CYLINDRICAL TUBE AND SURFACE-TENSION VISCOUS-FLOW MODELS IN THE ASSESSMENT OF CAPILLARY-FLOW AND LIQUID CONTACT ANGLES IN PHARMACEUTICAL POWDERS

The validity of the cylindrical tube (CT) model for the estimation of contact angles of irregular porous media using liquid penetration data is based on the linearity of l vs t1/2 plots which assumes the same g eometric contribution for liquids exhibiting different contact angles in the same medium. In recent criticisms it was observed that conforma tion of penetration data to t1/2 dependence has also been predicted fo r systems other than cylindrical tubes, and a method for obtaining con tact angles based on surface tension viscous flow (STVF) model and sca ling concepts was proposed. To assess the validity of these proposals, the rates of liquid penetration of a homologous series of alkanes, al cohol and carboxylic acids into fifteen pharmaceutical powders (Avicel PH101, Starch 1500, magnesium stearate, calcium carbonate, calcium ph osphate, calcium formate, calcium acetate, calcium lactate, calcium gl uconate, calcium stearate, p-hydroxybenzoic acid, methyl p-hydroxybenz oate, ethyl p-hydroxybenzoate, propyl p-hydroxybenzoate and butyl p-hy droxybenzoate) were determined. Contact angles were obtained according to the CT model. Plots of l vs t1/2 were linear as predicted by the C T model. Plots of reduced distance travelled as a function of reduced time, based on the STVF scaling concepts, did not superimpose on a sin gle curve, an indication that the systems studied were in dissimilar s tates, exhibiting differing contact angles. On the other hand, contact angles obtained from STVF scaling concepts were similar, thus indicat ing that the systems are in similar states. This contradicts the resul ts from plots of reduced distance vs time. The inability of both appro aches to reach the same conclusion questions the validity of the unive rsal application of the STVF model. The STVF model does not adequately describe liquid-powder systems which display finite and differing con tact angles when different liquids are used on the same powder. This m ay be due to the differences in wettability and surface energetics inh erent in such systems and not necessarily because of geometric factors .