Pretransitional Multiscale Structure Preceding Third-Phase Formation in Solvent Extraction Systems

Area of Science:

• Chemical Engineering
• Materials Science
• Nuclear Engineering

Background:

• Third-phase formation in liquid-liquid extraction complicates processes, reducing efficiency and safety.
• This phenomenon is particularly relevant in nuclear fuel recycling.

Purpose of the Study:

• To elucidate the pretransitional structures leading to third-phase formation during uranium extraction.
• To understand the role of aggregate formation and water in phase instability.

Main Methods:

• Combined ultrasmall-angle X-ray scattering (USAXS) and ultrasmall-angle neutron scattering (SANS).
• Analysis of uranium extraction by trioctylamine (TOA) in octane.
• Investigation across a wide scattering vector (q) range (0.004 to 3 Å⁻¹).

Main Results:

• Identified two distinct aggregate populations: small, reverse micelle-like structures (radius ~11 Å) and large, fluctuating nanodomains (>150 nm).
• Observed amplification of nanodomains with increasing TOA concentration, preceding phase separation.
• Demonstrated that phase instability arises from hierarchical condensation around uranium-loaded aggregates, including water.

Conclusions:

• Phase separation in solvent extraction is driven by multiscale phenomena, not just simple micelle growth.
• Accurate prediction of instability requires considering all species, including water.
• Findings offer new strategies to mitigate phase separation in critical industrial processes like nuclear fuel recycling.