Enthalpy stabilization

There have been a very large number of other thermodynamic studies involving the interaction of crowns with a wide range of metal-ions (Izatt et al., 1985). In general, as for the examples already discussed, the complexes are found to be enthalpy-stabilized with the entropy term also contributing to the stability in a number of cases. 

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC>3 from the binary ones (BaO and CC>2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature,... 

The relative stabilities of the dioxides, sesquioxides and monoxides for first period transition metals are given in Figure 7.11(c). The stability of the higher oxidation state oxides decreases across the period. As we will discuss later, higher oxidation states can be stabilized in a ternary oxide if the second metal is a basic oxide like an alkaline earth metal. The lines in Figure 7.11(c) can in such cases be used to estimate enthalpies of formation for unstable oxidation states in order to determine the enthalpy stabilization in the acid-base reactions see below. Finally, it should be noted that the relative stability of the oxides in the higher oxidation states increases from the 3d via 4d to the 5d elements, as illustrated for the Cr, Mo and W oxides in Figure 7.11(d). 

The Hg2+ complex of 18C6, is again weak, with a small entropy effect working against the enthalpy stabilization. For the Hg2+ complexes with... 

The thermodynamic data for the adduct formation equilibrium between uranyl chelates and neutral donors (Eq. 17) were evaluated from the solvent extraction data90,91 obtained at different temperatures and these are given in Table 23. It is seen that these adducts are mainly enthalpy stabilized and this is expected when the neutral donor is directly coordinated to the metal ion. However, caution should be exercised in interpreting such data since the diluent used is known to influence thermodynamic parameters appreciably215. ... 

With ethylenediamine complexes of the formula Ln(en)3X3 and Ln(en)4X3, where X = C1 , Br , NO, CIOJ have been characterized. IR data indicate that the tris and tetrakis complexes of the fighter lanthanides La-Sm, contain both ionic and coordinated nitrate groups. By contrast tetrakis complexes of heavier lanthanides, Eu-Yb contain ionic nitrate. This is possibly due to steric factors resulting from decreasing cationic radius that force the nitrate out of the coordination sphere of the lanthanides. A coordination number of 8 for tris complexes and a number of 9 for fighter lanthanide tetrakis complexes appears reasonable [234]. The thermodynamic parameters obtained show enthalpy stabilization for... 

Hydration enthalpy Stability or formation constant Overall and stepwise stability constants Chelate effect Macrocyclic effect Preorganization Equilibrium template effect Kinetic template effect Self-assembly... 

Macromolecular colloid solutions also play an important role in ensuring the stability of disperse systems (e.g. suspensions, emulsions). In the case of emulsions the polymer decreases the rate of separation by increasing viscosity on the one hand, and it has an enthalpy stabilizing effect by adsorption on the surface of the droplets on the other hand [3, 4, 7]. Depending on the concentration of the polymer, a protecting and flocculating effect can be observed during the interaction between suspensions and polymers. If the polymer concentration is low, the polymer adsorbed on the surface of the particles connects the particles into loose floccules. Thereby, the rate of... 

Physically, an external repulsive solute sphere that does not form any HB with the surrounding water molecules does not offer any enthalpie stabilization at all. Therefore, the surrounding water molecules reorder themselves to preserve as many HBs as possible. Therefore, the entropy of water deereases due to this ordering foreed on it by the solute. 

Fig. 7. Dependencies of thermal (peritectic decomposition temperature) and thermodynamic (dissolution enthalpy) stability of R123 lattices upon their geometrical stability, calculated using ionic radii of R (top...

Mizyed et al. [40] conducted a UV-Vis study in toluene solution of the CT complexes of Ceo with ferf-butylcalix[4]crowns 23a-c and 24 (Fig. 33.17) and also measured the thermodynamic data of the resulting 1 1 complexes. The K values were found to decrease with increasing size of the crown moiety of the calixcrown 23c 23a >23b >24. Most of the complexes were reportedly found to be enthalpy stabilized but entropy disfavoured. 

The methods of calculating stabilization or strain enthalpies cited in the introduction are open to criticism, largely on the choice of a reference state. Each method has, however, the virtues of simplicity and of basing theoretical constructs on enthalpies one can measure as distinct from things (ring currents, etc.) that we suppose are correlated with enthalpy. Stability is, in the final analysis, a thermodynamic property. 

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