Determination of very broad particle size distributions via interference optics in the analytical ultracentrifuge

Abstract

Dispersions play a big role in industry. Important characteristics such as viscosity and gloss depend on the particle size and the particle size distribution of these systems. Industrial dispersions often have a broad particle size distribution. If the particle size is in the submicron range, particle size distributions of this type can best be analysed by analytical ultracentrifugation as shown in a big round-robin test which was carried out by Bayer. When particles become smaller and their transmission increases, turbidity optics have to be substituted by interference optics. This leads to a more tedious way of transforming measured data into particle size distributions. This has to do with the enormous broad range of s values which often belongs to such a particle size distribution, for example from 1 to 300 Sv. As a consequence not only several interference pictures have to be taken at different times but also pictures at different rotor speeds. For this example the rotor speeds are 3,000, 10,000, 25,000 and 40,000 rpm. The new computer program for data evaluation converts the i = f(r) picture into a δ i = f (d) function using the well-known Stokes law. Then it collects the number of fringes for a given diameter interval. This concentration is computed for each of the 500–1,500 interference pictures and an average value is taken for the final result. By the application of our new program we can do our interference work in a semiautomated way and avoid the arbitrary man-made selection of pictures, which saves time and increases precision.