Iceberg water model

The solvation thermodynamics have been interpreted in a classical study by Frank and Evans in terms of the iceberg model . This model states that the water molecules around an nonpolar solute show an increased quasi-solid structuring. This pattern would account for the strongly negative... 

ICC Termination Act of 1995, 25 331, 326 Ice. See also Water entries elastic properties, 5 614t hydrogen-bonded structure of, 26 15 properties of, 26 17t Ice wines, 26 315 Iceberg model, 23 95 Ice formation, in food processing, 72 82 Iceland, bioengineering research program, 7 702... 

Stillinger (1980) suggested that disconnected icebergs should not be present, but that a more likely model is that of a random, three-dimensional network of hydrogen bonds, rather than long-lived clusters of molecules. Such networks of hydrogen-bonded water molecules and clusters are present when hydrates form or dissociate. These clusters are discussed relative to nucleation of hydrates in the following chapter. 

In the Frank and Evans iceberg model, ice-like structures form around hydrophobic entities, such as methane. In this model, the hydrophobic molecules enhance the local water structure (greater tetrahedral order) compared with pure water. Ordering of the water hydration shell around hydrophobic molecules has been attributed to clathrate-like behavior, in which the water hydration shell is dominated by pentagons compared to bulk liquid water (Franks and Reid, 1973). 

Fig. 17. Model of the clathrate structure of the water in clathrates I. The centre of the hole is occupied by hydrophobic guest molecules in gas-hydrates (model of the iceberg formation in aqueous solution )...

A consideration of thermodynamic properties of the aqueous solution of rare gases and hydrocarbons led to the iceberg model for water structure around nonpolar molecules [139], which later had to be abandoned (see Part IV, Chap. 23.4). The gas hydrate clathrate structures described in Part IV, Chap. 21 provided... 

The model of icebergs around nonpolar solute molecules in aqueous solution is clearly not a very realistic one. However, if solutions of hydrocarbons (or noble gases) are cooled, then the solid phase that sometimes separates out consists of a so-called gas hydrate (clathrate), in which water provides a particular kind of hydrogen-bonded framework containing cages that are occupied by the nonpolar solute molecules. Obviously, such gas hydrates (clathrates) represent more realistic models for the phenomenon of hydrophobic hydration [176]. 

Many models have been proposed, but none has adequately explained all properties of liquid water. Iceberg models postulated that liquid water contains disconnected fragments of ice suspended in a sea of unbounded water molecules. 

In this section we apply the above model for the description of the volumetric behavior of water and water + inert solvent mixtures at moderate to high temperatures and pressures. In this range of conditions we do not expect any predominance of the well-known picture of icebergs valid for the lower range of conditions. The formation of icebergs will be discussed in a later section. 

However, this iceberg model was discarded when many experiments showed that while water slowed down somewhat at the surface it never slows down to the extent that qualified it to be called an iceberg. 

Figure 8.1. Schematic representation of the old view of a protein molecule in aqueous environment, with a layer of strongly associated water (the hydration layer is an iceberg), suspended in aqueous solution. The hydration layer moves with the protein molecule (as proposed by the iceberg model), and beyond this layer the water molecules adapt to the normal tetrahedral geometry. Adapted with permission from Chem. Rev., 104 (2004), 2099-2123. Copyright (2004) American Chemical Society.

As we discussed above, the first model that attempts to explain this entropy loss was that of Frank and Evans, who proposed that water molecules in the first layer of the hydration shell form a eage-like strueture by forming HBs around the non-polar solute in a fence-like manner so as not to waste HBs by pointing them towards the solute. This ordering clearly eosts entropy. This iceberg model has sometimes been taken too literally, for example in understanding the hydration shell of proteins. The shell would certainly retain a eertain dynamic character, as it would be in dynamic equilibrium with the rest of the bulk. In fact, computer simulation studies indeed show that water molecules around methane or ethane have a residence time of a few tens of picoseconds at most, so the ieeberg model indeed has a limited validity. 

Of the many hypotheses of the structure of liquid water, that ofPople (1951), as modified by Sceats, Stavola and Rice (1979), agrees very well with all experimentally determined properties. In this model, water is formulated as a continuous polymer in which H2O units are united by a network of hydrogen bonds that extend throughout the whole liquid which becomes, in this sense, one large molecule. This formulation is compatible with all recorded physical properties. No support remains for older ideas of flickering clusters , icebergs , monomeric inclusions , or other types of discontinuity. 

The hydrophobic interaction term is used to describe the tendency of non-polar groups or molecules to aggregate in water solution. Hydrophobic interactions are believed to play a very important role in a variety of processes, specially in the behavior of proteins in aqueous media. The origin of this solvent-induced interactions is still unclear. In 1945 Frank and Evans proposed the so-called iceberg model where emphasis is made on the enhanced local structure of water around the non-polar solute. However, computational studies and ex-... 

The model of the ring of six water molecules which you showed surrounding a hydrocarbon chain is very similar to a six membered ring occurring in the crystal structure of ice. It appears that your proposed complex is not different from the clathrate or iceberg structures proposed quite some time ago to explain hydrocarbon-water interactions. 

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