Enthalpy of transfer of -CH 2- between water and organic solvents or mixed aqueous organic solvent systems

Abstract

The - CH 2- moiety is the basic building block of many hydrophobic molecules and of the hydrophobic parts of amphiphilic molecules. Thus understanding the solvation of - CH 2- is central to understanding hydrophobic interactions. In the present paper we report the enthalpies of solution of n-alcohols, C 2H 5OH to C 5H 11OH, in water, methanol, ethanol, acetonitrile and their aqueous mixtures (and of C 8H 17OH in those systems where solubility permits). These, and literature data for these solutes in a range of non-aqueous solvents and in aqueous N,N-dimethylformamide solvent systems are used to calculate the average enthalpies of transfer of - CH 2-, Δ t H θ(CH 2) , from the organic solvents to water and to the mixed aqueous solvents. It is found that the Δ t H θ(CH 2) values between the organic solvents and water increase with the length of the alkyl chain. The Δ t H θ(CH 2) are close to zero and depend significantly on the organic solvent involved but are positive into the mixed aqueous solvents, passing through maxima in the predominately aqueous solvent compositions. The magnitudes of the maxima vary with the organic component of the mixed solvent, as do the compositions at which the maxima occur.