Adsorption of dissolved species

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

Mixed monolayers have been widely employed in affinity biosensors, in order to prevent non-specific adsorption of dissolved species on the electrode surface, to reduce the steric hindrance between biological receptors, and to increase the degree of order of the bio-receptors on the electrode surface (Fig. 5.18). 

Equations (6.42), (6.43), (6.44), and (6.45) are established for the system which is not affected by precipitation of minerals such as Fe- and Mn-hydroxides, adsorption of dissolved species, colloids and fine grained particles onto minerals, uptake of elements by organisms and dissolution of sediments. Figure 6.17 shows an example of the system which is not affected by these processes. Observed concentration is in agreement with the calculated concentration based on Eq. (6.45) (Fig. 6.17). 

Reactions of dissolved species with particulate and colloidal suspended matter include adsorption/desorption, complexation, ion-exchange, precipitation/dissolution, coprecipitation during coagulation and flocculation (Morgan, 1966 Stumm and Morgan, 1981 Parks, 1975). These processes are particularly important at the land-sea boundary in estuaries (Duinker, 1980 Martin et al., this volume). The interaction with particles > 0.45 ym is not discussed here. 

The by far most widespread mechanism by which an N-NDR is hidden is the adsorption of a species that inhibits the main electron-transfer process. The species might be dissolved in the electrolyte, e.g., it might be the anion of the supporting electrolyte, or it is formed in a side reaction path, as it is the case in nearly all oxidation reactions of small organic molecules. Before we introduce specific examples of this type of HN-NDR oscillators, it is useful to study the dynamics of a prototype model. This will then help us to identify the essential mechanistic steps in real systems whose quantitative description requires more variables such that the basic feedback loops are not as obvious. 

The Eh values are too high to be explained by the formation of either PtS or PtS2. The rest potentials of the platinum electrode are dependent upon the concentration of dissolved hydrogen sulfide (z[H2S]]. This can be explained by selective adsorption of H2S species on the platinum followed by the discharge of the proton mediated via chemisorbed H2S. The corresponding reactions are (11] ... 

The steady adsorption of dissolved organic material onto a rising bubble, with the resulting increase in surface pressure, should progressively force the more water soluble and less surface-active species out of the bubble surface (35). Thus, for a given sized bubble, the proportion of various species varies with bubble age. This is refiected in the com-... 

Leckie [14] emphasized the advantage of chemical speciation over overall distribution coefficients in adsorption modeling. On the other hand, in many theoreticar studies of adsorption even the speciation in solution is neglected and only the total concentration of dissolved species is taken into account. One probable reason of paying no attention to well-known experimental facts is that some authors use adsorption equations borrowed from gas adsorption, and obviously these equations are not suitable to deal with multiple solution species involving the adsorbate. 

The treatment of such problems is more complicated than those involving only dissolved species, because one must choose an adsorption isotherm, which involves the introduction of additional parameters and, in general, nonlinear equations. In addition, the treatment must include assumptions about (a) the degree to which adsorption equilibrium is attained before the start of the electrochemical experiment (i.e., how long after the formation of a fresh electrode surface the experiment is initiated) and (b) the relative rate of electron transfer to the adsorbed species compared to that for the dissolved species. These effects complicate the evaluation of the voltammetric data and make the extraction of desired mechanistic and other information more difficult. Thus adsorption is often considered a nuisance to be avoided, when possible, by changing the solvent or changing concentrations. However, adsorption of a species is sometimes a prerequisite for rapid electron transfer (as in forms of electrocatalysis), and can be of major importance in processes of practical interest (e.g., the reduction of O2, the oxidation of aliphatic hydrocarbons, or the reduction of proteins). Our discussion here will deal with the basic principles and several important cases. 

The problem is a complex one for not only is there convection of the liquid by electroosmosis but if the dissolved contaminants are themselves charged, then an additional electromigration velocity will be imposed on them. Moreover, to the extent that concentration gradients are set up, there will also be transport of dissolved species by diffusion. In addition, there are chemical reactions in the bulk fluid and at the electrodes, together with adsorption or desorption at the soil surface. 

The liquid-solid interface supports the growth of 2D crystals on surfaces too. The liquid phase acts as a reservoir of dissolved species which can diffuse towards the substrate, adsorb, diffuse laterally and desorb. These dynamic processes favor the repair of defects. Under equihbrium conditions, relatively large domains of well-ordered patterns are formed. Large domains grow at the expense of small domains via a process which is called Ostwald ripening. Furthermore, the solvent plays a significant role in the network formation. The choice of solvent affects the mobility of molecules, especially, the adsorption-desorption dynamics via the solvation energy and possibly also via solvent viscosity. 

Carbonate. Schulthess and McCarthy (72) determined carbonate and acetate adsorption by 5-AI2O3 and concluded that competitive adsorption of carbonate must be considered for prediction of organic ion adsorption under field conditions. Competitive adsorption of dissolved CO2 species has also been established for chromate on am-Fe hydroxide (7i) and goethite (7-7). These titration/adsorption studies demonstrate and quantify the importance of adsorption of dissolved CO2 species but do not provide information as to the specific species and mode of bonding with the oxide surface. 

In addition to the reactions in solution mentioned above, molecular iodine dissolved in water can react with submerged surfaces. Irreversible chemical adsorption of iodine species on submerged surfaces could reduce the overall iodine inventory in the aqueous phase, and thereby reduce the release of molecular iodine to the containment atmosphere. 

The General Electric model (Lin et al., 1981) is a comprehensive analytical description of the activity transport as well as of the activity buildup. Iron transport and cobalt transport are treated in separate sets of equations of balance. The block diagram of Co/ Co transport is shown in Fig. 4.51. In this model, several interactions are considered to exist between dissolved ions and corrosion product particles, including adsorption of ionic species onto the surfaces of the particles. Moreover, both particulate and dissolved species are assumed to be deposited onto the surfaces of the fuel rods, with corrosion product particles playing an important role in the deposition of the ionic species. The fuel rod deposits are assumed to consist of two layers, loosely-adherent and tenacious ones, and a certain amount... 

The H entry into a metal fiom an aqueous electrolyte is believed to involve the same surface-bulk transfer step as in the gas phase, but the preliminary adsorption step is a more complex process because more H sources are involved in aqueous solution, allowing more possible H surface reactions, and also because of the specificity of the electrolyte-metal interface. Whereas H adsorption in the gas phase occurs by dissociative adsorption of gaseous H2 on the free sites of a bare metallic surface, H adsorption in aqueous solution may occur either chemically by dissociation of dissolved H2 or electrochemically from solvated (hydrated) protons or water molecules it takes place on a hydrated surface and thus implies the displacement of adsorbed water molecules or specifically adsorbed ions and local reorganization of the double layer [20] competition with the adsorption of oxygen species formed from the dissociation of water may also occur [21-23], The adsorbed H layer is also in interaction with surrounding water molecules, i.e., it is hydrated [8c,24,25],... 

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