Aggregation Behavior of Perfluorononanoic Acid/Sodium Dodecyl Sulfate Mixtures

Abstract

The mixed system of an anionic hydrocarbon surfactant, sodium dodecyl sulfate, and a perfluorinated surfactant, perfluorononanoic acid (PFNA), was investigated by a combination of methods. The critical micellar concentrations (CMC) were determined over a wide range of sample compositions by surface tension, conductivity and UV–visible spectrophotometry using N‐(4‐nitrophenyl)perfluorononanamide as a molecular probe. The values of CMC obtained by different techniques were in good agreement. In addition, the aggregation numbers were determined in the mixtures with a low content of hydrocarbon surfactant, by measuring the fluorescence quenching of pyrene. The hydrodynamic radii of the aggregates were estimated through the diffusion ordered 19F‐ and 1H‐NMR spectroscopy. The values obtained are in agreement with those expected according to the measured aggregation numbers. The analysis of the data with different aggregation models suggests the formation of a non‐ideal mixed micelle that is enriched in the perfluorinated surfactant when its mole fraction increases, and that is practically formed by PFNA only at mole fractions higher than 0.8.

Hydrolysis of p-nitrophenyl perfluoroctanoate in mixed surfactant systems

“…The value of the same ratio, (I 1 /I 3 ), for pure PFNA is also 1, indicating that aer a PFNA ¼ 0.5 the probe senses a similar polarity to that of an aggregate formed only by PFNA, reinforcing the concept that the micelles at high a PFNA in this mixture are mainly formed by PFNA as we previously established. 24 On the other hand, the same analysis for Brij-35-PFNA mixtures shows changes in polarity from 1.08 to 1.31 for a PFNA 0.1 to 0.9 respectively, i.e., an increase in the ratio with the increase in a PFNA . Considering that the value of the (I 1 /I 3 ) ratio is 1.59 in water, 37 the polarity of the Brij-35-PFNA interface is higher than that corresponding to SDS-PFNA.…”

“…Recently, we characterized two mixed surfactant systems, Brij-35-peruorononanoic acid (PFNA) 23 and SDS-PFNA. 24 We established that in the Brij-35-PFNA system two non-coexisting aggregates are formed (evidenced by the presence of two CMC). 23 We attributed the rst CMC to a mixed aggregate that changes its morphology when reaching the second CMC.…”

“…Contrary to the observed behaviour of the mixtures Brij-35-PFNA, SDS-PFNA mixtures show only one CMC. 24 However nuclear magnetic resonance results indicate that there are intramicellar demixing at high a PFNA . Table 5 summarizes the results of the hydrolysis of 1 in SDS-PFNA mixtures.…”

“…This is consistent with the fact that at low a PFNA the micelles are mixed while at high a PFNA the aggregates become almost of pure PFNA, as reported elsewhere. 24…”

“…Each value corresponds to the average of at least two determinations and the errors are standard deviations of the t. The values in brackets correspond to the CMC of the mixture at the corresponding a PFNA (ESI, Table S1 † and ref 24)…”