Influence of hydrophobic materials on dissolution of a nondisintegrating hydrophilic solid (potassium chloride).

A nondisintegrating hydrophilic solid was covered with coatings of hydrophobic materials as a model to study their influence on dissolution rates. Both solution and sublimation techniques were satisfactory for producing coatings of stearic acid. Only a sublimation method was applicable to magnesium stearate, but the resulting coating consisted mainly of a pyrolysis product, stearone. Reductions in dissolution rates in a continuous-flow system could best be interpreted by assuming that they reflected changes in the area of the hydrophilic solid exposed to the solvent. This concept is based on the assumptions that the intrinsic dissolution rate of uncoated potassium chloride did not change and that no dissolution occurred from coated areas. The most important factors controlling the dissolution rate were the strength of attachment of the hydrophobic coat to the surface of the hydrophilic substrate and the thickness of the coat.