Light scattering studies of a colloidal suspension of iron oxide particles

Abstract

A colloidal aqueous suspension of optically anisotropic and absorbing ellipsoidal particles of β-FeOOH has been characterized by means of laser light scattering (LLS) and transient electric birefringence (TEB). We measured the angular distribution of scattered intensity and of its time correlation function at different concentrations for both the polarized (VV component) and the depolarized (VH component) light scattering. The rotational relaxation times of the particles were measured by TEB at different concentrations and different electric field strengths. Furthermore, we were able to measure the depolarized light scattering spectrum at zero scattering angle in order to evaluate the value for the rotational diffusion coefficient. Due to the fact that β-FeOOH particles have a complex dielectric constant within the visible spectrum, we have taken into account the absorption effect as well as the high refractive index in our analysis of the static structure factor. Although our LLS and TEB measurements were performed in the lower shoulder of the absorption band, the effects on the particle scattering factor (or static structure factor) should be considered. In the absence of a theoretical solution for absorbing spheroids, a sphere-chain model consisting of a summation of Mie spheres is being proposed to partially correct for the absorption effect. By using a regularized inversion of the Laplace integral equation upon both the TEB decay function and the LLS autocorrelation function at small scattering angles, we were able to independently obtain distribution functions of the rotational diffusion coefficient and of the translational diffusion coefficient, respectively. From the cross sections of these two interdependent distributions, we could create a “space average” distribution of particle length and diameter. Average anisotropic factors at different wavelengths were obtained from the angular dependence of the scattered intensity ratio of the VV component and of the VH component. Finally, all of our results can be shown to be reasonably self-consistent and in agreement with electron microscopic measurements.