Does Microgravity Influence Self-Assembly??

The templated syntheses of TMA₂Sn3S7 and TBA2Sn4S9 (where TMA is tetramethylammonium and TBA is n-tetrabutylammonium) microporous layered tin(iv) sulfides have been carried out under both microgravity (μG) and earth (1G) conditions in order to elucidate the influence of gravity on the self-assembly and crystal-growth processes of this class of materials. The μG experiments were conducted on board the May 1996 Endeavour STS-77 NASA space-shuttle flight. It was determined that the long-range ordering of the porous layers and the population of defects but not the short-range ordering within the layers is influenced by gravity. Bulk and surface crystallinity, smoothness of crystal faces, optical quality, crystal habits, registry of the porous layers, and accessible void volume to adsorbates were found to be improved in the space-grown crystals. This is probably because the forces associated with the organization of the porous layers are expected to be weak and sensitive to the elimination of buoyancy-driven convective flows and Stokes sedimentation effects in a microgravity environment. One can draw an analogy to the weak forces between protein macromolecules and the established effect of microgravity on improving the diffraction quality of crystals harvested in space.