Phase behavior and rheological properties of salt-free catanionic TTAOH/DA/H2O system in the presence of hydrophilic and hydrophobic salts

In the cationic and anionic (catanionic) surfactant mixed system, tetradecyltrimethylammonium hydroxide (TTAOH)/decanoic acid (DA)/H(2)O, abundant phase behaviors were obtained in the presence of hydrophilic and hydrophobic salts. The microstructures of typical L(α) phases with the different compositions were characterized by the transmission electron microscope (TEM) images. Aqueous double-phase transition induced by addition of hydrophilic salts was observed when the cationic surfactant was in excess. Salt-induced reversible vesicle phases could be obtained when the anionic surfactant was excess, whereas the vesicle phase at lower salinity behaves highly viscoelastic but is much less viscoelastic with high salinity which was demonstrated by measuring their rheological properties. The L(α) phase with the positive membrane charges can be finally transferred into an L(1) phase with added salts. The ion specificity of hydrophilic and hydrophobic salts is discussed, and the order of cations is summarized, which is significant for the further study of the Hofmeister effects on catanionic surfactant mixed systems.     

Phase behavior and properties of reverse vesicles in salt-free catanionic surfactant mixtures

Salt-free 1:1 cationic/anionic (catanionic) surfactant mixture tetradecyltrimethylammonium laurate (TTAL) could be prepared by mixing equimolar tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA) in water. Given the condition of suitable range of weight fraction of TTAL in total surfactant, rho=WTTAL/(WTTAL+WLA), and at existence of a small amount of water, it was found that the mixtures of so-obtained TTAL and LA could spontaneously form stable reverse vesicles in various organic solvents including toluene, tert-butylbenzene, and cyclohexane. The reverse vesicle phase shows a blue color against room light and exhibits strong birefringence under polarized microscope. The reverse vesicles are very sensitive to temperature change. Increasing temperature could make the rho values within which reverse vesicles were constructed move to higher values. In organic solvents of alkanes such as n-heptane, reverse vesicles could still form but become unstable upon time and centrifugation. Increasing temperature could accelerate phase separation, and finally a gel-like bottom phase was usually observed. Interestingly, the stable reverse vesicles formed by so-called salt-free catanionic surfactant mixtures still show some resistance against adding inorganic salts. They can trap inorganic ions such as Zn2+ and S2- into their hydrophilic layers. This opens the door for template applications of reverse vesicles to prepare inorganic nanoparticles.     

Phase behavior of salt-free catanionic surfactant aqueous solutions with fullerene C60 solubilized

A salt-free catanionic surfactant system, tetradecyltrimethylammonium laurate (TTAL), was constructed by mixing tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA). The H+ and OH- counterions form water (TTAOH+LA-->TTAL+H2O), leaving the solution salt-free. The phase behaviors at fixing the total surfactant concentration (cTTAL) to be 33.0 and 55.0 mmol L(-1), respectively, were studied through varying the molar ratio of r=nLA/nTTAOH from 0.70 to 1.20. With an increasing value of r, one observed an L1-region, an Lalpha/L1 two-phase region with a birefringent Lalpha-phase at the top, and finally a single Lalpha-phase. The ability to solubilize a fullerene mixture of C60 and C70 of different phases in different regions was tested. The colloidal stability and phase behavior of different phases with embedded fullerenes were investigated as a function of r, cTTAL, and weight ratio of fullerene to surfactant (WF/WTTAL). The 33.0 or 55.0 mmol L(-1) zero-charged vesicle-phase at r=1.00 could solubilize a considerable amount of fullerenes without macroscopic phase separation and obvious vesicular structure breakage. However, these colloidal solutions became unstable at lower concentrations of surfactants, and a precipitate would be observed at the bottom. The micellar (L1-phase) solubilization at the TTAOH-rich side was less pronounced compared to the vesicular solubilization of the zero-charged vesicle-phase, and the solubilizing ability decreased at higher r values. In the Lalpha/L1 two-phase region, a brown or dark-brown Lalpha-phase was usually found at the top of a colorless or yellowish L1-phase, indicating that most of the fullerenes were embedded in the upper Lalpha-phase. The influence of fullerene incorporation on the property of the zero-charged TTAL vesicle-phase was also investigated, and evidence has been found that the system tended to be more fluid after fullerenes were incorporated into the hydrophobic microdomains of aggregates.     

