Fluorinated Polar Heads Can Strikingly Increase or Invert the Dipole Moments at the Langmuir Monolayer−Water Boundary: Possible Effects from Headgroup Conformations

The dipole potential of lipid monolayers and bilayers is positive toward their nonpolar moiety. In previous papers, we have shown that designed molecules with fluorinated polar heads can invert the polarity of un-ionized Langmuir films. Monolayers of long-chain trifluoroethyl ester RCOOCH2CF3 and trifluoroethyl ether ROCH2CF3 exhibit large negative ΔV values, shifted by 150−200% from the positive dipole potentials of their non-fluorinated analogs (Petrov and Möhwald J. Phys. Chem. 1996, 100, 18458; Petrov et al. J. Phys. Chem. B 2005, 109, 14102). Here we report large positive surface (dipole) potentials of monolayers of N-trifluoroethyl docosanamide RCONHCH2CF3 and a 300% ΔV shift with respect to the non-fluorinated N-ethyl docosanamide films. Comparing the dipole potentials and normal dipole moments of the RCONHCH2CF3 and RCOOCH2CF3 monolayers and the maps of the local electrostatic potential (MEP) and lipophilicity (MLP) of their molecules in vacuum, we conclude that the opposite ΔV shifts and the difference of 1480 mV between the films of these structurally similar amphiphiles seem to be due to strongly different conformations of their heads. The large positive ΔV values of the N-trifluoroethyl amide monolayer was related to the network of NH...OC bonds fixing the orientation of the hydrophobic δ+C−F3δ- dipoles toward water. The trifluoroethyl ester heads do not form H-bonds and can adjust their energetically optimal conformation orienting the hydrophobic δ+C−F3δ- dipoles toward air. The opposite signs of the dipole potential and the apparent normal dipole moments of the trifluoroethyl ester and ethyl ester monolayers were explained via energy minimization of 36 upright closely packed molecules with “hook-like” heads. The equilibrium architecture of this ensemble shows statistical distribution of the headgroup conformations and a nano-rough monolayer−water boundary as known from X-ray reflectivity experiments and molecular dynamic simulations of phospholipid monolayers and bilayers. The average of the vertical molecular dipole moments at equilibrium agree fairly well with the measured values of μ⊥, and the mean molecular area in the ensemble 19.3 Å2 matches the value of 18.9 ± 0.2 Å2 determined via X-ray diffraction at gracing incidence surprisingly well. These results reflect the balance of the attractive and repulsive forces between the closely packed “dry” amphiphilic molecules, but a more sophisticated molecular modeling explicitly including water would better serve to reveal the mechanism of the observed effects.