Study of perfluorooctyl-n-alkanes monolayers at the air-water interface

A series of semifluorinated n-alkanes (SFA), of the general formula: F(CF2)m(CH2)nH, (abbreviated as FmHn) where m = 8, 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 visualization of the film texture and estimation of the monolayer thickness at different stages of compression. Although homologues with short hydrogenated part (F8H8 and F8H10) are not stable at the surface and desorb upon compression, higher homologues (n ≥ 13) are good film-forming materials. The negative sign of the measured surface potential, ΔV, evidences for the orientation of all the investigated molecules, regardless the length of the hydrogenated unit, with their perfluorinated parts exposed towards the air. The analysis of the direction of the molecular dipole moment with 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 35° to the surface normal. The film thickness, inferred from the relative intensity measurements together with BAM images prove monomolecular structure of SFA spread on the water surface.