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PubMed1982

Transbilayer redistribution of phosphatidylethanolamine during fusion of phospholipid vesicles. Dependence on fusion rate, lipid phase separation, and formation of nonbilayer structures.

The effect of membrane fusion on the transbilayer distribution of dioleoyl- and dipalmitoylphosphatidylethanolamine (DOPE and DPPE, respectively) in phosphatidylserine (PS) vesicles was investigated. A 7-fold increase in the external pool of DOPE, as determined by labeling of the vesicle surface with 2,4,6-trinitrobenzenesulfonic acid, was observed when multilamellar vesicles (MLV) consisting of PS and DOPE were incubated with small unilamellar vesicles (SUV) of PS in the presence of Ca2+. In contrast, no significant redistribution of DPPE was seen when similar experiments were performed by using PS bilayers that contained DPPE instead of DOPE. Redistribution of neither DOPE nor DPPE could be detected during SUV-SUV fusion. By using the resonance energy transfer fusion assay for mixing of membrane lipids [Struck, D.K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099; Hoekstra, D. (1982) Biochemistry 21, 2833-2840], it was demonstrated that fusion between SUV and MLV had actually occurred. The results also suggested that (partial) fusion of internal bilayers within the multilamellar system must have occurred. Although Mg2+-induced fusion between SUV and MLV was also observed, no redistribution of DOPE was seen in this case. It is concluded that the observed translocation of DOPE during fusion was probably mediated via inverted micellar structures, which were formed when the lipid was converted to the hexagonal (HII) phase resulting from lipid phase separation between PS and DOPE. However, induction of the hexagonal phase per se, i.e., in the absence of fusion, did not cause substantial transbilayer redistribution of DOPE, suggesting that fusion was intimately involved in this phenomenon. It is suggested that fusion represents the "driving force" for transbilayer DOPE redistribution, requiring a (partial) overlap between the kinetics of phase separation and fusion.

Calcium↗

PubMed1990

Phase separation in short-chain lecithin/gel-state long-chain lecithin aggregates.

Small bilayer particles form spontaneously from gel-state long-chain phospholipids such as dipalmitoylphosphatidylcholine and 0.2 mol fraction short-chain lecithins (e.g., diheptanoyl-phosphatidylcholine). When the particles are incubated at temperatures greater than the Tm of the long-chain phosphatidylcholine (PC), the particles rapidly fuse (from 90-A to greater than or equal to 5000-A radius); this transition is reversible. A possible explanation for this behavior involves patching or phase separation of the short-chain component within the gel-state particle and randomization of both lipid species above Tm. Differential scanning calorimetry, 1H T1 values of proteodiheptanoyl-PC in diheptanoyl-PC-d26/dipalmitoyl-PC-d62 matrices of varying deuterium content, solid-state 2H NMR spectroscopy as a function of temperature, and fluorescence pyrene excimer-to-monomer ratios as a function of mole fraction diheptanoyl-PC provide evidence that such phase separation must occur. These results are used to construct a phase diagram for the diheptanoyl-PC/dipalmitoyl-PC system, to propose detailed geometric models for the different lipid particles involved, and to understand phospholipase kinetics toward the different aggregates.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2004

Dynamic domain formation in membranes: thickness-modulation-induced phase separation.

A simple model investigates the amplification of fluctuations on membranes constituted of two lipids having different lengths. Van der Waals and electrostatic interactions across the lipid bilayer result in a destabilization favoring thickness variations of the membrane. Close to spontaneous demixing of the two components, the additional gain in free energy due to thickness undulations shifts the stability boundary which promotes phase separation into domains. Interestingly, this effect can be induced by an applied electric field or membrane potential. In biological systems, the dynamic model presented here indicates that electric fields might be important for controlling phase separation and the formation of domains called "rafts".

Cell Membrane↗

PubMed2002

Absence of static phase separation in the high T(c) cuprate YBa(2)Cu(3)O(6+y).

We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.

Journal Article↗

PubMed2004

Phase separation in molecular layers of macromolecules at the champagne-air interface.

