Onion Phase Research Articles

Reversible nanodiamond-carbon onion phase transformations.

Because of their considerable science and technical interest, nanodiamonds (3-5 nm) are often used as a model to study the phase transformation between graphite and diamond. Here we demonstrated that a reversible nanodiamond-carbon onion phase transformation can become true when laser irradiates colloidal suspensions of nanodiamonds at the ambient temperature and pressure. Nanodiamonds are first transformed to carbon onions driven by the laser-induced high temperature in which an intermediary bucky diamond phase is observed. Sequentially, carbon onions are transformed back to nanodiamonds driven by the laser-induced high temperature and high pressure from carbon onions as nanoscaled temperature and pressure cell upon the laser irradiation process in liquid. Similarly, the same bucky diamond phase serving as an intermediate phase is found during the carbon onion-to-nanodiamond transition. To have a clear insight into the unique phase transformation the thermodynamic approaches on the nanoscale were proposed to elucidate the reversible phase transformation of nanodiamond-to-carbon onion-to-nanodiamond via an intermediary bucky diamond phase upon the laser irradiation in liquid. This reversible transition reveals a series of phase transformations between diamond and carbon allotropes, such as carbon onion and bucky diamond, having a general insight into the basic physics involved in these phase transformations. These results give a clue to the root of meteoritic nanodiamonds that are commonly found in primitive meteorites but their origin is puzzling and offers one suitable approach for breaking controllable pathways between diamond and carbon allotropes.

Dissipative particle dynamics simulation of onion phase in star-block copolymer

A dissipative particle dynamics simulation technique was used to investigate the effect of molecular architecture of star-block copolymer on the patterned structure in a nanodroplet. With increasing the ratio of solvophilic to block length to solvophobic block length(RH/T), solvophobic sphere, ordered hexagonal phase, onion phase, perforated onion phase and flocculent phase are formed, respectively. Since onion phase has potential application in controlled drug release, it has received wide attention experimentally and theoretically. Our simulation indicates onion phase forms at a certain RH/T(close to but less than 1). A star-block copolymer molecule has two conformations in onion phase: either fully located in a shell or shared by two neighboring shells. Central structure affects onion’s final shape. The molecular number of the copolymer in each shell is a quadratic function of the shell’s radius. The arm number of star-block copolymer has little influence on onion’s structure, but slightly affects the solvent content. Additionally, we studied the influence of arm length on onion’s structure.

Shear quench-induced disintegration of a nonionic surfactant C10E3 onion phase

The growth kinetics of a shear-induced onion structure in the nonionic surfactant C10E3 system after the shear quench is studied. In the small shear quench depth, the onion continuously grows up with a ripening behavior while discontinuous growth behavior is observed when the shear quench depth exceeds the threshold. In the discontinuous behavior, the initial onion first disintegrates into the planar lamellae, and then rebuilding of the new onion structure is achieved by Lα–onion transformation. We conclude that the driving force for the disintegration of the onion is the inhomogeneously stored bending energy at the edge of the polyhedral onion. The slow disintegration behavior observed for the stuffed onion with the colloidal particles indicates that the edges in the polyhedral onion are the origin of the instability after the shear quench.

Shear-induced Onion Formation of Triblock Copolymer-embedded Surfactant Lamellar Phase

Shear-induced onion phase formation is an unsolved topic in soft matter. It has been widely reported that many different surfactant lamellar systems show the shear-induced onion phase formation, since Roux et al. found it. In order to understand the shear-induced onion phase formation mechanism, Nettesheim et al. traced the formation process by using small angle neutron scattering under shear (RheoSANS) and found an intermediate structure during the structural transition. Kato et al. presented that the coherently buckled lamellae is plausible as such intermediate structure by carefully observing a small angle X-ray scattering profiles under shear (Rheo-SAXS). Although new experimental results have been steadily accumulated, the comprehensive studies on the critical condition and membrane flexibility necessary for the onion phase formation are lacked. Our finding in our previous study is that the development of the oily streak network density, which is a typical structural defect found in the smectic phase, is essential to achieve the shearinduced onion phase formation. As the oily streak network density is increased, the focal conic domains (FCD) with a negative Gaussian curvature are crowded and fulfill the space. FCDs have unfavorable energy contribution originating from the layer compression because of the inhomogeneous layer spacing. Excess increase in the FCD density seems to destabilize the lamellar phase and trigger the realignment of the layer orientation from FCD with the negative Gaussian curvature to new state so that the inhomogeneous layer spacing is eliminated. This could be a driving force for the shear-induced onion formation. The bilayer membrane curvature has to be reorganized from negative to positive for the onion formation (Fig. 1). We have previously studied the shear-induced lamellar/ onion transformation behavior of the amphiphilic triblock copolymer-grafted lamellar phase systems. In such block Shear-induced Onion Formation of Triblock Copolymer-embedded Surfactant Lamellar Phase

