5 - VISCOSITY

Abstract

The viscosity of solutions of rigid molecules is very sharply dependent on the length to breadth ratio of the molecules. The theory for this dependence has been fundamentally solved and successfully applied. With flexible chain molecules, viscosity also depends on length to breadth ratio and hence on chain length, but less directly than with rigid molecules, as the molecules are coiled in a random manner. This complicates theoretical treatments that are not always generally applicable and that are controversial to some extent. The molecules of globular proteins, which are rigid but not very elongated, give rise to relatively small viscosity effects. These are dependent only on the length to breadth ratio of the molecules and hence, provide information on the shape but not on the size of the molecules. If the molecular weight is determined by another method, the length and breadth of the molecules can be calculated from this and the viscosity data. When globular proteins “denature” and chains that were bound into a compact rigid structure separate as flexible chains, the viscosity of the solution normally increases.