Semiclathrate hydrate

A remarkable variation of the type-I clathrate superstructure is associated with the formation of semiclathrates in the Ge-P-Q (Q = Se, Te) systems [34, 35]. The term semiclathrate was first introduced for hydrates of quaternary ammonium salts, which show general structural and physical properties of normal clathrates but in addition feature a single hydrogen bond between host and guest substructures [50]. Up to now quite a number of semiclathrates-hydrates have been reported in the literature [51] compared to a very limited number of inorganic semiclathrates, which are exclusively cationic semiclathrates. 

At high pressures, different crystalline forms appear. One of the intriguing forms is so-called filled ice, which has a structure of known crystalline polymorphs of ice such as ice Ic and ice II. These ices have vacant spaces in which small molecules are incorporated. So far, each water molecule in clathrate hydrates is hydrogen-bonded with four neighbors as in ice. Other clathrate hydrate is also known, called semiclathrate hydrate in which guest molecules such as tetra-n-butylammonium bromide replace several water molecules so that some of the cages are broken [11]. Hereafter, we will not refer to those semiclathrate hydrates, which are beyond the scope of the present statistical mechanical theory. 

Babu P, Datta S, Kumar R, Linga P (2014) Impact of experimental pressure and temperature on semiclathrate hydrate formation for pre-combustion capture of CO2 using tetra-n-butyl ammonium nitrate. Energy 78 458 64... 

Park S, Lee S, Lee Y, Seo Y (2013) CO2 Capture from simulated fuel gas mixtures using semiclathrate hydrates formed by quaternary ammonium salts. Environ Sci Technol 47(13) 7571-7577... 

The latter elass of compounds elicited a great interest because of their ability to entrap gas molecules, of which carbon dioxide is a remarkable example. Parieaud and co-workers reported an interesting insight on the dissociation conditions of CO2 from several ammonium salts and tetra-n-butyl phosphonium bromide ([P4444]Br) semiclathrate hydrates. Ye et al deseribed a comparison between phase equilibrium and morphology eharacteristics of hydrates formed by and [P444JCI... 

The unusual cavities in Jeffrey s structure VI (4454,43596273) and structure VII (4668) have been observed only for single terf-butylamine guests and hexafluoro-phosphoric acid (HPF6-6H20) guests, respectively. tm-Butylamine, Me3CNH2 (Jeffrey and McMullan, 1967), is unique among amine hydrates since it forms a true clathrate rather than a semiclathrate. 

In Figure 5 the phase diagrams of BU3XO-H0O (X=N,P,As) systems are presented. According to their structural data 16] these hydrates are referred to as semiclathrates in Jeffrey s terms [4]. In spite of the fact that molecules hydrate formers mentioned have big hydrocarbon radicals and clathrate formation with their participation depends on the whole on the hydrophobic interaction, the hydrophilic interaction plays its part, too. The polarity increase of the functional groups CpK for oxides (CH3)3X0 equals to 4.5 [31], -0.5 [32] and 3.7 [31] for X=N,P and As, respectively) facilitates polyhydrate formation. 

Further, following up on Jeffrey s early work on amine hydrates (amine semiclathrates), numerous amines remain to be explored for hydrate phases. In particular, since it has also been observed that some amine clathrates as weU as stoichiometric alcohol hydrates show phase transitions to double clathrate hydrates in the presence of helpgases such as xenon, H2S, and methane, it becomes a worthwhile area for exploration. 

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