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PubMed2004

Direct imaging and mesoscale modelling of phase transitions in a nanostructured fluid.

The kinetics of phase transitions is essential for understanding pattern formation in structured fluids. These fluids play a key role in the morphogenesis of biological cells, and they are very common in pharmaceutical products and plastic materials. Until now, it has not been possible to follow phase transitions in structured fluids experimentally in real time and with high spatial resolution. Previous work has relied on static images and indirect experimental evidence from spatially averaging scattering experiments. Simulating the processes with computer models is a further challenge because of the multiple time and length scales involved. Our movies based on in situ scanning force microscopy show the time sequence of the elementary steps of a phase transition in a fluid film of block copolymer from the cylinder to the perforated lamella phase. The movies validate a versatile simulation model that gives physical insight into the nature of the process. Our approach provides a means of improving the study and understanding of pattern formation processes in nanostructured fluids. We expect a significant impact on nanotechnology where block copolymers serve as self-organized templates for the synthesis of inorganic nanostructured materials.

Computer Simulation↗
PubMed1988

Effects of platelet activating factor and related lipids on phase transition of dipalmitoylphosphatidylcholine.

Recent evidence localizing the inflammatory mediator, platelet activating factor, (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to the membranes of stimulated neutrophils raises the possibility that PAF may, in addition to its activities as a mediator, alter the physical properties of membranes. Accordingly, the effects of PAF and related alkyl ether and acyl analogs on phase transition thermodynamics of dipalmitoylphosphatidylcholine (DPPC) were studied using fluorescence polarization of the fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene (DPH). PAF, its ester analog (1-palmitoyl-2-acetylphosphatidylcholine) and both the corresponding alkyl and acyl lysophospholipid analogs (each at a concentration of 10 mol%) significantly decreased the phase transition temperature and broadened the phase transition of DPPC (P less than 0.05). The relative potency of the lipids in causing this effect was ester-PAF greater than or equal to PAF greater than or equal to lyso-PAF greater than lyso-PC suggesting that the fluidization of the synthetic membranes was attributable to both the 2-position acetyl group and the 1-position alkyl linkage. Furthermore, using various related compounds, increases in chain length and degree of unsaturation in the 2-position were shown to enhance the depression in transition temperature and broadening of the phase transition. Phase transition thermodynamics were also assessed using differential scanning calorimetry. Similar depression in the phase transition temperature was measured for PAF and both the alkyl and acyl lysophospholipids. Broadening of the phase transition for DPPC by the various analogs was assessed by calculation of transition peak width and cooperative unit. Data from fluorescence polarization and differential scanning calorimetry provide similar though not identical results and support the hypothesis that the unique features of PAF may alter membrane physical properties and could ultimately explain some of its biologic actions.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1983

Protein-mediated lipid transfer. The effects of lipid-phase transition and of charged lipids.

The protein-mediated phospholipid exchange between small unilamellar vesicles was investigated by fluorescence polarization measurements with diphenylhexatriene as optical probe. Thermotropic phase-transition measurements were taken after mixing two vesicle preparations of distinct and different phase-transition temperatures or having different states of charge. From the heights of each phase-transition step, we were able to follow the lipid-exchange process in the presence, as well as in the absence (natural exchange), of so-called transfer protein isolated from beef liver. A strong enhancement of the lipid transfer was observed at the corresponding lipid-phase-transition temperature, which is explained by the presence of fluctuating fluid and ordered domains co-existing at the lipid-phase-transition temperature. A unidirectional lipid transfer of the neutral component was observed between negatively charged phosphatidic acid and neutral phosphatidylcholine vesicles. Fluorescence polarization measurements showed the disappearance of the phosphatidylcholine phase transition, whereas the phosphatidic acid phase transition broadened and its phase transition temperature became lower.

Androgen-Binding Protein↗
PubMed2004

The effect of salt and pH on the phase-transition behaviors of temperature-sensitive copolymers based on N-isopropylacrylamide.

