Structure and Interactions in Perfluorooctanoate Micellar Solutions Revealed by Small-Angle Neutron Scattering and Molecular Dynamics Simulations Studies: Effect of Urea

Abstract

The self-assembly of surfactants in aqueous solution can be modulated by the presence of additives including urea, which is a well-known protein denaturant and also present in physiological fluids and agricultural runoff. This study addresses the effects of urea on the structure of micelles formed in water by the fluorinated surfactant perfluoro-n-octanoic acid ammonium salt (PFOA). Analysis of small-angle neutron scattering (SANS) experiments and atomistic molecular dynamics (MD) simulations provide consensus strong evidence for the direct mechanism of urea action on micellization: urea helps solvate the hydrophobic micelle core by localizing at the surface of the core in the place of some water molecules. Consequently, urea decreases electrostatic interactions at the micelle shell, changes the micelle shape from prolate ellipsoid to sphere, and decreases the number of surfactant molecules associating in a micelle. These findings inform the interactions and behavior of surface active per-and polyfluoroalkyl substances (PFAS) released in the aqueous environment and biota.

The Effect of Electrolytes and Urea on the Ethyl Lauroyl Arginate and Cellulose Nanocrystals Foam Stability

“…As it is known from the literature, urea increases critical micelle concentration of surfactants [34], increases counterion dissociation [35], displaces the water from the surface of ionic surfactants, helps solvate the hydrophobic micelle cores by localizing at their surfaces, and changes the micelle shape and the number of surfactants associating in the micelle [36]. Recent data show urea orients at the interface within the hydrogen-bonded water network.…”

Binding of Perfluorooctanoate to Poly(ethylene oxide)

“…SANS measurements of aqueous PFOA solutions in the absence and presence of PEO were performed on the NG-B 30 m SANS instrument at the Center for Neutron Research (NCNR), National Institute of Standards and Technology (NIST), Gaithersburg, MD (refer to the SI for information on SANS data collection and reduction). SANS has been widely used to determine the size and structure of surfactant micelles. ,− …”

“…The scattering intensities from PFOA + PEO mixed micelles are fitted using a combination of the core–shell ellipsoid form factor and Hayter rescaled mean spherical approximation (RMSA) structure factor with the correlation length model. The core–shell ellipsoid form factor and Hayter RMSA structure factor (details are provided in the subsequent text) have been widely used for describing ionic surfactant micelles. ,,, The correlation length model is incorporated into the overall scattering intensity to account for scattering (at low- q values) originating from a fraction of polymer molecules that are not forming complexes with PFOA and thus cannot be described by the core–shell form factor . The correlation length model is a combination of Lorentzian and power law terms and has been previously used to capture the scattering originating from nonionic polymers in aqueous solution. , The power law term describes Porod scattering from clusters, capturing the scattering behavior at low- q values.…”

“…All MD simulations were conducted using a nonpolarizable version of atomistic polarizable potentials for liquids, electrolytes and polymers (APPLE&P), which has been previously used in the investigation of perfluorinated surfactant self-assembly in water in the presence of ethanol or urea additives. , Studied systems contained 32 PFOA surfactant + counterion pairs and 4032 H 2 O molecules. This concentration (about 390 mM) is well above the CMC, as shown in previous simulations. , PFOA molecules form one large micelle with association number N agg = 32. To investigate the influence of PEO on the micelle structure, we considered a series of systems containing 1, 2, 3, or 4 PEO chains with a degree of polymerization of 50 (MW = 2202 g/mol), which correspond to 2.5, 5.0, 7.5, and 10 wt % PEO in solution, respectively.…”

Adsorption Mechanism of Perfluorooctanoate on Cyclodextrin-Based Polymers: Probing the Synergy of Electrostatic and Hydrophobic Interactions with Molecular Dynamics Simulations

“…All simulations were conducted using an in-house developed MD package with a nonpolarizable version of the Atomistic Polarisable Potential for Liquid, Electrolytes, and Polymers (APPLE&P) force field . This version of the force field has been previously used in simulation of PFOA and GenX aqueous solutions, − which proved to be in excellent agreement with the experimental results. The missing parameters to describe CD and DFB linkers were parametrized against quantum chemistry calculations (see the Supporting Information for parameters).…”