Langmuir, Vol.17, No.26, 8098-8103, 2001
Temperature effect on the adsorption of fluorooctanols at the hexane/water interface
The interfacial tensions of the hexane solution of fluorooctanols (1,1,2,2-tetrahydrotridecafluorooctanol, TFC8OH, and 1,1-dihydropentadecafluorooctanol, DFC8OH) against water were measured as a function of temperature and molality under atmospheric pressure. By drawing the interfacial pressure a vs mean area per adsorbed molecule A curves, it was concluded that the adsorbed film of TFC8OH exhibits a first-order phase transition between the gaseous and expanded states and that of DFC8OH shows the two types of phase transitions from the gaseous to the expanded state and from the expanded to the condensed one at the hexane/water interface. The comparison of the pi vs A curve between TFC8OH and DFC8OH shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the beta -carbon of TFC8OH. Furthermore, the difference in the transition pressure between DFC8OH and TFC10-OH (1,1,2,2-tetrahydroheptadecafluorodecanol) is explained by the differences in London dispersion force between hydrophobic chains and the dipole moment of their hydroxyl group. The partial molar entropy (s) over bar (H)(s) -s(s)(O) and energy (u) over bar (H)(s) - u(s)(O) changes of adsorption were evaluated and compared to those of TFC10OH. The (s) over bar (H)(s) - s(s)(O) value is negative and therefore alcohol molecules have smaller entropy at the interface than in the solution, which is attributable to the orientation of the molecules at the interface. The phase transition from the expanded to the condensed state in the adsorbed TFC10OH film causes larger decrease in partial molar entropy than that in the DFC8OH one. This may arise from the larger partial molar entropy of TFC10OH molecules due to the larger entropy of mixing of longer fluorocarbon chain with hexane in the expanded state and the smaller entropy of TFC10OH due to the stronger attractive interaction in the condensed state than that of DFC8OH molecules. The (u) over bar (H)(s) - u(s)(O) value is less negative for DFC8OH than for TFC10OH and therefore the energetical stabilization of DFC8OH accompanied by the adsorption from the solution is less than that of TFC10OH. Furthermore, it was concluded that the DFC8OH molecules are stabilized by forming the condensed film at the interface because of the strong molecular interaction between them, and the TFC8OH molecules form mainly tetramers in the hexane solution to lower the energetical state of the system.