The Thermodynamics of Chromatography - The Thermodynamic Analysis of the Dispersive Interactions that Can Take Place between Different Solutes and High Molecular Weigh > Page 18

Figure 7 shows that by taking a mean value for the slope of the curves in figure 6, there is close agreement between the calculated and experimental values. This confirms that the methylene groups can act as a reference group for thermodynamically analyzing solute retention. This procedure will now be used to analyze a simple n-alkanes series using the data obtained from the stationary phase n-heptadecane. As a result, it will be possible to identify the difference between the contribution of a methyl group and that of a methylene group on solute retention when retained exclusively by dipersive forces. The curves relating log(V'r(T)) to the number of methylene groups in each of the three n-alkanes for seven different temperatures are shown in figure 8.

 

 

Figure 8. Graph of Log(V'r(T)) against Number of Methylene Groups for a Series of n-Alkanes

 

It is seen from figure 8 that the expected straight lines are produced at each temperature and that the index of determination is very close to unity. Now as the slope represents the contribution to the standard energy from each methylene group and the intercept represents the contribution from two methyl groups, then the contribution from a single methyl group is seen to be significantly less than that from one methylene group.