Inverse gas chromatography: investigating whether the technique preferentially probes high energy sites for mixtures of crystalline and amorphous lactose.

PURPOSE: Inverse gas chromatography (IGC) is rapidly gaining popularity as a method for assessing powder surface energy. It is vital to understand what IGC measures if results are to be useful. This work examines the view that IGC preferentially measures high-energy sites on a powder surface.
METHODS: Mixtures of amorphous (high-energy) and crystalline (lower energy) lactose particles were prepared and tested using IGC with nonpolar probes. The surface area of the particles was measured in situ in the inverse gas chromatograph.
RESULTS: The results were weighted averages of the surface energy for amorphous and crystalline material until the amorphous content exceeded 15% w/w of the sample, after which the surface energy become equivalent to that of the amorphous form. The amorphous content dominated when the surface area was 40% of the total area. Given that the amorphous particles were much smaller and adhered to the crystalline ones, it is reasonable to conclude that many (most) of the binding sites on the surface of the crystalline particles were masked by the amorphous particles by the time that the amorphous content dominated the surface energy measurements.
CONCLUSIONS: IGC does not simply measure the high-energy sites in the packed column, but equally there is a complex process that results in measured data on mixtures not being a weighted mean of the surface energy of the two components.