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 gHE/RT is at most 0.01, manifesting the ideal mixing of molecules. This is in contrast to a positive deviation (gHE/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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry