Perfluoroalkyl-n-eicosanes at the Air-Water Interface-A Monolayer Study

Perfluoroalkyl-n-eicosanes of the general formula F(CF2)m(CH2)20H, where m = 4,6, 8,10,12 were spread at the air/water as Langmuir monolayers and studied at different experimental conditions, such as spreading volume, subphase temperature and compression speed. The Langmuir monolayer experiments (-A isotherms) have been complemented with quantitative Brewster angle microscopy results, which enabled estimation of the film thickness at different stages of compression. Although the investigated molecules do not possess any polar group and are purely hydrophobic, they form stable monomolecular layers at the free water surface. The negative sign of the measured surface potential, Delta V, evidences for the orientation of the molecules with their perfluorinated parts exposed towards the air, independently of the length of the perfluorinated moiety. The relative intensity measurements allow one to conclude that the molecules with shorter perfluorinated part (F4H20, F6H20 and F8H20) are oriented almost vertically (in respect to the interface) in the vicinity of film collapse, while F10H20 and F12H20 are tilted to the water surface.