Perfluorooctanoate in Aqueous Urea Solutions: Micelle Formation, Structure, and Microenvironment

Abstract

Fluorinated surfactants are used in a wide range of applications that involve aqueous solvents incorporating various additives. The presence of organic compounds such as urea is expected to affect the self-assembly of fluorinated surfactants, however, very little is known about this. We investigated the effect of urea on the micellization in water of the common fluorinated surfactant ammonium perfluorooctanoate (APFO), and on the structure and microenvironment of the micelles that APFO forms. Addition of urea to aqueous APFO solution decreased the critical micellization concentration (CMC) and increased the counterion dissociation. The observed increase in surface area per APFO headgroup and decrease in packing density at the micelle surface suggest the localization of urea at the micelle surface in a manner that reduces headgroup repulsions. Micropolarity data further support this picture. The results presented here indicate that significant differences exist between urea effects on fluorinated surfactant and on hydrocarbon surfactant micellization in aqueous solution. For example, the CMC of sodium dodecyl sulfate (SDS) increased with urea addition, while the increase in surface area per headgroup and packing density of SDS with urea addition are much lower than those observed for APFO. This study informs fluorinated surfactant fate and transport in the environment, and also applications involving aqueous media in which urea or similar additives are present.

Keywords: denaturation; perfluoroalkyl substances (PFAS); surfactant; urea; water.

Figure 1

Conductivity of ammonium perfluorooctanoate (APFO) aqueous solutions plotted versus surfactant concentration in the absence and in the presence of added urea at various concentrations (shown inside the graph) (24 °C). Linear fits to the data points below and above the break point have been applied in order to determine the critical micelle concentration (CMC).

Figure 2

Surface tension of APFO aqueous solutions in the absence and in the presence of 4 M urea (24 °C). Linear fits have been applied to the data points where the surface tension decreases prior to reaching the CMC.

Figure 3

(a) Pyrene fluorescence intensity I1/I3 ratio of APFO aqueous solutions plotted versus surfactant concentration in the absence and in the presence of urea (at various concentrations shown inside the graph) (22 °C); (b) variation with urea concentration of the pyrene fluorescence intensity I1/I3 ratio of APFO at various concentrations (shown inside the graph) in aqueous solution. The lines connecting the data points are meant as guides to the eye.

Figure 4

Relative viscosity, η_r, of APFO (a) and SDS (b) aqueous solutions in the absence and in the presence of 4 M urea, plotted as a function of micellized surfactant concentration. The lines through the viscosity data points are fits to Equation (6).

Figure 5

Pyrene monomer emission spectra of APFO aqueous solutions in the absence (a) and in the presence of urea (b). The spectra have been normalized by dividing the intensity at each wavelength with the intensity of the first peak (I1). The grey lines (——, ·········, - - - - -, — - — - ) indicate the spectra of pyrene for APFO concentrations below CMC, the red line (——) is for APFO concentration near CMC and the blue lines (——, ·········, - - - - -, — - — - ) are for APFO concentrations above CMC.