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

We report x-ray intensity fluctuation spectroscopy (XIFS) measurements of the equilibrium dynamics of colloidal suspensions of charge-stabilized Sb₂O₅ spheres of radius 250±40 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 ^-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, φ=0.18), the diffusion coefficient exhibits a minimum at the peak of the structure factor, indicating modifications to Brownian behavior due to interparticle interactions.