Adsorption irreversibility

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

The entrapment of various enzymes and proteins by clay minerals proceeds by weak interactions including electrostatic interactions, hydrogen and van der Waals bonding. Additivity of these various attractive forces renders the adsorption irreversible in some cases, but usually a leaching of enzyme is observed under working conditions. In order to fix the enzyme irreversibly at the surface of the clay layers different processes have been tried. In order to fix invertase on bentonite, Monsan and Durand [90] previously treated the clay mineral with a coupling agent,... 

Fig. 13. The dependence of the carbon monoxide saturation adsorption (total) and irreversible adsorption (irreversible) on the Cu/ZnO ratio in the binary copper-zinc oxide catalysts...

Sample Adsorption/irreversible Wrong working pH or too pH-control dilution and/or addition of... 

IRREVERSIBLE ADSORPTION. Irreversible adsorption with a constant mass-transfer coefficient is the simplest case to consider, since the rate of mass transfer is then just proportional to the fluid concentration. A truly constant coefficient is obtained only when all resistance is in the external film, but a moderate internal resistance does not change the breakthrough curve very much. Strongly favorable adsorption gives almost the same results as irreversible adsorption, because the equilibrium concentration in the fluid is practically zero until the solid concentration is over half the saturation value. If the accumulation term for the fluid is neglected, Eqs. (25.6) and (25.7) are combined to give... 

The size of a molecule is an important feature because proteins form multiple contacts with the surface (e.g., 77 contact points in the case of the albumin molecule and 703 contact points in the case of the fibrinogen molecule adsorbed on silica [10]). Multipoint binding usually causes adsorption irreversibility having a dynamic nature in the absence of irreversible denaturation. The rates of desorption are, as a rule, much lower than those of adsorption, and in many cases it is virtually impossible to attain the equilibrium state desorbing the adsorbed protein [11]. In other words, the formation of one or several bonds with the surface increases the probability of adsorption of neighboring sites of the same molecule. On the other hand, the desorption of a protein molecule requires the simultaneous rupture of a large number of bonds and, for kinetic reasons, equilibrium is not attained [12-14], This corresponds to a considerable difference between the activation energies for the adsorption and desorption processes [15,16],... 

It is likely that experimentally found values of molecular cross-sectional areas do not correspond to the equilibrium states of the protein layers but reflect only the transition state configuration, as assumed by MacRitchie [16] and depicted in Fig. 1. The problem of adsorption reversibility is basic for understanding the protein behavior at interfaces. The belief in the protein adsorption irreversibility is mainly based on drastic conformational changes in interfacial film and the great difficulty of desorbing a protein from this film [15], However, these criteria are not always a proof of irreversibility. It was shown in many cases [3,24,39-41] that proteins can be desorbed... 

Compare reversible adsorption, irreversible adsorption, and mineralization reactions with respect to the process. 

Asai M, Ohba T, Iwanaga T, Kanoh H, Endo M, Crunpos-Delgado J, Tenones M, Nakai K, Kaneko K (2011) Marked adsorption irreversibility of graphitic nanoribbons for CO2 and H2O. J Am Chem Soc 133 14880-14883... 

There are three different phenomena caused by solid support (inner capillary wall) activity reversible absorption (or semi-irreversible adsorption), irreversible adsorption and catalytic reaction of chromatographed compounds with stationary liquid phase, and solid support (inner walls of capillary tube), see, e.g. [4, 59]. In order to obtain reliable quantitative and qualitative results a chromatogra-pher must know for which compounds and under what chromatographic conditions the results will be influenced by these three types of activity and how to avoid its adverse influence. Column catalytic and adsorption activity is widely appreciated to be a function of temperature. Also, the thickness of... 

S.1.4.9 Summary of review of Dgbrowski et al. (2004) irreversible adsorption Irreversibility of phenol adsorption is important because of the need (if possible) for regeneration of the AC, the adsorbed (trapped) phenol having little (or zero) commercial value. 

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

This type of behavior can cause irreversibility in the adsorption isotherm as well as immobility on the surface [106]. 

An example of the time effects in irreversible adsorption of a surfactant system is shown in Fig. XI-8 for barium dinonylnapthalene sulfonate (an oil additive) adsorbing on Ti02 (anatase). Adsorption was ineversible for aged systems, but much less so for those equilibrating for a short time. The adsorption of aqueous methylene blue (note Section XI-4) on TiOi (anatase) was also irreversible [128]. In these situations it seems necessary to postulate at least a two-stage sequence, such as... 

Irreversible adsorption discussed in Section XI-3 poses a paradox. Consider, for example, curve 1 of Fig. XI-8, and for a particular system let the equilibrium concentration be 0.025 g/lOO cm, corresponding to a coverage, 6 of about 0.5. If the adsorption is irreversible, no desorption would occur on a small dilution on the other hand, more adsorption would occur if the concentration were increased. If adsorption is possible but not desorption, why does the adsorption stop at 6 = 0.5 instead of continuing up to 0 = 1 Comment on this paradox and on possible explanations. 

XVIII-11 (the paradox of desorption heat exceeding adsorption heat is explainable in terms of a partial irreversibility of the adsorption-desorption process). 

Jin X, Wang NHL, Tarjus G and Talbot J 1993 Irreversible adsorption on nonuniform surfaoes the random site model J. Phys. Chem. 97 4256-8... 

The swelling of the adsorbent can be directly demonstrated as in the experiments of Fig. 4.27 where the solid was a compact made from coal powder and the adsorbate was n-butane. (Closely similar results were obtained with ethyl chloride.) Simultaneous measurements of linear expansion, amount adsorbed and electrical conductivity were made, and as is seen the three resultant isotherms are very similar the hysteresis in adsorption in Fig. 4.27(a), is associated with a corresponding hysteresis in swelling in (h) and in electrical conductivity in (c). The decrease in conductivity in (c) clearly points to an irreversible opening-up of interparticulate junctions this would produce narrow gaps which would function as constrictions in micropores and would thus lead to adsorption hysteresis (cf. Section 4.S). 

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

A discussion of the adsorption of water on oxides would be incomplete without some reference to the irreversible effects which are often encountered when samples of oxide, hydroxide or oxide-hydroxide are exposed to the vapour. These effects ( low-temperature ageing ), which manifest themselves in changes in surface area, in pore structure and sometimes in the lattice structure itself, are complex and difficult to reproduce exactly. ... 

In the analytical chromatographic process, mixtures are separated either as individual components or as classes of similar materials. The mixture to be separated is first placed in solution, then transferred to the mobile phase to move through the chromatographic system. In some cases, irreversible interaction with the column leaves material permanently attached to the stationary phase. This process has two effects because the material is permanently attached to the stationary phase, it is never detected as leaving the column and the analysis of the mixture is incomplete additionally, the adsorption of material on the stationary phase alters the abiHty of that phase to be used in future experiments. Thus it is extremely important to determine the ultimate fate of known materials when used in a chromatographic system and to develop a feeling for the kinds of materials in an unknown mixture before use of a chromatograph. 

Most adsorption processes are exothermic (AH is negative). Adsorption processes involving nonspecific interactions are referred to as physical adsorption, a relatively weak, reversible interaction. Processes with stronger interactions (electron transfer) are termed chemisorption. Chemisorption is often irreversible and has higher heat of adsorption than physical adsorption. Most dispersants function by chemisorption, in contrast to surfactants, which... 

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