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We show that using mole fractions x s instead of activities a s in the Gibbs equation for analyzing surface tension curves for onset of amphiphilic association may be open for discussion. First, we determined the activity coefficients for the simple binary water-ethanol and water-n-propanol systems from vapor pressure measurements. The activity coefficients are neither unity nor are they constant in the range where the surface tensions σ of the binary systems decrease from the water value down to the value of the pure alcohols. Second, a break in the σ vs ln x s curves disappears when proper σ vs ln a s curves are constructed. The implication for determining critical micelle concentrations and headgroup areas of surfactants from surface tension curves is addressed.

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The interfacial tension γ of hexane solution of 1,1,2,2-tetrahydroheptadecafluorodecanol (FC 10 OH) against water was measured as a function of pressure p and molality m 1 at 298.15 K. The break points were observed on the γ vs p and γ vs m 1 curves. The interfacial density Γ 1 H increases and the volume change associated with the adsorption ∆ν decreases with increasing m 1 , and they change discontinuously at concentrations of the break points. By drawing the interfacial pressure π vs area A curve, it is concluded that two types of phase transitions take place from the gaseous to the expanded state and from the expanded to the condensed one in the adsorbed film of FC 10 OH. By examining our results by the Clapeyron type equations, the order of phase transition was considered first. The partial molar volume change associated with the adsorption of FC 10 OH was estimated and compared with that of 1-octadecanol (C 18 OH). It was found that FC 10 OH molecules were accompanied with the larger decrease in partial molar volume inherent in the interface than C 18 OH molecules when the phase transition takes place from the expanded to the condensed state in the adsorbed film. This suggests that the microscopic circumstance in the adsorbed film affects appreciably the volume change of adsorption. IntroductionIt is generally known that the interaction between fluorocarbon and hydrocarbon chains is weak and therefore a fluorocarbon surfactant mixes nonideally or often does not mix with a hydrocarbon surfactant in the adsorbed film and micelle. 1-6 Thus, the study on the adsorbed films of fluorocarbon compounds at a hydrocarbon oil/water interface is expected to be highly useful for understanding the interaction between fluorocarbon and hydrocarbon chains. In our latest paper, 7 we have studied the adsorbed film of 1,1,2,2-tetrahydrohenicosafluorododecanol (FC 12 OH) at the hexane/water interface from the viewpoints of entropy ∆s and energy ∆u of adsorption and claimed, on the basis of the discontinuous changes of ∆s and ∆u values, that the phase transition takes place at the interface. [8][9][10][11][12][13][14][15][16] Furthermore it was confirmed thermodynamically that the transition is the first order one from the gaseous to the condensed state. 7 In order to examine more closely the adsorption of fluoroalkanol from the viewpoint of the volume, this time we employed 1,1,2,2-tetrahydroheptadecafluorodecanol (FC 10 OH), because the deposit of FC 12 OH was observed in the measurement cell at a high-pressure region, and studied the pressure effect on its adsorption at the hexane/water interface. We will report the adsorption behavior of FC 12 OH in the absence and also in the presence of the deposit in this series. 17 The interfacial tension of hexane solution of FC 10 OH against water was measured as a function of pressure and molality at 298.15 K. The interfacial density and the volume change associated with the adsorption were evaluated by analyzing the experimental results thermodynamically. Experimental SectionFC 10 OH ...