A single parameter determines mesophase transitions in Swollen Liquid Crystals

We report how the control of a single parameter, the co-surfactant, determines the phase transitions of oil-in-water swollen liquid crystals (SLCs) prepared with cetyltrimethylammonium bromide (CTAB), from cubic to hexagonal, lamellar, and finally sponge-like structures. SLCs are complex mixtures (surfactant + co-surfactant + water + salt + oil) usually prepared to form hexagonal mesophases, with cell parameters tunable between 3 and 30 nm. These hexagonal mesophases were successfully used as nanoreactors to prepare a broad range of nanostructured materials. Because the potential of these mesophases as adaptive nanoreactors has not been extended to other liquid crystal geometries than the hexagonal, we studied in a first step the structure evolution of SLCs made with CTAB, cyclohexane, pentanol-1, water and different stabilising salts. We used small-angle X-ray scattering (SAXS), polarised light microscopy and Freeze-Fracture TEM to provide a partial phase diagram and list the different mesophases obtained as a function of composition. We report that the adjustment of a single parameter, the co-surfactant (pentanol-1), determines the phase transition between cubic, hexagonal, lamellar, and sponge-like structures, all other parameters such as the nature and concentration of salt, or amount of oil being constant.     

Copolymer-induced stabilizing effect of highly swollen hexagonal mesophases

We show that small amounts of copolymer that decorate an oil/water interface can greatly enhance the stability of swollen surfactant hexagonal phases, comprising oil tubes regularly arranged in a water matrix. Both the radius of the tubes and the thickness of the aqueous channel between the tubes can be controlled independently over large ranges. Such soft composite materials offer a potential interest for the synthesis of mesoporous materials.     

Polyelectrolyte-surfactant complexes with long range order

Mixtures of carboxymethyl cellulose (CMC) or hydrophobically modified CMC with an oppositely charged surfactant (benzyldimethyltetradecylammonium chloride) in water were prepared. When the global polymer concentration is 0.18% by weight and the surfactant content is high enough, a precipitate with hexagonal order is formed. The precipitate composition shows practically constancy in its water content and a slight diminution in polymer concentration when the global surfactant content is varied between 0.9 and 23 wt%. The lattice parameter in this phase decreases when the polymer/surfactant ratio in the phase increases; this variation is faster with CMC than with the hydrophobically modified CMC. In this way electrostatic and hydrophobic interactions are far from being additive. From the extrapolation to infinite dilution, the global interaction seems to depend on the substitution degree in the polymer. Additionally, the comparison between the radius at the polar-apolar interface in the cylinders and the lattice parameter as a function of polymer/surfactant ratio in the hexagonal phase is compatible with some of the alkyl chains belonging to the hydrophobically modified CMC being present in the aqueous zone.     

From faceted vesicles to liquid icoshedra: Where topology and crystallography meet

Many common amphiphiles spontaneously self-assemble in aqueous solutions, forming membranes and unilamellar vesicles. While the vesicular membranes are bilayers, with the hydrophilic moieties exposed to the solution, the structure formed by amphiphiles at the oil–water (i.e., alkane–water) interfaces, such as the surface of an oil droplet in water, is typically a monolayer. It has recently been demonstrated that these monolayers and bilayers may crystallize on cooling, with the thermodynamic conditions for this transition set by the geometry of the constituent molecules. While a planar hexagonal packing motif is particularly abundant in these crystals, a hexagonal lattice is incompatible with a closed-surface topology, such as a closed vesicle or the surface of a droplet. Thus, (at least) 12 five-fold defects form, giving rise to a complex interplay between the stretching and the bending energies of these two-dimensional crystals; in addition, a central role is also played by the interfacial tension. This interplay, part of which has been theoretically studied in the past, gives rise to a range of unexpected and counterintuitive phenomena, such as the recently-observed temperature-tunable formation of stable liquid polyhedra, and a tail growing and droplet-splitting akin to the spontaneous emulsification effect.     

Structural changes induced by addition of a hydrocarbon to water/amphiphile mixtures

The phase conditions in a system of water, hexadecane, sodium dodecyl sulphate, and di-ethylene glycol dodecyl ether showed theW/O microemulsions to be obtained first after destabilization of a liquid crystalline phase by addition of the hexadecane. The original lamellar liquid crystalline phase was moved towards higher surfactant/cosurfactant ratios and a new phase with inverse amphiphile cylinders in a hexagonal packing appeared.     

