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.

DSC and NMR results prove the existence of an amorphous acetaminophen nanolayer between acetaminophen nanocrystals (form I and form III) and pore walls in controlled porous glasses. This nanolayer controls the surface energy. This finding is highly relevant for approaches towards crystal engineering in nanopores and indicates similarities to the situation during the early stages of crystallization in polymorphic bulk samples.

No abstract

The influence of a hierarchically structured pore system of a silica sol–gel support for application as a functional component in an acetylcholinesterase‐based enzyme array, with respect to its efficiency (response time, in particular) is investigated. Careful adjustment of synthesis parameters and a novel drying method allow to prepare monolithic silica sol–gel membranes with monomodal or hierarchical pore structures. These supports enable direct comparison regarding the influence of morphological properties on maximum acetylcholinesterase (AChE) loading by a membrane and on the apparent reaction rate of the AChE‐catalyzed degradation of acetylcholine at identical enzyme loading. It is shown for the first time that the hierarchical, meso‐macroporous material is superior over the monomodal structures (of either mesopores or macropores) regarding combined functionality and transport efficiency, as reflected in the apparent reaction rates. The advantage of the mesopores in a hierarchical system is manifested in higher maximum enzyme loading than for purely macroporous material, while the presence of macropores results in less obstructed transport that for a purely mesoporous material, which in turn reduces the response time.

Autoclaved aerated concrete (AAC) as a building material is characterized by its very good thermal insulation. This is achieved by a very high porosity of the microstructure. The size of the pores ranges from millimeters to nanometers and depends on the position as well as on the origin of the pore. It can be distinguished in propellant pores, web pores, and intercrystalline or interparticle pores. Because of the broad pore size range it is not fully understood until now, which pore sizes significantly and to what extent affect the building material qualities. The AAC's entire pore size distribution can only be detected by the combination of different microstructure analysis methods. For this purpose the nitrogen sorption analysis, mercury intrusion, and 3-D microcomputer tomography was used as an effective combination. AAC samples that show different mineralogical phase compositions because of the differentiation of the formulation, were tested. On the basis of these samples and the pore size distribution, which is usually used for the description of the frost and frost-deicing salt resistance of concrete, initial insights into the influence of the respective pore size on building material properties of the AAC could be gained. K E Y W O R D Sautoclaved aerated concrete, 3-D microcomputer tomography, mercury intrusion, nitrogen sorption, porosity, specific surface