THE EFFECT OF SURFACE-TREATMENT ON THE VALUES OF CONTACT ANGLES MEASURED ON A COMPRESSED POWDER SURFACE

Contact angles formed by a series of alkanes, water and glycerol have been determined on compressed discs of ten powders (Avicel PH-101, starch, magnesium stearate, and the following calcium salts: carbonate, phosphate, stearate, formate, acetate, lactate and gluconate), by use of the drop height method. The contact angles were assessed on untreated compacts, and on compacts which had been cleaned in a vacuum chamber and re-equilibrated with air saturated with the vapour of the wetting liquid. Under both sets of conditions the beds were saturated with the test liquid prior to measuring a maximum height drop. There was no correlation between the surface tension of the alkanes and their contact angle on the powders. The contact angles formed by the alkanes were not subject to significant change after vacuum treatment of the surface. Contact angles formed by polar liquids (water and glycerol) were higher after vacuum treatment. The difference in values was greatest for the most hydrophobic powders. To assess the implications that the different values of the contact angle may have on the predicted performance of the powders, the spreading coefficients were calculated to examine the interaction between the powders and two polymeric binders (HPMC and PVP). The values of contact angles before and after treatment, yielded values for the spreading coefficients that did not correlate with each other (i.e. different rank orders). However, in only one case (magnesium stearate) would the prediction of best choice of binder change on the basis of the two sets of data. Thus, the results demonstrate that the surface treatment and environmental conditions to which a powder has been exposed will change the measured contact angle, and may invalidate predictions that are made from such data.