Ahfat, Nathalie Marilyn Wang; (1998) The role of surface energetics in the mixing of powders. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The mixing of powders is governed by the presence of interparticulate forces, which are dependent on the surface nature of powders. Surface energy, which provides a measure of the hydrophilic and hydrophobic nature of surfaces, was found to play a significant role in the behaviour of powders in mixes. Preliminary work involved the use of surface energetics to predict the spreading behaviour in binary mixes of iron oxide with three excipients; microcrystalline cellulose, magnesium stearate and titanium dioxide. Subsequent work was carried out on binary and tertiary mixes of an active compound, zamifenacin, with starch and/or lactose. The surface nature of the powders was determined using the Wilhelmy plate (DCA) technique, sessile drop technique (DAT), inverse gas chromatography (IGC) and triboelectric charging. These techniques (except for triboelectric charging) involve the use of non-polar and polar liquids to probe the surfaces of powders. Spreading coefficients determined from the DCA data, correctly predicted the spreading behaviour of powders in binary and tertiary systems of iron oxide, as confirmed by visual inspection and scanning electron microscopy of the mixes. The suitability of each technique in characterising surfaces of pharmaceutical powders was assessed and IGC was found to be superior to the DCA and DAT techniques since IGC relied on the direct interaction between the vapour probes and the powder surface. The interaction parameter (I) derived from IGC and the electrostatic charges developed on charging with stainless steel predicted that the interaction between zamifenacin and starch would be more favourable than between zamifenacin and lactose. These predictions were confirmed by homogeneity studies on binary and tertiary mixes of zamifenacin using high-pressure liquid chromatography and near-infrared spectroscopy (NIRS). NIRS was found to be a good alternative analytical technique for monitoring the mixing of powders, in particular the order of addition of excipients to zamifenacin.

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