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PubMed2006

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents.

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents is first theoretically investigated by using the real-space version of self-consistent field theory (SCFT). By varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration f(p), several morphologies are predicted and the phase diagram is constructed. Self-assembly morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thickness. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. We find that in a very dilute block copolymer solution, phase separation can be induced through polymer depletion as the solution becomes more confined. Our findings provide an interesting starting point for a renewed effort in both experimental and theoretical investigations of confined block copolymer solutions.

Journal Article↗
PubMed1986

Anesthetics release unfreezable and bound water in partially hydrated phospholipid lamellar systems and elevate phase transition temperature.

A dimyristoylphosphatidylcholine multilamellar system with varied water content was prepared by dessiccating sonicated vesicles in vacuo. The water content in the sample was determined by gas chromatography after dissolving the multilamellar system in water-free benzene. Differential scanning microcalorimetry revealed several endothermic peaks in the heating scan at subzero temperature, ranging from -25 to -3 degrees. The peaks that appeared in the subzero temperature range indicate the existence of water molecules bound to the lipid head groups, differing from free water that freezes at 0 degrees. The difference between the amount of water molecules that froze in calorimetry and the total amount of water detected by gas chromatography indicates the presence of unfreezable, tightly bound water molecules. The relative amount of free, intermediate, and unfreezable water was estimated by comparing the differential scanning microcalorimetry data with gas chromatography measurements. The addition of halothane and 1-hexanol significantly decreased the intermediately bound water peaks. The anesthetics dehydrated the lamellar system. The phase polymorphism of partially hydrated phospholipid multilayers is well known, and the temperature that corresponds to the main phase transition of fully hydrated lipid membranes shifts to a higher temperature. The addition of anesthetics increased the phase transition temperature when the water content was less than 18 wt%. This result is the complete reverse of the depressant action of anesthetics in fully hydrated lipid membranes. The present anesthetic effect upon the elevation of the transition temperature is apparently caused by anesthetic-induced dehydration of the lipid-water interface at the present experimental condition.

Anesthetics↗
PubMed1995

Can biological homochirality result from a phase transition?

The problem of chiral purity in living organisms is still one of the prominent difficulties in the study of the origins of life. In particular the parity non-conservation known to occur in weak interactions could not be related to this lack of symmetry: these physical forces, though universal, are very weak and up to now no amplification process had been proposed. In 1991, A. Salam remarked that, due to the attractive character of the parity violating force in electro-weak interactions, a phase transition at low temperature should exist, leading eventually to enantiomeric purity. We undertook then a series of experimental tests, looking for a sizeable change in the optical activity of cystine molecules. We found no evidence for the phase transition down to 0.01 K. The interpretation of these negative results will be discussed, and future experiments proposed.

Amino Acids↗
PubMed2005

Quater-, quinque-, and sexithiophene organogelators: unique thermochromism and heating-free sol-gel phase transition.

A series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl groups at the alpha-position, which are abbreviated as 4 T-(chol)(2), 5 T-(chol)(2), and 6 T-(chol)(2), respectively, have been synthesized. It has been found that these oligothiophene derivatives act as excellent organogelators for various organic fluids and show the unique thermochromic behaviors through the sol-gel phase transition. It was shown on the basis of extensive investigations, performed with UV-visible spectroscopy, circular dichroism (CD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), that these gelators self-assemble into the one-dimensional structures in the organogels, in which the pi-block moieties of the oligothiophenes are stacked in an H-aggregation mode. Surprisingly, an AFM image shows that 4 T-(chol)(2) forms unimolecular fibers in a left-handed helical sense, whereby one pitch of the helical fiber is constructed by 400-540 4 T-(chol)(2) molecules. Very interestingly, the conformational change in the oligothiophene moieties can be visually detected: for example, 6 T-(chol)(2) shows a specific absorption maximum in the gel (lambda(max) = 389 nm) and in the solution (lambda(max) = 439 nm). In addition, a sol-gel phase transition of the 6 T-(chol)(2) gel was implemented by addition of oxidizing and reducing reagents such as FeCl(3) and ascorbic acid, respectively. The stimuli-responsive functionality of the oligothiophene-based organogels makes them promising candidates for switchable opto- and electronic soft materials.

