Phase behavior, topology, and growth of neutral catanionic reverse micelles

The ternary catanionic system octylammoniumoctanoate/octane/water is studied by combined SANS, light scattering, conductivity, and phase diagram approach in the water-poor microemulsion region. The sphere-to-cylinder growth and branching depends on the concentration, the water-to-surfactant ratio, and the temperature. The unidimensional growth leads to a network of interconnected wormlike micelles. Like most studied linear nonionic surfactants, in this true catanionic system at equimolarity of anionic and cationic surfactant, the curvature toward water increases with temperature, making connections between cylinders less frequent.     

Phase behavior in mixtures of cationic and anionic surfactants in aqueous solutions

Phase behavior of cationic/anionic surfactant mixtures of the same chain length (n=10, 12 or 14) strongly depends on the molar ratio and actual concentration of the surfactants. Precipitation of catanionic surfactant and mixed micelles formation are observed over the concentration range investigated. Coacervate and liquid crystals are found to coexist in the transition region from crystalline catanionic surfactant to mixed micelles.The addition of oppositely charged surfactant diminishes the surface charge density at the mixed micelle/solution interface and enhances the apparent degree of counterion dissociation from mixed micelles. Cationic surfactants have a greater tendency to be incorporated in mixed micelles than anionic ones.     

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.     

Spontaneous vesicle formation in catanionic mixtures of amino acid-based surfactants: chain length symmetry effects

The use of amino acids for the synthesis of novel surfactants with vesicle-forming properties potentially enhances the biocompatibility levels needed for a viable alternative to conventional lipid vesicles. In this work, the formation and characterization of catanionic vesicles by newly synthesized lysine- and serine-derived surfactants have been investigated by means of phase behavior mapping and PFG-NMR diffusometry and cryo-TEM methods. The lysine-derived surfactants are double-chained anionic molecules bearing a pseudogemini configuration, whereas the serine-derived amphiphile is cationic and single-chained. Vesicles form in the cationic-rich side for narrow mixing ratios of the two amphiphiles. Two pairs of systems were studied: one symmetric with equal chain lengths, 2C12/C12, and the other highly asymmetric with 2C8/C16 chains, where the serine-based surfactant has the longest chain. Different mechanisms of the vesicle-to-micelle transition were found, depending on symmetry: the 2C12/C12 system entails limited micellar growth and intermediate phase separation, whereas the 2C8/C16 system shows a continuous transition involving large wormlike micelles. The results are interpreted on the basis of currently available models for the micelle-vesicle transitions and the stabilization of catanionic vesicles (energy of curvature vs mixing entropy).     

Probing the microstructure of nonionic microemulsions with ethyl oleate by viscosity, ROESY, DLS, SANS, and cyclic voltammetry

Microemulsions are important formulations in cosmetics and pharmaceutics and one peculiarity lies in the so-called "phase inversion" that takes place at a given water-to-oil concentration ratio and where the average curvature of the surfactant film is zero. In that context, we investigated the structural transitions occurring in Brij 96-based microemulsions with the cosmetic oil ethyl oleate and studied the influence of the short chain alcohol butanol on their structure and properties as a function of water addition. The characterization has been carried out by means of transport properties, spectroscopy, DLS, SANS, and electrochemical methods. The results confirm that the nonionic Brij 96 in combination with butanol as cosurfactant forms a U-type microemulsion that upon addition of water undergoes a continuous transition from swollen reverse micelles to oil-in-water (O/W) microemulsion via a bicontinuous region. After determining the structural transition through viscosity and surface tension, the 2D-ROESY studies give an insight into the microstructure, i.e., the oil component ethyl oleate mainly is located at the hydrophobic tails of surfactant while butanol molecules reside preferentially in the interface. SANS experiments show a continuous increase of the size of the structural units with increasing water content. The DLS results are more complex and show the presence of two relaxation modes in these microemulsions for low water content and a single diffusive mode only for the O/W microemulsion droplets. The fast relaxation reflects the size of the structural units while the slower one is attributed to the formation of a network of percolated microemulsion aggregates. Electrochemical studies using ferrocene have been carried out and successfully elucidated the structural transformations with the help of diffusion coefficients. An unusual behavior of ferrocene has been observed in the present microheterogeneous medium, giving a deeper insight into ferrocene electrochemistry. NMR-ROESY experiments give information regarding the internal organization of the microemulsion droplets. In general, one finds a continuous structural transition from a W/O over a bicontinuous to an O/W microemulsion, however with a peculiar network formation over an extended concentration range, which is attributed to the somewhat amphiphilic oil ethyl oleate. The detailed knowledge of the structural behavior of this type of system might be important for their future applications.     

