Correlation Among Crystal Structure, Mechanical Behavior, and Tabletability in the Co-Crystals of Vanillin Isomers

Tuning mechanical performance of
molecular materials is currently
attractive owing to their practical applications in pharmaceutical,
food, and fine chemical industries and optoelectronics. Here we employed
a crystal engineering approach to transform four food flavouring agents,
vanillin isomers, from brittle to soft solids by forming co-crystals
with 6-chloro-2,4-dinitroaniline (cda). The series includes
vanillin (van), ethylvanillin (evan), iso-vanillin (ivan), as well as a Schiff base
of ortho-vanillin (ovan) with ethylene
diamine (sb-ovan). All the co-crystals adopt flat two-dimensional
(2D) layer packing, except the sb-ovan:cda, which adopts
a corrugated layer packing with the presence of slip planes. The mechanical
properties of the co-crystals were studied by (1) a qualitative method,
(2) nanoindentation, and (3) powder compaction techniques, which allowed
for successfully establishing the relationship among crystal structure,
mechanical properties, and tablet tensile strength. The simple qualitative
mechanical (deformation) tests confirmed plastic shearing deformation
behavior in the cda co-crystals with van, evan, and ivan, while the co-crystal
of sb-ovan:cda showed plastic bending due to the presence
of slip planes formed by van der Waals interactions in the structure.
The measured tensile strengths of the vanillin isomers and their respective
co-crystals, which followed the order: sb-ovan:cda > evan > van > ivan:cda > evan:cda > van:cda > sb-ovan > ivan, confirmed that the plastically bendable
co-crystal, sb-ovan:cda, shows a significant improvement
in the compaction properties compared
to any other form studied. In contrast to the initial brittle forms
with isotropic structures, the new co-crystal solids show improved
plasticity due to their anisotropic 2D-layer structures with active
slip planes that facilitate the plastic deformation, which enhances
tabletability, particularly in the plastic bendable solid. The study
also suggests that the bending type crystals are potentially far better
suitable for tabletability than the shearing and brittle type crystals.