Entropy-driven structures

Consequently in the liquid obtained after breaking 13 — 20% of the H-bonds (i) all water molecules belong to one network and are connected to each other by a weakened network of H-bonds, (ii) there are some large cavities formed as a result of the rupture of H-bonds, and (iii) these cavities cannot be stable and are prone to entropy-driven structural transformations. These transformations contribute to the entropic and volumetric changes associated with ice melting. 

The dissolution of salt in water (2) is endothermic (AH > 0)—i. e., the liquid cools. Nevertheless, the process still occurs spontaneously, since the degree of order in the system decreases. The Na"" and Cl ions are initially rigidly fixed in a crystal lattice. In solution, they move about independently and in random directions through the fluid. The decrease in order (AS > 0) leads to a negative -T AS term, which compensates for the positive AH term and results in a negative AG term overall. Processes of this type are described as being entropy-driven. The folding of proteins (see p. 74) and the formation of ordered lipid structures in water (see p. 28) are also mainly entropy-driven. 

Sulfur forms a series of homoatomic dianions catena-S (x = 2-8), which, without exception, have unbranched chain structures in the solid state.The electrochemical reduction of cyclo-Sg in aprotic solvents occurs via an initial two-electron process to produce catenaS P In solution, catena- and other long-chain polysulfides, e.g. catena- and catenaSi, dissociate via an entropy-driven process to give radical anions S (x = 2-4), including the ubiquitous trisulfur radical anion (x = 3). This intensely blue species is the chromophore in the mineral lapis lazuli, which is used in the manufacture of jewellery. 

In capsides the self-assembling process is entropy driven. Of course, it would be of interest to carry out experiments in dissipative structures. 

From the temperature dependence of the equilibrium constant for proton exchange between some deuterated and undeuterated primary and secondary amines, monitored by high-pressure mass spectrometry, the reaction enthalpy, or difference in proton affinity, could be measured.101 Protonation of the deuterated amine is favored by 0.2kcalmol-1, varying with structure by 0.1 kcal mol-1 but with no obvious pattern. However, the equilibrium, at least for CH3CD2NHCH3, appears to be entropy driven, not enthalpy. 

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