A Generalized Refractive Index Equation for Estimating the Average Particle Size in Colloidal Suspensions

Abstract

ABSTRACT This paper proposes a rapid technique for characterizing the microstructure of dispersions and emulsions. The technique relies on a generalized equation that relates the refractive index of a colloidal suspension to the size and volume concentration of its dispersed phase. From the measured refractive index of the liquid one can calculate the volume-weighted average particle size, D 4,3, using this equation. Recent commercial high-precision critical-angle digital refractometers enable the accurate measurement of the refractive index for turbid dispersions and hence making this technique a viable rapid method for particle sizing without dilution. The possibility of measuring the refractive index of neat dispersions means that the calculated D 4,3 reflects the “real” particle size which could include clustered or flocculated particle structures. In flocculated samples the refractive index method reads the floc size which correlates better with the rheological and phase separation behavior of the system. The sizing results for latex dispersions, emulsions, and liposomal dispersions compare well with conventional methods in the absence of flocculation.