Enthalpic stabilization

Two simple thermodynamic considerations are suggested upon examination of Fig. 8. The first is that at temperatures below Tml the free energy of the bulk mesophase G m is in general bound to be lower than Gl, the free energy of the amorphous. In the limit of Class II mesophases, since AHml = 0, we will have Gm = Gl at T = 0 K while Gm < Gl at temperatures 0 < T < Tml since it is Sm > Sl at temperatures low enough as compared to Tml (Sect. 3.1). In the case of Class I mesophases AHml > 0. he., mesophases are enthalpically stabilized with respect to the liquid state, while Sm < Sl, so it will be Gm < Gl at temperatures T, with 0 < T < Tml- Note that the above consideration will in... 

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,... 

It is obvious from the definition of standard enthalpy of formation that these quantities do not represent the absolute enthalpic stability of compounds. They merely reflect their enthalpic stability relative to that of the chemical elements in standard reference states (to which AfH° = 0 has been arbitrarily assigned). It is thus unreasonable to state that a given substance is more stable than another just because it has a lower standard enthalpy of formation. We can only use AfH° values to make such direct comparisons when we are assessing the relative stability of isomers. 

Discuss polymer-colloid interactions and steric stability from a thermodynamic perspective. What is enthalpic stabilization What is entropic stabilization What is the critical flocculation temperature (CFT) ... 

Thermodynamic studies performed on a number of other guanosine adducts show some examples where binding represents an enthalpic destabilized, but entropy stabilized, process and others that are enthalpically stabilized, but entropically destabilized. The net thermodynamic result, however, appears to be a relatively modest localized free-energy destabilization that could form the basis for some recognition mechanism. 

Another interesting aspect of the data is the decrease in enthalpic stability of the complex relative to the free anion in moving from acetonitrile to A/,A/-dime-thylformamide. This observation could be explained in terms of the partial shielding of the anion by the ligand in the complex. 

Acetonitrile offers the optimal conditions for the complexation of 8-oca/J/ and the fluoride anion (equation (12)) in that both, reactants and product favourably contribute to complex formation in this solvent. Indeed, on the one hand, acetonitrile interacts weakly with the reactants while these undergo strong interaction with A/,A/-dimethylformamide to an extent that both, fluoride and receptor are reluctant to interact strongly between themselves. On the other hand, acetonitrile is a better solvating medium for the anion complex than A/,A/-dimethylformamide. As a result, the enthalpic stability for the complexation of fluoride and 8-aajSjS in acetonitrile is quite high in contrast with the relatively low ACH° value observed for this system in A/,A/-dimethylformamide. 

The relatively low thermodynamic stability of complexes of hemicarcerands or other container-type hosts is a direct consequence of structural aspects of the walls that make up the inner surface of such compounds. These walls are lined by aromatic subunits while free electron pairs of heteroatoms such as those of the ether oxygen atoms are preferentially oriented to the outside. Complexes are therefore enthalpically stabilized only by weak dispersive interactions. In the case of positively charged guests cation-re interactions can contribute to binding enthalpy as in a self-assembled calixarene-derived capsule [9], but directed interactions such as hydrogen-bonding interactions are usually absent. 

FIGURE 4.33 Enthalpic stabilization (a) particles with hydrated stabilizing polymer chains (b) overlapping stabilizing chains with released water molecules. 

A key contribution of molecular dynamics simulations to the imderstanding of mechanisms of selectivity and affinity in TBP-DNA complexes is the discovery of the active role of TBP in the formation of the complex. The view derived from crystal structures was that of a passive role for the TBP which only imposed a steric constraint on DNA shape. It appears now from the simulations that TBP can respond to the dynamics of the bound DNA sequence by adjusting its interdomain geometry, and this might be relevant for the construction of the final preinitiation complex. Furthermore, many of the contacts characterized in the crystal structures were found in the simulations to have an important dynamic component, as side chains switch rotamers rather frequently. This conformational freedom makes it possible for TBP to achieve suitable binding contacts with a variety of DNA moieties in a dynamic mode which contributes to enthalpic stabilization. However, the extent of preservation of side chain dynamics in the complex is dependent on the local structure. As it reduces the entropy loss upon complex formation, it provides an additional source of sequence-dependent gain in affinity that is revealed for the first time from the results of the molecular dynamics simulations. 

Entropic influences are less significant. However they can be used to selectively stabilize one product over others that have equal enthalpic stability. For example, at high solution concentrations entropy favors the formation of large structures over small ones. Thus, certain binuclear helicates can be transformed into their corresponding tri- or higher multinuclear circular helicates, simply by concentrating the reaction solution some metallocycles can be transformed into catenanes in the same way (vide infra, Sections 1.43.4.2 and 1.43.4.7). 

First, both AHp and may be positive so that the enthalpy of close approach favours stabilization whereas the corresponding entropy term promotes flocculation. If, however, the contribution of the enthalpy term to AGp exceeds that of the entropy term in absolute magnitude, AGp will be positive and close approach will be disfavoured. This is termed enthalpic stabilization since it is the net enthalpy of close approach that ensures stability. 

Note that certain limiting cases implicit in Table 7.1 can stifl result in a positive free energy of flocculation. IThiese are ASp=Q for enthalpic stabilization and AHp=0 for entropic stabilization. Note, too, that if AHp=ti, the system is said to be athermal. 

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