Halide exchange on Mg(II)-Al(III) layered double hydroxides: Exploring affinities and electrostatic predictive models

The crystalline chloride form of layered double hydroxide (LDH) with the formula Mg_0.75Al_0.25(OH)₂Cl _0.25·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.