Calorimetric investigation of the interaction between lithium perfluorononanoate and poly(ethylene glycol) oligomers in water

Aggregation behavior of Brij-35/perfluorononanoic acid mixtures

The mixed system of a nonionic hydrocarbon surfactant, polyoxyethylene (23) lauryl ether (Brij-35), and a perfluorinated surfactant, perfluorononanoic acid, was investigated by a combination of methods. The critical micelle concentrations (cmc's) have been determined over a wide range of sample compositions by fluorescence and UV-visible spectrophotometry using pyrene and N-(4-nitrophenyl) perfluorononanamide, respectively, as molecular probes. The values of the cmc's obtained were considerably different with the two techniques employed. Measurements of the (19)F nuclear magnetic resonance chemical shift of the same mixtures showed two breaks in the plots of Δδ(f) versus molar fraction of the perfluorinated surfactant. Conductivity and surface tension measurements also showed two breaks. The behavior is attributed to the formation of mixed micelles that change their composition when the fraction of the fluorinated compound increases and some segregation of the fluorinated compound takes place at a high total surfactant concentration.

Aggregation of perfluoroctanoate salts studied by 19F NMR and DFT calculations: counterion complexation, poly(ethylene glycol) addition, and conformational effects

The aggregation of perfluoroctanoate salts in H(2)O is studied by (19)F NMR on solutions of LiPFO, NaPFO, and CsPFO, without and with the addition of two poly(ethylene glycol) (PEG) oligomers of molecular weight 1500 and 3400 Da, respectively, and with the addition of suitable crown ethers. The (19)F chemical shift (cs) trends are monitored, at 25 °C, in a concentration range including the critical micellar concentration (cmc) or, in the presence of PEG, the critical aggregation concentration (cac). The cac values in the samples with PEG are lower than the cmc values of the corresponding samples without PEG; moreover, the (19)F cs trends above the cac and above the polymer saturation concentration reveal and help to explain some peculiarities of the aggregation process of PEG on PFO micelles, which, in the first step, seems to occur while the surfactant concentration in water is still increasing. Also in LiPFO/H(2)O or NaPFO/H(2)O solutions containing 12-crown-4 or 15-crown-5 ethers, suitable to complex Li(+) or Na(+) ions, respectively, the cmc decreases. On the other hand, the micellization process in the presence of crown ethers does not show other peculiarities. The prevailing conformations of the PFO chain are discussed on the basis of quantum-mechanical calculations. The theoretical chemical shifts were computed at the DFT level of theory, taking into account the effects of the environment by means of the IEF-PCM method. The helical structure is the most stable one, but anti conformations are easily accessible, in both the aqueous and fluorinated environment. The comparison between computed and experimental chemical shifts indicates that anti conformations are more important in the micelles than in water and in CsPFO micelles than in LiPFO or NaPFO ones.

Structure transition from hexagonal mesostructured rodlike silica to multilamellar vesicles

We studied the synthesis of siliceous structures by using a nonionic block copolymer (Pluronic P123) and perfluorooctanoic acid (PFOA) as cotemplates in an acid-catalyzed sol-gel process. Different siliceous structures were obtained through systematically varying the molar ratio (R) of PFOA/P123, and the resultant materials were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen sorption analysis, and Fourier-transform infrared spectroscopy. The results are consistent and reveal a structure transition from a highly ordered 2D hexagonal (HEX) mesostructure with a rodlike morphology to multilamellar vesicles (MLVs) with sharp edges when R is increased. The fact that the MLVs are initiated from the end of hexagonally mesostructured rods provides key evidence in such a novel structure transition. Our finding indicates that, at least in our observations, the MLVs are developed gradually from HEX structures, rather than by a direct cooperative self-assembly mechanism. It is suggested that PFOA molecules with rigid fluorocarbon chains closely interact with PEO. This interaction model may well explain (1) the "wall-thicken" effect in HEX mesostructures by enlarging the hydrophilic PEO moiety (R = 0-1.4), (2) the subsequent HEX to multilamellar structure transition by modifying the hydrophilic/hydrophobic volume ratio (R = 1.4-2.8), and (3) the formation of MLVs with sharp edges by increasing the bending energy. This model provides insight into the fabrication of novel porous materials by the use of block copolymers and fluorinated surfactant mixed templates.

Micellization of lithium perfluoroheptanoate and its aggregation on poly(ethylene glycol) oligomers in water

The interaction of lithium perfluoroheptanoate (LiPFHep) with poly(ethylene glycol) (PEG) of different molecular weights (300 < MW < 20 000 Da) was investigated in water at 298.15 and 308.15 K by the isothermal titration calorimetry (ITC). Density and sound velocity measurements were also performed at 288.15, 298.15, and 308.15 K, while viscosity and conductivity data were only collected at 298.15 K. The aggregation process of this surfactant on the PEG polymeric chain was found to be very similar to the process exhibited by the two homologous perfluorooctanoate and perfluorononanoate. Viscosity and ITC data indicated that the formation of polymer-surfactant complexes between PEG and LiPFHep also leads to a conformational change in the polymer. The aggregation of micelles of the lithium perfluoro surfactants on the PEG polymeric chain is characterized by a comparable thermodynamic stability, which results from a balance of enthalpy and entropy contributions, which both increase with the length of the surfactant hydrophobic chain.