The Thermodynamics of Chromatography - The Thermodynamic Analysis of the Dispersive Interactions that Can Take Place between Different Solutes and High Molecular Weigh > Page 22

 

 

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 values given in figure 10 (cf.  = 0.6084, c.f.  = 0.3789, calculated at 76˚C.) The standard energy contribution of one chlorine atom is shown to be nearly equivalent to 2 methylene groups. This explains why halogenated hydrocarbons provide much stronger dispersive interactions than a methylene group (which has been well established in both GC and LC.