Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems Kyuchul Shina, Rajnish Kumarb, Konstantin A. Udachina, Saman Alavia, and John A. Ripmeestera,1 aSteacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON, Canada K1A 0R6; and bNational Chemical Laboratory, Council of Scientific and Industrial Research, Pune 411008, India Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved July 26, 2012 (received for review April 12, 2012) There is interest in the role of ammonia on Saturn’s moons Titan Results and Enceladus as the presence of water, methane, and ammonia As a first step in this study we show that in the presence of other under temperature and pressure conditions of the surface and in- clathrate hydrate forming substances, ammonia indeed can be terior make these moons rich environments for the study of phases incorporated in cages within the clathrate lattice. Incorporation formed by these materials. Ammonia is known to form solid hemi-, of ammonia in a clathrate hydrate phase was accomplished by mono-, and dihydrate crystal phases under conditions consistent freezing a solution of the well known clathrate hydrate former with the surface of Titan and Enceladus, but has also been assigned tetrahydrofuran (THF) and approximately 5% aqueous ammonia a role as water-ice antifreeze and methane hydrate inhibitor which with an approximate THF:water mole ratio of 1∶17 at −10 °C. is thought to contribute to the outgassing of methane clathrate Crystals formed were harvested after a few days and those suita- hydrates into these moons’ atmospheres. Here we show, through ble for diffraction were identified and mounted on a diffract- direct synthesis from solution and vapor deposition experiments ometer at low temperatures. The above experimental conditions under conditions consistent with extraterrestrial planetary atmo- can be compared with those in the study of Dong et al. (15, 16) spheres, that ammonia forms clathrate hydrates and participates who used THF:water ratios of about 1∶63 with up to 5% ammo- synergistically in clathrate hydrate formation in the presence of nia to study methane hydrate formation in the presence of methane gas at low temperatures. The binary structure II tetra- ammonia. In their experiments, Dong et al. (15, 16) did not use hydrofuran + ammonia, structure I ammonia, and binary structure sufficient THF to lead to the direct formation of the binary CHEMISTRY I ammonia + methane clathrate hydrate phases synthesized have THF þ NH3 clathrate hydrate and ammonia hydrate formation been characterized by X-ray diffraction, molecular dynamics simu- was not observed. lation, and Raman spectroscopy methods. Structural data were recorded at 100 K (see Materials and Methods). The structure was shown to be the usual cubic structure ice ∣ single crystal X-ray diffraction ∣ hydrogen bonding ∣ II (sII) clathrate hydrate (28), space group Fd-3m with a unit cell hydrate inhibitors ∣ ethane edge of 1.71413(6) nm. The THF molecules occupy the large cages, as expected, and 39% of the small cages are filled with am- mmonia has long been seen as a key species in extraterres- monia molecules (Fig. 1). This measured small cage occupancy is Atrial space, both interstellar and on outer planets, moons, likely to be a function of the starting composition. Whether small and comets and the interplay of ammonia, methane, and water cages could be completely filled with a more ammonia-rich liquid has been the subject of a considerable number of studies and is uncertain and should be studied separately. One of the water speculation (1–8). The main role assigned to ammonia has been molecules in the small cages which encapsulate the ammonia guests has moved out of its normal position, 0.118 nm inward into that of an antifreeze for ice and clathrate hydrate formation, ⋯ modifying the stability region of the solid ice and methane clath- the small cage thus forming H2O H-NH2 hydrogen bonds (0.267–0.272 nm) with NH3 (as suggested by hydrogen positions rate hydrate phases as a thermodynamic inhibitor (2, 5, 9). How- ⋯ ever, ammonia is a methane-sized molecule, thus based on size on the ammonia guest). The formation of HOH NH3 hydrogen alone it has the potential for being a suitable guest for clathrate bonding could also lead to this water displacement. This displace- ment breaks the hydrogen bond of this water molecule with hydrate cages. Issues that may have prevented ammonia from ⋯ being considered as a suitable clathrate guest molecule include another water from an adjacent large (or small) cage (O O dis- tance 0.393 nm), thus distorting both large and small cages. The the notion that guest species need to be hydrophobic in order to equivalent O⋯O distance in the undistorted edges of the water be incorporated into clathrates, and the observation of a number polyhedra is 0.279 nm. It should be noted that the minimum