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PubMed2006

Phase separation of a fast rotating boson-fermion mixture in the lowest-Landau-level regime.

By minimizing the coupled mean-field energy functionals, we investigate the ground-state properties of a rotating atomic boson-fermion mixture in a two-dimensional parabolic trap. At high angular frequencies in the mean-field lowest-Landau-level regime, quantized vortices enter the bosonic condensate, and a finite number of degenerate fermions form the maximum-density-droplet state. As the boson-fermion coupling constant increases, the maximum density droplet develops into a lower-density state associated with the phase separation, revealing characteristics of a Landau-level structure.

Journal Article↗

PubMed2005

Evidence for vertical phase separation in densely grafted, high-molecular-weight poly(-isopropylacrylamide) brushes in water.

The detailed conformational change of poly(N-isopropylacrylamide) (PNIPAM) brushes at high grafting density in D2O was investigated as a function of temperature using neutron reflection. PNIPAM chains were grafted at high surface density from gold and silicon oxide surfaces by atom transfer radical polymerization. Whereas single layer profiles were observed for temperatures below and above the transition region, bilayer profiles were observed for a narrow range of temperatures near the transition. This nonmonotonic change in the concentration profile with temperature is discussed in the context of theoretical models of vertical phase separation within a brush.

Journal Article↗

PubMed1980

Phase separation in mixtures of ionized iron in a hydrogen plasma.

The accuracy of the hypernetted chain theory for mixtures of iron nuclei and protons in a charge-neutralizing background is demonstrated by comparison with molecular dynamics calculations. Near critical conditions the Debye-Hückel approximation fails to converge, even with second-order concentration corrections. The critical conditions, determined by assuming a uniform electron gas background, have a critical temperature more than a factor of 2 lower than present estimates of interior solar conditions. Qualitative arguments are made that including the polarization of the electron gas background would not change this result significantly and thus phase separation of iron in the interior of the sun is unlikely.

Journal Article↗

PubMed2006

Jahn-Teller solitons, structural phase transitions, and phase separation.

It is demonstrated that under common conditions a molecular solid subject to Jahn-Teller interactions supports stable Q-ball-like nontopological solitons. Such solitons represent a localized lump of excess electric charge in periodic motion accompanied by a time-dependent shape distortion of a set of adjacent molecules. The motion of the distortion can correspond to a true rotation or to a pseudorotation about the symmetric shape configuration. These solitons are stable for Jahn-Teller coupling strengths below a critical value; however, as the Jahn-Teller coupling approaches this critical value, the size of the soliton diverges signaling an incipient structural phase transition. The soliton phase mimics features commonly attributed to phase separation in complex solids.

Journal Article↗

PubMedDate not supplied

Microencapsulation of ketorolac tromethamine by means of a coacervation-phase separation technique induced by the addition of non-solvent.

Ketorolac tromethamine (KT) is a non-steroidal drug with potent analgesic and anti-inflammatory activity and is absorbed rapidly (Tmax < 1.0 h) with an efficiency > 87% following oral and intramuscular administration. The plasma half-life of ketorolac ranges from 1.1 to 6.0 h. Its oral bioavailability is estimated to be 80%. Ketorolac has been found 36 times more potent than phenylbutazone, approximately twice as potent as indomethacin, and three times more potent than naproxen in suppressing carrageenan-induced paw oedema in rat. In this study, microcapsules of KT were prepared by means of coacervation-phase separation technique induced by the addition of non-solvent, and release rates from microcapsules were studied. Eudragit S100 was used as the coating material. Coacervation was achieved by the addition of cyclohexane at 2 ml/min at 25 degrees C and 1:4 solvent: non-solvent ratio was used. The microcapsules were washed with cyclohexane to harden the wall and dried at room temperature. Microcapsules with core:wall ratio of 1:1 and 1:2 were prepared and the particles obtained by sieving with an average diameter of 177-500 microns were used. The yield was calculated and the release properties of KT were investigated by USP XXII paddle method and using UV spectrophotometry at 318 and 323 nm.

