Molecular-Scale Investigation of Heavy Metal Ions at a Charged Langmuir Monolayer*

Solvent extraction -- the surfactant-aided preferential transfer of a species from an aqueous to an organic phase -- is an important technique used in heavy and precious metal refining and reprocessing [1]. Solvent extraction requires transfer through an oil/water interface, and interfacial interactions are expected to control transfer kinetics and phase stability, yet these key interactions are poorly understood [2]. Langmuir monolayers with charged headgroups atop concentrated salt solutions containing heavy metal ions act as a model of solvent extraction interfaces; studies of ions at a charged surface are also fundamentally important to many other phenomena including protein solvation, mineral surface chemistry, and electrochemistry. We probe these charged interfaces using a variety of surface-sensitive techniques -- vibrational sum frequency generation (VSFG) spectroscopy, x-ray reflectivity (XRR), x-ray fluorescence near total reflection (XFNTR), and grazing incidence diffraction (GID). We integrate experiments with Molecular Dynamics (MD) simulations to uncover the molecular-level interfacial structure. [1] P. Tasker et al., Comp. Coord. Chem. II, 2003, 759. [2] W. Rock et al., Langmuir, 2016, 32 (44), 11532.