Phase behavior and rheological properties of lecithin/TTAOH/H<Subscript>2</Subscript>O mixtures

The phase behavior, structures, and rheological properties of lecithin/tetradecyltrimethylammonium hydroxide (TTAOH)/water system were investigated by cryogenic transmission electron microscopy (cryo-TEM), polarization optical microscope, ¹H and ³¹P nuclear magnetic resonance (NMR) spectra, surface tension, and rheological measurements. With the variation of mixing molar ratios and concentrations of lecithin and TTAOH, the system exhibits the phase transition from micelles (L₁ phase) to vesicles (L_α phase) through a phase separation region. The rod-like micelles, uni- and multilamellar vesicles were determined by means of cryo-TEM observations. The surface tension and rheological measurements were performed to follow the phase transition. The samples of L₁ phase region behave as Newton fluids at low concentration of lecithin. With the increase of the lecithin concentration, a shear-thinning L₁ phase at the shearing rate 100 s⁻¹ was found. The samples of \textL_a {{\text{L}}_{\alpha }} phase region show viscoelastic properties of the typical vesicles. The interactions between lecithin and TTAOH were monitored by ¹H and ³¹P NMR spectra. These results could contribute towards the understanding of the basic function of lecithin in biological membranes and membranous organelles.     

Aggregation behavior and adsorption properties of salt-free catanionic surfactant mixtures containing tetradecyltrimethyl ammonium salts

The aggregation behavior of salt-free catanionic surfactants, tetradecyltrimethyl ammonium hydroxide (TTAOH)/fatty acid (FA) including octanoic acid (OA), decylic acid (DA) and lauric acid (LA) in aqueous solutions were studied. The critical micelle concentration(cmc), surface tension at cmc (γ_cmc), surface excess (Г_max), mean molecular surface area (A_min), adsorption efficiency (pc₂₀) and surface tension reduction effectiveness (π_cmc) were obtained from surface tension isotherms. The influence of temperature on the surface tension of salt-free TTAOH/FA (TTAOF) systems was investigated. Data of adsorption dynamics indicated that at fixed adsorption time, the order of adsorption capacity was TTAOH?相似文献   

Unusual vesicle-micelle transitions in a salt-free catanionic surfactant: temperature and concentration effects

The spontaneous formation of vesicles by the salt-free surfactant hexadecyltrimethylammonium octylsulfonate (TASo) and the features of an unusual vesicle-micelle transition are investigated in this work. In a previous work, we have shown that this highly asymmetric catanionic surfactant displays a rare lamellar miscibility gap in the concentrated regime. Here, we analyze in detail the aggregation behavior in the dilute regime (less than 3 wt % surfactant) as a function of both concentration and temperature. The phase diagram is dominated by a two-phase region consisting of a dispersion of a swollen lamellar phase (Lalpha') in the excess solvent phase (L1). Stable vesicles form in this two-phase region, and upon temperature increase, a transition to a single solution phase containing only elongated micelles occurs. The structural characterization of the aggregates and the investigation of their equilibrium properties have been carried out by light microscopy, cryo-TEM, water self-diffusion NMR, and SANS. Similarly to the lamellar-lamellar coexistence, the changes in microstructure at high dilution and high temperature can be understood from solubility differences, electrostatic interactions, and preferred aggregate curvature. Surface charge in the aggregates stems from the higher solubility of the octylsulfonate (So-) ion as compared to that of the hexadecyltrimethylammonium ion (TA+). Upon temperature increase, the ratio of free So(-) relative to the neutral TASo increases. Consequently, the surface charge density of the aggregates increases, and this ultimately induces a transition to a higher-curvature morphology (elongated micelles). Vesicles can also be spontaneously formed by cooling solutions from the micellar region, and the mean size obtained is practically independent of cooling rate, suggesting that dissociation/charge effects also control this process.     

Fluorescence delineation of the surfactant microstructures in the CTAB-sOS-H2O catanionic system

A key feature of amphiphilic molecules is their ability to undergo self-assembly, a process in which a complex hierarchical structure is established without external intervention. Ternary systems consisting of aqueous mixtures of cationic and anionic surfactants exhibit a rich array of self-assembled microstructures such as spherical and rodlike micelles, unilamellar and multilamellar vesicles, planar bilayers, and bicontinuous structures. In general, multiple complementary techniques are required to explore the phase behavior and morphology of aqueous systems of oppositely charged surfactants. As a novel and effective alternative approach, we use fluorescence spectroscopic measurements to examine the microstructures of aqueous cationic/anionic surfactant systems in the dilute surfactant region. In particular, we demonstrate that the polarity-sensitive fluorophore prodan can be used to demarcate the surfactant microstructures of the ternary system of cetyltrimethylammonium bromide, sodium octyl sulfate, and water. As the fluorescence signature of this probe is dependent on the nature of the surfactant aggregates present, our method is a promising new approach to effectively map complex surfactant phase diagrams.     

