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.     

Salting-out effect induced by temperature cycling on a water/nonionic surfactant/oil system

This paper presents original effects induced by temperature cycling on the transitional phase inversion of emulsions, stabilized by a nonionic polyethoxylated C18E6 surfactant model. The phase inversion follow-up is performed by electrical conductivity measurements, which involves focusing the study on the shape and location of the emulsion inversion region. In that way, new observations are brought out as a gradual evolution of the emulsion inversion along the cycling process. Two alternative approaches are considered for tackling these results: (i) first, a molecular approach regarding the particular organization and rearrangement of water clusters surrounding the surfactant polymer polar head, and (ii) second, a thermodynamic approach only considering the whole Gibbs free energy of the system. The volumic approaches are transposed, here, to the water/oil interface, and disclose that the phase inversion zone is included in a metastable region, able to stabilize for a given temperature, either metastable O/W emulsions or stable W/O ones. In that way, this study proposes novel and complementary insights into the phenomena governing the emulsion phase inversion.     

Structure and phase behaviour of the ternary system water, n-heptane and the nonionic surfactant Igepal® CA520

The phase behavior of the system water, n-heptane and the nonionic surfactant Igepal^® CA520 has been studied by visual inspection, high-performance liquid chromatography, polarizing microscopy and freeze-fracture electron microscopy. The phase diagram at 25?°C contains two large homogeneous microemulsion phases, an extended region of a lamellar liquid crystalline structure and some two- and three-phase regions. The oil-rich part of the phase diagram has been investigated by static and dynamic light scattering in order to examine the behavior of the collective diffusion coefficient and the scattering intensity in the presence of increasing concentrations of water, starting from the binary system of n-heptane and Igepal^® CA520. The results suggested that at a very low water content the aggregates of the microemulsion are small. With the exception of this region the structure is bicontinuous.     

Globular and bicontinuous phases of nonionic surfactant films

Nonionic surfactants of the alkyloligoethylene oxide type form, with water and oil, a range of isotropic a liquid crystalline phases. We analyse the phase behaviour using the flexible surface model and argue that the strong temperature dependence is caused by the fact that the monolayer spontaneous curvature decreases strongly with increasing temperature. This is exemplified with the behaviour of bicontinuous microemulsions, showing a symmetric behaviour around the balanced state, globular microemulsions, behaving as hard spheres near the emulsification failure boundary, and sponge phases appearing when the monolayer spontaneous mean curvature is towards the abundant solvent. It is argued that there is a hierarchy of free energy contributions determining the preferred aggregate shape/phase. With a given oil-water ratio and a surfactant concentration that fixes the polar/apolar interfacial area, the most important free energy contribution comes from having a mean curvature close to the spontaneous curvature. The Gaussian curvature and the entropy terms become important when selecting between structures of similar mean curvature. At higher concentrations, surface forces and higher order elastic terms become significant.     

Overlapping of three-phase regions in a water/nonionic surfactant/triglyceride system

It was found that two types of three-phase regions containing surfactant phases (microemulsions) are overlapped and the four coexisting phases including excess water and oil phases appear in a three-component system of water/hexaethyleneglycol tetradecyl ether (R₁₄EO₆)/triglyceride (1,2,3-[tris(2-ethylhexanoyloxy)] propane, TEH). A schematic diagram of three- and four-phase behavior was constructed based on the real phase diagrams. One type of three-phase behavior is the same as that typically appearing over a wide range of water/oil ratios in a water/nonionic surfactant/hydrocarbon system. The other type of three-phase behavior is similar to that observed over a wide range of water/oil ratios in a system of water/nonionic surfactant/amphiphilic oil such as long-chain alcohols, fatty acids, and triglycerides. The result clearly shows how two three-phase regions interact with each other and are transferred from one to another.     

