A competitive relationship between wetting of oil lens and condensed film formation of fluorinated alkanol at the air-water interface

We have previously studied the wetting behaviour of hexadecane (C16) lens on the dodecyltrimethylammonium bromide (DTAB) aqueous solution from interfacial tensiometry and ellipsometry. It is found that the first order wetting transition between the partial wetting, where C16 molecules do not spread on water surface, and pseudo-wetting, where wetting film of molecular thickness coexists with excess amount of C16 lens, was induced by the gaseous-expanded phase transition at the air-water adsorbed film. In this study, we adopted the same experimental procedure to C16 lens in the presence of 2-perfluorooctylethanol, (CF3(CF2)(7)(CH2)(2)OH; FC10OH) and the results obtained were compared with those obtained for hexane (C6) lens. In the FC10OH-C16 system, the phase transition from gaseous to condensed film states was occurred at the air-water interface as increasing FC10OH concentration (m). Furthermore, the wetting transition to the pseudo-partial wetting was hindered by the weak van der Waals interaction between C16 molecules and FC10OH molecules in the condensed film. On the other hand, in the FC10OH-C6 system, the air-water interface first took the expanded state and then formed condensed film as m increased. Since the C6 and FC10OH molecules can be mixed in the expanded film, they form the pseudo-partial wetting film at low m and then the dewetting transition was induced by the condensed film formation of FC10OH at the air-water interface. In both cases, alkane molecules were expelled from the air-water interface by the condensed film formation of FC10OH, therefore, we concluded that, to realise molecularly thick wetting film, the formation of the expanded film is necessary and the condensed film formation of fluorinated alkane compete with the pseudopartial wetting.