A COMPARISON OF 2 CONTACT-ANGLE MEASUREMENT METHODS AND INVERSE GAS-CHROMATOGRAPHY TO ASSESS THE SURFACE ENERGIES OF THEOPHYLLINE AND CAFFEINE

The wettability of two powders has been assessed by three methods and values for surface energies have been determined. Two contact angle me thods have been used, both of which are variations of the Wilhelmy pla te approach. One method was to use compressed plates of powder, the ot her to stick uncompacted powder to a glass plate (as a support). It wa s found that the method using adhesion to the glass plate gave more re alistic values for the surface energy of the powders than did the comp acted plate approach. It can be concluded that the use of plates with powder adhered to the surface offers a significant advantage for conta ct angle assessment. The third method used was inverse gas chromatogra phy, which has seen little application to date in the pharmaceutical s ciences. It was found that the dispersion component of surface energy obtained by inverse gas chromatography (IGC) was almost identical to t he equivalent (Lifshitz-van der Waals) term obtained by use of the con tact angle data from the glass side method. The acid-base (polar) cont ributions to surface energy were not directly comparable, due to the d ifferent data manipulation methods associated with contact angle and I GC data. It can be concluded that the little used methods of adhering powder to an inert support for contact angle measurement and IGC have great promise as means of assessing the surface properties of powders. Both these techniques have the advantage of not requiring powder comp action (or any other surface damaging treatment) prior to use.