Phase behavior and self-assembly aggregation of hydrocarbon and fluorocarbon surfactant mixtures in aqueous solution

Surface Reconstruction of Fluoropolymers in Liquid Media

Surface reconstruction is the rearrangement of atoms or molecules at an interface in response to a stimulus, driven by lowering the overall free energy of the system. Perfluoroalkyl acrylate polymers with short side chains undergo reconstruction at room temperature when exposed to water. Here, we use contact angle aging to examine the liquid- and temperature- dependency of surface reconstruction of plasma polymerized perfluoroalkyl acrylates. We use a first order kinetic model to examine the dynamics of reconstructive processes. Our results show that, above the bulk melting point of the polymers, the contact angles of both polar and nonpolar (hydrocarbon) liquids show a time dependency well fit by the model. We conclude that surface reconstruction can be driven by the preferential segregation of hydrocarbon and fluorocarbon moieties as well as by polar interactions. This has implications in terms of using fluorocarbons to design oleophobic surfaces (and vice versa) and in terms of designing fluorocarbon and/or hydrocarbon surfaces with switchable wettability.

Surface Activity and Efficiency of Cat-Anionic Surfactant Mixtures

The surface activity of surfactant mixtures is critically analyzed. Cat-anionic systems, in which two ionic species are mixed in non-stoichiometric ratios, are considered. With respect to the solution behavior, where a substantial decrease of cmc is met compared to the pure components, a moderate effect on surface tension, γ, occurs. Compared to the pure species, the decrease of surface tension for such mixtures is not significant, and no clear dependence on the mole fraction anionic/cationic is met. The surface tension is grossly constant in the whole concentration range. Conversely, the interaction parameter for surfaces, β_surf (calculated by the regular solution theory), is more negative than that for micelle formation, β_mic. This fact suggests that the desolvation of polar heads of the two species at interfaces is largely different. Very presumably, the underlying rationale finds origin in the sizes and solvation of both polar head groups.

Glucose-Based Fluorinated Surfactants as Additives for the Crystallization of Membrane Proteins: Synthesis and Preliminary Physical-Chemical and Biochemical Characterization

We report herein the synthesis of a series of fluorinated surfactants with a glucose moiety as a polar head group and whose alkyl chain was varied in length and in fluorine/hydrogen ratio. They were synthesized in two or four steps in 20 to 50% overall yields allowing gram-scale synthesis. Their solubility in water is between 0.2 and 13.8 g/L, which indicates low water solubility. Two derivatives of the series were found to form micelles in water at ∼11 mM. Their hydrophilic-lipophilic balance was determined both by Griffin's and Davies' methods; they may exhibit a "harsh" character toward membrane proteins. This, combined with their low water solubility, suggest that they could advantageously be used in detergent mixtures containing a "mild" detergent. Finally, the potency of one of the derivatives, F₃H₅-β-Glu, to act as an additive for the crystallization of AcrB was evaluated in detergent mixtures with n-dodecyl-β-d-maltopyranoside (DDM). Among the six crystallization conditions investigated, adding F₃H₅-β-Glu improved the crystallization for three of them, as compared to control drops without additives. Moreover, preliminary tests with other compounds of the series showed that none of them hampered crystallization and suggested improvement for three of them. These novel glucose-based fluorinated detergents should be regarded as potential additives that could be included in screening kits used in crystallization.

Properties of some nonionic fluorocarbon surfactants and their mixtures with hydrocarbon ones

The adsorption of Zonyl FSN-100 (FSN100, having an average 14 oxyethylene units and 6 -CF₂ groups) and Zonyl FSO-100 (FSO100, having an average 10 oxyethylene units and 5 -CF₂ groups) as well as of their mixtures with p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycols) having 10, 16 and 8 oxyethylene groups in molecule (TX100, TX165, TX114) and cetyltrimethylammonium bromide (CTAB) at the solution-air and polytetrafluoroethylene (PTFE)-solution and polymethyl methacrylate (PMMA)-solution interfaces as well as the composition of the surface mixed layer was discussed based on the literature data. The adsorption properties of nonionic fluorocarbon surfactants were compared to those of the classical ones on the basis of the Gibbs standard free energy of adsorption determined by different ways and the intermolecular interactions of the surfactant molecules through the water phase. The synergetic effect in the reduction of the water surface tension by the mixture of fluorocarbon and classical nonionic surfactant was shown and explained by the comparison of the composition of the mixed surface layer to those in the bulk phase. The composition of the mixed fluorocarbon and classical surfactant layer at the solution-air interface was compared to that formed at the PTFE-solution and PMMA-solution interfaces. The changes of the surface tension of the aqueous solution of the fluorocarbon surfactants and their mixtures with classical hydrocarbon ones and their adsorption were analyzed taking into account the PTFE and PMMA surface wettability. This analysis was also based on the components and parameters of the head and tail of the surfactants surface tension as well as those of PTFE and PMMA. Apart from adsorption and wetting properties the aggregation of the fluorocarbon surfactants and their mixtures was discussed. A specific attention was paid to the possibility of two CMC values in the case of nonionic fluorocarbon surfactants as well as the synergism in CMC of mixtures of nonionic fluorocarbon and hydrocarbon surfactants.

