The interfacial structure of the room temperature ionic liquid methyltrioctylammonium bis­(trifluoromethylsulfonyl)­imide ([MTOA]+[NTF2]−) near a silicon electrode was investigated using specular X-ray reflectivity. Using this technique, we have previously observed “crowding”, i.e., formation of a thick anion layer on a positively charged electrode. We now report that this layer develops over time scales in the range ∼400–1100 s. This is different from the time scales reported in other experiments, and is inconsistent with most theoretical predictions. A tentative explanation is proposed which assumes that the formation and dispersion of the crowding layer requires collective reordering of anions/cations through the electrochemical cell. We suggest that because of the presence of multiple time scales in these systems, the observed time scales will vary depending on the time scale of the measurement.

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.

X-ray reflectivity was used to study the severalnanometer-thick "crowded" layers that form at the interfaces between a planar electrode and concentrated solutions of ionic liquids. The ionic liquid [P 14,6,6,6 ] + [NTf 2 ] − was dissolved in either strongly polar propylene carbonate or weakly polar dimethyl carbonate. In the range of 19−100 vol % ionic liquid, between working electrode potentials +2 and +2.75 V, uniform 2−7 nm thick interfacial layers were observed. These layers are not pure anions but contain three to five times as many anions as cations and about the same percentage of solvent as the bulk solution. On the other side of the layer, the density is that of the bulk solution. These features are inconsistent with a picture of the crowded layer as a region of pure, close-packed counterions. Not only the layer thickness but also the charge density decrease with increasing dilution at any given applied voltage. This appears to indicate, counterintuitively, that a thinner layer with lower net charge density will screen an electric field as effectively as a thicker layer with higher charge density.