Comprehending the Formation of Eutectics and Cocrystals in Terms of Design and Their Structural Interrelationships

The
phenomenon of cocrystallization, which encompasses the art of making
multicomponent organic solids such as cocrystals, solid solutions,
eutectics, etc. for novel applications, has been less studied in terms
of reliably and specifically obtaining a desired cocrystallization
product and the issues that govern their formation. Further, the design,
structural, and functional aspects of organic eutectics have been
relatively unexplored as compared to solid solutions and cocrystals
well-established by crystal engineering principles. Recently, eutectics
were proposed to be designable materials on par with cocrystals, and
herein we have devised a systematic approach, based on the same crystal
engineering principles, to specifically and desirably make both eutectics
and cocrystals for a given system. The propensity for strong homomolecular
synthons over weak heteromolecular synthons and vice versa during
supramolecular growth was successfully utilized to selectively obtain
eutectics and cocrystals, respectively, in two model systems and in
two drug systems. A molecular level understanding of the formation
of eutectics and cocrystals and their structural interrelationships
which is significant from both fundamental and application viewpoints
is discussed. On the other hand, the obscurity in establishing a low
melting combination as a eutectic or a cocrystal is resolved through
phase diagrams.