Triple-layer model site-binding

The discussion above pertains to the diprotic acid chemical model and the constant capacitance electrostatic model. It is interesting to note that in some applications of the triple layer model with site binding of electrolyte ions at the IHP, the... 

The original Smit model separates the surface plane into two sections one fiuction [i.e., 1 — /], of the overall surface (respectively the fraction of the surface sites) where only uncomplexed surface groups are present, and another fraction /, where only ion pairs formed with the electrolyte. For both sections, a different electrostatic model concept is introduced a Stem model (obviously without electrolyte binding) for the fraction 1 —/ and a triple-layer model for the fraction /. This separation is, of course, artificial. A mean value of the zeta potential is calculated from the equation given in Fig. 17i. Application of the model to experimental surface-charge data requires very low values for C2 One advantage of this model can be seen in the closer agreement of the model with the experimental observations quoted by Smit. 

The method is based on the chemical equilibrium program MINEQL (2), modified to include the coulombic energy of adsorption caused by the charged surface. First the principles of MINEQL are presented through a short example from solution chemistry and then the extension of the method to the constant-capacitance double-layer model of the surface/solution interface used by Stumm, Schindler, and co-workers (3,4) is demonstrated. Finally, the use of the method in the triple-layer site-binding model introduced by Yates, Levine, and Healy (5), and used by Davis, James, and Leckie (6), is shown. In each case the mathematics are described in suflBcient detail to be reproducible. 

Finally, we demonstrate the application of our general formulation of surface/solution equilibria to a more involved model of the surface/ solution interface, the triple-layer site-binding model of Yates, Levine, and Healy. Again we discuss the principles of the method using simple hydrolysis equilibria, but the extension to more complicated equilibria is straightforward. 

The triple-layer site-binding model now fits within the scheme of the general equilibrium problem given in Equations 1-3. Other adsorbed cations and anions can be included in the equilibria simply by adding the appropriate components and species. 

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