The effect of the comminution technique on the surface energy of a powder

Six samples of powdered aspirin were prepared from one crystalline source by use of ball, vibrating ball, hammer and fluid energy mills, either alone or in combination. These were compared with a sample of powdered aspirin obtained from Macarthy's. The surface energetics of the classified and milled samples were assessed with a vacuum microbalance and a microcalorimeter, to monitor water adsorption. Thermodynamic parameters and apparent rate constants for adsorption were obtained. Milling was shown to alter surface energetics in a manner related to the perceived energetics of the mill. Powders which were milled by two consecutive processes were found to retain the surface energetics imparted by the first process. Electron micrographs have been used to attempt an explanation of this phenomenon. The wetting process seems to be controlled by the availability of adsorption sites on the powder and the degree of order that is consequently imposed on adsorbed water; the entropy of adsorption is most directly related to wettability.