Diffusion-controlled dehydrations

The MTA trace exhibits a slow increase and decrease of m/e 18 (HaO) considered to be typical for diffusion controlled dehydration. Apparently it would have been possible to have carried out the dehydration at room temperature by reducing the pressure or applying a dessicant — a behavior atypical of true hydroxides. 

Fig. 13. Plot of variations of activation energy ( /kJ mole"1) with water vapour pressure (PHjO/Torr) for dehydration of calcium sulphate. Data from Ball et al. [281,590, 591] who discuss the significance of these kinetic parameters. Dehydrations of CaS04 2 H2O, nucleation ( ), boundary (o) and diffusion (e) control Q-CaSC>4 5 H2O, diffusion control, below (X) and above (+) 415 K j3-CaS04 5 H20, diffusion control ( ).

A number of recent developments in 129Xe NMR spectroscopy are presented with direct applications to the study of mesopore space in solids. This includes the establishment of a relationship between pore size and chemical shifts for a number of controlled pore glasses and the exploration of hyperpolarized (HP) xenon for a number of NMR and microimaging applications to porous solids. With HP xenon, the increase in experimental sensitivity is remarkable. Experiments illustrated include the rapid characterization of the void space in porous solids, including the in-situ study of processes such as diffusion and dehydration, and imaging with chemical shift resolution. 

OH, O and Nj radicals were reported to react with chloro-and hydroxy-derivatives of aniline at diffusion-controlled rates with k> dm mol s and the rates are nearly identical for all isomers of chloroaniline. The OH radical reaction involves both addition and direct H abstraction and the extent of the two reactions is determined by the position of the substituent. The initially formed OH adduct then undergoes dehydration to give the anilino radical. The attack of the OH radical at the carbon bonded to Cl is a minor pathway (not more than 15%) as was confirmed by the detection of Cl ions. 

The kinetics of dehydration [128] of Na2S203.5H20 were difficult to interpret because the course of the reaction was markedly influenced by the perfection of the initial reactant surface and the reaction conditions. No reliable Arrhenius parameters could be obtained. The mechanism proposed to account for behaviour was the initial formation of a thin superficial layer of the anhydrous salt which later reorganized to form dihydrate. The first step in the reaction pentahydrate - dihydrate was satisfactorily represented by the contracting area (0.08 < or, < 0.80) expression. The second reaction, giving the anhydrous salt, fitted the Avrami-Erofeev equation (n = 2) between 0.05 < 2< 0.8. The product layer offers no impedance to product water vapoiu escape and no evidence of diffusion control was obtained. The mechanistic discussions are supported by microscopic observations of the distributions and development of nuclei as reaction proceeds. 

The dehydration of Mn(HC00)2.2H20 is an anisotropic process [147] (as in the copper salt above) in which the reaction interface advances into the crystal as a contracting parallelogram. The value of for reaction (293 to 348 K) was 72 kJ mol, which is close to the enthalpy of dissociation. This value of is consistent with the requirements of the P-W equation. The rate decreased with increasing /j(H20) and no S-T effect was detected. Similar results were reported by Clarke and Thomas [148]. The anhydrous salt rehydrates, readily and completely, to the dihydrate by a diffusion-controlled process for which = 50 kJ mol. ... 

Water loss is identified [13] as being diffusion controlled across a dehydration layer of significant thickness that advances into the reactant particles (see Figure 8.1.). The Ca(OH)2 structure is maintained during the greater part of reaction [14]. The rate of this reaction is more deceleratory than the requirements of the contracting volume equation and data satisfactorily fit the first-order equation. The anhydrous residue later recrystallises to CaO but the principal water elimination zone and the phase transformation interface are separated and may advance independently. 

Giovanoli and Brutsch [37] emphasize the necessity for supplementing thermoanalytical kinetic data for the dehydration of y-FeOOH yFcjOj) with diffraction and electron microscopy studies. The deceleratory rate process can be described by either the first- order, or various diffusion-controlled expressions and thus a specific reaction model does not result from this kinetic analysis. Values of... 

Many food processes, which affect food quality and stability, are diffusion controlled (Karel et al., 1994 Roos, 1995). Transport of key penetrants such as water into or out of a polymeric food matrix can play a critical role in food quality and stability. Water is one of the major components and a very good plasticizer in foods. The quality and stability of dehydrated products, multi-domain foods, and the performance of biofilms and encapsulation and controlled release technologies are affected by moisture transport. The rates of molecular mobility and diffusion-limited reactions strongly depend on the factors surrounding the food. Temperature and water activity (fl ) pl y significant roles in penetrant diffusion. The physical state of the carrier matrix, chemistry, size, and structure of diffusing molecule and specific... 

Another critical consideration in protein delivery from hydrogel systems is the potential for protein denaturation in the device. For diffusion-controlled delivery systems, where water is the main transporting medium, the protein solution stability governs the type of device. Extended releasing times can be achieved with reservoir systems (Fig. 1) for highly stable proteins (Langer, 1990). Alternatively, dehydrated delivery systems... 

Dehydration reactions are typically both endothermic and reversible. Reported kinetic characteristics for water release show various a—time relationships and rate control has been ascribed to either interface reactions or to diffusion processes. Where water elimination occurs at an interface, this may be characterized by (i) rapid, and perhaps complete, initial nucleation on some or all surfaces [212,213], followed by advance of the coherent interface thus generated, (ii) nucleation at specific surface sites [208], perhaps maintained during reaction [426], followed by growth or (iii) (exceptionally) water elimination at existing crystal surfaces without growth [62]. 

In a discussion of these results, Bertrand et al. [596,1258] point out that S—T behaviour is not a specific feature of any restricted group of hydrates and is not determined by the nature of the residual phase, since it occurs in dehydrations which yield products that are amorphous or crystalline and anhydrous or lower hydrates. Reactions may be controlled by interface or diffusion processes. The magnitudes of S—T effects observed in different systems are not markedly different, which indicates that the controlling factor is relatively insensitive to the chemical properties of the reactant. From these observations, it is concluded that S—T behaviour is determined by heat and gas diffusion at the microdomain level, the highly localized departures from equilibrium are not, however, readily investigated experimentally. 

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