Standard free, entropy

Now, classical thermodynamics gives another expression for the standard free energy which separates it into two parts, the standard free enthalpy and the standard free entropy. 

Figure 9. Graph of Standard Free Entropy against Standard Free Enthalpy for an Ether, Thioether and Amine...

Introducing the functions for standard free enthalpy and standard free entropy. 

It is seen from equation (22) that there will, indeed, be a 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 free enthalpies of the two solutes between each solvent and the stationary phase, together with their standard free entropies. 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, if the solutes are similar in nature and are eluted with relatively small separation ratios (for example in the separation of enantiomers) then the standard enthalpies and entropies will be comparable, and the temperature/solvent-composition independence is likely be in a range that can be experimentally observed. 

Finally, it is necessary to select values for the thermodynamic constants that are to be used in equation (9). The data selected were that published by Beesley and Scott [2], for the two enantiomers, (S) and (R) 4-benzyl-2-oxazolidinone. The values for the standard free enthalpy and standard free entropy for the (R) isomer were... 

Temperature used = 298.15 K N = number of ligands AG°= standard free energy AH°= standard free enthalpy AS°= standard free entropy EDF = equilibrium driving force. 

To achieve a separation between two substances, thermodynamics has shown that their standard free energy of distribution must differ. As the difference between enantiomers are solely spatial and not structural, any separation must be achieved by primarily changing the relative standard free entropy contribution to the standard free energy of each isomer. It will be seen later that this does not exclude a significant contribution from a change in free enthalpy as well, but the primary effect must be entropic in order to realize the corresponding change in free enthalpy. This will be better understood when actual separations are discussed. Thus, in order to obtain some selectivity between enantiomers, the structure of the stationary phase must be such that one isomer will fit... 

In addition, the effect of temperature on column efficiency, is now being frequently exploited, particularly in size exclusion chromatography (SEC) and in other separations where the standard free entropy dominates the separation (i.e. all chiral separations and the separation of all closely eluting isomers). 

The same type of molecular forces are involved as those in GC, except that, as the solutes no longer need to be volatile, ionic interactions can now be used to control retention, in addition to dispersive and polar interactions, as in GC. It will be seen that temperature can also be used to control retention in LC, in a somewhat similar manner to GC. The distribution coefficient of a solute between the two phases in LC will always result from both standard free entropy and enthalpy changes during distribution, as in GC. In addition, the separation of enantiomers will also depend primarily on a difference in the standard free entropy between the two isomers, that results from spatial variations and which are then augmented by standard free enthalpy differences. 

The standard free entropy of micellization, Sh, was calculated from AG = AHl, - TASlt... 

---