Previous Fourier-transform infrared spectroscopic studies of adsorbates on the surface of ice nanocrystals have considered two categories of molecular adsorbates: (a) weak adsorbates that desorb rapidly into a vacuum at T < 100 K and that influence the vibrational modes of only the surface molecules of ice and (b) adsorbates that form significant H bonds to the unsaturated surface groups resist desorption at temperatures below ∼120 K and, without penetrating the ice, alter the subsurface ice spectrum. Here, we consider a third category of adsorbates which is characterized by molecules that form strong H bonds with the ice surface, penetrate the ice, and form hydrates. It is shown that above some minimum temperature of adsorption strong acids and Lewis base molecules, including ethers and amines, penetrate the ice and ultimately form known crystalline hydrates of the adsorbate. Results are reported for HCl, ethylene oxide, and ammonia as representative of these three subclasses of penetrating/reacting adsorbates. Ethylene oxide is shown to convert nanocrystals of ice to nanocrystals of the type I clathrate hydrate at 130 K. Similarly, nanocrystals of ice, exposed to NH 3 at 120 K, convert slowly to nanocrystals of the monohydrate of ammonia by a molecular reaction mechanism. By contrast, HCl, following adsorption in the 60-125 K range, converts ice nanocrystals to the amorphous ionic monohydrate which crystallizes upon warming above 135 K. However, below 50 K, adsorbed HCl exists primarily as the molecular adsorbate with a behavior similar to that observed for adsorption on amorphous ice. Disorder associated with the surface reconstruction of bare ice and the presence of strained H bonds between water molecules of the relaxed surface are likely critical factors in the interaction/reaction with adsorbates, such as atmospherically important HCl. However, the ability of strong proton donor/acceptor molecules to break and replace stable H bonds to water molecules is considered a prerequisite for the observed complete conversion of ice nanocrystals to hydrates at cryogenic temperatures.

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.