Phase Coexistence in a Dynamic Phase Diagram

Metastability and phase coexistence are important concepts in colloidal science. Typically, the phase diagram of colloidal systems is considered at the equilibrium without the presence of an external field. However, several studies have reported phase transition under mechanical deformation. The reason behind phase coexistence under shear flow is not fully understood. Here, multilamellar vesicle (MLV)‐to‐sponge (L₃) and MLV‐to‐L_α transitions upon increasing temperature are detected using flow small‐angle neutron scattering techniques. Coexistence of L_α and MLV phases at 40 °C under shear flow is detected by using flow NMR spectroscopy. The unusual rheological behavior observed by studying the lamellar phase of a non‐ionic surfactant is explained using ²H NMR and diffusion flow NMR spectroscopy with the coexistence of planar lamellar–multilamellar vesicles. Moreover, a dynamic phase diagram over a wide range of temperatures is proposed.     

Shear-induced morphology transition and microphase separation in a lamellar phase doped with clay particles

We report on the influence of shear on a nonionic lamellar phase of tetraethyleneglycol monododecyl ether (C12E4) in D2O containing clay particles (Laponite RD). The system was studied by means of small-angle light scattering (SALS) and small-angle neutron scattering (SANS) under shear. The SANS experiments were conducted using a H2O/D2O mixture of the respective scattering length density to selectively match the clay scattering. The rheological properties show the familiar shear thickening regime associated with the formation of multilamellar vesicles (MLVs) and a shear thinning regime at higher stresses. The variation of viscosity is less pronounced as commonly observed. In the shear thinning regime, depolarized SALS reveals an unexpectedly strong variation of the MLV size. SANS experiments using the samples with lamellar contrast reveal a change in interlamellar spacing of up to 30% at stresses that lead to MLV formation. This change is much more pronounced than the change observed, when shear suppresses thermal bilayer undulations. Microphase separation occurs, and as a consequence, the lamellar spacing decreases drastically. The coincidence of the change in lamellar spacing and the onset of MLV formation is a strong indication for a morphology-driven microphase separation.     

Elongational flow induced ordering in surfactant micelles and mesophases

We have used small angle neutron scattering, SANS, to investigate the elongational flow induced ordering in surfactant micelles and mesophases. Spatially resolved SANS measurements have been used to determine the distribution of orientational ordering over the flow velocity pattern in an elongational flow cell, and comparison with the effects of shear flow are made. Two different surfactant systems have been studied, the charged wormlike mixed micelles of hexaethylene monododecyl ether, C16E6/hexadecyl trimethylammonium bromide, C16TAB (3% C16E(6)/5 mol% C16TAB), and the Lalpha lamellar phase of C16E6 (50.6 wt% C16E6 at 55 degrees C), and a substantially different response is observed. The orientational distribution of the Lalpha lamellar phase of C16E6 reflects the flow velocity pattern distribution within the cell, whereas for the wormlike mixed micelles of C16E6/C16TAB this is not the case, and this is associated with the shear thinning behavior of that system.     

Self-assembly in complex mixed surfactant solutions: the impact of dodecyl triethylene glycol on dihexadecyl dimethyl ammonium bromide

The impact of the nonionic surfactant, dodecyl triethyleneglycol ether (C(12)E(3)) on the solution microstructure of the dialkyl chain cationic surfactant, dihexadecyl dimethyl ammonium bromide, (DHDAB) has been investigated. The variation in solution microstructure has been studied using a combination of small angle neutron scattering, ultra small angle neutron scattering, optical texture and photon correlation spectroscopy. At low surfactant concentrations (1.5 mM) the microstructure takes the form of bilamellar vesicles (BLV) for compositions containing less than 20 mol % of added C(12)E(3). Multilamellar vesicles (MLV) are the predominant microstructure for solutions richer in composition than 20 mol % C(12)E(3). At more than 80 mol % C(12)E(3), the solution microstructure reverts to that of a lamellar phase dispersion consistent with studies on the pure nonionic surfactant. At higher concentrations (60 mM) a wide continuous L beta phase region is observed for compositions in the range 20 to 80 mol % C(12)E(3). The fine details of the phase diagram were obtained from quantitative analysis of the SANS data using a well-established lamellar membrane model. Irrespective of the nonionic content, the bilayers are in general highly rigid, consistent with those stabilized by charge interactions. Furthermore estimates of the product of membrane moduli (compressibility and bending modulus) indicate that the different phase regions have very different membrane properties, however the magnitude of the variations observed are not predicted using existing theoretical treatments.     

