Bengt Kronberg

General thermodynamic analysis of the dissolution of non-polar molecules into water. Origin of hydrophobicity

The Gibbs energy, enthalpy, entropy and heat capacity of transfer from the pure non-polar liquid into water are analysed in detail. It is found that if the combinatorial contribution to the Gibbs energy and entropy of transfer is subtracted from the experimental values, all non-polar solutes in water behave in a universal manner, i.e. all of their thermodynamic transfer functions can be studied with their molecular surface area as the only parameter. This is illustrated with the alkylbenzene series, for which experimental Gibbs energies of transfer in a wide temperature range have been obtained recently. A new interpretation scheme for the thermodynamic transfer functions is presented and contrasted with that due to Privalov and Gill. It is considered that water molecules around the solute undergo a relaxation process which lowers the Gibbs energy, enthalpy and entropy of the system and is responsible for the large heat capacity of transfer. This relaxation process is described here using a two-state model for water molecules obtained from first principles. The negative relaxation contribution to the Gibbs energy promotes solubility, but is overcome by a large positive contribution arising from the creation of a cavity in water and the large differences between solute–solute, water–water and solute–water interactions. The origin of hydrophobicity lies then in the high cohesive energy of water. The proposed interpretation scheme is used to (a) predict the solubility of alkanes in water, (b) understand the origin of the solubility minimum appearing in aqueous solutions of non-polar solutes, (c) rationalize the experimental finding that the enthalpy of transfer becomes zero in a narrow temperature range for many non-polar solutes, (d) discuss the significance of entropy of transfer vs. heat capacity of transfer plots often used to understand the nature of the hydrophobicity of non-polar solutes and proteins, and (e) account for the expected change in sign (with temperature) of the water proton NMR chemical shifts discussed earlier in the literature.

Thermodynamics of the hydration of non-polar substances

We re-examine the numerical value and physical significance of T(S) the temperature where the entropy of transfer Delta(L)(W)S from the pure hydrocarbon liquid into water is zero. It is shown that the numerical value of T(S) depends on the convention adopted for calculating Delta(L)(W)G from solubility data at 25 degrees C and on the Delta(L)(W)C(P) fitting function. It is concluded that the interpretation of T(S) as the temperature where hydration ceases cannot be sustained. As previously reported [R.L. Baldwin. N. Muller, Proc. Natl. Acad. Sci. USA, 89 (1992) 7110], hydration must vanish at a temperature T > T(S), where its experimental manifestation, i.e., Delta(L)(W)C(P), is zero. We discuss the concept of water relaxation around a non-polar solute molecule and its relation to the hydration process.

Accurate measurements of solubility and thermodynamic transfer quantities using reversed-phase liquid-liquid chromatography

Abstract A reversed-phase high-performance liquid chromatographic system, consisting of polydimethylsiloxane coated on non-porous glass beads as the stationary phase and pure water as the mobile phase, was used to measure the absolute solubility and the temperature dependence of the solubility of a series of alkylbenzenes in water in the temperature range 0–80°C. The system and the method of analysis provide accurate values of the molar free energy, enthalpy, entropy and reasonable values of the heat capacity of transfer of the alkylbenzenes from their own liquids into water. The thermodynamic data were analysed in terms of the Flory-Huggins theory giving combinatorial and non-combinatorial, i.e. , interactional, contributions to the free energy of transfer. All the data were found to agree very well with literature values. The success of the system is attributed to the liquid nature of the stationary phase, the low surface area-to-volume ratio offered by the support material chosen and the availability of the absolute value of the volume of the stationary phase to calculate the phase ratio.

Thermodynamics of aliphatic and aromatic hydrocarbons in water

Makhatadze and Privalov have analyzed the thermodynamics of transfer of aliphatic and aromatic hydrocarbons from the gas phase into water. Finding that the hydration free energy of aliphatic and aromatic hydrocarbons have different signs, they conclude that the mechanism causing hydrophobicity of these solutes is of a different nature. Here, we offer an alternative analysis of the dissolution of these non-polar compounds into water based on a recently published interpretation scheme for thermodynamic transfer functions. Our analysis shows that the hydrophobicity of aromatic and aliphatic hydrocarbons is qualitatively the same, i.e. its causes are the same namely the extremely high cohesive energy of water which overcomes the favorable solute-solute and solute-water interactions. However, both analyses conclude that the experimentally observed quantitative difference between the interactions of water with aliphatic and aromatic hydrocarbons, can be assigned to the formation of aromatic ring-water H-bonds.