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How Temperature Rise Can Induce Phase Separation in Aqueous Biphasic Solutions
Gautier Meyer1, Ralf Schweins2, Tristan Youngs3
1Université Grenoble Alpes, CNRS, LIPhy, 38000 Grenoble, France.
The Journal of Physical Chemistry Letters
|March 21, 2022
Summary
Ionic-liquid-based acidic aqueous biphasic solutions exhibit a lower critical solution temperature, enabling metal recycling. Microscopic mechanisms reveal micelle aggregation and chloride ion adsorption drive phase separation upon heating.
Area of Science:
- Materials Science
- Physical Chemistry
- Chemical Engineering
Background:
- Ionic-liquid-based acidic aqueous biphasic solutions (AcABSs) are a novel approach for metal recycling.
- These systems exhibit a lower critical solution temperature (LCST), facilitating phase separation with a modest temperature increase.
Purpose of the Study:
- To elucidate the microscopic mechanisms governing the phase separation in AcABSs.
- To understand the role of ionic liquid structure, acid concentration, and temperature in phase behavior.
Main Methods:
- Small-angle neutron scattering (SANS) was employed to study micelle formation and aggregation.
- Systematic variation of acid concentration and temperature to probe phase transitions.
Main Results:
- Spherical micelle formation was observed in binary ionic liquid/water solutions.
- Acid addition screened electrostatic repulsion, promoting micelle aggregation.
- Increased acid concentration and temperature led to micelle flocculation and phase separation.
- Chloride ion adsorption at micelle surfaces was identified as the key step for phase separation.
Conclusions:
- The phase separation in AcABSs is driven by micelle flocculation induced by electrostatic screening and chloride adsorption.
- Exothermic chloride adsorption compensates for entropic costs, explaining the counterintuitive LCST behavior.
- These findings offer insights into designing advanced separation systems and can be generalized to other LCST systems.
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