Induced birefringence in colloidal dispersions. Reformulation of the optical anisotropy parameter of large particles
The Peterlin and Stuart formulation of the optical anisotropy parameter (ga–gb) is discussed in the case of large rigid macromolecules or colloidal particles in dilute solutions or dispersions. Whereas the Peterlin and Stuart expressions indicate that (ga–gb) is independent of particle size for a given particle in a given continuous medium, this has not been found experimentally. Another formulation of (ga–gb) is proposed, based on the anomalous diffraction theory of light scattering by large particles. In contrast to the Peterlin and Stuart expressions, this anomalous diffraction expression predicts a strong particle size dependence for wavelength-sized colloidal particles. The particle size dependence of (ga–gb) found recently in kaolinite clay dispersions receives a satisfactory explanation in terms of the anomalous diffraction theory. The relevance of the anomalous diffraction theory to the theories of all induced birefringences is discussed, as these now appear to be in error for large particles. Similarly, particle sizing methods which assume that (ga–gb) is particle size independent appear to be invalid for large particles.