Thermodynamic study on phase transition in adsorbed film of fluoroalkanol at the hexane/water interface. 7. Pressure effect on the miscibility in the adsorbed film of alkanol and fluoroalkanol mixture

The Miscibility of 1-icosanol (C₂₀OH) and 1,1,2,2-tetrahydroheptadecafluorodecanol (FC₁₀OH) in the adsorbed film at the hexane/water interface was studied by measuring the pressure dependence of interfacial tension at various compositions and analyzing the experimental results thermodynamically. The criterion of ideal mixing in the adsorbed film with respect to the volume of adsorption Δ_w^Hv was constructed and used to examine the nonideal mixing of C₂₀OH and FC₁₀OH in the adsorbed film. It is found that the Δ_w^Hv value deviates positively from the ideal mixing line, and therefore the mixing of C₂₀OH and FC₁₀OH molecules causes a larger increase in volume in the adsorbed film than in the bulk solution. Furthermore, the excess volume of adsorption v^H,E was positive and its magnitude was comparable to that of the liquid alkane and perfluoroalkane mixture. This result is probably due to the increase in the occupied area of individual components at the interface. To gain a better understanding of the miscibility of C₂₀OH and FC₁₀OH in the adsorbed film, the phase diagram of adsorption (PDA) was constructed at various pressures. We found that these alcohol molecules become less miscible and therefore the activity coefficients of alcohols become larger in the expanded state with increasing pressure. This is in accord with the positive v^H,E value. Furthermore the C₂₀OH and FC₁₀OH molecules were practically immiscible in the condensed state at high pressure.