Coated Ice Nanocrystals from Water−Adsorbate Vapor Mixtures:  Formation of Ether−CO₂ Clathrate Hydrate Nanocrystals at 120 K

Abstract

The extraordinary surface area of ice nanocrystals permits their rapid conversion to hydrates in the presence of strong H-bonding adsorbates such as acids, methanol, ammonia, and small ethers at cryogenic temperatures. In the case of small ether molecules, the product is a structure I or II clathrate hydrate. Since the ice nanocrystals, originally formed at 70 K as a suspended network with average diameters near 20 nm, grow through Ostwald ripening at elevated temperatures, the reaction to form hydrates is best observed at temperatures below 135 K so that the particle growth is limited. It is shown here that transport of a reactant to the icehydrate interface at such low temperatures can be greatly facilitated for relatively nonvolatile substances, such as tetrahydrofuran (THF) and methanol, by growing the ice nanocrystals under conditions such that they are formed with a coat of the adsorbate. The hydrates then form upon warming the coated ice nanocrystals to temperatures above ∼120 K. Using this approach, networks of ∼30 nm crystals of the clathrate hydrates of THF, ethylene oxide, and dimethyl ether have been prepared on a time scale of an hour. Ether molecules prefer to occupy the large clathrate cages so, if the enclathration occurs in the presence of a second smaller guest molecule such as CO 2 , the smaller cages (and to some degree the larger cages) become populated by this second adsorbate. In some instances it is also possible to load the small cages by exposure to the vapor of the second guest following formation of nanocrystals of the ether clathrate hydrate. Such behavior signals a completely reversible loading of the cages that allows quantitative measurements of enclathration/ decomposition rates, energetics, and pressures at cryogenic temperatures. The results are also consistent with a recent report reversing the long held view that small cage occupation by CO 2 is unimportant under equilibrium conditions.

Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems

“…The ammonia guest has moved a water molecule out of its normal position by pulling it into the small cavity by forming a H 2 N-H⋯OH 2 or H 3 N⋯HOH hydrogen bond. nanoparticles at low temperatures in the presence of hydrogen bonding ether guest molecules have been observed (34).…”

Water surface is acidic

“…Because of the surface and subsurface disorder, protonated H 2 O configurations there should be similar to those in amorphous ice, liquid water, and the amorphous (H 2 O) 48 H ϩ cluster. On the other hand, proton transfer from the surface to the crystalline bulk is expected to be more endothermic than for the liquid, because, in contrast to water, crystal ice is a very poor solvent (1,25). Further, one may note past computational results favoring a greater surface than bulk proton concentration, which showed that the O⅐H interaction between hydronium embedded in ice, and the proton-donating neighbor water molecule is repulsive (26).…”

Structure, spectra, and mobility of low‐pressure ices: Ice I, amorphous solid water, and clathrate hydrates at T < 150 K

“…Examples include CO2 CHs of structure I and II [Fleyfel and Devlin, 19911. 5. Ice nanocrystal arrays may be converted to CH arrays using small proton-acceptor guest molecules with/without nonpolar guests [Delzeit et al, 1997b;Hernandez et al, 1998]. …”

“…The action of the proton-acceptor guest is demonstrated in the complete conversion of ice nanocrystals to CHs containing large populations of nonpolar guest molecules. For example, although exposure of pure ice nanocrystals to CO2 (g) for long periods at 120 K results in only adsorbed CO2, this nonpolar guest rapidly fills the small cages of the tetrahydrofuran structure II CH at this temperature [Hernandez et al, 1998]. Mobile defects, induced by the proton-acceptor guest, are imagined to loosen the CH structure, facilitating the diffusion of the CO2, both into and away from the hydrate cages.…”

“…"Strong" adsorbates, such as NH3, methanol, small ethers, and strong acids, for which the adsorbate-water bonding is comparable in strength to water-water bonding within ice, also reorder the ice surface and subsurface in the manner noted for intermediate adsorbates. However, strong adsorbaies, at exposure levels beyond "surface saturation", are uniquely able to penetrate the ice and convert it to known hydrates [Hernandez et al, 1998]. Since these processes involve the breaking and making of H bonds by molecular agents, each hydrate can be viewed as a product of "H bond chemistry."…”