The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Mono-layer Adsorption > Page 43

It is seen from equation (29) that the stationary phase surface, with which the molecules of solute will interact during their passage along the LC column, may be quite complex. If a solvent mixture is employed as the mobile phase, then the stationary phase may have two or more different surface fractions, each covered with a different solvent. The relative proportion of each surface will change as the composition of the mobile phase is changed. However, changing the mobile phase composition will also change the character of the interactions of the solute with the mobile phase. The use of ternary mixtures exacerbates the problem and the difficulties are further compounded when the solvents themselves associate with each other as can happen in the case of aqueous mixtures (e.g. methanol/water mixtures). The adsorption isotherm for chloroform adsorbed on a silica gel from a solution in n-heptane (13) is shown in figure 18. The variables given in the graph are not in the form of equation (29), but relate concentration of chloroform on the silica gel to the concentration of chloroform in the mobile phase. It is seen that the silica gel surface does not become completely covered with chloroform (and thus, have consistent interactive properties with respect to any solute) until the concentration of chloroform in the mobile phase is in excess of 40%w/w.

Figure 18. Langmuir Isotherm for Chloroform on Silica Gel