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Different portions of the standard free energy of distribution can be allotted to different parts of a molecule and, thus, their contribution  to solute retention can be disclosed. In addition, from the relative values of the standard enthalpy and standard entropy of each portion or group, the manner in which the different groups interact with 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

Most van't Hoff curves have positive slopes and the negative intercepts. A negative intercept indicates that the standard entropy change results from the production of a less random and more orderly system during the distribution process.      The Analysis of the Standard Energy of Distribution The standard energy of distribution (DG) can be divided into different parts each representing different energy sources that contribute to the equilibrium process. There are two major modes of standard energy distribution; portions of standard energy can be allotted to specific types of

nbsp; The linear relationship between the standard enthalpy and standard entropy indicates that, if either the standard enthalpy or standard entropy was known, the other can be calculated. It is also shown that there is a linear relationship between the atomic polarizability of the interacting atom and its standard enthalpy of interaction. Thus, there is also the possibility of calculating the standard enthalpy, hence the standard entropy and thus the retention of a solute, from its molecular structure and the physical and electrical properties of it component atoms. However, at this time, there appears to be a relatively large contribution to the standard enthalpy of interaction that is independent of the polarizability of the interacting atom. This may be due to some other physical characteristic of the distribution system that contributes to the standard enthalpy. Alternatively,

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

and standard entropy,                          Thus                    (35) Equation (35) is an expression for the temperature at which the separation ratio of the two solutes will be independent of the solvent composition. The temperature is determined by the relative values of the standard enthalpies of the two solutes between each solvent and the stationary phase, together with their standard entropies between each solvent and the stationary phase. If the separation ratio is very large, there will be a considerable difference between the respective standard enthalpies and entropies of the two solutes. As a consequence, the temperature at which the separation ratio becomes independent of solvent composition may well be outside the practical chromatography range. However,

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