Electroneutral surface layers

Model of electroneutral surface layers of ionised molecules... 

The advantages of the electroneutral surface layer model presented above can be regarded also as a deficiency, because this model cannot describe the structure of the surface layer, electric potential of the surface, etc. In addition, no satisfactory analysis of the adsorption of proteins... 

Let us consider now the case when a solution contains a mixture of two anionic (or cationic) surfactants (for example, homologues RiX and R2X with a eommon eounterion X ) with addition of inorganic electrolyte XY. In such systems the counterion concentration is given by the sum of concentrations of RiX, R2X and XY. For simplicity, the saturation adsorptions of the two homologues will be taken as equal, i.e., o)ix= o)2x=2too. After consideration of the surface-to-bulk distribution of both electroneutral combinations of ions, the surface layer equation of state for the Frumkin-type non-ideality of a mixture of two ionic surfactants can be written in a form similar to Eq. (2.35), where it is assumed that l/tO, = Corresponding... 

It was shown in [77] that, irrespective of the electrolyte concentration, for a surface coverage exceeding 20%, the portion of adsorbed surface active l l-charged ions bonded to counterions in the S-H layer is approximately 90%, that is, the surface layer is almost electroneutral. The calculations performed in [80] for sodium tetradecyl sulphate solutions, with or without added NaCl, revealed that the extent of bonded counterions, while being somewhat lower than that reported in [77], is quite high, and amounts up to 80% for concentrated solutions. These results... 

As the surface layer is electroneutral, and therefore X j = JP y, then from Eqs (12) and (40) for nonideal (Frumkin) surface layers and nonideal bulk solutions of one ionic surfactant, wifli or without additional nonsurface active electrolyte, the adsorption isotherm follows... 

It was shown in Refs 75 and 80 that the portion of adsorbed siuface active 1 1 charged ions which becomes bound to the counterions within the S-H layer is approximately 70—90%, that is, the surface layer is almost electroneutral. These results explain why Lucassen-Reynders theory can be successfully applied to those systems. It can be shown additionally that for compositions of ion the effect produced by the DEL vanishes. For compositions of ions Eq. (55) can be presented in the form ... 

These reaction formulae indicate that the electron transfer taking place at the metal I polymer interface is accompanied by ionic charge transfer at the polymer Isolation interface, in order to maintain the electroneutrality within the polymer phase. Counterions usually enter the polymer phase, as shown above. However, less frequently the electroneutrality is established by the movement of co-ions present in the polymer phase, e.g., in so-called self-doped polymers. Oxidation reactions are often accompanied by deprotonation reactions, and H+ ions leave the film, removing the excess positive charge from the surface layer. It should also be mentioned that simultaneous electron and ion transfer is also typical of electrochemical insertion reactions however, this case is somewhat different since the ions do not have lattice places in the conducting polymers, and both cations and anions may be present in the polymer phase without any electrode reaction occurring. The es-... 

However, attention has to be paid not only to the electronic charging of the polymer film (i.e., to the electron exchange at the metal/polymer interface and the electron transport through the surface layer) since, in order to preserve electroneutrality within the film, ions will cross the film/solution interface. The motion of counterions (or less frequently that of the coions) may also be the rate-determining step. The thermodynamic equilibrium between the polymer phase and the contacting solutions requires fi (film) = /zj (solution) for all mobile species. In fact, we may regard our film as a membrane or a swollen polyelectrolyte gel (i.e., the... 

At any interface between two different phases there will be a redistribution of charge in each phase at the interface with a consequent loss of its electroneutrality, although the interface as a whole remains electrically neutral. (Bockris considers an interface to be sharp and definite to within an atomic layer, whereas an interphase is less sharply defined and may extend from at least two molecular diameters to tens of thousands of nanometres the interphase may be regarded as the region between the two phases in which the properties have not yet reached those of the bulk of either phase .) In the simplest case the interface between a metal and a solution could be visualised as a line of excess electrons at the surface of the metal and an equal number of positive charges in the solution that are in contact with the metal (Fig. 20.2). Thus although each phase has an excess charge the interface as a whole is electrically neutral. 

Since Bis via Gauss s Law of electrodynamics proportional to the local excess free charge it follows that the term fjeV VGj is proportional to the net charge stored in the metal in region G. This net charge, however, was shown above to be zero, due to the electroneutrality of the backspillover-formed effective double layer at the metal/gas interface and thus Dfje w.Gj must also vanish. Consequently Eq. (5.47) takes the same form with Eq. (5.19) where, now, O stands for the average surface work function. The same holds for Eq. (5.18). 

FIG. 9 Schematic illustration of adsorption of poly(styrenesulfonate) on an oppositely charged surface. For an amphiphile surface in pure water or in simple electrolyte solutions, dissociation of charged groups leads to buildup of a classical double layer, (a) In the initial stage of adsorption, the polymer forms stoichiometric ion pairs and the layer becomes electroneutral, (b) At higher polyion concentrations, a process of restructuring of the adsorbed polymer builds a new double layer by additional binding of the polymer. 

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. 

On the basis of crystallochemistry consideration and taking into account electron microscopy observations of the surface of crystals upon which some polymer was formed,99 Arlman and Cossee13 concluded that the active sites are located on crystal surfaces different from the basal (001) ones. In particular, these authors considered in detail active sites located on crystal surfaces parallel in the direction a — b of the unit cell defined as in Ref. 98. Figure 1.13 illustrates that, if we cut a TiCl3 layer parallel to the direction defined above, which corresponds to the line connecting two bridged Ti atoms, electroneutrality conditions impose that each Ti atom at the surface of the cut be bonded... 

Grahame derived an equation between a and based on the Gouy-Chapman theory. We can deduce the equation easily from the so-called electroneutrality condition. This condition demands that the total charge, i.e. the surface charge plus the charge of the ions in the whole double layer, must be zero. The total charge in the double layer is /0°° pe dx and we get [59]... 

The arrows above and the symbols below the interfaces indicate the transfer of the charge at each interface when the concentration of NaF in the sample is abruptly increased. It is possible to estimate the actual number of ions that are required to establish the potential difference at the interfaces. A typical value for the doublelayer capacitor is 10 5 F cm 2. If a potential difference of n = 100 mV is established at this interface, the double-layer capacitor must be charged by the charge Q = nCdi = 10 6 coulombs. From Faraday s law (6.3), we see that it corresponds to approximately 10 11 mol cm 2 or 1012 ions cm 2 of the electrode surface area. Thus, a finite amount of the potential determining ions is removed from the sample but this charge is replenished through the liquid junction, in order to maintain electroneutrality. 

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