(77d) Investigating Surface Energy Heterogeneity of Processed Lactose Powders

Particle surface properties are known to be affected by the processing routes and processing conditions used for generating particles. Inverse Gas Chromatography (IGC), particularly Finite-Dilution Inverse Gas Chromatography (FD-IGC), has been shown to be a versatile technique for surface energy characterisation of solids. In this study, lactose powders processed by both milling and sieving the similar crystalline α-lactose monohydrate starting material were used. The sieved samples used in this study were divided into top, middle and bottom fractions. FD-IGC was used to examine the surface energy heterogeneity of these processed crystalline α-lactose monohydrate powders. The dispersive component of surface energy at different surface coverages of lactose was determined in this work. The results indicated that all lactose samples investigated showed some degree of heterogeneity. Unmilled, coarse lactose powders showed lower dispersive surface energy values and fine powders, produced by milling, were found to have the highest dispersive surface energy values. The surface energy distribution obtained from the deconvolution of the surface energy data obtained from FD-IGC using a Boltzmann distribution model and simulation-fitting approach suggested possibly three different surface energy regions on the processed lactose samples. The surface energy region with highest dispersive surface energy ranges from 44-49 mJ/m² followed by a 38-45 mJ/m² region and a region with lowest dispersive surface energy of 26-31 mJ/m². For sieved samples, the surface energy distributions for the top and medium sieved fractions suggested that the surface energy of large, unbroken α-lactose monohydrate crystals consists of roughly equal amounts of higher and lower energy surfaces whereas, the bottom fraction with smaller particles had greater proportion of high energy sites and less of the lower energy sites than the other two sieved fractions. Overall, FD-IGC for energetic heterogeneity determination helped in characterising the changes in surface properties of powders produced by different processing operations. Moreover, the experimental analysis combined with modelling approach and surface energy deconvolution were able to quantifiably determine different surface energy regions on the processed lactose samples.