Evaluation of the surface free energy of inhalable powders

J T Pinto, S Zellnitz, M Bresciani, J G Khinast, E Roblegg, A Paudel

A successful dry powder inhalation (DPI) formulation necessitates an in depth understanding of the solid-state, surface and particulate proprieties of powder particles. In particular, surface energy could be critical when developing a successful formulation with the desired inter-particle interaction as these can considerably impact the manufacturing process and pharmaceutical performance. In the present work, a Wilhelmy plate-based contact angle measurement approach was used in order to assess the nature of the powder surfaces of three model carriers (Inhalac 70, Capsulac 60 and Pearlitol 300 DC) and one model API (salbutamol sulphate). The salbutamol sulphate of inhalable size was produced by jet milling and spray drying and the impact of each technique in particulate energetics was further evaluated. The Good and van Oss equation was used to determine the surface energy and the theoretical work of cohesion of the selected powders and work of adhesion between drug and carrier were calculated according to the Young-Dupré equation. The evaluated carriers showed similar trends for the contact angles measurements between the different used liquids, i.e. the increase in the values of contact angle with the increase in solvent polarity. A marked difference in surface free energy between the milled particles and the spray-dried particles of API was shown. Finally, data on work of adhesion/cohesions suggests that Inhalac 70/spray dried sabutamol sulphate pair may present the most promising outcome regarding aerosolization performance.

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