The Thermodynamics of Chromatography - The Thermodynamic Analysis of the Dispersive Interactions that Can Take Place between Different Solutes and High Molecular Weigh > Page 20

 

The curves provide relative values for the standard enthalpy ( and ) and standard entropy ( and  ) of distribution for each group and the relative magnitudes of which give some indication as to how they interact with the stationary phase and the relative processes that contribute to retention.

Although the standard energy of interaction of the methylene group is much greater than that of the methyl group, the standard enthalpies of both groups are very similar. However, the entropy term for the methyl group is nearly 150% greater than that of the methylene group and, as this acts in opposition to the standard enthalpy contribution, it reduces the free energy associated with the methyl group by about 30% relative to that of the methylene group. This entropy difference between the two groups is due to the methylene group being situated in a chain (more rigidly held) and has, initially, a much lower entropy before solution in the stationary phase. In contrast, the methyl group, situated at the end of the chain, is much less restricted and thus, on interaction with the stationary phase molecules (where it is held more rigidly) the entropy change is much greater. It follows that the introduction of a methylene group into a solute molecule will increase its dispersive interactive capability (leading to greater retention) more than the introduction of a methyl group due to the greater change in entropy associated with the methyl group.