Shear-induced onion formation of polymer-grafted lamellar phase

We study the shear-induced lamellar/onion transformation of the polymer-grafted lamellar phase composed of nonionic surfactant and amphiphilic triblock copolymers. Increase in the mole fraction of polymer XP and the degrees of polymerization of hydrophilic chain NEO remarkably affects the shear-induced onion phase formation behavior. From the viewpoint of the defect-mediated rheology, the shear modulus G′ is attributed to the oily streak network density. Linear increase in G′ with preshear rate prior to the onion formation indicates that the increase in the oily streak network density is essential as a pretransition. The oily streak is composed of the focal conic domains (FCDs), which locally have inhomogeneous layer spacing. The critical shear stress σc is well scaled by the effective increment of the bending modulus theoretically predicted for the polymer-grafted membranes, Δκ ∼ XPN1.27EO. Experimental results suggest that the shear-induced onion phase is achieved by reorganizing the layer orientation of FCDs with the negative Gaussian curvature in order to eliminate the inhomogeneous layer spacing.

Elasticity of smectic liquid crystals with focal conic domains

We study the elastic properties of thermotropic smectic liquid crystals withfocal conic domains (FCDs). After the application of the controlled preshearat different temperatures, we independently measure the shear modulusG′ and theFCD size L. We find out that these quantities are related by the scaling relationG′≈γeff/L, whereγeff is the effective surface tension of the FCDs. The experimentally obtained value ofγeff shows the same scaling as the effective surface tension of the layered systems , where K and B are the bending modulus and the layer compression modulus, respectively. The similarity ofthis scaling relation to that of the surfactant onion phase suggests an universal rheologicalbehavior of the layered systems with defects.

Order−Disorder Transition of Nonionic Onions under Shear Flow

We have investigated the shear-induced ordering of multilamellar vesicles (onions) in a nonionic surfactant (C(12)E(4)) system using a small-angle light scattering (shear-SALS) and a small-angle X-ray scattering (shear-SAXS) technique. In a narrow shear rate-temperature space, the onions form a two-dimensional (2D) hexagonally close-packed structure that shows characteristic hexagonal scattering patterns in both SALS and SAXS. In the dynamic phase diagram, the ordered onion phase is surrounded by disordered onion phase, indicating reentrant behavior against the temperature. The disorder-order transition is accompanied by a jump in onion size by a factor of 5-6. In the disordered onion phase, by applying a shear flow, the planar lamellar membranes transform to an intermediate structure, multilamellae cylinders or a coherent stripe buckling, and then the intermediate structure develops to the isotropic onions. On the other hand, in the ordered onion phase, the intermediate structure breaks to onions stretched in the shear velocity direction, and then the stretched onions aligned gradually to form the 2D hexagonal close packing.

Multilamellar Structures Induced by Hydrophilic and Hydrophobic Ions Added to a Binary Mixture ofD2Oand 3-Methylpyridine

A phase transition is observed between a one-phase disordered phase and an ordered phase with multilamellar (onion) structures in an off-critical mixture of D2O and 3-methylpyridine (3MP) containing 85 mM of a salt in the absence of a surfactant. The salt consists of hydrophilic cations and hydrophobic anions that interact asymmetrically with the solvent composition fluctuations inducing mesophases. The structure factor of the composition distribution obtained from small-angle neutron scattering has a peak at an intermediate wave number in the disordered phase and multiple peaks in the ordered phase. Lamellar layers forming onions are composed of solvation-induced charged membranes swollen by D2O. The onion phase is realized only for small volume fractions of 3MP (in D2O-rich solvent).