Acrylic Resins↗

PubMed1985

Lipid phase separations induced by the association of cholera toxin to phospholipid membranes containing ganglioside GM1.

The interactions of cholera toxin and their isolated binding and active subunits with phospholipid bilayers containing the toxin receptor ganglioside GM1 have been studied by using high-sensitivity differential scanning calorimetry and steady-state and time-resolved fluorescence and phosphorescence spectroscopy. The results of this investigation indicate that cholera toxin associates with phospholipid bilayers containing ganglioside GM1, independent of the physical state of the membrane. In the absence of Ca2+, calorimetric scans of intact cholera toxin bound to dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles containing ganglioside GM1 result in a broadening of the lipid phase transition peak and a slight decrease (less than 5%) in the transition enthalpy. In the presence of Ca2+ concentrations sufficient to cause ganglioside phase separation, the association of the intact toxin to the membrane results in a significant decrease of enthalpy change for the lipid transition, indicating that under these conditions the toxin molecule perturbs the hydrophobic core of the bilayer. Calorimetric scans using isolated binding subunits lacking the hydrophobic toxic subunit did not exhibit a decrease in the phospholipid transition enthalpy even in the presence of Ca2+, indicating that the binding subunits per se do not perturb the hydrophobic core of the bilayer. On the other hand, the hydrophobic A1 subunit by itself was able to reduce the phospholipid transition enthalpy when reconstituted into DPPC vesicles. These calorimetric observations were confirmed by fluorescence experiments using pyrene phospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗

PubMed2005

Diffusion-limited phase separation in eukaryotic chemotaxis.

The ability of cells to sense spatial gradients of chemoattractant factors governs the development of complex eukaryotic organisms. Cells exposed to shallow chemoattractant gradients respond with strong accumulation of the enzyme phosphatidylinositol 3-kinase (PI3K) and its D3-phosphoinositide product (PIP(3)) on the plasma membrane side exposed to the highest chemoattractant concentration, whereas PIP(3)-degrading enzyme PTEN and its product PIP(2) localize in a complementary pattern. Such an early symmetry-breaking event is a mandatory step for directed cell movement elicited by chemoattractants, but its physical origin is still mysterious. Here, we propose that directional sensing is the consequence of a phase-ordering process mediated by phosphoinositide diffusion and driven by the distribution of chemotactic signal. By studying a realistic reaction-diffusion lattice model that describes PI3K and PTEN enzymatic activity, recruitment to the plasma membrane, and diffusion of their phosphoinositide products, we show that the effective enzyme-enzyme interaction induced by catalysis and diffusion introduces an instability of the system toward phase separation for realistic values of physical parameters. In this framework, large reversible amplification of shallow chemotactic gradients, selective localization of chemical factors, macroscopic response timescales, and spontaneous polarization arise naturally. The model is robust with respect to order-of-magnitude variations of the parameters.

Chemotaxis↗

PubMed2005

Phase separation of ternary self-assembled monolayers into hydrophobic 1-dodecanethiol domains and electrostatically stabilized hydrophilic domains composed of 2-aminoethanethiol and 2-mercaptoethanesulfonic acid on Au(111).

Ternary self-assembled monolayers (SAM) composed of 2-aminoethanethiol (AET), 2-mercaptoethanesulfonic acid (MES), and 1-dodecanethiol (DDeT) form two types of domains as if it were a two-component SAM: DDeT-rich hydrophobic domains and electrostatically stabilized hydrophilic domains composed of MES and AET on Au(111). MES and AET behave virtually as a single surface-active species. Two distinct reductive desorption peaks in cyclic voltammograms (CV) and binarized images of scanning tunneling microscopy clearly show nanometer scale, yet macroscopically distinguishable, phase separation over a wide range of the mixing ratio of DDeT and MES-AET in the bathing solution. X-ray photoelectron spectroscopy measurements indicate that the ratio of MES to AET in the hydrophilic domains is unity and that both terminal groups are in the charged states, that is, the sulfonate group and the ammonium group. With decreasing the total concentration of the thiols, the mole fraction of DDeT in the bathing solution at which the surface coverage of MES-AET domains is equal to that of DDeT domains dramatically decreases. This suggests that the adsorption kinetics plays a crucial role in the formation of the domains structure.

