Rapid mixing methods

Measurements of kinetic parameters of liquid-phase reactions can be performed in apparata without phase transition (rapid-mixing method [66], stopped-flow method [67], etc.) or in apparata with phase transition of the gaseous components (laminar jet absorber [68], stirred cell reactor [69], etc.). In experiments without phase transition, the studied gas is dissolved physically in a liquid and subsequently mixed with the liquid absorbent to be examined, in a way that ensures a perfect mixing. Afterwards, the reaction conversion is determined via the temperature evolution in the reactor (rapid mixing) or with an indicator (stopped flow). The reaction kinetics can then be deduced from the conversion. In experiments with phase transition, additionally, the phase equilibrium and mass transport must be taken into account as the gaseous component must penetrate into the liquid phase before it reacts. In the laminar jet absorber, a liquid jet of a very small diameter passes continuously through a chamber filled with the gas to be examined. In order to determine the reaction rate constant at a certain temperature, the jet length and diameter as well as the amount of gas absorbed per time unit must be known. 

H. Hikita, S. Asai, H. Ishikawa, et ah, The kinetics of reactions of carbon dioxide with monoethanolamine, diethanolamine and triethanolamine by a rapid mixing method, Chem. Eng. Journal, 1977, 13, 7-12. 

H. Roder, K. Maki, R. F. Latypov, H. Cheng and M. C. R. Shastry, Early Events in Protein Folding Explored by Rapid Mixing Methods , WILEY-VCH Verlag GmbH Company KGaA, Weinheim, 2005. 

Analysis of steady-state kinetics can also reveal those cases in which rapid mixing methods are likely to fail or at least be more difficult to perform. For example, because one must examine a single enzyme turnover, enzymes with exceedingly high values (greater than 1(X)0 sec ) are not amenable to rapid... 

Composites of PANI-NFs, synthesized using a rapid mixing method, with amines have recently been presented as novel materials for phosgene detection [472]. Chemiresistor sensors with nanofibrous PANI films as a sensitive layer, prepared by chemical oxidative polymerization of aniline on Si substrates, which were surface-modified by amino-silane self-assembled monolayers, showed sensitivity to very low concentration (0.5 ppm) of ammonia gas [297]. Ultrafast sensor responses to ammonia gas of the dispersed PANI-CSA nanorods [303] and patterned PANI nanobowl monolayers containing Au nanoparticles [473] have recently been demonstrated. The gas response of the PANI-NTs to a series of chemical vapors such as ammonia, hydrazine, and triethylamine was studied [319,323]. The results indicated that the PANI-NTs show superior performance as chemical sensors. Electrospun isolated PANI-CSA nanofiber sensors of various aliphatic alcohol vapors have been proven to be comparable to or faster than those prepared from PANI-NF mats [474]. An electrochemical method for the detection of ultratrace amount of 2,4,6-trinitrotoluene with synthetic copolypeptide-doped PANI-NFs has recently been reported [475]. PANI-NFs, prepared through the in situ oxidative polymerization method, were used for the detection of aromatic organic compounds [476]. 

As stated previously, kinetic studies showed that the enzyme-catalyzed reaction follows an ordered ternary complex mechanism in which the binding of NAD occurs prior to that of lactate, and release of pyruvate precedes that of NADH. The use of rapid mixing methods allowed rates of many of the individual steps in the reaction sequence to be elucidated, including the hydride ion-transfer step in the catalytically active ternary complex. 

Roder, H, Maid, K, Cheng, H, and Shastry, MGR, Rapid mixing methods for exploring the kinetics of protein folding. Methods 34 (2004) 15-27. 

Some techniques were presented for studying fast reactions that cannot be studied by classical experimental techniques. These techniques included continuous flow and stopped-flow techniques, which are rapid mixing methods, as well as relaxation techniques. The relaxation techniques included shock-tube methods, flash photolysis, and T-jump and P-jump methods. Equations were derived for the relaxation of a reaction after a small perturbation, giving an exponential relaxation for a variety of rate laws. 

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