Adsorption and micellisation of partially- and fully-fluorinated surfactants

Effects of chemical structure on adsorption and aggregation behaviour have been investigated for sodium perfluorononanoate (C8F17COO- Na+ or Na PFN) a nd 9-H perfluorononanoate (H-C8F16COO- Na+ or H Na PFN). Replacing the term inal F for H in this way gives rise to a permanent dipole moment at the hyd rophobic tip of the chain. The critical micelle concentrations, determined by surface tension and electrical conductivity, were 10 and 40 mmol dm(-3) for the Na PFN and H Na PFN, respectively. Tensiometric and neutron reflect ion (NR) methods were employed to investigate adsorption at the air-water i nterface. Problems associated with the experimental measurements, and data interpretation, are discussed. In particular effects on surface tensions of trace levels of Ca2+ +/- Mg2+ and Ba2+, as well as the sequestering agent ethylenediaminetetraacetic acid (EDTA), were investigated. The background l evels of a principal contaminant, Ca2+, were directly measured by atomic ab sorption spectrophotometry, indicating a typical ratio of Ca2+:Na+ of 1:10( 4) in these surfactant solutions. Hence, an appropriate level of added EDTA , sufficient to chelate unwanted polyvalent metals but enough not affect th e surface tension, was chosen and this was EDTA:surfactant 1:333. For both compounds the surface excess Gamma obtained from NR data was consistently h igher than that derived from tensiometry, using a Gibbs pre-factor of 2. Ho wever, better agreement was found if this was changed to 1.7, which would b e consistent with around 30% dissociation of counterions from the him. At t he critical micelle concentration (cmc) the fully fluorinated surfactant ad sorbs most strongly, in that NR gave areas per molecule for the Na PFN and H Na PFN of 41 and 44 Angstrom(2), respectively, whereas drop volume tensio metry gave 43 and 51 Angstrom(2). Taking together ail the different methods , at their cmc's the average values were 43.1 +/- 3.4 and 48.4 +/- 5.2 Angs trom(2) for the Na PFN and H Na PFN, respectively. These changes are consis tent with weaker intermolecular interactions in the layer, and stronger ads orption, in the absence of the H-CF, dipole. Aggregation in the bull: was a lso investigated by small-angle neutron scattering (SANS), and the data wer e modelled in terms of charged spherical micelles. At a volume fraction emp ty set(mic) similar to 0.05, the micellar radii and net charges were simila r, being 15 and 14 Angstrom - 10 and - 13, for Na PFN and H Na PFN, respect ively. These values suggest that the ion dissociation in micelles is betwee n 25 and 45%, and this is similar to the ionisation at the surface suggeste d by the adsorption measurements. These results will help improve understan ding about properties of fully- and partially-fluorinated surfactants, whic h are often used in speciality applications, such as water-in-CO2 microemul sions. (C) 1999 Elsevier Science B.V. Ah rights reserved.