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

Area of Science:

• Surface Chemistry
• Separation Science
• Lanthanide Chemistry

Background:

• Surfactants are crucial for ion attraction to water surfaces in separation processes.
• Understanding ion-surfactant interactions at interfaces is key for developing efficient separation techniques.

Purpose of the Study:

• To investigate the interaction of lanthanide ions with dihexadecyl phosphate monolayers at air-water interfaces.
• To determine the conditions leading to monolayer versus bilayer formation based on lanthanide properties.

Main Methods:

• Studied lanthanide ion interactions with dihexadecyl phosphate monolayers at air-water interfaces.
• Utilized X-ray fluorescence, X-ray reflectivity, and grazing incidence X-ray diffraction.
• Performed explicit solvent all-atom molecular dynamics simulations.

Main Results:

• Heavier lanthanides (Z ≥ 65) induced bilayer formation, while lighter lanthanides (Z < 65) formed only monolayers.
• Bilayer formation was concentration-dependent, occurring above ~10⁻⁵ M for heavier lanthanides.
• Grazing incidence X-ray diffraction and simulations revealed lateral ion-ion correlations in bilayers, not monolayers.

Conclusions:

• A distinct bilayer structure forms with heavier lanthanides, isolating them from the aqueous subphase.
• This lanthanide-induced bilayer formation presents a potential mechanism for selective separation of heavier lanthanides.