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

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.

Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

“…Unlike the lanthanide ions where ligands interact electrostatically, transition metals can show significant selectivities due to ligand–metal coordination bonds that are directional. Finally, a paper published while our manuscript was under review confirms the static inverted bilayer structure we propose for the lutetium system …”

“…Finally, a paper published while our manuscript was under review confirms the static inverted bilayer structure we propose for the lutetium system. 74 The kinetics of monolayer to inverted bilayer transition with Lu 3+ in the subphase is strongly temperature dependent with an activation barrier of ~ 40 kcal/mol. This relatively large energy barrier is in correspondence with the low temperature required to arrest the heavy lanthanide species at the dodecane-water interface.…”

Relevance of surface adsorption and aqueous complexation for the separation of Co(II) and Ni(II)

“…It has been demonstrated that the type of metal ion and aqueous pH are crucial to maintain a stable DHDP monolayer. 29,33 Here, we found that a low pH value of 2.5 is essential when stabilizing a DHDP monolayer on the surface of aqueous solutions containing Fe(III).…”

“…[17][18][19] Recent studies have used X-ray and neutron scattering and non-linear optical techniques to probe ion-extractant complexes at liquid surfaces and interfaces. 8,[20][21][22][23][24][25][26][27][28][29] Although the importance of complexation in the bulk aqueous solution had been dismissed in earlier work, 17 our recent studies of lanthanide ion complexation with HDEHP in the bulk aqueous phase noted that such complexation can be antagonistic to the intended selectivity of the extraction process. 30 Heavy lanthanides will be held back by HDEHP in the bulk water, which may hinder the extraction efficiency and separation selectivity with light lanthanides.…”

Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers: Charge Reversal or Water Reorganization?

“…3 Chemical separations require selective ion adsorption and transfer. [4][5][6][7][8][9][10][11][12][13] A molecular scale understanding of ion-surface-water interactions is crucial for addressing the key challenges in these fields.In the classical picture of a charged interface, including Stern and diffuse layers of electric double layer (EDL), the total charge of the counterions adsorb at the interface cannot exceed the surface charge. However, in the last two decades many experimental and theoretical studies have demonstrated that the charge of adsorbed ions can exceed the surface charge, leading to a charge reversal or overcharging.…”