Constant charge surfaces

The assessment of surface area may sometimes present difficulties, e.g. with smectites N2 adsorption measurements grossly underestimate the area that is exposed in solution when the layers are fully expanded. In an attempt to overcome this problem a simple theoretical model has recently been developed (j4) for deriving double-layer potentials for the clay-solution interface from co-ion exclusion measurements. The results of this work suggest that the surfaces of montmorlllonlte and illite have constant potentials and do not behave like constant-charge surfaces as is generally assumed. 

A computer program which evaluates the interaction of two charged plates in a 2 1 1 electrolyte solution for either case of constant charge surfaces or constant potential surfaces as a function of their separation is available from the author. 

Figure 3.23. Schematic of electric potential distribution for two electrolyte concentrations as a function of distance from a constant charge surface (from Dixon and Weed, 1977, with permission).

Constant Charge Surface pH Dependent Charge Surface... 

Denitrification, 340,473 Desorption, 221,362 Diffuse double layer, 141 Model, 142-146 Thickness, 145 Variable-charge surfaces, 146 Constant-charge surfaces, 143-146 Diffusion, 298, 398 Film diffusion, 398 Particle diffusion, 398 Solution diffusion, 398 Dioctahedral silicates, 122 Dispersion, 367... 

For constant charge, surfaces with carges of equal sign repel each other at all distances. For constant potential surfaces of equal sign in potential also repel each other at large distances. If the surface potentials are not equal for very short distance the force becomes attractive. (Figure 4.8) [426]. 

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