Miscibility and multilayer formation of fluoroalkane-α,ω-Diol mixtures at the air/water interface

The surface tension γ of the aqueous solution of 1H,1H,6H,6H- perfluorohexane-1,6-diol (FC₆diol) and 1H,1H,8H,8H-perfluorooctane-1, 8-diol (FC₈diol) mixtures was measured as a function of total molality m and composition of FC₈diol in the mixture X₂ at 293.15 K under atmospheric pressure. The γ vs m curves except at X ₂ = 0 and 0.05 have a distinct break point due to a phase transition in the adsorbed film. The surface pressure ? vs mean area per adsorbed molecule A curves consist of two parts connected by a discontinuous change. The curve was almost vertical just below the phase transition, and the variation of the A value with film composition X2H was linear corresponding to the fact that FC₆diol and FC₈diol molecules orient parallel to the surface and are densely packed with the same areas of individual condensed films. Above the phase transition, the A value further decreases to around 0.12-0.19 nm² which is much smaller than the cross-sectional area of the fluorocarbon chain, indicating the multilayer formation at the surface. The phase diagram of adsorption (PDA) in the condensed film showed that the m vs film composition X2H curve is almost linear and the excess Gibbs energy of adsorption g^HE/RT is at most 0.01, manifesting the ideal mixing of molecules. This is in contrast to a positive deviation (g^HE/RT ∼0.12) observed in the condensed film of the mixture of 1H,1H,2H,2H- perfluorodecanol (FC₁₀OH) and 1H,1H,2H,2H-perfluorododecanol (FC ₁₂OH) with perpendicular molecular orientation. The loss of dispersion interaction between different species having different chain lengths is more appreciable in the perpendicular condensed films and thus leads to less miscibility of FC₁₀OH and FC₁₂OH. In the parallel condensed film, on the other hand, FC₆diol and FC₈diol molecules can arrange their position as close as possible to minimize the loss of dispersion interaction. The X2H value in the multilayer is close to unity, and thus, the multilayer consists of almost FC₈diol molecules which form a multilayer in the single-component system. Furthermore, the condensed monolayer-multilayer phase transition was accompanied by a large increase in surface density of FC₈diol and a small decrease in that of FC ₆diol, indicating that FC₈diol molecules pile preferentially to form a multilayer.