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Abstract
The adhesion force between salmeterol base and a series of chemically related salmeterol salts to various macroscopically flat substrate materials has been investigated. The substrates varied in their surface roughness, surface free energy, and Young's modulus. The results suggest that the adhesion force of a particulate material to a macroscopically flat substrate depends on the physical properties of the substrate such as surface roughness, surface free energy and Young's modulus, but there is no universal relationship. The properties of both the particles and the substrates have to be considered for possible interactions. A knowledge of the Hansen-solubility parameters of the particulate materials allows a prediction of rank order of the adhesion strength of particulate materials to a given substrate, but requires a knowledge of similar parameters for the substrate material also. The general rule is that materials which have, for example, similar hydrogen bonding terms or similar dispersion terms adhere strongly to each other, whereas the least adhesion strength will be obtained between materials of different polarities. However, the rank order of adhesion strength of a given particulate material to various macroscopically flat substrates cannot be predicted theoretically. Only the experimental assessment of the adhesion force allows a judgement as to whether or not a substrate material is the appropriate choice.