Water, reversible absorption

A OT/Cd(NO )2Hsooctanel Water Reverse Micelles in the Presence and the Absence of HMP. In the presence of an excess of cadmium ions, [Cd2+]/[S2 l = 2, the absoiption spectra obtained at various water content, w, in Na( AOT) reverse micelles in the presence and in the absence of HMP show a red shift with increasing the water content, w. For a given w value, a blue shift in the presence compared to the absence of HMP is observed. As described in the litterature (28,34,37), the average size of the particles can be deduced from the absorption onset. The size of the semiconductor is always less than that obtained in aqueous solution. The presence in reverse micelles of HMP as a protecting agent allows a reduction in the size of the particle. 

In physical absorption no chemical reactions take place and the absorbed material (absorbate) is held by the absorbing material (absorbent) only by the forces of cohesion or capillary action in the pores of the solid. Physical absorption is a reversible process. As examples may be cited the absorption of gases such as nitrogen or oxygen in water, and absorption by soda lime or KOH of carbon dioxide... 

When the solvent is to be distilled after standing over a desiccant, the drying agent should be filtered off before distillation if it removes water reversibly, e.g. by hydrate formation (MgSOa, CaCh), or by absorption (molecular sieves). The solvent can be distilled without removal of the desiccant in cases where water removal is irreversible (CaH2, P2O5). 

Sorption processes are very effective and include adsorption/desorption (reversible binding at the solid-water interface), absorption (diffusion of pollutants into the solid matrix), precipitation and coprecipitation (incorporation into a freshly formed solid), and occlusion (sequestration of adsorbed pollutants during mineral growth). The most important factors for retention processes are pollutant concentration, the composition of the solid matrix, solution composition (e.g., complexing agents) and E/pH conditions (Brady and Boms 1997). 

The data required for input into the groundwater flow models to predict the hydrodynamic flow velocity include the porosity of the soil, the water table, rainfall, reversible absorption/desorption phenomena, irreversible sorption, chemical reactions, and microbial degradation kinetics 37). Mixing with seawater, air, or steam may also be considered. Based on these models, estimates of leaching and pollutant distribution can be made many years into the future although significant amounts of computer time are usually required (57). 

FIGURE 4.18 Reversible water desorption/absorption for oxidized polymer sPSO2-220 in the temperature range 7 = 105°C-180°C at a water pressure of p(H20)=l atm (ICP Pa) as obtained by TGA (heating and cooUng rate 0.2°C/min). (Reprinted from Schuster, M. et al.. Macromolecules, 42(8), 3129, 2009. With permission.)... 

One chemical that is often overlooked is H2O, water. Most of us think of water as an inert material, but for some materials, such as raw iron, exposure to water causes an immediate chemical reaction. Fortunately, most thermoplastics do not chemically react with water. But there are some thermoplastics, such as nylon, which absorb water. This absorption process, which is fully reversible, causes the material to swell, and also acts as a plasticizer, making the material tougher, more flexible, and more ductile although it also reduces its strength. 

Sorption and Desorption Processes. Sorption is a generalized term that refers to surface-induced removal of the pesticide from solution it is the attraction and accumulation of pesticide at the sod—water or sod—air interface, resulting in molecular layers on the surface of sod particles. Experimentally, sorption is characterized by the loss of pesticide from the sod solution, making it almost impossible to distinguish between sorption in which molecular layers form on sod particle surfaces, precipitation in which either a separate soHd phase forms on soHd surfaces, covalent bonding with the sod particle surface, or absorption into sod particles or organisms. Sorption is generally considered a reversible equdibrium process. 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

It should be noted that the highest possible absorption rates will occur under conditions in which the hquid-phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. Such situations would exist, for instance, for NH3 absorption into an acid solution, for SO9 absorption into an alkali solution, for vaporization of water into air, and for H9S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, wrien both the hquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pres-... 

Water generally is used for gases fairly soluble in water, oils for light hydrocarbons, and special chemical solvents for acid gases such as CO9, SO9, and H9S. Sometimes a reversible chemical reaction will result in a veiy high solubility and a minimum solvent rate. Data on actual systems are desirable when chemical reactions are involved, and those available are referenced later under Absorption with Chemical Reaction. ... 

Barrels and Heaters These are also similar to those in extruder machines. In recent years, vented barrels have become available to facilitate the moulding of water sensitive plastics without the need for pre-drying. Water sensitivity in plastics can take several forms. If the plastic absorbs water then dimensional changes will occur, just as with wood or paper. The plastic will also be plasticised by the water so that there will be property changes such as a reduction in modulus and an increase in toughness. All these effects produced by water absorption are reversible. 

Certain chemicals (sorbents) have the ability to absorb moisture from a gas they may be either solid or liquid. Performance of a chemical dehumidifi cation device depends on the sorbent used. The sorbent must t>e able to attract and remove the sorbate, such as water, from the gas stream, Stirbems absorb water on the surface of the material by adsorption or by chemically combining with water (absorption). If the unit is regenerative, the process is reversible, allowing water to be removed. This is achieved by a sorbent such as silica gel, alumina gel, activated alumina, lithium chloride salt, lithium chloride solution, glycol solution, or molecular sieves. In the case of nonregenerative equipment, hygroscopic salts such as calcium chloride, urea, or sodium chloride are used. 

This action is reversible, so that to maintain the ester value it is obvious that the removal of water is advantageous. As a matter of fact, if transpiration is increased, or the absorption of moisture by the roots is diminished, the esterification is more rapid. 

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

In addition to the reactions of luciferin shown in Fig. 7.2.3, Trainor (1979) made an interesting discovery that arylsulfatase from Pattela vulgata (Sigma) converts luciferin into a violet compound (treatment conditions pH 5.05, 37°C, 4 hr). The compound showed absorption peaks at 344 nm and 560 nm (Fig. 7.2.7), and the peaks shifted reversibly to 347 nm and 547 nm in acid, and to 364 nm and 720 nm in an alkaline solution. The violet compound was also obtained from the pink compound by treatment with arylsulfatase, or from luciferin by heating in 50% trifluoroacetic acid at 100°C for 1 hr, followed by an addition of oxygenated water. These results, together with the... 

Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

The microenvironment in water-containing AOT-reversed micelles has a marked effect on the spectral properties of flnorescein. The absorption peaks are red-shifted by about 10 nm from the corresponding positions in aqueous solution, the absorption extinction coefficient increases with R, and the fluorescence is more effectively quenched in AOT-reversed micelles than in aqueous solution [149],... 

The protein-containing colloidal solutions of water-in-organic solvents are optically transparent. Hence, absorption spectroscopy, circular dichroism spectroscopy and fluorescence spectroscopy are found to be convenient for studying biocatalysis [53]. The reversed micelles are interesting models for studying bioconversion, since the majority of the enzymes in vivo act inside or on the surface of biological membranes. 

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