The Interactive Energy Difference Between that of a Methylene Group and that of a Methyl Group with an Alkane Stationary Phase Before discussing some examples of the use of thermodynamics in chromatography, it should be pointed out that there is an immense amount of accurate and precise retention data available in the literature. Unfortunately, much of this data was published before 1980 and, thus, is not identified by many computer search engines and needs to be found by older and conventional literature search methods. So much retention data

processes that contribute to retention. Although the standard energy of interaction of the methylene group is much greater than that of the methyl group, the standard enthalpies of both groups are very similar. However, the entropy term for the methyl group is nearly 150% greater than that of the methylene group and, as this acts in opposition to the standard enthalpy contribution, it reduces the free energy associated with the methyl group by about 30% relative to that of the methylene group. This entropy difference between the two groups is due to the methylene group being situated in a chain (more rigidly held) and has, initially, a much lower entropy before solution in the stationary phase. In contrast, the methyl group, situated at the end of the chain, is much less restricted and thus, on interaction with the stationary phase molecules (where it is held more rigidly) the entropy change is much greater. It follows that the introduction of a methylene group into a solute

nbsp;   Figure 10. Graph of Intercept and Slope from [Log(V'r(T))/Number of CH2 Groups Curves] for a Series of 1-Chlorohydrocarbons 1/T   The difference between the methyl group, methylene group, and the chlorine atom is quite striking. The enthalpy and entropy values for the methylene group are again very close to those obtained from the n-alkane series. As would be expected, the chlorine atom has both a higher enthalpy term and a higher entropy term than the methylene group. The high enthalpy contribution probably arises from its larger mass and size which would be expected to provide stronger interactions with the stationary phase molecules. Its increased entropy contribution arises from it being a terminal atom as opposed to a group, consequently, prior to interaction with the stationary phase, it has much greater freedom. Th  e contribution of the methylene group and the chlorine atom can be calculated from the enthalpy and entropy

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

Adopting the procedure of dividing (DGo) into different parts so that the each portion represents the standard free energy associated with specific parts of a molecule, consider the distribution of the standard free energy throughout an n-alkane molecule. If a portion of (DGo) is allotted to each methylene group and also to the two methyl groups, then, algebraically, this can be expressed as follows. RTLn(V'r(T)) = -nDGo(MethyleneGroup) -mDGo(Methyl Group)     DGo(Methylene Group) is standard free energy of the methylene group,   DGo(Methyl Group) is standard free energy of the methyl group,   (n) is the number of methylene groups,   and, (m) is the number of methyl groups (m=2 for an n-alkane).  

;                                 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