Model solvent extraction system confirms enhancement of lanthanide extraction by bilayer structures.*

Understanding of the interactions of lanthanide ions with surfactants is essential to the development of improved solvent extraction and separation processes. However, the underlying microscopic mechanisms are still poorly understood. It is known that at the air-water interface, the amphiphilic molecule dihexadecyl phosphate (DHDP) forms bilayer structures with heavier lanthanides upon compression, while monolayers are observed when only light lanthanides are present in the subphase ^[1],[2] . We have studied solvent extraction of lanthanides from aqueous solutions to organic solvents containing model extractants, and correlated the results to the interfacial structures. The selectivity, extraction rates and extraction efficiencies all support the postulate that the formation of hydrophobic bilayers leads to faster and better extraction and potentially to better selectivity in rare earth element separations. A likely reason is the bilayers are hydrophobic and thus more soluble in the organic phase.

 

 

[1] Nayak, Srikanth, et al. “Spontaneous and Ion-Specific Formation of Inverted Bilayers at AIR/Aqueous Interface.” Langmuir, vol. 38, no. 18, 2022, pp. 5617–5625., https://doi.org/10.1021/acs.langmuir.2c00208.

[2] Yoo, Sangjun, et al. “Specific Ion Effects in Lanthanide–Amphiphile Structures at the Air–Water Interface and Their Implications for Selective Separation.” ACS Applied Materials & Interfaces, vol. 14, no. 5, 2022, pp. 7504–7512., https://doi.org/10.1021/acsami.1c24008.