Smith-Topleys Effect

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 

It seems that we can explain this effect, in all the cases, by a modification of the equilibrium constant (i.e. dissociation pressure of the hydrate) with the water pressure applied even if this modification does not always come from the same cause. Indeed, this variation of equilibrium constant can We at least two causes. With the first one, we eiqilain the effect on copper sulfate and with the other one the effect on lithium sulfate or manganese oxalate. 

The equilibrium pressure of the system can vary because we do not cany out the same reaction at all water pressures it is the case of copper sulfate. Indeed, we noted (section 3.8) that according to the range of pressure, we could carry out different dehydrations. Thus from penta-hydrated copper sulfate, we produce monohydrate for pressures lower than 40 Pa, that is, we ejqness the reaction  

In a more general way, Hartoulari and Dufour [DUF 69, DUF 70, DUF 88] assume that the solid phases formed at the interface are disoi nized with a Gibbs energy excess conqrared with the stable phase of larger size arrd thus each period of the disorganization has its own equihbriitm pressure. 

In the following section, we will return for the effect on the curve-specific frequency of nucleation pressure. 

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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 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 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]. 

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... 

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

Figure 13.7. Smith-Topley s effect on hydrated lithium sulfate (a) for growth, (b) for nucleation...

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.)...

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