Dissolution at porous interfaces. IV. Pore effects in natural convection flow

The dissolution rate of a nicotinic acid tablet surface was measured in a dissolution system where mass transport in the solvent is governed by a combination of natural and forced convection. While such experimental conditions were chosen that natural convection outweighted forced convection, the dissolution rate increasing effect of large pores in the tablet surface was studied. To that end cylindrical pores were drilled into the tablet. The increase in dissolution rate was measured as a function of the number of pores, their depth and diameter and their position with respect to each other and to the centre of the dissolving surface. The results are discussed with regard to the hydrodynamic conditions near the pores. In addition to the regularly arranged cylindrical pores a more irregular configuration of cubic pores was investigated. These pores were created by embedding cubic sodium chloride crystals in the tablet surface during the compression procedure. After the sodium chloride particles had dissolved rapidly a porous surface of much slower dissolving nicotinic acid remained. The dissolution rate of the porous surface was determined and a comparison was made between surfaces with and without pores of several sizes.