Polystyrene latex fractionation and particle size determination: The use of density gradients in a low-speed centrifuge

Abstract

The technique of low-speed centrifugation in density gradients can be used to determine polystyrene latex particle diameters. Particles in the 0.1–0.5 μm diameter range were successfully analyzed, using Ficoll density gradients generated by osmocentrifugation and zonal centrifugation speeds of 3000 rpm. Using this technique, the supernatant of an aged, coagulated latex (nominal diameter 109 nm) was fractionated in six bands, corresponding to singlets, doublets and up to hexaplets. Using another latex, to which NaCl was added to give various salt concentrations (above and below the critical coagulation concentration), latex zonal centrifugation revealed various bands, even at [NaCl]=5 × 10 −5 M. A turbidimetric scan of cell contents, in samples containing NaCl, showed that there are significant particle concentrations in the spaces between successive bands; this may be due to particle-particle reversible association or to aggregate sedimentation coefficient polydispersity, arising from geometrical (shape, interparticle distance) changes, among aggregates.