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 (FC10diol) at the hexane solution/water interface was investigated by the measurement of interfacial tension gamma 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,2H-perfluorodecanol (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 pi 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 FC10diol 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 nu1H.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 nu1H,M was very close to the molar volume of solid FC10diol nu1S and smaller than that of nu1H.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 Delta(nu)(S) evaluated from the gamma 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 Gamma1H,S and thus due to the minimum in nu1H,M.