title: Why fluorination of the polar heads reverses the positive sign of the dipole potential of Langmuir monolayers: a vibrational sum frequency spectroscopic study.
abstract: Natural nonionic amphiphiles forming monolayers, bilayers, micelles, or biomembranes create a positive dipole potential at the boundary with water. In a series of papers we have reported on Langmuir monolayers with CF3 terminals of the polar heads, which show a negative surface dipole potential ΔV (Petrov , J. G.; Andreeva, T. D.; Kurt, D. K.; Möhwald, H. J. Phys. Chem. B 2005, 109, 14102). Here we use vibrational sum frequency spectroscopy (SF) to study the origin of the opposite ΔV signs of Langmuir films of CH3(CH2)20COCH2CH3 (ethyl ether, EE) and CH3(CH2)20COCH2CF3 (fluorinated ethyl ether, FEE). The vibrational sum frequency spectra are recorded at the same film density of the S-phase of the EE and FEE monolayers and analyzed in the spectral regions of OH, COC, CH3, and CF3 stretching vibrations because these functional groups could be responsible for the different dipole potentials. We compare the rearrangement of the pure water surface by EE and FEE monolayers and the conformations of EE and FEE polar heads. The analysis is performed according to the three-capacitor model of the dipole potential of Langmuir monolayers (Demchak, R. T.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191). The results show that reversal of the ΔV sign caused by fluorination of the polar heads originates from the upward-oriented CF3 terminals of the FEE heads, whose negative normal dipole moment component determines the negative dipole potential of the FEE monolayer.
authors.affiliation: Institute of Physical and Theoretical Chemistry, University of Regensburg, Regensburg, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Karageorgiev
authors.forename: Jordan G
authors.initials: JG
authors.lastname: Petrov
authors.forename: Hubert
authors.initials: H
authors.lastname: Motschmann
authors.forename: Helmuth
authors.initials: H
authors.lastname: Moehwald
mesh_terms: D000388:Air; D004986:Ether / Q000737:chemistry*; D006845:Hydrocarbons, Fluorinated / Q000737:chemistry*; D015394:Molecular Structure; D013057:Spectrum Analysis; D013499:Surface Properties; D014732:Vibration; D014867:Water / Q000737:chemistry
chemical_list: D006845:Hydrocarbons, Fluorinated; D014867:Water; D004986:Ether