Miscibility and Multilayer Formation of Fluoroalkane-α,ω-Diol Mixtures at the Air/Water Interface

The surface tension gamma of the aqueous solution of 1H,1H,6H,6H-perfluorohexane-1,6-diol (FC(6)diol) and 1H,1H,8H,8H-perfluorooctane-1,8-diol (FC(8)diol) mixtures was measured as a function of total molality m and composition of FC(8)diol in the mixture X(2) at 293.15 K under atmospheric pressure. The gamma vs in 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 pi 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 X(2)(H) was linear corresponding to the fact that FC(6)diol and FC(8)diol 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 nm(2) 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 at vs film composition X(2)(H) 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 similar to 0.12) observed in the condensed film of the mixture of 1H,1H,2H,2H-perfluorodecanol (FC(10)OH) and 1H,1H,2H,2H-perfluorododecanol (FC(12)OH) 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 FC(10)OH and FC(12)OH. In the parallel condensed film, on the other hand, FC(6)diol and FC(8)diol molecules can arrange their position as close as possible to minimize the loss of dispersion interaction. The X(2)(H) value in the multilayer is close to unity, and thus, the multilayer consists of almost FC(8)diol 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 FC(8)diol and a small decrease in that of FC(6)diol, indicating that FC(8)diol molecules pile preferentially to form a multilayer.