Viscoelasticity of Onion Phase Composed of Complex Surfactant Bilayers

Onion (multilamellar vesicles) has a quite fascinating structure organized by amphiphilic bilayer membranes. According to the unique structure, a decrease in the onion radius is considered to be a serial event induced by peeling off the outer shell. However, it is not well known how onion grows up, when a shear rate is changed. We studied its mechanism by viscoelastic measurements. When the shear rate is quenched with a large depth, onion structure is degraded with exponential decay in the viscosity, followed by onion reformation. However, small quench depth in the shear rate does not cause exponential decay. Threshold shear rate for this mechanism is determined by the viscosity of lamellar phase in Newtonian region.

Onion Phases of PEG-8 Distearate

The aims of the present study were to formulate stable onion phases of the biodegradable surfactant PEG-8 Distearate (PEG8DS) and evaluate application of the onion phases in encapsulating sumatriptan succinate, a BCS class III potent antimigraine drug. Drug loaded and placebo onion phases were prepared by shearing lyotropic lamellar phases of the surfactant. Effect of drug/surfactant ratio, shear rate and shear time on particle size, and encapsulation efficiency were studied. The onion phases were characterized by polarized microscopy, small angle neutron scattering (SANS), NMR, rheology, and FTIR. Formation of onion phases of PEG8DS was confirmed by the presence of maltese crosses under a crosspolarized microscope and further by SANS studies. The onion phases revealed an increase of inter-bilayer spacing by 7 A after drug incorporation. NMR studies revealed location of drug in the aqueous phases of the multilamellar vesicles (MLVs). FTIR study revealed no interaction between drug and surfactant. Drug loaded onion phases exhibited high encapsulation efficiency ( approximately 90%) and rapid in vitro drug release (>90% in 10 min). Onion phases stored at 5 degrees C +/- 3 degrees C revealed no significant drug leakage at the end of 3 months suggesting adequate stability. Sumatriptan succinate loaded stable onion phases of PEG8DS with high entrapment efficiency and rapid drug release suggests potential application of the onion phases in drug delivery.

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.

Onion Phases as Biomimetic Confined Media for Silica Nanoparticle Growth

Phospholipid onion phases were investigated as biomimetic media for the synthesis of silica in a confined environment. Stable multilamellar nanovesicles incorporating sodium silicate solutions could be obtained. Upon aging, silica condensation occurs in the onion interlayer space while preserving the initial multilamellar organization. The hybrid structure consists of an array of apparently unconnected silica nanoparticles in the 20-30 A size range packed in the vesicular 50 A interlayer space, suggesting that the silica growth was efficiently controlled by its confinement in the onion lamellar organization.

An Onion Phase in Salt‐Free Zero‐Charged Catanionic Surfactant Solutions

An Onion Phase in Salt‐Free Zero‐Charged Catanionic Surfactant Solutions

An Onion Phase in Salt‐Free Zero‐Charged Catanionic Surfactant Solutions

No added salt: Transmission electron microscopy and small-angle X-ray scattering studies have shown that uni- and multilamellar vesicles with radii of 25.5 and 366 nm (see schematic diagram; the hydrophilic cation/anion groups are shown as green spheres and the hydrocarbon chains in blue), respectively, are formed from trimethyltetradecylammonium hydroxide and oleic acid in water. The multilamellar phase consists of seven bilayers.

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 °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.

Lamellar phases confined in quasicylindrical pores: lattice model results.

A two-dimensional (2D) vector lattice model of microemulsions is applied to study the structure of lamellar phases confined in long rectangular pores. One-point distribution functions are calculated within mean field approximation. The effects of pore geometry and surface fields are considered. A 2D analog of an onion phase is favored by a pore with strongly hydrophilic walls. For neutral walls, far from the phase boundaries, the lamellar phase is stable inside the pore. By contrast, close to the lamellar-tubular phase boundary a pore with neutral walls favors a 2D tubular phase. This is the analog of capillary condensation. In all cases the excess pressure is calculated as a function of the pore geometry.

