The pore morphology of a porous host
may determine which polymorph
a crystallizable guest preferentially forms and may influence the
kinetics of solid/solid transitions. Slow cooling of the drug acetaminophen
(ACE) inside the straight cylindrical pores of anodic aluminum oxide
(AAO, tortuosity = 1) in contact with a bulk ACE surface film preferentially
yields uniformly oriented form II and/or form III crystals. The occurring
orientations of form II and form III crystals are characterized by
high structural registry along the AAO pores. The uniformly oriented
form III crystals inside the AAO pores were readily converted into
likewise uniformly oriented form II crystals by a solid/solid transition.
Thus, we obtained uniformly oriented form II crystals in AAO at high
yields. We suggest that sporadic heterogeneous nucleation at bulk
crystals formed in the bulk surface film on top of the AAO coupled
with kinetic selection of crystal orientations results in fast growth
of properly oriented crystals along the 100 μm deep AAO pores.
This mechanism is suppressed in controlled porous glass (CPG) having
isotropic spongelike pores (tortuosity > 1.5) with free growth
paths
on the order of 100 nm, where form I formed instead. Moreover, the
transition from form III to form II is suppressed in CPG. Possible
reasons may include impingement of the propagation front of the solid/solid
transition on the CPG pore walls after short propagation paths and
inevitable formation of form II grains with different orientations
separated by energetically disadvantageous grain boundaries. The results
reported here are relevant to mesoscopic crystal engineering aimed
at controlled drug release from nanoscale delivery systems. Polymorphs
not accessible otherwise in nanoscale containers may be produced at
high yields. The principles reported here may be transferred to areas
such as nanowire-based organic electronics.
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