Molecular orientation and miscibility of fluorinated α,ω-alkanediol and alcohol at the hexane/water interface

The interfacial tension γ of the hexane solution of 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol) and 1H,1H,2H,2H-perfluorododecanol (TFC12OH) mixture against water was measured as a function of the total molality m and composition of TFC12OH in the mixture X2 at 298.15 K under atmospheric pressure. The interfacial pressure π vs. mean area per adsorbed molecule A curves and the phase diagram of adsorption (PDA) were constructed in order to shed light on the effect of the molecular orientation on the state of and the miscibility of FC10diol and TFC12OH in the adsorbed film. The π vs. A curves demonstrated that the mixed system exhibits three kinds of film states, parallel-condensed, normal-condensed, and multilayer states, depending on m and X2. The PDA indicated that film composition X2H in the parallel-condensed state never exceeds 0.14 even when X2 is increased to 0.9. This limited miscibility is probably due to a microphase separation that FC10diol and TFC12OH molecules are not dispersed molecularly but the parallel-condensed domains of FC10diol and the normal-condensed domains of TFC12OH are mixed at the interface. In the normal-condensed state, on the other hand, the FC10diol molecules may stand upright and be mixed with TFC12OH molecules. The multilayer is much richer in FC10diol than in TFC12OH when the multilayer is evolved from the parallel-condensed monolayer below X2 = 0.9, while the fraction of TFC12OH increases very rapidly when it is evolved from the normal-condensed monolayer above X2 = 0.925. Below X2 = 0.9, the X2H value in the multilayer is similar to that in the parallel-condensed monolayer. Thus it is plausible that FC10diol molecules are preferentially piled with parallel orientation in the upper layer of the multilayer. Above X2 = 0.925, the multilayer may not be homogeneous but heterogeneous in thickness. These views were confirmed from the X2 dependence of molality mieq of each component at the phase transition points.