Adsorption of solvents

There are some theoretical complications discussed in Refs. 91 and 92. Experimental complications include adsorption of solvent or of film on the electrode [93,94] the effect may be used to detect atmospheric contaminants. The atmosphere around the electrode may be flushed with dry nitrogen to avoid condensation problems [87]. 

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

However, as follows from the results presented in Fig. 1(b), the behavior of the PMF for the case of adsorbed dispersion in the matrix at Pm< m — 0.386 contains interesting features in addition to those shown in Fig. 1(a). We observe that the PMF is modulated by the presence of solvent species and in addition is modulated by the presence of matrix particles. The structural repulsive barrier appears, due to matrix particles. An additional weak attractive minimum exists at separations corresponding to matrix-separated colloids. It is interesting that the effects of solvent modulation of the PMF in the adsorbed dispersion are seen for matrix separated colloids. The matrix particles are larger than colloids adsorption of solvent species on the surface of a matrix particle is stronger than on the surface of a colloid. Therefore, the solvent modulating effects of the PMF result from colloids separated by a matrix particle covered by a single layer of solvent species. 

The vast majority of modem liquid chromatography systems involve the use of silica gel or a derivative of silica gel, such as a bonded phase, as a stationary phase. Thus, it would appear that most LC separations are carried out by liquid-solid chromatography. Owing to the adsorption of solvent on the surface of both silica and bonded phases, however, the physical chemical characteristics of the separation are more akin to a liquid-liquid distribution system than that of a liquid-solid system. As a consequence, although most modern stationary phases are in fact solids, solute distribution is usually treated theoretically as a liquid-liquid system. 

Mono-Laver Adsorption of Solvents on the Surface of Silica Gel... 

Solvent adsorption. The adsorption of solvent molecules is manifested in their orientation and ordered arrangement at the interface. 

Qualitative and quantitative elemental analysis of polymers can be carried out by the conventional methods used for low-molecular-weight compounds. So a detailed description is not needed here. Elemental analysis or determination of functional groups is especially valuable for copolymers or chemically modified polymers. For homopolymers where the elemental analysis should agree with that of the monomer, deviations from the theoretical values are an indication of side reactions during polymerization. However, they can also sometimes be caused by inclusion or adsorption of solvent or precipitant, or, in commercial polymers, to the presence of added stabilizers. The preparation of the sample for... 

Adsorption of solvent on silica gel. Attempts to use this adsorbent failed since activated silica adsorbs moisture from air more strongly than alcohol and ether vapour. 

Adsorption of solvent with activated charcoal. This is the most modern method of solvent recovery. It was introduced after World War I and immediately attracted attention by the exceptionally high recovery obtained, amounting to approximately 98% of the solvent entering the plant. The first plants using activated charcoal were used extensively in the oil industry for separating methane from heavier fractions in natural gas. 

The simplest device for the adsorption of solvent on charcoal is shown in Fig. 226. The air containing alcohol and ether is introduced from below through valve (7) into a cylindrical container (adsorber) filled with activated charcoal. Alcohol and ether are adsorbed on the charcoal and the solvent-free air escapes through... 

Fig. 227. Schematic view of an installation for the adsorption of solvent vapours (alcohol and ether) with activated charcoal by the Acticarbone method.

Calculated from molecular sieve weight increase during adsorption assuming only oil and wax adsorbed. This calculation ignores water adsorption during molecular sieve transfer and adsorption of solvent. Consequently, these values are low. 

When two such surfaces approach each other, layer after layer is squeezed out of the closing gap (Fig. 6.12). Density fluctuations and the specific interactions then cause an exponentially decaying periodic force the periodic length corresponds to the thickness of each layer. Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [168], Periodic solvation forces across confined liquids were first predicted by computer simulations and theory [168-171], In this case, however, the experimental proof came only few years afterwards using the surface forces apparatus [172,173]. Solvation forces are not only an important factor in the stability of dispersions. They are also important for analyzing the structure of confined liquids. 

Many organic liquids have been suspected of exercising more than a solvent action on coal. An indication of chemical interaction is the observation that the total weight of products sometimes exceeds the original weight of coal, although up to about 5% may be the result of strong adsorption of solvents on the residue and extract. With mixed solvents, the potential for interaction may be increased. 

The physical adsorption of solvent molecules in catalytic reactions carried out in solutions is often overlooked. The author cannot resist the impression that in many of Maxted s (387) experiments on the poisoning of catalysts the adsorption of solvent molecules plays a more important role than Maxted thinks. It may be expected that a critical study of the adsorption of the solvent molecules and their influence on the adsorption of the poison molecules will lead to even more important conclusions on poisoning than have already been reached by Maxted and collaborators in their systematic and elaborate work. 

Adsorption of solvent vapors on commercial activated carbon. Ind. Eng. Chetn., 36 894-900. 

The ability of monomeric Schiff base complexes or polymeric Schiff base ligands to absorb solvent molecules is well-known. Sawodny et al.27) found that the polymer Schiff base complexes exhibit this property much more markedly. The polymers were prepared by the polycondensation reaction of polymer ligands of the Schiff base (29) with metal salts (Eq. 8). The adsorption of solvent is reversible and the ability is dependent upon the nature of the central metal atom and of the ring size around the metal. This profile suggests that the above polymers can be used as a molecular sieve. 

Measurement of the adsorption of solvent vapours by films of polymer and of polymer mixtures can be used to obtain information about interactions within mixtures, The theoretical basis for the technique is similar to I.G.C. but, being a static rather than dynamic method it is in principle easier to reduce the effects of surface adsorption and diffusion limitation inherent in I.G.C. The technique is however not so fast and easy as I.G.C., but in principle gives information about the interaction between two polymers over the whole range of solvent composition. 

The adsorption of solvent molecules by chelates (126 M) from (123 c) and (124 e) with bivalent metal cations of high coordination number, e.g. Cr was studied. So molecules such as pyridine, CCI4, benzene, CH3OH, n-pentane with a critical diameter of < 7 A are adsorbed. The capacity is higher than using various molecular sieves (except for water). 

Dickert and co-workers [18] have made an innovative advance by applying polyurethane-based MIPs to quartz crystals for the selective detection of solvent vapours (see also Section 20.2.5.8. and Chapter 21). Electrodes may be attached to quartz crystals to form quartz crystal microbalances (QCMs), wherein minute increases in the mass of the device (for example, upon adsorption of solvent vapour) result in a decrease of the resonant frequency of the crystal. For crystals... 

Activated carbon utilized for gas purification, e.g. for the adsorption of solvent vapor, can be regenerated by desorption with steam at 120 to 140°C. The activated carbon then has to be dried. In special cases other types of regeneration are possible. 

For adsorption of solvent, the surrounding phase of interest is typically the vapor phase. Hygroscopicity is a special case of adsorption in which water is preferentially adsorbed onto the crystal surface. Materials that exhibit this behavior are called hygroscopic materials. Deliquescence is a phenomenon in which a hygroscopic material liquefies after adsorbing a certain amount of water onto the crystal surface. Hygroscopicity and deliquescence are commonly encountered in pharmaceuticals. 

In 1978, the EPA established Control Techniques Guidelines (CTG), leading to requests for statements of Reasonably Attainable Control Technology (RACT). The goal of the EPA was to reduce VOC emissions to set limits or to achieve an 85% reduction of existing VOC. VOC levels were to be reduced by one or more of four methods adjustment of LEL (lower explosive limit) of curing ovens solvent scrubbing solvent incineration and carbon adsorption of solvents. 

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