The self‐aggregation of sodium perfluorooctanoate in aqueous solution at different temperatures

Abstract

Electrical conductivities of sodium perfluorooctanoate (SPFO) in aqueous solutions were measured at different temperatures (range 293-328 K). Critical micelle concentrations (CMC) and the degree of ionization (α) of the micelles were derived from such data. The results revealed that temperature dependence of CMC is U-shaped with a minimum at 316 K. Gibbs free energies, enthalpies, and entropies of micelle formation as a function of temperature were estimated from the CMC and α values using the charged pseudo-phase separation model. To correlate the enthalpic and entropic contributions, the compensation phenomenon was studied, with a compensation temperature of 309 K and an intercept of −27.7 kJ⋅mol −1 . Apparent molar volumes and adiabatic compressibilities of SPFO were determined from density and ultrasound velocity measurements in the same temperature range as conductivities. Positive deviation from the Debye-Hückel limiting law of the apparent molar volume in the range of temperatures studied evidenced hydrogen bonding-type interactions between monomers and the existence of dimers in the premicellar region. With micellization, the apparent molar volumes decrease with rising temperature, indicating that the structure of micelles is looser than that of monomers. The isentropic apparent molar adiabatic compressibilities following micellization were positive, indicating the predominant role of the decrease in hydrophobic hydration in the association process.The substitution of hydrogen by fluorine produces dramatic changes in hydrogenated chains (1). The size of the fluorine atom (van der Waals radius ≈1.47 Å) (2) is significantly larger than the hydrogen atom (≈1.20 Å) (2), so perfluoroalkyl chains are bulkier than hydrogenated ones [with cross sections of ≈30 and ≈20 Å (2), respectively] (3,4). Owing to the low polarizability of fluorine (5), the van der Waals interactions between fluorinated chains are weak; consequently, the liquid fluorocarbons show low cohesive energy densities (6,7). This is reflected in very low surface tensions, high fluidities, low dielectric constants, high vapor pressures, high compressibilities, and high gas solubilities (8-10). Fluorinated amphiphiles have a greater tendency to self-assemble in water and to collect at interfaces than their hydrogenated analogs. For example, the incremental change in the free energy of adsorption for the transfer of a CF 2 group from water to the air/water interface is −5.1 kJ⋅mol −1 vs. −2.6 kJ⋅mol -1 for a CH 2 group (11,12). Fluorinated surfactants have lower critical micelle concentrations (CMC) than their hydrogenated counterparts. For instance, the contribution of a CF 2 group toward micellization of perfluorinated carboxylic acids is equivalent to 1.6 times that of a CH 2 group (13). Micelles of fluorinated amphiphiles tend to be rod-shaped, as predicted from calculations based on their geometry (3) and shown by experimental results (14,15). These specific characteristics of fluorinated surfactants are of special interest in a wide variety of a...

A self-assembly toolbox for thiophene-based conjugated polyelectrolytes: surfactants, solvent and copolymerisation

“…Therefore, the more weakly-charged PFOS may also be expected to penetrate more deeply within the neutral P3HT blocks of the diblock copolymer than SDS. 76 Methanol is a better solvent for device fabrication than water due to its higher volatility. While the optical properties of P3HTPMe 3 and P3HT-b-P3HTPMe 3 in MeOD showed only a minor dependence on the nature of the counterion, the aggregate structures formed strongly resemble those in D 2 O, although with subtle differences (Fig.…”

Cationic–anionic vesicle templating from fluorocarbon/fluorocarbon and hydrocarbon/fluorocarbon surfactants

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Supramolecular “Leeks” of a Fluorinated Hybrid Amphiphile That Self-Assembles into a Disordered Columnar Phase

“…4 Furthermore, fluorocarbons display weak van der Waals interactions that lead to weaker intermolecular cohesive forces and enhanced hydrophobicity compared to the hydrogenated analogues. 2,5,6 Because of these reasons, the physical properties of fluorinated surfactants, like phase behavior, lyotropic LC phase morphology, and rheology, are drastically modified. 2 The LLC phase behavior of fluorinated surfactants and hybrid surfactants with fluorocarbon and hydrocarbon tails has not been studied as extensively as that of hydrocarbon-based surfactants.…”