
TY  - JOUR
A1  - Oakenfull, DG 
A1  - Fenwick, DE 
T1  - Thermodynamics and mechanism of hydrophobic interaction
JF  - Australian Journal of Chemistry
JO  - Aust. J. Chem.
Y1  - 1977
VL  - 30
IS  - 4
SP  - 741
EP  - 752
KW  - 
UR  - https://doi.org/10.1071/CH9770741 
N2  - .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 (&#916;G&#176;HI) over the temperature range 278-328 K. The value of &#916;G&#176;HI becomes more negative (stronger hydrophobic interaction) with increasing temperature. The temperature coefficient of &#916;G&#176;HI was used to calculate the enthalpy (&#916;H&#176;HI) and entropy (&#916;S&#176;HI) of hydrophobic interaction. At low temperature the entropic contribution to the free energy is the larger but &#916;H&#176;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 &#177; 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. 
ER  -