Decrease of droplet size of the reverse microemulsion 1-butyl-3-methylimidazolium tetrafluoroborate/Triton X-100/cyclohexane by addition of water

In the present contribution, results concerning the role of small amounts of water in the 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane ionic liquid (IL) reverse microemulsions are reported. Dynamic light scattering (DLS) revealed that the size of microemulsion droplets decreased remarkably with increasing water content although water is often used as a polar component to swell reverse microemulsions. It was thus deduced that the number of microemulsion droplets was increased which was confirmed by conductivity measurements. The states of dissolved water were investigated by Fourier transform IR (FTIR) spectroscopic analysis showing that water molecules mainly act as bound water. 1H NMR along with two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) further revealed that water molecules were mainly located in the periphery of the polar core of the microemulsion droplets and behave like a chock being inserted in the palisade layer of the droplet. This increased the curvature of the surfactant film at the IL/cyclohexane interface and thus led to the decrease of the microemulsion droplet size. The order of surfactant molecules arranged in the interface film was increased and thus induced a loss of entropy. Isothermal titration calorimetry (ITC) indicated that an enthalpy increase compensates for the loss of entropy during the process of microstructural transition.     

非离子表面活性剂Brij 30诱导离子液体型表面活性剂C16imC8Br蠕虫状胶束向凝胶的转变

Wormlike micelles and low-molecular-weight hydrogels are composed of three-dimensional networks that endow them with viscoelasticity, but their viscoelastic properties are markedly different. The viscosity of wormlike micelles is attributed to a transient network, while that of gels is due to a stable network. Under certain conditions, wormlike micelles can undergo transition to gels with an increase in the density of the network. In our previous study, we found that the wormlike micelle formed by the ionic liquid-type surfactant 1-hexadecyl-3-octyl imidazolium bromide ([C₁₆imC₈]Br) without any additive has high viscoelasticity. The inclusion of a nonionic surfactant polyoxyethylene lauryl ether (Brij 30) is expected to enhance the viscoelasticity of [C₁₆imC₈]Br wormlike micelles via electrostatic shielding and strong hydrophobic interactions, which may be the driving factor for the wormlike micelle-to-gel structural transition. The morphology and viscoelasticity of [C₁₆imC₈]Br wormlike micelles with Brij 30 were studied as a function of concentration by rheological measurements and freeze-fracture transmission electron microscopy. The thermal stability and gel-sol transition temperature of the Brij 30/[C₁₆imC₈]Br gels were studied using rheology. The interaction between Brij 30 and [C₁₆imC₈]Br was studied by zeta potential measurements and nuclear magnetic resonance (NMR) spectroscopy. Upon the inclusion of Brij 30 into the [C₁₆imC₈]Br wormlike micelles, the viscoelasticity of the Brij 30/[C₁₆imC₈]Br samples first increased and then decreased with an increase in the Brij 30 concentration, at different initial concentrations of [C₁₆imC₈]Br. At a certain Brij 30 concentration, the Brij 30/[C₁₆imC₈]Br samples rheologically behaved as a gel. The maximum viscoelasticity of the [C₁₆imC₈]Br (4.06% (w))/Brij 30 gel was observed at a Brij 30/[C₁₆imC₈]Br molar ratio of 4.55. The viscoelasticity of the Brij 30/[C₁₆imC₈]Br gels was positively correlated with the activation energy of the gels. The gel-sol transition temperature of the Brij 30/[C₁₆imC₈]Br gels also increased first and then decreased with an increase in the Brij 30 concentration. The highest gel-sol transition temperature of the Brij 30/[C₁₆imC₈]Br (4.06% (w)) gel was observed at a Brij 30/[C₁₆imC₈]Br molar ratio of 2.93. The Brij 30 concentration had a notable impact on the viscoelasticity, thermal stability, and gel-sol transition temperature of the Brij 30/[C₁₆imC₈]Br gels. The zeta potential and ¹H NMR measurements revealed that the neutral Brij 30 molecules are inserted into the palisade layer of the [C₁₆imC₈]Br wormlike micelles via hydrophobic interactions. This decreased the electrostatic repulsion between the [C₁₆imC₈]Br headgroups, which in turn induced the rapid growth of wormlike micelles and the formation of a stiffer network structure. Finally, the wormlike micelles underwent a structural transition to gels. The obtained results would aid in better understanding the relationship between wormlike micelles and gels, and may be of potential value for industrial and technological applications.     

