Specific Ion Effects in Lanthanide-Amphiphile Structures at the Air-Water Interface and Their Implications for Selective Separation

Sangjun Yoo, Baofu Qiao, Travis Douglas, Wei Bu, Monica Olvera De La Cruz, Pulak Dutta*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The use of surfactants to attract dissolved ions to water surfaces and interfaces is an essential step in both solvent-based and solvent-free separation processes. We have studied the interactions of lanthanide ions in the aqueous subphase with monolayers of dihexadecyl phosphate at air-water interfaces. With heavier lanthanides (atomic number Z ≥ 65) in the subphase, the floating layer can be compressed to an area/molecule of about half the molecular cross section, indicating bilayer formation. X-ray fluorescence and reflectivity data support this conclusion. In the presence of lighter lanthanides (Z < 65), only monolayers are observed. Subphase-concentration-dependent studies using Er3+ (heavier) and Nd3+ (lighter) lanthanides show a stepwise progression, with ions attaching to the monolayer only when the solution concentration is >3 × 10-7 M. Above ∼10-5 M, bilayers form but only in the presence of the heavier lanthanide. Grazing incidence X-ray diffraction shows evidence of lateral ion-ion correlations in the bilayer structure but not in monolayers. Explicit solvent all-atom molecular dynamics simulations confirm the elevated ion-ion correlation in the bilayer system. This bilayer structure isolates heavier lanthanides but not lighter lanthanides from an aqueous solution and is therefore a potential mechanism for the selective separation of heavier lanthanides.

Original languageEnglish (US)
Pages (from-to)7504-7512
Number of pages9
JournalACS Applied Materials and Interfaces
Volume14
Issue number5
DOIs
StatePublished - Feb 9 2022

Keywords

  • Langmuir trough
  • X-ray reflectivity
  • air-water interface
  • lanthanides
  • solvent extraction
  • specific ion effects
  • surfactant structure
  • synchrotron X-rays

ASJC Scopus subject areas

  • Materials Science(all)

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