The Solubilities of Hydrocarbons in Concentrated Salt Solutions and Their Correlation with the Clathrate Water Theory, Ion-Ion Associations, and Structures of Polymers

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Abstract
The solubility of benzene, benzoic acid, and hexanol in aqueous solutions containing various salts or dipolar molecules was examined. Both the anion and cation of guanidinium or other salts determines the salting-in properties. The solubility of benzene was found to be a linear function of the molar concentration of salt, CS, to the saturation point of the salt; i.e., the solubility constant kS remains the same. In other words, the interaction of benzene with hydrated ions is the same at extremely low ionic strengths as at high ionic strengths. This result agrees with the proposal that the B region (or negatively hydrated region) of ions consists of a single monomolecular layer of water molecules. Moreover, since in such concentrated salt solutions all water molecules are hydrated, the disappearance of any proposed clathrate structure in concentrated salt solutions should alter the value of kS. Consequently, the constant kS values show that water clathrate structures do not surround hydrocarbons. The solubility studies in concentrated salt solutions at 25 and 60°C also show that clathrate water structures cannot exist around hydro-carbons, since in all cases the solubility of the hydrocarbon increases with an increase in temperature. The association of NH4 + and OH− ions to form the dipolar NH4 δ+OHδ− molecule reduces the amount of electrostatic charge per surface area, and hence the solubility of benzene in NH4OH solutions is increased. In concentrated solutions of NH4Cl (or HCl) there appears to be a similar electro-static interaction between the NH4 + and Cl− (or H+ and Cl−) ions which reduce the charge per unit surface area on these ions. From these results it is concluded that solubility studies of benzene can be used to detect ion association reactions. Thiourea salts-in more than urea because of less charge per unit surface area, indicating that urea as well as thiourea exists as a zwitterion. Positive hydration is shown to exist for fluoride ions. These results are applied to the properties of proteins and other polymers.

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