Salt Effect on the Aggregation Behaviors of an Anionic Carboxylate Gemini and a Cationic Surfactant

The synthesis method and conditions for the carboxylate Gemini surfactant O,O′‐bis(sodium 2‐lauricate)‐p‐benzenediol (C₁₁pPHCNa) were explored. Surface tension data of the single and mixed systems of catanionic (cationic with anionic) surfactants with different salt concentrations were used to determine the CMC values and to obtain the information of self‐assembly behaviors of the surfactants. Aggregates' morphologies were observed by transmission electron microscopy (TEM) and speculation was made according to the viscosity measurement results. The results show that large spherical aggregates formed in the mixed solution, which tend to transfer into branched and wormlike mixed micelles with the increases of the salt concentration. The viscosity of the mixed solution was found to increase gradually corresponding to the change of the catanionic surfactant mixtures' morphology.     

Molecular dynamics simulations of mixed cationic/anionic wormlike micelles

Simulations of mixed cationic/anionic wormlike micellar systems have been carried out for a wide range of compositions, including pure anionic and cationic systems. It was found that the wormlike micelle formed by only cationic surfactant molecules is unstable and transforms to a set of small spherical micelles. Adding anionic surfactants with a short hydrophobic chain (only eight carbon atoms) results in stable wormlike micelles. The 34/66 cationic/anionic worm is stable and symmetrical, while the 50/50 mixture yields a flattened worm, indicating a phase transition to the lamellar phase. All these observations are in excellent agreement with the experimental results of Raghavan et al. (Langmuir 2002, 18, 3797), and they provide a molecular mechanism for their observations. The addition of octyltrimethylammonium chloride increases the radius of the worm due to the bigger hydrophobic part. Meanwhile, the length of the worms decreases with the concentration of cationic surfactant and reaches a minimum for the 50/50 mixture. The latter system is of special interest due to a zero surface charge density. The worm with the electrostatically neutral surface was used to investigate intermicellar interactions. The molecular dynamics (MD) simulations show that the merging process requires a substantial activation energy even in the case of reduced electrostatic repulsion.     

Microemulsion droplets decorated by Brij700 block copolymer: phase behavior and structural investigation by SAXS and SANS

The phase behavior and structure of a four-component microemulsion system forming droplets with an oil core surrounded by the non-ionic C12E5 surfactant in water and "decorated" by long PEO chains using the block copolymer/surfactant Brij 700 has been studied. The surfactant-to-oil volume ratio, the coverage density of the droplets with decorating molecules, and the temperature were varied. For a surfactant-to-oil volume ratio of 2, the solutions form isotropic and clear solutions at room temperature, and the addition of Brij molecules stabilize the micelles: the transition to an opaque phase is shifted to higher temperatures as the surface coverage increases. At a surfactant-to-oil ratio of 1, the isotropic microemulsion phase is confined to a very narrow range of temperature, which location is shifted to increasing temperature, as the amount of Brij at the surface of the droplet is increased. For large surface coverages, the lower emulsification boundary varies roughly linearly with the surface coverage. The structure of the droplet phase was investigated by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). For a surfactant-to-oil ratio of 2, the SANS data revealed a transition from rodlike to spherical particles when Brij molecules are added to the system, which induces a larger curvature of the surfactant film. For a surfactant-to-oil ratio of 1, the droplets are nearly spherical at all surface coverages. The intermicellar interactions effects become increasingly more pronounced as Brij is added, due to the introduction of the highly swollen corona. A quantitative analysis of some of the SAXS data was done using an advanced model based on Monte Carlo simulations. It demonstrates the strong chain-chain interactions within the corona and confirms the increased interparticle interactions, as the coverage density is increased.     

Horseradish peroxidase activity in a reverse catanionic microemulsion

In this paper we present the first results of enzymatic activities in a reverse microemulsion medium based on a mixture of an anionic and a cationic surfactant, called catanionic microemulsion. The studied system is composed of sodium dodecyl sulfate (SDS)/dodecyltrimethylammonium bromide (DTAB)/n-hexanol/citrate buffer/n-dodecane, with high SDS/(SDS + DTAB) weight fractions. It turns out that the results are similar to those obtained in classical reverse microemulsions, except that the presence of DTAB exerts an inhibiting effect on the enzyme. Nevertheless, enzymatic superactivities are found even at a DTAB to total surfactant ratio of 15%, corresponding to 3% weight fraction of cationic surfactant in the microemulsion. The influence of pH and hexanol content on the enzymatic activities is also studied.     

