Nonexponential relaxation of density fluctuations in charge-stabilized colloids.

Abstract

The relaxation of density fluctuations in charge-stabilized colloidal suspensions is characterized by the dynamic structure factor S(q,t), which can be measured by dynamic light scattering. Whereas the short-time behavior of this quantity is well understood, its characteristics at longer times are more difficult to determine due to memory effects, which lead to a nonexponential decay of S(q,t). A suitable measure of the overall nonexponential decay of S(q,t) is the reduced memory function \ensuremath{\Delta}(q). Formally exact results for \ensuremath{\Delta}(q) can be obtained on the basis of the many-body Smoluchowski equation, but for its evaluation one has to introduce approximations. Earlier calculations of \ensuremath{\Delta}(q) based on a particular form of the mode-coupling approximation were found to be in qualitative disagreement with experimental results. In particular, for monodisperse suspensions it was predicted that \ensuremath{\Delta}(q\ensuremath{\rightarrow}0)=0, whereas positive values for \ensuremath{\Delta}(0) are extrapolated from experimental data. It will be shown that surprisingly small amounts of polydispersity can give rise to finite values of the measured reduced memory function at q=0. For this reason, we have improved the mode-coupling approximation and extended the theory to moderately polydisperse suspensions. Our results are in good qualitative agreement with available experimental data. We have also studied how the nonexponential decay of S(q,t) is affected by the amount of added electrolyte. The reduced memory function is found to depend significantly on the ionic strength. \textcopyright{} 1996 The American Physical Society.