The Thermodynamics of Chromatography - Other Thermodynamic Methods that are Used for Studying Chromatographic Systems > Optimum Operating Conditions for Chiral Separations in Liquid Chromatography > The Effect of Temperature and Solvent Composition on the Separation Ratio of the Two Enantiomer

 

The Effect of Temperature and Solvent Composition on the Separation Ratio of the Two Enantiomers

Curves demonstrating the change in separation ratio of the two enantiomers with temperature and solvent composition, calculated from equation (49)  are shown in figure (21). Despite the dominant effect of solvent composition on capacity ratio, the effect of solvent composition on the separation ratio is much smaller, and the dominant effect is now the operating temperature. This stresses the importance of temperature for selectivity control in chiral separations. It is very interesting to note that there is a temperature at which the solvent composition has no effect on the separation ratio whatsoever (ca 43˚C).

Figure 21. Curves Relating the Separation Ratio of the Two Enantiomers with Temperature and Solvent Composition

It is clear that there is a temperature at which the ratio of the capacity factors are constant at all solvent compositions (with the caveat that volume fractions of ethanol above 0.5 were not examined). Consequently, the difference between the logarithm of the capacity factors, and thus the difference in standard energies of the enantiomers, must also remain constant. As the standard enthalpies of the two enantiomers differ, then the standard energies of the two enantiomers can only be independent of solvent composition, if the solvent concentration only controls the probability of interaction, and has no influence on the energy of interaction. This clearly indicates that relating the logarithm of the distribution coefficient to the solvent composition is fundamentally unsound, as it assumes that the solvent composition controls, in some way, the energy of distribution, and not solely the probability of interaction. The calculated values of (k'S), (k'R) and (aSR) can be used in equations (39), (40), (42) (43) and (44) to determine the required column efficiency, the minimum variance per unit length, the optimum column length, the optimum velocity and the analysis time respectively. In this way, the effect of temperature and solvent composition on each of these parameters can be considered individually.