Temperature Effect on the Adsorption of Florooctanols at the Hexane/Water Interface

摘要

The interfacial tensions of the hexane solution of fluorooctanols (1,1,2,2-tetrahydrotridecafluorooctanol, TFC_8OH, and 1,1-dihydropentadecafluorooctanol, DFC_8OH) against water were measured as a function of temperatue and molality under atmospheric pressure. By drawing the interfacial pressure #pi# vs mean area per adsorbed molecule A curves, it was concluded that the adsorbed film of TFC_8OH exhibits a first-order phase transition between the gaseous and expanded states and that of DFC_8Oh shows the two types of phae trnasitions 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 TCF_8OH and DFC_8OH shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the #beta#-carbon of TFC_8OH. Furthermore, the difference in the transition pressure between DFC_8OH and TFC_(10)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 between hydrophoibic chains and the dipole moment of their hydroxyl group. The partial molar entropy s_s~H-s_s~O and u_s~H-u_sO changes of adsorption were evaluated and compared to those of TFC_(10)OH. The s_s~h-s_s~O value is negatie and therefore alcohol molecules have smaller entrpy at the interface than in the solution, which is attributable to the orientation of the molecules at he interface. The phase transition from the expanded to the condensed state in the adsorbed TFC_(10)OH film causes larege decrease in partial molar entropy than that in the DFC_8OH one. This may arise from the larger partial molar entropy of TFC_(10)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_(10)OH due to the stronger attractive interaction in the condensed state than that of DFC_8OH moleules. The u_s~H-u_s~O value is less negatie for DFC_8OH than for TFC_(10)OH and therefore the energetical stabilization of DFC_8OH accompanie by the adsorption from the solutioon is les than that of TFC_(10)OH. Furthermore, it was concluded that the DFC_8OH molecules are stabilized by forming the condensed film at the interface because of the strong molecular interaction between them, and the FCT_8OH molecules form mainly tetramers in the hexane solution to lwer the energetical state of the system.