Structural characterisation of complex layered double hydroxides and TGA-GC-MS study of thermal response and carbonate contamination in nitrate- and organic-exchanged hydrotalcites

Layered double hydroxides (LDH) are versatile materials used for intercalating bioactive molecules, in pharmaceutical, nutraceutical and cosmetic fields, with the purpose of protecting them from degradation, enhancing their water solubility to increase bioavailability, to improve pharmacokinetics properties and formulation stability. Moreover LDH’s are used in various technological applications, to improve stability and processability. The crystal chemistry of hydrotalcite-like compounds is investigated by X-ray powder diffraction (XRPD), automated electron diffraction tomography (ADT) and hyphenated TGA-GC-MS to shed light on the mechanisms involved inion exchange and absorption of contaminants, mainly carbonate anions. For the first time ADT allowed to obtain a structural model of LDH_NO3 from experiment,shedding light on the conformation of nitrate insideLDH and on the loss of crystallinity due to the layer morphology.The ADT analysis of a hybrid LDH sample (LDH_EUS) clearly revealed the increase ofdefectivity in this material.XRPD demonstrated that the presence of carbonate is able to influence the intercalation of organic molecules into LDH, since CO3 contaminated samples tend to assume d-spacings roughly multiples of LDH_CO3d-spacing. TGA-GC-MS allowed distinguishing and quantifyingintercalated and surface adsorbed organic molecules, confirming the presence and amount of carbonate, especially at low (below 2% in weight) concentrations and separating the different types and strength of adsorption, in relation with the temperature of elimination.