Properties of Langmuir monolayers from semifluorinated alkanes

The aim of this study was to characterize several semifluorinated alkanes (SFA), of the general formula F(CF2)m(CH2)nH (in short FmHn), containing 25 carbon atoms in total (pentacosanes) differing in the m/n ratio, as Langmuir monolayers at the free water surface. The following compounds have been studied: F6H19, F8H17, F10H15 and F12H13. Surface pressure (π) and electric surface potential (ΔV) isotherms were recorded in addition to quantitative Brewster angle microscopy results. The negative sign of ΔV evidenced for the orientation of all the investigated semifluorinated pentacosanes, regardless the length of the hydrogenated segment, with their perfluorinated parts directed towards the air. As inferred from apparent dipole moment values and relative reflectivity results, the fluorinated pentacosanes with shorter perfluorinated fragment (F6H19 and F8H17) were found to be vertically oriented at the air/water interface, while those with longer perfluorinated moiety (F10H15 and F12H13) remain titled even in the vicinity of the film collapse.