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 parti tioning experiments. The ''signature'' of hydrophobicity is a large ne gative entropy at room temperature and a large positive heat capacity upon transferring a nonpolar solute into water. These unusual thermody namic properties are usually attributed to the water phase. Here we sh ow that we can completely change the thermodynamics by regulating a pr operty of the oil phase, namely the degree of alkyl chain alignment, I n reversed-phase liquid chromatography experiments, we measured the te mperature-dependent partition coefficients of the 20 natural amino aci ds as a function of the surface bonding density of the alkyl chains, w hich controls the degree of alkyl chain alignment. We find that the th ermodynamics of partitioning amino acids into grafted aligned-chain oi ls is very different than into bulk-phase oils: it is enthalpy-driven at room temperature, and the heat capacity of transfer is determined b y the bonding density of the stationary-phase chains. We suggest a mod el whereby solutes may be squeezed toward the ends of grafted aligned alkyl chains with increasing temperature. This model may also contribu te toward an explanation for the ''nonclassical hydrophobic effect'' o r ''bilayer effect'' that has been observed for solute partitioning in to lipid bilayer membranes.