Water adsorption, selectivity

This cany-off of film drops, which may also occur with breakiug foam, in certain cases can partially convert water pollution into air pollution. if such is the case, it may be desirable to recirculate the gas. Such recirculation is also indicated if hydrocarbon vapors or other volatiles are incorporated in the gas stream to improve adsorptive selectivity [Maas, Sep. Sci., 4, 457 (1969)]. 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

Solutions must be concentrated or the constituents must be isolated before trace amounts of the various organics present as complex mixtures in environmental water samples can be chemically analyzed or tested for toxicity. A major objective is to concentrate or isolate the constituents with minimum chemical alteration to optimize the generation of useful information. Factors to be considered in selecting a concentration technique include the nature of the constituents (e.g., volatile, nonvolatile), volume of the sample, and analytical or test system to be used. The principal methods currently in use involve (1) concentration processes to remove water from the samples (e.g., lyophilization, vacuum distillation, and passage through a membrane) and (2) isolation processes to separate the chemicals from the water (e.g., solvent extraction and resin adsorption). Selected methods are reviewed and evaluated. 

Selection of the proper filler is based on a number of factors. The most important, of course, is the improvements in processing and end properties that it will provide. However, other important considerations when one is selecting a filler include availability, cost, surface chemistry, water adsorption, oil adsorption, density, and other physical or chemical properties of the filler grade itself. 

Dilute fluorine gas (0-20%) can be used to treat zeolites at near-ambient temperature and pressure. Most of the resulting materials retain very high crystallinity even after 600°C postcalcination for two hours. Both framework infrared spectra and X-ray powder diffraction patterns clearly show structural dealumination and stabilization. The hydrophobic nature of the fluorine-treated and 600sC-calcined material is shown by a low water adsorption capacity and selective adsorption of n-butanol from a 1 vol.% n-butanol-water solution. Fluorination also changes the catalytic activity of the zeolite as measured by an n-butane cracking method. 

A series of silica gels were synthesised from TEOS, selecting the experimental conditions (pH and water/TEOS ratio) in order to obtain samples with different porosity. Porous texture characterisation of these samples was done by gas adsorption (N2 and CO2 adsorption at 77 K and 273 K, respectively) (Quantachrome. Autosorb 6). The samples were degassed at 623 K under vacuum, until 10 torr. Water adsorption studies were carried out at 298 K in an automatic volumetric gas adsorption instrument (Belsorb 18). Experimental data was corrected for adsorption on inner wall of apparatus. Additionally, a blank experiment on all bulbs used showed that water adsorption on the inner surface of glass was negligible. Previous to water adsorption... 

The surface chemical properties of the carbon materials were characterized as follows measurement of pH of carbon slurries (in 0.1 M NaCl solution) [89] neutralization with bases of different strength and dilute HCl according to Boehm s method [63,66] determination of total oxygen/nitrogen content by elemental analysis (with an accuracy of 0.2%) [170] mass loss of carbon samples after heat treatment in a vacuum. Additionally, the number of primary adsorption centers (a,)) was determined from water vapor adsorption isotherms according to the Dubinin-Serpinsky method [171], as was the heat of immersion in water for selected samples [111,172]. The results of these operations are pre.sented in Table 3. For all samples transmission Fourier Transform Infrared (FTIR) spectra and X-ray photoelectron spectra (XPS) were recorded. 

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. 

The surface excess isotherm for adsorption of bulk water (0 < Xi < 1) from a binary liquid mixture can be U (water selectively adsorbed at all values of Xi) or S (water not selectively adsorbed at aU values of xi) shaped [20], Figure 8 shows an example of U shaped isotherm for adsorption of water-alcohol binary mixture on Alcoa H-152 alumina... 

Thus, the mechanism of the supporting influence of cationic surfactants on microflotation and flotation can be different. In microflotation the electrostatic barrier can be decreased, in flotation the contact angle can be increased. Naturally, both effects manifest themselves simultaneously. Li Somasundaran (1990, 1992) observed a bubble recharge due to adsorption of multivalent inorganic cations. Thus, their application is recommended in order to increase the contact angle and to stabilise bubble-particle aggregates. Naturally, selective adsorption of multivalent ions at the water-air interface is important. But even in the absence of adsorption selectivity under equilibrium conditions a deviation from equilibrium can happen due to the increase of adsorption within the r.s.c. This is important for the precalculation of increase of the contact angle caused by cation adsorption. 

Virtually any water-insoluble substance that satisfies the required characteristics discussed in Section 4.7.2.1 can serve as a solid support. However, no ideal matrix has been introduced yet. But an important factor is the rapid development of solid supports. If, at present, some carrier is neglected owing to some undesirable quality, it may be suitable after appropriate modification. It is almost impossible to find a matrix material which does not interact with biological active molecules by hydrogen bonding or ionic or hydrophobic interaction. Occasionally, these interactions fortuitously improve adsorption selectivity, but according to the opinion of Egly and... 

The adsorption energy of a sample molecule may be a function not only of the strengths of individual adsorbent sites and their concentrations, but also of their geometrical distribution (Section 3-3). In cases where surface geometry is important to the adsorption of certain sample molecules, and where water deactivation selectively changes the geometry or... 

Grand canonical ensemble Monte Carlo simulations of the adsorption properties of several model faujasite zeolites were performed using the statistical bias method. The results enable a better understanding of the effect of cation exchange in the selective adsorption of binary mixtures of para and meta xylene isomers. We predict that adding a small amount of water molecules could enhance the adsorption selectivity in favour ofp-xylene. 

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