Molecular orientation and multilayer formation in the adsorbed film of 1H,1H,10H,10H-perfluorodecane-1,10-diol at the hexane/water interface; Pressure effect on the adsorption of fluoroalkane-diol

The adsorption of 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC ₁₀diol) at the hexane solution/water interface was investigated by the measurement of interfacial tension γ as a function of pressure p and concentration m1 and the thermodynamic data analysis. The results obtained were compared with those of 1H,1H,2H,2Hperfluorodecanol (TFC₁₀OH) in order to clarify the effect of molecular orientation on the structure and property of the adsorbed film from the viewpoint of volume change of adsorption. The interfacial pressure π versus mean area per adsorbed molecule A curve revealed two types of phase transitions among expanded, parallel condensed, and multilayer states. The A value in the condensed state and the transition pressure between the expanded and condensed states were larger for FC ₁₀diol than for TFC₁₀OH, which manifests the different molecular orientation that the dispersion interaction between hydrophobic chains is weaker in the parallel orientation of FC₁₀diol than in the perpendicular orientation of TFC₁₀OH. The partial molar volume of FC₁₀diol in the condensed state ν₁ ^H.C is slightly larger than that of TFC₁₀OH, although the partial molar volume in the hexane solution is much smaller for FC₁₀diol than for TFC₁₀OH. This supports the view that the fluorocarbon chains of FC₁₀diol remain in their contact with hexane even in the condensed film because of the parallel molecular orientation. The partial molar volume in the multilayer ν₁ ^H,M was very close to the molar volume of solid FC₁₀diol ν₁^S and smaller than that of ν₁^H.C at the condensed-multilayer phase transition, and increased gradually with molecular piling. This substantiates that FC₁₀diol molecules are densely packed in a first few layers just above the phase transition and a little loosely packed in the upper layers of the multilayer with increasing molecular piling. Furthermore, the volume change associated with adsorption from the solid FC₁₀diol Δν(S) evaluated from the γ versus p curve under the existence of solid deposit was positive and showed a minimum against concentration for the multilayer state. This is primarily due to the minimum in interfacial density at the solubility limit Γ₁^H,S and thus due to the minimum in ν₁^H,M.