HPC/H2O/H3PO4 tertiary system: a rheological study

This study characterizes the rheological behavior of the HPC/H₂O/H₃PO₄ tertiary system based on the 3-D phase diagram that was obtained in our earlier work. The effects of frequency, temperature, HPC concentration, liquid composition (H₂O/H₃PO₄ ratio), and phase status on the rheological parameters were thoroughly investigated. The most useful parameter for distinguishing the isotropic (I) and liquid crystalline (LC) phases was tanδ. Agglomeration in the cloudy suspension (CS) phase at high temperature was too severe to allow a smooth flow, so the tanδ and η* represented significant damping, which is a good indicator of the presence of the CS phase. With an increase in temperature, the viscosity of the flow with a single homogeneous phase—either the I phase or the LC phase—or a combination of two homogeneous phases in sequence, obeyed the Arrhenius model. In contrast, once the temperature rose to that of the formation of heterogeneous CS phase, the Arrhenius model was no longer valid. The activation energy E of the I phase was greater, and more sensitive to the HPC concentration, than the LC phase. Finally, the concentration of the sol/gel transition (SGT) declined as temperature increased but increased as the H₃PO₄ content increased. Furthermore, this tertiary system exhibited no clear order of the onsets of the formations of SGT phase, the LC phase, and the CS phase as HPC concentration was varied.     

Vesicles of a new salt-free cat-anionic fluoro/hydrocarbon surfactant system

Weakly basic tetradecyldimethylaminoxide (C14DMAO) molecules can be protonated to form a cationic surfactant, C14DMAOH+, by an acidic fluorocarbon surfactant, an 8-2-fluorotelomer unsaturated acid (C7F15CF==CHCOOH), to form a salt-free cationic and anionic (cat-anionic) fluoro/hydrocarbon surfactant system in aqueous solution. The high Krafft point of C7F15CF==CHCOOH was largely reduced as a result of being mixed with a C14DMAO micelle solution. A study of the phase behavior of the new salt-free cat-anionic fluoro/hydrocarbon surfactant system clearly indicates the existence of a birefringent Lalpha-phase region at (25.0+/-0.1) degrees C. The birefringent Lalpha phase consists of vesicles, which include uni- and multilamellar vesicles with one to dozens of shells, and oligovesicular vesicles, as demonstrated by freeze-fracture and cryo-transmission electron microscopy (FF- and cryo-TEM) images. The size distribution and structural transitions in the salt-free cat-anionic fluoro/hydrocarbon surfactant system were studied by dynamic light scattering (DLS) and 1H and 19F NMR spectroscopy. The formation of a salt-free cat-anionic vesicle phase could be induced by the strong electrostatic interaction between the cationic hydrocarbon C14DMAOH+ and the anionic fluorocarbon C7F15CF==CHCOO-, which provided evidence that the electrostatic interaction between the cationic and anionic surfactants is larger than the nonsynergistic interaction between the stiff fluorocarbon and the soft hydrocarbon chains of the surfactants.     

Phase and rheological behavior of the polymerizable surfactant CTAVB and water

The phase and rheological behaviors of the polymerizable surfactant, cetyltrimethylammonium benzoate (CTAVB), and water as a function of surfactant concentration and temperature are investigated here. The critical micelle concentration (cmc) and the (cmc(2)), as well as the Krafft temperature (T(K)), are reported. A large highly viscous micellar solution region and hexagonal- and lamellar-phase regions were identified. The micellar solutions exhibit shear thickening in the dilute regime, below the overlapping or entanglement concentration. At higher concentrations, wormlike micelles form and the solutions show strong viscoelasticity and Maxwell behavior in the linear regime and shear banding flow in the nonlinear regime. The linear viscoelastic regime is analyzed with the Granek-Cates model, showing that the relaxation is controlled by the kinetics of reformation and scission of the micelles. The steady and unsteady responses in the nonlinear regime are compared with the predictions of the Bautista-Manero-Puig (BMP) model. Model predictions follow the experimental data closely.     

Densely stacked multilamellar and oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles of a salt-free catanionic extractant and surfactant system

In the phase diagram of an excellent extractant of rare earth metal ions, di(2-ethylhexyl) phosphate (HDEHP, commercial name P204), mixing with a cationic trimethyltetradecylammonium hydroxide (TTAOH) in water, a birefringent Lalpha phase was found, which consists of densely stacked multilamellar vesicles. The densely stacked multilamellar vesicles are remarkably deformed, as observed by means of cryotransmission electron microscopy (cryo-TEM). Further, self-assembled structures-oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles-were also observed. The self-assembled phase is transparent, anisotropic, and highly viscous, possessing elastic properties determined by rheological measurements. This is the first time that birefringent Lalpha phase with remarkably deformed amphiphilic bilayer membranes has been constructed through combining a hydrophobic organic extractant having double chains with a water-soluble surfactant having a single chain, which may direct primarily toward acquiring an understanding of the mechanism of salt-free catanionic vesicles and secondarily to determine if vesicle-extraction technology utilizing extractants is possible.     

