Non-polar gas adsorption

In the special case of the physisorption of a simple non-polar gas by a homogeneous microporous solid, as Everett (1970) has pointed out, we may relate the Henry s law constant, kH, to the potential energy of adsorption, , by an expression analogous to... 

Simply calculating specific surface areas from the values in Tables 3-5 leads to apparent specific surface areas of approximately 300-500 Specific surface areas obtained from similar analyses of non-polar gas (nitrogen or krypton) adsorption studies, however, are typically in the range of Im /g, independent of sample pretreatment. 

Chemical separations may first be accomplished by partitioning on the basis of polarity into a series of solvents from non-polar hexane to very polar compounds like methanol. Compounds may also be separated by molecular size, charge, or adsorptive characteristics, etc. Various chromatography methods are utilized, including columns, thin layer (TLC) gas-liquid (GLC), and more recently, high pressure liquid (HPLC) systems. HPLC has proven particularly useful for separations of water soluble compounds from relatively crude plant extracts. Previously, the major effort toward compound identification involved chemical tests to detect specific functional groups, whereas characterization is now usually accomplished by using a... 

Reactants and reagents can be conveniently loaded into the dry zeolite by adsorption. This can be accomplished by intimately mixing the solid or liquid reactant and the powdered zeolite, by absorption from the gas phase, or by diffusion in a solvent slurry containing the zeolite and dissolved reactant. The choice of solvent for the slurry method is critical. It must be volatile enough to be removable at a pressure and temperature that does not result in evacuation of the reactant or its decomposition. In addition, the reactant must have a greater affinity for the interior of the zeolite than for the slurry solvent itself. The lack of affinity for the interior of the zeolite is an acute problem for non-polar hydrocarbons that lack binding sites for the intrazeolitic cations. The use of fluorocarbons such as perfluorohexane as slurry solvents takes advantage of the fluorophobicity of many hydrocarbons and has alleviated this problem to some extent.29... 

Procedures for determining fatty acids in sediments involved liquid-liquid extraction, liquid-solid adsorption chromatography followed by gas liquid chromatographic analysis [10-12], Liquid extractions have been performed with methanol-chloroform [13], methylene chloride [14] and benzene-methanol [15, 16]. Typical liquid-solid adsorbents are silicic acid. Standard gas chromatographic separations for complex mixtures employ non-polar columns packed with OV-1, OV-17, OV-101, SE-30, or glass capillary columns containing similar phases. 

The problem of the interaction of an atomic system with the surface of a metal at large distances is of significant interest for the theory of gas and vapor adsorption on solids surfaces. Just as in the interaction of two atomic systems, the universal attraction at large distances for neutral and non-polar systems may be obtained only in the second approximation of perturbation theory [1], Hitherto, only one attempt has been made in this direction, but the untenability of the assumptions, methods, and results of this attempt were obvious [2]. 

The model has been tested for a wide variety of gas-zeolite combinations. Gases of increasing complexity were considered Ar(non-polar), (quadrupole moment, no dipole moment), NgO (quad-rupole moment, small dipole moment), and NHg (large dipole moment, small quadrupole moment). The zeolites tested were all in the synthetic faujasite family however, they ranged from the cation-rich zeolite X to the cation-poor zeolite Y. Cation geometries considered in the tests were those typical of the dehydrated zeolite form and those typical of the hydrated geometry (associated with NHg adsorption). Two forms of representative cations were considered, Li and Na+. 

As discussed in Section 2.2, surfactant has a tendency to adsorb at interfaces since the polar head group has a strong preference for remaining in water while the hydrocarbon tail prefers to avoid water. The surfactant concentration affects the adsorption of surfactants at interfaces. Surfactant molecules lie flat on the surface at very low concentration. Surfactant molecules on the surface increase with increasing surfactant concentration in the bulk and surfactant tails start to orient towards gas or non-polar liquid since there is not enough space for the surfactant molecules to lie flat on the surface. Surfactant molecules adsorb at the interface and form monolayer until the surface is occupied at which point surfactant molecules start forming self-assembled structures in the liquid (Section 2.3). 