Synthesis and Characterization of a Novel Class of Zwitterionic Fluorocarbon Surfactants Based on Perfluorobutyl

Perfluorooctane sulfonate (PFOS) and its derivatives had been banned due to their potential environmental hazards, although they possessed excellent surface activity. An effective method to solve this problem was to shorten the fluorocarbon chain of these surfactants from C°H17 to C4F9. As previous studies had shown, zwitterionic surfactants possess higher surface activity but have lower toxicity compared to other types of surfactants. In view of this, a class of novel zwitterionic fluorocarbon surfactants (n-CFNA-Br) with perfluorobutyl moiety was synthesized in this work. Their structures were characterized by FTIR, 1H NMR, 13C NMR, 19F NMR and MS. The results showed that all synthesized n-CFNA-Br had almost the same minimum surface tension, but their critical micelle concentration (CMC) decreased with increasing length of hydrophobic carbon chain. In pure water, the surface tension at the CMC (γCMC) of the four n-CFNA-Br were about 20 mN/m, and the CMC values were 7.73 mmol/L for 1-CFNA-Br, 4.70 mmol/L for 2-CFNA-Br, 4.13 mmol/L for 3-CFNA-Br, and 3.36 mmol/L for 4-CFNA-Br, indicating high efficiency and effectiveness. In 0.1 mol/L NaCl, the CMC values reduced to less than half of the CMC values measured in the pure aqueous surfactant solution, while the surface tensions γCMC remained almost unchanged, indicating good salinity tolerance of the synthesized surfactants. The acidic surfactant solutions exhibited similar CMC values to the saline solutions, but the surface tension γCMC increased slightly to 25 mN/m. However, further investigation showed that the n-CFNA-Br surfactants exhibited poor surface activity in alkaline solution (0.1 mol/L NaOH). In the pH range of 6.6 to 10.4, white precipitates appeared in the surfactant solutions after some time, indicating that the n-CFNA-Br are not suitable for use in alkaline systems with pH greater than 6.6.

Environmental applications of a biodegradable cysteine-based surfactant

An improved method has been developed for the efficient synthesis of octanoyl-cysteine in single-chain form (N-octanoyl-cys) which operates as a surfactant over a wide pH range, is easily decomposed into natural products and has a high product yield. The compound offers an environmentally acceptable agent for the adsorption of a range of heavy metals from contaminated waters/soils, and it could also be used in general household detergents or personnel-cleaner formulations, and even in toothpastes. The surfactant was used as a co-surfactant for flotation of perfluorooctanoic acid (PFOA), a per/poly-fluoroalkyl substance (PFAS). The new surfactant produced significant foaming and removed 70% of the PFOA after 30min of foam fractionation. The compound is also potentially useful in facilitating the release of PFAS compounds; these are negatively charged and often bound to charged particles in sand, clay, and humic-acid-coated materials and microorganisms via bridging multivalent ions, such as Ca²⁺, Mg²⁺, Al³⁺ and Fe³⁺, as well as a range of other heavy-metal ions present in soil to varying degrees. In soils (and also in contaminated water), the common chelating agent EDTA is often used to encapsulate these ions (e.g. Ca²⁺, the dominant ion in soil) at moderately high pH to aid in the release of the bound PFAS compounds. However, it would be more environmentally acceptable to use this new biodegradable surfactant, which could combine chelation with foam-fractionation separation of surface-active (e.g. PFAS) components in soils.

Synergistic Properties of a Mixed Micelle System Consisting of a Nonionic Fluorinated Surfactant and a Cationic Surfactant

The physicochemical properties of aqueous surfactant mixtures containing a nonionic fluorosurfactant (undecafluoro-n-pentyldecaoxyethylene ether (C5F11EO10)) and various amounts of a cationic surfactant (decyltrimethylammonium bromide (DeTAB)) were determined by surface tension and conductivity measurements. All values of the critical micelle concentrations of the mixtures turn out to be smaller than those of both pure surfactants revealing the presence of a significant synergy for all DeTAB proportions in the mixed system. The analysis of the experimental data was performed on the basis of three different well established thermodynamic models of mixed micelle formation to determine several relevant parameters, especially the micelle composition, the interaction parameters, and the free energy of micelle formation. The results indicate that the dominant interactions between DeTAB and C5F11EO10 molecules are attractive. The main reason for this behavior could be attributed to the complexation between the polyoxyethylene chain of C5F11EO10 and the quaternary ammonium group of DeTAB giving rise to stable structures.

