Assessment of Electrostatic Interactions in Dense Colloidal Suspensions with Multiply Scattered Light

Abstract

The electrostatic repulsive force between particles impacts the static structure of colloidal systems and dramatically hinders visible light scattering. In this investigation, we employed frequency domain photon migration (FDPM) to measure the isotropic scattering coefficients of dialyzed polystyrene latex suspensions at 687 and 828 nm as a function of ionic strengths between 1 and 120 mM NaCl equivalents. Measured isotropic scattering coefficients increased with decreasing ionic strength of the suspensions, suggesting that changes in electrostatic interactions can be evaluated in ensemble measurements of visible light scattering. At 120 mM NaCl equivalents, the isotropic scattering coefficients were accurately predicted by Mie theory with the Percus−Yevick (PY) polydisperse model to account for hard-sphere (HS) interactions. At lower ionic strengths, the isotropic scattering coefficients at varying colloidal volume fractions were regressed to scattering theory which incorporated the mean spherical approximation (MSA) with the hard sphere Yukawa (HSY) interaction for monodisperse suspensions in order to yield an effective surface charge given the size and ionic strength. The estimates of surface charges obtained from scattering data at 687 and 828 nm were consistently similar but varied with ionic strength. A three-component MSA primary model (PM) was used in an attempt to account for sample polydispersity but yielded poor fit and estimates of effective surface charges which varied for data taken at the two wavelengths.