Bubble and foam stability, which are essential for the hallmark of champagne, rely on the concentration of amphiphilic macromolecules originating from the grape, which form molecular layers at the interface between champagne and gas. Ellipsometry and Brewster angle microscopy experiments were conducted at the air-champagne interface to analyse the lateral organization of the layers of macromolecules. Several kinds of phase separations - leading in some cases to two-dimensional foams - were identified. At the beginning of layer formation, condensed domains develop at the expense of dilute domains. Thereafter, phase separations occur within the condensed domains. These findings may allow advances in the implementation of methods predicting bubble and foam stability of champagnes.

Adsorption↗

PubMed1999

Ibuprofen agglomerates preparation by phase separation.

The compression ability and dissolution rate of ibuprofen are poor. There are many processes to optimize these properties through adapted formulations. However, it would be more satisfactory to obtain directly during the crystallization step crystalline particles that can be directly compressed and quickly dissolved. This was the aim of this work. Ibuprofen spherical agglomerates were obtained using a very simple method based on the difference of solubility of ibuprofen in ethanol and in water. By cooling down an ibuprofen-saturated solution in an ethanol/water 50/50 mixture from 60 degrees C to room temperature under stirring, a phase separation occurs. Ibuprofen crystallizes in separated water droplets. After separation by sieving and drying, spherical agglomerates were obtained. A study of the physical properties of ibuprofen agglomerates was carried out using electron scanning microscopy and X-ray powder diffraction. The compression ability was tested using an instrumented tablet machine, and the dissolution rate was measured using continuous flow cells. An improvement in compression and dissolution properties of the spherical agglomerates produced was observed. The process of crystallization in a separated dispersed phase could be envisaged each time a drug exhibits opposite solubilities in two miscible solvents.

Anti-Inflammatory Agents, Non-Steroidal↗

PubMed2002

Observation of magnetic domain structure in phase-separated manganites by lorentz electron microscopy.

Magnetic domain structure in manganites was investigated by Lorentz electron microscopy, in order to understand some unusual physical properties, such as a colossal magnetoresistance (CMR) effect and a metal-to-insulator (MI) transition. In particular, we examined the spatial distribution of the charge/orbital ordered (CO/OO) insulator state and the ferromagnetic (FM) metallic state in phase-separated manganites, (La5/8-xPrx)Ca3/8MnO3 for x = 0.375, by obtaining both the dark-field and Lorentz images. We found an unusual coexistence of the CO/OO and FM metallic states with micrometer size below a MI transition temperature of 60 K. Our experimental findings provide direct evidence of the phase separation found in CMR manganites.

Journal Article↗

PubMed2001

Kinetics of phase separation in polymer-solvent mixtures.

We investigate the kinetics of phase separation in systems with a strong asymmetry in the mobility. This simple model system mimics the segregation kinetics of polymer-solvent mixtures, where the polymer-rich phase forms a low-mobility gel. We obtain detailed numerical results for this model, both without and with thermal noise. In the absence of thermal noise, we find that domain growth is slowed down drastically when the low-mobility phase percolates the system. However, thermal noise restores rapid domain growth through a Brownian coalescence mechanism.

Journal Article↗

PubMed1976

Cytochrome c induced lateral phase separation in a diphosphatidylglycerol-steroid spin-label model membrane.

The extrinsic membrane protein cytochrome c binds to lipid mixtures containing negatively charged phospholipids such as diphosphatidylglycerol (DPG). In this study the effect of cytochrome c on the lipid distribution in a DPG-steroid spin-label (3-doxyl-5alpha-cholestane) model membrane system is examined. The electron spin resonance (ESR) line-shape changes indicate that cytochrome c induces lateral phase separation at room temperature. The resulting two-dimensional lipid distribution is nonrandom, consisting of clusters of phospholipids bound to cytochrome c and patches of steroid spin-label molecules. Phase separations are also observed in the three-component system: DPG, phosphatidylcholine, and 3-doxyl-5alpha-cholestane.

Animals↗

PubMed2005

An interfacial instability in a transient wetting layer leads to lateral phase separation in thin spin-cast polymer-blend films.

