Reconciling the Magnitude of the Microscopic and Macroscopic Hydrophobic Effects

The magnitude of the hydrophobic effect, as measured from the surface area dependence of the solubilities of hydrocarbons in water, is generally thought to be about 25 calories per mole per square angstrom (cal mol$^{-1}$ $^\circle{A}$^{-2}$). However, the surface tension at a hydrocarbon-water interface, which is a "macroscopic" measure of the hydrophobic effect, is ≈72 cal mol$^{-1}$ $^\circle{A}$^{-2}$. In an attempt to reconcile these values, alkane solubility data have been reevaluated to account for solute-solvent size differences, leading to a revised "microscopic" hydrophobic effect of 47 cal mol$^{-1}$ $^\circle{A}$^{-2}$. This value, when used in a simple geometric model for the curvature dependence of the hydrophobic effect, predicts a macroscopic alkane-water surface tension that is close to the macroscopic value.