Optical properties of surfaces covered by latex particles with a bimodal size distribution: comparison with theory

Abstract

Scanning angle reflectometry around the Brewster angle has been used to investigate the optical properties of an assembly of polystyrene latex particles deposited on a silica–water interface. The surfaces are partially covered with particles of radius 120 nm, yielding stable films. A second particle sample, with a radius ranging between 50 and 300 nm, is then introduced. A simple model based on the Mie theory of particle light scattering describes the data well, both for the monomodal and bimodal particle size distributions, with only two parameters in the analysis: one particle size and one particle coverage (the other being known and fixed). The determined particle size is in good agreement with values determined by electron microscopy. The polydispersity within each particle size distribution may easily be taken into account within this model: polydispersity in the range of several tens of nanometers has no significant effect on the particle size determined from the analysis but leads to an underestimation of the particle density by a quantity that depends only on the mean size and the size distribution for the particles.