Viscosity reduction in liquids

Insofar as liquid solvents are concerned, the most important factor governing / is viscosity 77, as the Stokes equation clearly demonstrates. Therefore any systematic effort to increase separation speed requires close attention to viscosity, with an emphasis on finding solvents and conditions for which viscosities are minimal. The reduction of viscosity can be pursued systematically in place of a hit or miss search for low viscosity solvents. The approach below was developed by the author and his colleagues [32] for use in optimizing size-exclusion chromatography, but the conclusions are generally applicable to separations. 

It has long been known that liquid viscosity decreases exponentially with absolute temperature [27] 

The reduction in viscosity as one approaches and then exceeds the normal boiling point can be easily approximated. To do this we write Eq. 4.51 in the following form 

It is known that Ev is approximately 0.3 AHv where AHv is the enthalpy of vaporization. Trouten s rule gives HV/Tb = 21 cal/mol deg, which means that 6 cal/mol deg. Writing the gas constant as 91 = 1.987 cal/mol 

The value of the viscosity relative to its value at the boiling point is therefore roughly approximated by 

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