Effect of Molecular Orientation on Monolayer and Multilayer Formations of Fluorocarbon Alcohol and Fluorocarbon-α,ω-diol Mixture at the Hexane/water Interface

The effect of molecular orientation on the miscibility and structure of the adsorbed film of the 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol)-1H,1H,2H,2H-perfluorodecanol (FC10OH) mixture at the hexane/water interface were examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension and X-ray reflectivity at the hexane solution/water interface were measured as a function of total molality m and composition of FC10OH in the mixture X2 under atmospheric pressure at 298.15 K. The interfacial pressure vs mean area per molecule curves showed that two kinds of condensed monolayers (C1 and C2) and multilayer (M) states appeared depending on m and X2. In the pure component systems, it was found that FC10OH forms condensed monolayer in which the molecules orient almost normally to the interface, and FC10diol orients parallel and is densely packed in the condensed monolayer and then piles spontaneously to form multilayer. In the mixed system, the phase diagram of adsorption indicated that FC10OH molecules are richer in C2 than in C1 state. The X-ray reflectivity measurements manifest that the condensed monolayer below X2 = 0.985 is heterogeneous in which the normal- and parallel-oriented domains coexist at the interface (C1 state), and that above X2 = 0.985 seems to be homogeneous with normal molecular orientation (C2 state). The structure of M state depends on those of condensed monolayers, on which the molecules pile spontaneously. The heterogeneous structure in C1 state is compared to that previously observed in the mixed system of FC10diol-FC12OH (1H,1H,2H,2H-perfluorododecanol), where FC12OH has longer fluorocarbon chain length than FC10OH and is discussed in terms of domain line tension.