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.     

Self-assembly in mixed dialkyl chain cationic-nonionic surfactant mixtures: dihexadecyldimethyl ammonium bromide-monododecyl hexaethylene glycol (monododecyl dodecaethylene glycol) mixtures

The self-assembly of dialkyl chain cationic surfactant dihexadecyldimethyl ammonium bromide, DHDAB, and nonionic surfactants monododecyl hexaethylene glycol, C(12)E(6), and monododecyl dodecaethylene glycol, C(12)E(12), mixtures has been studied using predominantly small-angle neutron scattering, SANS. The scattering data have been used to produce a detailed phase diagram for the two surfactant mixtures and to quantify the microstructure in the different regions of the phase diagram. For cationic-surfactant-rich compositions, the microstructure is in the form of bilamellar, blv, or multilamellar, mlv, vesicles at low surfactant concentrations and is in an L(beta) lamellar phase at higher surfactant concentrations. For nonionic-rich compositions, the microstructure is predominantly in the form of relatively small globular mixed surfactant micelles, L(1). At intermediate compositions, there is an extensive mixed (blv/mlv) L(beta)/L(1) region. Although broadly similar, in detail there are significant differences in the phase behavior of DHDAB/C(12)E(6) and DHDAB/C(12)E(12) as a result of the increasing curvature associated with C(12)E(12) aggregates compared to that of C 12E 6 aggregates. For the DHDAB/C(12)E(12) mixture, the mixed (blv/mlv) L(beta)/L(1) phase region is more extensive. Furthermore, C(12)E(12) has a greater impact upon the rigidity of the bilayer in the blv, mlv, and L(beta) regions than is the case for C(12)E(6). The general features of the phase behavior are also reminiscent of that observed in phospholipid/surfactant mixtures and other related systems.     

The microstructure of di-alkyl chain cationic/nonionic surfactant mixtures: observation of coexisting lamellar and micellar phases and depletion induced phase separation

The evolution of the microstructure and composition occurring in the aqueous solutions of di-alkyl chain cationic/nonionic surfactant mixtures has been studied in detail using small angle neutron scattering, SANS. For all the systems studied we observe an evolution from a predominantly lamellar phase, for solutions rich in di-alkyl chain cationic surfactant, to mixed cationic/nonionic micelles, for solutions rich in the nonionic surfactant. At intermediate solution compositions there is a region of coexistence of lamellar and micellar phases, where the relative amounts change with solution composition. A number of different di-alkyl chain cationic surfactants, DHDAB, 2HT, DHTAC, DHTA methyl sulfate, and DISDA methyl sulfate, and nonionic surfactants, C12E12 and C12E23, are investigated. For these systems the differences in phase behavior is discussed, and for the mixture DHDAB/C12E12 a direct comparison with theoretical predictions of phase behavior is made. It is shown that the phase separation that can occur in these mixed systems is induced by a depletion force arising from the micellar component, and that the size and volume fraction of the micelles are critical factors.     

Ellipsometry studies of nonionic surfactant adsorption at the oil--water interface

In the presented study we have developed and implemented a methodology for ellipsometry measurements at liquid interfaces that makes it possible to determine the amount adsorbed without assumptions of refractive index or thickness of the adsorbed layer. It was demonstrated that this is possible by combined measurements from different aqueous phases, H(2)O and D(2)O, which were shown to have sufficiently different refractive indices. The methodology was tested by studying adsorption of two types of nonionic poly(ethylene glycol) alkyl ether surfactants, C(n)H(2)(n)(+1)(OC(2)H(4))(m)OH or C(n)E(m) at the decane--aqueous interface, where C(12)E(5) was adsorbed from the oil phase and C(18)E(50) from the aqueous phase. The observed plateau values of the adsorbed amounts were 1.38 and 0.93 mg/m(2) for C(12)E(5) and C(18)E(50), respectively, which is in agreement with the corresponding values of 1.49 and 1.15 mg/m(2) obtained from applying the Gibbs equation to interfacial tension data for the same systems. We will briefly discuss the adsorption behavior in relation to the molecular structure of the surfactant and the phase behavior of the oil--surfactant--aqueous systems in relation to our experimental results.     

