Interplay of enthalpy and entropy

Self-assembly is a thermodynamically controlled process. The formation of the capsules and the exchange of guest molecules proceed within seconds to hours, sometimes days, but finally, an equilibrium is reached which is governed by a finely balanced interplay of enthalpy and entropy. A... 

See also Internal Energy (E), Enthalpy, Interplay of Enthalpy and Entropy... 

See also Secondary Structure (General), Globular Proteins, Factors Determining Secondary and Tertiary Structure, Clathrate Structure of Water, Enthalpy, Entropy, Interplay of Enthalpy and Entropy... 

Summary of Interplay of Enthalpy and Entropy (Figure 3.4, Table 3.3)... 

Not only is the extent of the equilibrium reaction, that is, the ratio of the concentrations of the products to those of the reactants, governed by the initial composition and the equilibrium constant, affected by the solvent, so also is the temperature dependence, that is, the enthalpy and entropy of the reaction. There is again a complicated interplay in the solvation of the two species between polarity and hydrogen-bonding abilities of the solvents. The negative entropies of this reaction, that become more negative along the same sequence, are due more to the solute properties than the solvent ones. 

In general, the outcome of polymer melt intercalation is determined by the interplay of entropic and enthalpic factors." The confinement of the polymer chains inside the silicate galleries results in a decrease in the overall entropy of the polymer chains, and the increased conformational freedom of the tethered ammonium cations compensates the entropy loss as the silicate layers separate with each other. However, the small increase in the gallery spacing does not affect the total entropy change rather, the total enthalpy will drive the intercalation. The enthalpy of mixing has been classified into two components the interaction between polymer and ammonium cations and the interaction between the layered polar silicates and the polymer chains. In most conventional organo-modified silicates, the tethered ammonium cations are apolar. The apolar interactions between the polymer and ammonium cations are unfavorable to the... 

A fundamental limitation for fhe apphcation of geometric fitting procedures is that fhe complexation free energies are fhe sum of enthalpic and entropic contributions, with the consequence that selectivity can be inversed at different temperatures. Positive cooperativity between different interactions in a complex will usually lead to tighter association at fhe expense of motional freedom and fhus of entropy [51]. The interplay and often observed compensation of enthalpic and entropic contributions have been discussed in several reviews [10, 52, 53], particularly with emphasis on biological systems, and cannot be dealt with in detail here. Unfortunately, many published enthalpy-entropy compensations are blurred by possible artifacts, as the two underlying parameters do not represent independent variables [54]. 

Fullerenes and metallofullerenes were for the first time observed in the gas phase about 20 years ago and then prepared in crystalline form about 15 years ago. An enormous amount of observed and computed data has been obtained during the period. The chapter surveys various computational aspects of fullerene science including rich isomerism and the enthalpy-entropy interplay both of which represent essential features of fullerene and metaUofuUerene formations. 

Although the inter-isomeric separation energies are important, they alone cannot predict the relative stabilities of the isomers. Owing to very high temperatures, entropy contributions can even over-compensate the enthalpy terms. Hence, the enthalpy-entropy interplay represents an essential feature of fullerenes and metallofullerenes. 

In the case of NaCl and especially NH4NO3 dissolving in water, AH o r > but the increase in entropy that occurs when the crystal breaks down and the ions mix with water more than compensates for the increase in enthalpy. (The enthalpy/entropy interplay in physical and chemical systems is covered in depth in Chapter 20.)... 

There are several related phenomena that will not be treated in this chapter. They include the well documented transformation of nematic phases into cholesteric (twisted nematic) phases by adding small amounts of optically active molecules to a nematogen [171], creation of smectic phases from mixtures of molecules which alone form only nematic phases [172,173], and the presence of reentrant phases [174] due to molecular reorganizations based upon the relative importances of various short- and long-range intermolecular interactions in different temperature regimes (as mediated by the interplay of entropy and enthalpy terms) [175 -177]. Each has been exploited to create interesting and novel systems and devices based upon mesomorphism. 

Similarly, a solution usually has higher entmpy than the pure solute and pure solvent in other words, solutions form naturally but pure solute and pure solvent do not. The number of ways to distribute the raiergy and the fi eedom of motion of the particles are related to the number of different interactions between the particles, and there are fer more interactions possible when solute and solvent are mixed than when they are pure thus, > (5s iute + solvent) or ASs n > 0. The solution process involves the interplay of the change in enthalpy and the change in entropy systems tend toward a state of lower enthalpy and higher entropy, and the relative magnitudes of AH oin and ASs n determine whether a solution forms. 

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