This study explores neodymium (Nd) adsorption on Langmuir monolayers of octadecylphosphonic acid (ODPA) at the air/water interface. Using sum frequency generation spectroscopy and X-ray fluorescence near total reflection, how Nd ions enhance ODPA deprotonation and drive interfacial water ordering is revealed. The findings challenge classical models and offer insights into optimizing rare earth separation processes.

The increasing need for rare earth separations requires a detailed understanding of trivalent ion behavior at charged aqueous interfaces. Here, neodymium (Nd) adsorption on Langmuir monolayers of octadecylphosphonic acid (ODPA), a single-chain phosphonic acid capable of double deprotonation, at the air/water interface, is investigated. Combining sum frequency generation (SFG) spectroscopy with X-ray fluorescence near total reflection (XFNTR), both the interfacial water ordering and ion density are examined. Under ambient conditions, Nd ions induce enhanced deprotonation of ODPA headgroups, leading to interfacial ion densities as high as 1 Nd per 30 Ų. This adsorption behavior arises from a complex interplay between direct electrostatic interactions, ion pairing, and hydration effects, which cannot be fully captured by classical Gouy–Chapman–Stern models. These insights into trivalent ion adsorption mechanisms provide a pathway toward more effective separation processes for rare earth metals.