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.     

Studies on the phase properties of lyotropic liquid crystals of Brij35/sodium oleate/oleic acid/water system:By means of polarizing microscope,SAXS,~2H-NMR and rheological methods

The pseudo-quaternary phase diagram of Brij35/sodium oleate/oleic acid/water systems has been investigated, and the liquid crystal area has been identified, which covers about two thirds of the whole phase diagram. The liquid crystal structure and behavior have been also studied by using polarizing texture, small angle X-ray scattering, 2H-NMR and rheometer etc. The result shows that when the composition of the system changes along the line of AA′ in this large liquid crystal region, the structural change is cubic→cubic/lamellar→lamellar→lamellar/hexagonal→hexagonal. Meanwhile, we made the first attempt of systematic study of the rheological properties of the above system. The lattice constants of cubic and hexagonal liquid crystals are 10.53 and 5.68 nm, respectively.     

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.     

Studies on the phase properties of lyotropic liquid crystals of Brij35/sodium oleate/oleic acid/water system: By means of polarizing microscope, SAXS, 2H-NMR and rheological methods

The pseudo-quaternary phase diagram of Brij35/sodium oleate/oleic acid/water systems has been investigated, and the liquid crystal area has been identified, which covers about two thirds of the whole phase diagram. The liquid crystal structure and behavior have been also studied by using polarizing texture, small angle X-ray scattering, ²H-NMR and rheometer etc. The result shows that when the composition of the system changes along the line of AA’ in this large liquid crystal region, the structural change is cubic→cubic/lamellar→lamellar→lamellar/hexagonal→hexagonal. Meanwhile, we made the first attempt of systematic study of the rheological properties of the above system. The lattice constants of cubic and hexagonal liquid crystals are 10.53 and 5.68 nm, respectively.     

Influence of the electrical interface properties on the rheological behavior of sonicated soy lecithin dispersions

A significant effect, on the rheological behavior, due to the electrical properties of vesicles formed from concentrated soy lecithin dispersions have been studied in this work. The rheopectic behavior of concentrated soy lecithin dispersions (120, 150, 180, 210 and 240 g L-1) prepared by swelling-light sonication-freezing-unfreezing procedure is studied and it is specially emphasized on the transition under steady shear from lamellar phase of planar sheets to closed structures as multilamellar vesicles. Samples have been exposed to a different number of sonication cycles (from 0 to 100) and the changes in the hysteresis loop area, the apparent viscosity and the electrophoretic mobility have been studied. When the number of sonication cycles increases, the size and number of bilayers of these multilamellar vesicles decrease and therefore the total number of the vesicles and the volume fraction occupied by them show an increase.     

Phase equilibria and structures in ternary systems of a cationic surfactant (C16 TABr or (C16 TA)2SO4), alcohol,and water

The dependence of alcohol chain length on the isothermal phase behavior of the ternary systems hexadecylrrimethylammonium bromide/alcohol/water has been investigated. A liquid crystalline phase (the normal hexagonal one) occurs in the phase diagrams along the surfactant/water axis and this phase extends in the interior of the diagrams.When the alcohol is methanol, ethanol or butanol, there is in the ternary phase diagram a continuous solution region from the water to the alcoholic corner, and in the butanol case, in addition, a small region of lamellar liquid crystalline phase in the interior of the diagram. When the alcohol chain length is increased, the continuous solution region is divided into two subregions, an aqueousL ₁ and an alcoholicL ₂. The lamellar phase occupies the center of the phase diagrams and has the capability to incorporate large amounts of water under one-dimensional swelling. On the alcoholic side of the lamellar phase occur a reversed hexagonal liquid crystalline phase and a cubic liquid crystalline phase in the octanolic system; in the decanolic system the cubic phase is missing, but instead another liquid crystalline phase, presumably with rod-structure, occurs in addition to the reversed hexagonal phase.In a decanolic system where the monovalent bromide ion is replaced by the divalent sulphate ion there are the same solution regionsL ₁ andL ₂, and phase regions with liquid crystalline normal hexagonal and lamellar structures. The lamellar phase has lost much of its capability of incorporating water. That is in analogy with the conditions in anionic systems where the counterion charge has been increased. There is no reversed hexagonal phase, but on the alcoholic side of the lamellar phase, there is the same foreign liquid crystalline phase with a presumed rod-structure as in the monovalent system.     

Polymer-induced structural changes in lecithin/sodium dodecyl sulfate-based multilamellar vesicles

Aqueous concentrated lecithin mixtures (asolectin from soybean) show typical lamellar liquid crystalline behavior and the individual lamellae tend to form spherical supramolecular structures, i.e., multilamellar vesicles. When part of the lecithin is replaced by the anionic surfactant sodium dodecyl sulfate (SDS), the compact multilamellar vesicles disappear and the viscosity decreases. By adding poly(diallyldimethylammonium chloride) (PDADMAC) to the lecithin/SDS system, the formation of multilamellar vesicles can be induced again and the viscosity increases. However, one characteristic feature of these polymer-modified systems is a temperature-dependent phase transition from a compact multilamellar vesicle phase to a more swollen liquid crystalline phase. The polymer-modified multilamellar compact vesicles are of interest for utilization as new thermosensitive drug delivery systems.     

