Concentration fluctuations in the binary mixture hexane-nitrobenzene with static and dynamic x-ray scattering.

We report on a small-angle x-ray scattering study of the hexane-nitrobenzene binary fluid mixture near its critical point. The use of an ultrabright x-ray undulator synchrotron source enabled us to measure the temperature dependence of the static structure factor with unprecedented contrast, and the large coherent flux of this source provided a probe for the fluctuation dynamics via the x-ray photon correlation spectroscopy technique. We find that the intensity and correlation lengths diverge with the expected three-dimensional Ising critical exponents, and the dynamical correlation function decays exponentially with correlation times as small as 250 micros. In the range of wave vector studied here (1.2-2.6 x 10(-3) A(-1)) the concentration fluctuations relaxed diffusively with a diffusion constant consistent with that determined from visible light scattering measurements.