Thermodynamics and mechanism of hydrophobic interaction

Abstract

.In the mixed solvent, 0.1 mole fraction ethanol-water, long-chain decyltrimethylammonium carboxylates form ion pairs. Ion-pair association constants (and hence the free energy of ion-pair formation) can be measured conductometrically. It is possible to separate the hydrophobic from the electrostatic contribution to the free energy of ion-pair formation by systematically varying the hydrocarbon chain length.

We report measurements of the free energy of hydrophobic interaction (ΔG°_HI) over the temperature range 278-328 K. The value of ΔG°_HI becomes more negative (stronger hydrophobic interaction) with increasing temperature. The temperature coefficient of ΔG°_HI was used to calculate the enthalpy (ΔH°_HI) and entropy (ΔS°_HI) of hydrophobic interaction. At low temperature the entropic contribution to the free energy is the larger but ΔH°_HI, dominates at temperatures above c. 324 K.

The volume change of hydrophobic interaction was similarly estimated from the volume change of ion-pair formation. We obtained values of apparent molar volume of the decyltrimethylammonium carboxylates (over a range of concentrations) from very precise density measurements. These could then be combined with the appropriate ion-pair association constant (from the conductance measurements) to give the partial molar volumes of the free ions and the ion pair. Hydrophobic interaction was found to be accompanied by a substantial increase in volume amounting to 10.2 ± 0.3 ml mol^-1 for each pair of interacting methylene groups.

Our results support the view that hydrophobic interaction occurs with a further ordering of water molecules over and above that which exists in the hydrophobic hydration layer surrounding an isolated hydrophobic molecule.