Combined enthalpic-entropic

The third mechanism of imparting steric stabilization differs from the preceding two in that the enthalpy and entropy contributions to the free energy of flocculation act in concert instead of in opposition. Both now contribute to stability since AHp is positive and dSp is negative. The magnitudes of their respective contributions to AGp are no longer important This is termed combined enthalpic-entropic stabilization. 

As both the entropy and enthalpy terms conspire to impart stability, it is not possible to induce flocculation merely by changing the temperature in a dispersion exhibiting combined enthalpic-entropic stabilization. As discussed below, however, it may well be possible by temperature and/or pressure changes to convert the dispersion into one displaying either enthalpic or entropic stabilization. Flocculation by temperature changes could thereupon become possible. 

Iliese conditions herald the onset of entropic stabilization. The intervening quadrant is specified by positive values for AHp, coupled with negative values for ASp. It corresponds to combined enthalpic-entropic stabilization. 

It is not possible, as was noted previously, to move from a domain of entropic stabilization to a domain of enthalpic stabilization without passing through a region of combined enthalpic-plus-entropic stabilization. Again the enthalpy term promoting stability in the combined region is that associated with the free volume dissimilarity. The entropy term that contributes to stability is the combinatorial term, just as it is in the purely entropic domain. 

The enhanced stability of chelating ligands comes from a combination of entropic (AS°) and enthalpic (AH°) factors that lower the total complexation free energy (AG°), as follows (where T is the temperature in Kelvin) ... 

Electroneutral combinations are often favored by enthalpic against entropic contributions, however with noticeable exceptions. Thus, crown ether 18-C-6 complexes with e.g. free amines in methanol show stability constants around lgK= 2.5 0.1, quite independent of the amine structure, but very sizeable differences in AH, ranging from 1 or 2 kJ/mol for secondary or tertiary amines to 30 kJ/mol for primary amines with surprisingly favorable entropic TAS values of up to 13 kJ/mol. Protonated amines show increased lg K values of up to 4.4, mainly due to enthalpic advantages, and not. as one might expect for the more polar combinations, due to entropic effects, although these are also sizeable.[38]... 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

As for the supraminols and related superstructures covered in this chapter, entropic and enthalpic forces may combine with steric factors, recognition, and complementarity to ultimately decide the architecture, organization, dimensional-... 

Guillet and coworkers (8-10) have determined the solubility parameter of polymers from the probe-polymer interaction coefficients. They separated the interaction parameter into entropic and enthalpic contributions, such that Xi2=X h+Xs to yield, in combination with Hildebrand s solution theory, the following expression ... 

Guillet approach we did not separate )f12 into entropic and enthalpic components. Instead, a more general term, 0 (23) was introduced to represent a combination of interaction terms. By introducing the contact energy per unit volume, B (23), a simpler form of the expression is obtained ... 

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