Lyotropic liquid crystalline phases formed in ternary mixtures of 1-cetyl-3-methylimidazolium bromide/p-xylene/water: a SAXS, POM, and rheology study

The phase behavior of ternary mixtures of 1-cetyl-3-methylimidazolium bromide (C(16)mim-Br)/p-xylene/water is studied by small-angle X-ray scattering (SAXS), polarized optical microscopy (POM), and rheology measurements. Two types of lyotropic liquid crystalline phases are formed in the mixtures: hexagonal and lamellar. The structural parameters of the lyotropic liquid crystalline phases are calculated. Greater surfactant content in the sample leads to denser aggregation of the cylindrical units in the hexagonal liquid crystalline phase. The increase in lattice parameter and thickness of the water layer in lamellar phase are attributed to the increase of water content, and the area per surfactant molecule at the hydrophobic/hydrophilic interface for lamellar phase is found to be larger than that for hexagonal phase. The structural parameters of the liquid crystalline phases formed from the cetyltrimethylammonium bromide (CTAB) system are larger than those for the C(16)mim-Br system. The rheological properties of the samples are also found to be related to the structure of the liquid crystalline phases.     

A review of the rheology of the lamellar phase in surfactant systems

The rheology of the liquid crystal lamellar phase has been studied in a wide range of systems including non-ionic, anionic and cationic surfactants and block co-polymers. This review summarises the main advances in this area over the past twenty years and includes examples of the rheo-optical techniques, which help to elucidate the changes in microstructural conformation taking place in the lamellar phase during shear. Particular emphasis is given to the microstructural change of the lamellar phase from sheet-like bilayers to dispersed multilamellar vesicles (droplets). Examples of this transition are provided for both surfactant and block co-polymer systems. The review highlights similarities in the rheological signatures of the transition for different systems and also summarises the variation in behaviour of the lamellar phase at different surfactant concentrations and on the addition of salt.     

Cubic phase prepared in an anionic/amphoteric surfactant/oleic acid/decane/water system and the relationship with the neighboring phase

The formation, properties, and structure of discontinuous cubic phase in the pseudo-ternary system consisting of N'-carboxyethyl N'-hydroxyethyl N-aminoethyl dodecylamide (imidazoriniumbetain), sodium and triethanol amine salt of polyoxyethylene (1.5 mol) myristyl ether sulfate, oleic acid, decane, and water at a constant surfactant/water ratio of 4/6 were studied by means of small-angle X-ray scattering, freeze-fracture transmission electron microscopy, static light scattering, and dynamic rheology to gain an insight in its origin and interrelation with neighboring phases. It was found that the cubic phase occupied a rather wide region in a constructed ternary phase diagram, including from 25 to 45% of decane. Its properties and structural parameters varied with changing the oil content. The decane addition caused the swelling of spherical micellar aggregates. This resulted in an increase of their diameter up to 35 nm, which was ca. nine times larger than that of the initial micelles, and micellar volume fraction (packing fraction) up to 72 vol. %, which was close to the theoretically possible value of 74 vol. % for the close-packed spherical particles. The cubic phase was surrounded by a micellar L1 phase from the water-rich side (separated by a short two-phase region), two-phase region (cubic + oil) from the oil-rich side, and a lamellar phase from the surfactant-rich side. A transition from the L1 phase to the cubic state at the packing fraction of 60 vol. % was caused by an increase in the packing density of micellar aggregates, occurring with the decane addition. When it reached 72 vol. %, the oil started forming a separated phase owing to the inability of micelles to dissolve it. The important observation is that the adjacent phase from the surfactant-rich side was a lamellar one made up of flat bilayers. The preliminary data showed that the lamellar phase coexisted with cylindrical micelles in the intermediate two-phase region separating the cubic and lamellar phases.     

表面活性剂溶致液晶的流变学性质

系统阐述了三种溶致液晶(六角状、立方状和层状液晶)的流变性质,概括了各自的流变性特点并给出了其理论模型,特别对立方相的流变学模型和层状相的剪切诱导转变作用进行了较详细的说明.讨论了因为这种转变而导致的囊泡的形成,并且在表面活性剂和嵌段共聚物中均可观察到剪切诱导的结构转变.     

