The Thermodynamics of Chromatography - Other Thermodynamic Methods that are Used for Studying Chromatographic Systems > Optimum Operating Conditions for Chiral Separations in Liquid Chromatography > Effect of Temperature and Solvent Composition on the Optimum Velocity > Page 81

  

In addition, the viscosity of a binary mixture of unassociating solvents, canbe calculated using an equation developed by Moore et al. [20], viz,

 

                                         (52)

where (y1) is the mole fraction of solvent (1),
(y2) is the mole fraction of solvent (2),
(h1) is the viscosity of solvent (1),
and (h1) is the viscosity of solvent 2.

By employing the viscosity values calculated for a range of temperatures and solvent mixtures in equation (50) the effect of temperature and solvent composition on the optimum mobile phase velocity could be calculated. A practical initial value for () was assumed to be 2.5 x 10-5 cm2s. This value was taken as the probable estimate from publishd data for similar compounds (21,22) for the diffusivity of the (R)-enantiomer at 20˚C in a solvent mixture containing 10%v/v of ethanol and consequently may not be a highly accurate value. Nevertheless, it will allow the relative effect of temperature and solvent composition on the optimum mobile phase velocity and analysis time to be demonstrated. The optimum mobile phase velocities were calculated using the above equations and experimental data, and the results obtained are shown as graphs relating optimum velocity to temperature for a range of different solvent mixtures in figure 25.