Micelles and aggregates of fluorinated surfactants

High energy radiation - Induced cooperative reductive/oxidative mechanism of perfluorooctanoate anion (PFOA) decomposition in aqueous solution

The mechanism of high-energy radiation induced degradation of perfluorooctanoate anion (PFOA, C₇F₁₅COO^-) was investigated in aqueous solutions. Identification and quantification of transient species was performed by pulse radiolysis and of final products by gas and ion chromatography, electrochemical method using fluoride ion-selective electrode and ESI-MS after γ-radiolysis. Experimental data were further supported by kinetic simulations and quantum mechanical calculations. Radiation induced degradation of PFOA includes as a primary step one-electron reduction of PFOA by hydrated electrons (e^-_aq) resulting in formation of [C₇F₁₅COO^-]^●-. The rate constants of this reaction were found to be in the range 7.7 × 10⁷-1.3 × 10⁸ M^-1s^-1 for ionic strength of the solutions in the range 0.01-0.1 M and were independent of pH of the solutions. At pH > 11 [C₇F₁₅COO^-]^●- tends to defluorination whereas at lower pH undergoes protonation forming [C₇F₁₅COOH]^•-. A sequence of consecutive reactions involving [C₇F₁₅COOH]^•- leads to PFOA regeneration what explains a high radiation resistance of PFOA at moderately acidic solutions. A simultaneous presence of oxidizing transient species (^●OH) in the irradiated system enhanced decomposition of (C₇F₁₄)·COO^- as well as [C₇F₁₅COOH]^•-. The key steps in this complex radical mechanism are the reactions of both these radical anions with ^●OH leading to semi-stable products which further undergo consecutive thermal reactions. On the other hand, direct reactions of PFOA with ^●OH and ^●H were found to be relatively slow (7 × 10³ and <4 × 10⁷ M^-1s^-1, respectively) and do not play relevant role in PFOA degradation. Collected for the first time results, such as dependence of selected reaction rate constants and selected products radiation chemical yields on pH as well as finding of several semi-stable products, missing in previous studies, indicate incompleteness of published earlier reaction pathways of PFOA degradation. The presented overall mechanism explains experimental results and verifies previously suggested mechanisms found in the literature.

Headgroup-Specific Interaction of Biological Lipid Monolayer/Water Interface with Perfluorinated Persistent Organic Pollutant (f-POP): As Observed with Interface-Selective Vibrational Spectroscopy

Perfluoro compounds are widely used in various manufacturing processes, which leads to their bioaccumulation and subsequent adverse effects on human health. Using interface-selective vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation (HD-VSFG)), we have elucidated the molecular mechanism of the perturbation of lipid monolayers on the water surface using a prototype perfluorinated persistent organic pollutant, perfluoroheptanoic acid (PFHA). PFHA disrupts the well-ordered all-trans conformation of a cationic lipid (1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP)) monolayer and reduces the interfacial electric field at the lipid/water interface. In contrast, the hydrophobic packing of an anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phospoglycerol (DPPG)) monolayer remains largely unaffected in the presence of PFHA, though the interfacial electric field is reduced. For a zwitterionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC))/water interface, both alkyl chain ordering and interfacial electric field are fairly perturbed by PFHA. Lipid headgroup-specific interaction of PFHA and the repulsive interaction of oleophobic fluoroalkyl chain with the lipid alkyl chains govern these distinct perturbations of the lipid monolayers on the water surface.

Infrared spectra of hydrogen bond network in lamellar perfluorocarboxylic acid monohydrates