Phase behavior and rheological properties of DNA-cationic polysaccharide mixtures

Associative aqueous mixtures over a range of concentrations of double- (ds) or single- (ss) stranded DNA with dilute or semidilute solutions of two cationic derivatives of hydroxyethyl cellulose (cat-HEC and cat-HMHEC,(1) the latter carrying grafted hydrophobic groups), were studied. The phase behavior showed an interesting asymmetry: Phase separation occurred immediately when small (sub-stoichiometric) amounts of cationic polyelectrolyte were added to the DNA solution, but redissolution into a single cat-(HM)HEC/DNA/H(2)O phase occurred already with a modest charge excess of the cationic polyelectrolyte, at a charge ratio approximately independent of the overall polyelectrolyte concentration. Cat-HEC/dsDNA/H(2)O and cat-HEC/ssDNA/H(2)O systems presented a considerable difference in the extension of the phase separation region. The one-phase samples with excess cationic polyelectrolyte were studied by rheology. The presence of DNA strengthened the viscoelastic behavior of the solutions of the cationic polyelectrolytes, reflected in an increase in storage modulus and viscosity. Differences in phase behavior and rheology were observed, particularly between systems containing cat-HEC or cat-HMHEC, but also between dsDNA and ssDNA. Thus, these systems allow for the preparation of DNA formulations with widely variable rheology and water uptake.     

The peroxidase/H2O2 system as a free radical-generating agent for gelling maize bran arabinoxylans: rheological and structural properties

The oxidative gelation of maize bran arabinoxylans (MBAX) using a peroxidase/H(2)O(2) system as a free radical-generating agent was investigated. The peroxidase/H(2)O(2) system led to the formation of dimers and trimer of ferulic acid as covalent cross-link structures in the MBAX network. MBAX gels at 4% (w/v) presented a storage modulus of 180 Pa. The structural parameters of MBAX gels were calculated from swelling experiments. MBAX gels presented a molecular weight between two cross-links (Mc), a cross-linking density (ρ(c)) and a mesh size (x) of 49 × 103 g/mol, 30 × 10-6 mol/cm3 and 193 nm, respectively.     

Metal-ligand coordinated Ca(DS)_2/C_(14)DMAO/H_2O system:Phase behavior and rheological property

A metal-ligand coordinated surfactant system formed by calcium dodecylsulfate(Ca(DS)2)/tetradecyldimethylamine oxide (C14DMAO)/H2O was studied in terms of surface tension,conductivity,negative-staining TEM,phase behavior and rheological operation.In C14DMAO solution,when Ca(DS)2 is added,metal-ligand complexes form between the Ca 2+and N→O group of C14DMAO.Under this metal-ligand driving force,different phases can be obtained at different concentrations and different ratios of Ca(DS)2 and C14DMAO.At the fix...     

Phase formation and transition in a xanthan gum/H2O/H3PO4 tertiary system

Phase formation and transition in a xanthan gum (XG)/H₂O/H₃PO₄ tertiary system were characterized by polarized optical microscopy, light transmission detection and rheological methods. Three distinct phases and a transition region—the completely separated (S) phase, the liquid crystalline (LC) miscible phase, the isotropically (I) miscible phase and the S plus LC region—were identified. The presence of H₃PO₄ in the XG/H₂O system inhibited the evolution of both the S and LC phases. The S and LC phases contained less than 73 and 62 wt% of H₃PO₄, respectively. As the temperature increased over 65 °C, the LC phase in the H₃PO₄-rich and H₂O-poor region seriously shrunk owing to the breakup of hydrogen bonds among the XG helical structure. At the same XG loading, the viscosity of the XG solutions in LC phase was found to be much higher than that in I phase. It indicated the existence of numerous XG intermolecular interactions in the LC phase that suppress the movement of liquid. A study of the kinetics demonstrated that the shrinkage relaxation time (τ) depended strongly on temperature and was fitted by the Volgel-Fulcher-Tammann (VFT) expression. The potential energy barrier of this liquid was quite low at approximately 3.0 kJ mol^?1, falling in the range of hydrogen-bond disassociation. The light absorbance test in heating mode revealed a biphasic transitional region between the LC phase and I phase. The contour of this region depended on the heating rate, and this fact was explained again by the relaxation behavior of XG helices at temperatures higher than 65 °C.     