Three-phase behavior in a mixed nonionic surfactant system

The effect of monodisperse solubilities of each surfactant in an excess oil phase on the three-phase behavior was investigated in a water/octaethyleneglycol dodecyl ether (R₁₂EO₈)/tetraethyleneglycol dodecyl ether (R₁₂EO₄)/heptane system. The mid temperature of the three-phase region is defined as the HLB temperature. The HLB temperature is largely skewed to higher temperature in a dilute region due to the difference in the distribution of each surfactant between excess oil and microemulsion (surfactant) phases forming the three-phase body. Taking account of the monodisperse solubilities, the equation for the HLB temperature was obtained on the basis of geometrical calculation of a particular three-phase triangle. The equation well describes the three-phase behavior for a mixed surfactant system in a space of compositions and temperature.In the mixed surfactant system, the monodisperse solubility of R₁₂EO₈ in oil phase forming a three-phase body is monotonously increased with the rise in temperature, whereas that of R₁₂EO₄ is first increased and then is decreased. Consequently, the sum of both solubilities does not change greatly in a wide range of temperature.     

Anomalous decrease in lamellar spacing by shear flow in a nonionic surfactant/water system

We study the effects of shear flow on the structure of a lamellar phase in a C16E7 [hepta(oxyethylene glycol)-n-hexadecyl ether]/water system (40-55 wt % of C16E7) at 70 degrees C using small-angle neutron scattering in the range of shear rate of 10(-3)-30 s(-1). At the shear rate 0.1-1 s(-1), the repeat distance (d) is decreased significantly (down to about 40% of d at rest in the most significant case) and discontinuously with increasing shear rate. With the further increase in the shear rate, d increases through a sharp minimum (referred to as d*). Such a shear rate dependence of d is obtained for all the principal orientations of lamellae. As the concentration of C16E7 decreases from 55 to 40 wt %, d increases from 6.5 to 8.5 nm at rest whereas d* remains almost constant (approximately equal to 5 nm). Moreover, d* is found to be almost equal to the thickness of bilayers obtained from the line shape analysis of small-angle X-ray scattering at rest. The results strongly suggest that the water layer is excluded by shear flow and that the lamellar phase segregates into surfactant-rich and water-rich regions, although these regions do not reach macroscopic size.     

Distribution of metal chelates between aqueous and surfactant phases separated from a micellar solution of a nonionic surfactant

A dilute micellar solution of poly(oxyethylene) 4-nonylphenyl ether with oxyethylene units 7.5 (PONPE-7.5) was separated into two phases (aqueous and surfactant phases) at room temperature. The partition constants of several chelating reagents and their metal chelates between the two phases were determined at 293 K and ionic strength 0.1 (NaClO₄). The partition constants of the neutral metal chelates depend on the kind of metal ions and were considerably smaller than those expected from the regular solution theory. These facts suggested that the chelates were incorporated into a hydrocarbon environment in the surfactant phase, whereas the chelating reagents were distributed in the poly(oxyethylene) part of PONPE-7.5. A brief review was also presented on the analytical applications to the extraction of metal ions and organic compounds.     

Structural transitions induced by shear flow and temperature variation in a nonionic surfactant/water system

In this study, we investigate structural transitions of tetraethylene glycol monohexadecyl ether (C(16)E(4)) in D(2)O as a function of shear flow and temperature. Via a combination of rheology, rheo-small-angle neutron scattering and rheo-small-angle light scattering, we probe the structural evolution of the system with respect to shear and temperature. Multi-lamellar vesicles, planar lamellae, and a sponge phase were found to compete as a function of shear rate and temperature, with the sponge phase involving the formation of a new transient lamellar phase with a larger spacing, coexisting with the preceding lamellar phase within a narrow temperature-time range. The shear flow behavior of C(16)E(4) is also found to deviate from other nonionic surfactants with shorter alkyl chains (C(10)E(3) and C(12)E(4)), resembling to the C(16)E(7) case, of longer chain.     

Vesicle formation from temperature jumps in a nonionic surfactant system

When heating a dilute sample of the binary system of tetraethyleneglycol dodecyl ether (C12E4) and water from the micellar phase (L1) into the two-phase region of a lamellar phase (L(alpha)), and excess water (W) vesicles are formed. During heating, one passes a region of phase separation in the micellar phase (L1' + L1') where the initial micelles rapidly fuse into larger aggregates forming the concentrated L1 phase (L1') with a structure of branched cylindrical micelles, a so-called "living network". The static correlation length of the micelles are increasing with increasing concentration, from ca. 10 nm to 80 nm in the concentration range of 0.0001 g/cm3-0.0035 g/cm3. The overlap concentration was determined to 0.0035 g/cm3. When the temperature reaches the L1' + L(alpha) region the network particles transform into bilayer vesicles with a z-average apparent hydrodynamic radius in the order of 200 nm depending on the composition. The size of the final vesicles depends on the extent of aggregation/fusion in the L1' + L1' region and hence on the rate of heating. The aggregation/fusion in the L1' + L1' is slower than diffusion-limited aggregation, and it is shown that 1/100 of the collisions are sticky results in the fusion event.     