Journal Article↗
PubMed1985

Thermodynamic reversibility of phase transitions. Specific effects of alcohols on phosphatidylcholines.

The gel-to-fluid phase transitions of several phosphatidylethanolamines (PE's) and phosphatidylcholines (PC's) have been investigated in the presence of three short-chain alcohols. The effects of the alcohols on the thermodynamic reversibility of these transitions was studied and it was found that the transitions for PC's are not thermodynamically reversible at relatively high alcohol concentrations. The PE transitions are thermodynamically reversible for all alcohol concentrations, and the PE's do not exhibit the biphasic effects of alcohol on the transition temperature previously reported for the PC's (Rowe, E.S. (1983) Biochemistry 22, 3299-3305). The biphasic transition temperature effects and the thermodynamic irreversibility of PC transitions at high alcohol concentrations appear to be correlated with the induction of a fully interdigitated gel phase recently reported in the literature (Simon, S.A. and McIntosh, T.J. (1984) Biochim. Biophys. Acta 773, 169-172). The biological significance of these findings is discussed.

1-Propanol↗
PubMed2000

Structural phase transition of aluminum induced by electronic excitation

The dynamics of a structural phase transition induced by interband electronic excitation in aluminum is studied by determining the time evolution of the dielectric constant at 1.55 eV through the measurement of the transient reflectivity induced by an ultrafast pump pulse. The threshold fluence and the time scale for this transition are significantly less than the values necessary for ultrafast heat-induced melting, indicating that this phase change is caused by band structure collapse and lattice instability resulting from strong electronic excitation.

Journal Article↗
PubMedDate not supplied

[Phase transition in lipids and the problem of homoiothermia].

Why such a high stability of temperature is necessary for warm-blooded animals? Why the range of body temperatures of various warm-blooded species is several-fold narrower than the range of environmental temperatures? What were the physicochemical factors that determined such results of the biological evolution? A hypothesis presented in this short communication provides answers to these basic questions of the problem of homoiothermia. The hypothesis implies that the Ca(2+)-induced chain-ordering phase transition in the lipid component of synaptic membranes plays a key role at the last step of the mechanism of synaptic transmission, namely, the step of neurotransmitter release. The physicochemical substantiation of a possible molecular mechanism of the release involving the phase transition is presented, and the main kinetic and evolutionary issues of the mechanism are considered in brief.

Animals↗
PubMed1999

The phase transitions and crystal structures of Ba3RM2O7.5 complex oxides (R = rare-earth elements, M = Al, Ga).

The structures of alpha-Ba(3)RAl(2)O(7.5) and beta-Ba(3)RM(2)O(7.5) complex oxides (R = rare-earth elements, M = Al, Ga) have been studied by a combination of X-ray diffraction, electron diffraction (ED) and high-resolution electron microscopy (HREM). The alpha and beta forms have cell parameters related to the perovskite subcell: a = 2a(per), b = a(per)(2)(1/2), c = 3a(per)(2)(1/2), however, the alpha form has an ortho-rhombic unit cell whereas the beta form adopts monoclinic symmetry. The crystal structure of monoclinic Ba(3)ErGa(2)O(7.5) was refined from X-ray powder data (space group P2/c, a = 7.93617(9), b = 5.96390(7), c = 18.4416(2) Å, beta = 91.325(1) degrees, R(I) = 0.023, R(P) = 0.053), the structure of the alpha form (space group Cmc2(1)) was deduced from ED and HREM data. The important feature of the alpha and beta structures is the presence of slabs containing strings of vertex-sharing tetrahedral Al(2)O(7) pairs. Two almost equivalent oxygen positions within the strings can be occupied either in an ordered manner leading to the low-temperature beta phase or randomly resulting in the high-temperature alpha structure. The critical temperature of this order-disorder phase transition was determined by high-temperature X-ray diffraction and by differential thermal analysis (DTA). In situ ED and HREM observations of the second-order phase transition confirmed the symmetry changes and revealed numerous defects (twins and antiphase boundaries) formed during the phase transformation.