The salt and pH effects on the phase transition behaviors of temperature-sensitive hydrophilic and amphiphilic polymers containing copolymers of N-isopropylacrylamide as hydrophilic segments were investigated by UV and dynamic light scattering studies. The polymers used for this study include two hydrophilic polymers of HO-terminated poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (1) and HO-functionalized poly(N-isopropylacrylamide-co-N-hydroxylmethylacrylamide) (3), and their amphiphilic derivatives of poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide)-b-poly(D,L-lactide) (2) and cholesteryl-grafted poly(N-isopropylacrylamide-co-N-hydroxylmethylacrylamide) (4). With the increase of NaCl concentration, significant salt effects on their phase transition behaviors were observed. (1) Significant decreases in the lower critical solution temperatures of all polymers were observed, the salt effects were also characterized by a newly defined 'phase transition salt concentration'; (2) time-course UV spectroscopic studies showed that the phase transitions became faster and more complete with the increase of NaCl concentration, which were demonstrated by a newly defined 'phase transition constant'; (3) the salt exerted stronger effects on phase transition behaviors of the amphiphilic polymers than that of the hydrophilic polymers, and on polymer 4 than polymer 2. In contrast, the phase transition behaviors of the polymers were less sensitive to pH. The present studies provide important information for the design and synthesis of temperature-sensitive polymers and polymeric micelles that are important drug delivery carriers for hydrophobic drugs.

Acrylamides↗
PubMed2006

Unconventional continuous phase transition in a three-dimensional dimer model.

Phase transitions occupy a central role in physics, due both to their experimental ubiquity and their fundamental conceptual importance. The explanation of universality at phase transitions was the great success of the theory formulated by Ginzburg and Landau, and extended through the renormalization group by Wilson. However, recent theoretical suggestions have challenged this point of view in certain situations. In this Letter we report the first large-scale simulations of a three-dimensional model proposed to be a candidate for requiring a description beyond the Landau-Ginzburg-Wilson framework: we study the phase transition from the dimer crystal to the Coulomb phase in the cubic dimer model. Our numerical results strongly indicate that the transition is continuous and is compatible with a tricritical universality class, at variance with previous proposals.

Journal Article↗
PubMed1978

The effect of free fatty acids on the thermotropic phase transition of dimyristoyl glycerophosphocholine.

The effect of free fatty acids on the phase transition characteristics and fluidity of bilayers of dimyristoyl glycerophosphocholine were studied by pyrene eximer fluorescence and differential scanning calorimetry. High melting saturated fatty acids with chain lenghts of 12--18 carbon atoms raise the phase transition temperature and enhance the ability of pyrene to form clusters in the gel state while not affecting the fluidity of the membrane in the liquid crystal state. Low melting unsaturated fatty acids lower the phase transition temperature and decrease the ability of pyrene to form clusters in the gel state while not affecting the fluidity of the membrane in the liquid crystal state. The effects of the very long chain fatty acids, arachidic (C 20) and behenic (C 22) appears to be similar to those of cholesterol in that they cause a broadening of the phase transition with a lowering of the transition enthalpy but have little effect on the temperature at which the phase transition occurs.

Calorimetry↗
PubMed1998

Trapping of short-lived intermediates in phospholipid phase transitions: the L* alpha phase.

Time-resolved small-angle X-ray diffraction of liquid-crystalline phospholipid-water systems under temperature or pressure jump conditions has demonstrated the existence of an ordered, intermediate L alpha phase, with a sub-second lifetime, designated as the L* alpha-phase. The lamellar repeat spacing is, universally, 0.3 nm smaller than that of the parent phase, irrespective of the lipid composition and of the jump conditions, provided that the jump leads to a net volume expansion of the phase. The presence of salts, most notably LiCl, leads to a prolongation of the lifetime. The results suggest a non-monotonic potential function for the interbilayer water thickness.

Membranes, Artificial↗
PubMed2000

Orientational phase transitions in the hexagonal phase of a diblock copolymer melt under shear flow

We generalize the earlier theory by Fredrickson [J. Rheol. 38, 1045 (1994)] to study the orientational behavior of the hexagonal phase of diblock copolymer melt subjected to steady shear flow. We use symmetry arguments to show that the orientational ordering in the hexagonal phase is a much weaker effect than in the lamellae. We predict the parallel orientation to be stable at low and the perpendicular orientation at high shear rates. Our analysis reproduces the experimental results by Tepe et al. [Macromolecules 28, 3008 (1995)] and explains the difficulties in experimental observation of the different orientations in the hexagonal phase.

Journal Article↗
PubMedDate not supplied

Control by ganglioside GD1a of phospholipase A2 activity through modulation of the lamellar-hexagonal (HII) phase transition.

