Thermodynamic Study on Phase Transition in Adsorbed Film of Fluoroalkanol at the Hexane/Water Interface. 6. Pressure Effect on the Phase Transition in the Adsorbed Film of Alkanol and Fluoroalkanol Mixture

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 X₂ = 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 FC₁₀-OH 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, ν̄^H_i-ν^O_i, 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 ∂ν^O₂/∂p > ∂ν^O₁/∂p > ∂ν̄^H₂/∂p ≈ ∂ν̄^H₂/∂p. Furthermore, it was found that the transition pressure π^eq 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 ν̄^H_i-ν^O_i values at the phase transition point.