Particle growth in aqueous suspensions: the influence of surface energy and polarity

Abstract

The aggregation of drug particles in suspensions can cause products to be spoiled. In this paper the role of surface energetics in controlling the dispersion of powders in water is considered, by use of a spreading coefficient. The contact angles, surface energies and the polarities were determined for a series of barbiturate powders. Contact angles were measured using a Wilhelmy plate technique. Aqueous suspensions of each powder were prepared and the change in particle size was monitored over a period of hours and days. By extrapolation the limiting size of the aggregates was calculated and this was related to the original particle size of each powder. The powders were largely hydrophobic in nature, and as such were difficult to disperse in the liquid. It was determined that dispersion was possible only if the negative spreading coefficient of water over the powder did not have a significantly larger value than the dispersion component of the water. This has been explained in terms of the hydrophobic powder tending to interact with the dispersive, rather than the polar, entities in the water. For powders which spontaneously dispersed in the water a good correlation existed between the increase in particle size noted in the suspension and the spreading coefficient.