Novel intermediate phases in an ionic fluorocarbon surfactant/water system

This small angle X-ray scattering study of the lyotropic phases in the binary tetramethylammonium perfluorodecanoate/water system shows that there are no classical lyotropic mesophases present. Much of the liquid crystal region is taken up with a random mesh intermediate phase, Mh₁(0) and a phase with rhombohedral symmetry which is probably a rhombohedral mesh intermediate phase, Mh₁(R3m). This behaviour is unusual since previously these mesh phases have been associated with hydrocarbon surfactants or diblock copolymer melts. All the mesophases found have non-uniform interfacial curvature and a sufficiently strong inter-layer interaction to ensure the long range correlation of structures in some phases.     

Hexagonal close packing of nonionic surfactant micelles in water

The knowledge of the exact shapes of micelles in various micellar phases found in both lyotropic and thermotropic liquid crystals is very important to our understanding of the underlying principles of molecular self-assembly. In the current paper we present a detailed structural study of the hexagonal close packed (hcp, space group P63/mmc) micellar phase, observed in the binary mixtures of nonionic surfactant C12EO8 and water. The reconstructed electron density map of the phase shows perfectly spherical micelles. A spherical core/shell model of micelles, which fits the observed X-ray diffraction pattern satisfactorily, is subsequently constructed. The results confirm the previous assumption that the hcp phase consists of spherical close contacting micelles, each of which contains a low-density core of aliphatic parts and a high-density shell of hydrated ethylene oxide segments, with the gaps between the micelles filled by pure water.     

Peroxidase-catalyzed oxidative polymerization of phenol with a nonionic polymer surfactant template in water

Peroxidase-catalyzed oxidative polymerization of phenol has been examined using the template, poly(ethylene glycol) monododecyl ether (PEGMDE), in water. The role of this template, which forms a complex with phenol in the polymerization, was verified by UV measurements. During the reaction, a complex of the resulting polymer and PEGMDE was precipitated in high yields. The amount and the PEG chain length of PEGMDE strongly affected the polyphenol yield. The unit molar ratio of polyphenol and PEG of the template was about 1:0.9. The presence of the PEGMDE template in the aqueous medium greatly improved the regioselectivity of the polymerization, yielding polyphenol with a phenylene unit content close to 90%. FT-IR, DSC, and XRD analyses confirmed the formation of the miscible complex of polyphenol and PEGMDE by hydrogen-bonding interactions.     

Micellization and interaction of anionic and nonionic mixed surfactant systems in water

Interaction between binary surfactant mixtures containing anionic surfactants viz. sodium dodecyl sulphates (NaDS) and magnesium dodecyl sulphates (Mg(DS)₂) and a nonionic surfactants viz. dodecyl dodecapolyethylene glycol ether (C₁₂E₁₂) and dodecyl pentadecapolyethylene glycol ether (C₁₂E₁₅) in water at different mole fractions (0–1) were studied by surface tension, viscometry and dynamic light scattering (DLS) methods. The composition of mixed micelles and the interaction parameter, β evaluated from the CMC data obtained by surface tension for different systems using Rubingh's theory were discussed. Activity coefficient (f₁ and f₂) of metal dodecyl sulphates (MDS)/C₁₂E_m (m = 12, 15) mixed surfactant systems were evaluated, which shows extent of ideality of individual surfactant in mixed system. The estimated interaction parameter indicates an overall attractive interaction in the mixed micelles, which is predominant for NaDS as compared to Mg(DS)₂. Counter ion valency has specific effect on the mixed micelles, as Mg(DS)₂ has less interaction with nonionic surfactants in comparison to NaDS due to strong condensation of counter ion. The stability factors for mixed micelles were also discussed by Maeda's approach, which was justified on the basis of steric factor due to difference in head group of nonionic surfactant. DLS measurements and viscosity data reveals the synergism in mixed micelles, showing typical viscosity trends and linearity in sizes were observed.     

