NONEXPONENTIAL RELAXATION OF DENSITY-FLUCTUATIONS IN CHARGE-STABILIZED COLLOIDS

The relaxation of density fluctuations in charge-stabilized colloidal suspensions is characterized by the dynamic structure factor S(q,t), w hich can be measured by dynamic light scattering. Whereas the short-ti me behavior of this quantity is well understood, its characteristics a t 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 funct ion Delta(q). Formally exact results for Delta(q) can be obtained on t he basis of the many-body Smoluchowski equation, but for its evaluatio n one has to introduce approximations. Earlier calculations of Delta(q ) based on a particular form of the mode-coupling approximation were f ound to be in qualitative disagreement with experimental results. In p articular, for monodisperse suspensions it was predicted that Delta(q- ->0)=0, whereas positive values for Delta(0) are extrapolated from exp erimental data. It will be shown that surprisingly small amounts of po lydispersity can give rise to finite values of the measured reduced me mory function at q=0. For this reason, we have improved the mode-coupl ing approximation and extended the theory to moderately polydisperse s uspensions. Our results are in good qualitative agreement with availab le experimental data. We have also studied how the nonexponential deca y of S(q,t) is affected by the amount of added electrolyte. The reduce d memory function is found to depend significantly on the ionic streng th.