Sheridan, Louise; (1994) Assessment of the surface properties of pharmaceutical powders. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The purpose of this study was to evaluate and improve some of the available methods of assessing the surface properties of powders. The value of an automated Wilhelmy plate contact angle measurement technique was investigated. This technique has not previously been used extensively for powders. It was found to be a valuable and simple approach which is suitable for a wide range of materials. The resolution of this technique is not sufficient to allow discrimination between the surface properties of powders, which are similar in nature. Reproducibility of data could not be improved by controlling the experimental procedure extensively. Surface energies were calculated from contact angle data using four of the available theories. The harmonic and geometric mean equations provided the most suitable approaches for pharmaceutical systems. A microcalorimetry method, using a novel cell was evaluated for the investigation of powder/water interactions. Data obtained was highly reproducible and therefore, it was possible to detect differences in the wetting behaviour of three commercial brands of α-lactose monohydrate, which was not possible using contact angle data. A new microbalance technique was tested, however, no reliable data were obtained using this method. A study was also carried out to investigate the relationship between surface properties (contact angles against water and/or surface energies) of pharmaceutical powders and several molecular orbital indices. A relationship between the contact angle against water and the surface energy and the superdelocalisability index was elucidated. Two empirical equations were proposed which would enable workers to determine the surface properties of pharmaceutical powders from molecular structure. The limitations of this approach were discussed. Calculated molecular orbital parameters provide an extremely good first indication of material surface properties, but should be substantiated by experimental methods. Contact angle measurement offers a relatively fast and simple indication of surface properties. However, where it is necessary to discriminate between the wettability of powders of a similar nature, flow microcalorimetry is the method of choice.

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