Hydration of krypton

For the gas hydrates it is not possible to make an entirely unambiguous comparison of the observed heat of hydrate formation from ice (or water) and the gaseous solute with the calculated energy of binding of the solute in the ft lattice, because AH = Hfi—Ha is not known. If one assumes AH = 0, it is found that the hydrates of krypton, xenon, methane, and ethane have heats of formation which agree within the experimental error with the energies calculated from Eq. 39 for details the reader is referred to ref. 30. 

Ashbaugh, H. S. Asthagiri, D. Pratt, L. R. Rempe, S. B., Hydration of krypton and consideration of clathrate models of hydrophobic effects from the perspective of quasichemical theory, Biophys. Chem. 2003,105, 323-338... 

Villard measured hydrates of Ar, and proposed that N2 and O2 form hydrates first to use heat of formation data to get the water/gas ratio deForcrand and Thomas sought double (W/H2S or H2Se) hydrates found mixed (other than IpSx) hydrates of numerous halohydrocarbons mixed with C2H2, CO2, (pHg de Forcrand first used Clausius-Clapeyron relation for AH and compositions tabulated 15 hydrate conditions Scheffer and Meyer refined Clausius-Clapeyron technique de Forcrand measured hydrates of krypton and xenon... 

Hydrates of Ar, Kr, and Xe were first synthesized by Villard in 1896 [141]. They were further studied, as well as hydrates of krypton and xenon, by de Forcrand [142]. Several structures for noble gas hydrates are known [143-146]. All the hydrate structures are different from that of ordinary hexagonal ice. In the two fundamental structures, the water molecules form pentagonal dodecahedra which are stacked with different degrees of distortion from their ideally regular forms [146]. The two types of structures are shown in Fig. 26a and 26b [140]. One structure contains 46 water molecules in the unit cell with 2 small and 6 larger cavities. The other structure has 136 water molecules in the unit cell with 16 small and 8 larger cavities. The formation of the two fundamental types of hydrates depends mainly on the size of the guest species. More detailed data for the two principal clathrate hydrate structures are available from the literature [147]. 

This method is plagued by large uncertainties in calculated configurational energies that grow with system size. Nevertheless, it has been used successfully, most recently by Durell and Wallqvist to investigate the hydrophobic hydration of krypton." ... 

In their study of krypton hydration, Durell and Wallqvist also reported a calculation of the enthalpy of hydration evaluated by the direct method of Eq. [31]." Both constant volume and constant pressure enthalpies were determined by varying the volume of the krypton solution. Their results are displayed in Table 1. The enthalpy of hydration in the constant volume case (—6.3 1.3 kcal/mol) is significantly more exothermic than in the constant pressure case (—3.4 1.3 kcal/mol). The latter number agrees very well with the experimental value of —3.3 kcal/mol, also obtained at constant pressure. The calculated enthalpies of solvation were decomposed into solute-water and water-water (solvent reorganization) terms. The solute-water contribution is comparable and favorable (—5.4 kcal/mol) in both the constant volume and constant pressure calculations. The solvent reorganization term, in contrast, shows a large ensemble dependence. In the constant-pressure case, the solvent reorganization term has a value of 2.0 1.3 kcal/mol. The overall favorable enthalpy of hydration of krypton at constant pressure therefore results from the solute-water attractions rather than from a... 

Handa, Y.P. (1986b). Calorimetric determinations of the compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of xenon and krypton. J. Chem. Thermodynamics, 18 (9), 891-902. 

Physics and Chemistry of Ice showing the recent results of krypton hydrate, nitrogen... 

As for krypton hydrate, Desgreniers et aU studied the pressure-induced phase transformations of krypton hydrate at room temperature by using X-ray diffraction measurements. They found that the initial cubic sll of krypton hydrate (KH-I) successively transformed to the cubic si (KH-II), the hexagonal structure (KH-III), and the sO (KH-IV) at 0.3 GPa, 0.6 GPa, and 1.8 GPa, respectively. They also found that the sO phase decomposed at pressures above 3.8 GPa. 

The best-known noble gas clathrates are hydrates, hydroquinone and phenol clathrates, which have found an increasing number of uses [131]. Clathrates may serve as convenient storage for noble gases. Because of the different affinity hydroquinone clathrate prepared from an equal mixture of krypton and xenon liberates 3 times the amount of Xe than Kr [132]. Clathrates are also of interest for nuclear technology. Radioactive isotopes of argon, xenon and krypton can more easily be handled in the compact form of a solid rather than in gas form [133-136]. 

