Halide Exchange on Mg(II)–Al(III) Layered Double Hydroxides: Exploring Affinities and Electrostatic Predictive Models

Abstract

The crystalline chloride form of layered double hydroxide (LDH) with the formula Mg0.75Al0.25(OH)2Cl0.25·mH2O was gradually exchanged with F–, Br–, or I– up to a total displacement of Cl–. For the three anions, both the exchange isotherms as well as the structural changes were inspected along the whole range of chloride displacement. The bulkier Br– and I– followed an ideal exchange behavior isotherm while F– denoted strong deviations from the ideal regime as well as phase segregation. The exchange constants recorded herein were contrasted with bibliographic data belonging to an analogous LDH host, revealing a strong linear free energy correlation. Higher Al(III) to Mg(II) ratios, or layer charge densities, favor a stronger selectivity for smaller halides. For both hosts, the exchange free energy was satisfactorily described in terms of strictly electrostatic-based models.