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

Langmuir isotherms were recorded for 1-(perfluorobutyl)undecanoic acid (F3C(CF2)3(CH2)10CO2H), 1-(perfluorohexyl)undecanoic acid (F3C(CF2)5(CH2)10CO2H) and 1-(perfluorooctyl)undecanoic acid (F3C(CF2)7(CH2)10CO2H), their hydrocarbon analog, perfluorododecanoic and perfluorotetradecanoic acid after spreading onto hydrochloric acid (pH=1.9–2.0) at 32°C. 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 π-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.