Reversible size of shear-induced multi-lamellar vesicles

We have investigated the reversibility in the shear-induced multi-lamellar vesicle (MLV) size during stepwise cycling of the shear rate by employing common rheometry, polarized light microscopy and rheo-optic techniques. We thus address the question whether there is a true MLV steady state, irrespective of history. The system studied, was the nonionic surfactant triethylene glycol decyl ether (C₁₀E₃) with a concentration of 40 wt.% in D₂O and a constant temperature of 25°C. It was found that the MLV size varies reversibly with varying shear rate, and hence there exists a true steady state in the presence of shear flow. The experimental observations of reversibility are however restricted to higher shear rates. Because the transformation of the size results from the shear strain, the process is very slow at lower shear rates, where the steady state cannot be reached within a reasonable experimental time.     

Effect of hydrophobically modified polymers on shear-induced multilamellar vesicles

The effects of polymer concentration, polymer molecular weight, and hydrophobe substitution level of modified poly(acrylic acid) polymers on the formation, size, and viscoelastic properties of shear-induced multilamellar vesicles (onions) are studied by rheology and light diffraction. The onions are close-packed, space-filling vesicles formed by shearing aqueous lamellar phases of C12E5 surfactant to produce phases with sufficient order and size uniformity (O(1-3 microm)) to diffract light. The addition of hydrophobically modified polymers enhances the rate of formation, uniformity, and stability independent of hydrophobe substitution level. Onion size decreases with increasing shear rate as observed for pure surfactant onion systems, but the shear-rate dependence is changed by the polymer. The onion phase has a plateau modulus that increases with polymer concentration but is independent of hydrophobe substitution level or molecular weight. The model presented by Panizza et al. that relates the plateau modulus of the onion phase to membrane rigidity and the compression modulus is consistent with independent measurements of membrane properties from SANS.     

Shear-induced structural transformation of pentaethylene glycol <Emphasis Type="Italic">n</Emphasis>-dodecyl ether and lithium perfluorooctane sulfonate mixed-surfactant lamellar solution

The viscoelastic behavior of the shear-induced structural transformation from the lamellar phase to multilamellar vesicles (MLVs) of a mixed-surfactant system was investigated. The transformation was divided into two processes on the basis of the strain dependence of the apparent viscosity. The first stage is a lamellar-to-intermediate structure transformation. It was found that a strain, not an applied shear rate, governed this process. The second stage is an intermediate-to-MLV phase transformation, which was not controlled by the strain. These structure developments were found in the shear-thickening viscosity regime. The MLV phase formed by applying shear flow exhibited shear-thinning viscosity behavior and reversible response to shear flow. The viscoelastic properties of the MLV phase were investigated by dynamic viscoelastic measurements. Under oscillating shear deformation, the amplitude dependence of the dynamic modulus indicated that the viscoelasticity of the MLV depended on the initial structure, such as the number of vesicle shells and the size of the MLV, which is governed by the preshear rate.     

Nonaqueous lyotropic liquid-crystalline phases formed by gemini surfactants in a protic ionic liquid

The aggregation behaviors of three Gemini surfactants [(C(s)H(2s)-α,ω-(Me(2)N(+)C(m)H(2m+1)Br(-))(2), s = 2, m = 10, 12, 14] in a protic ionic liquid, ethylammonium nitrate (EAN), have been investigated. The polarized optical microscopy and small-angle X-ray scattering (SAXS) measurements are used to explore the lyotropic liquid crystal (LLC) formation. Compared to the LLCs formed in aqueous environment, the normal hexagonal and lamellar phases disappear. However, with increasing the surfactant concentration, a new reverse hexagonal phase (H(II)) can be mapped over a large temperature range except for other ordered aggregates including the isotropic solution phase and a two-phase coexistence region. The structural parameters of the H(II) are calculated from the corresponding SAXS patterns, showing the influence of surfactant amount, alkyl chain length, and temperature. Meanwhile, the rheological profiles indicate a typical Maxwell behavior of the LLC phases formed in EAN.     

