Centrifugation in Particle Size Analysis

Abstract

Abstract Millions of tons per year of submicron particles are produced in industrial processes around the world. They are also referred to as nanoparticles, colloids, latices, polymer dispersions, mini‐emulsions or micro‐emulsions. These solid or liquid particles, which are mostly dispersed in an aqueous medium, have a diameter (D) in the 10 < D < 3000 nm range. They are used as dyes and pigments, in paints, adhesives and coatings applied to paper, as impact modifiers in plastics, as catalysts, in cosmetic and photographic emulsions, metal sols and electrodeposition coatings applied to automobiles, as excipients for pharmaceuticals, etc. Most of the valuable practical properties of these nanoparticle dispersions/emulsions result from their extremely small size and their high specific surface area. This implies that it is essential to be able to control and measure the particle size in order to produce nanoparticle systems of consistent quality. The particle size distribution is just as important as the average particle diameter. The practical consequences of changing from a very narrow to a very broad particle size distribution with the same average diameter can be quite dramatic in many applications. For example, the viscosity of a high‐solid polymer latex will decrease, the impact strength of plastic material will increase, surfaces coated with emulsion paints will become more level, etc. Many methods, such as light scattering, electron microscopy, field flow fractionation (FFF), capillary hydrodynamic chromatography and ultracentrifugation, are used to measure the average particle diameters and overall particle size distribution of nanoparticles. Analytical ultracentrifugation (AUC) is the most versatile method with the broadest diameter range (1–5000 nm) and the highest resolution. The nanoparticles are forced to move through a predominantly aqueous medium inside an AUC measuring cell under a centrifugal force of 500–500 000 × g (rotor speed 100–60 000 rpm). Sedimentation causes fractionation to take place, because the larger particles move faster. The sedimentation velocities s i of the different fractions i are measured with refractive index, ultraviolet (UV)‐absorption or light scattering/turbidity detectors. The diameters D i of the different fractions can be calculated from the difference in s i , i.e. the complete particle size distribution, according to Stokes' law. The resolution of the AUC method is so high because, according to Stokes, s i is proportional to the square of D i . The AUC particle size distribution method can give spurious results if D is too high, i.e. if the particles form a sediment too quickly or if they display a broad chemical heterogeneity (i.e. no uniform particle density).