Investigation of the glidant properties of compacted colloidal silicon dioxide by angle of repose and X-ray photoelectron spectroscopy

The optimal flow-enhancing effect of a new compacted, hydrophilic colloidal silicon dioxide (AEROSIL® 200 VV Pharma) on microcrystalline cellulose (Avicel® PH 101) and pregelatinized starch (Starch 1500®) was found to be 0.25% under gentle and 0.125% under strong mixing conditions, as measured by the angle of repose. The effect could be explained by X-ray photoelectron spectroscopy (XPS) investigations. The Si 2p signals of the silicon dioxide indicated that the coverage of the excipient surface with AEROSIL® was greater for all mixtures produced under strong mixing conditions and corresponded to a higher degree of de-agglomeration of the AEROSIL® aggregates. The more extensive surface coverage of Avicel® PH 101 as compared to Starch 1500 could be explained by the particle morphology on the one hand and by the XPS C 1s signals on the other. Due to the different conformation of the two excipients, Avicel® PH 101 offers a higher density of hydroxyl groups on its surface which are available for hydrogen bonding with the surface hydroxyl groups of hydrophilic colloidal silicon dioxide.