Analysis of fluorinated surfactants

Analytical techniques are employed to determine the purity or the concentration of a fluorinated surfactant and to characterize a fluorinated surfactant and its solutions. Because most fluorinated surfactants are mixtures of homologs, the term purity has to be redefined for each particular case. In most cases, the determination of purity begins with the analysis of intermediates used to synthesize the surfactant. Usually, the intermediates can be readily analyzed by chromatography and the homolog distribution determined. Gas chromatography has only a limited value for the analysis of fluorinated surfactants proper because most fluorinated surfactants are not sufficiently volatile for gas chromatography. 

In general, the concentration of a fluorinated surfactant in solution can be determined by conventional volumetric or spectroscopic methods used for hydrocarbon-type surfactants [1-5], In addition to the functional groups utilized for the analysis of hydrocarbon-type surfactants, the fluorine content is a unique feature useful for the determination of fluorinated surfactants. If the fluorinated surfactant is the only fluorine-containing species in a solution or a substrate, then the fluorine content indicates the concentration of the fluorinated surfactant. 

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Raman spectroscopy has been very useful for conformation studies of hydrocarbon-type and partially fluorinated surfactants but has only a limited value for perfluorinated surfactants. Unlike a strong IR absorption band arising from the CF stretching mode, the intensity of Raman bands is low for these vibrations. In contrast to the characteristic CH stretching mode, the CF and CF2 modes are in a region where other molecular modes occur and complicate absorption patterns. Amorim da Costa and Santos [72] have nevertheless been able to show Raman spectroscopy to be useful for structure and conformation analysis of fluorinated surfactants (Fig. 9.6). 

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