Observation of a Rare Earth Ion−Extractant Complex Arrested at the Oil−Water Interface During Solvent Extraction Wei Bu,* ,† Hao Yu, † Guangming Luo, ‡,⊥ Mrinal K. Bera, † Binyang Hou, † Adam W. Schuman, † Binhua Lin, ∥ Mati Meron, ∥ Ivan Kuzmenko, § Mark R. Antonio, ‡ L. Soderholm,* ,‡ and Mark L. Schlossman* ,† † Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, United States ‡ Chemical Sciences and Engineering Division, and § XSD, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ∥ Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States * S Supporting Information ABSTRACT: Selective extraction of metal ions from a complex aqueous mixture into an organic phase is used to separate toxic or radioactive metals from polluted environments and nuclear waste, as well as to produce industrially relevant metals, such as rare earth ions. Selectivity arises from the choice of an extractant amphiphile, dissolved in the organic phase, which interacts preferentially with the target metal ion. The extractant-mediated process of ion transport from an aqueous to an organic phase takes place at the aqueous−organic interface; nevertheless, little is known about the molecular mechanism of this process despite its importance. Although state-of-the-art X-ray scattering is uniquely capable of probing molecular ordering at a liquid−liquid interface with subnanometer spatial resolution, utilizing this capability to investigate interfacial dynamical processes of short temporal duration remains a challenge. We show that a temperature-driven adsorption transition can be used to turn the extraction on and off by controlling adsorption and desorption of extractants at the oil−water interface. Lowering the temperature through this transition immobilizes a supramolecular ion−extractant complex at the interface during the extraction of rare earth erbium ions. Under the conditions of these experiments, the ion−extractant complexes condense into a two-dimensional inverted bilayer, which is characterized on the molecular scale with synchrotron X- ray reflectivity and fluorescence measurements. Raising the temperature above the transition leads to Er ion extraction as a result of desorption of ion−extractant complexes from the interface into the bulk organic phase. XAFS measurements of the ion− extractant complexes in the bulk organic phase demonstrate that they are similar to the interfacial complexes. 1. INTRODUCTION The transport of ions across liquid−liquid interfaces underlies a wide variety of biological, environmental, and industrial processes. 1−3 Although there have been studies focused on various aspects of metal−ion phase transfer, a paucity of relevant data has prohibited the development of a molecular- level understanding of events occurring at the interface. Recent advances in X-ray interface-sensitive techniques 4 now extend the capability to probe molecular-level speciation and organization to liquid−liquid interfaces. Herein, we apply these techniques to probe molecular-level organization at an aqueous−organic solution interface with specific relevance to solvent extraction. Solvent extraction, an important industrial process for separating, isolating, and thus purifying metal ions, involves the transfer of a targeted species between two immiscible solution phases. 5−8 The process involves contact of an aqueous phase containing a mixture of ionic species with an organic phase to which the targeted metal species is to be selectively transferred by complexation with an amphiphilic extractant molecule, which serves to solubilize the metal cation in the nonpolar phase. Until recently, the major energetic driver for this process has been thought to center on small differences in molecular energetics favoring the molecular metal−extractant complex. 9 New evidence is changing this perspective, suggesting instead that efficient metal extraction processes may involve the formation of extractant-based clusters in the organic phase. 10,11 Described in terms of reverse micelles, these clusters are comprised of extractants that self-organize. Critical to, but missing from this model, is knowledge of how the micelles are formed and how the targeted ions become enclosed in these supramolecular structures. Our X-ray studies provide the basis for a molecular-level understanding of interactions and organization at the liquid−liquid interface through evidence of an interfacial route to such supramolecular structures. Although this work focuses on a system of relevance to solvent extraction, the results provide new insight into molecular-level processes at liquid−liquid interfaces. Received: June 6, 2014 Revised: August 18, 2014 Published: August 19, 2014 Article pubs.acs.org/JPCB © 2014 American Chemical Society 10662 dx.doi.org/10.1021/jp505661e | J. Phys. Chem. B 2014, 118, 10662−10674