Intercalation Behavior of Salicylic Acid into Calcined Cu-Al-Layered Double Hydroxides for a Controlled Release Formulation

Owing to their layered structure and memory effect property, layered double hydroxides (LDHs), also called anionic clays, have received considerable attention in recent years. We have studied the effect of the operating conditions on the formation of the Cu-Al-CO3 by co-precipitation and a series of Cu-AlLDHs has been synthesized by this method. The Cu/Al molar ratio varied between 0.5 and 3 and the pH between 6 and 12. The effect of the aging time on the cristallinity of the Cu-Al-CO3 hydrotalcite was also investigated. By the reconstruction method, the calcined Cu-LDHs were tested for their ability to intercalate salicylic acid into the interlayer space. Powder X-ray diffractogrammes show that the basal spacing of the Cu-Al-LDHs bearing salicylate as the intergallery anion expanded from 7.52 angstrom in the precursors to 15.7 angstrom, these values suggesting the organic molecules form bilayers in the interlayer space. The Fourier transform infrared study further confirmed intercalation of salicylate into the interlayers of the LDHs. The thermal stability of the intercalated salicylic acid is significantly enhanced compared with the pure form before intercalation. Using the X-ray diffraction results combined with a molecular simulation model, a possible representation of the salicylate anion positioning between the lamellar layers has been proposed. The in vitro release rate was remarkably lower than that from the corresponding physical mixture at pH 7.5. The kinetic analysis shows the importance of the diffusion through the control of the drug release rate. The obtained results demonstrate show that hydrotalcites may be used to prepare modified release formulations.