![]() ![]() Important measurement categories concerning moisture are the relative humidity of the air, the water activity, and the equilibrium moisture content of a powder sample. Multilayer water adsorption consists of water uptake into pores and capillary spaces, dissolution of solutes, and finally the mechanical detention of water. Thus, moisture is adsorbed as mono- or multilayers or may be present as normally condensed moisture. Diffusional forces exceed the binding forces as more water molecules adhere to the surfaces and moisture is transferred into the material. Water molecules first adsorb onto the surfaces of dry material to form a monomolecular layer, which is subjected to both surface binding and diffusional forces. ![]() The adsorption process occurs with the water forming hydrogen bonds with the hydrophilic sites on the surface of the solid. Īiraksinen reviewed the interactions between moistures and solids. If water is adsorbed onto the surface of the powder particles, the water distribution will depend on the hydrophilic properties of the powder. But the humidity can also simply be integrated into the powder particle, which can be seen for lignite. If the water is absorbed, it can lead to a physicochemical change of the powder material, e.g., formation of a hydrate, such as with anhydrous theophylline. The amount of water sorbed is a function of the affinity between the surface and the water molecules, temperature, relative humidity (RH), and in case of adsorption, and also the amount of exposed surface area. Water can interact only at the surface of solids (adsorption), or water can penetrate the bulk solid structure (absorption). The effect of moisture on the flowability depends on the amount of water and its distribution. Pharmaceutical solids may be exposed to water during storage in an atmosphere containing water vapor, or in a dosage form consisting of materials that contain water (e.g., excipients) and are capable of transferring in to other ingredients. Additionally, the previous history of the material, which means the previous consolidation stress, can have an influence on powder flow ability. Powder flow is influenced by environmental factors, such as moisture and static electricity, as well as powder related factors, such as morphology, size, size distribution, true density, surface area, and chemical composition. An electrostatic force, F es, only participates if the particles and/or surfaces have been actively charged. The value of F e is a constant for two materials in contact, and F e cannot arise if moisture is present in the gap between the contiguous surfaces. Consequently, in a bulk of smaller powder particles, higher capillary forces are acting. ![]() Capillary forces are inversely proportional to the particles’ size, but the effect described above is also valid for these forces. The capillary force has been identified as the force which dominates over any other force component, once the relative humidity of the air has reached a value of 65 to 75%. Contributing factors are the interfacial tension and capillary pressure. The capillary force or liquid bridges depends on the percent of water and its distribution. But as the number of particle contacts per square unit is indirectly proportional to the square of particle size, the smaller the particles are, the higher the sum of the van der Waals’ forces are. The van der Waals’ force is proportional to the particle size. propose to include solid bridges to the cohesive forces. Where F ad is the force of adhesion, F vdw is the Lifshitz-van der Waals’ force, F c is the capillary force, F e is the electrical force, and F es is the electrostatic (Coulomb) force. ![]()
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