Free Radicals on the Surface

Besides the formerly considered orientations of cyclohexane, the multiplet theory treats one more orientation (Fig. 21a). The CH2 parts of the molecule get into the deepest valleys, situated on the (111) facet of the Al lattice in conformity with the CH2 links of the ring. According to what has been stated previously, this must lead to an activated adsorption which is often accompanied by the break of a bond. Here also a break of bonds takes place and free radicals, CH2, are formed, which can travel about the Leonard-Johnes valleys. The CH2 radical is known to be fairly stable, resembling the molecules in 

Zelinskii and Shuikin (236) explained by the formation of CHa radicals the production of methylcyclohexane and toluene, etc., from cyclohexane on Ni under rigorous conditions. Eidus and Zelinskii 237) found methylcyclohexane and toluene in the formation of syntine from water gas in the presence of cyclohexane and benzene, supporting in this way the hypotheses of Orlov and of Fisher and Tropsch, that syntine is formed through the CH2 radicals. With hydrogen CH2 gives 

and with water, CO2 (or CO) and H2. According to the multiplet theory, such a decomposition, e.g., of butane with formation of CH2 radicals is shown by the model of Fig. 21b, which is similar to that of Fig. 21a. 

The CH2 radical seems also to be an intermediate product in the interesting type of joint dehydrogenation of hydrocarbons with more than four hydrocarbon atoms and with water vapor, which was found by Slovokhotova (238) and simultaneously by Haensel [patent (239) ]. 

Reactions of this type between hydrocarbons and CO2 have also been studied (244). 

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Chemical interactions also occur in the condensed phases. Some of these are expected to be quite complex, e.g., the reactions of free radicals on the surfaces of or within aerosol particles. Simpler sorts of interactions also exist. Perhaps the best understood is the acid-base relationship of NH3 with strong acids in aerosol particles and in liquid water (see Chapter 16). Often, the main strong acid in the atmosphere is H2SO4, and one may consider the nature of the system consisting of H2O (liquid), NH3, H2SO4, and CO2 under realistic atmospheric conditions. Carbon dioxide is not usually important to the acidity of atmospheric liquid water (Charlson and Rodhe, 1982) the dominant effects are due to NH3 and H2SO4. The sensitivity the pH of cloud (or rainwater produced from it) to NH3 and... 

Catalyst accelerates the decomposition of hydroperoxide to free radicals on the surface. The free radicals then diffuse into the reactant bulk and initiate the chain oxidation of the oxidized substance. 

The principle of surface activation is to generate free radicals on the surface of the wood this then provides bonding sites on the wood surfaces for resin-free board production. The use of biological systems to generate surface radicals will be considered later, this section being concerned with the use of chemical systems for surface activation. 

The dangling bonds and polymer free radicals (on the surface of glow discharge treated polymer) capture molecular O2 or H2O, producing hydroxyl and carbonyl groups as schematically shown ... 

The cession of a cr-bond yields two free radicals. The free radical on the surface subsequently reacts with ambient oxygen when the treated substrate is exposed to air rendering the surface hydrophilic (in the case of Ar plasma). The high energy of impinging entities (electrons and/or ions) tends to yield excessive cession of bonds, which creates a weak boundary layer below the top surface. This situation could be visualized by the trends that plasma treatment of hydrophobic surface makes the surface paintable, but the paint does not adhere well because the paint could delaminate through the weak boundary sublayer. 

If the catalytic oxidation of alkane molecule starts with the formation of a free radical on the surface of an active catalyst particle and its escape to the gas phase, the complete reaction network includes both homogeneous and heterogeneous steps of the transformation of primary (CnH2n+l) and secondary radicals. Since all these processes are sufficient for the formation of the final products, the analysis of the influence of different factors on the... 

For surface chain reactions in catalysis, which have not been known till quite recently, the theory of Voievodskii et al. (260) based on electronic conceptions can hold. From what has been said above it is evident that this theory should be complemented by structural considerations. The multiplet theory deals with the latter. In Section I,B [Consequence (10)] the conditions under which free radicals on the surface can be formed according to the multiplet theory were considered. 

Figure 3.3 shows graft polymerization occurs when the monomer reacts with a free radical on the surface of the fabric. The free radical attacks the double bond of the monomer, opening the bond and forming a new bond between the fabric and the monomer, through Reaction 1. This leaves the free radical on the monomer, which may... 

Figure 25.19 shows the maximum unpaired spin concentration versus specific surface area of various carbon blacks. The increasing order of the free radical concentration of various types of carbon blacks is TM-15 < HAF < SRF < FEF < ISAF. The results are related to the concentration of free radicals on the surface of carbon blacks, i.e. concentration of free radicals for the carbon blacks become higher for the carbon blacks having higher specific surface area. This indicates that the interfacial interactions between NR matrix and carbon black are governed by the active surface area of the carbon blacks. 

Figure 10.4. Photografting mechanisms of HEMA or MAA on the surface of PHBHV film according to the graftingfrom technique. (I) Photolysis ofTAS and production of both thiyls and phenyl radicals. (2) Hydrogen abstraction from PHBHV film and generation of free radicals on the surface of the PHBHV film. (3) Free radicals initiate covalent attachment of HEMA or MAA on the surface, followed by polymerization of the monomer. Reproduced widt permission from The Royal Society of Chemistry [VER12]...

The plasma used for treating material surfaces is called cold plasma, which means its temperature is about room temperature. Cold plasma is created by introducing the desired gas into a vacuum chamber (Fig. 14.5), followed by radio frequency (13.56 MHz) or microwave (2450 MHz) excitation of the gas. The energy dissociates the gas into electrons, ions, free radicals, and metastable products. Practically any gas may be used for plasma treatment but oxygen is the most common. The electrons and free radicals created in the plasma collide with the polymer surface and rupture covalent bonds thus creating free radicals on the surface of the polymer. The free radicals in the plasma may then recombine to generate a more stable product. After a predetermined time or temperature is reached the radio frequency is turned off. The gas particles recombine rapidly and the plasma is extinguished. 

There is numerous evidence of formation of free radicals on the surfaces of heterogeneous catalysis and their further desorption to the gas phase. The majority of the data that indicates an involvement of gas-phase reactions into a heterogeneous catalytic process... 

Freshly fractured quartz-silica contains free radicals on the surface that have the ability to regulate cytokine gene expression (112,113). 

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