Ganglioside GD1a has a dual effect on the formation of the HII phase in mixtures of dioleoylphosphatidyl choline (DOPC)-dioleoylphosphatidylethanolamine (DOPE) (1:5). Below 1 mol% ganglioside in the mixture the formation of the HII phase is facilitated as indicated by a decrease of the lamellar-HII phase transition temperature; above 1 mol% the presence of ganglioside GD1a opposes formation of the HII phase and causes an increase of the phase transition temperature; the latter is completely abolished at 3 mol% GD1a or above. The rate of activity of phospholipase A2 against these mixtures showed a temperature variation in coincidence with the establishment of the lamellar-hexagonal II phase transition. In the presence of GD1a at 0.5 and 1 mol% in the mixture, the maximum activity shifted in correspondence with the ganglioside-induced displacement of the lamellar-hexagonal II phase transition. The temperature variation of the enzymatic activity did not show any maximum for pure DOPC of for DOPC-DOPE (1:5) mixtures containing 3 mol% GD1a or above, in which no lamellar-hexagonal II phase transition occurs. The effects of GD1a are probably due to the geometrical features of the ganglioside molecule that allow a composition-dependent compensation of the structural defects required for the formation of the HII phase which are detected by phospholipase A2.

Calorimetry↗
PubMed2004

Unusual phase transitions in ferroelectric nanodisks and nanorods.

Bulk ferroelectrics undergo structural phase transformations at low temperatures, giving multi-stable (that is, multiple-minimum) degenerate states with spontaneous polarization. Accessing these states by applying, and varying the direction of, an external electric field is a key principle for the operation of devices such as non-volatile ferroelectric random access memories (NFERAMs). Compared with bulk ferroelectrics, low-dimensional finite ferroelectric structures promise to increase the storage density of NFERAMs 10,000-fold. But this anticipated benefit hinges on whether phase transitions and multi-stable states still exist in low-dimensional structures. Previous studies have suggested that phase transitions are impossible in one-dimensional systems, and become increasingly less likely as dimensionality further decreases. Here we perform ab initio studies of ferroelectric nanoscale disks and rods of technologically important Pb(Zr,Ti)O3 solid solutions, and demonstrate the existence of previously unknown phase transitions in zero-dimensional ferroelectric nanoparticles. The minimum diameter of the disks that display low-temperature structural bistability is determined to be 3.2 nm, enabling an ultimate NFERAM density of 60 x 10(12) bits per square inch-that is, five orders of magnitude larger than those currently available. Our results suggest an innovative use of ferroelectric nanostructures for data storage, and are of fundamental value for the theory of phase transition in systems of low dimensionality.

Journal Article↗
PubMed2003

Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100).

Magnetic phase transitions in coupled magnetic sandwiches of Cu/Co/Cu/Ni/Cu(100) and Cu/Co/Fe/Ni/Cu(100) are investigated by photoemission electron microscopy. Element-specific magnetic domains are taken at room temperature to reveal the critical thickness at which the magnetic phase transition occurs. The results show that a coupled magnetic sandwich undergoes three types of magnetic phase transitions depending on the two ferromagnetic films' thickness. A phase diagram is constructed and explained in the process of constructing Monte Carlo simulations, which corroborate the experimental results.

Journal Article↗
PubMed1982

Fluorescence depolarization studies of the phase transition in multilamellar phospholipid vesicles exposed to 1.0-GHz microwave radiation.

The phase transition in multilamellar dimyristoylphosphatidylcholine (DMPC) vesicles was studied during exposure to continuous wave 1.0-GHz microwave radiation. Fluorescence depolarization measurements using a lipid-seeking molecular probe, diphenylhexatriene (DPH), were performed as a function of temperature. Semilog plots of microviscosity versus temperature illustrate the phase transition which shows a 5 degree C shift when the vesicles are treated with chloroform as a positive control. No shift of the phase transition was found during exposure to microwave radiation at specific absorption rates between 1 and 30 W/kg. Samples were exposed in a rectangular transmission line (TEM cell), and specific absorption rates were calculated from electrical measurements of incident, reflected, and transmitted power. Samples were exposed to increasing intensities of radiation, while the temperature was maintained at either 23.5 or 25.5 degree C; these temperatures represented the two ends of the phase transition region for these vesicles. No statistically significant difference was found between exposed and control samples. These results are in contrast to those of others using laser Raman spectroscopy to measure the phase transition in similar multilamellar vesicles exposed to microwave radiation.

Dimyristoylphosphatidylcholine↗
PubMed2005

Effect of deuterium oxide on the thermodynamic quantities associated with phase transitions of phosphatidylcholine bilayer membranes.

