Small-Angle Rayleigh Scattering by Relatively Large Latex Particles

According to the Rayleigh−Debye (RG) theory of light scattering, the intensity of forward scattering is proportional to the volume-squared of the scatterers, independent of their shape or orientation. This makes small-angle light scattering (SALS) attractive as a tool for studying the kinetics of flocculation of model latexes, where the conformation of elemental particles in any floc is unknown. In preparation for such a study using a modified version of the apparatus of Lips and Willis (1973), we experimentally determine the limits under which SALS produced by a He−Ne laser can be used for sizing of monodisperse polystyrene latexes. For every particle in the sample volume to experience the same intensity of incident light (i.e. for negligible extinction), the particle concentration must be less than c max, where c max ld 6 = (1.08 ± 0.09) × 10-26 m4, l is the pathlength and d is the particle diameter. For c < c max, the scattering per particle at 2° is proportional to d 6 provided d is less than 1 μm, which is considerably larger than for scattering at larger angles. We speculate that the scattering by flocs will be proportional to the square of the number of elemental particles in dependent of their conformation provided the floc is also smaller than 1 μm.