Mechanism, chemical

See Chemical Engineering News, April 24, 1989, p. 37. la. W. N. Unertl and M. Grunze, Physical and Chemical Mechanisms of Tribology, special issue, Langmuir, 12, 4481-4610 (1996). 

The generally low chemical, mechanical and thennal stability of LB films hinders their use in a wide range of applications. Two approaches have been studied to solve this problem. One is to spread a polymerizable monomer on the subphase and to polymerize it either before or following transfer to the substrate. The second is to employ prefonned polymers containing hydrophilic and hydrophobic groups. 

Complex chemical mechanisms are written as sequences of elementary steps satisfying detailed balance where tire forward and reverse reaction rates are equal at equilibrium. The laws of mass action kinetics are applied to each reaction step to write tire overall rate law for tire reaction. The fonn of chemical kinetic rate laws constmcted in tliis manner ensures tliat tire system will relax to a unique equilibrium state which can be characterized using tire laws of tliennodynamics. 

The description of chemical reactions as trajectories in phase space requires that the concentrations of all chemical species be measured as a function of time, something that is rarely done in reaction kinetics studies. In addition, the underlying set of reaction intennediates is often unknown and the number of these may be very large. Usually, experimental data on the time variation of the concentration of a single chemical species or a small number of species are collected. (Some experiments focus on the simultaneous measurement of the concentrations of many chemical species and correlations in such data can be used to deduce the chemical mechanism [7].)... 

The industrial value of furfuryl alcohol is a consequence of its low viscosity, high reactivity, and the outstanding chemical, mechanical, and thermal properties of its polymers, corrosion resistance, nonburning, low smoke emission, and exceUent char formation. The reactivity profile of furfuryl alcohol and resins is such that final curing can take place at ambient temperature with strong acids or at elevated temperature with latent acids. Major markets for furfuryl alcohol resins include the production of cores and molds for casting metals, corrosion-resistant fiber-reinforced plastics (FRPs), binders for refractories and corrosion-resistant cements and mortars. 

Fig. 2. Examples of ozone isopleths generated using four different chemical mechanisms (a) EPA (b) ESM (c) CBII (d) ELSTAR NMOC = nonmethane...

Peroxy and hydroperoxy radicals play important roles ia the knock process. A number of good reviews have discussed the details of the chemical mechanisms (16). Ignition delay (tau) has also been used for description of the chemical tendency to knock (17). The chemical factors affecting knock are... 

High demands are placed on the substrate material of disk-shaped optical data storage devices regarding the optical, physical, chemical, mechanical, and thermal properties. In addition to these physical parameters, they have to meet special requirements regarding optical purity of the material, processing characteristics, and especially in mass production, economic characteristics (costs, processing). The question of recyclabiUty must also be tackled. 

An ink is considered dry when a print does not stick or transfer to another surface pressed into contact with it. Drying is accompHshed by one or more of the following physical or chemical mechanisms absorption, evaporation, precipitation, oxidation, polymerization, cold setting, gelation, and radiation curing. 

Delignification Chemistty. The chemical mechanism of sulfite delignification is not fully understood. However, the chemistry of model compounds has been studied extensively, and attempts have been made to correlate the results with observations on the rates and conditions of delignification (61). The initial reaction is sulfonation of the aUphatic side chain, which occurs almost exclusively at the a-carbon by a nucleophilic substitution. The substitution displaces either a hydroxy or alkoxy group ... 

Reductive dechlorination of chlorinated aUphatic hydrocarbons, eg, lindane (11) (eq. 17) is extremely facile and occurs almost exclusively via chemical mechanisms, although microorganisms are typically the source of electron donors (30). 

Representation of Atmospheric Chemistry Through Chemical Mechanisms. A complete description of atmospheric chemistry within an air quaUty model would require tracking the kinetics of many hundreds of compounds through thousands of chemical reactions. Fortunately, in modeling the dynamics of reactive compounds such as peroxyacetyl nitrate [2278-22-0] (PAN), C2H2NO, O, and NO2, it is not necessary to foUow every compound. Instead, a compact representation of the atmospheric chemistry is used. Chemical mechanisms represent a compromise between an exhaustive description of the chemistry and computational tractabiUty. The level of chemical detail is balanced against computational time, which increases as the number of species and reactions increases. Instead of the hundreds of species present in the atmosphere, chemical mechanisms include on the order of 50 species and 100 reactions. 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

R.A. Graham, Shock Compression of Solids as a Physical-Chemical-Mechanical Process, in Shock-Waves in Condensed Matter—1987 (edited by S.C. Schmidt and N.C. Holmes), Elsevier Science, Amsterdam, 1988, pp. 11-18. 

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. Resorcinol reacts readily with formaldehyde to produce resins (Fig. 2) which harden at ambient temperatures if formaldehyde is added. The initial condensation reaction, in which A-stage liquid resins are formed, leads to the formation of linear condensates only when the resorcinol/formaldehyde molar ratio is approximately 1 1 [119]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [120]. However, reaction with the remaining reactive but sterically hindered site (2-positiori) between the hydroxyl functions also occurs [119]. In relation to the weights of resorcinol-formaldehyde condensates which are isolated and on a molar basis, the proportion of 4- plus 6-linkages relative to 2-linkages is 10.5 1. However, it must be noted that the first-mentioned pair represents two condensa-... 

Atmospheric particulates (sea salt, carbonaceous soot, and sulfuric acid aerosols) are known to provide a condensed phase for conq>lex heterogeneous chemistry to occur. Although the presence of atmospheric particulates are known to alter trace gas concentrations, details of the specific chemical mechanisms for condensed phase chemistry have not been identified. 

Moisture can significantly affect loose materials, particularly their flowability. Low temperatures, particle bridging, and caking can alter interparticle void fractions and cause dramatic changes in bulk density. Moisture becomes bound to solids because of mechanical, physicochemical, and chemical mechanisms. Moisture retained... 

Hyland, L., Toma.szek, T, and Meek, T, 1991. Human immunodeficiency viru.s-1 protease 2 Use of pH rate. studies and solvent isotope effects to elucidate details of chemical mechanism. Biochemistry 30 8454-8463. 

Based on the action of thiamine pyrophosphate in catalysis of the pyruvate dehydrogenase reaction, suggest a suitable chemical mechanism for the pyruvate decarboxylase reaction in yeast ... 

Nothing in nature is lost that was the message of the 1840s. Chemical, mechanical, and electrical... 

Bolts. Bolts shall conform to one of the applicable SAE, ASTM, or AISC specifications. Other bolts not covered by these specifications may be used provided the chemical, mechanical, and physical properties conform to the limits guaranteed by the bolt manufacturer. Certified reports shall constitute sufficient evidence of conformity with the specification. Bolts of different mechanical properties and of the same diameter shall not be mixed on the same drilling or servicing structure to avoid the possibility of bolts of relatively low strength being used where bolts of relatively high strength are required. 

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