Analysis of dense colloidal dispersions with multiwavelength frequency domain photon migration measurements

摘要

Frequency domain photon migration (FDPM) measurements are used to studythe properties of dense colloidal dispersions with hard sphere and electrostatic interactions,which are otherwise difficult to analyze due to multiple scattering effects.Hard sphere interactions were studied using a theoretical model based upon apolydisperse mixture of particles using the hard sphere Percus Yevick theory. Theparticle size distribution and volume fraction were recovered by solving a non linearinverse problem using genetic algorithms. The mean sizes of the particles of 144and 223 nm diameter were recovered within an error range of 0-15.53% of the meandiameters determined from dynamic light scattering measurements. The volume fractionwas recovered within an error range of 0-24% of the experimentally determinedvolume fractions.At ionic strengths varying between 0.5 and 4 mM, multiple wavelength (660, 685,785 and 828 nm) FDPM measurements of isotropic scattering coefficients were madeof 144 and 223 nm diameter, monodisperse dispersions varying between 15% - 22%volume fraction, as well as of bidisperse mixtures of 144 and 223 nm diameter latexparticles in 1:3, 1:1 and 3:1 mixtures varying between volume fractions of 15% - 24%.Structure factor models with Yukawa potential were computed by Monte Carlo (MC)simulations and numerical solution of the coupled Ornstein Zernike equations.In monodisperse dispersions of particle diameter 144 nm the isotropic scattering coefficient versus ionic strength show an increase with increasing ionic strength consistentwith model predictions, whereas there was a reversal of trends and fluctuationsfor the particle diameter of 223 nm.In bidisperse mixtures for the case of maximum number of smaller particles,the isotropic scattering coefficient increased with increasing ionic strength and thetrends were in conformity with MC simulations of binary Yukawa potential models.As the number of larger diameter particles increased in the dispersions, the isotropicscattering coefficients depicted fluctuations, and no match was found between themodels and measurements for a number ratio of 1:3.The research lays the foundation for the determination of particle size distribution,volume fractions and an estimate of effective charge for high density of particles.