Odidi, Isa O.; (1990) Contact angle and surface energetics assessment of powders. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The wetting of pharmaceutical dosage forms such as tablets, capsules and powders is an initial step in the process of drug dissolution. This is followed by penetration of the liquid into capillaries of the powders that make up the dosage form. The rate of penetration may be limited by unfavourable surface energetics in which case disintegration and dissolution becomes difficult. In order to understand, predict and control the wetting process, studies on the powder surface energy and interfacial free energy of a liquid-powder system are essential. In this work such studies have been carried out through contact angle measurements using data from liquid penetration into powder beds and drop heights measured on compressed discs of powder. Eighteen powders were studied in this work, microcrystalline cellulose. Starch 1500, calcium carbonate, calcium phosphate, p-hydroxybenzoic acid, methyl p- hydroxybenzoate, ethyl p-hydroxybenzoate, propyl p- hydroxybenzoate, butyl p-hydroxybenzoate, calcium formate, calcium acetate, calcium lactate, calcium gluconate, calcium stearate, magnesium o stearate, stearic acid, polytetrafluoroethylene, and hard paraffin wax. The effect of vapour phase adsorption on liquid penetration and contact angle has been investigated. A detailed study of liquid transport in porous media has been carried out, and a comparison made between cylindrical tube model of Washburn (1921) and the STVF model of Miller and Miller (1955) in the interpretation of liquid penetration data. The two basic theories for the estimation of powder and powder-liquid surface energies i.e., the theory of surface energy components and the equation of state theory of interfacial tension, have been extensively studied. It has been established that there is a multiplicity of components of surface energy particularly of polar origin (comprising of electron donor and electron acceptor parameters which are asymmetric). It is impossible to determine surface energy from a single equation using only one liquid as the equation of state theory suggests. The existence and meaning of negative interfacial tension has been examined. A methodology for the determination of surface pressure encompassing electron donor and electron acceptor parameters has been introduced and the true meaning of surface pressure proposed.

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