THE INFLUENCE OF CONSTANT AND CHANGING RELATIVE-HUMIDITY OF THE AIR ON THE AUTOADHESION FORCE BETWEEN PHARMACEUTICAL POWDER PARTICLES

The influence of the press-on force applied and the relative humidity of the air during the storage of autoadhesion samples made from lactos e monohydrate or salmeterol xinafoate has been studied using the centr ifuge technique. Capillary forces can be obtained for lactose monohydr ate if subjected to a relative humidity of 75% or more. The resulting strong autoadhesion forces are reversible by subsequently storing the autoadhesion samples at a low humidity environment. The strength of th e capillary forces depends on the storage time, and to reach an equili brium level requires more than 72 h. For the less polar salmeterol xin afoate the performance of capillary forces could not be proved with ce rtainty. It appears that a very small increase in autoadhesion force a l higher relative humidity levels is more likely caused by adsorbed wa ter acting as a plasticizer. Model calculations of capillary forces di d not prove successful in predicting the autoadhesion force of the mat erials if stored at a high relative humidity of the air. The knowledge of the surface free energy and its polar component gave a better basi s for a prediction. Copyright (C) 1996 Elsevier Science B.V.