Mercury porosimetry of microcrystalline cellulose tablets: effect of scanning speed and moisture

Abstract

The effect of pretreatment and scanning speed of mercury porosimetry on the porosity result of microcrystalline cellulose tablets was studied. The porosity parameters followed were total pore volume, mean and median pore size, and volume pore size distribution. Scanning speed did not affect the total pore volume of tablets compressed from microcrystalline cellulose. With increasing speed, the smallest pores of powder tablets were not properly determined, which increased the mean pore size. The median pore size of tablets compressed from powder and granules decreased and the maximum at the pore size range 500–1000 nm changed towards smaller pores with increasing scanning speed. Scanning speed appears to affect in different ways the samples with different physical structures. In tablet samples, scanning speed affects the volume of the pores at the whole pore size range determined. Thus, it is important to use about the same, reasonably low scanning speed in the measurements when comparing the samples. Swelling of microcrystalline cellulose in tablet samples is observed by mercury porosimetry measurement; a change in the pore structure is detected after storage at 88% relative humidity as increased total pore volumes and median pore sizes. Due to swelling, the maximum at the pore size range 500–2000 nm changed towards larger pores with increasing moisture. Swelling is observed similarly in tablets manufactured from powder and granules. When storing in humid conditions, water fills the smallest pores of microcrystalline cellulose powder tablets, hinders the intrusion of mercury and, thus, the mean pore size increases. Contrary to this, the volume of the smallest pores of granule tablets compressed with the highest compression pressure increased with increasing moisture. Careful pretreatment before the measurements is important.