The infrared spectra of the long-chain perfluorocarboxylic acid monohydrates differ markedly from those of the anhydrous dimers. Consequently, the structure of the solid perfluorocarboxylic acid monohydrates must differ from any known dimer-containing carboxylic acid crystals. Consideration of the significant features of the infrared spectra of the long-chain perfluorocarboxylic acid monohydrates, supplemented by their Raman spectra, and comparison with the spectra of auxiliary substances have led us to conclude that the rather strong neutral carboxyl-hydroxyl to water bonding can best explain the observations. The infrared spectra indicate the presence of fairly short hydrogen bonds connecting the water molecules to the carbonyl groups. In the construction of the hydrogen bonding pattern of the perfluorocarboxylic acid monohydrates, the oxalic acid dihydrate plays the key role. The striking similarity between the infrared spectra of the oxalic acid dihydrates and the perfluorocarboxylic acid monohydrates in the regions characteristic of water and OH⋯O vibration suggests that the structure of the hydrated carboxyl groups is the same in both crystals. These regions are characterized by the sharp doublet at 3539 cm^-1 and 3464 cm^-1, which is due to the H₂O ν₁ and ν₃ stretching vibrations, respectively, and the broad absorption between 3000 cm^-1 and 1500 cm^-1 with the intense band at 1970 cm^-1, both associated with the vibration of the OH⋯O group. The later peak consists of two band components at near 1980 cm^-1 and 2020 cm^-1. These band components show different behaviour when the temperature, polarization or deuteration is changed. In general, the infrared spectra of long-chain perfluorocarboxylic acids represent the system with very short hydrogen bonds connecting the water molecules to the carboxylates. This hydrogen bond pattern should be very similar to that found in the crystals of α-oxalic acid dihydrate.

Thermodynamics of aqueous perfluorooctanoic acid (PFOA) and 4,8-dioxa-3H-perfluorononanoic acid (DONA) from DFT calculations: Insights into degradation initiation

Modern fluorosurfactants introduced during and after perfluoroalkyl carboxylates/sulfonates phase-out present chemical features designed to facilitate abatement, hence reducing persistence. However, the implications of such features on environmental partitioning and stability are yet to be fully appreciated, partly due to experimental difficulties inherent to the handling of their (diluted) aqueous solutions. In this work, rigorous quantum chemistry calculations were carried out in order to provide theoretical insights into the thermodynamics of hydroperfluorosurfactants in aqueous medium. Estimates of acid dissociation constant (pK_a), standard reduction potential (E⁰), and bond dissociation enthalpy (BDE) and free energy (BDFE) were computed for perfluorooctanoic acid (PFOA), 4,8-dioxa-3H-perfluorononanoic acid (DONA) and their anionic forms via ensemble averaging at density functional theory level with implicit solvent models. A ‹pK_a› in the neighborhood of zero and a E⁰ of about 2.2 V were obtained for PFOA. Predictions for the acidic function of DONA compare well with PFOA's, with a pK_a of 0.8-1.5 and a E⁰ of 2.07-2.15 V. Deprotonation thus represents the dominant phenomenon at environmental conditions. Calculations indicate that H-abstraction of the aliphatic proton of DONA by a hydroxyl radical is the thermodynamically favored reaction path in oxidative media, whereas hydrolysis is not a realistic scenario due to the high dissociation constant. Short intramolecular interactions available to the peculiar hydrophobic tail of DONA were also reviewed, and the relevance of the full conformational space of the fluorinated side chain discussed.

Optical and Magnetic Resonance Imaging Using Fluorous Colloidal Nanoparticles

Improved imaging of cancerous tissue has the potential to aid prognosis and improve patient outcome through longitudinal imaging of treatment response and disease progression. While nuclear imaging has made headway in cancer imaging, fluorinated tracers that enable magnetic resonance imaging (¹⁹F MRI) hold promise, particularly for repeated imaging sessions because nonionizing radiation is used. Fluorine MRI detects molecular signatures by imaging a fluorinated tracer and takes advantage of the spatial and anatomical resolution afforded by MRI. This manuscript describes a fluorous polymeric nanoparticle that is capable of ¹⁹F MR imaging and fluorescent tracking for in vitro and in vivo monitoring of immune cells and cancerous tissue. The fluorous particle is derived from low-molecular-weight amphiphilic copolymers that self-assemble into micelles with a hydrodynamic diameter of 260 nm. The polymer is MR-active at concentrations as low as 2.1 mM in phantom imaging studies. The fluorinated particle demonstrated rapid uptake into immune cells for potential cell-tracking or delineation of the tumor microenvironment and showed negligible toxicity. Systemic administration indicates significant uptake into two tumor types, triple-negative breast cancer and ovarian cancer, with little accumulation in off-target tissue. These results indicate a robust platform imaging agent capable of immune cell tracking and systemic disease monitoring with exceptional uptake of the nanoparticle in multiple cancer models.