Hydrogen, which is highly volatile and non-polar or polarizable, is, when mixed with various impurities, practically unadsorbable, and is hence easy to purify by this method. The regeneration of adsorbent beds which have fi.xed the other components is usually carried out by raising the temperature obtained by a stream of hot gas which also acts as the desorbent the restoration of adsorption conditions then requires the beds to be cooled. These heat transfers are slow, making the process inapplicable to rapid cycles, and restricts it to the separation of small amounts of impurities. 

Another potentially interesting zeolite characteristic is the nature of gas diffiision in the intracrystalline pores. It has been suggested in the literature that certain adsorbed gas molecules close in size to the zeolite pores float within non polar zeolite crystals, instead of the standard adsorption-desorption mechanism. This concept opens the possibility that under certain circumstances, the emission of desorbed gas molecules may be directionally coherent as it emerges fi om each zeolite crystal face. Such a coherent gas emission - "a molecular laser" - may find applications in catalytic combustion or in other applications benefitting from "non thermalized" gas emissions. 

The selectivity of adsorption (S = niyj/njyi) of water vapour (component 1, mole fraction yi) on aluminas over component j (mole fraction yj) of a gas mixture can be complex functions of adsorbate loadings (ni,nj), system temperature and pressure. There is a scarcity of published data on water adsorption from multicomponent gas mixtures on alumina. Typically, it is assumed that water is exclusively adsorbed on aluminas (S — oo,nj —> 0) from non- polar gases such as air or natural gas. The assumption may not be valid when the gas mixture contains polar components. The mixed gas Langmuir or Toth models may be used to describe multicomponent Type I equilibria on aluminas [6,7]. No isotherm model is available to describe adsorption of water from gas mixtures when there is partial condensation of water in the mesopores of the alumina. 

Applications of gas-liquid chromatography - GLC is used to separate volatile, non-polar compounds substances with polar groups must be converted to less polar derivatives prior to analysis, in order to prevent adsorption on the column, resulting in poor resolution and peak tailing. 

Gas-phase adsorption of polar (methanol, acetonitrile, acetone) and non-polar adsorbates (hexane) on the surface of non-polar (hexadecanethiol) and polar (11-mercaptoundecanol) monolayers on gold was reported by Blanchard and Karpovich. The adsorption curves were fitted by a BET isotherm. The data suggest that an octadecanethiol surface is to some extent permeable for the adsorbates409. 

As the CFD method is a potential source of large errors, it is necessary to take steps to avoid them. A very useful procedure, making it possible to ascertain the presence of impurities ( chemical noise ), is to run a blank experiment. It is also necessary to use a sample of known composition to check the technique elaborated. This check should be performed repeatedly, especially when different batches of reagents are used. Special precautions should be taken when impurities are analysed. Kaiser [66] pointed out the possibility of the results being greatly distorted in the determination of impurities of non-polar compounds in a polar medium (and vice versa) because of their adsorption on the gas-liquid and liquid—solid (container walls) interfaces. It is also necessary to remember that stoppers can be a source of impurities and, possibly, of large errors [65]. 

Alumina has found wide application in GSC. It is a highly polar material with a typical surface area of 250 m g . It interacts strongly with polar molecules such as water, and also has some catalytic activity, for example, it may convert acetone to diacetone alcohol or dehydration may occur. Much of the early work on alumina was carried out by Scott who subsequently concentrated his research on surface modified materials [37,38]. He measured polarity in terms of the retention of ethylene relative to non-polar ethane and propane. Activation by heating at temperatures up to 500°C increased the polarity by loss of water and adsorption of water reduced polarity until a minimum was reached when the amount required for a monolayer had been adsorbed. Further water continued to reduce the activity (by reducing the surface area) but increased the polarity [39-41]. Scott later extended his work on alumina to substances modified with sodium hydroxide and obtained results similar to those obtained for the water-modified alumina without the need to presaturate the carrier gas. Retention of benzene relative to heptane increased markedly on alumina modified with sodium salts in the order OH > Cl > Br > I . A recent comparative study of the modifier effects alkali... 

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