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, TFC<INF>8</INF>OH, and 1,1-dihydropentadecafluorooctanol, DFC<INF>8</INF>OH) against water were measured as a function of temperature and molality under atmospheric pressure. By drawing the interfacial pressure π vs mean area per adsorbed molecule A curves, it was concluded that the adsorbed film of TFC<INF>8</INF>OH exhibits a first-order phase transition between the gaseous and expanded states and that of DFC<INF>8</INF>OH 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 π vs A curve between TFC<INF>8</INF>OH and DFC<INF>8</INF>OH shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the β-carbon of TFC<INF>8</INF>OH. Furthermore, the difference in the transition pressure between DFC<INF>8</INF>OH and TFC<INF>10</INF>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 &smacr;<INF>s</INF>^H − s<INF>s</INF>^O and energy ū<INF>s</INF>^H − u<INF>s</INF>^O changes of adsorption were evaluated and compared to those of TFC<INF>10</INF>OH. The &smacr;<INF>s</INF>^H − s<INF>s</INF>^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 TFC<INF>10</INF>OH film causes larger decrease in partial molar entropy than that in the DFC<INF>8</INF>OH one. This may arise from the larger partial molar entropy of TFC<INF>10</INF>OH molecules due to the larger entropy of mixing of longer fluorocarbon chain with hexane in the expanded state and the smaller entropy of TFC<INF>10</INF>OH due to the stronger attractive interaction in the condensed state than that of DFC<INF>8</INF>OH molecules. The ū<INF>s</INF>^H − u<INF>s</INF>^O value is less negative for DFC<INF>8</INF>OH than for TFC<INF>10</INF>OH and therefore the energetical stabilization of DFC<INF>8</INF>OH accompanied by the adsorption from the solution is less than that of TFC<INF>10</INF>OH. Furthermore, it was concluded that the DFC<INF>8</INF>OH molecules are stabilized by forming the condensed film at the interface because of the strong molecular interaction between them, and the TFC<INF>8</INF>OH molecules form mainly tetramers in the hexane solution to lower the energetical state of the system.