Discovery of stable nanodomains in an ionic liquid between 20 and 50˚C could enhance separations

Model for an arbitrary type of separation between two phases.
A model for an arbitrary type of separation between one phase, α, and another “phase,” β comprised of two nanodomains. The analyte, A, can: diffuse in α or in the two domains, D1 and D2 in β; cross between α and the domains in β; and cross adjacent domains in β.  A can also interact with the interface between the domains and the solid substrate, phase γ.
Scientific Achievement

Previous investigations have focused on the small (~1-nm) domains formed by ionic liquids.  We observe two large domains in tetradecyltrihexyl phosphonium chloride that are stable up to 50˚C, which is attributed to the critical point for a liquid-liquid phase transition.

Significance and Impact

Ionic liquids (a class of liquid salts) have exceptional properties in separations that may benefit the DOE mission in sequestering CO2 or rare earth elements.  Understanding their structure provides a means of exploiting them.

Research Details
  • Fluorescence correlation spectroscopy using probes of varying sizes fixed the lower limit of the size of the two domains.
  • Single-molecule tracking experiments reveal two domains between 20 and 50℃.  Above 50℃, there is only one.
  • Computational studies are consistent with the nanodomains arising from a first-order, liquid-liquid phase transition.

Opare-Addo, J et al., Nanodomains and Their Temperature Dependence. J. Phys. Chem. B. https://doi.org/10.1021/acs.jpcb.4c05184

Maddala, B.G. et al., Evidence for Nanostructures of at Least 20 nm.  J. Phys. Chem. B. 2024, 128, 11197-11207. https://doi.org/10.1021/acs.jpcb.4c04950

V. Nguyen; et al., Automated Characterization of Spatial and Dynamical Heterogeneity in Supercooled Liquids via Implementation of Machine Learning. J. Phys. Condens. Matter 2023, 35, 465401. https://doi.org/10.1088/1361-648x/acecef