Semifluorinated alcohols in Langmuir monolayers--a comparative study

A series of semifluorinated alcohols differing in the proportion of the perfluorinated to hydrogenated chains length was synthesized and investigated in Langmuir monolayers using surface pressure and surface potential measurements. All the investigated semifluorinated alcohols were found to be capable of stable floating monolayer formation. The stability of monolayers was found to be higher upon increasing the length of the perfluorinated segment. A lower stability of the monolayers from alcohols having shorter perfluorinated fragment was attributed to the aggregation process, which was visualized with Brewster angle microscopy (BAM). Most condensed monolayers were formed by compounds with longer perfluorinated moiety, whereas monolayers composed by molecules with an iso-branched perfluorinated segment were found to be more expanded. The change of electric surface potential was negative along the whole compression. The maximum absolute values of DeltaV varies, depending on the number of CF(2) groups, from ca. -400 mV for F6H10OH to ca. -700 mV for F10H10OH. The dipole moments of free molecules were calculated with Hyperchem, and the obtained values were approximately the same (within the experimental error), i.e., 2.8D for all the investigated molecules, independently on the perfluorinated fragment length. The dipole moment vector was found to be virtually aligned to the main molecular axis for the studied compounds. Therefore, the observed differences in the measured values of DeltaV can result from a different dielectric permittivity of a particular monolayer.