Journal Article↗

PubMed2006

Phase separation and percolation of reversibly aggregating spheres with a square-well attraction potential.

Reversible aggregation of spheres is simulated using a novel method in which clusters of bound spheres diffuse collectively with a diffusion coefficient proportional to their radius. It is shown that the equilibrium state is the same as with other simulation techniques, but with the present method more realistic kinetics are obtained. The behavior as a function of volume fraction and interaction strength was tested for two different attraction ranges. The binodal and the percolation threshold were determined. The cluster structure and size distribution close to the percolation threshold were found to be consistent with the percolation model. Close to the binodal phase separation occurred through the growth of spherical dense domains, while for deep quenches a system spanning network is formed that coarsens with a rate that decreases with increasing attraction. We found no indication for arrest of the coarsening.

Computer Simulation↗

PubMed1995

Increased permeability of phase-separated liposomal membranes with mixtures of ethanol-induced interdigitated and non-interdigitated structures.

It has been suggested by many workers using model membranes that the interdigitated structure formation, in which the acyl chains fully interpenetrate the hydrocarbon chains of the opposing monolayer, plays an important role in regulating many functions of biomembranes. In the present study the control of permeability was focused on as one of the biomembrane functions, and the effects of ethanol on the permeability of large unilamellar vesicles made by the extrusion technique (LUVET) (average diameter: about 250 nm), composed of dipalmitoyl or egg yolk phosphatidylcholines, were studied by monitoring the leakage of fluorescent dye, calcein, entrapped in the inner aqueous phase of the LUVET. The permeability was estimated from the apparent rate constant of calcein leakage at 25 degrees C. Large permeabilities were observed in the region of 0.6 M to 1.3 M ethanol, with a concentration dependence. In this range of ethanol concentrations the normal bilayer and interdigitated structure coexist and the membrane is in a phase-separated state. The large permeability is due to the instability of the boundary regions, the interdigitated membrane being characterized by a thinner structure and more rigid hydrocarbon regions in the layer than its non-interdigitated counter part. These results suggest the possibility of biomembrane-permeability regulation by interdigitated membrane formation.

Ethanol↗

PubMed2004

Phase separation of cholesterol from phosphatidylserine-cholesterol mixtures in the presence of the local anesthetic tetracaine.

Addition of the local anesthetic tetracaine (TTC) to multilamellar dispersions of natural phosphatidylserine (PS) causes changes in the thermotropic properties of the membrane, which can be detected by differential scanning calorimetry, and in the structure of the membrane as detected by X-ray diffraction. At molar ratio [PS]/ [TTC] approximately 8.5, the melting temperature of the phospholipid shifts downwards by approximately 2.5 degrees C. The melting endotherm is broadened; however, there is little change in the enthalpy of melting. In ternary mixtures (PS-TTC-cholesterol), the thermotropic changes are enhanced. At [PS]/ [TTC] approximately 13, the onset of phase separation of cholesterol crystals from PS in the liquid crystalline state occurs at molar fraction cholesterol (Xchol) approximately 0.28, marginally smaller than that found in the absence of the anesthetic.

Anesthetics, Local↗

PubMed2005

Crosslinking a lipid raft component triggers liquid ordered-liquid disordered phase separation in model plasma membranes.

The mechanisms by which a cell uses and adapts its functional membrane organization are poorly understood and are the subject of ongoing investigation and discussion. Here, we study one proposed mechanism: the crosslinking of membrane components. In immune cell signaling (and other membrane-associated processes), a small change in the clustering of specific membrane proteins can lead to large-scale reorganizations that involve numerous other membrane components. We have investigated the large-scale physical effect of crosslinking a minor membrane component, the ganglioside GM1, in simple lipid models of the plasma membrane containing sphingomyelin, cholesterol, and phosphatidylcholine. We observe that crosslinking GM1 can cause uniform membranes to phase-separate into large, coexistent liquid ordered and liquid disordered membrane domains. We also find that this lipid separation causes a dramatic redistribution of a transmembrane peptide, consistent with a raft model of membrane organization. These experiments demonstrate a mechanism that could contribute to the effects of crosslinking observed in cellular processes: Domains induced by clustering a small number of proteins or lipids might rapidly reorganize many other membrane proteins.

