Heterogeneous clathrates

In Part III heterogeneous equilibria involving clathrates are discussed from the experimental point of view. In particular a method is presented for the reversible investigation of the equilibrium between clathrate and gas, circumventing the hysteresis effects. The phase diagrams of a number of binary and ternary systems are considered in some detail, since controversial statements have appeared in the literature on this subject. 

When studying heterogeneous equilibria involving clathrates, one is faced with peculiar difficulties owing to the hysteresis effects mentioned in the introduction the solute in a clathrate crystal of hydroquinone, for instance, will not come to thermodynamic equilibrium with the vapor in which it is placed. Consequently it is impossible, or at least very difficult, to measure the equilibrium vapor pressure of the solute in a clathrate by placing some crystals in a tensometer (cf. the experiments of Wynne-Jones and Anderson,58 and those of Leech and Richards reported by Powell33). 

Although AF(G)Ps and LDHIs are distinct, they both inhibit the growth of crystals. Neither AFGP nor PVP are reported to significantly affect ice nucleation, and similary, we have shown that AFPs and PVP did not affect homogeneous nucleation of THF hydrate. It is not known if these two types of inhibitors can adsorb to other hydrophilic surfaces, however silica is an ubiquitous impurity and common to both these systems. Thus, it is of interest to determine the effects of these inhibitors on heterogeneous nucleation of ice/clathrate hydrate. 

Shepelev, Yu.F., Smolin, Yu.L, Ershov, A.S., Rademacher, O., and Scheler, G. (1987) Crystal sbucture determination of the sodium-tetramethylammonium silicate Na.7[N(CH3)4]SigO20.54H2O a-120 =frcC, Kristallogrc a,i2,n99- AQ (1987). Wiebeke, M., and Roller, H. (1992) Single-Crystal X-ray Dif action and Variable-Temperature MAS NMR Study on the Heterogeneous Network Clathrate Na.7[N(CH3)4]SigO20-54 HjO, Acta C/yst, 848,449-458. 

Soft chemical methods for the preparation of solid-state materials possess many advantages over conventional synthesis approaches. The introduction of additional synthetic variables allows for additional ways to influence the resulting products that form, and the comparatively low temperatures used in these reactions allow access to kinetically stable (thermodynamically metastable) compounds that cannot be prepared by conventional methods. The use of low temperature chemical oxidation of precursors in heterogenous reactions was recently introduced as an effective method for synthesis of intermetallic clathrates (see also Chap. 2 of the present volume) [83, 84]. We summarize here the compositions obtained by this approach. 

The responsive behavior of ELRs has been defined as their ability to respond to external stimuli. This property is based on a molecular transition of the polymer chain in the presence of water at a temperature above a certain level, known as the Inverse Temperature Transition (ITT). This transition, whieh shares most of the properties of the lower critical solution temperature (LCST), although it also differs in some respects, particularly as regards the ordered state of the folded state, is clearly relevant for the application of new peptide-based polymers as molecular devices and biomaterials. Below a specific transition temperature (T,), the free polymer chains remain as disordered, random coils [20] that are fully hydrated in aqueous solution, mainly by hydrophobic hydration. This hydration is characterized by ordered, clathrate-like water structures somewhat similar to those described for crystalline gas hydrates [21, 22], although somewhat more heterogeneous and of varying perfection and stability [23], surrounding the apolar... 

---