Characterising surface energy of pharmaceutical powders by inverse gas chromatography at finite dilution

Objectives The objectives of this project were the use of surface energy distributions in: distinguishing the effects of magnesium stearate on the surface energy of lactose processed by two methods: mixing in a Turbula and mechanofusion; characterising surface energy of materials before and after micronisation; and understanding surface energy changes of micronised lactose before and after storage at high relative humidity (RH). Methods Heptane, octane and nonane were used to determine nonpolar surface energy, and dichloromethane and ethyl acetate were used to determine polar surface energy in inverse gas chromatography at finite dilution. Key findings The total surface energy of lactose decreased more after mechanofusion with magnesium stearate than mixing in Turbula. The nonpolar surface energy of indometacin increased while polar and total surface energies decreased after micronisation. The nonpolar, polar and total surface energies and work of cohesion of micronised lactose decreased after storage at 75 RH for three months. Conclusions The surface energy distributions determined at finite dilution successfully distinguished and revealed more information than infinite dilution on surface energy changes in materials undergoing different pharmaceutical processes such as mixing, mechanofusion, micronisation and storage at high RH.