Mechanism of ionic-liquid-based acidic aqueous biphasic system formation

Area of Science:

• Materials Science
• Chemical Engineering
• Separation Science

Background:

• Ionic-liquid-based acidic aqueous biphasic systems (IL-based AcABS) are emerging as sustainable alternatives to traditional solvent extraction for critical metal recovery.
• These systems enable a 'one-pot' approach, combining metal leaching and separation by replacing inorganic salts with acids.
• A deeper understanding of AcABS formation mechanisms is crucial for optimizing their application.

Purpose of the Study:

• To elucidate the formation mechanisms of IL-based AcABS by comparing them with traditional aqueous biphasic systems (ABS).
• To identify key structural drivers for ionic liquid (IL) applicability in AcABS.
• To investigate deviations from established principles like the cationic Hofmeister series and explore thermomorphic properties.

Main Methods:

• Extensive screening of various ILs for AcABS formation.
• Systematic study of tributyltetradecylphosphonium chloride ([P44414]Cl) with different chloride salts and acids.
• Analysis of cation charge shielding and Hofmeister effects.
• Investigation of thermomorphic behavior by varying temperature.

Main Results:

• Cation charge shielding was identified as the primary structural driver for IL suitability in AcABS.
• A significant deviation from the cationic Hofmeister series was observed for IL-based ABS.
• The weaker salting-out ability of H3O+ compared to Na+ was attributed to stronger interactions with the [P44414]+ micelle surface.
• Remarkable thermomorphic properties were observed, with increased biphasic region upon heating.

Conclusions:

• The findings expand the applicability of ABS to new acidic systems, demonstrating their versatility and tunability.
• Understanding the fundamental mechanisms of AcABS formation facilitates the design of efficient metal recovery processes.
• The study highlights the importance of cation structure and ion interactions in designing IL-based separation systems.