# The Thermodynamics of Chromatography - The Thermodynamic Analysis of the Dispersive Interactions that Can Take Place between Different Solutes and High Molecular Weigh > Page 16

This procedure is applicable to any homologous aliphatic series including alcohol's, amines, etc. However, before proceeding further, the validity of using the methylene group as the reference group requires to be established. The retention data used to demonstrate the validity of the procedure is taken from reports of Martire and his group [3-7] and includes data from the thesis of some of his students.

** **

**Figure 6. Graph
of Log(V'r) against Number of Methylene Groups for Different Aliphatic Series**

The stationary
phases used were all *n*-alkanes and included extensive data from the
stationary phase *n*-octadecane. The specific data was the specific
retention volumes of the different solutes at 0˚C (V'_{r(To)}) and
thus, (V'_{r(T)}) was calculated for any temperature (T_{1}) as
follows,

_{}

where (r_{T1}) is the density of the stationary phase at (T_{1})

Graphs
relating log(V'_{r(T)}) to the number of methylene groups in a molecule
is shown in figure 6 for a range of different solute types. Figure 6 shows that
the slopes of each linear curve (which will be related to the contribution of
each methylene group to the total standard free energy) are very similar for
all the series. In contrast, the intercepts (standard free energy contributions
from other groups and atoms) differ considerably. By averaging the values for
the slopes, and taking the *average value*
so obtained, in conjunction with the *appropriate* *number of methylene groups* together with the *actual* values for the *intercepts*,
it is possible to calculate the theoretical values for log(V'r_{(T)})
for each value in each series. The calculated values of log(V'r_{(T)})
are shown plotted against those experimentally measured in figure 7.