Atomic force microscopy study of adsorbed moisture on lactose particles

The thickness of the adsorbed moisture layer on particulate surfaces is of interest because it directly influences the interaction between individual particles and, as a corollary, the mechanical behavior of the bulk powder assembly. A technique has been developed to measure the thickness of these layers on individual particles using atomic force microscopy (AFM). The approach takes account of the presence of an adsorbed layer on the AFM probe tip and the deformation or bulging of the liquid layers due to attractive van der Waals forces as they approach one another. Results are presented for three samples of alpha -lactose monohydrate originating from different sources. The measurements of film thickness differ between the three samples. The differences can, in part, be explained in terms of different levels of surface roughness, although some variation due to impurities is also suspected. The absolute magnitude of the measured layer thickness is greater than that measured using a gravimetric technique. Work is ongoing to identify and resolve the discrepancy.