Dove, Jason Willoughby; (1998) A comparison of the physicochemical characterisation of powders using a selection of current methods. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The surface characterisation of six powders (Caffeine, Ethacrynic acid, Ibuprofen, Salicylic acid, Saquinavir and Theophylline) was compared using a selection of traditional and novel methods; one liquid penetration and two Wilhelmy plate methods utilising contact angles, inverse gas chromatography (IGC), isothermal microcalorimetry (IM) and dynamic vapour sorption (DVS). All the techniques distinguished between the chemically different powders, although only IGC, IM and DVS were sensitive enough to detect batch to batch variation of the Saquinavir after it had been wetted and dried under different conditions. Of the three methods using contact angle analysis, the powder coated Wilhelmy plate method was the most versatile and accurate, as reproducible contact angles were consistently obtained with all powder and probe liquid combinations. Surface energy data were calculated using two theories, one producing a more complete analysis, with non-polar, polar, and acid-base values. IGC, DVS and IM discovered small variations with Saquinavir between batches, IM being the most sensitive. IGC produced non-polar surface energies that were directly comparable to the contact angle analysis, whilst it only gave a qualitative indication of acid-base nature. DVS and IM measured water vapour adsorption, useful in their own right, which were combined to achieve a detailed thermodynamic analysis of the adsorption process with changing relative humidity. In conclusion, the ideal surface characterisation method depends on many factors, for example, sensitivity required, powder quantity available and whether the surface energy is being evaluated or compared for batch to batch variation. It is evident that batch to batch variation is not readily detectable with contact angle techniques, which should therefore be reserved for a quick indication of the magnitude of a powder's surface energy. Whereas DVS, IM and IGC provide straightforward and highly sensitive methods of analysing surface properties and the degree of variation between batches of the same drug.

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