.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.
Solvent and Salt Effect on Lithium Ion Solvation and Contact Ion Pair Formation in Organic Carbonates: A Quantum Chemical Perspective
