Abstract
The surface free energy of solids is important to processes and product performance in a number of industries. As only a few scientists in the pharmaceutical industry have training in surface and/or adhesion science, surface free energy is not often recognized as a source of product variation. In this paper, the determination of the surface free energy of solids is reviewed. In particular, the determination of surface free energy by IGC (inverse gas chromatography) is discussed. Determination of surface free energy by IGC is, furthermore, compared to determination using contact angles and AFM (atomic force microscopy). IGC is highly suitable to pharmaceutical powders. Unlike contact angle measurements, it does not require large surfaces for the measurements to be made. The solubility of pharmaceutical materials in the various probe liquids is also of concern as dissolution can affect the measured contact angle. Research has determined that the surface free energy of various pharmaceutical powders depends on their mechanical processing history. Particle milling and crystallization from various solvents are examples of processes that may affect the surface free energy. Variation in surface free energy can affect the performance of dry powder inhalers and the tensile strength of tablets. In the future, the surface free energy and its effects on process and product performance need further attention.
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