The influence of constant and changing relative humidity of the air on the autoadhesion force between pharmaceutical powder particles

Abstract

The influence of the press-on force applied and the relative humidity of the air during the storage of autoadhesion samples made from lactose monohydrate or salmeterol xinafoate has been studied using the centrifuge technique. Capillary forces can be obtained for lactose monohydrate 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 the capillary forces depends on the storage time, and to reach an equilibrium level requires more than 72 h. For the less polar salmeterol xinafoate the performance of capillary forces could not be proved with certainty. It appears that a very small increase in autoadhesion force at higher relative humidity levels is more likely caused by adsorbed water acting as a plasticizer. Model calculations of capillary forces did not prove successful in predicting the autoadhesion force of the materials if stored at a high relative humidity of the air. The knowledge of the surface free energy and its polar component gave a better basis for a prediction.