Journal Article↗
PubMed2006

Surface order-disorder phase transitions and percolation.

In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in the presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square, and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold theta(c) of the adlayer, which presents an interesting dependence with w/k(B)T (w, k(B), and T being the lateral interaction energy, the Boltzmann constant, and the temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a nonpercolating region is determined.

Journal Article↗
PubMed1978

Interacting enzyme systems at steady state: location of the phase transition in approximations of the mean field type.

We consider a phase transition "loop," obtained from a mean field type of approximate treatment of a closed steady-state Ising system. Where is the cut (stable path) across the loop located? The general procedure, in answering this question, is to pass to an open version of the same system and use the cut that appears automatically in this case (no loop is possible in an open system). This is equivalent to finding the point at which the two phases have equal total probability in the open system. It is shown here that this procedure, when applied to a system of two-state enzyme molecules, is formally equivalent to well-known thermodynamic methods (Maxwell's theorem, etc.). These can be applied directly to the closed system without considering the open system explicitly. However, for enzyme molecules with more than two states, the "thermodynamic" method generally fails and one must fall back on the open system procedure mentioned above. Practical implementation of this procedure is not easy.

Enzymes↗
PubMed2005

New electric-field-driven mesoscale phase transitions in polarized suspensions.

We report the discovery of a new class of an electric field-driven bulk phase transition due solely to dipolar interactions in a suspension under the action of a uniform ac field where the effects of other competing forces are suppressed. This transition appears after the well-known chain-column formation and causes the uniform suspension of columns to rearrange into a cellular pattern consisting of particle-free domains surrounded by particle-rich walls. Interestingly, the characteristic size of these domains scales linearly with the interelectrode spacing and remains insensitive to the size of the particles.

Colloids↗
PubMed2000

Structures and phase transitions of the A7PSe6 (A = ag, Cu) argyrodite-type ionic conductors. III. alpha-Cu7PSe6

The crystal structure of the third polymorph of the Cu(7)PSe(6) argyrodite compound, alpha-Cu(7)PSe(6), heptacopper phosphorus hexaselenide, is determined by means of single-crystal diffraction from twinned crystals and X-ray powder diffraction, with the help of extensive NMR measurements. In the low-temperature form, i.e. below the last phase transition, alpha-Cu(7)PSe(6) crystallizes in orthorhombic symmetry, space group Pna2(1), with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) A, V = 1028.590 (9) A(3) (deduced from powder data, T = 173 K) and Z = 4. Taking into account a twinning by reticular merohedry, the refinement of the alpha-Cu(7)PSe(6) structure leads to the residual factors R = 0.0466 and wR = 0.0486 for 127 parameters and 3714 observed, independent reflections (single-crystal data, T = 173 K). A full localization of the Cu(+)d(10) element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of two phase transitions for Cu(7)PSe(6), to be compared with only one for Ag(7)PSe(6), is attributed to the d(10) element stability in a low coordination environment, copper being less prone to lower coordination sites than silver, especially at low temperature.

Journal Article↗
PubMed2004

Theory of bulk, surface and interface phase transition kinetics in thin films.