Photochemical control of molecular assembly formation in a catanionic surfactant system

Photochemical control of vesicle disintegration and reformation in aqueous solution was examined using a mixture of 4-butylazobenzene-4'-(oxyethyl)trimethylammonium bromide (AZTMA) as the photoresponsive cationic surfactant and sodium dodecylbenzenesulfonate (SDBS) as the anionic surfactant. Spontaneous vesicle formation was found in a wide-ranging composition of the trans-AZTMA/SDBS system. AZTMA molecules constituting vesicles underwent reversible trans-cis photoisomerization when irradiated with ultraviolet and visible light. Transmission electron microscopy observations using the freeze-fracture technique (FF-TEM) showed that UV light irradiation caused the vesicles to disintegrate into coarse aggregates and visible light irradiation stimulated the reformation of vesicles (normal control). A detailed investigation of the phase state and the effects of UV and visible light irradiation on the AZTMA/SDBS system with the use of electroconductivity, dynamic/static light scattering, and surface tension measurements and FF-TEM observations revealed that in the AZTMA-rich composition (AZTMA/SDBS 9:1) a micellar solution before light irradiation became a vesicular solution after UV light irradiation and visible light irradiation allowed the return to a micellar solution (reverse control). Thus, we could photochemically control the disintegration (normal control) and reformation (reverse control) of vesicles in the same system.     

Sugar-derived tricatenar catanionic surfactant: self-assembly and aggregation behavior in the cationic-rich side of the system

The aggregation behavior of the cationic-rich side of a sugar-based tricatenar catanionic mixture was investigated in water, and it was shown that the excess of cationic sugar-based surfactant enhanced vesicle stability as well as encapsulation properties. Moreover, when the system was diluted, the vesicular solution collapsed into a lamellar phase, whereas, when it was concentrated, no major impact on the shape and stability of the aggregates was observed. We also showed that both an increase in temperature and the addition of salt induced reversible vesicle aggregation, which appeared to be salt-specific, following the direct order of the Hofmeister series. A proper adjustment of these parameters should then enable better control of the shape, stability, and even encapsulation ability of the aggregates formed by these tricatenar cationic/anionic mixtures.     

Dynamical and rheological properties of fluorinated surfactant films adsorbed at the pressurized CO2-H2O interface

The dynamics of adsorption, interfacial tension, and rheological properties of two phosphocholine-derived partially fluorinated surfactants FnHmPC, designed to compensate for the weak CO(2)-surfactant tail interactions, were determined at the pressurized CO(2)-H(2)O interface. The two surfactants differ only by the length of the hydrocarbon spacer (5 CH(2) in F8H5PC and 11 CH(2) in F8H11PC) located between the terminal perfluoroalkyl chain and the polar head. The length of this spacer was found to have a critical impact on the adsorption kinetics and elasticity of the interfacial surfactant film. F8H5PC is soluble in both water and CO(2) phases and presents several distinct successive interfacial behaviors when bulk water concentration (C(W)) increases and displays a nonclassical isotherm shape. The isotherms of F8H5PC are similar for the three CO(2) pressures investigated and comprise four regimes. In the first regime, at low C(W), the interfacial tension is controlled by the organization that occurs between H(2)O and CO(2). The second regime corresponds to the adsorption of the surfactant as a monolayer until the CO(2) phase is saturated with F8H5PC, resulting in a first inflection point. In this regime, F8H5PC molecules reach maximal compaction and display the highest apparent interfacial elasticity. In the third regime, a second inflection is observed that corresponds to the critical micelle concentration of the surfactant in water. At the highest concentrations (fourth regime), the interfacial films are purely viscous and highly flexible, suggesting the capacity for this surfactant to produce water-in-CO(2) microemulsion. In this regime, surfactant adsorption is very fast and equilibrium is reached in less than 100 s. The behavior of F8H11PC is drastically different: it forms micelles only in the water phase, resulting in a classical Gibbs interface. This surfactant decreases the interfacial tension down to 1 mN/m and forms a strongly elastic interface. As this surfactant forms a very cohesive interface, it should be suitable for formulating stable water-in-CO(2) emulsions. The finding that the length of the hydrocarbon spacer in partially fluorinated surfactants can drastically influence film properties at the CO(2)-H(2)O interface should help control the formation of microemulsions versus emulsions and help elaborate a rationale for the design of surfactants specifically adapted to pressurized CO(2).