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 (T1) as follows,
where (rT1) is the density of the stationary phase at (T1)
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.