Molecular Orientation and Multilayer Formation in the Adsorbed Film of 1H,1H,10H,10H-Perfluorodecane-1,10-diol at the Hexane/Water Interface; Temperature Effect on the Adsorption of Fluoroalkane-diol

Abstract

The adsorption of 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol) at the hexane/water interface was investigated by the measurement of temperature dependence of interfacial tension and the thermodynamic data analysis in order to know the effect of two hydroxyl groups at both ends of the hydrophobic chain and the rigidity of the hydrophobic chain on the adsorption of fluorocarbon alcohol at the interface. The curves of interfacial tension versus temperature and concentration show break points corresponding to the phase transitions in the adsorbed FC10diol film. The interfacial pressure versus mean area per adsorbed molecule curve shows three kinds of states connected by two discontinuous changes. The area value after the first phase transition is very close to the calculated cross-sectional area of the FC10diol molecule along its major axis, and thus the FC10diol molecules form a condensed monolayer with molecular orientation parallel to the interface. Another noticeable point is that the value after the second phase transition point decreases furthermore to 0.12 nm2, which is much smaller than the cross-sectional area of the fluorocarbon chain, 0.28 nm2, with increasing interfacial pressure. This suggests that FC10diol molecules pile spontaneously and successively form a multilayer above the second phase transition. Furthermore, the partial molar entropy and energy change of adsorption in the expanded and condensed states were evaluated and compared to those of 1H,1H,2H,2H-perfluorodecanol (TFC10OH), which orients almost perpendicular to the interface. In addition to the contact of two hydroxyl groups with hexane in the bulk solution, the results are explained by the dependence of partial molar entropy and energy at the interface on the following factors resulting from the parallel orientation of FC10diol at the interface; (a) hydrogen bonding of two hydroxyl groups with water molecules, (b) hydrogen bonding between two hydroxyl groups facing each other, and (c) the fluorocarbon chain−water contact. The adsorbed FC10diol film is stabilized by factors a and b, which overwhelm the energetic disadvantage caused by factor c. Furthermore, the entropy change of adsorption Δs in the multilayer is compared to the Δscal calculated on the assumption that the condensed monolayer piles to form the multilayer. It was suggested that FC10diol molecules are not so densely packed in the multilayer compared to the first condensed monolayer and therefore the multilayer is not simply formed by the piling of condensed monolayers.