nbsp; The curves provide relative values for the standard enthalpy ( and ) and standard entropy ( and  ) of distribution for each group and the relative magnitudes of which give some indication as to how they interact with the stationary phase and the relative 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

the stationary phase may also be revealed. Another interesting relationship arises from the above treatment and that is the standard entropy term tends to increase with the standard enthalpy term. This relationship between entropy and enthalpy has been reported many times in the literature. An example of a graph relating (DHo) to (DSo), produced by Martire and his group (K), is shown in figure 11. From a theoretical point of view, this relationship between standard enthalpy and standard entropy is to be expected. Any increase in enthalpy indicates that more energy is used up in the association of the solute molecule with the molecules of the stationary phase.     Figure 11. Graph of Standard Free Entropy against Standard Free Enthalpy for an Ether, Thioether and Amine  

thermodynamic terms, is said to be "energy driven". In contrast, for distribution system (B) there is only a small enthalpy change , but a high entropy contribution . Thus, the distribution is not predominantly controlled by molecular forces. The entropy is a measure of the degree of randomness that a solute molecule experiences in a particular phase. The more random and 'more free' the solute molecule is in a particular phase, the greater its entropy. A large negative entropy change means that the solute molecules are more restricted or less random in the stationary phase (B). and this loss of freedom is responsible for the greater solute retention. The change in entropy in system (B) is the major contribution to the change in free energy, so

nbsp;   Figure 14 Graph of Standard Entropy against Standard Enthalpy for Each Element A negative value or (DHo) means that heat is evolved when interaction takes place in the stationary phase as a result of the forces between the atom and the n-octadecane. From table 3 it is seen that the standard entropy term increases with the standard enthalpy term. This relationship between standard entropy and standard enthalpy  is shown in figure 14.      It is seen that there is an impressive clean linear correlation between (DHo) and (DSo) (index of determination 1.000). The excellent correlation is due to the condition that only one interactive process is significantly active in the distribution (i.e., dispersive interactions). As already discussed, a linear relationship between standard enthalpy and standard entropy is to be expected. An increase in enthalpy

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.

;                        DGo = DHo - TDSo                                 (2)     where (DHo) is the standard enthalpy,     and (DSo) is the standard entropy. The standard enthalpy and standard entropy represent two distinctly different portions of the energy associated with distribution and are related to quite different parts of the distribution processes.   The enthalpy term represents the energy involved when the solute molecules break their interactions with the mobile phase and interact with, and enter, the stationary phase. These interactions result from intermolecular forces that are electrical in nature (see book 7