Small-angle scattering from hexagonal liquid crystals

In this paper, we discuss the scattering behavior of two-dimensional hexagonal liquid crystals with micellar cylinders as a building unit. We treat the hexagonal phase as an accumulation of ordered domains of finite size that typically consists of one hundred parallel cylinders whose axes are perpendicular to the lattice plane. When we suppose that no specific orientation is preferred, the lengths of the cylinders are rather large compared to their diameter, and the polydispersity of the size of the cylinders is negligible; it is therefore possible to split the scattering intensity into a product of the so-called form factor and the structure factor. This product approximation is the basic condition for the use of the generalized indirect Fourier transformation (GIFT) method and the deconvolution (DECON) method to evaluate the small-angle scattering data of hexagonal phases. The GIFT method provides the parameters of the structure factor model and the pair distance distribution function of the cylinders. Via the DECON technique, we can calculate the radial contrast profile of the cylinders from the pair distance distribution function that is obtained by the GIFT method.     

Solubilization of triglycerides in liquid crystals of nonionic surfactant

The solubilization of triglycerides [1,2,3-tributanoylglycerol (TBG) and 1,2,3-trihexanoylglycerol (THG)] in water/octa(oxyethylene) dodecyl ether (C(12)EO(8)) systems has been investigated. Oil-induced changes in the structure of liquid crystals in water/C(12)EO(8) system have been studied by optical observation and small-angle X-ray scattering (SAXS) measurements. In the water/C(12)EO(8)/oil systems, solubilization of THG and TBG induces a transition between H(1) (hexagonal) and L(alpha) (lamellar) liquid crystals at high C(12)EO(8) concentrations, whereas at low surfactant concentrations a H(1)-I(1) (discontinuous micellar cubic phase) transition occurs. This anomalous behavior is attributed to the partitioning of solubilized oil in the micelles. At low surfactant concentrations THG is mainly solubilized into the hydrophobic cores of the surfactant micelles, indicating high swelling or low penetration tendency, resulting in a steep increase in the radius of the aggregates (r(H)), thereby inducing a rod-sphere transition. At high surfactant concentrations, THG is not mainly solubilized into the core but distributed between the palisade layer and the core of the aggregates. The TBG is considerably solubilized into the surfactant palisade layer, indicating a high penetration tendency, resulting in an increase in the effective cross-sectional area per surfactant molecule, a(s). The thermal stability of the I(1) phase increases with the solubilization of THG into the aggregate cores. The percentage deviation of the experimental interlayer spacings (P(d)) from complete swelling was also evaluated for different triglycerides in the H(1) and L(alpha) phases or different surfactant concentrations. It is found that the penetration tendency of triglycerides could be used as a tuning parameter for I(1) phase formation depending on the surfactant concentration and the molecular weight of the oil.     

Lamellar Liquid Crystals Formed in Ternary DEP/Oil/Water Systems

Lamellar liquid crystals of the nonionic surfactant dodecyl polyoxyethylene (4) polyoxypropylene (5) ether (DEP) were investigated by means of phase diagram and rheological technique at 25°C in the presence of different solvents. The aqueous medium includes pure water, physiological saline and 20% sucrose solution, and the oil phase includes isopropyl myristate (IPM), oleic acid, and geraniol, which are all pharmaceutically accepted chemicals. The steady and dynamic rheological property analyses of the lamellar liquid crystals formed in DEP/H₂O/IPM system indicate that the lamellar samples constructed by this special surfactant behave as pseudo-plastic fluid with relatively high elasticity and possess defects in their assembled structure. Also in this system, the elastic character gets decreased with increase in the water content. Furthermore, the comparison investigations show that the relatively high polar oil component increases the network strength of the lamellar phase, while increase in the polarity of water phase enhances the structure defects.     

