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

Abstract

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.