The solubility of gaseous hydrocarbons and the hydrophobic effect in aquaeous solutions of alkyl-ureas

Abstract

The solubility of methane and ethane in aqueous solutions of some alkylderivatives of urea have been studied in order to evaluate the hydrophobic effect according to the Ben-Naim model [1]. The cosolutes used are monomethylurea (MMU), monoethylurea (MEU), symmetric dimethylurea (1,3-DMU) and symmetric diethylurea (1,3-DEU). The study is of potential interest for its biochemical and bio-physical implications [1, 2]. In fact, hydrocarbons can be considered as model molecules of some of the side chains of proteins and, at the same time, it is known the capability of alkylureas in denaturing globular proteins [3–5]. The standard free energies of solution for the gases are deduced from the Ostwald absorption coefficient γ s: ▪ From the temperature derivatives of the free energies, the standard enthalpies and entropies of solubilization are also obtained. It is already known that the last two quantities are negative for the apolar gases in water, and we found that the addition of the studied series of cosolutes do not introduce drastic variations in this behaviour. The signs of the enthalpy and entropy were ascribed [6] to an enhancement of water structure, promoted by the apolar gases. A quantitative scale of comparison for the intensities of the hydrophobic interactions was proposed by Ben-Naim and coworkers [1]. The mechano-statistical treatment leads, with certain assumptions, to the simple relation: ▪ where Δμ° Et and Δμ° Me are the free energies associated with the process of transfer, at constant P and T, of two molecules of methane from fixed positions at infinite distance to a defined distance R 1 in the presence of the solvent respect to the vacuum. The charges of δ G HI with the concentration of the cosolutes (alkylureas) and with the temperature, hence the signs and values of the corresponding δ H HI and δS HI, are analyzed and compared with the free energies of transfer Δμ° tr. Qualitative conclusions are tried on the mechanisms ruling the complex phenomenon of the solubilization of gaseous hydro-carbons in aqueous mixed solvents and on the self and cross interactions among the different species in solution.