Water structural entropy

Table 3.6 Water structural entropy Astiuc /J mol and heat capacity AstiucCp/J mol effects of representative ions according to three treatments. Adapted from (Marcus 2009) with kind permission of The American Chemical Society...

TABLE 5.2 Factors, AG Water Structural Entropy A j 5j/JK- moI" and Structural Heat Capacity mol- Effects [51] and the Changes of the Hydrogen Bond Geometrical hb(i), of Representative Ions [53] ... 

Usually, dissolution of a small amount of one compound in a pure liquid is enthalpically unfavourable and driven by an increase in (mixing) entropy. At room temperature, the opposite is true for the dissolution of a small apolar compound in water. This unexpected behaviour is referred to as the hydrophobic effect [4]. Classically, this effect has been rationalised by ordered water structures around apolar compounds (entropy reduction) and the increase in number... 

Free energy variations with temperature can also be used to estimate reaction enthalpies. However, few studies devoted to the temperature dependence of adsorption phenomena have been published. In one such study of potassium octyl hydroxamate adsorption on barite, calcite and bastnaesite, it was observed that adsorption increased markedly with temperature, which suggested the enthalpies were endothermic (26). The resulting large positive entropies were attributed to loosening of ordered water structure, both at the mineral surface and in the solvent surrounding octyl hydroxamate ions during the adsorption process, as well as hydrophobic chain association effects. 

A large portion of the calculated entropy change (+ or -) results from the relative ordering or disrupting of water structure during adsorption. 

The cause of a positive entropy of micellisation is not entirely clear. A decrease in the amount of water structure as a result of micellisation may make some contribution. A more likely contribution, however, involves the configuration of the hydrocarbon chains, which probably have considerably more freedom of movement in the interior of the micelle than when in contact with the aqueous medium. 

The behavior of hA in real micellar systems is more complex as seen in Fig. 2.12. Similar data have been obtained for several other amphiphiles148,149). The deviations in hA from the standard value at infinite dilution appear clearly below the CMC, but at these concentrations one has a compensating change in the partial molar entropy. This effect might be due to a repulsive interaction between the hydrophobically hydrated alkyl chains leading to a breakdown of the water structure with a concomitant increase in entropy. 

When hydrophobic side-chains of amino acids are in contact with water, they can only be accommodated in the aqueous phase if water structure is altered. Water near nonpolar groups forms what are sometimes referred to as cages or clathrates around these groups. Water molecules in these cages possess a relatively high amount of structure compared to the bulk water of the solution. It should be clear, then, why an increase in the entropy of the sys-... 

The actual amount and structure of this "bound" water has been the subject of debate (83), but the key factor is that in water, PVP and related polymers are water structure oiganizers, which is a lower entropy situation (84). Therefore, it is not unexpected that water would play a significant role in the homopolymerization of VP, because the polymer and its reactive terminus are more rigidly constrained in this solvent and termination k is reduced (85). 

The associated enthalpy and entropy quantities, 5m//f and 6mS, show more marked variations. Both 8mHf and T, 8mSf increase rapidly as x2 increases and, in the case of ethane in ethyl alcohol + water mixtures, there is an endothermic maximum near x2 = 0-2 (Fig. 41). In water-rich mixtures the solubility data reflect the impact of water structure (Cargill and Morrison, 1975). A key observation is the tendency for the solubility of an apolar solute to decrease (i.e. 8m[i rises to a maximum) as an organic co-solvent is slowly added. This salting-out of an apolar solute accounts for the enhancement of protein structure by low mole fractions of TA solvents, for example (Brandts and Hunt, 1967). A further clear... 

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