Adsorption effects, separation

The regular and rigid pore structures also make MOFs good shape and size selectivity for HPLC separation. Ahmad et al. reported the separation of benzene, naphthalene, anthracene, phenanthrene, pyrene, 1,3,5-triphenylbenzene, and 1,3,5-tris(4-bromophenyl)benzene on MOF-199- and MOF-5-packed columns based on the combination of molecular sieving and adsorption effects. Separation of ethylbenzene and styrene with very similar dimensions was also achieved on MOF-199 column. Unlike shape and size selectivity mechanisms, the separation of ethylbenzene and styrene was ascribed to the jt-complexation of styrene with Cu metal site in MOF-199. 

Adsorption Processes. More recendy, pressure swing adsorption (PSA) processes utilizing a high selectivity copper adsorbent have been utilized to effectively separate carbon monoxide from blast furnace gas and coke oven gas (97—101). 

Adsorption of bath components is a necessary and possibly the most important and fundamental detergency effect. Adsorption (qv) is the mechanism whereby the interfacial free energy values between the bath and the soHd components (sofld soil and substrate) of the system are lowered, thereby increasing the tendency of the bath to separate the soHd components from one another. Furthermore, the soHd components acquire electrical charges that tend to keep them separated, or acquire a layer of strongly solvated radicals that have the same effect. If it were possible to foUow the adsorption effects in a detersive system, in all their complex ramifications and interactions, the molecular picture of soil removal would be greatly clarified. 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Excess collector can also reduce the separation by forming micelles in the bulk which adsorb some of the colhgend, thus keeping it from the surface. This effect of the micelles on Ki for the colhgend is given theoretically [Lemhch, Principles of Foam Fractionation, in Periy (ed.). Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1] by Eq. (22-44) [Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972] if F, is constant when C, > C-... 

The complexity of the system increases with the number of solvents used and, of course, their relative concentrations. The process can be simplified considerably by pre-conditioning the plate with solvent vapor from the mobile phase before the separation is started. Unfortunately, this only partly reduces the adsorption effect, as the equilibrium between the solvent vapor and the adsorbent surface will not be the... 

TSK-GEL PW type columns are commonly used for the separation of synthetic water-soluble polymers because they exhibit a much larger separation range, better linearity of calibration curves, and much lower adsorption effects than TSK-GEL SW columns (10). While TSK-GEL SW columns are suitable for separating monodisperse biopolymers, such as proteins, TSK-GEL PW columns are recommended for separating polydisperse compounds, such as polysaccharides and synthetic polymers. 

Current use of statistical thermodynamics implies that the adsorption system can be effectively separated into the gas phase and the adsorbed phase, which means that the partition function of motions normal to the surface can be represented with sufficient accuracy by that of oscillators confined to the surface. This becomes less valid, the shorter is the mean adsorption time of adatoms, i.e. the higher is the desorption temperature. Thus, near the end of the desorption experiment, especially with high heating rates, another treatment of equilibria should be used, dealing with the whole system as a single phase, the adsorbent being a boundary. This is the approach of the gas-surface virial expansion of adsorption isotherms (51, 53) or of some more general treatment of this kind. 

Figures 6, 7 and 9 show calibration curves using two multi-column combinations and illustrate the degree of "optimization obtained in this system. The mobile phases for Figures 6 and 7 contained 0.025 g polyethylene oxide and ion exclusion and adsorption effects should therefore be largely eliminated. Figure 6 shows that reasonably good resolution can be obtained with a combination of five columns but does exhibit some loss of peak separation at the low cuid high MW ends. In Figure 7 the effect of adding a sixth column of small pore size is illustrated and it is seen that resolution at the low MW end is thereby somewhat improved. This calibration curve is effectively linear with a change of slope at 500,000 MW. It should provide a useful aqueous GPC system for MW and MWD determination of nonionic polyacrylamides.

A sharp decrease in adsorption enthalpy between 10 and 30% surface coverage of SAL can also be seen in Figure 2. This decrease may indicate that only a small number of surface sites are favorably oriented for SAL-goethite bond formation, although possible SAL-SAL interactions on the surface may also have an effect. Separate measurements of SAL adsorption on goethite, gave relatively small adsorption maxima (when compared to the phosphate and fluoride adsorption maxima discussed above) of 22 and 11 pmol/g at pH 4.8 and 6.3, respectively, in either 0.001 M NaN0 or 0.001 M KC1 06). J... 

In cases, of acute emulsion-problems substances like-anion exchangers alumina or silicagel are used specifically to resolve the problem by adsorption of the emulsifying agents. In fact, it would be advisable to employ the technique of column chromatography for the effective separation of the analyte as compared to an extraction process. 

The adsorptive effects of the polar adsorbents are often due to the presence of hydroxyl groups and the formation of hydrogen bonds with the solute molecules. The strength of these bonds and hence the degree of adsorption increases as the polarity of the solute molecule increases. In order to separate the solute from the adsorbent it is necessary to use a solvent in which the solute will dissolve and which also has the ability to displace the solute from the adsorbent. Solvents that are too polar will overwhelm the adsorptive effects and result in the simultaneous elution of all the components of a mixture. Table 3.3 lists various classes of compounds and some common solvents in order of polarity. 

Cellulose is itself polar in nature and can cause some adsorption, which may result in the tailing of zones. However, this adsorptive effect may contribute to the separation process in some instances and the use of a polar mobile phase can enhance this effect further, e.g. the separation of amino acids using an aqueous solution of ammonia as the mobile phase. The combination of partition and adsorption generally influences separations on cellulose thin-layer plates, which have superseded paper chromatography in most instances and offer increased speed and resolution. 

Zonal techniques are the most frequently used form of electrophoresis and involve the application of a sample as a small zone to a relatively large area of inert supporting medium which enables the subsequent detection of the separated sample zones. A wide range of supporting media have been developed either to eliminate difficulties caused by some media (e.g. the adsorptive effects of paper) or to offer additional features (e.g. the molecular sieving effects of polyacrylamide gel). 

Packings based on silicas for separation by molecular size are also presently available. However, the molecular size range available is more limited and possibility of encountering adsorption effects is more likely. Such columns must be throughly evaluated for each new type of sample for which a separation and/or molecular weight distribution is desired. 

Finally, we note that the x and solubility parameters of the O-butylated extract are noticeably absent in Table VI. We have attempted to analyze the sorption kinetics according to the Berens-Hopfenberg model in order to correct for adsorption effects, but the treatment yielded unreasonable x parameters. The reason for this is not clear, but we believe it may be due to the fact that di sion into the extract is so rapid. Hole-filling and swelling may have comparable rates so that a separation of the two processes is not possible. 

The cation exchanger in the H fornt required to separate Ca 2 ions from 1 dm3 of 01N CaC12 are The adsorption effect of an ion is characterised by the distribution coefficient... 

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