Size and temperature dependence of hydrocarbon solubility in concentrated aqueous solutions of urea and guanidine hydrochloride

At 25°C, methane and ethane are more soluble in water than in 7 M aqueous urea or 4.9 M aqueous guanidine hydrochloride (GuHCl); the reverse is true for larger hydrocarbons. In addition, the hydrocarbon solubility in 7 M aqueous urea or 4.9 M aqueous GuHCl increases compared with that in water on raising the temperature in the range of 5–45°C. These experimental data have not yet been rationalized. Using a well-founded theory of hydrophobic hydration, the present analysis indicates that the transfer of hydrocarbons from water to 7 M aqueous urea or to 4.9 M aqueous GuHCl is favored by the difference in the solute–solvent van der Waals interaction energy, and contrasted by the difference in the work of cavity creation. At room temperature, on increasing the hydrocarbon size, the first contribution rises in magnitude more rapidly than the second contribution, accounting for the threshold size occurrence. Moreover, the second contribution decreases in magnitude with an increase in temperature, becoming less unfavorable, while the first contribution is practically constant in the range of 5–45°C. The different temperature dependence of the work of cavity creation in such solvent systems is due to the fact that the density of 7 M aqueous urea and 4.9 M aqueous GuHCl decreases more rapidly than that of water when raising the temperature. The relationship between the density of a liquid and the work to create a cavity in it is discussed in detail.Key words: work of cavity creation, solute-solvent van der Waals interaction energy, H-bond reorganization.

Solvation of a water molecule in cyclohexane and water

Reliable values for the thermodynamic functions associated with the solvation of a water molecule in its own liquid phase and in cyclohexane, at 25°C, are obtained using experimental data from different investigations. They are successfully rationalized by means of a general theory of solvation. The standard Gibbs energy change is given by the balance of two contrasting terms: the work to create a cavity in the solvent suitable to host the water molecule; and the work to turn on the water-solvent interactions. It proves that the work of cavity creation is largely overwhelmed by the formation of two H-bonds in liquid water, whereas it is almost exactly counterbalanced by the establishment of van der Waals interactions in liquid cyclohexane.Key words: water, cavity creation, H-bonds, van der Waals interactions.

Solvation thermodynamics of cyclohexane

The solvation thermodynamics of cyclohexane in pure liquid phase and in water is analyzed by means of the theoretical approach developed by Lee. The sum of the work of cavity creation and the dispersive solute–solvent interaction energy reproduces well the experimental Gibbs energy values over the whole temperature range 5–100°C. This implies that the purely structural solvent reorganization is an exactly compensating process in both liquids. The dispersive solute–solvent interaction energy is larger in magnitude in cyclohexane than in water, whereas the work of cavity creation is larger in water than in cyclohexane. Therefore, both terms contrast the transfer of cyclohexane from pure liquid phase to water, determining its hydrophobicity. This mechanism qualitatively corresponds to that operative in the case of benzene.Key words: hydrophobicity, cavity creation, excluded volume, dispersive interactions, enthalpy–entropy compensation.