Surface chemistry is an important, yet underappreciated and poorly understood material characteristic within the pharmaceutical industry, which can affect handling and processing of powders. The intricate nature of solid surfaces makes surface chemistry a difficult variable to study and comprehend. The principal objectives of this work were fourfold: first, to successfully prepare a range of materials exhibiting different surface chemistries using a pharmaceutical grade crystalline powder, D-mannitol, and to quantify the surface chemistry alterations through XPS and ToF-SIMS; second, to use these materials to characterise the effect of surface chemistry on wettability behaviour and surface energy via contact angle and IGC experiments; third, to use this knowledge to understand the multi-faceted bulk powder flow behaviour; and forth, to relate surface chemistry, surface energy and powder flow behaviour to tabletability performance.
Primarily, the silanisation reactions carried out were thus intended to only alter surface chemistry, without affecting other variables, in order to facilitate direct correlations between the powder behaviours studied. Surfaces rendering phenyl-, methyl- and fluoro- chemical functionalities were prepared and the hydroxyl- rich surface of the untreated material was used as a control. The successful silanisation was validated by XPS characterisation which revealed that upon silanisation, two C 1s peaks were observed, one corresponding to the carbons present in the substrate environment, and one corresponding to the environment generated by the silane reagent. The materials’ wettability behaviour and surface energy were linked to the surface hydrophobicity rendered by the different surface chemical functional groups being present on the surface of the powders.
In this work, the importance of surface chemistry on powder flow performance has been determined for a range of crystalline powders exhibiting a vast number of surface chemistries. The dynamic, bulk and shear powder flow behaviours were linked to the surface chemistry of the four materials. Importantly, the electrostatic character of the fluorinated powder had an overriding effect on the powder flowability, emphasising that in certain conditions, surface chemistry is not the driving force for the performance of a material. Although surface chemistry effects have been recognised to affect powder flow behaviour as characterised by FT4 Powder Rheometer®, the proof-of-concept tabletability study revealed that when larger compression forces are applied to a powder, the surface properties, specifically surface chemistry, become a less important driving force for the tabletability performance. Instead, bulk powder properties become the dominant factors in producing powder compacts.