Combined enthalpic-entropic stabilization

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]... 

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