Physical and dielectric properties of pharmaceutical powders

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The limited availability of published physical and dielectric property data for pharmaceutical powders hinders the design of processing systems, particularly dryers. In this study, the physical properties (solubility and boiling point) and dielectric properties, in terms of temperature rise, dielectric constant, and dielectric loss factor, of selected pharmaceutical powders were measured. The pharmaceutical actives, paracetamol and aspirin, and selected common bulk excipients, together with a range of solvents including water, ethanol, methanol, and acetone, were used. The solubility of the powders was found to be solvent dependent and increased with increasing temperature. Solute concentration was also shown to affect the solvent boiling point, with the boiling points for certain systems being elevated or reduced by up to 11 and 33 °C, respectively. Research into the dielectric properties of pharmaceutical materials has, in particular, been prompted by widespread interest in microwave radiation as an alternative energy source for drying. The temperature rise induced in the powders and solvents during microwave heating was shown to give an effective indication of the relative dielectric properties. Greater temperature rises were observed in liquid and ionic materials. The dielectric constant and loss factor of a dry mixture was found to be dependent on the relative volumetric fraction and dielectric properties of the components. The temperature rise increased with increasing moisture content; however, the solubility of the solute in the solvent had a significant effect on the dielectric properties of the mixture. In general, the dielectric constant and loss factor increased with increasing moisture content. However, for selected powders, the dielectric properties at the critical moisture content were significantly greater than those at higher moisture contents.