The Thermodynamics of Chromatography - Thermodynamics Basics > Page 9
There are some reports in the literature that feign to show nonlinear van't Hoff curves. Nonlinear van't Hoff curves are, in fact, a contradiction in terms. To give an example, a curve relating log(V'r) against 1/T for solutes eluted from a reverse bonded phase by a mixed solvent will sometimes be nonlinear. Graphs relating log(V'r) against 1/T, however, can only be termed van't Hoff curves if they apply to an established and constant equilibrium system, where the interactive character of the system does not change with temperature. In a reversed phase system, the solvents themselves are also differentially distributed between the two phases in addition to the solute. As a consequence, (depending on the actual concentration of solvent) if the temperature changes, so will the relative amount of solvent adsorbed on the stationary phase surface alter, and so the interactive character of the stationary phase will also change. It follows, that the curves relating log(V'r) against 1/T will not be linear and, more importantly, as the distribution system itself is varying, the curves will not constitute van't Hoff curves. This effect is well known, an early example is afforded by work carried out by Scott and Lawrence (1).
Scott and Lawrence examined the effect of water vapor as a moderator on the surface of alumina in the gas/solid separations of some n-alkanes. Examples of the results obtained by those authors are shown in figure 3. The alumina column was moderated by a constant concentration of water vapor (constant partial pressure of water) contained in the carrier gas.