The Thermodynamics of Chromatography - The Analysis of the Standard Energy of Distribution > Distribution of Standard Energy Between Different Chemical Groups > Page 12

 

 

Figure 4 Graph of Log (Specific Retention Volume) against Number of Carbon Atoms for a Series of Alkanes Eluted from n-Heptadecane, Including a Value for Methanol

The linear relationship between Log(K) and carbon number is clearly demonstrated. The stationary phase was n-heptadecane which offers only dispersive interactions with the solutes. This is confirmed by the elution position of methanol at an equivalent carbon number of 2.7. This position is to be expected if only dispersive interactions are operative. The methyl group in methanol would be approximately equivalent to one methylene group (probably slightly greater due to the extra hydrogen atom) and as the oxygen atom is significantly larger than the carbon atom, it would be expected to act dispersively as equivalent to something between that of 1 to 2 methylene groups. Methanol is an extremely polar solute and, if any polar or induced polar interactions with the stationary phase were functioning, the elution position of methanol would be significantly greater than 2.7. Thus, from the results for the highly polar solute methanol, if any polar or induced-polar interactions were possible with an alkane chain (and the indications are that this is highly improbable), their net contribution to retention would be extremely small and not significant.