Reversible mobile adsorption

Here we consider the simple adsorption-desorption reaction equilibrium of a reversible mobile adsorption process without any chemical reactions ... 

The given description of the gas phase transport in a tube with a temperature gradient is only valid for adsorption equilibria of reversible mobile adsorptions without any superimposed chemical reaction. The temperature profile along the chromatography column is approximated to be linear by ... 

The compatibility of electrochemical detection with the various modes of liquid chromatography is limited. For all practical purposes, electrochemical detection is not suitable for use with normal phase adsorption or partition chromatography due to the solvents of low dielectric constant used as the mobile phase. On the other hand, reverse-phase adsorption and partition (including ion-exchange or ion-pairing systems) are highly com-... 

Because of their large surface-to-volume ratio and high metabolic activity, microorganisms are important vectors in the introduction of heavy metal and radionuclide pollutants into food webs. As discussed in Chapter 5, heavy metals in soils and sediments tend to be immobilized by precipitation at neutral to alkaline pH and/or adsorption to cation exchange sites of clay minerals. Microbial production of acid and chelating agents can reverse this adsorption and mobilize toxic metals. Microbial metabolism products that can chelate metals include... 

Small net adsorption enthalpies represent a prerequisite to reversible non-localized mobile adsorption processes without reactions of the species adsorbed on the stationary surfaces. The experimental proof of such correlations for defined classes of pure substances is essential for the prediction of adsorption properties of transactinides and their compounds. Therefore, a variety of gas adsorption chromatographic experiments were carried out with carrier free amounts of different radioisotopes using selected modified surfaces as stationary phases. The use of carrier free amounts is necessary to experimentally obtain adsorption conditions at nearly zero surface coverage. 

If the mixture to be separated contains fairly polar materials, the silica may need to be deactivated by a more polar solvent such as ethyl acetate, propanol or even methanol. As already discussed, polar solutes are avidly adsorbed by silica gel and thus the optimum concentration is likely to be low, e.g. l-4%v/v and consequently, a little difficult to control in a reproducible manner. Ethyl acetate is the most useful moderator as it is significantly less polar than propanol or methanol and thus, more controllable, but unfortunately adsorbs in the UV range and can only be used in the mobile phase at concentrations up to about 5%v/v. Above this concentration the mobile phase may be opaque to the detector and thus, the solutes will not be discernible against the background adsorption of the mobile phase. If a detector such as the refractive index detector is employed then there is no restriction on the concentration of the moderator. Propanol and methanol are transparent in the UV so their presence does not effect the performance of a UV detector. However, their polarity is much greater than that of ethyl acetate and thus, the adjustment of the optimum moderator concentration is more difficult and not easy to reproduce accurately. For more polar mixtures it is better to explore the possibility of a reverse phase (which will be discussed shortly) than attempt to utilize silica gel out of the range of solutes for which it is appropriate. 

As the solvent concentration increases, the PIC reagents will interact more strongly with the mobile phase and will be less strongly adsorbed on the reverse phase surface. As a consequence, there will be less ion exchange material on the stationary phase surface. This is clearly demonstrated by the adsorption isotherm of octane sulfonate shown in figure 10. 

Immobilization by adsorption onto a surface such as activated carbon or to an ion-exchange resin gives a reversible and relatively weak bond, but this can be sufficient to increase the retention time in a flow system to acceptable levels. Recall Section 10.6 where it is shown that the residence time of an adsorbed species can be much larger than that of the mobile phase, in essence giving more time for catalysis. 

The form of the effective mobility tensor remains unchanged as in Eq. (125), which imphes that the fluid flow does not affect the mobility terms. This is reasonable for an uncharged medium, where there is no interaction between the electric field and the convective flow field. However, the hydrodynamic term, Eq. (128), is affected by the electric field, since electroconvective flux at the boundary between the two phases causes solute to transport from one phase to the other, which can change the mean effective velocity through the system. One can also note that even if no electric field is applied, the mean velocity is affected by the diffusive transport into the stationary phase. Paine et al. [285] developed expressions to show that reversible adsorption and heterogeneous reaction affected the effective dispersion terms for flow in a capillary tube the present problem shows how partitioning, driven both by electrophoresis and diffusion, into the second phase will affect the overall dispersion and mean velocity terms. 

From the general framework of the Snyder and Soczewinski model of the linear adsorption TLC, two very simple relationships were derived, which proved extremely useful for rapid prediction of solute retention in the thin-layer chromatographic systems employing binary mobile phases. One of them (known as the Soczewinski equation) proved successful in the case of the adsorption and the normal phase TLC modes. Another (known as the Snyder equation) proved similarly successful in the case of the reversed-phase TLC mode. 

Prus and Kowalska [75] dealt with the optimization of separation quality in adsorption TLC with binary mobile phases of alcohol and hydrocarbons. They used the window diagrams to show the relationships between separation selectivity a and the mobile phase eomposition (volume fraction Xj of 2-propanol) that were caleulated on the basis of equations derived using Soezewiriski and Kowalska approaehes for three solute pairs. At the same time, they eompared the efficiency of the three different approaehes for the optimization of separation selectivity in reversed-phase TLC systems, using RP-2 stationary phase and methanol and water as the binary mobile phase. The window diagrams were performed presenting plots of a vs. volume fraetion Xj derived from the retention models of Snyder, Schoen-makers, and Kowalska [76]. 

At a Pd(l 11) surface at room temperature, the chemisorption state is disordered when the NO pressure is less than 3 x 10-6 Torr with very noisy STM images due to the high mobility of the adsorbed molecules.14 With increasing pressure (and coverage), the c(4 x 2) state, which is reversible, is locked-in and immobile. The adsorption at lower temperatures (150-200 K), where the coverage exceeds that at room temperature, the c(4 x 2) state coexists with a p(2 x 2) and a c(8 x 2) phase the latter is only present when it coexists with the c(4 x 2) and p(2 x 2) states. 

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). 

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