The structure of xenon hydrate and the hydrates of argon, krypton, methane, chlorine, bromine, hydrogen sulfide, and some other substances is shown in Figure 9-10. The cubic unit of structure has edge about 1200 pm and contains 46 water molecules. Chloroform hydrate, CHCla-I7H2O, has a somewhat more complicated structure, in which the chloroform molecule is surrounded by a 16-sided polyhedron formed by 28 water molecules. 

Oxygen, nitrogen and air, like argon and krypton (1), are found to preferentially form gas hydrates of structure II, rather than structure I as previously expected for gas hydrates of small guest molecules. Lattice parameters from X-ray diffraction are given in the Table. 

If the critical temperature of the solute is below room temperature, the phase diagram is similar to the one described for the system hydroquinone-argon. No temperature can then be indicated above which hydrates cannot exist. This situation arises for the following solutes argon,48 krypton,48 xenon,48 methane,3 and ethylene.10... 

Radon forms a series of clathrate compounds (inclusion compounds) similar to those of argon, krypton, and xenon. These can be prepared by mixing trace amounts of radon with macro amounts of host substances and allowing the mixtures to crystallize. No chemical bonds are formed the radon is merely trapped in the lattice of surrounding atoms it therefore escapes when the host crystal melts or dissolves. Compounds prepared in this manner include radon hydrate, Rn 6H20 (Nikitin, 1936) radon-phenol clathrate, Rn 3C H 0H (Nikitin and Kovalskaya, 1952) radon-p-chlorophenol clathrate, Rn 3p-ClC H 0H (Nikitin and Ioffe, 1952) and radon-p-cresol clathrate, Rn bp-CH C H OH (Trofimov and Kazankin, 1966). Radon has also been reported to co-crystallize with sulfur dioxide, carbon dioxide, hydrogen chloride, and hydrogen sulfide (Nikitin, 1939). 

The virial isotherm equation, which can represent experimental isotherm contours well, gives Henry s law at low pressures and provides a basis for obtaining the fundamental constants of sorption equilibria. A further step is to employ statistical and quantum mechanical procedures to calculate equilibrium constants and standard energies and entropies for comparison with those measured. In this direction moderate success has already been achieved in other systems, such as the gas hydrates 25, 26) and several gas-zeolite systems 14, 17, 18, 27). In the present work AS6 for krypton has been interpreted in terms of statistical thermodynamic models. 

NEON. [CAS 7440-01-9], Chemical element, symbol Ne, at. no. 10. at. wt. 20 183, periodic table group 18,mp —248,68 C. bp —246.0UC, density 1.204 g/cm3 (liquid). Specific gravity compared with air is 0.674. Solid neon has a face-centered cubic crystal structure. At standard conditions, neon is a colorless, odorless gas and does not form stable compounds with any other element, Due to its low valence forces, neon does not form diatomic molecules, except tn discharge tubes. It does form compounds under highly favorable conditions, as excitation m discharge tubes, or pressure in the presence of a powerful dipole, However, the compoundforming capabilities of neon, under any circumstances, appear to be far less than those of argon ur krypton. No knuwn hydrates have been identified, even at pressures up to 260 atmospheres. First ionizadon potential, 21.599 eV. 

Barrer, R. M., Edge, A. J. V. (1967) Gas hydrate containing argon, krypton, and xenon Kinetics and energetic of formation and equilibrium. Proc. Roy. Soc., London, A300, 1-24. 

The tense and relaxed forms of a-cyclodextrin mimic the induced-fit model proposed for enzymes. In the inclusion complexes of a-cyclodextrin, the macrocycle adopts a round, relaxed shape with all the 0(2),0(3) hydroxyls engaged in intramolecular 0(2)- (3 ) hydrogen bonds (Fig. 18.8) [577]. The average O - - O distance of around 3.0 A is consistent with an H 0 separation of about 2.0 A, indicating relatively weak interactions. This form of a-cyclodextrin persists even when an included guest molecule, such as krypton (3.8 A diameter) is too small to fully occupy the 5.0 A cavity and is disordered over several sites [578], analogous to the gas hydrate structures discussed in Part IV, Chapter 21. 

Figure 4 Comparisons of phase transformation pressures between the X-ray diffraction (XRD) and neutron diffraction (ND) studies and the present results by microscopic observations and in-situ Raman scattering measurements for krypton hydrate, nitrogen hydrate, and methane hydrate.

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