Two-dimensional pH mapping of release kinetics of silica-encapsulated drugs

The encapsulation of pharmaceutical drug molecules in silica gels during the sol-gel synthesis and their kinetic release profile in aqueous solutions were systematically investigated in dependence of synthesis pH(S) and extraction pH(E) values. Six pH values in the range from 1 to 6 were chosen in a 6 × 6 two-dimensional matrix to screen the first-order initial dissolution rate constant and the total amount of released drug. Characteristic differences are discovered in such two-dimensional pH mapping profiles for the molecules with different ionization behavior and they are explained by surface imprinting and encapsulation processes. Remarkably, these encapsulations must occur either in the particles of the sol solutions or during the rapid liquid-solid transition of the spray-drying process employed in this study. This pH mapping method is suggested as a novel tool to probe noncovalent imprinting and encapsulation processes in sol-gel-derived materials with embedded guest molecules.