Aqueous surfactant two-phase systems in a mixture of cationic gemini and anionic surfactants

The phase behavior as well as the microstructures of the cationic gemini surfactant and anionic conventional surfactant aqueous two-phase system (ASTP) have been studied. The ASTP formation can be attributed to the coexistence of different kinds of aggregates in the upper and lower phases. The effects of temperature, shearing, surfactant concentration and mixing molar ratio on the phase separation of the ASTP-forming systems are systematically investigated. The ASTP can be destroyed by applying shear and increasing temperature. In this process, the lamellar structures (flat bilayers) in the ASTP are transformed into vesicles. Variation of surfactant structure also affects the phase behavior and the aggregates transformation. Appropriate molecular packing is crucial for the formation of ASTP.     

Phase behavior and microstructure of microemulsions with a room-temperature ionic liquid as the polar phase

Microemulsions of nonionic alkyl oligoethyleneoxide (CiEj) surfactants, alkanes, and ethylammonium nitrate (EAN), a room-temperature ionic liquid, have been prepared and characterized. Studies of phase behavior reveal that EAN microemulsions have many features in common with corresponding aqueous systems, the primary difference being that higher surfactant concentrations and longer surfactant tailgroups are required to offset the decreased solvophobicity the surfactant molecules in EAN compared with water. The response of the EAN microemulsions to variation in the length of the alkane, surfactant headgroup, and surfactant tailgroup has been found to parallel that observed in aqueous systems in most instances. EAN microemulsions exhibit a single broad small-angle X-ray scattering peak, like aqueous systems. These are well described by the Teubner-Strey model. A lamellar phase was also observed for surfactants with longer tails at lower temperatures. The scattering peaks of both microemulsion and lamellar phases move to lower wave vector on increasing temperature. This is ascribed to a decrease in the interfacial area of the surfactant layer. Phase behavior, small-angle X-ray scattering, and conductivity experiments have allowed the weakly to strongly structured transition to be identified for EAN systems.     

Phase behavior and preparation of mesoporous silica in aqueous mixtures of fluorinated surfactant and hydrophobic fluorinated polymer

The phase behavior and formation of self-assemblies in the ternary water/fluorinated surfactant (C(8)F(17)EO(10))/hydrophobic fluorinated polymer (C(3)F(6)O)(n)COOH system and the application of those assemblies in the preparation of mesostructured silica have been investigated by means of phase study, small angle X-ray scattering, and rheology. Hexagonal (H(1)), bicontinuous cubic (V(1)) with Ia3d symmetry, and polymer rich lamellar (L(alpha)(')) are observed in the ternary diagram. C(8)F(17)EO(10) molecules are dissolved in polymer rich aggregates, whereas (C(3)F(6)O)(n)COOH molecules are practically insoluble in the surfactant lamellar phase due to packing restrictions. Hence, two types of lamellar phases exist: one with surfactant rich (L(alpha)) and the other with polymer rich (L(alpha)(')) in the water/C(8)F(17)EO(10)/(C(3)F(6)O)(n)COOH system. As suggested by rheological measurements, worm-like micelles are present in C(8)F(17)EO(10) aqueous solutions but a rod-sphere transition takes place by solubilization of (C(3)F(6)O)(n)COOH. C(8)F(17)EO(10) acts as a structure directing agent for the preparation of hexagonal mesoporous silica by the precipitation method. The addition of (C(3)F(6)O)(n)COOH induces the formation of larger but disordered pores.     

Memory effects across surfactant mesophases

We report a detailed analysis of deuteron NMR spectra of micellar, lamellar, cubic, and hexagonal mesophases in the aqueous non-ionic surfactant system C(12)E(6)/water. Samples are prepared with and without shear. Particular attention is paid to an interesting temperature-driven phase sequence that includes all of the above phases that are studied before and after shear parallel or perpendicular to the magnetic field direction. Surprising memory effects are found across mesophase transitions. These memory effects provide clues to the structure of the various phases.     

2H NMR evidence for the formation of random mesh phases in nonionic surfactant-water systems

Random mesh phases share many common features with the classical lamellar phase in that they are layered phases; but crucially, they possess nonuniform interfacial curvature, since the lamellae are pierced by water-filled pores. The introduction of curvature into the lamellae has been posited as a transitional precursor for other lyotropic phases. In this paper, we show that simple 2H nuclear magnetic resonance (NMR) experiments provide strong indication for the formation of the random mesh phase and the NMR data correlate well with literature results from small-angle X-ray scattering. The thermal evolution of the recorded quadrupolar splitting (DeltanuQ) is monitored within the lamellar phase of two nonionic surfactants, C16E6 and C12E5, as the samples are cooled or heated, and a marked and reversible change in the evolution of DeltanuQ is observed. Data from heavy water and deuterium labeled surfactant show the same temperature dependence and consequently report on the same structural changes with temperature. The formation of the random mesh phase is quantified in terms of an effective order parameter that is unity in the classical lamellar phase and takes values of <1 in the random mesh phase, reaching 0.6 at lower temperatures.     