Solubilization and phase equilibria of water-in-oil microemulsions : II. Effects of alcohols, oils, and salinity on single-chain surfactant systems

The solubilization and phase equilibria of w/o microemulsions have been shown to be dependent on two phenomenological parameters, namely the spontaneous curvature and elasticity of the interfacial film, when interfacial tension is very low. The spontaneous curvature of an interface is basically determined by the geometric packing of surfactant and cosurfactant molecules at the interface, whereas the interfacial elasticity is related to the energy required to bend the interface. The droplet size and solubilization of microemulsions is mainly determined by the radius of spontaneous curvature, and is further influenced by interfacial elasticity and interdroplet interactions. A w/o microemulsion with a highly curved and relatively rigid interfacial film can exist in equilibrium with excess water at the solubilization limit due to the interfacial bending stress. Increasing the natural radius and fluidity of the interface can increase the droplet size and hence the solubilization in the microemulsion. On the other hand, a w/o microemulsion with a highly fluid interfacial film can exist in equilibrium with an excess oil phase containing a low density of microemulsion droplets due to attractive interdroplet interaction. Increasing the interfacial rigidity and decreasing the natural radius in this case can increase water solubilization in the microemulsion by retarding the phase separation process. Thus, a maximum water solubilization in a w/o microemulsion can be obtained by minimizing both the interfacial bending stress of rigid interfaces and the attractive interdroplet interaction of fluid interfaces at an optimal interfacial curvature and elasticity. The study of phase equilibria of microemulsions can serve as a simple method to evaluate the property of the interface and provide phenomenological guidance for the formulation of microemulsions with maximum solubilization capacity.     

Solubilization of Polar Oils in Surfactant Self-Organized Structures

The cloud temperature of 2 wt% C(12)EO(8) aqueous solutions decreases upon addition of sarcosinate-lauroyl isopropyl (SLIP), 1-dodecanol, and m-xylene, whereas it increases in glycerol tris(2-ethylhexanoic) ester (TEH), isopropyl myristate (IPM), and saturated hydrocarbon systems. A three-phase microemulsion is formed at equal weights of water and oil in the IPM system, but a lamellar liquid crystal (L(alpha)) is present in the SLIP system at the balanced temperature. The effect of added oil on the phase transition of the hexagonal (H(1)) phase was also investigated by means of SAXS study. The H(1)-L(alpha) transition occurs upon addition of SLIP or 1-dodecanol whereas the H(1)-I(1) (discontinuous micellar cubic) phase transition takes place in TEH or IPM systems. These differences in phase behavior are attributed to the placement of solubilized oil in micelles: In the former systems, oil tends to penetrate in the surfactant palisade layer and induces the surfactant layer curvature in micelles to be less positive, while the penetration tendency is small and the opposite effect on the curvature is induced upon addition of the latter oils. Copyright 2001 Academic Press.     

Pressure-induced hexagonal phase in a ternary microemulsion system composed of a nonionic surfactant, water, and oil

The pressure-induced phase transition in a microemulsion, consisting of pentaethylene glycol mono-n-dodecyl ether, water, and n-octane, was investigated by means of small-angle neutron scattering. A pressure-induced phase transition from a lamellar structure to a hexagonal structure was observed. The temperature-pressure phase boundary shows a positive slope with dTdP approximately 0.09 KMPa. The structure unit of the high-pressure hexagonal phase was an oil-in-water cylinder with the membrane thickness of 15.5 A, identical to the low-temperature hexagonal phase. Pressurizing was found to have the same effect by decreasing temperature. This behavior was satisfactorily explained with the pressure dependence of the spontaneous curvature of surfactant membranes. That is, the volume change of surfactant tails plays a dominant role in the structure change of the microemulsion with applying pressure.     

Small-Angle Neutron Scattering Study of Nonionic Surfactant Micelles and Phase Transitions in w/o Microemulsion

The structure of micelles formed by a four component water-in-oil nonionic microemulsion surfactant polyoxyethene (20) sorbitan monoleate (Tween 80), sorbitan monolaurate (Span 20) at ethyl oleate and deuterated water interface have been probed by small-angle neutron scattering (SANS). The total surfactant concentration in each of the samples studied (Tween 80: Span 20) is fixed at 3:2. The deuterated water content is variable at 5–60% w/w. The experimental SANS data from all the seven samples are fit well by spherical micelles interacting with hard sphere potential. Increased deuterated water leads to spherical to lamellar and rod-like micelle geometry featured in the SANS scattering data. The observed change in micelle geometry supports the characterization of phase transition between the self-assembled micelles of the nonionic microemulsion.      

