Particle diffusional layer thickness in a USP dissolution apparatus II: A combined function of particle size and paddle speed

Abstract

This work was to investigate the effects of particle size and paddle speed on the particle diffuisonal layer thickness h app in a USP dissolution apparatus II. After the determination of the powder dissolution rates of five size fractions of fenofibrate, including <20, 20–32, 32–45, 63–75, and 90–106 µm, the present work shows that the dependence of h app on particle size follows different functions in accordance with the paddle speed. At 50 rpm, the function of h app is best described by a linear plot of $h_{{rm app}} = 9.91sqrt d - 23.31$ ( R 2 = 0.98) throughout the particle diameter, d , from 6.8 to 106 µm. In contrast, at 100 rpm a transitional particle radius, r , of 23.7 µm exists, under which linear relationship h app = 1.59 r ( R 2 = 0.98) occurs, but above which h app becomes a constant of 43.5 µm. Thus, h app changes not only with particle size, but also with the hydrodynamics under standard USP configurations, which has been overlooked in the past. Further, the effects of particle size and paddle speed on h app were combined using dimensionless analysis. Within certain fluid velocity/particle regime, linear correlation of h app / d with the square-root of Reynolds number $(dvarpi /upsilon )^{1/2} $ , that is, $h_{{rm app}} /d = 1.5207 - 9.25 times 10^{ - 4} (dvarpi /nu )^{1/2} $ ( R 2 = 0.9875), was observed. © 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:4815–4829, 2008