# 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.