VISCOSITY AND MOLECULAR STRUCTURE

This chapter describes the viscosity of molecules and the structure of molecules. Within the framework of a corresponding states system, the viscosity of liquids can be examined most easily if comparisons are confined to specific regions. The viscosity in the high-temperature region is dominated by molecule dimension and intermolecular force constants, but even in reduced coordinates, significant effects of molecular structure can be observed. The largest difference, between rigid and flexible molecules, can be handled quantitatively through the use of the number of external degrees of freedom excited in the liquid state, an independently determined molecular property. Smaller specific structure effects exist within these two groups but cannot yet be quantified in terms of molecule properties. The viscosity in the low-temperature region, although still affected by molecule dimension, intermolecular force, and the number of external degrees of freedom per molecule, is dominated by what may be loosely called the equilibrium glass transition temperature. The relation between the viscosity of mixtures and the molecular structure of their components appears to be broadly understood and can be treated reasonably quantitatively. That treatment involves the correction of the interaction potential by means of the excess free energy of mixing. Hence, mixture viscosity may be predictable to the extent to which the thermodynamic properties of the mixture can be predicted from molecular-structure information.