Inhibition, chemistry

The inhibition chemistry has been extensively studied. As shown in equation 1, the prevulcanization inhibitor (retarder) reacts with MBT (1) before it can form polysulfides. 

Fristrom, R. M. Sawyer, R. E Flame Inhibition Chemistry Advisory Group for Aerospace Research and Development Conference Proceedings, No. 84 on Aircraft Fuels, Lubricants and Fire Safety, AGARD-CP-84-71, Section 12, North Atlantic Treaty Organization, 1971. 

Deacon, G.B., Forsyth, M., Junk, RC., and Leeb, W.W. (2008) From chromates to rare earth carboxylates a greener take on corrosion inhibition. Chemistry in Australia, 75 (9), 18-21. 

ADMET Properties of Drugs Cytochrome P450 Monooxygenases, Chemistry of Drug Metabolizing Enzymes, Chemistry of Enzyme Inhibition, Chemistry and Mechanisms of Polymorphisms, Detection of... 

These two steps remove the intermediates which propagate the chain reaction and stop the chain. Inhibition chemistry is the centerpiece of the application of antioxidant packages in lubricants. 

Bencharit, S., Morton, G. L., Hyatt, J. L., Kuhn, P., KDanks, M. K, Potter, P. M., et al. (2003). Crystal structure of human carboxyles-terase 1 complexed with the alzheimer s drug tacrine from binding promiscuity to selective inhibition. Chemistry 6 Biology, 10, 341-349. 

For a positive resist based on dissolution inhibition chemistry such as the DNQ/novolac resist system, the rate of the reaction tr between the developer with the resist (comprised of resin R, a photoactive compound that acts as a dissolution inhibitor M, but which is converted to product P on exposure to UV light, which in turn enhances the dissolution rate of the resin) is given by... 

These requirements eliminate many interesting possibilities for scale inhibition chemistry that are very useful in other crystal growth applications. For example, polymers containing hydrophobic character can be quite effective for inhibiting crystal growth in a simple system, but are generally not suitable for oilfield applications. 

The obtained data suggest very intense chemical reactions of inhibition in the flames doped with alkali metal compoxmds. The inhibition chemistry of the alkali metals has been studied insufficiently and may become an object of future research. 

Summarizing the obtained data on inhibition chemistry, we came to following conclusions. 

The inhibition chemistry of essentially rich hydrocarbon flames with equivalence ratio close to the flammability limit is not adequately investigated. It is explained by very complex combustion chemistry of soot and its precursors formation, which interaction with inhibitor was not explored. 

Oxidation Inhibition Chemistry. The most commonly used additive to inhibit oxidation is thiosulfate. This ion was initially added to FGD systems in the form of sodium thiosulfrite solution. However, in late 1987, tests at an operating utility plant deiuonstraled that elemental sulfur could be used at a cost which is only about 20% of the cost of adding sodium thiosulfate (Moser et al., 1990). The sulfur is converted to thiosulfrite by the following reaction. Conversion efficiency is on the order of 50% ... 

Enediynes hold substantial promise as anti cancer drugs because of their potency and selectivity Not only do they inhibit cell growth they have a greater tendency to kill cancer cells than they do normal cells The mechanism by which enediynes act involves novel chemistry unique to the C C—C=C—C C unit which leads to a species that cleaves DNA and halts tumor growth... 

Both solvent-iaduced swelling and oxygen inhibition ate characteristic of all cross-linking negative resists based on free-radical chemistry. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Petroleum sulfonates are widely used as solubilizers, dispersants (qv), emulsifiers, and corrosion inhibitors (see Corrosion and corrosion inhibitors). More recentiy, they have emerged as the principal surfactant associated with expanding operations in enhanced oil recovery (66). Alkaline-earth salts of petroleum sulfonates are used in large volumes as additives in lubricating fluids for sludge dispersion, detergency, corrosion inhibition, and micellar solubilization of water. The chemistry and properties of petroleum sulfonates have been described (67,68). Principal U.S. manufacturers include Exxon and Shell, which produce natural petroleum sulfonates, and Pilot, which produces synthetics. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

Clavulanic acid has only weak antibacterial activity, but is a potent irreversible inhibitor for many clinically important P-lactamases (10—14,57,58) including penases, and Richmond-Sykes types 11, 111, IV, V, VI ([Bacteroides). Type I Cephases are poorly inhibited. Clavulanic acid synergizes the activity of many penicillins and cephalosporins against resistant strains. The chemistry (59—63), microbiology (64,65), stmcture activity relationships (10,13,60—62,66), biosynthesis (67—69), and mechanism of action (6,26,27,67) have been reviewed. 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

Chelation is a feature of much research on the development and mechanism of action of catalysts. For example, enzyme chemistry is aided by the study of reactions of simpler chelates that are models of enzyme reactions. Certain enzymes, coenzymes, and vitamins possess chelate stmctures that must be involved in the mechanism of their action. The activation of many enzymes by metal ions most likely involves chelation, probably bridging the enzyme and substrate through the metal atom. Enzyme inhibition may often result from the formation by the inhibitor of a chelate with a greater stabiUty constant than that of the substrate or the enzyme for a necessary metal ion. 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

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