Spin-coating is a very widely used technique for making uniform thin polymer films. For example, the active layers in most experimental semiconducting polymer-based devices, such as light-emitting diodes and photovoltaics, are made this way. The efficiency of such devices can be improved by using blends of polymers; these phase separate during the spin-coating process, creating the complex morphology that leads to performance improvements. We have used time-resolved small-angle light scattering and light reflectivity during the spin-coating process to study the development of structure directly. Our results provide evidence that a blend of two polymers first undergoes vertical stratification; the interface between the stratified layers then becomes unstable, leading to the final phase-separated thin film. This has given us the basis for establishing a full mechanistic understanding of the development of morphology in thin mixed polymer films, allowing a route to the rational design of processing conditions so as to achieve desirable morphologies by self-assembly.

Coated Materials, Biocompatible↗

PubMed1998

Phase separation in phosphatidylcholine/anionic phospholipid membranes in the liquid-crystalline state revealed with fluorescent probes.

The mixing properties of anionic phospholipids such as phosphatidic acid (PA), phosphatidylserine (PS), and phosphatidylglycerol (PG) with phosphatidylcholine (PC) were examined in the liquid-crystalline state of membranes using extrinsic fluorescent probes incorporated into lipid bilayers. The excimer to monomer (E/M) fluorescence ratio of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine (PPC) was higher for the PA and PS matrices as compared to that for the PC matrix. When PC was replaced with PA or PS, the E/M ratio of PPC also increased in a concentration-dependent manner. When the concentration of PA or PS was increased in the PC membrane, the fluorescence of 1-palmitoyl-2-[12-[7-nitro-2,1, 3-benzodiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine decreased, indicating the occurrence of lipid clustering. Direct evidence for the PA or PS-induced phase separation in the PC/PA or PC/PS system was provided by the resonance energy transfer between 2-(4,4-difluoro-5-methyl-4-boro-3a, 4a-diaza-s-indacene-3-dodecanoly)-1-hexadecanoly -sn-glycero-3-phospho choline and PPC. The fluorescence polarization of 1,6-diphenyl-1,3, 5-hexatriene further supported the lateral organization of membranes by PA and PS. PA and PS also reduced the polarity of lipid bilayers, as measured by the emission fluorescence of 6-lauroyl-2-dimethylaminonaphthalene. On the other hand, PG had very little effect on the PC matrix, suggesting the ideal miscibility with PC molecules. The results suggest that the mixing properties of PA and PS in the PC matrix are not random but that phase separation occurs in the liquid-crystalline phase of membranes.

Anions↗

PubMed2006

Observation of phase separation in a strongly interacting imbalanced fermi gas.

We have observed phase separation between the superfluid and the normal component in a strongly interacting Fermi gas with imbalanced spin populations. The in situ distribution of the density difference between two trapped spin components is obtained using phase-contrast imaging and 3D image reconstruction. A shell structure is clearly identified where the superfluid region of equal densities is surrounded by a normal gas of unequal densities. The phase transition induces a dramatic change in the density profiles as excess fermions are expelled from the superfluid.

Journal Article↗

PubMed2004

Concentration dependence of droplet deformation in a phase separation process under an electric field.

We show how deformation of droplets, arising in a phase separation process of a two-phase dielectric or conducting liquid composite material under a low frequency electric field, depends on their volume fraction. The electric interactions between distorted particles is taken into account in a self-consistent way based on the effective medium approach. It follows from our model that the main physical mechanism responsible for the dependence of droplets' deformation on the volume fraction is the formation of an effective anisotropic medium. A two-parameter condition of the droplet instability was derived as a function of the dielectric permittivity or conductivity mismatch between the two phases and the volume fraction of the inclusions.

Journal Article↗

PubMed2001

Critical properties and phase separation in lattice Boltzmann fluid mixtures.

Basic equilibrium properties of lattice Boltzmann (LB) fluid mixtures (coexistence curve, surface tension, interfacial profile, correlation length) are calculated to characterize the critical phenomena occurring in these model liquids and to establish a reduced variable description allowing a comparison with real fluid mixtures. We observe mean-field critical exponents and amplitudes so that the LB model may be useful for modeling high molecular weight polymer blends and other fluid mixtures approximated over a wide temperature range by mean-field theory. We also briefly consider phase separation under quiescent and shearing conditions and point out the strong influence of interacting boundaries on the qualitative form of the late-stage phase-separation morphology.

