Argon hydroquinone clathrate

Fig. 1. The crystal structure of a hydroquinone clathrate according to Palin and Powell. 8 The balls inside the transparent spheres represent argon atoms encaged in the cavities formed by the two interpenetrating lattices, (photograph kindly supplied by Dr. Powell).

The composition of the hydrates varies with temperature along the three-phase lines H ice G and HL2G in a way similar to that described for the system hydroquinone-argon. But as already noted in Section II.D this variation is smaller than for the hydroquinone clathrates. Accordingly, a cross section through the P-T-x diagram at constant temperature below 0°C would reveal a two-phase area G+H in which the composition of the latter is less sensitive to pressure than found in the corresponding case for the hydroquinone clathrates (cf. Fig. 4). 

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]. 

Let us first consider the three-phase equilibrium ( -clathrate-gas, for which the values of P and x = 3/( +3) were determined at 25°C. When the temperature is raised the argon content in the clathrate diminishes according to Eq. 27, while the pressure can be calculated from Eq. 38 by taking yA values following from Eq. 27 and the same force constants as used in the calculation of Table III. It is seen that the experimental results at 60°C and 120°C fall on the line so calculated. At a certain temperature and pressure, solid Qa will also be able to coexist with a solution of argon in liquid hydroquinone at this point (R) the three-phase line -clathrate-gas is intersected by the three-phase line -liquid-gas. At the quadruple point R solid a-hydroquinone (Qa), a hydroquinone-rich liquid (L), the clathrate (C), and a gas phase are in equilibrium the composition of the latter lies outside the part of the F-x projection drawn in Fig. 3. The slope of the three-phase line AR must be very steep, because of the low solubility of argon in liquid hydroquinone. 

Along the three-phase line liquid-clathrate-gas the variation of the composition with temperature is considerable (cf. CD in Fig. 3), because when applying Eq. 27 to this equilibrium, the relatively small quantity AH = 0.16 kcal/mole has to be replaced by the much larger difference/ —//ql between the partial molar heat functions of / -hydroquinone and the liquid phase, which amounts to about —6 kcal/mole. The argon content of the solid reaches a minimum at the quadruple point. 

In the P-T projection the difference in slopes of the three-phase lines -clathrate-gas and liquid-clathrate-gas at the quadruple point R is determined by the heat of fusion of the number of moles of hydroquinone associated with one mole of argon in the clathrate under the conditions prevailing at R. If we extrapolate the three-phase line liquid-clathrate-gas to lower pressures (where it is no longer stable), the value of yA decreases until it becomes zero when we are dealing with pure / -hydroquinone. Hence, the metastable part of this three-phase line ends in the triple point B of /1-hydro-... 

The equilibria between clathrate and gas, and Qa, clathrate, and gas could be determined by using w-propanol as the auxiliary solvent.53 In the latter equilibrium, the composition of the clathrate is found from the amount of gas required for the conversion of a given amount of solid a-hydroquinone suspended in the propanol solution into clathrate at constant temperature and pressure. The dissociation pressure of the clathrate is given by the total pressure of the four-phase equilibrium -clathrate-solution-gas, corrected for the vapor pressure of w-propanol saturated with a-hydroquinone. Using this technique it was found that the equilibrium clathrates of hydroquinone and argon have yA = 0.34 at 25°C63 and 0.28 at 60°C.28... 

The most important host for clathrates is hydroquinone. Three molecules, held together by hydrogen bonding, make a cage in which one molecule of the guest fits. Typical guests are methanol (but not ethanol), SO2, CO2, and argon (but not neon). 

Krypton is an inert gas element. Its closed-shell, stable octet electron configuration allows zero reactivity with practically any substance. Only a few types of compounds, complexes, and clathrates have been synthesized, mostly with fluorine, the most electronegative element. The most notable is krypton difluoride, KrF2 [13773-81-4], which also forms complex salts such as Kr2F3+AsFe [52721-23-0] and KrF+PtFF [52707-25-2]. These compounds are unstable at ambient conditions. Krypton also forms clathrates with phenol and hydroquinone. Such interstitial substances are thermodynamicahy unstable and have irregular stoichiometric compositions (See Argon clathrates). 

Much clathrate research has involved studies at only ambient temperature and pressure. There is considerable scope for widening these experimental parameters. Indeed, Powell used high pressure to obtain the clathrates of hydroquinone with argon, krypton, and xenon way back in 1950, and solvothermal methods are now widely used for metal containing systems. Protective coating of unstable crystals and... 

Attempts to generate real argon compounds such as ArF2 respect ArF have failed [2.9]. Together with water, argon forms a hydrate with a dissociation pressure of 106 bar at 0 °C. At 0 °C and above 106 bar, this hydrate is stable. Moreover, with hydroquinone an inclusion compound is known (clathrates) however containing no real chemical bonds [2.8]. 

---