Routes to the preparation of mixed monolayers of fluorinated and hydrogenated alkanethiolates grafted on the surface of gold nanoparticles

The use of binary blends of hydrogenated and fluorinated alkanethiolates represents an interesting approach to the construction of anisotropic hybrid organic-inorganic nanoparticles since the fluorinated and hydrogenated components are expected to self-sort on the nanoparticle surface because of their reciprocal phobicity. These mixed monolayers are therefore strongly non-ideal binary systems. The synthetic routes we explored to achieve mixed monolayer gold nanoparticles displaying hydrogenated and fluorinated ligands clearly show that the final monolayer composition is a non-linear function of the initial reaction mixture. Our data suggest that, under certain geometrical constraints, nucleation and growth of fluorinated domains could be the initial event in the formation of these mixed monolayers. The onset of domain formation depends on the structure of the fluorinated and hydrogenated species. The solubility of the mixed monolayer nanoparticles displayed a marked discontinuity as a function of the monolayer composition. When the fluorinated component content is small, the nanoparticle systems are fully soluble in chloroform, at intermediate content the nanoparticles become soluble in hexane and eventually they become soluble in fluorinated solvents only. The ranges of monolayer compositions in which the solubility transitions are observed depend on the nature of the thiols composing the monolayer.

Amphiphile self-assemblies in supercritical CO2 and ionic liquids

Supercritical (sc) CO2 and ionic liquids (ILs) are very attractive green solvents with tunable properties. Using scCO2 and ILs as alternatives of conventional solvents (water and oil) for forming amphiphile self-assemblies has many advantages. For example, the properties and structures of the amphiphile self-assemblies in these solvents can be easily modulated by tuning the properties of solvents; scCO2 has excellent solvation power and mass-transfer characteristics; ILs can dissolve both organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. Therefore, the amphiphile self-assemblies in scCO2 and ILs have attracted considerable attention in recent years. This review describes the advances of using scCO2 or/and ILs as amphiphile self-assembly media in the last decade. The amphiphile self-assemblies in scCO2 and ILs are first reviewed, followed by the discussion on combination of scCO2 and ILs in creating microemulsions or emulsions. Some future directions on the amphiphile self-assemblies in scCO2 and ILs are highlighted.

Controlling the morphology of membranes by excess surface charge in cat-anionic fluorinated surfactant mixtures

The segregation and phase sequence of semifluorinated cat-anionic surfactant membranes at different excess surface charges was investigated by freeze-fracture transmission electron microscope (FF-TEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR). The thermal behavior of the membranes was evaluated by conductivity, rheology, and deuterium nuclear magnetic resonance ((2)H NMR). The experimental results show that the cat-anionic fluorinated surfactant mixtures can form faceted vesicles and punctured lamellar phase when there is excess surface charge. The cationic and anionic fluorinated surfactants are stiff in the membranes, like phospholipids in the frozen "crystalline" or "gel" phase. For the system with excess cationic surface charge, the gel-like faceted vesicles and punctured lamellae can transform into smooth-shaped vesicles at 65 °C. However, for the system with no excess charge or with excess anionic surface charge, no phase transformation occurs even at 90 °C. A model was established to demonstrate the mechanism of the formation and transition of the aggregates with different morphologies. The segregation-crystallization mechanism works well with other cosmotropic counterions from the Hofmeister series. The observations provide a better understanding of how to control the membrane morphology of the aqueous solutions of cat-anionic surfactant mixtures.

Line tension and line activity in mixed monolayers composed of aliphatic and terphenyl-containing surfactants

Custom-designed surfactants, known as "linactants", have the ability to reduce the line tension between coexisting phases within mixed monolayers of chemically dissimilar compounds at the air-water interface. Thus far, linactants have been successfully identified for only one type of chemical dissimilarity, involving mixed monolayers of hydrocarbon and fluorocarbon surfactants. In the present work, we have pursued a more general interpretation of linactant compounds by extending the concept to a new system that is comprised of a mixture of aliphatic (pentadecanoic acid) and aromatic (p-terphenyl carboxylic acid) compounds. We found that the "bare" line tension between phases of this mixed monolayer was ~4 pN, and within the same order of magnitude as our previous measurement in mixed monolayers containing hydrocarbons and fluorocarbons. Furthermore, we examined a homologous series of potential linactant compounds possessing an aliphatic tail of variable length and a p-terphenyl block. We determined that linactants with longer tails were able to reduce the line tension more efficiently and effectively. In particular, the addition of only 0.14% of a linactant with an 11-carbon chain reduced the line tension by more than a factor of 2. We hypothesize that the efficiency of this particular linactant is associated with its long tail; this creates strong van der Waals interactions with the aliphatic chains and enables the tail to adopt conformations that facilitate π-stacking interactions with the aromatic compounds within the monolayer.