VARIATIONS IN THE ADHESION FORCE BETWEEN A DRUG AND CARRIER PARTICLESAS A RESULT OF CHANGES IN THE RELATIVE-HUMIDITY OF THE AIR

The influence of the press-on force applied and the relative humidity of the air during storage of adhesion samples made from lactose monohy drate surfaces (as a model carrier) and salmeterol xinafoate (as a mod el drug) particles has been studied using the centrifuge technique. Ca pillary forces were observed for samples which were subjected to a rel ative humidity of 75 or 95%. The resulting strong adhesion forces betw een drug and carrier particles cannot be reduced by a restorage of the samples at a low relative humidity. An equilibrium Value of the capil lary force was reached within 72 h. Experiments where particles adhere d to compacted powder surfaces lead to the same conclusions as experim ents where the particles adhered to carrier particles. Model calculati ons have shown that it is possible to establish a relationship between adhesion forces and the interfacial energy of the materials in contac t. If a minimisation of the adhesion strength is sought, the interfaci al energies between the surfaces should be as small as possible when L ifshitz-van der Waals' forces are the main adhesion force component, w hereas in the case when capillary forces dominate the adhesion process , the interfacial energy between the contiguous surfaces should be lar ge. (C) 1997 Elsevier Science B.V.