The dihedral angle of an alcohol lens at the air/water interface and the three kinds of interfacial tensions of air/1-octanol/water and air/1-decanol/water systems have been measured as a function of temperature from 288.15 to 313.15 K at 2.5 K intervals under atmospheric pressure. In order to measure a dihedral angle, the new experimental apparatus was constructed and the new procedure was adopted. By comparing the dihedral angles measured with those calculated by applying Neumann's relations to the interfacial tension values, it was concluded that the dihedral angle measurement was performed with a satisfactory accuracy. The properties of the interfacial film and also the occurrence of the intruding of water phase on the air/alcohol interface were discussed.
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 π 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 π vs A curve between TFC8OH and DFC8OH shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the β-carbon of TFC8OH. Furthermore, the difference in the transition pressure between DFC8OH and TFC10OH (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 s̄ s H − s s O and energy ū s H − u s O changes of adsorption were evaluated and compared to those of TFC10OH. The s̄ s H − 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 ū s H − 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.
The dihedral angle of an alcohol lens floating on the air/water interface and the three kinds of interfacial tensions of air/1-undecanol/water and air/1-dodecanol/water systems were measured as a function of temperature under atmospheric pressure. By applying the thermodynamics of interfaces to the experimental results of the interfacial tension measurement, it was found that the phase transitions between the expanded and the condensed states take place in the interfaces. It was found that there are break points on the dihedral angle versus temperature curves corresponding to the phase transitions of the interfacial films. The mutual relation among the states and the phase transition of the interfacial film, the wetting behavior, and the intruding phenomenon of the water phase on the air/alcohol interface was discussed. It was shown that the dihedral angles measured coincide with those calculated by Neumann's equation with a satisfactory accuracy except at very low temperatures. The discrepancy at the low temperatures was proved to be attributable to the meniscus of the air/water interface.
The phase transition in the adsorbed film of 1-icosanol (C20OH) and 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH) mixture at the hexane/water interface was studied by measuring the pressure dependence of interfacial tension at a specific composition of FC10OH X 2 = 0.280 and 298.15 K and evaluating the volume change of adsorption. By drawing the interfacial pressure π vs mean area per adsorbed molecule A curve, it was clarified that three types of first-order phase transitions (gaseous−expanded, expanded−FC10OH condensed, and FC10OH condensed−C20OH condensed) take place in the adsorbed film. The phase transition from the FC10OH condensed to the C20OH condensed state was accompanied by a decrease of the volume change associated with adsorption Δν. By evaluating the partial molar volume change of adsorption, − , and comparing its pressure dependence between the FC10OH condensed and C20OH condensed states, it was suggested that the pressure dependence of the partial molar volume is in the order of ∂ /∂p > ∂ /∂p > ∂ /∂p ≈ ∂ /∂p. Furthermore, it was found that the transition pressure πeq of the FC10OH condensed−C20OH condensed transition (second transition) increases and the corresponding one of the expanded−FC10OH condensed transition (first transition) decreases with increasing pressure. This difference was explained by means of the − values at the phase transition point.
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