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Determination of the Polar and Total Surface Energy Distributions of Particulates by Inverse Gas Chromatography Shyamal C. Das,* Ian Larson, David A. V. Morton, and Peter J. Stewart Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville campus), 381 Royal Parade, Victoria 3052, Australia Received October 13, 2010. Revised Manuscript Received November 25, 2010 This Letter reports a technique of measuring polar surface energy distributions of lactose using inverse gas chromatography (IGC). The significance of this study is that the total surface energy distributions can now be characterized by combining the already known dispersive surface energy distribution with polar surface energy distribution determined in this study. The polar surface energy was calculated from the specific free energies for surface interactions with a monopolar basic probe, ethyl acetate, and a monopolar acidic probe, dichloromethane.
Introduction Surface energy is a combination of dispersive (i.e., due to longrange forces such as van der Waals forces) and specific or polar interactions (i.e., due to acid-base interactions).1 Surface energy is directly related to interparticulate interactions or forces of adhesion.2,3 An understanding of surface energy is, therefore, important in any industrial processing that involves interparticulate interactions such as coating, wetting, agglomeration of solids, powder handling, dispersion of liquids, and catalysis processes.4-6 For example, changes to powder surfaces by coating7,8 may change the surface energy, resulting in a change in bulk properties such as flow and angle of repose. Inverse gas chromatography (IGC) has been used for determining surface energy.9 In IGC, both nonpolar and polar probes are passed through a packed column of a solid at a very low concentration or infinite dilution (where the retention of probe is independent of injection volume and, therefore, the isotherm is linear). The dispersive surface energy is calculated from the retention volumes of a homologous series of alkanes using either the Dorris-Gray10 or Schultz approach,11 whereas the specific free energies are calculated from the retention volumes of polar probes. Surface energy measurements at infinite dilution measure only the highest energy sites of a material surface which is often