Micelle Formation in Liquid Ammonia

Perfluorinated long chain alkyl amides aggregate in liquid ammonia with increasing concentration which reflects micelle-type formation based on changes in (19)F NMR chemical shifts. The critical micelle concentrations (cmc) decrease with increasing chain length and give Kleven parameters A = 0.18 and B = 0.19. The micelles catalyze the ammonolysis of esters in liquid ammonia. The corresponding perfluorinated long chain alkyl carboxylates form ion pairs in liquid ammonia, but the equilibrium dissociation constants indicate favorable interactions between the chains in addition to the electrostatic forces. These perfluorinated carboxylates form micelles in aqueous solution, and their cmc's generate a Kleven B-value = 0.52 compared with 0.30 for the analogous alkyl carboxylates. The differences in hydrophobicity of CH2 and CF2 units in water and liquid ammonia are discussed, as is the possible relevance to life forms in liquid ammonia.

Uranium(IV) halide (F-, Cl-, Br-, and I-) monoarene complexes

The syntheses of four nearly isostructural uranium(IV) monoarene complexes, supported by the arene anchored tris(aryloxide) chelate, [((Ad,Me)ArO)3mes](3-), are reported. Oxidation of the uranium(III) precursor [(((Ad,Me)ArO)3mes)U], 1, in the presence of tetrahydrofuran (THF) results in THF coordination and distortion of the equatorial coordination sphere to afford the uranium(IV) η(6)-arene complexes, [(((Ad,Me)ArO)3mes)U(X)(THF)], 2-X-THF, (where X = F, Cl, Br, or I) as their THF adducts. The solvate-free trigonally ligated [(((Ad,Me)ArO)3mes)U(F)], 2-F, was prepared and isolated in the absence of coordinating solvents for comparison.

Prediction of aqueous solubility, vapor pressure and critical micelle concentration for aquatic partitioning of perfluorinated chemicals

The majority of perfluorinated chemicals (PFCs) are of increasing risk to biota and environment due to their physicochemical stability, wide transport in the environment and difficulty in biodegradation. It is necessary to identify and prioritize these harmful PFCs and to characterize their physicochemical properties that govern the solubility, distribution and fate of these chemicals in an aquatic ecosystem. Therefore, available experimental data (10-35 compounds) of three important properties: aqueous solubility (AqS), vapor pressure (VP) and critical micelle concentration (CMC) on per- and polyfluorinated compounds were collected for quantitative structure-property relationship (QSPR) modeling. Simple and robust models based on theoretical molecular descriptors were developed and externally validated for predictivity. Model predictions on selected PFCs were compared with available experimental data and other published in silico predictions. The structural applicability domains (AD) of the models were verified on a bigger data set of 221 compounds. The predicted properties of the chemicals that are within the AD, are reliable, and they help to reduce the wide data gap that exists. Moreover, the predictions of AqS, VP, and CMC of most common PFCs were evaluated to understand the aquatic partitioning and to derive a relation with the available experimental data of bioconcentration factor (BCF).

Phase behaviors and self-assembly properties of two catanionic surfactant systems: C(8)F(17)COOH/TTAOH/H(2)O and C(8)H(17)COOH/TTAOH/H(2)O

Two fatty acids, perfluorononanoic acid (C(8)F(17)COOH) and nonanoic acid (C(8)H(17)COOH), were mixed with a cationic hydrocarbon surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), in aqueous solutions for comparative investigation. Phase behaviors of the two systems are quite different because of the special properties of the fluorocarbon chains. For the C(8)H(17)COOH/TTAOH/H(2)O system, a single L(α) phase region with phase transition from planar lamellar phase (L(αl) phase) to vesicle phase (L(αv) phase) was observed. For the C(8)F(17)COOH/TTAOH/H(2)O system, two single phases consisting of vesicles were obtained at room temperature. One is a high viscoelastic gel phase consisting of vesicles with crystalline state bialyers at the C(8)F(17)COOH-rich side, which was confirmed by freeze-fracture transmission electron microscope (FF-TEM) and differential scanning calorimetry (DSC) measurements. With the increase of TTAOH proportion, another vesicle phase consisting of liquid state bilayers was observed after the two-phase region. The fluorosurfactant systems prefer to form vesicle bilayers than the corresponding hydrocarbon ones because of the rigid structure, the stronger hydrophobicity, and the larger volume of fluorocarbon chains.