Micellar shape transition under dilute salt-free conditions: promotion and self-fluorescence monitoring of stimuli-responsive viscoelasticity by 1- and 2-naphthols

Effect of 1 and 2-naphthols on the shape transition of cetyl trimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) micelles are studied. Stimuli-responsive viscoelastic gels of long wormlike micelles are formed at low surfactant concentrations in the presence of neutral naphthols, where H-bonding plays a key role in micellar shape transition in the absence of any charge screening. Micelle-embedded naphthols also act as novel self-fluorescence probes for monitoring viscoelasticity of the system as a function of applied shear. 1H NMR study shows that the solubilization sites of naphthols in the micelle are located near the surface. While UV absorption and Fourier transform infrared studies confirm the presence of intermolecular H-bonds in micelle embedded naphthols, transmission electron micrographs of vacuum-dried samples at room temperature demonstrate the transition in shape from sphere to rodlike micelles.     

反相微乳液中苯酚硝化反应选择性的研究

采用气相色谱内标法研究了苯酚在AOT/异辛烷/水、CTAB/正癸醇/异辛烷/水、DBSA/异辛烷/水3种反相微乳液中进行硝化反应的选择性;考察了表面活性剂种类、反应时间、反应温度以及反相微乳液的含水量等因素对反应选择性的影响.研究结果表明,苯酚在微乳液体系中的硝化反应具有明显的邻位选择性,阴离子表面活性剂DB-SA体系的邻位选择性最高,这与它同时具有微乳催化和酸催化作用有关.     

Giant micellar worms under shear: a rheological study using SANS

Flow-SANS experiments were performed on viscoelastic aqueous solutions of erucyl bis(hydroxyethyl) methylammonium chloride in the presence of potassium chloride. This cationic surfactant has the ability to form very long and flexible wormlike micelles upon addition of salt. The effects of the key-parameters-shear rate, temperature, surfactant and salt concentration-on the ability of the micelles to align in the flow-field were investigated. The scattering data were analyzed in terms of an anisotropy factor (Af). It was found that the wormlike micelles aligned in the direction of the applied shear rate and that the anisotropy factor increased with shear rate. In addition, an increase in temperature caused a decrease of the anisotropy factor (Af) due to the formation of shorter worms. Furthermore, the branching of the micelles at high ionic strength caused the anisotropy factor to decrease in comparison with the values obtained from linear wormlike micelles, hence revealing that the formation of 3-way junctions restricts the alignment of the micelles in the shear-flow. Furthermore, the total surfactant concentration was found to affect the shear-induced patterns significantly, and different behaviors were observed depending on the ionic strength.     

Percolative phenomena in lecithin reverse micelles: the role of water

 The role played by the solvation water molecules on the macroscopically observed sol–gel transition in lecithin/cyclohexane/water reverse micelles is investigated by quasielastic neutron scattering, dielectric relaxation and conductivity measurements. The experimental results are juxtaposed to those from spherical Aerosol OT reverse micelles. It is shown how the results from lecithin-based system can be interpreted only assuming that, in contrast to Aerosol OT systems, the water molecules are entrapped at the interfaces without coalescing into an inner water pool. It is suggested that, in the case of lecithin, the solvation water can induce a change in the surface curvature, in such a way promoting the formation of branch points. Such a hypothesis is supported by the temperature dependence of the conductivity which agrees with the hypothesis of an intermicellar bond percolation. The investigation of the structures imposed by an external electric field is also studied. The observed electrorheological behaviour seems to confirm the existence of a percolated transient network in the gel phase. Received: 21 March 2001 Accepted: 24 August 2001     

Structural characterization of frozen n-heptane solutions of metal-containing reverse micelles

The microstructure of temperature-quenched solutions of reverse micelles formed by sodium, cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate in n-heptane has been investigated by SAXS and EXAFS. Some changes in the X-ray absorption spectra with respect to the same systems at room temperature have been observed. The analysis of the SAXS spectra leads to the hypothesis that at 77 K the closed spherical structure of reverse micelles is retained and that during the temperature quench they undergo a clustering process involving the transition from a quite random dispersion to the formation of more or less large clusters of strongly packed reverse micelles. This behavior is attributed to competitive effects caused by the temperature decrease. The prevalence of intermicellar attractive interactions with respect to Brownian motions leading to a collapse to more compact structure is in competition with the rapid decrease of reverse micelle diffusion rate involving a freezing of the local structures. In the case of cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate reverse micelles, further information from EXAFS measurements indicates that within the reverse micelle core exists a quite ordered nanosized domain composed of water, surfactant counterions, and oxygen atoms of the SO3- head groups. The conservation of local order and inverse structure during the clustering phenomenon that results from the fast freezing with liquid nitrogen of solutions of reverse micelles could have biological implications, i.e., the preservation of tissue samples at cryogenic temperatures.