Fine powder flow under humid environmental conditions from the perspective of surface energy

• Amorphous and crystalline pharmaceutical fine powders of hydrophilic and hydrophobic nature studied for flow and surface energetics at varying RH.
• Surface energy and powder flow of hydrophilic powders affected by relative humidity (RH) whereas remained unaffected for hydrophobic drug.
• Not much variation in γLW but γSP of hydrophilic excipients increased with RH.
• Evidence of electrostatic charge or frictional forces at low RH and increased capillary forces at high RH.
• γSP component of surface energy an effective indicator of liquid induced flow behavioral changes in fine hydrophilic powders.
• Identified different regimes of humidity-dependent adhesion forces.

The influence of humidity on surface energetics and flow behavior of fine pharmaceutical powders was investigated. Amorphous and crystalline fine powders with hydrophilic (Corn starch and Avicel PH105) and hydrophobic (ibuprofen) nature were considered for this study. The surface energy was determined using surface energy analyzer and flow behavior was measured in terms of unconfined yield stress (UYS) using a shear tester. The study showed that unlike hydrophobic ibuprofen powder, surface energy and flow of hydrophilic excipient powders were affected by relative humidity (RH). The Lifshitz–van der Waals dispersive (γLW) component of surface energy barely changed with varying RH for all pharmaceutical powders. For hydrophilic excipients, the specific component of surface energy (γSP) was found to increase with increasing RH. Furthermore, for these excipients, flow deterioration at elevated RH was observed due to increased capillary bridge formation. Detailed analysis showed that γSP component of surface energy can be an effective indicator for flow behavior of fine powders under varying humid conditions. The present study also brought out the existence of different regimes of probable interparticle forces which dictate the bulk flow behavior of fine hydrophilic powder under humid conditions.

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