Solid-solution interactions adsorption

Solute adsorption is usually restricted to a mono molecular layer, since the solid-solute interactions, although strong enough to compete successfully with the solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers. Multilayer adsorption has, however, been observed in a number of cases, being evident from the shape of the adsorption isotherms and from the impossibly small areas per adsorbed molecule calculated on the basis of monomolecular adsorption. 

In adsorption from solution, physisorption is far more common than chemisorption, although the latter is sometimes possible. Solute adsorption is usually restricted to a monomolecular layer, since the solid-solute interactions, although strong enough to compete successfully with solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers, because the interaction is screened by the solvent molecules. Thus, multilayer adsorption has only rarely been observed in a number of cases, and identified, when the number of adsorbate molecules exceeds the number of mono-layer molecules possible on the total adsorbent surface area. However, this analysis cannot be applied to polymer adsorption, because it is generally impossible to determine the surface area of a monomolecular layer of a polymer adsorbed flat on the solid surface. This is because the adsorbed polymer can only be anchored to the surface at a few points, with the remainder of the polymer in the form of loops and ends moving more or less freely in the liquid phase. 

The adsorption of nonelectrolytes at the solid-solution interface may be viewed in terms of two somewhat different physical pictures. In the first, the adsorption is confined to a monolayer next to the surface, with the implication that succeeding layers are virtually normal bulk solution. The picture is similar to that for the chemisorption of gases (see Chapter XVIII) and arises under the assumption that solute-solid interactions decay very rapidly with distance. Unlike the chemisorption of gases, however, the heat of adsorption from solution is usually small it is more comparable with heats of solution than with chemical bond energies. 

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir-Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11-16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17[. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. 

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). 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

The nature of soil-phenolic acid interaction adsorption-desorption. Adsorption of a solute from solution onto a solid matrix results in a higher solute concentration at the fluid-solid interface than in the solution. Huang and coworkers (27) observed a high sorption capacity of the mineral fraction of four latosols for phenolic acids. On the basis of their results, distribution coefficients,... 

Figure 19.12. Batch parametric processing of solid-liquid interactions such as adsorption or ion exchange. The bottom reservoir and the bed interstices are filled with the initial concentration before pumping is started, (a) Arrangement of adsorbent bed and upper and lower reservoirs for batch separation, (b) Synchronization of temperature levels and directions of flow (positive upward), (c) Experimental separation of a toluene and n-hcptane liquid mixture with silica gel adsorbent using a batch parametric pump. (Reprinted from Wilhelm, 1968, with permission of the American Chemical Society), (d) Effect of cycle time t on reservoir concentrations of a closed system for an NaCl-H20 solution with an ion retardation resin adsorbent. The column is initially at equilibrium with 0.05M NaCl at 25°C and a = 0.8. The system operates at 5° and 55°C. [Sweed and Gregory, AIChE J. 17, 171 (1971)J.

Recently, considerable progress has been made on the calculation of electrostatic and hydrophobic interactions in biochemical systems 189 19 >. We can expect such calculations to become common for protein-solid surface interactions. Thus we can expect approximate values for the adsorption energy in selected systems to appear in the near future. The problem of time-dependent conformational adaptation of the protein to the surface (and vice versa) will be much more difficult. Initially, we will have to resort to crude measures of the structural stability of a protein, such as the temperature at which thermal denaturation occurs, the urea molar concentration for solution denaturation, etc. One or more of the models given in Fig. 13 should apply. 

A statistical thermodynamic equation for gas adsorption on synthetic zeolites is derived using solid solution theory. Both adsorbate-adsorbate and adsorbate-adsorbent interactions are calculated and used as parameters in the equation. Adsorption isotherms are calculated for argon, nitrogen, ammonia, and nitrous oxide. The solution equation appears valid for a wide range of gas adsorption on zeolites. 

There have been few studies reported in the literature in the area of multi-component adsorption and desorption rate modeling (1, 2,3., 4,5. These have generally employed simplified modeling approaches, and the model predictions have provided qualitative comparisons to the experimental data. The purpose of this study is to develop a comprehensive model for multi-component adsorption kinetics based on the following mechanistic process (1) film diffusion of each species from the fluid phase to the solid surface (2) adsorption on the surface from the solute mixture and (3) diffusion of the individual solute species into the interior of the particle. The model is general in that diffusion rates in both fluid and solid phases are considered, and no restrictions are made regarding adsorption equilibrium relationships. However, diffusional flows due to solute-solute interactions are assumed to be zero in both fluid and solid phases. 

Phenol and dodecyl benzene sulfonate are two solutes that have markedly different adsorption characteristics. The surface diffusion coefficient of phenol is about fourteen times greater than that for dodecyl benzene sulfonate. The equilibrium adsorption constants indicate that dodecyl benzene sulfonate has a much higher energy of adsorption than phenol (20,22). The adsorption rates from a mixture of these solutes can be predicted accurately, if (1) an adequate representation is obtained for the mixture equilibria, and (2) the diffusion rates in the solid and fluid phases are not affected by solute-solute interactions. 

Solute-solute Interactions may affect the diffusion rates In the fluid phase, the solid phase, or both. Toor (26) has used the Stefan-Maxwell equations for steady state mass transfer In multicomponent systems to show that, in the extreme, four different types of diffusion may occur (1) diffusion barrier, where the rate of diffusion of a component Is zero even though Its gradient Is not zero (2) osmotic diffusion, where the diffusion rate of a component Is not zero even though the gradient Is zero (3) reverse diffusion, where diffusion occurs against the concentration gradient and, (4) normal diffusion, where diffusion occurs In the direction of the gradient. While such extreme effects are not apparent in this system, it is evident that the adsorption rate of phenol is decreased by dodecyl benzene sulfonate, and that of dodecyl benzene sulfonate increased by phenol. 

A mathematical model has been developed to describe the kinetics of multicomponent adsorption. The model takes into account diffusional processes in both the solid and fluid phases, and nonlinear adsorption equilibrium. Comparison of model predictions with binary rate data indicates that the model predictions are in excellent for solutes with comparable diffusion rate characteristics. For solutes with markedly different diffusion rate constants, solute-solute interactions appear to affect the diffusional flows. In all cases, the total mixture concentration profiles predicted compares well with experimental data. 

The removal of Ra by adsorption has been attributed to ion exchange reactions, electrostatic interactions with potential-determining ions at mineral surfaces, and surface- precipitation with BaSO 4. The adsorptive behavior of Ra2+ is similar to that of other divalent cationic metals in that it decreases with an increase in pH and is subject to competitive interactions with other ions in solution for adsorption sites. In the latter case, Ra is more mobile in groundwater that has a high total dissolved solids (TDS) content. It also appears that the adsorption of Ra + by soils and rocks may not be a completely reversible reaction (Benes et al. 1984, 1985 Landa and Reid 1982). 

The S-shaped isotherm has an initial slope that increases with increasing equilibrium solute concentration and has two causes. Giles et al. (1974) attributed the S-shape to cooperative adsorption due to solute-solute interactions. These interactions stabilized the solute at the solid surface, and therefore the first adsorbed molecules enhance the adsorption of the next solute molecules. At high concentration, when the sites of the solid surface are saturated with solute the slope of adsorption isotherm start to decrease again. Sposito (1984) explained the S-shaped isotherm by a competing reaction within the solution. Solution ligands compete with surface... 

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