Abstract
The Washburn method was used to determine the dynamic contact angles of theophylline and salicylic acid powders. The suitability of the materials for liquid intrusion studies was assessed by packing studies. The liquids used were distilled water, methanol, 1-propanol, 1-butanol, 1-octanol and n-hexane. Liquid intrusion was studied with saturated solutions. Despite similarities in the morphology of the particles theophylline presented good packing properties whereas salicylic acid formed a heterogeneous powder bed structure. 1-Propanol was the best wetting liquid for both substances. Water did not penetrate into the salicylic acid powder bed. The contact angle of 1-propanol and the powders was estimated to be 0°. With theophylline the following values for the other liquids were obtained: water, 70.3°; hexane, 28.7°; octanol, 26.6°; methanol, 22.0°; and butanol, 12.3°. With salicylic acid the values obtained were: methanol, 81.7°; hexane, 73.7°; octanol, 71.5°; and butanol, 59.7°. Using the contact angle data obtained, the surface free energy parameters of theophylline, salicylic acid and, with data from a previous study, of lactose, were determined. The parameters were calculated with the geometric mean, harmonic mean and geometric-harmonic mean methods. The validity of the calculated surface free energy parameters is discussed on the basis of spreading coefficients. The dynamic contact angles do not correlate to the calculated surface free energy parameters, whereas the slope of distance squared vs time plots of liquid intrusion showed a relationship with the spreading coefficients.
Published Version
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