High-speed X-ray Photon Correlation Spectroscopy studies of dynamics in liquid-liquid extraction systems

X-ray Photon Correlation Spectroscopy (XPCS) and newly available high-speed X-ray detector systems provide a golden opportunity for shedding new light on a problem of both practical and fundamental importance – the dynamics of fluctuations near critical points in complex fluids. Here, we present a high-speed XPCS study of critical fluctuation dynamics in an important class of complex organic fluids designed for chemical separations. In these liquid-liquid extraction systems, an amphiphilic extractant molecule is used to selectively transfer targeted species from an aqueous phase into an organic phase through the formation of nanoscale molecular complexes. The most efficient extraction typically occurs near a critical point where the correlation length of the complexes becomes large. Our XPCS studies reveal the microsecond dynamics of the molecular assemblies both away from and at the critical temperature T_c. The observed fluctuation dynamics can be characterized as a function of Q and T using predictions from the theory of critical phenomena. In particular, we see that the dynamic critical exponent z describing the Q dependence of tau ~ Q^-z changes from 2 to 3 as the critical temperature Tc is approached