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11 Solubility of Organic Molecules in Water (A Sureace Tension-Cavity Model Theory) 

In the following, one may consider the process of solubility of any substance in water as follows  

FIGURE 1.34 Pure water (W) and a foreign solute (organic) molecule (S) is dissolved in water. The cavity is created for S (see text). 

This indicates that some kind of structure will be present in liquid water surrounding CH4 or alkane molecule, which would be indicative of hydrate structure (Tanford, 1980). These data are of current interest in the future (from nonconventional reservoirs) gas recovery from methane hydrate reservoirs. 

If a salt exhibits a maximum solubility (called saturation solubility at a specific temperature and pressure) of 10 mol/L in water, then it corresponds to ca. 10 mol of salt 55 mol of water (ratio of 1 5.5). On the other hand, an alkane molecule may show a maximum solubility of 0.0001 mol/L in water (0.0001 55 mol), or a ratio of 1 550.000. Many decades ago, this model was found to be able to predict the solubilities of both simple organic molecules, and for the case of more complicated. In most simple case, the solubility of heptane is lower than that of hexane, due to the addition of one -CH2- group. In the case of alkane molecules, a linear relation between the solubility and the number of -CH2- groups (Acree, 2004 Birdi, 1997 Kyte, 2003 Tanford, 1980). 

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