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.

Perfluoroalkyl sulfonic and carboxylic acids: a critical review of physicochemical properties, levels and patterns in waters and wastewaters, and treatment methods

Perfluorinated acids (PFAs) are an emerging class of environmental contaminants present in various environmental and biological matrices. Two major PFA subclasses are the perfluorinated sulfonic acids (PFSAs) and carboxylic acids (PFCAs). The physicochemical properties and partitioning behavior for the linear PFA members are poorly understood and widely debated. Even less is known about the numerous branched congeners with varying perfluoroalkyl chain lengths, leading to confounding issues around attempts to constrain the properties of PFAs. Current computational methods are not adequate for reliable multimedia modeling efforts and risk assessments. These compounds are widely present in surface, ground, marine, and drinking waters at concentrations that vary from pg L(-1) to microg L(-1). Concentration gradients of up to several orders of magnitude are observed in all types of aquatic systems and reflect proximity to known industrial sources concentrated near populated regions. Some wastewaters contain PFAs at mg L(-1) to low g L(-1) levels, or up to 10 orders of magnitude higher than present in more pristine receiving waters. With the exception of trifluoroacetic acid, which is thought to have both significant natural and anthropogenic sources, all PFSAs and PFCAs are believed to arise from human activities. Filtration and sorption technologies offer the most promising existing removal methods for PFAs in aqueous waste streams, although sonochemical approaches hold promise. Additional studies need to be conducted to better define opportunities from evaporative, extractive, thermal, advanced oxidative, direct and catalyzed photochemical, reductive, and biodegradation methods. Most PFA treatment methods exhibit slow kinetic profiles, hindering their direct application in conventional low hydraulic residence time systems.

Experimental pKa determination for perfluorooctanoic acid (PFOA) and the potential impact of pKa concentration dependence on laboratory-measured partitioning phenomena and environmental modeling

An accurately measured equilibrium acid dissociation constant (pKa) is essential for understanding and predicting the fate of perfluorocarboxylic acids (PFCAs) in the environment. The aqueous pKa of perfluorooctanoic acid (PFOA) has been determined potentiometrically using a standard water-methanol mixed solvent approach and was found to be 3.8 +/- 0.1. The acidity of PFOA is thus considerably weaker than its shorter-chain PFCA homologues. This was attributed to differences in molecular and electronic structure, coupled with solvation effects. The pKa of PFOA was suppressed to approximately 2.3 at higher concentrations because of the aggregation of perfluorooctanoate (PFO). Often, PFCA partion coefficients are determined at concentrations above those found in the environment. Thus, it was suggested that a pKa correction factor, which accounts for this concentration-dependent shift in acid/base equilibrium, should be applied to PFCA partition efficients before they are implemented in environmental fate models. A pKa of 3.8 +/- 0.1 suggests that a considerable concentration of the PFCA exists as the neutral species in the aqueous environment for example, in typical Ontario rainwater, it is approximately 17%. Transport, fate, and partitioning models have often ignored the presence this species completely. The environmental dissemination of PFCAs could, in part, be explained by considering the role of the neutral species.

Spontaneous self-assembly of water-soluble nucleotide-calixarene conjugates in small micelles coalescing to microspheres

Spontaneous self-assembly of calix[4]arenes bearing four 2'-deoxythymidine or 2'-deoxyadenosine nucleotide pendants is investigated using (1)H NMR, exchange NMR, and diffusion ordered NMR spectroscopies and dynamic light scattering. In aqueous medium, the nucleotide-calixarene conjugates, by noncovalent interactions involving both nucleobases and calixarene skeleton, form dimers which self-organize in micelles by increasing the concentration. Microscopic images (scanning electron microscopy and transmission electron microscopy) show that the nucleobase affects the aggregate morphology in the solid state.

