A comparison of two 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 methods have been used, both of which are variations of the Wilhelmy plate approach. One method was to use compressed plates of powder, the other to stick uncompacted powder to a glass plate (as a support). It was found that the method using adhesion to the glass plate gave more realistic values for the surface energy of the powders than did the compacted plate approach. It can be concluded that the use of plates with powder adhered to the surface offers a significant advantage for contact angle assessment. The third method used was inverse gas chromatography, which has seen little application to date in the pharmaceutical sciences. It was found that the dispersion component of surface energy obtained by inverse gas chromatography (IGC) was almost identical to the equivalent (Lifshitz-van der Waals) term obtained by use of the contact angle data from the glass side method. The acid-base (polar) contributions to surface energy were not directly comparable, due to the different data manipulation methods associated with contact angle and IGC 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 compaction (or any other surface damaging treatment) prior to use.