation and anion concentrations by ESCA impossible. 

The electrochemistry of Mb has been achieved by using mercury electrodes [93], methyl-viologen-modified gold electrodes [94], and ultraclean and hydrophilic indium... 

M. Tominaga, T. Kumagai, S. Takita, and I. Taniguchi, Effect of surface hydrophilicity of an indium oxide electrode on direct electron transfer of myoglobins. Chem. Lett. 10, 1771-1774 (1993). 

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

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