Investigating the applicability of inverse gas chromatography to binary powdered systems: an application of surface heterogeneity profiles to understanding preferential probe-surface interactions.

The aim of this study was to investigate the applicability of surface energy characterization tools such as inverse gas chromatography for the analysis of binary systems. Drug substance was coated with two grades of silicon dioxide and the surface energy characteristics determined using a surface energy analyser. The results demonstrated that the measured dispersive surface energy of such intermediate samples were as a consequence of probe interactions with both constituent components, however, the degree and order of interaction with each species was related to surface energy heterogeneity and surface availability. A method to predict the degree of probe-surface preferentiality within the intermediate samples was applied to the data, demonstrating to closely match the measured data whilst suggesting notable differences in probe-surface preferentiality. Specific probe interactions were also assessed and the results suggested that probe surface preferentiality was not equivalent to that of the dispersive probes, possibly due to differences in ranges of the dispersive/specific forces. An equivalent physically mixed sample was analysed and the results demonstrated that the measured heterogeneity curve mirrored that of the pure drug substance suggesting that the driver for probe interaction is different for the physically mixed and the coated intermediate samples.