Water-Guest Interactions Under Clathrate Hydrate Formation Conditions: A Matrix-Isolation Approach.
Abstract: The eventual depletion of fossil fuels, coupled with the effects of
greenhouse gas emission on the global climate, has called for exploration of alternative
technologies to address increasing energy demand. Clathrate hydrates, crystalline solids
composed of gas and water, is a class of materials with enormous potential both as an
energy source and as a medium for gas storage. These ... read morepeculiar compounds occur naturally in
the permafrost and the deep oceans, and are estimated to contain more carbon than all the
oil found on Earth. There have been numerous investigations on the kinetics and
thermodynamics of clathrate formation; however, a molecular-level understanding of this
phenomenon is far from complete. A major challenge has been characterization of the
water-guest interactions at the beginning stages of nucleation. Infrared spectroscopy is
among the most sensitive probes for detecting interactions involving water, but it is
rarely chosen for hydrate studies due to the high absorbance of bulk aqueous samples. This
work employs a room-temperature matrix isolation technique to circumvent the opacity
problem. Carbon tetrachloride is used as the solvent to disperse water into isolated
monomers with ambient thermal energies, providing an excellent environment to investigate
interactions with hydrate guest molecules at typical clathrate formation conditions
(temperatures near 0 oC and pressures between 1-55 atm). Propane
is one of the simplest hydrocarbons that form a stable clathrate hydrate under relatively
mild conditions. In carbon tetrachloride, the water monomer signature consists of the
symmetric stretch, the asymmetric stretch, and rotational wings associated with the
asymmetric stretch. Interaction of water with propane causes a dangling OH
(d-OH) peak to appear between the symmetric and asymmetric
stretches, indicating a guest-induced water cluster. A combination of isotopic substitution
and density functional theory calculations has established the presence of an attractive
interaction between the methylene groups of propane and the electron lone pairs of water.
Aside from their potential role in energy applications, hydrate plugs formed during
transportation and processing of natural gases have been a longstanding nuisance for the
petroleum industry. A number of thermodynamic inhibitors, primarily methanol, have been
used to prevent hydrate formation in natural gas pipelines for many years, but their
interactions with the hydrate constituents are not very well identified. Matrix-isolation
experiments are hence performed to elucidate the mutual interactions between methanol,
water and propane in CCl4. Experimental data indicate that while
methanol forms hydrogen bond to water via donation of the hydroxyl group (binding constant
K = 4.4 x 102), it does not have any specific interaction with
propane. These results are consistent with a picture in which methanol disrupts formation
of the water-guest complex by competing with propane for the oxygen lone pairs in water.
Interactions between water and pentacyclic guests such as tetrahydrofuran (THF) and
cyclopentane are subsequently examined. Despite the distinct difference in aqueous
solubilities (THF is totally miscible with water and cyclopentane is hydrophobic), both
compounds form stable clathrates above 0 oC at ambient pressure.
Knowledge of the nature of the water structure surrounding the cyclic guests is thus a
critical component for a deeper understanding of the nucleation mechanism. Experimental
results show that compared to propane, cyclopentane gathers significantly larger clusters
of water in carbon tetrachloride. The presence of a bridging water molecule is also
observed for the cyclopentane-water cluster, providing compelling evidence for arrangement
of water molecules in a local five-membered ring motif. The rigidity of the cyclic guest in
this instance partially immobilizes water molecules in its vicinity, and reduces the
entropic cost for forming a complete hydrate lattice.
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Chemistry.
Advisor: Mary Jane Shultz.
Committee: Elena Rybak-Akimova, Arthur Utz, and Thomas Keyes.
Keyword: Physical chemistry.read less
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