heavy of stoichiometric nonclathrate phases of ammonia and water atom N⋯O distance in the clathrate hydrate is smaller than in the obtained upon cooling aqueous ammonia solutions (2). Previous ammonia hydrates, where the more uniform local environment of experimental work on the water-ammonia and water-methane- the NH3 molecule results in N⋯O distances of 0.305 to 0.330 nm. ammonia systems had not shown evidence for the enclathration Because structural evidence for the mixed THF þ NH3 sII of ammonia (5, 10–19). Close inspection, however, shows that the hydrate showed that it is possible to incorporate ammonia into low pressure ammonia dihydrate (18) and the high pressure phase II of ammonia monohydrate (19) have structural features in com- mon with canonical clathrate and semiclathrate structures. Author Contributions: R.K., K.S., and J.A.R. performed the vapor deposition experiments; Recent structural analysis and molecular simulations have K.S., K.A.U., and J.A.R. did the powder and single-crystal XRD experiments; S.A., K.S., and shown that some guest molecules which form strong hydrogen J.A.R. performed the molecular dynamics simulations; and J.A.R., S.A., and K.S. wrote the manuscript. bonds with the water framework of the clathrate hydrate lattice The authors declare no conflict of interest. may nonetheless produce stable phases (20–23). It is therefore reasonable to consider that ammonia has potential as a clathrate This article is a PNAS Direct Submission. guest molecule. Furthermore, previous experimental and com- Data deposition: The single-crystal XRD structure of the binary tetrahydrofuran and ammonia structure II clathrate hydrate has been deposited in the Cambridge Structural putational studies have shown that in the gas phase, water forms Database, Cambridge Crystallographic Data Centre, Cambridge CB2 1EZ, United Kingdom dodecahedral cages around ammonia and ammonium ions. (CSD reference no. 864781). These structures may be indicative of a geometric tendency to- 1To whom correspondence should be addressed. E-mail: [email protected]. wards clathrate hydrate structures for the ammonia and ammo- This article contains supporting information online at www.pnas.org/lookup/suppl/ nium molecules (24–27). doi:10.1073/pnas.1205820109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1205820109 PNAS Early Edition ∣ 1of6 Downloaded by guest on September 29, 2021 Fig. 1. The large and small cages of the cubic structure II THF-ammonia binary clathrate hydrate from single crystal X-ray diffraction. The ammonia guest has moved a water molecule out of its normal position by pulling it into the small cavity by forming a H2N-H⋯OH2 or H3N⋯HOH hydrogen bond. clathrate cages, attempts were made to form clathrate hydrates with only ammonia as guest. Pure ammonia clathrate hydrate synthesis cannot be done by simply cooling aqueous ammonia solutions, as a variety of stoichiometric hydrates of ammonia are known to form preferentially (2, 11, 18, 19). Other ways of form- ing clathrate hydrates include vapor deposition of water at low temperatures to yield amorphous ice, followed by exposure of the ice to a pressure of guest gas and annealing (29), or vapor code- position of water and the potential guest material at low tempera- tures, again, followed by annealing (23, 30, 31). It has been shown that below approximately 140 K ice surfaces are relatively inert unless strong hydrogen-bond donors or acceptors are present. For example, molecules normally hydrolyzed by water such as formal- dehyde will not do so when in contact with ices at low tempera- ture and will template clathrate hydrate lattices when amorphous ice is annealed (30). Following the latter procedure, described in detail in Materials and Methods, vapor codeposits of ammonia and water were pre- pared at T < 20 K. The codeposited crystals were harvested at 77 K and kept in liquid nitrogen until appropriate measurements Fig. 2. PXRD patterns of NH3 and H2O co-deposition sample (A) recorded at could be made. A portion of each amorphous product was 100, 140, and 150 K; (B) expansion of the pattern at 150 K. Lattice parameters mounted in a low temperature cell on a powder X-ray diffract- (150 K) are: a ¼ 0.4527ð1Þ nm, b ¼ 0.5586ð1Þ nm, c ¼ 0.9767ð3Þ nm for am- 2 2 2 ¼ 0 4496ð1Þ ¼ ometer and the samples annealed stepwise, increasing tempera- monia monohydrate (space group: P 1 1 1); a . nm and c 0.7339ð1Þ nm for ice Ih (space group P63∕mmc); a ¼ 1.1818ð2Þ nm for sI ture from approximately 100 K. Fig. 2 shows the transformation ¼ 0 8404ð2Þ ¼ A ammonia clathrate hydrate (space group Pm-3n); a . nm, b of the amorphous deposit upon annealing. Fig. 2 shows the pow- 0.8441ð3Þ nm, and c ¼ 5.345ð1Þ nm for ammonia hemihydrate (space group der X-ray diffraction (PXRD) pattern for amorphous ice with Pbnm); and a ¼ 0.7125ð1Þ nm for ammonia dihydrate (space group P213).