Thermodynamics of liquid-liquid partition and hydration of aliphatic ethers

Interaction between water and ethers in dilute aqueous solutions was investigated by water-heptane partition of twelve linear aliphatic ethers containing various number of carbon atoms (from 2 to 8), and one branched homologue. Standard free energy (Gibbs) and enthalpy of partition were calculated from partition constants and from their temperature dependence. The differences in the free energy and entropy result from different hydrophobic hydration of these amphiphilic solutes in the aqueous phase. The position of the oxygen atom in an ether molecule affects the hydrophilic-hydrophobic properties of the molecule. The methylene groups in the alpha and beta positions in respect to the oxygen atom are distinctly less hydrophobic than those more distant, therefore, different are their hydrophobic contributions into standard thermodynamic functions of hydration. The commonly accepted group contribution models, which assume equal contributions from the same fragments irrespectively of their position (additivity principle), should be revised.