Determination of Particle Size Distributionsof Swollen (Hydrated) Particlesby Analytical Ultracentrifugation

Abstract

The classic idea of a particle is that of a hard particle for example a hard sphere. Deviations from this idea may refer to differences in shape. It may have the shape of an ellipsoid or a cylinder. These cases have in common, that the density of the particles is still well defined: It is the density of the solid material these particles consist of.

In industrial practice however a great number of dispersions consists of particles, which have absorbed solvent, due to the fact, that they contain a few percent of soluble polymers. In the case of an aqueous dispersion then they are called hydrophilic. In these cases the density of these particles is unknown, it is between the density of the dry particle and that of the solvating medium. Applying the well-known ultracentrifugation method for determining particle size distributions from sedimentation velocity using the dry density results then in apparent diameters, which are smaller than the real diameters. To overcome this problem the degree of hydration has to be taken into account. So it is possible to calculate from the apparent diameter the diameter of the unswollen, compact particles as well as the diameter of the hydrated, swollen particles.

The degree of hydration (or swelling) can be determined by preparative ultracentrifugation by pelleting the material and determining the weight of the wet material and after drying of the dry substance. To achieve equilibrium hydration pelleting is carried out at low concentrations and the particles are allowed to swell back at rest in the serum of the dispersion for several hours. Several examples are given.