We report a theoretical study of phase transition kinetics in confined two-dimensional systems, motivated by recent experimental results on the amorphous-to-crystalline transition in supported, thin amorphous water films [E.H.G. Backus, M.L. Grecea, A.W. Kleyn, and M. Bonn, Phys. Rev. Lett. (to be published)]. We generalize and extend existing theories to simultaneously describe the converted (crystalline) fractions in the bulk, at the sample-vacuum surface, and at the sample-support interface as a function of time. The general approach presented here results in expressions for the time-dependent converted bulk, surface, and interface fractions, for arbitrary desorption rate from the thin film, nucleation and growth rates and also includes finite nucleation grain size. The converted bulk, surface, and interface fractions are calculated for nucleation of the new phase occurring (i) in the bulk, (ii) at the support-sample interface, and (iii) at the sample surface (sample-vacuum interface), resulting in nine expressions. The results demonstrate the advantage of monitoring bulk, surface and interface fractions simultaneously to make definite statements regarding the location of the nucleation, and to reliably determine the values of the relevant crystallization parameters.

Journal Article↗
PubMed1997

DNA length and concentration dependencies of anisotropic phase transitions of DNA solutions.

Critical concentrations for the isotropic to cholesteric phase transitions of double-stranded DNA fragments in simple buffered saline (0.1 M NaCl) solutions were determined as a function of DNA contour length ranging from approximately 50 nm to 2700 nm, by solid-state 31P NMR spectroscopy and polarized light microscopy. As expected for semirigid chains, the critical concentrations decrease sharply with increasing DNA length near the persistence length in the range from 50 to 110 nm, and approach a plateau when the contour length exceeds 190 nm. The biphasic region is substantially wider than observed for xanthan, another semirigid polyelectrolyte approximately twice as stiff as DNA, primarily because of low critical concentrations for first appearance of the anisotropic phase, C(i)*, in DNA samples > or =110 nm (320 base pairs) long. The limiting C(i)* for DNA > or =490 nm long is exceptionally low (only 13 mg/ml) and is substantially lower than the C(i)* of approximately 40 mg/ml reported for the stiffer xanthan polyelectrolyte. The much higher values of the critical concentrations, C(a)*, for the disappearance of the isotropic DNA phase (> or =67 mg/ml) are modestly higher than those observed for xanthan and are predicted reasonably well by a theory that has been applied to other semirigid polymers, if a DNA persistence length in the consensus range of 50-100 nm is assumed. By contrast, the broad biphasic region and low C(i)* values of DNA fragments > or =190 nm long could only be reconciled with theory by assuming persistence lengths of 200-400 nm. The latter discrepancies are presumed to reflect some combination of deficiencies in current theory as applied to chiral, strong polyelectrolytes such as DNA, and sequence-dependent variations in DNA properties such as flexibility, curvature, or interaction potential. The propensity of DNA to spontaneously self-order at low concentrations well in the physiological range may have biological significance.

Anisotropy↗
PubMed2004

Self-assembled monolayers of 4-mercaptopyridine on Au(111): a potential-induced phase transition in sulfuric acid solutions.

In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-induced phase transition over the range 0.40-0.15 V versus saturated calomel electrode. Analysis of the data was consistent with the presence of a (5 x square root(3)) and (10 x square root(3)) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, theta(loc), is about 0.2 (two 4-MPy molecules per unit cell), which compresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, theta(loc) ca. 0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 x square root(3)) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular pi-pi ring interactions, resulting in higher local coverages.

Gold↗
PubMed1975

Effects of proteins on thermotropic phase transitions of phospholipid membranes.

