Determination of particle size distributions of latices by velocity ultracentrifugation using absorption optics

Abstract

A method has been developed for the determination of the weight distribution of latices by velocity ultracentrifugation using absorption optics. Since the ratio of the light scattered by a latex particle to its weight depends upon its size, the concentration in a sedimenting boundary, in the case of polydispersity, is not linearly proportional to the measured optical density. The problem of determining the concentration has been solved by dividing the boundary region in a number of narrow sections, then firstly calculating the particle radius from the measured sedimentation coefficient, and secondly determining the proportionality factor between the increase in concentration and the increase in optical density from the radius, using Mie's theory. By a suitable extrapolation procedure the influence of radial dilution is eliminated. The experiments were greatly facilitated by the use of a newly constructed direct-reading scanning absorption attachment to the ultracentrifuge. A short description of this piece of equipment is given. Results show that our method for the determination of size distributions is reliable. The limits of particle sizes that can be determined are discussed. The method is compared with a similar one recently developed by Cantow (1).