INFLUENCE OF RELATIVE-HUMIDITY OF STORAGE AIR ON THE ADHESION AND AUTOADHESION OF MICRONIZED PARTICLES TO PARTICULATE AND COMPACTED POWDER SURFACES

The influence of the relative humidity of storage air on the median ad hesion force at the interface of lactose monohydrate and salmeterol xi nafoate has been investigated by the centrifuge technique using partic le-on-particle and particle-on-surface experiments. The relative humid ity of storage air can strongly affect the median adhesion force. Cond ensed moisture was found to result in strong irreversible solid bridge bonds or in weak solid bridge bonds that are of comparable strength t o Lifshitz-van der Waals forces obtained under conditions of low relat ive humidity up to 35%. The effect was dependent on the structure of t he substrate surface, the solubility of the particles adhered, and the solubility of the surface material. The use of different substrate su rfaces such as particles and compacted powder disks of identical mater ials led to different results. Hence, if the influence of relative hum idity of storage air on the adhesion between particles in, for example , an interactive mixture is to be predicted, the adhesion measurements have to be performed using particle-on-particle adhesion force measur ements. Relative humidity of the air greater than 75% results in irrev ersible, strong forces acting at the interface of the particles. Below this humidity level, capillary forces can be removed by storing the m ixtures at a very low relative humidity of about 5% for at least 72 h. (C) 1997 Academic Press.