Synthesis and langmuir isotherms of difluorostearic acids

The CF2 containing 6,6- and 10,10-difluorooctadecanoic acids 7 and 8, respectively have been synthesised and their Langmuir isotherms evaluated. In marked contrast to the behaviour of octadecanoic acid (stearic acid) 1 it is demonstrated that spreading on an aqueous subphase generates an unstable monolayer which reorganises to a more stable bilayer structure. Grazing incidence X-ray scattering and ellipsometric measurements after deposition of 8 from the subphase onto a silicon wafer, confirmed a bilayer structure. These observations reinforce our earlier conclusions with 12,12-difluorooctadecanoic acid that substitution of CH2 by CF2 leads to increased conformational flexibility of a hydrocarbon chain.