Structures in a microemulsion system of an ethoxylated polymethylsiloxane surfactant, water, and oil studied by NMR self-diffusion measurements

Microemulsion samples of an ethoxylated polymethylsiloxane surfactant, water, and 1-dodecanol or 1-decanol as the oil component are investigated using pulsed field gradient NMR to determine the components' self-diffusion coefficients. It is demonstrated that the structure of the liquids depends heavily on their composition, in that, for low water content, the structure is water-in-oil (w/o), gradually changing to a bicontinous structure in a concentration range ca. 40-60 wt % water, and, finally, to an oil-in-water (o/w) structure for more water rich samples. In the water poor samples, the surfactant molecules apparently do not form extended aggregates (inverted micelles). In the water rich samples, the surfactant and oil (if present) form ordinary micelles, and it is demonstrated for the binary water/surfactant system that the micelles are spherical at very low surfactant concentrations and grow into oblate (disk) shaped aggregates at surfactant concentrations above ca. 5 wt %. From density and viscosity measurements of binary mixtures of oil (1-decanol) and surfactant, it is demonstrated that these components form solutions that are not far from ideal.     

Distribution features of Triton-X100, a nonionic surfactant,in the quartz-water-cyclohexane system during preferential wetting

The distribution of TX-100, a nonionic surfactant, over solid surfaces in cyclohexane (CH), with quartz being preferentially wetted by aqueous solutions of TX-100, is studied using radioactive labels and wetting. From low-concentration solutions, the surfactant is adsorbed from the aqueous phase on the quartz surface preferentially near the three-phase contact line. At higher TX-100 concentrations, adsorption on the solid surface occurs from both liquid phases. The TX-100 distribution on quartz influences the kinetics of acquisition of the contact angle of preferential wetting.     

Mixed vesicle formation on a ternary surfactant system: didodecyldimethylammonium bromide/dodecylethyldimethylammonium bromide/water

The formation of mixed aggregates has been investigated on a ternary system consisting of two cationic surfactants with similar polar heads and two and/or one 12 carbon atom hydrophobic tail, respectively, didodecyldimethylammonium bromide and dodecylethyldimethylammonium bromide and water. The study has been carried out by means of conductivity, zeta potential, and cryogenic transmission electronic microscopy (cryo-TEM) experiments on the very diluted region. A variety of mixed aggregates, microaggregates, vesicles, and micelles has been found, depending on system composition and total surfactant concentration. Mixed critical microaggregate concentration and mixed critical vesicle concentration have been determined from conductivity data. Furthermore, zeta potential and cryo-TEM experiments allow for the characterization of the aggregates/solution interface and of the shape and size of the aggregates. This experimental evidence has also been analyzed in terms of the theoretical packing parameter, P.     

红外光谱研究以非离子型表面活性剂所组成微乳液的水结构

由烷基聚氧乙烯醚(AEO9)/正己醇/正十六烷/水所组成的微乳液,采用红外光谱对水内核的微观结构进行研究。以水分子的OH伸缩振动谱带, 由高斯分布曲线面积得出不同结构水的含量。只有少量水与表面活性剂结合, 另有部分水束缚于聚氧乙烯链段之间, 这些水与水相中的自由水呈动态平衡。当体系在剧烈振动后, 少量结合水转为束缚水, 静止后又恢复原状。     

Microemulsions from silicone oil with an anionic/nonionic surfactant mixture

Microemulsion phases have been prepared for the first time from the silicone oil "M(2)" (hexamethyldisiloxane) and a surfactant mixture of a nonionic surfactant "IT 3" (isotridecyltriethyleneglycolether) and an ionic surfactant Ca(DS)(2) (calciumdodecylsulfate). For such a surfactant mixture the hydrophilicity of the system can be tuned by the mixing ratio of the two components. With increasing IT 3 content, the surfactant mixtures show a L(1)-phase, a wide L(α)-region and a narrow L(3) sponge phase. For constant temperature, two single phase channels exist in the microemulsion system. The lower channel (low IT 3 content) ends in the middle of the phase diagram with equal amounts of water and oil, the upper channel begins with the L(3)-phase and passes all the way to the oil phase. Conductivity data show that the upper channel has a bi-continuous morphology up to 40% oil while the lower channel consists of oil droplets in water. In contrast to previous studies on nonionic systems, the two single phase channels are not connected and microemulsions with equal amount of oil and water do not have a bicontinuous structure.