Rare Earth Selectivity and Electric Potentials at Mica Interfaces

Controlling materials' composition and structure to selectively adsorb rare earth elements (REE) is critical for better separations. Understanding how local electric potentials affect REE adsorption and how they can be modified via chemical substitution is of fundamental importance. We present calculated mean inner potentials for muscovite and phlogopite micas in excellent agreement with measured values of +10.6 V. Natural substituents for aluminum significantly influence the electric potentials at the basal surface, altering the REE adsorption energies (<i>E</i>_ad). Nd³⁺ adsorption is generally favored over that of Yb³⁺ on both micas. Substituents with lower charge than aluminum increase the Lewis basicity of adjacent oxygen atom lone electron pairs, enhancing <i>E</i>_ad by 20 to 30 kcal/mol. These substitutions modify the electric potential's magnitude and spatial extent, highlighting the role of high-resolution electron holography/tomography and the importance of understanding atomic-scale variations in electric potentials for improving REE cation adsorption and selectivity on micas. In summary, the adsorption energies of REE cations are correlated with the substituent charge and local electrostatic potential variations because these factors dictate the electrostatic interactions between the cations and the interface.