Stem model, potential changes

For present purposes, the electrical double-layer is represented in terms of Stem s model (Figure 5.8) wherein the double-layer is divided into two parts separated by a plane (Stem plane) located at a distance of about one hydrated-ion radius from the surface. The potential changes from xj/o (surface) to x/s8 (Stem potential) in the Stem layer and decays to zero in the diffuse double-layer quantitative treatment of the diffuse double-layer follows the Gouy-Chapman theory(16,17 ... 

In fact an inner layer exists because ions are not really point charges and an ion can approach a surface only to the extent allowed by its hydration sphere. The Stern model specifically incorporates a layer of specifically adsorbed ions bounded by a plane, the Stem plane see Figure 13 and refs. 14, 31, and 45). In this case the potential changes from ij/ at the surface, to ilf 8) at the Stern plane, to (/f = 0 in bulk solution. 

The most speculative of the relationships in the original model is the viscosity factor (Vi)—an oversimplification that accounts for a combination of effects. The inclusion in the model of the factor stem cell potentiation to allow for changes in the sensitivity of erythropoietin places less dependence on the viscosity factor to produce a reasonable fit between model response and experimental data. In the original model the viscosity factor took effect when hematocrit was greater than 0.5 (Hctk) and resulted in a viscosity factor (Vi) of 0.8 when the hematocrit equaled 0.75. In the present model Hctk was set at 0.6 so that no viscosity effect occurs until the hematocrit is greater than 0.6. In addition, Ke in Equation 5 is set to produce a viscosity factor of 0.9 when the hematocrit is 0.75. 

Farkas A, Coker SJ (2002) Limited induction of torsade de pointes by terikalant and erythromycin in an in vivo model. Eur J Pharmacol 449 143-153 Gibson JK, Bronson J, Palmer C, Numann R (2014) Adult human stem cell-derived cardiomyocytes detect drug-mediated changes on action potentials. J Pharmacol Toxicol Methods 70 255-267... 

The description of the double layer properties by the Stem-Gouy model is a very crude one. A veiy weak point is the assumption that the dielectric contact suddenly changes from that of the solution to that of the Helmholtz double layer. The main information comes, therefore, from the minimum which indicates the potential of zero excess charge on the metal. This is, however, only correct in the absence of specific adsorption of ions. If ions are adsorbed, the counter charge for the diffuse double layer is the sum of the surface charge in the metal and of the adsorbed ions. Since the concentration of adsorbed ions also varies with the applied potential, this effect increases the apparent capacity of the Helmholtz double layer. 

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