Langmuir monolayers characteristic of (perfluorodecyl)-alkanes

A series of semifluorinated n-alkanes (SFAs) of the general formula F(CF2)(m)(CH2)(n)H, (in short FmHn) where m = 10 and n = 6-20, have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were obtained under a variety of experimental conditions such as spreading volume, subphase temperature, and compression speed. The Langmuir monolayer experiments have been complemented with Brewster angle microscopy results which enabled both direct visualization of the monolayers structure and estimation of the monolayer thickness at different stages of compression. Our results show that these "nonclassical" film-forming materials, which are completely hydrophobic in nature and do not possess any polar group in their structure, are capable of monolayer fonnation at the air/water interface. It has been observed that with the increase in the molecule's length, its stability at the free water surface increases. The negative sign of the measured surface potential, DeltaV, proves that SFA molecules are oriented at the air/water interface with their perfluorinated parts directed toward the air. The effective dipole moments reach the value of -0.65 +/- 0.1 D at the minimum for all stable SFAs. The analysis of the direction of the molecular dipole moment in respect to the main axis enabled us to estimate that the minimum effective dipole moment is achieved for a molecule oriented at the angle of about 35degrees to the surface normal. The relative intensity measurements allow one to conclude that film molecules are tilted in respect to the surface normal at the vicinity of collapse.