Synthesis, characterization, adsorption, and interfacial rheological properties of four-arm anionic fluorosurfactants

Four-arm oligo(fluorooxetane) tetraols containing -CF3 and -C2F5 groups were prepared in reasonable yields by cationic, ring-opening polymerization of fluorinated oxetane monomers using a tetrafunctional, alkoxylated polyol as initiator and BF3.THF as catalyst. The tetraols were then converted to ammonium sulfate salts using oleum followed by neutralization with ammonium hydroxide in excellent yields. The four-arm oligo(fluorooxetane) sulfates (1=-CF3, 2=-C2F5) have an architecture characterized by a hydrophobic core of oligo(fluorooxetane) arms with a hydrophilic sulfate shell and initiator. The four-arm anionic oligo(fluorooxetane)s are surface active with critical micelle concentration values approximately 4.2x10(-6) and 2.4x10(-6) mol/L for 1 and 2, respectively. Surface tension isotherms in pH 8 buffered solution were measured and data fitted parametrically to the Davies surface tension isotherm equation. Molecular areas at saturation were estimated to be approximately 89 and approximately 85 A2 with DeltaGads=-12.7 and -13.2 kcal/mol for 1 and 2, respectively. The results are compared to two-arm, bolaamphiphilic analogues of 1 and 2 and a small molecule, long perfluoroalkyl-chain (-C8F17), anionic fluorosurfactant (Kausch, C. M.; Kim, Y.; Russell, V. M.; Medsker, R. E.; Thomas, R. R. Langmuir 2003, 19, 7182). Dynamic surface tension data for 1 and 2 were analyzed using the Ward-Tordai mass transport equation to yield concentration-dependent diffusion coefficients. In the concentration range approximately 10(-6) mol/L, diffusion coefficients were estimated to be approximately 1-3x10(-5) cm2/s. Dilational interfacial rheological parameters for 1 and 2 were measured. Values of |E| and E' were found to be larger than those of the two-arm analogues of the same perfluoroalkyl chain length while E' 'and phi were found to be smaller. The magnitude of these values reflects the difference in adsorption strength and mass transport and/or relaxation between the two different architectures.

A Cationic Fluorocarbon Surfactant DEFUMACl and Its Mixed Systems with Cationic Surfactants: 19F NMR and Surface Tension Study

A cationic fluorocarbon surfactant system of diethanolheptadecafluoro-2-undecanolmethylammonium chloride (DEFUMACl) and both mixed systems of DEFUMACl/cationic dodecyltrimethylammonium chloride (DTACl) and DEFUMACl/cationic Gemini copolymer was investigated by 19F NMR spectroscopy and surface tension measurements. The critical micelle concentration (cmc) of DEFUMACl by 19F NMR is about 3.40 mmol/L, which is completely consistent with that obtained by the surface tension method. The studies of salt and temperature on the cmc values of DEFUMACl suggest that both salt addition and temperature increase decrease the cmc values of DEFUMACl. 19F NMR measurements provide much richer information on both mixed systems. For the DEFUMACl-DTACl system, two break points were observed with increased total surfactant concentration. The first break point means the DEFUMACl and DTACl mixed micelles and the second one implies the individual DEFUMACl micelles. Results of 19F NMR and surface tension measurements for DEFUMACl/cationic Gemini copolymer mixtures show three peculiar break points, corresponding to the critical association concentration (cac) of DEFUMACl, the concentration where cationic Gemini copolymer molecules become saturated by DEFUMACl micelles, and the concentration where DEFUMACl micelles and cationic Gemini copolymer coexist. These peculiar points in the cationic-fluorocarbon and cationic-copolymer systems were first reported by 19F NMR and surface tension measurements. These results should broaden the useful information for a better understanding of the mechanism of interaction and the behavior of surfactant-polymer mixtures.

Probing the initial events in the spontaneous emulsification of trans-anethole using dynamic NMR spectroscopy

The spontaneous emulsification of alcoholic solutions of trans-anethole (t-A) in water is investigated using EXSY and DOSY NMR techniques. The system investigated (5-10 mM t-A in 5% EtOH/H2O solution) is exceptional in providing sharp, clearly resolved signals for both t-A that is dissolved in the aqueous phase (free t-A) and t-A that is incorporated in aggregates (3-6 nm diameter) thus allowing both fractions to be probed simultaneously. This feature is utilized to explore the initial events that occur during the spontaneous emulsification process. Upon mixing, the majority of the t-A (ca. 75%) undergoes nucleation to form small aggregates (ca. 10 nm diameter), while 15% (corresponding to [t-A] = 7.5 x 10(-4) M) is dissolved in the aqueous phase. The kinetic rates governing the exchange process between aggregated and free t-A are found to be time-dependent and slow on the NMR time scale (k = 0.8-2 s(-1)). DOSY experiments indicate that the initially formed small aggregates undergo rapid coalescence to form larger droplets. Ostwald ripening of these droplets at the expense of the remaining small aggregates is responsible for the subsequent, slower time-evolution of the system.