Phase behavior of sucrose monoalkanoate in a water-long-chain alcohol system

Phase diagram of a water/sucrose monododecanoate (SE)/hexanol system was determined at 30°C. Aqueous micellar, reverse micellar, normal hexagonal liquid crystalline, and lamellar liquid crystalline phases appear in the phase diagram. The change in interlayer spacing and interfacial section area of surfactant in the liquid crystalline phases was investigated by small-angle x-ray scattering. Upon addition of water, the section area and the radius of cylindrical aggregates are almost constant in a hexagonal liquid crystal, whereas the distance between each cylinder is separated on the water-SE axis. The interlayer spacing slightly decreases or is almost unchanged on the surfactant-hexanol axis, because alcohol molecules penetrate into the palisade of bilayers. Although the average section area decreases with increasing alcohol content, each section area of SE and alcohol molecules are kept constant. Since the interfacial section area of alcohol is less than the section area of hydrocarbon chain, the phase transition from lamellar liquid crystal to reverse micelle occurs in an alcohol-rich region.     

Visualization of an adsorption model for surfactant transport from micelle solutions to a clean air/water interface using fluorescence microscopy

The micro- and mesoscopic structure of reverse Pluronic 25R4 in aqueous mixtures has been studied by SANS, SAXS and shear rheology. These techniques have been able to give a deep insight into the complex structure of the system phase diagram, that includes an isotropic water-rich liquid phase L(1), and liquid crystalline phases with hexagonal, E, or lamellar order, D. Particular attention has been paid to the isotropic water-rich phase L(1), which has a large stability region in the temperature-composition phase diagram. This region is crossed by a large "cloudy zone". Below it, namely at low temperature and composition, SANS data show the presence of polymer unimers in a gaussian coil conformation. Above the "cloudy zone", at higher temperature and composition, the L(1) phase is structured as a network of interconnected multimeric micelles. Rheology adds information about the structuring of the L(1) phase showing its incipient hexagonal pre-structuring. This technique is also able to highlight the defective structure of the E phase itself. In the temperature and concentration ranges in which a lamellar phase D is present, SANS and SAXS results are in complete agreement, showing how interlamellar distance is influenced by both polymer composition and temperature according to an "ideal deswelling" or a "not ideal swelling" mechanism. In addition, in the D phase rheology suggests the presence of densely packed vesicles.     

Rheological properties of lyotropic liquid crystals encapsulating curcumin

This study constructed new curcumin-loaded lyotropic liquid crystals containing pharmaceutically accepted oil, and ethyl oleate (EtOL). Three liquid crystalline phases including lamellar, hexagonal, and cubic phases were identified by means of the polarized optical microscopy and rheology method. By analyzing the shear viscosity (η_0.1), the viscosity of curcumin-liquid crystals is smaller than those without curcumin. Dynamic rheological results show that: Dissolved curcumin in EtOL can make the elastic modulus of hexagonal and cubic phase increase compared with that without curcumin, while the elastic modulus of lamellar phase decreases. Dissolved curcumin in Brij 97 can lead to the decreasing of the elastic modulus for cubic and lamellar phases, whereas it has little influence on hexagonal phase. When the curcumin is solubilized in both EtOL and Brij 97, the elastic modus of hexagonal phase increase, the elastic modus of lamellar and cubic phases decrease compared with that without curcumin. Furthermore, three temperature turning points were identified by the change in the slope of tanδ (G″/G′) for curcumin-hexagonal liquid crystal. These studies might be a help to study the storage of drug carrier and in vitro release properties of lyotropic liquid crystals containing curcumin.     

Microstructure and rheological properties of liquid crystallines formed in Brij 97/water/IPM system

The phase diagram of Brij 97/water/IPM systems was determined at 25 degrees C. Rich liquid crystalline phases including Lalpha, H1, and cubic Fd3m phases were identified by means of small angle X-ray scattering (SAXS). Microstructure transitions of liquid crystals with changes in surfactant concentration and oil content are explained qualitatively by the surfactant packing parameter (vL/aSlc). Dynamic rheological results indicate that all three kinds of liquid crystals investigated show high elasticity. The lamellar, Lalpha, phases formed in Brij 97/water with two different oils, oleic acid and geraniol, were also studied in comparison with those of Brij 97/water/IPM systems. The strength of the network of lamellar phases formed in Brij 97/water/oleic acid and Brij 97/water/geraniol systems are appreciably stronger than for Brij 97/water/IPM systems, indicated by the smaller area of surfactant molecules at the interface and the higher moduli (G' and G').     

Effect of shear on vesicle and lamellar phases of DDAB/lecithin ternary systems

The influence of shear flow on bilayer structures (vesicle and planar lamellar phases, L(α)), formed in DDAB/lecithin ternary systems, is studied by means of conventional rheology, Rheo NMR, and optical microscopy. The vesicles in the diluted (Lam(1)) phase are polydisperse multilamellae which turn into smaller monodisperse vesicles under shear. The concentrated (Lam(2)) phase is formed by non-oriented lamellae that do not surprisingly exhibit any pronounced shear-induced alignment prior to the transition into giant multilamellar vesicles. The biphasic region (Lam(1)+Lam(2)) shows a mosaic texture with a powder pattern indicating the prevalence of lamellae that transform into onions under shear.     

