Characterization of solid excipients: delving deeper by probing the surface.
Performance differences in different lots of solid excipients often cannot be detected by bulk physicochemical methods. This is because some of these (performance) differences originate at the surface of individual excipient particles, accounting for attributes that comprise a disproportionately large part of the surface but a very small fraction of the total amount of the material. Because surface interactions play a significant role in the formulation and stability of solid dosage forms, it should not be surprising that excipients that
may be indistinguishable at the compendial, and/or bulk level, may yet behave differently due to their different surface characteristics. Surface acidity, surface free energy, surface charge, surface roughness and surface
amorphicity (crystallinity) of a bulk excipient powder represent attributes that are not usually measured as part of routine raw material testing by the end user. In addition to the apparent values of these surface characteristics, their intra- and inter-lot heterogeneity can influence functionality, and in some instances, significantly cause it to change.