Concentration changes during film

Fig. 2. Concentration changes of MB under UV-A (left) and fluorescent light (right) by untreated and H2+Ar plasma treated Xi02 films (blank refers to the reference line for the calibration of concentration changes due to the evaporation of water by lights during the measurements)...

Qualitatively, this picture is relatively simple to understand in terms of the mobilities of the anions involved. Quantitatively, the picture is rather complex, particularly when the ion sizes are not too different. In the case of p-toluenesulfonate (with a small solution cation), anion transfer is the thermodynamically preferred process, but the greater mobility of a small cation makes the latter transfer kineticaUy more facile. Consequently, the dominant process is timescale dependent. Furthermore, as indicated in Sect. 2.7.3.7.1, the presence of salt within the film is electrolyte concentration dependent. Cations (coions) can only be ejected from the fihn if they are present in the first place, that is, if the film is nonpermselective. The complexities of this process have only recently been unraveled [149, 150) and the EQCM response (film composition) is dependent upon the experimental timescale (e.g. voltammetric scan rate), film history (first or subsequent redox cycle after equi-hbration), and electrolyte concentration. Fihn mass changes during redox cycling are nonmonotonic on short timescales cation (sodium) transfer is the predominant mode of satisfying electroneutrahty... 

The macropore diffusion of nc adsorbates is described by the Maxwell-Stefan equation as learnt in Chapter 8 (Section 8.8). The micropore diffusion in crystal is activated and is described by eq. (10.6-11), and the adsorption process at the micropore mouth is assumed to be very fast compared to diffusion so that local equilibrium is established at the mouth. Adsorption and desorption of adsorbates are associated with heat release which in turn causes a rise or drop in temperature of the pellet. We shall assume that the thermal conductivity of the pellet is large such that the pellet temperature is uniform and all the heat transfer resistance is located at the thin film surrounding the pellet. How large the pellet temperature will change during the course of adsorption depends on the interplay between the rate of adsorption, the heat of adsorption and the rate of heat dissipation to the surrounding. But the rate of adsorption at any given time depends on the temperature. Thus the mass and heat balances are coupled and therefore their balance equations must be solved simultaneously for the proper description of concentration and temperature evolution. 

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