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 ice−hydrate 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 CO2, 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 CO2 is unimportant under equilibrium conditions.