Mixed adsorption mechanisms

For a modelling of adsorption processes the well-known integro-differential equation (4.1) derived by Ward and Tordai [3] is used. It is the most general relationship between the dynamic adsorption r(t) and the subsurface concentration e(0,t) for fresh non-deformed surfaces and is valid for kinetic-controlled, pure diffusion-controlled and mixed adsorption mechanisms. For a diffusion-controlled adsorption mechanism Eq. (4.1) predicts different F dependencies on t for different types of isotherms. For example, the Frumkin adsorption isotherm predicts a slower initial rate of surface tension decrease than the Langmuir isotherm does. In section 4.2.2. it was shown that reorientation processes in the adsorption layer can mimic adsorption processes faster than expected from diffusion. In this paragraph we will give experimental evidence, that changes in the molar area of adsorbed molecules can cause sueh effectively faster adsorption processes. 

Obviously the influence of micelles must be taken into account in the first approximation only for diffusional and mixed adsorption mechanisms. If the rate-limiting step consists in crossing the adsorption barrier by the surfactant monomers, all the relations derived for non-micellar solutions hold for c > CMC too. 

Fig. 5.17. Dependence of the damping coefficient ot capillary waves on DACh concentration at the frequency of 200 Hz [168] curves are calculated according to Eq. (5.256) for the diffusion - controlled adsorption mechanism (solid line), for the mixed adsorption mechanism (dotted line), and for the barrier -controlled adsorption mechanism (dashed line).

Figures 5 and 6 show that the concentration of the two surfactants in the effluents increases simultaneously with the production of the desorbent, which confirms the mixed micellization mechanism described above. Figure 5, where the three additives are produced lately, illustrates the phenomenon particularly well. At the lower pH corresponding to strong adsorption conditions for sulfonate (test 4), the one pore-volume micellar slug would have been entirely consumed by the medium in the absence of any desorbent.

The second part of the book covers zeolite adsorptive separation, adsorption mechanisms, zeolite membranes and mixed matrix membranes in Chapters 5-11. Chapter 5 summarizes the literature and reports adsorptive separation work on specific separation applications organized around the types of molecular species being separated. A series of tables provide groupings for (i) aromatics and derivatives, (ii) non-aromatic hydrocarbons, (iii) carbohydrates and organic acids, (iv) fine chemical and pharmaceuticals, (v) trace impurities removed from bulk materials. Zeolite adsorptive separation mechanisms are theorized in Chapter 6. 

A few papers have been published recently on the problem of surfactant adsorption maxima on solids in the region of the CMC (1-5). Scamehorn et al. (1,2) and Trogus et al. (3) expTTined the origin of these maxima by various radios of the surfactant solution to the solid, in connection with isomeric impurity of the surfactant. Ananthapadmanabhan and Soniasundaran (4) examined critically the presence of such maxima from the viewpoint of various proposed adsorption mechanisms. They have shown that a mechanism including micellar exclusion, mixed micelle formation and properties of solids, such as the pore size, cannot explain satisfac-... 

Another problem associated with LLC is that of mixed retention mechanisms. Ideally, the solid support in LLC binds the molecules of the stationary phase with strong adsorptive forces, but it does not exert these forces on solute molecules. Clearly, this ideal situation can never be realized completely [315]. 

It has been demonstrated that mixed oxides obtained from calcined LDHs have the ability to act as sorbents for a variety of anionic compounds from aqueous solution. This ability is because of the propensity for the mixed oxide to hydrate and re-form an LDH in such conditions and is of particular interest for the decontamination of waste-water. Hermosin et al. have found, for example, that MgAl-LDHs calcined at 500 °C are potential sorbents for the pollutants trinitrophenol and trichlorophenol from water [208, 209]. The adsorption mechanism was shown, using PXRD, to involve reconstruction of the LDH, with the uptake of the phenolate anions into the interlayers. Similarly, the ability of calcined MgAl-LDHs to remove nitriloacetate anions from solution has been demonstrated [210]. Calcined LDHs have been utilized also for the sorption of radioactive anions, such as 111, from aqueous solution [211]. A particularly attractive feature of the use of calcined LDHs for the remediation of waste-water is that the sorption capacity of the material may be regenerated via calcination of the rehydrated LDH. 

Wilson and co-workers developed a statistical mechanical model for single component surfactant adsorption (29-31) and expanded it to a binary system (2,3). Different adsorption curves were generated by varying the Van der Waals interaction parameters. The mixed adsorption equations that were developed were very complex and were not applied to experimental data. 

In such a model, the distribution of a chemical element is controlled by mixing between an upper and lower boundary and some production or removal process. The assignment of a mechanism to explain this process is to some extent intuitive (for instance, a deficit over atmospheric equilibrium is attributed to respiration) however, for many metal ions, the explanation of the observed removal rate by an adsorptive mechanism in the deep aphotic ocean appears to be most likely. 

