Investigating sizing induced surface alterations in crystalline powders using surface energy heterogeneity determination

Abstract Particle sizing is the most commonly employed and critical unit operation across powder processing industries. In this work, we show the surface energy changes prompted by the sizing operations like milling and sieving in α-lactose monohydrate powders using Finite Dilution Inverse gas chromatography (FD-IGC) analysis. Three separate sieved fractions of α-lactose monohydrate powder were divided into a top, middle and bottom fraction from the same starting material. Similarly, a custom grade α-lactose monohydrate was milled for a different duration to produce two milled samples with different median particle sizes. Sieved sample results showed that the bottom fraction exhibited higher heterogeneity with higher dispersive (γd) surface energy values ranging from 42.5 mJ/m2 to 45.9 mJ/m2 compared to the top and middle fractions. The finest fraction contains more cleaved surfaces that are exposed during the preparation process, i.e. milling, of the material resulting in different surface properties. Furthermore, the surface energy analysis of the milled samples revealed slight but vital differences in the γd heterogeneity profiles. The site-specific distribution of energies was obtained using the Boltzmann probability distribution model and revealed two distinct regions for crystalline α-lactose monohydrate. Thus, our work confirmed that sizing operations like milling and sieving affect the surface energy of the particulate solids due to the changes in properties like size, shape, exposure of internal cleavage planes, etc. and that the surface energy heterogeneity determination using FD-IGC helped in characterising these changes.