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 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.