Behavior of partially fluorinated carboxylic acids at the air-water interface

Langmuir isotherms were recorded for 1-(perfluorobutyl)undecanoic acid (F3C(CF2)(3)(CH2)(10)CO2H), 1-(perfluorohexyl)undecanoic acid (F3C(CF2)(5)(CH2)(10)CO2H) and 1-(perfluorooctyl)undecanoic acid (F3C(CF2)(7)(CH2)(10)CO2H). their hydrocarbon analog, perfluorododecanoic and perfluorotetradecanoic acid after spreading onto hydrochloric acid (pH = 1.9-2.0) at 32 degreesC. All acids formed stable monolayers at the air-water interface, 1-(Perfluorobutyl)undecanoic acid shows some similarity to the compression isotherm of tetradecanoic acid while lacking the phase transition of pentadecanoic acid. The isotherm for 1-(perfluorohexyl)undecanoic acid shows a temperature -dependent phase transition similar to the liquid expanded-liquid condensed transition of pentadecanoic acid. The pi -A isotherm of 1-(perfluorooctyl)undecanoic acid resembles the highly condensed monolayers of hepta- and nonadecanoic acid. Thus, the partially fluorinated acids seem to exhibit isotherm characteristics similar to hydrocarbon acids with shorter chain lengths. The three partially fluorinated acids have a higher limiting area compared to hydrocarbon and perfluorocarbon acids, which may be atributed to the strong dipole moment of the -CF2-CH2- linkage. Within this series of partially fluorinated carboxylic acids. the limiting area decreases with chain lengthening and a higher degree of fluorination. The collapse pressures of the fluorinated acids are smaller compared to their respective hydrocarbon analog. (C) 2001 Elsevier Science B.V. All rights reserved.