Interpreting the appearance of dispersed systems: I. Model dispersions of polymer latex microspheres

Measurements of total absorbance at wavelengths 350–780 nm of aqueous dispersions of polymer latex microspheres of diameters 0.091 μm, 0.254 gmm, 0.325 μm, and 1.10 μm were used to interpret systematic observations of them. Light scattering dissymmetries and scattering ratios of dispersions of the 0.091 μm microspheres were measured at varying concentration and path length at 546 nm and 436 nm. Spectroturbidimetry and observations were also made in binary mixtures of the above particle sizes and in dispersions of microspheres with added dye, the sodium salt of methyl red. For ab‐sorbance due to scattering, Ascat, exceeding 0.04 but not 2, the ab‐sorbance and its wavelength dependence yield reliable estimates of particle size, even though the dissymmetry and the scattering ratio do not. Observations of nonabsorbing systems under ordinary illum‐ination are most reliably interpreted with 0.1 < Ascat < 1, i.e., when the systems look translucent to translucent‐turbid, even though multiple scattering predominates in this range. That the Tyndall effect, or a variant of it when absorption is important, is visible im‐plies that particles smaller than 0.1 μm are present. To estimate particle sizes in milky dispersions in which A > 2, it is necessary to decrease the path length — or the concentration, if tolerable — so that the absorbance falls in that optimal range. Outside this range, the literature rules are unreliable. Because observers and illumination conditions vary among laboratories, it seems essential that model systems such as the microspheres and the dye employed here be used to simulate scattering and absorption features of dispersed systems. By direct comparisons of model systems to systems of interest, observations can be standardized and interpretation of appearances can become less subjective. Moreover, combining obser‐vations with spectroturbidimetry provides a much more potent tool for estimating sizes simply and quickly.