A variety of proteins have been studied for their ability to interact and alter the thermotropic properties of phospholipid bilayer membranes as detected by differential scanning calorimeter. The proteins studied included: basic myelin protein (A1 protein), cytochrome c, major apoprotein of myelin proteolipid (N-2 apoprotein), gramicidin A, polylysine, ribonuclease and hemoglobin. The lipids used for the interactions were dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol. The interactions were grouped in three catagories each having very different effects on the phospholipid phase transition from solid to liquid crystalline. The calorimetric studies were also correlated with data from vesicle permeability and monolayer expansion. Ribonuclease and polylysine which exemplify group 1 interactions, show strong dependence on electrostatic binding. Their effects on lipid bilayers include an increase in the enthalpy of transition (deltaH) accompanied by either an increase or no change in the temperature of transition (Tc). In addition, they show minimal effects on vesicle permeability and monolayer expansion. It was concluded that these interactions represent simple surface binding of the protein on the lipid bilayer without penetration into the hydrocarbon region. Cytochrome c and A1 protein, which exemplify group 2 interactions, also show a strong dependence on the presence of net negative charges on the lipid bilayers for their binding. In contrast to the first group, however, they induce a drastic decrease in both Tc and deltaH of the lipid phase transition. Furthermore, they induce a large increase in the permeability of vesicles and a substantial expansion in area of closely packed monolayers at the air-water interface. It was concluded that group 2 interactions represent surface binding followed by partial penetration and/or deformation of the bilayer. Group 3 interactions, shown by proteolipid apoprotein and gramicidin A, were primarily non-polar in character, not requiring electrostatic charges and not inhibited by salt and pH changes. They had no appreciable effect on the Tc but did induce a linear decrease in the magnitude of the deltaH, proportional to the percentage of protein by weight. Membranes containing 50% proteolipid protein still exhibited a thermotropic transition with a deltaH one half that of the pure lipid, and only a small diminution of the size of the cooperative unit. It was concluded that in this case the protein was embedded within the bilayer, associating with a limited number of molecules via non-polar interactions, while the rest of the bilayer was largely unperturbed.

Animals↗
PubMed1996

Effects of ganglioside GD1a on the fluidity and phase transition of phosphatidylcholine model membrane immobilized on porous cellulose acetate filter.

The thermotropic effect of ganglioside GD1a on the phosphatidylcholine membrane immobilized on the porous cellulose acetate filter was investigated by ESR spin-labeling and DSC. The spin-labeled GD1a having 12-DOXYL-stearic acid instead of the long acyl chain of the ceramide portion (GD1a*) was incorporated into the model membrane. An ESR examination of the membrane showed that GD1a* undergoes an anisotropic rotation in the wide range of temperature -30-70 degrees C. By monitoring the overall splitting (2T parallel) of spin-labeled phosphatidylcholine (PC*), the model membranes were found to show decreased fluidity in accordance with the GD1a content. The phase transition temperature (Tm) of distealoylphosphatidylcholine (DSPC) model membranes could be estimated by the measurement of ESR and DSC. The effects of GD1a were found to be more significant in broadening the phase transition rather than in elevating the Tm of DSPC model membranes.

Animals↗
PubMed2006

Synthesis and volume phase transitions of glucose-sensitive microgels.

By the functionalization of poly(N-isopropylacrylamide-co-acrylic acid) microgels with 3-aminophenylboronic acid (APBA) via carbodiimide coupling, nearly monodisperse glucose-sensitive P(NIPAM-PBA) microgels with a diameter of several hundred nanometers were synthesized in aqueous media. Dynamic laser light scattering was used to study the glucose-sensitive and thermosensitive behaviors of the resultant microgels under various conditions. The introduction of the hydrophobic phenylboronic acid (PBA) group significantly decreases the volume phase transition temperature of the resultant microgels. As a result, the P(NIPAM-PBA) microgels with a 10.0 mol % PBA content are in a collapsed state at room temperature. However, the addition of glucose makes the microgels swell dramatically. The glucose-sensitivity of the PBA-containing microgels relies on the stabilization of the charged phenylborate ions by binding with glucose, which can convert more hydrophobic PBA groups to the hydrophilic phenylborate ions. The presence of glucose also induces a two-stage volume phase transition of the P(NIPAM-PBA) microgels, which is explained by the core-shell-like heterogeneous structure of the microgels induced by the formation of the unique glucose-bis(boronate) complex in the "core" area of the microgels. The effects of pH, ionic strength, and PBA content on the glucose sensitivity of the P(NIPAM-PBA) microgels were investigated.

Gels↗