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Although molecules like methane are hydrophobic and thus only sparingly soluble (10-4 molar) in water, methane constitutes 5% of the mass of typical clathrates. Clathrates are metastable at atmospheric pressure, but melt at above zero degrees. These are fascinating materials in addition to their importance in the environment, their potential as a source of carbon, and their interference with natural gas pipelines at low temperatures. (Clathrates form a waxy plug that clogs the pipe preventing gas transport.)

There are several motivations for understanding how these substances form:

  • preventing formation in pipelines
  • stabilizing the structures to open the supply of carbon
  • forming analogous substances to sequester CO2
  • forming similar materials to contain and store hydrogen

Our approach to investigate the fundamental steps in clathrate formation is to investigate the interaction between small hydrocarbons and water as a function of pressure in carbon tetrachloride. Due to the low solubility in CCl4, water is dispersed into isolated monomers with ambient thermal energies. The IR spectrum of water (pictured below) consists of the symmetric stretch, the asymmetric stretch, and the free symmetry-axis rotational structure. These features are due to an interaction between the oxygen lone pair and the electropositive carbon in CCl4.

Introduction of propane (figure below) results in an emerging resonance at 3643 cm-1 right between the symmetric and asymmetric stretch. This is characteristic of a dangling OH bond and is a signature of water cluster formation. Note that water itself does not cluster under pressure.

To identify the interaction between water and propane that is responsible for this clustering effect, isotopically substituted CD3CH2CD3 is used and its spectra with and without water is examined (figure below). Here the two peaks at 2870 and 2930 cm-1 are attributed to the CH2 symmetric and asymmetric stretches, respectively. Addition of water causes a third peak to appear at 2960 cm-1, on the blue side of the asymmetric stretch. This new feature is assigned to the C-HO interaction. The inset of Figure 3 shows the methyl C-D stretching modes, which are virtually unaffected by the presence of water. Hence, the initial interaction that nucleates propane clathrate is between the water oxygen and the propane methylene hydrogens.

Methanol has long been used to inhibit clathrate formation in natural gas pipelines. Here the interest is to investigate the interaction between methanol and the propane-water complex that leads to this inhibitory effect. The spectrum of methanol (below, panel a) shows a sharp peak at 3643 cm-1 due to the free O-H stretch and a weak shoulder at 3528 cm-1 due to methanol clusters. Upon addition of water (panel b), the difference spectrum (water/methanol minus water) reveals an H-bonded resonance at 3440 cm-1 in which methanol acts as a proton donor to water. This relatively strong H-bond between water and methanol breaks up the van der Waals attraction between water and propane and inhibits formation of water-propane cluster.

Formation of a complete clathrate hydrate lattice hinges on the making of five- and six-membered rings and the arrangement of these rings into a cage structure. Cyclopentane and THF are among a few organic compounds that form stable clathrates at atmospheric conditions. Since both species are themselves rigid 5-membered rings, it is conjectured that water molecules in the vicinity of these solutes also form a pentacyclic structure that could serve as a template for hydrate nucleation. The picture below shows a macroscopic sample of cyclopentane clathrate at 3 oC in contrast to a phase-separated pentane/water mixture (note that the aqueous solubility of pentane is 5 times higher than that of cyclopentane). The flexible methyl end groups of pentane could potentially block the approach of new water from forming H-bonded clusters, thereby preventing the ordered structuring of water around the guest species. Combined evidence from experimental results and theoretical calculations of the cyclopentane/water system reveals the presence of a bridging water molecule in the guest-induced cluster (see bottom figure). This configuration restricts the motion of the water molecules to some degree and thus bears some characteristic of the hydrate cavity and reduces the entropic cost for forming complete clathrate cages.

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