Modified layered double hydroxides in polycaprolactone as a tunable delivery system: in vitro release of antimicrobial benzoate derivatives

2,4-Dichlorobenzoate (BzDC) and para-hydroxybenzoate (p-BzOH), having antimicrobial activity, have been intercalated into [Zn-0.65 Al-0.35(OH)(2)](NO3)(0.35) center dot 0.6H(2)O layered double hydroxide (ZnAl-LDH), via anion-exchange reactions. The obtained nanohybrids (ZnAl-BzDC and ZnAl-p-BzOH) have been dispersed into polycaprolactone (PCL), giving rise to different composite morphologies: an exfoliated nanocomposite with ZnAl-BzDC and a microcomposite with ZnAl-p-BzOH. The different dispersion of the fillers was related to the different interactions between the two molecular anions intercalated and the LDH layers. It has been found that films of micro- and exfoliated composites and of PLC containing sodium 2,4-dichlorobenzoate or p-hydroxybenzoate simply dispersed in the polymer, release the antimicrobial species in different ways when in contact with physiologic solutions. The investigated release of the antimicrobial moieties "free dispersed" into the polymer was much faster than that of the molecular anions fixed on the inorganic compound and occurred in one step. At variance, the release from the nanohybrids occurred in two stages: the first, rapid as a "burst", occurring in the first days, the second, very slow, extending up to many months. The diffusion of the active molecules out of the microcomposite was always slower than in the case of the exfoliated nanohybrid. Moreover a hysteresis of chemical absorption was shown by the microcomposite and this parameter has to be taken into account when tuning the release. (C) 2011 Elsevier B.V. All rights reserved.