SMALL-ANGLE RAYLEIGH-SCATTERING BY RELATIVELY LARGE LATEX-PARTICLES

According to the Rayleigh-Debye (RG) theory of light scattering, the i ntensity of forward scattering is proportional to the volume-squared o f the scatterers, independent of their shape or orientation. This make s small-angle light scattering (SALS) attractive as a tool for studyin g the kinetics of flocculation of model latexes, where the conformatio n of elemental particles in any floc is unknown. In preparation for su ch a study using a modified version of the apparatus of Lips and Willi s (1973), we experimentally determine the limits under which SALS prod uced by a He-Ne laser can be used for sizing of monodisperse polystyre ne 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) x 10(-26) m(4), l is the pathlength and d is the parti cle diameter. For c < c(max), the scattering per particle at 2 degrees is proportional to d(6) provided d is less than 1 mu m, which is cons iderably larger than for scattering at larger angles. We speculate tha t the scattering by flocs will be proportional to the square of the nu mber of elemental particles in dependent of their conformation provide d the flee is also smaller than 1 mu m.