Structural Approach to the Solvent Power of Water for Hydrocarbons; Urea as a Structure Breaker

Abstract

The finding of Wetlaufer et al., that addition of urea to water increases the (mole fraction) solubility of hydrocarbon gases (except methane) while making them dissolve with a smaller evolution of heat, is interpreted as a primarily statistical phenomenon. For this purpose, it is treated in terms of a skeleton model in which not only is water represented as a two-species mixture of dense and bulky constituents but the dissolved hydrocarbon is represented as dissolving separately in these constituents, as if it were distributed between two phases, i.e., between a quasiclathrate solution in the bulky constituent and a quasilattice or “regular” solution in the dense one. Added urea is pictured as being able to enter only one of the solutions, the quasilattice one in the dense water, with the result that it acts as a structure breaker. A statistical-thermodynamic analysis of this model leads to equations for the chemical potentials and the partial molal enthalpies and entropies of the hydrocarbon and the urea solutes, and it is found that very simple assumptions suffice to give calculated values for log γ and L̄ of urea, in binary aqueous solutions, in good agreement with experiment. The choice of parameters to fit the results of the Wetlaufer transfer experiments (of hydrocarbon from water to urea-water) leads to calculated values for the partial molal enthalpies and entropies of the hydrocarbons in their hypothetical binary solutions in the bulky (quasiclathrate solution) and dense (quasilattice solution) waters and comparison of these quantities with properties of appropriate real physical systems makes them appear to be acceptable. One inference from this successful result is that the hypothesis of Frank and Evans, that the “extra” negativeness of the S̄2 for hydrocarbon gases in water arises from a structure shift in the latter, may not be unique, but that a similar effect might also arise from the fact that water received some of the hydrocarbon solute into a different type of environment from what it would find in a “normal” solvent.