The rare earth elements (REEs) are critical metals for modern, green technologies whose separation and purification are challenging as REEs typically have similar charge and differ only weakly in size. Currently, the separation of REEs is performed by liquid-liquid extraction processes in which extractant molecules in organic solvent interact with the REEs at organic-aqueous interfaces and extract them into the organic phase. The process has poor selectivity, is environmentally burdensome, and is energy intensive due to the many stages required to achieve the desired purity. We propose a green, efficient REE separation process that employs peptide surfactants (PEPS) to extract Ln3+ in a froth flotation process. PEPS forms a multidentate binding loop that selectively binds a single Ln3+ cation. When this PEPS:Ln3+ complex adsorbs to the air-water interface it is crucial that the binding loop is not strongly perturbed, releasing the selectively bound Ln3+. Ideally, the selectivity of the bulk binding will be mirrored at the interface. To probe PEPS:Ln3+ complexation at the air-water interface we incorporate the non-natural, fluorescent amino acid acridon-2-yl-alanine (Acd) into the PEPS structure and image solutions of PEPS-Acd with a confocal laser scanning microscope. By collecting a z-stack of micrographs through the bulk solution and planar air-water interface, we can measure a spatial fluorescence profile from Acd emission, which allows us to calculate PEPS-Acd surface concentration. Furthermore, Acd emission is sensitive to the bound Ln3+, allowing the differentiation between differing REE cations present at the interface. We develop analysis that allows us to assess the selectivity of the PEPS at the interface and in the bulk phases to assess the binding loop integrity and selectivity. Exploitation of these peptides in foam-based separation schemes for pairs of cations of interest in REE recycling is discussed.