Membrane crumpling and restricted swelling in onion phases

We have observed a deviation from one-dimensional swelling behaviour in onion phases. This is explained in terms of crumpling of each membrane due to its flexibility together with a lateral membrane tension which depends on the degree of dilution.

Influence of Salt Concentration and Surfactant Concentration on the Microstructure and Rheology of Lamellar Liquid Crystalline Phases

The microstructures and linear rheological properties of onion phases prepared from sodium dodecylbenzenesulfonate, a C13-15 ethoxylated alcohol with on average 7 EO groups at a fixed weight ratio of 7:3, sodium citrate and water were investigated over a broad concentration range of surfactant and salt. At low salt concentration a lamellar phase in equilibrium with a micellar phase is found. When more salt is added (9−15%), a colloidally stable, swollen onion phase is found. The repulsion between the surfactant bilayers is caused by thermal undulations. The effect of the undulations can be observed on electron microscope photographs. At salt concentrations between approximately 15 and 20% the appearance of the onion phase changes from translucent to milky white and the onions are (weakly) flocculated. The hydration of the surfactant molecules, the bilayer repeat distance, and, as a consequence of this, the obtained volume fraction of onions decrease rapidly. Thermal undulation of the bilayers is no longer observed because of stiffening of the layers. At even higher salt concentrations between 20 and 30% severe flocculation of the onions occurs. For the linear rheological behavior of the colloidally stable onion dispersions at 9.5 and 12.7% salt, a model taking into account the undulation potential as given by Helfrich and the length scale of the affine deformation is derived. The model satisfactorily explains the magnitude of the found elastic modulus G‘ at low frequencies, suggesting affine motion at a length scale larger than the size of the onions. The curved parts of the onions are assumed to be relaxed. The volume fraction dependence of G‘ of the onion dispersions at 16.9 and 28% salt is similar to that of a flocculated dispersion. At salt concentrations of 9.5 and 12.7% and onion volume fractions of 0.6 and above a broad relaxation transition in G‘ ‘ is observed. The strength of the relaxation (ΔG) can be explained if we assume that the transition is caused by the inclusion of the curved part of the onions. At constant salt level, the characteristic time (τ) of the transition and ΔG vary proportionally, showing that the viscosity (Δη) associated with the transition is constant. Going from 9.5 to 12.7% salt Δη increases by a factor of 2.5. Some possible explanations for this effect are offered.

Swelling and dissolution of onion phases: the effect of temperature

Contact experiments have been performed between an onion lamellar phase and brine in the SDS/octanol/brine system. Using video microscopy we have studied the non-equilibrium behaviour of the swelling and dissolution process of onions. Experiments at T=20 and 30°C showed that temperature has a strong effect on their behaviour. At low temperature onions are observed to diffuse away from the onion phase and only swell slightly. However, by increasing the temperature we induce the formation of sponge phase ( L 3) at the onion/brine interface. Onions that initially swell then dissolve into L 3 phase expel a stable core which moves to the micellar phase and remains there. Over a longer period of time (several days) we have also observed coalescence leading to the formation of large onions of up to ∼100 μm diameter. These huge onions have a radial distribution of domains, or solvent cavities, within them.

Shear-Induced Transition of Originally Undisturbed Lamellar Phase to Vesicle Phase

We have studied the transformation of an Lα phase composed of stacked bilayers to a vesicle phase under the influence of shear. The ionically charged stacked bilayer phase was prepared by protonation of an L3 phase from tetradecyldimethylamine oxide hexanol, and water. The protonation was accomplished by a hydrolysis reaction of an ester solubilized in this phase. By the protonation of the bilayers the L3 phase becomes unstable and, in the absence of shear, is transformed into a classical lamellar phase. This low-viscosity phase was then transformed into an onion phase by shear. The structural changes during this transformation process were monitored by means of scattering experiments and by rheological and conductivity measurements under shear. The following results were obtained when the shear rate was varied: (a) the transition from stacked lamellae to vesicles is irreversible and does not go back; (b) the formation of multilamellar vesicles requires a total strain of about 2000 (for a given surfactan...