47195-GB10
Expansion and Shrinkage of Water Cages in Structure II Clathrate Hydrates: The Effects of Temperature and Guest
Research conducted over the last reporting period can be divided into three parts: hydrate synthesis, neutron diffraction experiments, and data analysis. We have focused primarily on structure II hydrates of propylene oxide (PO) and cyclopentane (CP) without additives, and tetrahydrofuran (THF) with additives, and the structure H hydrate of 4,4-dimethyltetrahydropyran (44DMTHP). This work was performed with the assistance of two summer undergraduates, Divij Mathew, class of 2009 and Thomas Nevers, class of 2010. Xu Ke assisted with the synthesis of cyclopentane hydrate during the 2007-2008 school year.
Hydrate Synthesis
All students had primary responsibility for synthesizing clathrate hydrates for both in-house (with H2O) and neutron scattering (with 2H2O) studies. Their work revealed several of the challenges of synthesis. First, because of the stochastic nature of the growth process, hydrate growth times are unpredictable and sometimes lengthy. To address these difficulties, we employ agitation, seed the solutions with small ice crystals, use shock wave techniques, and maximize the degree of undercooling to speed up the crystallization process.
Secondly, solidified products usually contain a mixture of ice and hydrate in the case of THF and ice, hydrate, and liquid organic in the case of CP. Propylene oxide seems to form in pure form even when flash frozen, possibly due to its formation as a congruently-melting phase. It was therefore necessary to develop annealing procedures in which the frozen samples are kept at temperatures close to their melting points, and above the melting point of water in the case of the THF and cyclopentane hydrates.
Our experiences developing annealing protocols lead us to the third problem encountered, that of the unknown melting points of the hydrates with deuterated hosts. From previous early studies of phase behavior of clathrate hydrates, estimates of the melting points of the hydrogenated versions are available. These data formed the basis for our first attempts at setting conditions for hydrate synthesis of both hydrogenated and deuterated forms. However, although clathrate hydrates with deuterated hosts (and guests) have been the preferred specimens for structure determinations from neutron scattering data, similar studies of their phase behavior have not been performed. To characterize hydrate phase behavior in various states of deuteration, we are now performing differential scanning calorimetry in collaboration with Prof. Jae Lee at the City College of New York.
Neutron Diffraction Experiments
Experiments related to this grant period were performed July 5-7, 2007 and October 16-17, 2008 at the NIST Centerfor Neutron Research. In 2007, we transported a sample of structure H 44DMTHP hydrate with Xe as a “help gas,” an additional guest molecule needed to fill the small cages and provide stability for the structure. The hydrate and the 44DMTHP itself were synthesized by Thomas Nevers and is a putative guest molecule designed to fit the 51268 cage of the structure H hydrate by a former student, Michael Flanders, during the summer of 2006. In 2008, the PI transported CP hydrate to NIST for a diffraction experiment. Unfortunately, the neutron diffraction patterns for these samples showed them to be mostly ice. At first, we attributed the presence of ice to failure of our storage conditions; however, with the knowledge we have accumulated on the stability of hydrogenated and deuterated hydrates, we must assume a combination of factors and are working to understand these and modify our sample processing and handling procedures.
Data Analysis
If the conditions for the stability of hydrogenated and deuterated hydrates display marked differences, what differences might the crystal structures themselves display? This is an important issue in light of the fact that structures derived from neutron scattering data involve the use of deuterated components. This past summer, the PI and student Thomas Nevers performed a detailed study of the water host of structure II clathrate hydrate with PO as the guest. The data for this study came from high-resolution, constant-wavelength neutron powder diffraction data measured prior to this grant. The structure is as expected, cubic in space group Fd3m, with lattice parameters slightly above 0.17 nm. At low temperatures, where guest motion is negligible, the bond distances and angles are most similar to those of 2H2O ice Ih (Fig. 1) Above 100 K, the structure of 2H2O water in the hydrate host varies significantly from that of ice Ih. Although these temperature-dependent distortions are related to the size, shape, and dynamics of the guest molecules as well as the dynamics of the host itself, they might also be a result of an isotope effect on the water dynamics. In addition, variations in the bond angles and distances may provide a quantitative indicator of the strength of guest-host interactions or the deviation of the thermodynamic properties of hydrate hosts from that of ice Ih; here again, isotope effects will have to be taken into consideration. A manuscript of this work is being written for publication.
