Mechanism of glidants: Investigation of the effect of different colloidal silicon dioxide types on powder flow by atomic force and scanning electron microscopy



The effect of hydrophilic and hydrophobic colloidal silicon dioxide types (CSD) on the flow characteristics of microcrystalline cellulose (MCC) under different mixing conditions was macroscopically measured using the angle of repose method, the bulk and tapped densities. CSD ameliorated the flow characteristics in general, but hydrophobic CSD was more effective compared to the hydrophilic types under gentle mixing conditions. The macroscopic effect was explained on the particle level by scanning electron (SEM) and atomic force microscopy (AFM) studies. The CSD distribution on the MCC surface was more uniform for the hydrophobic type and was independent from the mixing conditions used in this study. From the cumulative adhesion force distributions of the mixtures, determined by AFM, the mean and the standard deviation of the adhesion force were calculated. The means were 44.8 nN for MCC alone, 25.2 and 28.3 nN for mixtures containing the two hydrophilic types, and 13.8 N for the hydrophobic CSD under gentle mixing conditions in a Turbula mixer. Stronger mixing in a plowshare mixer led to a further reduction to 17.5 and 17.4 nN for the two hydrophilic types, while the hydrophobic CSD showing a value of 13.9 nN was unchanged. A linear correlation between the angle of repose and the adhesion force could be established, indicating that for routine measurements of the efficiency of a glidant the simple angle of repose method is sufficient. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:2635–2644, 2004