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 adhesion force at the interface of lactose monohydrate and salmeterol xinafoate has been investigated by the centrifuge technique using particle-on-particle and particle-on-surface experiments. The relative humidity of storage air can strongly affect the median adhesion force. Condensed moisture was found to result in strong irreversible solid bridge bonds or in weak solid bridge bonds that are of comparable strength to Lifshitz-van der Waals forces obtained under conditions of low relative humidity up to 35%. The effect was dependent on the structure of the substrate surface, the solubility of the particles adhered, and the solubility of the surface material. The use of different substrate surfaces such as particles and compacted powder disks of identical materials led to different results. Hence, if the influence of relative humidity 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 measurements. Relative humidity of the air greater than 75% results in irreversible, strong forces acting at the interface of the particles. Below this humidity level, capillary forces can be removed by storing the mixtures at a very low relative humidity of about 5% for at least 72 h.