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We have performed combined dynamic light scattering (DLS) and dynamic x-ray scattering (DXS) experiments on dense colloidal suspensions. The intermediate scattering functions obtained with these two techniques are compared directly. In the case of optically index matched samples, the comparison demonstrates that DXS yields accurate and reliable results. It is shown that the hydrodynamic interaction H(q) can be determined experimentally, without taking recourse to any theoretical model, by combining DXS and DLS. The combination of the two methods probes the dynamics over more than one decade in scattering vector. Experiments on optically opaque samples, where DLS fails, demonstrate the necessity to use x rays in these systems.

The dynamics of concentrated, charge‐stabilized colloidal silica suspensions was studied over a wide range of wave‐vectors. The short‐time diffusion coefficient, D(Q), was measured for concentrated suspensions up to their solidification points by photon correlation spectroscopy with coherent X‐rays and compared to free particle diffusion D0, studied by Dynamic Light Scattering (DLS) in the dilute case. Small angle X‐ray scattering (SAXS) was used to determine the static structure factor S(Q). D0/D(Q) peaks for Q values corresponding to the maximum of the static structure factor showing that the mostly likely density fluctuations decay the slowest. The data allow one to estimate the diffusion coefficient D(Q) in the Q → 0 and Q →∞ limits. Thus, hydrodynamic functions can be derived free from any modeling of the static or dynamic properties. The effects of hydrodynamic interactions on the diffusion coefficient in charge‐stabilized suspensions are presented for volume fractions 0.075 < Φ < 0.28.

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