The bilayer phase transitions of three kinds of phospholipids, dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC) and dihexadecylphosphatidylcholine (DHPC), in deuterium oxide (D(2)O) and hydrogen oxide (H(2)O) were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The DSC measurements showed that the substitution of H(2)O by D(2)O affected the pretransition temperatures and the main-transition enthalpies of all PC bilayers. The temperature-pressure phase diagrams for these PC bilayer membranes in both solvents were constructed by use of the data of light-transmittance measurements. Regarding the main transition of all PC bilayer membranes, there was no appreciable difference between the transition temperatures in D(2)O and H(2)O under high pressure. On the other hand, the phase transitions among the gel phases including the pretransition were significantly affected by the solvent substitution. The thermodynamic quantities of phase transitions for the PC bilayer membranes were evaluated and the differences in thermodynamic properties by the water substitution were considered from the difference of interfacial-free energy per molecule in the bilayer in both solvents. It was proved that the substitution of H(2)O by D(2)O causes shrinkage of the molecular area of phospholipid at bilayer interface due to the difference in bond strength between deuterium and hydrogen bonds and produces the great influence on the bilayer phase with the smaller area. Further, the induction of bilayer interdigitation in D(2)O turned out to need higher pressures than in H(2)O.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1978

Studies on the effect of the lipid phase transition on the interaction of glucagon with dimyristoyl glycerophosphocholine.

Glucagon is found to interact with dimyristoyl glycerophosphocholine both above and below the phase transition temperature of the lipid. Above the phase transition temperature the interaction is manifested by an increase in the rate of vesicle aggregation and by an increased permeability of unilamellar vesicles to Eu3+ and to Fe(CN)3-6. However, no stable lipoprotein complex can be detected by gel filtration. Below the phase transition glucagon can form stable complexes with dimyristoyl glycerophosphocholine vesicles but cannot rapidly rearrange these vesicles to disk-shaped particles until the phase transition temperature is approached. The energy of activation for the dissociation of glucagon from the disk-shaped lipoprotein particle is 29 kcal/mol at temperatures above 36 degrees C but increases markedly at lower temperatures, as the region of the lipid phase transition is approached. This increase in energy of activation at lower temperatures is most probably due to the larger amount of energy required to rearrange gel-state lipid in the transition state and provides an explanation for the unusual kinetic stability of the glucagon-dimyristoyl glycerophosphocholine lipoprotein complex only at temperatures below the phase transition of the lipid.

Chemical Phenomena↗
PubMed1974

Electrostatic effects on lipid phase transitions: membrane structure and ionic environment.

Ordered --> fluid phase transitions in bilayers of charged lipids are accompanied by a decrease in electrostatic free energy mainly as a result of bilayer expansion. For uniform charge distribution the Gouy-Chapman theory of the electrical double layer predicts a decrease of the transition temperature with increasing charge density. We studied the effects of pH and mono- and divalent cations on the phase transition of lecithin, cephalin, phosphatidylserine, and phosphatidic acid bilayers. Phosphatidic acid with two ionizable protons was selected for a systematic investigation. A change in pH from 7 to 9 increases the charge per polar group from one to two elementary charges. This lowers the transition temperature by about 20 degrees C in agreement with the theory. In this pH region rather small changes in pH suffice to induce the phase transition at constant temperature. Divalent cations (Mg(++) and Ca(++)) increase the transition temperature by charge neutralization and thus can be used to induce the fluid --> ordered transition at constant temperature. In contrast, monovalent cations (Li(+), Na(+), K(+)) lower the transition temperature, or fluidize the bilayer structure at a given temperature. Rather small changes in ionic environment can induce gross alterations in bilayer structure; divalent and monovalent cations have antagonistic effects. This result parallels current theories on nerve excitation and sensory transduction where cation-induced structural changes in biomembranes are invoked.

Calcium↗
PubMed2002

First-order Fréedericksz transition above the nematic-smectic-a phase transition.

The Fréedericksz transition is investigated for thin homeotropic cells of a negative dielectric anisotropy liquid crystal above its nematic-smectic-A phase transition temperature in a spatially uniform electric field. For very thin cells, a crossover from second- to first-order Fréedericksz behavior is observed; no additional fields are necessary. The results are consistent with the theory of Shi [Liq. Cryst. 29, 121 (2002)]].

Journal Article↗