Solvent adsorption oxygenated solvents

Curing mechanism Solvent evaporation, oxygen adsorption... 

This can be seen from the calculated adsorption isotherms plotted in Figure A2-20 for several oxygenated solvents with different sfruaures, and data of trichloroethylene. 

Figure A2-20 Calculated Adsorption Performance of Oxygenated Solvents...

The nature of the attraction between solvents and adsorption sites depends upon the nature of the solvents being adsorbed. Solvents with polar centers (chlorine atoms in trichloroethylene, for example) are attracted by mutual alignment of electron structures. Solvents with oxygen atoms (alcohols or ethers, for example) are attracted through hydrogen bonding... 

Difficulties in the experimental measurements that have not always been acknowledged or ameliorated include the following (a) Since polycyclic aromatic hydrocarbons often readily react with oxygen and other atmospheric components in both the synthesis and workup processes, fullerenes also should be similarly reactive. Furthermore, for many fullerene derivatives, there are numerous isomers that are difficult to separate, (b) Fullerenes have numerous exposed atoms and so considerable surface area (i.e., they are molecular forms of activated charcoal). As such, adsorption of solvent or other extraneous compounds is hard to avoid, (c) As mentioned before in this review in the context of polycyclic aromatic hydrocarbons, problems arise in determining sublimation enthalpies for species of low volatility. Reliable, as well as reproducible, formation enthalpies for fullerenes are thus hard to come by. 

Highly pure / -hexane can be produced by adsorption on molecular sieves (qv) (see Adsorption, liquid separation) (43). The pores admit normal paraffins but exclude isoparaffins, cycloparaffins, and aromatics. The normal paraffins are recovered by changing the temperature and/or pressure of the system or by elution with a Hquid that can be easily separated from / -hexane by distillation. Other than ben2ene, commercial hexanes also may contain small concentrations of olefins (qv) and compounds of sulfur, oxygen, and chlorine. These compounds caimot be tolerated in some chemical and solvent appHcations. In such cases, the commercial hexanes must be purified by hydrogenation. 

Sulfiir-anchored SAMs and thin films, mostly from organosulfiir precursors, have been discussed at length by a number of authors [10, 181]. SAMs of organosulfiir compounds (thiols, disulfides, sulfides) form on gold substrates by spontaneous adsorption from either the liquid or the vapor phase. A number of experimental factors can affect the formation and structure of SAMs such as choice of solvent, temperature, concentration, immersion time, purity of adsorbate, oxygen concentration in solution, cleanliness, and structure of the adsorbate. Interestingly, the... 

The enhanced adsorption of anions and other substances that occurs at increasingly positive potentials causes a gradual displacement of water (or other solvent) molecules from the electrolyte layer next to the electrode. This leads to a markedly slower increase in the rate of oxygen evolution from water molecules and facilitates a further change of potential in the positive direction. As a result, conditions arise that are favorable for reactions involving the adsorbed species themselves (Fig. 15.9). In particular, adsorbed anions are discharged forming adsorbed radicals ... 

We outline experimental results and provide theoretical interpretation of effect of adsorption of molecular oxygen and alkyl radicals in condensed media (water, proton-donor and aproton solvents) having different values of dielectric constant on electric conductivity of sensors. We have established that above parameter substantially affects the reversible changes of electric conductivity of a sensor in above media which are rigorously dependent on concentration of dissolved oxygen. 

It should be noted that dissociation of surface complexes of oxygen in polar solvents on semireduced ZnO films is presumably justified from the thermodynamic point of view as oxygen adsorption heat on ZnO and electron work function are [58] 1 and approximately 5 eV respectively while the energies of affinity of oxygen molecules to electron, to solvation of superoxide ion and surface unit charge zinc dope ions are 0.87, 3.5, and higher than 3 eV, respectively [43]. 

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

In case of weaker dependence of the rate of oxygen adsorption on ZnO Kds on the value s of solvent, compared to that of Kads as a function of e (which is supported by experimental results), we can deduce the following equation on the basis of the expressions (3.28) and (3.29) ... 

At room temperature, unsaturated vapours of the above specified polar and nonpolar liquids do not influence considerably the rate of adsorption and chemical activity of not only adsorbed oxygen layers, but also of acceptors of semiconductor electrons of another type, namely, of alkyl radicals [54]. This is seen from the electric conductivity of ZnO films with adsorbed alkyl radicals or oxygen being invariable in the atmosphere of the saturated vapours of the above specified solvents. In the case of oxygen, this can be also seen from the fact that the oxygen concentration features no decrease. 

The new phenomenon discovered in these experiments consists in different chemical activity revealed by one and the same kind of adsorbed particles in contact with one and the same kind of molecules of the medium, but at different nature of the interface either interface of a solid (ZnO film) with a polar liquid or interface of the solid with vapours of the polar liquid. This difference is caused by the fact that in the case of contact of the film with an adsorbed layer (oxygen, alkyl radicals) with a polar liquid, the solvated ion-radicals O2 chemically interact with molecules of the solvent (see Chapter 3, Section 3.4). In the case where alkyl radicals are adsorbed on ZnO film, one can assume, by analogy with the case of adsorbed oxygen, that in the process of adsorption on ZnO, simple alkyl radicals from metalloorganic complexes of the type... 

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

The opposing theories were reviewed in detail by Garten and Weiss (73, 111). To summarize the facts acid is adsorbed only in the presence of oxygen (16), the adsorption is dependent on oxygen pressure, at least at low pressures (22,112), and a part of the adsorbed acid can be removed by solvents such as toluene (109, 111). According to Burstein and Frumkin (113), hydrogen peroxide is liberated when acid is ad.sorbed in the presence of oxygen ... 

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