Change in surface energy of lactoses after storage at high relative humidity deterined by inverse gas chromatography

Dry poweder inhalers (DPI) show significant difference in performance after storage at high relative humidity (RH) (1). This could be due to changes in drug and exipient surface energy which can influence the relative strength of particle adhesion and agglomeration. Lactoses are the most commonly used excipeints in DPIs. The use of micronised lactose (ML) in addition to coarse lactose (CL) has been shown to significantly improve DIP efficiency (2). However, surface energies of micronised and un-micronised lactoses could vary due to differences in surface crystal dislocation and the development of amorphous domains. Therefore, while stored at high RH, the change in crystallinity could be different for CL and ML leading to different surface energy changes. Thus, they can influence perfomance differently after storage. Surface energy consists of dispersive/non polar and specific polar components. The aims of this research were to evaluate the effect of RH on the surface energies of CL and ML using infinite and finite dilution methods and to compare the heterogeneity profiles.