Journal Article↗

PubMed2006

Interplay between two phase transitions: crystallization and liquid-liquid phase separation in a polyolefin blend.

The interplay between liquid-liquid phase separation (LLPS) and crystallization at several compositions in statistical copolymer blends of poly(ethyleneco-hexene) and poly(ethylene-cobutene) has been examined by optical microscopy (OM), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). The phase contrast optical microscopy shows interconnected bicontinuous structures for deeply quenched LLPS, characteristic of spinodal decomposition. After a second quench to a temperature below the melting point, an overwhelming change in crystallization kinetics has been clearly observed, which is caused by the increase of the nucleation rate assisted by concentration fluctuations due to the spontaneous spinodal LLPS. We propose a new mechanism of "fluctuation assisted nucleation" in the crystallization process for such interactive process in a blend system. The experimental results from OM, AFM, and DSC measurements at various conditions are all consistent with the fluctuation assisted nucleation model.

Journal Article↗

PubMed2001

Hydrodynamic bubble coarsening in off-critical vapor-liquid phase separation.

Late-stage coarsening in off-critical vapor-liquid phase separation is reexamined. In the limit of bubbles of vapor distributed throughout a continuous liquid phase, it is argued that coarsening proceeds via inertial hydrodynamic bubble collapse. This replaces the Lifshitz-Slyozov-Wagner mechanism seen in binary liquid mixtures. The arguments are strongly supported by simulations in two dimensions using a novel single-component soft-sphere fluid.

Journal Article↗

PubMed1999

Phase Separation, Creaming, and Network Formation of Oil-in-Water Emulsions Induced by an Exocellular Polysaccharide.

We have investigated the effect of an exocellular polysaccharide (EPS) on the phase behavior and the rheology of oil-in-water emulsions. Already at low EPS concentrations the phase separation occurs. The phase line can be described by depletion interaction theory. At high EPS concentrations and dispersed phase volume fractions above 10% there is a stable "gel"-like region in the phase diagram. A kinetic study showed that the rate of creaming decreases with increasing oil content due to hydrodynamic effects. This rate depends strongly on the concentration of EPS, which is related to the strength of the depletion interaction and the viscosity of the continuous phase. At low EPS concentration the creaming rate strongly increases with EPS concentration because of the stronger attraction. At higher EPS concentrations creaming is slowed down by the viscosity increase of the continuous phase and of the particle network which is formed. At high EPS concentrations this network becomes so strong that the gel prevents creaming. The rheological behavior of the "gel" was studied by measuring flow curves, which could be interpreted by a theoretical model for weakly aggregating particles. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

Late stage kinetics of a phase separation induced by a cw laser wave in binary liquid mixtures.

cw laser waves can be used to locally induce phase transitions. We investigate experimentally and theoretically a laser-driven liquid-liquid phase separation in a micellar phase of microemulsion and analyze its late stage kinetics. The medium is optically quenched in composition in the metastable region of the phase diagram. Two processes can lead to these concentration variations: electrostriction and thermodiffusion. The first originates from induced dipolar couplings in a field gradient. The second corresponds to a variation in concentration driven by a small thermal gradient. Since the nucleated droplets are optically trapped by the beam, we show that it becomes possible to experimentally analyze an academic situation, i.e., the diffusion-driven growth of a single droplet in compensated gravity. The late stage of this kinetics can be divided into two parts, a bulk behavior and a regime controlled by the finite transverse size of the beam. The bulk regime is totally analogous to that observed in classical situations (R proportional, variant t(1/3), where R is the droplet radius and t the time), and the scaling of the amplitudes in terms of reduced length and time scales is in total agreement with the expected behaviors for fluids belonging to the same Ising universality class. Moreover, the Gaussian beam behaves as an optical bottle with "soft walls" in which the absence of rigid boundaries, and thus of wetting couplings, allows an intrinsic description of the influence of finite-size effects on the kinetics. The beam size breaks the dynamic universality when the growing domains start to feel it. We experimentally investigate the resulting slowing down, and a diffusion-driven model of the growth inside a laser wave is built for comparison. The good agreement observed for the bulk regime and during its modification induced by finite-size effects opens a promising field for the development of this new application of laser waves to control out-of-equilibrium liquid mixtures.

Journal Article↗