May 15, 2006
Pressurizing crystals locks host and guest lattices together
Researchers at the University of Rennes and CEA-CNRS in France and the University of the Basque Country, Spain, have applied pressure to an aperiodic crystal consisting of a host molecule and guest molecules with slightly different lattice parameters. The pressure caused the guest molecule lattice parameter to shrink to that of the host molecule in the first observation of an effect called commensurate lock-in.
"We retained a compound where the period of the guest was close to the pitch of urea [the host], just slightly larger," Flavien Aubert and Bertrand Toudic of the University of Rennes told nanotechweb.org. "Pressure only reduces the parameter of the guest, which is much softer, and so we were able to make the parameter of the host and of the guest equal. (Actually, the guest parameter is twice the host parameter). Now, and over many kilobars, the two host and guest systems are 'attached' as in normal crystals. It is a direct proof of a lock-in energy term in such self-assembled supramolecular [crystals]."
The composite self-assembled crystal consisted of a host molecule of helicoidal urea, which contains roughly cylindrical channels with an available diameter of 5.5 Ångstrom and alkane guest molecules inside the channels. The team used neutron diffraction to measure the lattice parameters of the molecules.
"If you have incommensurability, the two subsystems are not 'attached' together and you can slide them without any energy cost," said Toudic. "You can imagine the importance of such aperodicity in dynamics on the nanoscale and especially in nanopores."
By applying pressure to the crystal Toudic and colleagues tuned its properties, moving the crystal from an incommensurate to a commensurate state.
"The continuous control of the guest repeat gives a unique tool for tuning one-dimensional properties of confined compounds," said Toudic. "Our results open a broad field of scientific subjects that can be tackled combining the low friction of incommensurate nanoporous materials with the conformational, optic and electronic properties of guest molecules. These topics include nanofluidics and nanotribology, both of which have very important practical applications. Nanometre-sized containers also have huge potential for use in molecular manipulation and chemical reactions."
The researchers reported their work in Physical Review Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.