Although there is evidence that complexation with silver ions is the governing interaction in Ag-TLC, other factors should also be considered. Thus sihca gel, which is the most widely used supporting material, possesses appreciable polarity and adsorption activity. Therefore, in many cases, an impact of mixed retention mechanism on migration, geometry of spots, and selectivity of resolution is to be expected. Also, the mobile-phase solvents are active elements of the chromatographic system and interactions both with the supporting material and FA is possible this may also have a serious effect on the whole separation process. 

Higher aliphatic amines (butylamine, pentylamine, diethylamine, etc.) had larger retention volumes and values well above 1.0. A mixed retention mechanism involving hydrophobic adsorption and steric effects was observed for these compounds. Aromatic amines were found to be retained almost solely by a reversed-phase mechanism involving interaction of the solute with the unfunctionalized regions of the stationary phase. Retention of these solutes could be manipulated most easily by addition of acetonitrile to the eluent. 

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

Further models of adsorption kinetics were discussed in the literature by many authors. These models consider a specific mechanism of molecule transfer from the subsurface to the interface, and in the case of desorption in the opposite direction ((Doss 1939, Ross 1945, Blair 1948, Hansen Wallace 1959, Baret 1968a, b, 1969, Miller Kretzschmar 1980, Adamczyk 1987, Ravera et al. 1994). If only the transfer mechanism is assumed to be the rate limiting process these models are called kinetic-controlled. More advanced models consider the transport by diffusion in the bulk and the transfer of molecules from the solute to the adsorbed state and vice versa. Such mixed adsorption models are ceilled diffusion-kinetic-controlled The mostly advanced transfer models, combined with a diffusional transport in the bulk, were derived by Baret (1969). These dififiision-kinetic controlled adsorption models combine Eq. (4.1) with a transfer mechanism of any kind. Probably the most frequently used transfer mechanism is the rate equation of the Langmuir mechanism, which reads in its general form (cf. Section 2.5.),... 

OV-275. For solutes retained solely by gas-liquid partitioning (e.g. nitromethane, dioxane and ethanol) on Carbowax 20M the plots have a zero slope. The n-alkanes are retained by a mixed retention mechanism on Carbowax 20M as indicated by the slope. The relatively large intercept is an indication that gas-liquid partitioning makes a significant contribution to the retention mechanism. Interfacial adsorption is important for all compounds on OV-275 and is dominant for the n-alkanes, which have a near zero intercept, indicating that gas-liquid partitioning is of minor importance to their retention. The lack of a reliable method to estimate the surface area of the liquid phase prevents Eq. (2.3) from being used to determine the gas-liquid adsorption coefficient. 

When fillers and rubbers are dry mixed and the resulting samples solvent extracted, the amount of rubber bound is clearly the result of all possible adsorption mechanisms as well as of intermolecular cross-linking. Because of the hysteretic nature of physical adsorption, chemical interactions are not necessary for the development of some bound rubber. In many cases, however, chemisorption aixl grafting dominate the physical adsorption. Sircar and Voet (97) used low molecular weight SBR and polyisobutylenes 50,000 and less) to minimize the... 

For the practical use a criterion is needed to decide whether it is justified that such a kinetic equation is applied. This criterion must ensure that the kinetic constants are independent of the parameters of the adsorption process, mainly the surfactant concentration and the monolayer coverage. Experimental data for various surfactants show that for surface lifetimes shorter than 20 ms the reduced desorption rate constant k , = kj /T is nearly constant and of the order of 100 s [16]. This important result allows to define a simple criterion for a non-diffusional adsorption mechanism by comparing the characteristic times of diffusion and adsorption kinetics according to the model of Eq. (4.15). The condition for mixed or kinetic controlled... 

Chromate. Increasing chromate adsorption shifted the PZC of amorphous iron oxide to increasingly lower pH value indicating an inner-sphere adsorption mechanism (54). FTIR analyses of freeze-dried samples containing adsorbed chromate mixed with KBr showed shifts in absorption bands from those... 

Small organic molecule inhibitors often function by an adsorption mechanism, the extent of adsorption being dependent on a number of variables including the potential of the metal relative to its potential of zero charge (pzc) (for oxide surfaces pzc is dependent on pH value), the structure and the charge (or polarizability) of the inhibitor (adsorbate), the structure of the metal (oxide) surface, and the presence of other species in the electrolyte [22]. Such inhibitors are often of the mixed type, reducing the rate of both cathodic and anodic reactions [22]. It is interesting to note that aniline [24], pyrrole [25], thiophene [26], and the functionalized forms of these molecules exhibit the ability to inhibit... 

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