THE NATURE OF COLLOID AND INTERFACE OF MISCIBLE PHASE SYSTEM IN ALKALINE-SURFACTANT COMBINATION FLOODING

Several representative systems of surfactant + alkaline combination flooding are chosen to study in this paper, these systems are characterized in two points: The first: oil is miscible with water only by reversing them five to ten times, this miscible process is close to spontaneous. Interfacial tension between oil and aqueous is ultra-low. The second: this type of systems is high dispersion and can be stabilized in a period of time. These systems are observed by inicroscopy, polarizing microscopy, microcalorimetry, laser particles analysis instrument and interfacial tension meter, it is found that small particles are main constituent and special liquid crystals distributed on the surface of small particles exist. Small particles and special liquid crystals are responsible for producing ultra-low interfacial tension. The middle phase microemulsion of sodium dodecylsulfate is studied. By comparing difference and similarity between middle phase microemulsion and miscible system, mechanisms of forming low interfacial tension in miscible system and middle phase microemulsion are discussed initially.     

Polymerization in lyotropic liquid crystals. I. Change of structure during polymerization

Polymerization was made at 60°C in a lyotropic liquid crystal of sodium undecenoate and water. The liquid crystalline structure prior to polymerization was identified by optical microscopy and low-angle x-ray diffraction as an array of hexagonal closely packed cylinders with the hydrophobic part of the soap in the center of the cylinders. During polymerization the structure became isotropic at 60°C. Cooling to 20°C transformed the structure to a lamellar liquid crystal–a reversible transition. The structure of the lamellar phase was interpreted as a polyethylene backbone from which deformed decanoate chains reached toward the aqueous layer. Molecular models showed the model to accept head-tail, head-head, and tail-tail configurations in cis and trans conformations with the exception of the cis tail-tail configuration.     

Phase behavior of polyion-surfactant ion complex salts: effects of surfactant chain length and polyion length

The aqueous phase behavior of a series of complex salts, containing cationic surfactants with polymeric counterions, has been investigated by visual inspection and small-angle X-ray scattering (SAXS). The salts were alkyltrimethylammonium polyacrylates, CxTAPAy, based on all combinations of five surfactant chain lengths (C6, C8, C10, C12, and C16) and two lengths of the polyacrylate chain (30 and 6 000 repeating units). At low water contents, all complex salts except C6TAPA6000 formed hexagonal and/or cubic Pm3n phases, with the hexagonal phase being favored by lower water contents. The aggregate dimensions in the liquid crystalline phases changed with the surfactant chain length. The determined micellar aggregation numbers of the cubic phases indicated that the micelles were only slightly aspherical. At high water contents, the C6TAPAy salts were miscible with water, whereas the other complex salts featured wide miscibility gaps with a concentrated phase in equilibrium with a (sometimes very) dilute aqueous solution. Thus, the attraction between oppositely charged surfactant aggregates and polyions decreases with decreasing surfactant chain length, and with decreasing polyion length, resulting in an increased miscibility with water. The complex salt with the longest surfactant chains and polyions gave the widest miscibility gap, with a concentrated hexagonal phase in equilibrium with almost pure water. A decrease in the attraction led to cubic-micellar and micellar-micellar coexistence in the miscibility gap and to an increasing concentration of the complex salt in the dilute phase. For each polyion length, the mixtures for the various surfactant chain lengths were found to conform to a global phase diagram, where the surfactant chain length played the role of an interaction parameter.     

SYNERGETIC EFFECT OF SUCROSE AND ETHANOL ON FORMATION OF TRIGLYCERIDE MICROEMULSIONS

The phase behavior of soybean oil, a nonionic surfactant (ethoxylated monodiglycerides) and an aqueous phase of water containing ethanol, and sucrose was investigated at 35 and 40°C. A minimum concentration of 20 wt% ethanol was required for the formation of isotropic solutions. Addition of sucrose to the aqueous phase decreased the amount of ethanol required to form these solutions. The solubilization mechanism of the oil was investigated by small angle x-ray diffraction and polarized light microscopy. A stable lamellar liquid crystalline phase was formed for a mixture of 75/25 surfactant/sucrose solution (2.5 wt% sucrose). This phase was destabilized with increased concentrations of sucrose and liquid crystalline phases having hexagonal structures were favored at 8.75 wt% sucrose. At a ratio of 55/45 wt% of surfactant/sucrose solution (9 wt% sucrose) hexagonal structures were formed and could be destabilized or destroyed by addition of ethanol. The concept of stabilization and destabilization of liquid crystalline mesophases was applied to the solubilization of triglycerides in aqueous solutions. Two microemulsion regions were identified; oil-in-water (L₁) and water-in-oil (L₂) in systems containing soybean oil, ethoxylated monodiglycerides, and 20 wt% ethanol solution. At 55/45 wt% surfactant/20 wt% ethanol solution,7.5 wt% of soybean oil was solubilized. Addition of 10, 20, and 30 wt% sucrose, at the same ratio of surfactant to ethanol solution, increased the solubility of the oil to 9, 13.5, and 18 wt% respectively. In addition, the size of the L₁ phase increased and moved to the aqueous corner of the phase diagram and the size of the L₂ phase decreased.     