Adaptor Proteins, Signal Transducing↗

PubMed2006

Numerical simulations of phase separation dynamics in a water-oil-surfactant system.

We have studied numerically the dynamics of the microphase separation of a water-oil-surfactant system. We developed an efficient and accurate numerical method for solving the two-dimensional time-dependent Ginzburg-Landau model with two order parameters. The numerical method is based on a conservative, second-order accurate, and implicit finite-difference scheme. The nonlinear discrete equations were solved by using a nonlinear multigrid method. There is, at most, a first-order time step constraint for stability. We demonstrated numerically the convergence of our scheme and presented simulations of phase separation to show the efficiency and accuracy of the new algorithm.

Journal Article↗

PubMed2006

Novel method to prepare morphologically rich polymeric surfaces for biomedical applications via phase separation and arrest of microgel particles.

We outline here a simple method to prepare polymeric surfaces of controlled surface topography on the micrometer scale, via assembly and arrest of microgel particles, for use in a range of biological applications to modify cell adhesion and spreading. In previous work by other groups, it has transpired that topography on the nanoscale is unlikely to be useful for this purpose, as roughness on this scale is often covered or coated by serum derived proteins during the early stages of cell adhesion and cells can easily bridge nanoscale roughness. Therefore, in our work, we have focused on roughness or topographic variations on the micrometer length scale. The basic idea is to modify the interactions between particles, thereby causing the microgel particles to phase separate into particle-dense and particle-dilute domains and to arrest these domains on the surface. The result is the creation of surfaces with controlled topography. By changing the particle size, it is possible to alter the size of the pores formed and their distribution in the film. Preliminary results show that the system can readily be arrested into a homologous series of such structures (formed from microgel particles of the same size and same chemical structure) with biological implications. At the extremes of this series, large phenotypic differences are observed between cells, ranging (at one end) from localization of the cells in the pores to (at the other end) cells that avoid such localization, and remain extended, growing along the ridges between the pores. This constitutes a sort of cell localization transition on a surface with identical chemical components, where only the morphology has been adjusted.

Acrylamides↗

PubMed2001

Electroresistance and electronic phase separation in mixed-valent manganites.

The sensitivity of transport in colossal magnetoresistance (CMR) manganites to external electric and magnetic fields is examined using field effect configurations with La(0.7)Ca(0.3)MnO(3) (LCMO), Na(0.7)Sr(0.3)MnO(3), La(0.7)Ba(0.3)MnO(3), and La(0.5)Ca(0.5)MnO(3) (0.5-doped LCMO) channels, and ferroelectric PbZr(0.2)Ti(0.8)O(3) (PZT) or dielectric (SrTiO(3)) gates. A large electroresistance (ER) of approximately 76% at 4 x 10(5) V/cm is found in LCMO with PZT-ferroelectric gate, but the magnitude of the effect is much smaller (a few percent) in the other three channels. The ER and CMR effects are remarkably complimentary. The size and systematics of the effect strongly favor a percolative phase separation picture.

Journal Article↗

PubMed2004

Chemical force microscopy of mixed self-assembled monolayers of alkanethiols on gold: evidence for phase separation.