Aggregation of Cesium Perfluorooctanoate on Poly(ethylene glycol) Oligomers in Water

The interaction of cesium perfluorooctanoate (CsPFO) with poly(ethylene glycol) (PEG) of different molecular weight (300 < or = MW < or = 20000 Da) has been investigated at 298.15 K by isothermal titration calorimetry (ITC), density, viscosity, and conductivity measurements. Calorimetric titrations exhibited peculiar trends analogous to those already observed for sodium dodecyl sulfate (SDS). Micelles of the perfluorosurfactant, as compared to those of SDS, yield complexes with the polymer of similar thermodynamic stability but are able to interact with shorter PEG oligomers. The average number of surfactant molecules bonded per polymer chain at the saturation is about twice that observed for SDS. ITC data at 308.15 K indicate a larger thermodynamic stability of the aggregates but an almost constant stoichiometry. The peculiar thermal effects and the viscosity trend observed during the titration of an aqueous PEG solution with the surfactant appear consistent with a conformational change of the polymer. The PEG chain would evolve from a strained to an expanded conformation, induced by the growing of the surfactant micellar clusters bonded to the polymer, as suggested in a previous study of the PEG/SDS/H2O system.

Hybrid fluorocarbon-hydrocarbon CO2-philic surfactants. 1. synthesis and properties of aqueous solutions

Six different hybrid fluorocarbon-hydrocarbon (F-H) sulfate and sulfonate surfactants, with variations in the relative F/H carbon chain length, have been synthesized and characterized in aqueous solution. These compounds have been targeted for potential activity in densified CO2. Tensiometric data and chemical analyses were consistent with surfactants of high chemical purity. Fluorination in terms of the F/H ratio exerts a strong control over all the surfactant physicochemical properties, including critical micelle concentrations (cmc's) and adsorption isotherms. One of these partially fluorinated surfactants (the sulfonate phi-F6H4) achieves very low surface tensions in water (gamma(cmc) approximately 19 mN m(-1)) more reminiscent of fully fluorinated double-chain compounds. Detailed 19F NMR studies revealed that omega'-CF3 groups can exhibit separate signals for monomeric and micellized forms, hence facilitating cmc determinations. Small-angle neutron scattering investigations confirmed the presence of ellipsoidal or extended disklike micelles, depending on the F-H chain asymmetry. For example, a symmetric hybrid F8H8 generates disklike micelles, whereas chain asymmetry in F8H4 or phi-F6H4 tends to drive cylindrical aggregation structures. These changes are consistent with variations in the surfactant packing parameter caused by the different chain F/H ratios. Hence, adsorption and aggregation are shown to respond in a predictable way to the molecular structure of these unusual surfactants.

Synthesis of ether-linked fluorocarbon surfactants and their aggregational properties in organic solvents

A series of single- and double-tailed hydrocarbon-fluorocarbon (HF) surfactants were prepared to evaluate the effect of molecular structure on aggregate formation in organic solvents. The molecules were designed with ether linkages to permit facile syntheses of both sets of molecules. Solvent foaming studies were used to rapidly assess the surface-active properties of the surfactants, while dynamic light scattering provided quantitative critical micelle concentrations (CMC) and hydrodynamic radius (R(h)) measurements of the aggregates in solution. The single-tailed surfactants did not produce any foaming action in a number of hydrocarbon solvents, nor was any micellar formation observed up to 100 mM concentrations. Double-tailed surfactants, on the other hand, gave low CMC values in dodecane but with R(h) values that indicated a tight micelle structure. Bilayer formation was expected but not observed for these molecules, which is believed to be due to their unusual structural geometry, imparted by the glycerol backbone. No thermotropic liquid crystalline (LC) behavior was observed for any of the single- or double-tailed molecules. These data contrast with the known behavior of perfluorinated alkanes and other fluorinated surfactants, suggesting that the ether linkage plays an important role in the self-organizing behavior of these molecules.

