The Thermodynamics of Chromatography - The Analysis of the Standard Energy of Distribution > Distribution of Standard Energy Between Different Chemical Groups > Page 13

 

Figure 5. Graph of Log (Distribution Coefficient) against Number of Carbon Atoms in the Solute for a Series of Alkanes Eluted from PEG 350, Including a Value for Methanol

This is important, as the results indicate that if an alkane stationary phase is chosen, then solute interactions with the stationary phase will be exclusively dispersive in character and be independent of the polar character of the solute. Consequently, employing alkane stationary phases in GC, dispersive interactions can be examined exclusively and individually, and independent of any other type of interaction.

The converse effect is shown in figure 5 where the same type of solutes are examined using a very polar stationary phase. It is seen that the same linear curve is obtained but methanol is now eluted at an equivalent carbon number of 8.7 (close to the elution position of n-nonane). It is seen that the polar interactions between the OH group of the methanol and the OH groups of the PEG 350 are extremely strong and equivalent to a dispersive interaction equivalent to at least 6 carbon atoms. Thus, by employing a similar procedure, the difference between  the dispersive interactive energy of a methylene group and that of a methyl group can be determined even in cases where the solutes may have a dipole of significant strength.