Nonelectrolyte adsorption

A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

The recent work by Li and coworkers [18] provides a good illustration of the importance of the surface chemistry and pore texture of carbon materials on nonelectrolyte adsorption. They studied the adsorption of trichloroethene (TCE) and methyl ieri-butyl ether (MTBE) on different commercial activated carbons and activated carbon fibers with different porosity and surface chemistry. TCE is a relatively hydrophobic planar molecule. MTBE is tetrahedron-like and relatively hydrophilic. The results of the adsorption from aqueous solutions on the more hydrophobic carbons showed that TCE adsorption was controlled by a pore volume ranging from 0.7 to 1 nm width, as shown in Fig. 25.2. MTBE was primarily adsorbed in pores with widths between 0.8 and 1.1 nm. These micropore ranges were between 1.3 and 1.8 times the kinetic diameter of the adsorptives. 

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

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. 

Kruyt and van Duin (Kolloidchemie Beihefte, v. 269,1914) have examined the alteration in sensitiveness to electrolytes of a suspension of arsenious sulphide to which various non-electrolytes had been added. They found that the influence of various nonelectrolytes on the sensitiveness of the suspension ran parallel to the adsorption of the non-electrolytes from aqueous solution by powdered charcoal and that the most capillary active non-electrolyte exerted the greatest effect on the liminal concentration required for precipitation. Further it was observed that the addition of non-electrolytes lowered the liminal concentrations, i.e. increased the sensitiveness of the suspension to uni- and trivalent ions but increased the liminal concentrations, i.e. decreased the sensitiveness for divalent and tetravalent cations as will be noted from the following tables. 

Kipling, J. J., Adsorption from Solutions of Nonelectrolytes, Academic, New York, 1965. 

Kipling, J. J. Adsorption from solutions of nonelectrolytes, New York Academic Press 1965... 

SCHAY,G., Adsorption of solutions of nonelectrolytes , in reference 9 2,155-211 (1969) Thermodynamics of adsorption from solution , in reference 15 ... 

Adsorption (Chemical Engineering) Chemical Thermodynamics Electrolyte Solutions, Thermodynamics Kinetics (Chemistry) Nonelectrolyte Solutions, Thermodynamics... 

Park, J.H. and Lee, H.J., Estimation of bioconcentration factor in fish, adsorption coefficient for soils and sediments and interfacial tension with water for organic nonelectrolytes based on the linear solvation energy relationships, Chemosphere, 26, 1905-1916, 1993. 

Refs. [i] Wang J (1989) Voltammetry after nonelectrolytic preconcentration. In Bard AJ (ed) Electroanalytical chemistry, vol. 16. Marcel Dekker, New York, p 1 [ii] Kakiuchi T (2001) Adsorption at polarized liquid-liquid interfaces. In Volkov AG (ed) Liquid interfaces in chemical, biological, and pharmaceutical applications. Marcel Dekker, New York, p 105... 

D.H. Everett, R.T. Podoll, Adsorption at the Solid-Liquid Interface Nonelectrolyte systems, in Specialist Periodical Report. Colloid Science. The Chemical Society (London), Vol. 3 (1979) 63. (Literature review up to 1977, especially for homogeneous surfaces.)... 

Chapter 7 of our landmark reference [6] discusses various aspects of the adsorption of weak electrolytes and nonelectrolytes from aqueous solution. In particular an attempt is made to elucidate the mechanism of adsorption of undissociated aromatic compounds from dilute aqueous solutions. As discussed in detail in Section VI, the authors concluded that the aromatic ring of the adsorbate inter-... 

Section 3 presents the chapters both on adsorption from nonelectrolyte mixtures and on ion adsorption at the oxide/electrolyte interface. This interface is probably the most important in science, life and technology. Moreover, the ionic surfactant adsorption, mainly from aqueous solutions onto various inorganic sorbents has been considered. 

In the following sections I attempt to offer a consistent explanation of the importance of the carbon surface properties that influence the adsorption processes, which is valid for different organic solutes, from nonelectrolytes to polyelectrolytes and bacteria. 

All these results show the importance of the carbon surface chemistry and pore texture on the adsorption of nonelectrolytic organic solutes. Thus, for hydrophobic carbons, which generally have a low content of surface oxygen complexes, the adsorption of organic molecules is by dispersion and hydrophobic interactions, and the pores involved in the adsorption depend on the molecular size of the adsorptive. Conversely, when the adsorbent s content of surface oxygen complexes increases or its hydrophobicity decreases, there is a preferential adsorption of water on these complexes, which reduces the adsorption capacity of the adsorbent. 

The adsorption of organic electrolytes is a more complicated process than that of nonelectrolytes because it is a complex interplay between electrostatic and nonelectrostatic interactions. In this section, I present results obtained with three representative types of organic electrolytes phenol and its derivatives, dyes, and surfactants. These observations demonstrate the importance of the surface chemistry of carbons on the adsorption processes. 

This chapter shows that a unified explanation can be given of the adsorption from dilute aqueous solutions of different organic solutes, from nonelectrolytes to electrolytes, polyelectrolytes, and bacteria. Thus, the adsorption process is a complex interplay between electrostatic and nonelectrostatic interactions. Electrostatic interactions depend on the solution pH and ionic strength. The former controls the charge on the carbon surface and on the adsorptive... 

Larionov and co-workers (Institute of Physical Chemistry, the U.S.S.R. Academy of Sciences, Moscow) (308-312) carried out systematic theoretical and experimental investigations of the adsorption from liquid solutions of nonelectrolytes on silica adsorbents. They studied the adsorption of individual substances and binary liquid solutions (benzene/carbon tetrachloride, carbon tetrachloride/isooctane, benzene/isooctane, etc.) on Si02 samples with different degrees of porosity but identical surface chemical properties. The experimental results were compared with the theoretical calculations carried out by the Gibbs method. This procedure made it possible to calculate the dependence of the enthalpy, entropy, and free energy of wetting on the concentration and to obtain expressions describ-... 

The mathematical models that have been applied to the physical adsorption from liquid solutions are generally extensions of the theories that have been developed to describe the sorption of gases on solid surfaces with modifications to account for the competition between the solute and solvent for the adsorption sites. Two of these models have been applied to the adsorption isotherms of nonelectrolytes from solution they are the Langmuir model and the Brunauer, Emmett, and Teller (BET) model in addition the Freundlich empirical equation has also been used. In the Langmuir model it is assumed that the adsorbed species forms a monolayer on the surface of the adsorbent, that the adsorbed molecules... 

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