THE MONOMOLECULAR THERMOTROPIC PHASE-BEHAVIOR OF 17-METHYL-OCTADECANOIC ACID AT THE AIR-WATER-INTERFACE

The thermotropic phase behavior of monomolecular films of the methyl iso-branched fatty acid, 17-methyl-octadecanoic acid, over the temperature range 11.3 to 37.9°C has been studied using a variety of techniques. A comparison with the behavior of three straight-chain acids (tetradecanoic, hexadecanoic, and octadecanoic acids) in both condensed and expanded states is made. While phase transitions between the condensed states of straight-chain fatty acids show little evidence of changing hydrogen bonding, those of 17-methyl-octadecanoic acid do. Observations from compressional isotherms, from viscosity studies, and from ATR FTIR studies of Langmuir-Blodgett-deposited monolayers in the range 11 to 27°C indicate a more expanded rigid condensed state (C1) and one which is more condensed but fluid (C11). These data are explained in terms of a model where the C1 state is postulated to be more hydrogen bonded due to an increase in the close-packed areas/molecule of the acid by methyl iso-branching. Internal energies for the C1/C11 transition, however, indicate that the degree of hydrogen bonding is much less than the one hydrogen bond/molecule theoritically possible. The liquid-expanded states of 17-methyl-octadecanoic acid, somewhat surprisingly, are less expanded than those of the corresponding straight-chain acids. Coupled with the more expanded condensed states the expanded/condensed transition is much smaller than that for straight-chain acids and iso-branched acids go from a fully condensed to a fully expanded state over a much smaller temperature span. © 1990.