Structure of the Langmuir monolayers with fluorinated ethyl amide and ethyl ester polar heads creating dipole potentials of opposite sign

This study experimentally checks our previous hypothesis (Petrov, J. G.; Polymeropoulos, E. E.; Moehwald, H. Langmuir-2007,23,2623) that different conformations of the fluorinated heads of RCONHCH2CF3 and RCOOCH2CF3 monolayers cause the opposite signs and the striking difference of 1.480 V between their surface potentials AV. In Situ X-ray diffraction at grazing incidence (GIXD) shows that both monolayers form orthorhombic lattices with closely packed chains tilted to the next-nearest neighbors in the RCONHCH2CF3 film and upright in the RCOOCH2CF3 monolayer. The packing of the chains in the plane perpendicular to them, which excludes the effect of the tilt, shows the same distance between the next-nearest neighbors, but significantly closer nearest neighbors in the RCONHCH2CF3 film. This difference implies a specific anisotropic attraction between the adjacent amide heads. IR reflection absorption spectroscopy (IRRAS) shows that the -CONHCH2CF3 heads have trans conformation and participate in H-bonds forming a -NH center dot center dot center dot O = C- lateral network. We speculate that such structure hinders the energetically optimal orientation of the hydrophobic -CH2CF3 terminals toward air, so that the C delta+-(F delta-)(3) dipoles at the monolayer/water boundary yield a strong positive contribution to AV. In contrast, most of the unbounded by H-bonds -COOCH2CF3 heads statistically orient their hydrophobic C delta+-(F delta-)(3) dipoles toward air, yielding a negative average dipole moment at the monolayer/water boundary and negative Surface dipole potential.