Electrostatic potential diffusion effect

Calculations of the variations expected in the fluorescent-yield (FY) profiles as a function of the distribution model parameters are shown in Figure 7.19. When the species of interest resides predominantly at the solid surface, the FY profile shows a maximum at the critical angle for total external reflection. As the ratio of the surface-bound species to the total number of species in the solution volume adjacent to the surface decreases, the FY distribution broadens at the low angles. A similar effect is noted when a diffuse layer accumulation arises due to an interfacial electrostatic potential. 

The influence of a cut-off relative to the full treatment of electrostatic interactions by Ewald summation on various water parameters has been investigated by Feller et al. [33], These authors performed simulations of pure water and water-DPPC bilayers and also compared the effect of different truncation methods. In the simulations with Ewald summation, the water polarization profiles were in excellent agreement with experimental values from determinations of the hydration force, while they were significantly higher when a cut-off was employed. In addition, the calculated electrostatic potential profile across the bilayer was in much better agreement with experimental values in case of infinite cut-off. However, the values of surface tension and diffusion coefficient of pure water deviated from experiment in the simulations with Ewald summation, pointing out the necessity to reparameterize the water model for use with Ewald summation. 

Fig. 3.6 The Gouy-Chapman model of the double layer, (a) Arrangement of the ions in a diffuse way (b) Variation of the electrostatic potential, 0, with distance, x, from the electrode, showing effect of ion concentration, c. (c) Variation of Cd with potential, showing the minimum at the point of zero charge Ez.

As a consequence of the selective adsorption of ions with a higher affinity for the stationary phase than their counterions electrostatic theories assume the formation of a surface potential between the bulk mobile phase and stationary phase. The adsorbed ions constitute a charged surface, to which is attracted a diffuse double layer of strongly and weakly bound oppositely charged ions equivalent in number to the adsorbed surface charges to maintain electrical neutrality. Because of repulsion effects the adsorbed ions are expected to be spaced evenly over the stationary phase surface and at a concentration that leaves the properties of the stationary phase largely unaltered except for its electrostatic potential. The transfer of solutes from the bulk mobile phase to the... 

The activity coefficient exponent, v- 1 -i- Ioiy, for the solute ion, Y y, influences the solute activity coefficient through the solution properties, kay, and the ion valence, Vy. These effects are the familiar Debye-Huckel behavior resulting from screening of the electrostatic potential of a solute ion by the diffuse counter ion layer. Since Z < 1.0, will vary inversely with the activity coefficient due to solute ion screening by neighboring solute ions. To understand this behavior better it is instructive to examine electrostatic potential screening of solute ions. 

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