Reports: GB10
47195-GB10 Expansion and Shrinkage of Water Cages in Structure II Clathrate Hydrates: The Effects of Temperature and Guest
Over the past year, we conducted a study of the freezing and melting behavior of tetrahydrofuran (THF)-water mixtures containing the additives NaF and KF. The motivation for these studies was three-fold: (1) to investigate a model hydrate-forming system containing ions such as those found in ocean environments, (2) to investigate the occurrence of stabilizing behavior similar to that observed in semiclathrates, and (3) to investigate new techniques for probing hydrate structure. A striking example of hydrate stability is provided by the tetraalkyl-ammonium halide semiclathrate hydrates, in which tetraalkylammonium cations are guests and halide ions are incorporated into the host network. Among semiclathrates, the fluorides possess the highest melting points, due to the ability of F- to fit into the tetrahedral H-bonded host network better than the larger halide anions, Cl-, Br-, and I-. We considered whether halide-ion incorporation could be achieved in the clathrate hydrate framework. Here I have results to report on the NaF-containing hydrate.
Our preliminary results indicate that (1) NaF produces a more pronounced depression, of the freezing point in THF hydrate than it produces with cyclopentane hydrate; both are less than that for pure water and binary THF-H2O mixtures. Also, (2) the addition of low concentrations of NaF to hydrate-forming solutions causes a slight increase in the melting point in both hydrates. One possible explanation is that we have observed the effect of halide-ion incorporation into the hydrate hosts. To test this hypothesis, we are exploiting two properties of fluorine, the fact that it has only one naturally-occurring isotope, F-19, and the large chemical shift anisotropy of F-19 makes it extremely sensitive to its local environment, including the identity and isotope ratio of the solvent, and concentration of dissolved salts.
NMR is an established technique for studying guest dynamics and determination of cage occupancy. Studies have been conducted on proton, deuterium, carbon-13, and xenon-129. To our knowledge, there has never been a type II hydrate study involving F-19 binding in the host. We conducted a preliminary F-19 magic-angle spinning (MAS) solid-state NMR study at the National High Field Magnet Laboratory (NHFML) in collaboration with Prof. M. Cotten, a biophysical chemist at Hamilton and Dr. Riqiang Fu at NHMFL. We measured tetrabutylammonium fluoride (TBAF) hydrate powders with TBAF:water molar ratios of 28 and 32, as these are known to crystallize in cubic and tetragonal lattices. In addition, we measured THF hydrate powders prepared with aqueous NaF, 0.05 molal in H2O and D2O.
In comparing the spectra, we noted that the tetragonal 32-hydrate has more features than the cubic 28-hydrate, indicating that the 32-hydrate has more inequivalent F- ion environments. Both share resonances in common with the spectrum of the cubic type II THF hydrate made with 0.05 molal NaF in H2O. The spectra of the TBAF 28-hydrate and THF 17-hydrate are the most similar; in particular, (3) the THF 17-hydrate has 3 inequivalent oxygen sites (multiplicity ratio of 96:32:8), matching the number of F- environments indicated in 17-hydrate and 28-hydrate spectra. With this in mind, (4) the similarities may indicate that the TBAF 28-hydrate structure (not published) has a symmetry and geometry similar to that of the type II hydrate. Moreover, (5) the appearance of the spectra indicates that the F- ions present in the samples exist in relatively symmetric environments. And (6) based on the chemical shifts, F- does not appear to be in direct contact with Na+.
Additional support for our hypothesis is found in studies of fluoride hydration. Hydrated F- ions assume a highly-structured hydrogen-bonded configuration in the first coordination sphere. Evidence for this comes from calculated radial distribution functions (RDF), which show the first sharp peak for F-H is almost 0.1 nm shorter than the F-O distance. A sharp second peak also occurs in the F-H RDF at a distance slightly larger than that of the first peak, and the intensity between the first and second peaks falls close to zero, indicating strong ion-solvent interactions and thus a tightness of the first shell and low probability for intermediate structures. Beyond 0.4 nm, the number of H bonds in the bulk remained close to the value for bulk pure water, so from these calculations it does not appear that F- is a structure-making ion beyond the first coordination sphere. The F-O distance has been calculated to be 0.263-0.322 nm with 4-8 nearest neighbors.
Coordination numbers from MC and MD are 4.09-6.3. First principles MD simulations produce a coordination number of around 4. There is no evidence of hydrogen bonding between water molecules in the first hydration sphere for clusters containing up to five water molecules. This is another consequence of the strong F-H bonding: F- interacts more strongly with water than any other halide ion, stronger than the water-water interaction as well. This strong interaction increases water-water repulsion and thus weakens hydrogen bonds. Unlike Cl-, the bonding energy in F-(H2O)n clusters is not approximated very well by an electrostatic model, indicating that some additional F-H bond formation is taking place. The nature of the cation can affect significantly the coordination state of F-, and the role and location of the cations in the hydrate is unknown. Thus, we have compelling evidence from F-19 SS-NMR that F- ions incorporate into the host network, where their lack of influence on water structure beyond the first coordination sphere of water may provide a passive probe of hydrate formation, structure, and memory effect. Moving forward from these results, we will undertake follow-up experiments to verify our preliminary results, and conduct isotope substitution studies to understand the striking difference between the spectra with H2O and D2O.
