Dynamics of concentrated colloidal suspensions probed by x-ray correlation spectroscopy

Abstract

We report x-ray intensity fluctuation spectroscopy (XIFS) measurements of the equilibrium dynamics of colloidal suspensions of charge-stabilized ${\mathrm{Sb}}_{2}{\mathrm{O}}_{5}$ spheres of radius $250\ifmmode\pm\else\textpm\fi{}40 \mathrm{\AA{}}$ in glycerol. The dynamics were investigated for times between 4 and several thousand seconds, and for wave vectors, $Q$, from 0.003 to 0.012 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$, well beyond wave vectors that can be studied with visible light. The unique advantages of XIFS for studies of concentrated suspensions composed of refractive-index-mismatched components have not previously been explored. Here, we study a dilute sample (sample I) and a concentrated sample (sample II). For sample I, the diffusion coefficient is independent of $Q$, with a value consistent with the Stokes-Einstein relation. For sample II (effective hard-sphere volume fraction, $\ensuremath{\varphi}=0.18)$, the diffusion coefficient exhibits a minimum at the peak of the structure factor, indicating modifications to Brownian behavior due to interparticle interactions.