Aging of oil-in-water emulsions: the role of the oil

Controlling stability and aging of emulsions is important from commercial and scientific perspectives. Achieving such control comes through gaining an understanding of the relationship between emulsion constituents and microstructure and how these influence the kinetics and mechanism of destabilisation. We present here an investigation determining the rate of destabilisation as a function of time for a series of water/n-alkane/Triton X-100 oil-in-water emulsions. The time dependence of the emulsions was investigated using static light scattering, PFG-NMR and measurement of gross phase separation. By changing the chain length of the oil from hexane to tetradecane, an almost five orders of magnitude variation in emulsion lifetime could be achieved, while maintaining most of the other chemical and physical characteristics of the emulsions. Further, we show that while Ostwald ripening is the dominant destabilisation mechanism, two distinct regimes are evident. Initially, we observed an enhanced Ostwald ripening regime due to the presence of oil-swollen micelles in the aqueous continuum, that is a depletion flocculation mechanism is followed. The presence of oil-swollen micelles was confirmed using PFG-NMR. The micelles aid the gross oil transport between the discrete oil domains. Upon phase separation the oil-swollen micelles are predominantly removed from the emulsion along with the excess water resulting in a concomitant reduction in the ripening rate, producing the more general Ostwald ripening cubic dependence of droplet radius as a function of time for the lower molecular weight oils. The oils with higher molecular weight (decane and above), however, were observed to switch over to destabilisation via creaming. PFG-NMR was shown to be a powerful technique to fully probe emulsion microstructure as a function of time with droplet size and spacing being directly obtained from the data.     

Characterization and aging of aqueous vesicular dispersions of sodium 4-(1′-heptylnonyl)benzenesulfonate

Vesicles formed by sonication of aqueous dispersions of liquid crystals of the double-tailed surfactant sodium 4-(1′-heptylnonyl)benzenesulfonate (SHBS) are examined with several techniques. The average diameter of the vesicles prepared in water is about 450 Å. The average size decreases when prepared in NaCl or at higher surfactant concentrations. The presence of a few large liquid crystallites in the dispersion, as detected by fast-freeze cold-stage transmission electron microscopy, is shown to severely bias the measurement of vesicle sizes by quasi-elastic light-scattering techniques. The commonly used techniques of gel-permeation chromatography and ultrafiltration are shown to be ineffective in separating liquid crystals from SHBS vesicle dispersions. Vesicle preparation in the presence of uranyl acetate is shown to dramatically reduce the vesicle size. The spontaneous, irreversible reversion of vesicles to liquid crystallites as the dispersions age is documented and proves that SHBS vesicles are not equilibrium structures in water or brine.     

Complexation-mediated crystallization. Crystallization of sodium acetate trihydrate needles from cyclohexane solution

The phenomenon of complexation-mediated crystallization is introduced and proposed as a new means for the preparation of novel forms of materials. Attempted crystallization of complexes of salts with crown ethers often results in the precipitation of the uncomplexed salt rather than the complex. In this study, a solution of the 15-crown-5 complex of sodium acetate in cyclohexane precipitated uncomplexed sodium acetate trihydrate as single crystals. The crystal morphology (long needles) of this crystalline sodium acetate stands in sharp contrast to the hexagonal plates known to form from aqueous solution. X-ray diffraction analysis revealed that sodium acetate as crystallized from cyclohexane solution adopted a crystal lattice identical to that in sodium acetate crystallized from aqueous solution. Possible origins of the change in crystal morphology are discussed.     

