Structure and Dynamics of Composition Fluctuations in a Liquid–Liquid Extraction Organic Phase with a Mixture of Two Extractants

Here, we investigate composition fluctuations in the TRUEX organic phase, a chemical system developed for liquid–liquid extraction (LLE) of transuranic elements from acidic aqueous media. This organic phase consists of two amphiphilic extractants, TBP (tributyl phosphate) and CMPO (octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide) dissolved in dodecane. Previous studies of similar LLE systems involving only a single extractant have found critical fluctuations associated with phase separation within the organic phase that are consistent with the 3D Ising universality class. Here we use small-angle X-ray scattering (SAXS) to observe the growth of equilibrium fluctuations as a function of temperature approaching the instability (spinodal). We also use X-ray photon correlation spectroscopy (XPCS) to characterize the dynamics of the fluctuations. Both the statics and dynamics obey scaling laws from the theory of critical fluctuations but with parameters that differ from those expected for binary fluids. Viewing the mesoscale structures in these LLE systems in terms of critical fluctuations provides a general framework for understanding their function.