Mixed self-assembled monolayers formed by the coadsorption of hydroxyl- and methyl-terminated alkanethiols with similar chain lengths have been characterized by friction force microscopy. Friction coefficients have been determined by assuming a fit to Amonton's law. The friction coefficients vary linearly with the fraction of polar-terminated adsorbates in the self-assembled monolayer (SAM). With carboxylic acid-terminated tips, the coefficient of friction increases with the fraction of hydroxyl-terminated thiols, while with methyl-terminated tips it decreases. Similar trends are observed for pull-off forces, which increase and decrease as a function of the fraction of polar-terminated adsorbates for carboxylic acid- and methyl-terminated adsorbates, respectively. Analysis of histograms of adhesion forces has yielded insights into the phase structure of mixed SAMs. Single-component monolayers yield histograms that may be fitted to symmetric Gaussian distributions, irrespective of the nature of the terminal group on either the tip or the SAM. However, mixed monolayers yield broad, asymmetric distributions that could not be fitted with a Gaussian distribution. The best explanation for these data is that mixed SAMs of hydroxyl- and methyl-terminated alkanethiols of similar chain length form phase-separated structures.

Journal Article↗

PubMed2004

A study of phase separation in peptide-loaded HPMC films using T(zero)-modulated temperature DSC, atomic force microscopy, and scanning electron microscopy.

Despite the widespread use of drug-loaded polymeric systems, there is still considerable uncertainty with regard to the nature of the distribution of the drug within the polymer matrix. The aim of this investigation was to develop thermal and microscopic techniques whereby the miscibility and spatial distribution of a model peptide, cyclosporin A (CyA), in hydroxypropyl methylcellulose (HPMC) films may be studied. The new technique of T(zero)-modulated temperature differential scanning calorimetry (T(zero) MTDSC), scanning electron microscopy (SEM), and pulse force mode atomic force microscopy (PFM-AFM) were used in conjunction to study films prepared using a solvent evaporation process, with a solvent extraction study performed to elucidate the nature of the observed phases. T(zero) MTDSC studies showed glass transitions for both the HPMC and CycA, with the T(g) for the HPMC and CycA seen for the mixed systems. SEM showed two spherical phases of differing electron density. PFM-AFM also showed spheres of differing adhesion that increased in size on addition of drug. Pixel intensity analysis indicated that the smaller spheres corresponded to CycA. Exposure of the films to dichloromethane, in which CycA is soluble but HPMC is not, resulted in the presence of voids that corresponded well to the spheres suggested to correspond to the drug. It was concluded that the system had undergone extensive or complete phase separation, and that the thermal and microscopic techniques outlined above are an effective means by which this issue may be studied.

Calorimetry, Differential Scanning↗

PubMed1988

Interactions of proteins with ganglioside-enriched microdomains on the membrane: the lateral phase separation of molecular species of GD1a ganglioside, having homogeneous long-chain base composition, is recognized by Vibrio cholerae sialidase.

The thermotropic behavior (studied by high-sensitivity differential scanning calorimetry) and susceptibility to Vibrio cholerae sialidase hydrolysis of large unilamellar vesicles of dipalmitoyl-phosphatidylcholine, containing native GD1a ganglioside or the molecular species of GD1a containing C18:1 or C20:1 long-chain base (C18:1 GD1a; C20:1 GD1a), were studied. Vesicles containing ganglioside (10% in molar terms) showed the presence in the heat capacity function of a second minor peak besides the phospholipid main transition peak. The presence of a second peak is much more evident with C20:1 GD1a than with C18:1 GD1a, the difference being potentiated by Ca2+ and indicating a different tendency of the CD1a molecular species to undergo lateral phase separation. The scans of vesicles containing native GD1a showed the features of those obtained with C18:1 GD1a and C20:1 GD1a, indicating that the main components of native GD1a, C18:1 GD1a and C20:1 GD1a, maintain their individual aggregative properties. V. cholerae sialidase affects vesicle-bound GD1a at a much higher rate (17-25-fold) than it does micellar GD1a, the activation by Ca2+ being 3- and 2-fold, respectively. The Vmax values were identical on C18:1 GD1a and C20:1 GD1a in micellar dispersions, whereas they were markedly higher (from 20 to 50%) on C18:1 GD1a than on C20:1 GD1a in vesicular dispersions. Exhaustive sialidase hydrolysis of vesicles carrying native GD1a produced C18:1 GM1 and C20:1 GM1 in the same proportion as the C18:1 and C20:1 species present in native GD1a (53.9% and 46.1%).(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dipalmitoylphosphatidylcholine↗