An NMR study of translational diffusion and structural anisotropy in magnetically alignable nonionic surfactant mesophases

The diffusion of both water and surfactant components in aqueous solutions of the nonionic surfactant "C12E6"--which includes hexagonal, cubic, lamellar, and micellar mesophases--has been studied by pulsed-field-gradient NMR. Diffusion coefficients were measured in unaligned samples in all of these phases. They were also obtained in the hexagonal and lamellar phases in oriented monodomain samples that were aligned by slow cooling from the micellar phase in an 11.7 T magnet. Measured water and soap diffusion coefficients in the NMR-isotropic cubic and (high-water-content) micellar phases as well as diffusion anisotropy measurements in the magnetically aligned hexagonal phase were quantitatively consistent with the constituent structures of these phases being identical surfactant cylinders, with only the fraction of surface-associated water varying with the water-soap molar ratio. The values of the water and soap diffusion coefficients in the oriented lamellar phase suggest an increase in defects and obstructions to soap diffusion as a function of increasing water content, while those in the low-water-content micellar phase rule out the presence of inverse micelles.     

含有十二烷基酚聚氧乙烯(10)醚的溶致液晶体系的研究

本文以非离子表面活性剂十二烷基酚聚氧乙烯(10)醚(TX-10)/苯乙烯/水组成的三元体系为研究对象, 绘制了三元相图, 选取液晶区域作为研究对象, 配制系列样品, 摄制了纹理照片, 用小角X光衍射法测定了液晶中各种组分变化时间的层间距, 并结合^2H NMR谱图和纹理照片的对照以及互为补充的分析, 为精确区分液晶结构提供了新的途径。这不仅对于基础理论研究, 同时对于日用化工和帮次采油都具有一定指导意义。     

Hexagonal liquid crystalline phases formed in ternary systems of Brij 97-water-ionic liquids

Phase diagrams of two ionic liquids: hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate (bmim-PF(6)) and relatively hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF(4)) in aqueous solutions of Brij 97 were determined at 25 degrees C. Two hexagonal liquid crystalline phases formed in bmim-PF(6)- and bmim-BF(4)-containing ternary systems were investigated by means of small-angle X-ray scattering (SAXS) and rheological techniques, with comparison of composition and temperature effects. From analysis of the SAXS data, bmim-PF(6) is dominantly penetrated between the oxyethylene chains of surfactant molecules, whereas bmim-BF(4) is mainly located in the water layer of hexagonal phases. The strength of the network of hexagonal phase formed in the Brij 97/water/bmim-BF(4) system is appreciably stronger than that of the Brij 97/water/bmim-PF(6) system, indicated by the smaller area of the surfactant molecule at the interface and the higher moduli (G', G' '). Temperature has a converse effect on the lattice parameters of the two hexagonal phases.     

Rheology and microstructure studies of SDS/CTAB/H2O system

Phase behavior of mixed sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) aqueous solution was studied. The rheological properties and microstructure were investigated using a rheostat and freeze-fracture technique and are shown to be closely related to the phase behavior. Experimental investigations reveal two symmetrical aqueous two-phase systems (ATPS) in the ternary phase diagram of SDS/CTAB/H₂O system. In the surfactant rich phase of ATPS or in the adjacent stoichiometric state of ATPS, the system has high viscosity because of its long range ordered structure. Lamellar phase was found in the high viscosity samples in which the cationic and anionic surfactant are in 1: 3 or 3: 1 stoichiometry. In addition, the viscosity has a tendency to increase when salt was added to the solution. The viscosity increase is due to the salt can screen the repulsion between different charged headgroups and thus reduces the effective size of surfactants and facilitates the spherical or rod likes micelles to be transformed to worm-like micelles which can form hexagonal or liquid crystal phases. Large-size salt ions like sodium sulfate (especially organic salt ions) have more significant effect on the surfactant solution viscosity. The text was submitted by the authors in English.     

STRATIFICATION IN EMULSION FILMS

Formation of films possessing a layered or stratified structure has been observed with foam films from liquid crystals, from concentrated surfactant solutions and in liquid layers on the surface of water. The stratifying films have a structure similar to that of the smectic phase which soaps are known to form in the bulk solution at high concentrations. The repeating units of which such films are built are the so-called black films: each unit consists of two surfactant layers interleaved by a thin aqueous core. In the study presented here we have observed that stratification can also take place in emulsion films from concentrated aqueous surfactant solutions. We have compared these results with those obtained for foam films using the same surfactant, i.e. sodium dodecylsulfate.