Oil/Water Partitioning Has a Different Thermodynamic Signature When the Oil Solvent Chains Are Aligned Than When They Are Amorphous

The hydrophobic effect has been widely studied through oil/water partitioning experiments. The “signature” of hydrophobicity is a large negative entropy at room temperature and a large positive heat capacity upon transferring a nonpolar solute into water. These unusual thermodynamic properties are usually attributed to the water phase. Here we show that we can completely change the thermodynamics by regulating a property of the oil phase, namely the degree of alkyl chain alignment. In reversed-phase liquid chromatography experiments, we measured the temperature-dependent partition coefficients of the 20 natural amino acids as a function of the surface bonding density of the alkyl chains, which controls the degree of alkyl chain alignment. We find that the thermodynamics of partitioning amino acids into grafted aligned-chain oils is very different than into bulk-phase oils: it is enthalpy-driven at room temperature, and the heat capacity of transfer is determined by the bonding density of the stationary-phase chains. We suggest a model whereby solutes may be squeezed toward the ends of grafted aligned alkyl chains with increasing temperature. This model may also contribute toward an explanation for the “nonclassical hydrophobic effect” or “bilayer effect” that has been observed for solute partitioning into lipid bilayer membranes.