Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 88

Both the Knox equation and the Van Deemter equation predict a linear relationship between the value of the (B) term (the longitudinal diffusion term) and solute diffusivity. A plot of the (B) term against diffusivity for benzyl acetate and hexamethylbenzene is shown in figure 23. It is seen that the predicted linear relationship is indeed realized. However, it can be shown that the values for the (B) term from the Knox equation curve fit also give a linear relationship with solute diffusivity so the linear curves shown in figure 23 do not exclusively support the Van Deemter equation.

The stationary phase, for a silica gel column would consist almost entirely of that portion of the 'mobile phase' that is 'static' and trapped in the pores of the silica gel. As a consequence, in equation (49) it would appear reasonable to assume that,

                                               Dm = Ds


However, this will be an approximation as, if a binary mixture of solvents is used for the mobile phase, then a layer of the more polar solvent would be adsorbed on the surface of the silica gel (25) and the mean composition of the solvent in the pores of the silica gel would not be exactly the same as that of the mobile phase. Nevertheless, it would be reasonable to assume that,