Pores occlusion in MCM-41 spheres immersed in SBF and the effect on ibuprofen delivery kinetics: a quantitative model

MCM-41 silica particles have been synthesized with size in the low submicron range, loaded with ibuprofen and characterized by means of XRD, N2 adsorption and scanning electron microscopy, coupled with EDS analysis both before and after contact with different volumes of simulated body fluid (SBF) at 37 .C up to 10 h. The particles do not show any change in morphology, composition and mesostructure as a consequence of soaking. MCM-41 spheres, though, are not inert towards SBF. Two processes take place, showing features independent from the soaking volume: (i) one within 1.2 h, bringing about dissolution of silica into the liquid phase up to a concentration of 2.2mM and no change in the mesopore volume; (ii) the second, after an induction period of 1.2 h, bringing about a limited increase in the concentration of dissolved silica, but affecting severely the mesoporous volume, which decreases exponentially with time. Delivery curves differ significantly when varying the volume of SBF used. To account for release kinetics under the circumstances observed, a mathematical model is proposed, based on the standard Noyes.Whitney equation, taking into account both the SBF volume used and the mesopores occlusion, this latter through a time-dependent diffusion coefficient. A satisfactory agreement is observed, without the intervention of any adjustable parameter.