Liquid Chromatography - Solvent/Solute Interactions with the Slica Gel Surface 2
The adsorption isotherms of the more polar solvents, ethyl acetate, isopropanol and tetrahydrofuran from n-heptane solutions on to the silica gel surface did not fit the simple mono-layer adsorption equation but did fit the bi-layer adsorption isotherm which is a simple extension of the monolayer formation process. The bi-layer adsorption isotherm for ethyl acetate on silica gel is shown in figure 41. The curve is theoretical and the points experimental.
The individual isotherms for the two adsorbed layers of ethyl acetate are included in figure 41. The two curves, although of the same form, are quite different in magnitude. The first layer is very strongly held to the surface and is complete when the concentration of ethyl acetate in the mobile phase is no more than 1%w/w. As the concentration of ethyl acetate starts to rise above 1%w/w the second layer is only just being formed. The formation of the second layer of ethyl acetate is much slower and obviously the interactions between the solvent molecules with those already adsorbed on the surface are much weaker than their interaction with the silica gel silanol groups.
Figure 41. The Individual and Combined Adsorption Isotherms for Ethyl Acetate on Silica Gel