Kinetics dehydration

Grant and coworkers [8] studied the dehydration kinetics of piroxicam monohydrate using both model-free and model-fitting approaches in an effort to understand the effects of lattice energy and crystal structure. The dehydration kinetics was found to differ when determined under isothermal and nonisothermal conditions. Ultimately, the dehydration behavior of piroxicam monohydrate was determined by details of the crystal structure, which was characterized by an absence of channels and a complicated hydrogen-bonding network, and ab initio calculations proved useful in understanding the structural ramifications of the dehydration process. 

X-ray powder diffractometry can be used to study solid state reactions, provided the powder pattern of the reactant is different from that of the reaction product. The anhydrous and hydrated states of many pharmaceutical compounds exhibit pronounced differences in their powder x-ray diffraction patterns. Such differences were demonstrated earlier in the case of fluprednisolone and carbamazepine. Based on such differences, the dehydration kinetics of theophylline monohydrate (CvHgN H20) and ampicillin trihydrate (Ci6H19N304S 3H2O) were studied [66]. On heating, theophylline monohydrate dehydrated to a crystalline anhydrous phase, while the ampicillin trihydrate formed an amorphous anhydrate. In case of theophylline, simultaneous quantification of both the monohydrate and the anhydrate was possible. It was concluded that the initial rate of this reaction was zero order. By carrying out the reaction at several... 

Dehydration kinetics of the four alochols were followed using two distinct types of catalytic reactors a static FTIR spectrometer cell, in which the concentration of alcohol adsorbed by the catalyst was adjusted to be less than or equal to the concentration of the active sites and a flow microreactor, which allowed the escaping products (and reactant) to be identified by gas chromatography. Kinetic measurements conducted with the FTIR cell refer to the... 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

Note that, at steady-state, the dehydration kinetics were always zero-order with respect to alcohol in the feeding flow, signifying that, under these conditions, only a very small fraction of the active sites are free. As schematized in... 

The effect of various thermohygrometric conditions on grape dehydration kinetics has been recently studied by Barbanti et al. (2008). Nonetheless, improved ways of controlling this delicate process are always looked for, with the aim to better clarify the role of Botrytis in the metabolic changes that occur in the fruttaio, and to choose the best conditions to favor the development of noble rot. 

Johnson, K. S. (1982) Carbon dioxide hydration and dehydration kinetics in seawater. Limnol. Oceanogr. 27, 849-55. 

Complementary measurements have been made of the dehydration kinetics of three distinct lithium potassium tartrate hydrates those containing the d and dt anions were monohydrates and the meso salt was a dihydrate. These reactants were of interest because they contained chemically identical components, but had different crystal structures and reactivities [150]. 

The dehydration kinetic measurements of HC03, catalyzed by these cobalt(II) complexes, were also investigated.392... 

The transition of sulfaguanidine from a monohydrate form to an anhydrous form in the presence of differing water vapor pressures586 conformed to different rate equations among those listed in Table 10. Similarly, the dehydration kinetics of hydrated theophylline614 of different particle sizes was also described by different rate equations. Polymorphic transitions of anhydrous theophylline in tablets conformed to different rate equations depending on the tablet size and pore size.583-584... 

D. Toledo-Velasquez, H. T. Gaud, and K. A. Connors, Misoprostol dehydration kinetics in aqueous solution in the presence of hydroxypropyl methylcellulose, J. Pharm. Sci. 81, 145-148(1992). 

Ritschel Notari RE and Caiola SM, Catalysis of streptovitacin A dehydration Kinetics and mechanism,/. Pharm. Sci., 58,1203-1208 (1969). NB The structure is a substituted 2,4,6-trialkylphenol. 

Pavlyuchenko MM, Borisenko EM (1970) Dehydration kinetics of calcium phosphate monohydrate. In Pavlyuchenko MM, Prodan EA (eds) Heterogeneous chemical reactions. Nauka Tekhnika, Minsk, (in Russian), pp 181—189... 

Localization of reactions and appearance of decomposition patterns, specific features of formation of crystalline and X-ray amorphous dehydration products, nucleation, self-cooling - these are only a few examples of research subjects for crystalline hydrates [43, 45]. Recently the interest in dehydration kinetics was expressed in an attempt (unfortunately, unsuccessful) to use the dehydration of Li2S04 H2O as a kinetics standard [73]. 

Many other examples could have been selected in the area of solvate investigation. Numerous reports of dehydration kinetic studies on drug hydrates are found in the pharmaceutical literature. An instructive recent case concerns piroxicam monohydrate (PM) [67]. Model-free kinetics showed that the dehydration kinetics of PM under isothermal and non-isothermal conditions were different. Complementary structural studies revealed a complex hydrogen-bonding pattern that was reconciled with the observed dehydration behaviour of PM. 

F. Nsonzi and H.S. Ramaswamy, Osmotic dehydration kinetics of blueberries. Drying TechnoL, 16(3/5) 725 (1998). 

A. Paijoko, S.M. Rahman, K.A. Buckle, and C.O. Perera, Osmotic dehydration kinetics of pineapple wedges nsing palm sugar, LWT, 29 452 (1996). 

A. Eisenberg, T. Yokoyama and E. Sambilido, Dehydration kinetics and glass transition of poly(acrylic acid), J. Polytn. Sci., A-1, 7 1717 (1969). 

Most recently, methanol dehydration kinetics were examined over a g-Al203 catalyst at 0.15 MPa in the temperature range 290360°C. A kinetic equation assuming surface reaction control with dissociative adsorption of methanol gave the best fit to the experimental results [80]. The dissociative adsorption is consistent with many previous kinetic models in which the rate of DME formation is found to proportional to the square root of the methanol concentration [80]. 

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