Evidence of formation of ammonium perfluorononanoate/2H2O multilamellar vesicles: morphological analysis by rheology and rheo-2H NMR experiments

Rheology and rheo-(2)H NMR measurements are presented for 30 wt % ammonium perfluorononanoate (APFN)/(2)H(2)O mixture in the temperature range 20-70 °C. A first-order lamellar-to-nematic transition occurs at 42 °C, and a first-order nematic-to-isotropic transition occurs at 49 °C. Different rheological behaviors of the lamellar phase were observed with increasing the temperature. The lamellar structure at low temperature (Lα(-)) has a clear gel-like viscoelasticity, while at high temperature the lamellar structure (Lα(+)) has a liquid-like response. In this study we have observed for the first time, along with the lamellar phase of a surfactant containing fluorinated fatty acid, the formation of multilamellar vesicles (MLVs) ("onions") induced by shear. With the aid of nonlinear rheology and rheo-NMR techniques, onion formation was found to occur in both temperature regimes of the lamellar phase, but at different strain units. It is suggested that the lamellar phase consists of smectic structures in both Lα(-) and Lα(+), but with different percentages of defect density.     

Morphological transitions in model membrane systems by the addition of anesthetics

The mechanism of anesthetic action on membranes is still an open question, regardless of their extensive use in medical practice. It has been proposed that anesthetics may have the effect of promoting pore formation across membranes or at least switching transmembrane channels. In both cases this may be the result of changes in the interfacial curvature of the membrane due to the presence of anesthetic molecules. Aqueous solutions of surfactants display phases that mimic, in a simplified manner, real biological membranes. Therefore, in this study, two nonionic surfactant systems C16E6/H2O in concentrated solution and C10E3/H2O in dilute solution have been used as model membranes for the investigation of the effects of six common anesthetics (halothane, sodium thiopental, lidocaine base form and hydrochloride, prilocaine hydrochloride, and ketamine hydrochloride). Both binary surfactant-water systems exhibit phase transitions from the lamellar phase, Lalpha, that has zero spontaneous curvature and zero monolayer curvature to phases with more local interfacial curvature. These are the random mesh phase, Mh1(0), which consists of lamellae pierced by water-filled pores with local areas of positive interfacial curvature and the sponge phase, L3, that consists of the lamellar phase with interlamellae attachments, often referred to as a "melted" cubic phase, possessing negative monolayer curvature. Small-angle X-ray scattering and 2H NMR experiments upon the C16E6/2H2O system and optical observations of the C10E3/H2O system showed that all anesthetics employed in this study cause a shift in the Mh1(0) to Lalpha phase transition temperature and in the Lalpha to L3 transition temperature, respectively. All of the anesthetics studied bind to the interfacial region of the surfactant systems. Two types of behavior were observed on anesthetic addition: type I anesthetics, which decreased interfacial curvature, and type II, which increased it. However, at physiological pH both types of anesthetics decreased interfacial curvature.     

Shear-induced phase transitions in sucrose ester surfactant

The behavior of a commercial sucrose stearate blend has been examined by means of various experimental techniques (differential scanning calorimetry, light polarization and electron microscopy, and rotational rheometry). A partial phase diagram in water has been established. It shows that the binary system forms a lamellar lyotropic mesophase and that the melting behavior is characterized by a lamellar gel-lamellar liquid crystalline phase transition. The identification of the liquid crystalline phase has been carried out from textural observation using polarization microscopy and freeze-fracture electron microscopy. At low surfactant concentrations, the phase transition has been followed through rheological experiments. Furthermore, a shear-induced transition, from the lamellar phase (sheets of surfactant bilayers including a few large multilamellar vesicles) to an onion phase, has been observed above a critical temperature of 43 degrees C. The vesicles so obtained did not relax over more than 3 weeks. The presence of a small ratio of distearate in the sugar ester blend seems to be the key to vesicle formation at low surface-active material concentration.