First anionic micelle with unusually long lifetime: self-assembly of fluorocarbon-hydrocarbon hybrid surfactant

The exchange of a fluorocarbon-hydrocarbon hybrid surfactant between monomer and micelle states in deuterium oxide has been investigated through 19F NMR and 1H NMR experiments. The CF3 group in the surfactant gives two kinds of 19F NMR signals corresponding to the monomer and micelle states, indicating slow surfactant exchange on NMR time scale. The lifetime (taumic) of micelle, estimated by line shape analysis of the signals, is 2.0 ms at cmc, 102 to 103 times longer than that of general surfactant micelles. Pulsed-gradient spin-echo (PGSE) experiments show that the hybrid surfactant forms considerably small micelles with a hydrodynamic radius of 0.6 nm. In contrast, at a higher concentration where no slow surfactant exchange is observed, the micelle radius increases to 1.1 nm. The interdigitation between the surfactant molecules in the micelle will contribute to the unusually long lifetime, in other words, slow surfactant exchange on the NMR time scale.

Nature of the liquid crystalline phase transitions in the cesium pentadecafluorooctanoate (CsPFO)-water system: the nematic-to-isotropic transition

Deuterium NMR spectroscopy of 2H2O has been used to monitor the magnetic-field-induced order on approaching a transition to a nematic phase in isotropic solutions of disklike micelles of cesium pentadecafluorooctanoate. Highly accurate data on the phase boundaries and spinodals have been obtained for solutions with volume fraction concentration straight phi between 0.078 and 0.201. The quantity TIN-T*, where T* is the spinodal limit of the isotropic phase and TIN is the temperature at which the nematic phase first appears on cooling, decreases linearly with decreasing concentration, extrapolating to zero only at zero concentration. Thus, there is no evidence to support the presence of a Landau point along the transition line as has previously been conjectured. The values for (TIN-T*)/TIN are in the range 10(-5)-10(-4), up to two orders of magnitude smaller than corresponding values reported for calamitic thermotropic nematics. The transition gap (phiNI-phiIN)/phiIN approximately 0.33% for phi<0.20 is also very small, although finite as required for a first-order phase transition. These data, when combined with previously measured properties, present an intriguing picture of the isotropic-to-nematic phase transition in a paradigmatic system of self-assembled diskotic particles. However, it is not completely clear, within the context of current theoretical understanding, whether the behavior of this system is explicable by hard-particle models, or if the self-assembly plays a crucial role in weakening the phase transition.

Article

Kinetic studies through stopped-flow spectroscopy were undertaken in the dilute solution range of anionic surfactants where pronounced rate enhancement or inhibition of Ni2+-ligand complexations is often observed at surfactant concentrations much below the critical micelle concentration (CMC). The results are interpreted in terms of Ni-surfactant micelles as the agents responsible for the rate changes in dilute surfactant solution. At higher surfactant concentrations these micelles are transformed into mixed micelles (counterion and size changes), eventually becoming normal surfactant micelles close to the CMC. Surface tension, dye solubility, conductivity, and fluorescent probe investigations support this interpretation.Key words: micellar catalysis, sodium dodecyl sulfate, micelles, critical micelle concentration, premicelles, Ni2+-ligand complexations.

Surfactosomes: a novel approach to the reconstitution and 2-D crystallisation of membrane proteins

The formation of vesicle-like structures (termed surfactosomes) and lamellar sheets from solutions containing ammonium perfluoroocanoate (APFO) is illustrated using conventional and cryo-transmission electron microscopy. It is shown how this detergent can be used for the solubilisation, reconstitution and 2-D crystallisation of membrane proteins as demonstrated for the major protein of the membrane sector of the V-type H+-ATPase (16-kDa protein). Electron microscopical analysis of 2-D crystals of the 16-kDa protein (a=b=13.0+/-0.2nm with gamma = 90 degrees and p4 projection symmetry) revealed a unit cell comprising four dimeric complexes of the 16-kDa protein the significance of which is discussed.