Clathrates Flashcards
What are clathrate hydrates?
Formation of crystalline complexes with water network acting as host for a non-polar guest.
What are the crystallisation outcomes of urea?
alpha - close-packed form, unable to complex guests
beta - formation of hexagonal channels and pores. stable so long as guest is included.
How can urea and thiourea act as clathrates?
Urea and thiourea are good H-bond acceptors and donors and form entwined helices and hexagonal channels. Hydrogen bonding between molecules gives the structures, with long-chain hydrocarbons acting as guests.
How does binding in urea clathrates work?
Binding very weak due to lack of H-bonding to guests - all used up in formation of beta-crystallised form. Urea host lattice works as an inclusion compound with pores for molecules to fall through. Urea host lattice stable to guest exchange.
How can guest exchange in clathrates be followed?
Raman spectroscopy. Can map movement of guests by taking Raman spectra across a single crystal of urea.
What is ferroelasticity?
Change of properties on addition of mechanical force. Squeezing of channels can cause individual molecular reorientation to undo pressure disturbance, reorientation of host and guest can cause distortion leading to visual changes.
What technique can be used to see ferroelasticity?
Looking at a host-guest crystal under a microscope, can see the different polarisations when under mechanical force and not.
What are two important reactions in industry to do with gas storage and separation?
Water gas shift and steam reforming
WGS: CO + H2O <=> CO2 + H2
SR: CH4 + H2O <=> CO + 3H2
What types of isotherms are generally seen for supramolecular materials?
Type I: microporous solids. Amount of absorption increases until there is no space left and then levels off with increasing pressure.
What is a strange observation relating to calixarenes and gas sorption?
Calixarene fills with air under ambient conditions. Evidence: crystallisation by sublimation to get guest-free calixarene under vacuum. As liquid added, bubbles are seen.
Conclusion: Air fills holes after removal from vacuum. Calixarenes exhibit type I isotherms, with immediate absorption of air.
What is the purpose of a porous liquid?
Example: permanent pore cyclophane dissolved in liquid 15-crown-5. Collection of rigid holes in solution.
Liquids very good at solubilising gases, improves absorption of CH4.