Hydration of dimethyldodecylamine-N-oxide: enthalpy and entropy driven processes

Dimethyldodecylamine-N-oxide (DDAO) has only one polar atom that is able to interact with water. Still, this surfactant shows very hydrophilic properties: in mixtures with water, it forms normal liquid crystalline phases and micelles. Moreover, there is data in the literature indicating that the hydration of this surfactant is driven by enthalpy while other studies show that hydration of surfactants and lipids typically is driven by entropy. Sorption calorimetry allows resolving enthalpic and entropic contributions to the free energy of hydration at constant temperature and thus directly determines the driving forces of hydration. The results of the present sorption calorimetric study show that the hydration of liquid crystalline phases of DDAO is driven by entropy, except for the hydration of the liquid crystalline lamellar phase which is co-driven by enthalpy. The exothermic heat effect of the hydration of the lamellar phase arises from formation of strong hydrogen bonds between DDAO and water. Another issue is the driving forces of the phase transitions caused by the hydration. The sorption calorimetric results show that the transitions from the lamellar to cubic and from the cubic to the hexagonal phase are driven by enthalpy. Transitions from solid phases to the liquid crystalline lamellar phase are entropically driven, while the formation of the monohydrate from the dry surfactant is driven by enthalpy. The driving forces of the transition from the hexagonal phase to the isotropic solution are close to zero. These sorption calorimetric results are in good agreement with the analysis of the binary phase diagram based on the van der Waals differential equation. The phase diagram of the DDAO-water system determined using DSC and sorption calorimetry is presented.     

Rheology and dynamics of micellar cubic phases and related emulsions

The rheological behavior of micellar cubic phases in C12EO25 systems and related emulsions has been investigated. In the aqueous C12EO25 binary system, the transition from the cubic phase to the micellar solution is associated with a sudden drop in viscosity and with a small enthalpy of transition. The elastic modulus and viscosity of the cubic phases show a maximum with concentration but remain very high within the range of existence of the cubic phase. Several relaxation processes seem to be present in binary cubic phases, and some of them occur in a time scale that can be followed by both rheology and dynamic light scattering measurements. Upon addition of a small amount of oil (decane), the rheological behavior changes remarkably. As the oil fraction increases, the relaxation times also increase and, finally, highly concentrated, gel-like emulsions are obtained. Contrary to conventional concentrated emulsions, the viscosity of cubic-phase-based emulsions is decreased by increasing the fraction of the dispersed phase. The non-Maxwellian rheological behavior at low oil fractions is described according to the model of slipping crystalline planes, modified by using a distribution of bulk relaxation times, and good fitting to the experimental data is obtained.     

STUDIES ON THE PHASE DIAGRAM OF THE SURFACTANT/1-PENTANOL / WATER TERNARY SYSTEM II: THE LIQUID CRYSTALLINE PHASE

The phase behavior of liquid crystalline in the ternary system of dodecyl dimethyl ammonium hydroxyl propyl sulfonate(DDAHPS)/1pentanol(C5HnOH) / water deuteron(D₂O) has been investigated by polarizing optical microscopy, ²H NMR spectroscopy methods. The results indicate that two kinds of liquid crystals (the lamellar, and the hexagonal) exist in the liquid crystalline phase region. In this paper, we also use the polarized Raman spectroscopy method to measure the values of the order/ disorder parameters and the values of the environment polarity parameters for the samples selected from the liquid crystalline phase region, and compare these two parameters of the samples with those of solid state DDAHPS and liquid state pentan-l-ol.     

Phase Equilibria in Systems of Water,Naphthenic Acids,and Phenols

A partial phase diagram has been determined for the system based on 5‐phenylvalerate, 4‐pentyphenol, and water at 25 °C. The system showed a very rich phase behavior in which many different isotropic solutions and liquid crystals were found. Both normal and reverse self‐assembly structures of the micellar and hexagonal types were noted. In the middle of the phase diagram, a lamellar liquid crystalline phase with a large swelling capacity was observed. When the aromatic alcohol was replaced by a long‐chain alcohol the reverse hexagonal structure disappeared. The effect of temperature and salinity on the phase behavior was also studied. Raising the temperature increased the micellar regions, while the lamellar phase was slightly reduced and the reverse hexagonal phase disappeared. Addition of salt gave the lamellar phase a smaller region of existence and the large extension towards the water apex disappeared. Introduction of an acid to the system resulted in a remarkable change of the phase behavior: both the normal micellar and lamellar regions were significantly reduced, while the reverse micellar region was significantly increased.