The Smith—Topley effect

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by 

The form of the (da/df)D—PH2o curve and the position of the maximum may vary considerably with temperature and S—T behaviour may not be detected under certain conditions [596]. These effects [282,291] may 

No single mechanistic explanation of the S— T effect has been accepted as possessing general validity the salient features of the several alternative reaction models which remain under discussion are summarized below. There also exist the possibilities that there may be concurrent, consecutive or intermediate behaviour and that different mechanisms may operate for different solids. 

A variation of this approach has recently been provided by Lyakhov et al. [598] who, from measurements of water adsorption on CuS04 5 H20, on MgS04 7 H20, and on their respective dehydration products, discern a correlation between strengths of surface bonding and S—T behaviour. At low surface coverages, the mutual dipole—dipole repulsions in the adsorbed layer inhibit water loss, in part by a blocking action on loss of water of crystallization and in part by polarization effects which provide a 

It has also been shown [599] that a maximum on the (da/dt)D Ph2o plot can result from a catalytic effect by the product gas (water) on the desorption process. 

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At least three major interrelated aspects of kinetic behaviom are important in investigating dehydration reactions, (i) Kinetic equations based upon nucleation and growth models have been developed and found application to a wide range of reactants [31], (ii) Theoretical explanations of the magnitudes of calculated Arrhenius parameters have been proposed, (iii) The influence of water vapour pressure on reaction rates has been investigated in detail (the Smith-Topley effect). Topics (ii) and (iii) are expanded below,... 

A gravimetric study of the thermal dehydration of Y(HC00)j.2H20 was undertaken [149] in various water vapour pressures between 5 x 10" to 8 Torr (387 to 407 K). Following an initial short acceleratory process, the reaction predominantly fitted the contracting volume equation. The reaction rate increased with pQi20), reached a maximum value and thereafter decreased to a constant value. This is a pattern of behaviour similar to the Smith-Topley effect. The results are explained on the basis of the crystallinity of the dehydrated residual product. 

The dehydration that precedes the decomposition of manganese(II) oxalate exhibits the Smith-Topley effect. The residual product of decomposition, MnO, is readily oxidized and the temperature of reaction is decreased in the presence of oxygen. The decomposition of anhydrous manganese(II) oxalate in vacuum [63] can be represented as ... 

The Smith-Topley (S-T) effect. The characteristic pattern of variation of the rate of some dehydrations with prevailing water vapour pressure [2,21,48,49]. 

This is often referred to as the Smith-Topley s effect. 

More quantitative measurements of the systematic variations of dehydration rates with/7(H20), referred to as Smith-Topley behaviour, could lead to support for one or more of the several theoretical explanations that have been proposed [2,21,49,54,63] based on recrystallization of sohd product, local self-cooling and/or diffusion (effects expected to occur in all dehydration reactions) and adsorption of the volatile product. Dehydrations may also involve the intervention of a zeolitic residue and/or an amorphous phase, the formation and reciystallization of one or more lower hydrates as intermediates, and diffusive esc e of water through various channels of the barrier layer of product may be slow. 

Prodan et al. [75] studied the low pressure (lO" Torr), low temperature (fi om 273 to 373 K) dehydration of Na5P30,Q.6H20 in the form of fine crystals. Reaction occurred in two stages (with = 56 and 84 kJ mol ) both of which were diffiision controlled. The activation energy increased with extent of reaction. The rate of reaction of this salt was enhanced [76] by water vapour, attributed to its ability to reorganize the diffusion layer. This effect (Smith-Topley behaviour) has been noted in many dehydration reactions (Chapter 7). 

Smith-Topley s effect on the influence of water vapor in dehydration reactions... 

As we note in section 13.3.2.2, Smith-Topley s effect is characterized by the presence of a maximum and a minimum in the curve giving the motion velocity of... 

Figure 13.9. Smith-Topley s effect in the case of only one dehydration reaction...

As shown in Figure 13.7b, it seems that nucleation follows Smith-Topley s effect in the same ranges of pressures that the reactivity of growth. 

Using the fact that the critical size of the nucleus varies with the pressure and temperature [GRU 74], Bouineau [BOU 98] shows, via a relation such as [13.10], that the equilibrium constant of nucleation of the reaction is modified, and consequently, as in the case of the growth, it is necessary to change curves giving y according to P (or 7). Thus, Smith-Topley s effect would be found on the curves of nucleation for the same fundamental reason (modification of the equilibrium constant) as that on the curves of reactivity. 

The CDV mechanism is proposed [55]. Interpretation of coefficients of vaporization [56[. Modelling of the Topley-Smith effect [57]... 

Product Structure A feature frequently observed in the decomposition of crystalline hydrates, which has not yet been given a convincing interpretation in the framework of universally accepted ideas, is the formation of solid products in either an amorphous or a crystalline state, depending on the actual water vapour pressure in the reactor. This phenomenon was observed by Kohlschiitter and Nitschmann in 1931 [35] and has been the subject of numerous publications, including the study of Volmer and Seydel [36], who used it as a basis for explaining the Topley-Smith (T-S) effect, and a series of articles by Frost et al. [37-39]. Dehydration of many crystalline hydrates in vacuum entails formation of an X-ray amorphous (finely dispersed) residue and, in the presence of water vapour, formation of a crystalline product. The highest H2O pressure at which an amorphous product can still form varies for different hydrates from a few tenths to a few Torr (Table 2.4). As the decomposition temperature increases, the boundary of formation of the crystalline product shifts towards higher H2O pressures. 

Where E parameters had been determined at different pressures Ph20j the result corresponding to the highest Ph20 magnitude was chosen (beyond Pmo magnitudes that are typical for the Topley-Smith effect). 

Fig. 7.1 The Topley-Smith effect. The dehydration rate of MnC204 2H2O at 76° C in the presence of water vapour. (Reproduced from [2], with permission.)...

Fig. 7.6 The Topley-Smith effect for CaCOs. Two sets of experimental data (points) were taken from [19], with permission. Theoretical curves were calculated for different emittance factors (above) 0.002 and (below) 0.0015...

Basic achievements The proposed thermochemical approach to the kinetics of solid and melt decompositions rested on the CDV mechanism revealed the interconnection of two traditional, but distinct, scientific approaches (after Arrhenius and after Knudsen-Langmuir). Some unusual or mysterious phenomena (including misconceptions), revealed or formed during the more than a one-hundred-year period of studies in these traditional fields, have been interpreted. The most significant achievements are the explanation of the mechanism of the nucleation and growth of nuclei and the interpretation of such effects as localization of the decomposition process, the Topley-Smith effect, the difference between the rates of decomposition under effusion (after Knud-sen) and free-surface (after Langmuir) conditions, a decrease in the decomposition rate upon melting of the reactant, differences in the crystalline structure... 

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