Isothermal Transition Research Articles

Solution properties of exocellular polysaccharides of rhizobium leguminosarum. Static and dynamic light‐scattering: Characterization in dilute solution

AbstractThree polysaccharides, Rhizobium leguminosarum 8002 EPS(I), Rhizobium trifolii TA1‐EPS (II), Rhizobium leguminosarum 127K87 EPS (III), produced by bacteria of Rhizobium genus have been investigated by static and dynamic light scattering combined with chirooptical measurements. All three polymers have the same backbone but differ in the length of the side chains and in the content of minor substituents. An isothermal conformational transition coil → helix was observed with I and II by adding salt (NaCl). The molecular parameters of the polysaccharides in the ordered state were determined by light‐scattering data. Increasing the ionic strength a shrinking of the helix was observed accompanied by a corresponding decrease in the radius of gyration. An extraordinary chain stiffness in terms of Kuhn segment lengths was found in both cases, similar to that already observed for other microbial polysaccharides. In the case of III no disorder → order transition was induced by the salt, and the scattering behavior corresponds to that of a rather flexible polymer with a characteristic ratio C∞ = 24. The incapability of III to form a helical structure is attributed to the effect of the very long side chain. The analysis of the time correlation functions revealed typical flexible chain behavior for all three polysaccharides. This behavior for the two ordered polymers is in agreement with a recent theory by Maggs and is due to bending modes of the rods. ©1995 John Wiley & Sons, Inc.

Gas-phase secondary flow in horizontal, stratified and annular two-phase flow

Experiments were performed and semi-empirical correlations were developed to both prove the existence of gas-phase secondary flow and to predict annular to stratified transition limits for isothermal and heated horizontal annular two-phase flows in pipes. At the low vapor velocities, where this transition from stratified flow occurs and the entrainment/deposition mechanism is insignificant, secondary flow in the vapor ccore plays the principal role in the full development of the liquid annulus. Direct measurements of secondary flow are presented along with a simple model to correlate its behavior. The secondary flow model is used in conjuction with the fluid mechanics of the film to derive a boundary model for the onset of annular flow in this transition region. The resulting boundary agrees well with both the thermal and isothermal transition data.

Phase transitions in gravitating systems and the formation of condensed objects

The thermal equilibrium of a self-gravitating classical fluid with local equation of state corresponding to a system of hard spheres is studied numerically. When the volume effectively occupied by the particles is much smaller than the accessible volume, a phase transition occurs at which the system can shuttle between a quasi-uniform state and one in which a highly condensed nucleus is immersed in a dilute atmosphere. Under isothermal contact conditions, the two states have different energies; under isoenergetic conditions, they have different temperatures. The isothermal transition bridges a region of negative specific heat in the family of isoenergetic systems. The phase transitions mark nonlinear fluid stability thresholds. These can differ by orders of magnitude from the traditional linear ones, i.e. the gravothermal catastrophe and Jeans instability, which only mark the stability limits of thermally metastable regions. It is discussed how phase transitions may give the proper onset criteria for the formation of condensed objects from the size of planetoids up to stars.

Investigations Regarding the Determination of Glass Transition Temperatures from Moisture Sorption Isotherms

Moisture sorption isotherm and glass transition temperature (Tg data on polyvinylpyrrolidone (PVP) of molecular weights less than or equal to 40,000 as well as PVP containing added plasticizers were collected to examine the possibility of extracting T, values from moisture sorption data. Moisture sorption isotherms generated for PVP of various molecular weights were similar to those previously reported. The moisture sorption isotherm for PVP-K15 containing added vinylpyrrolidone (VP) and methylpyrrolidone (MP) were predicted reasonably well by simple addition of their respective isotherms with that of PVP-K15. The Tg values for PVP as a function of moisture was similar to that found previously. Decreasing polymer molecular weight or the addition of plasticizers (VP or MP) reduced the Tg of the polymer system, displacing the Tg to lower temperatures. When the moisture content to give a Tg of 25°C (Wg) was indicated on the isotherm, Wg shifted downward with decreasing molecular weight or increasing plasticization by VP or MP while the shape of the isotherm was not distinguishably altered. Consequently, Wg, and thus Tg, would be difficult to extract from the moisture sorption isotherm.

Phase equilibria in phospholipid-water systems

This work is a critical account of phase studies of phospholipid-water mixtures. The Phase Rule and equations of thermodynamics of heterogeneous systems are applied in the analysis of calorimetric and dilatometric results for these systems. It is inferred that the approach in which a lipid-water mixture of low lipid content is regarded as a one-component system is misleading and unhelpful. A mixture containing water and a pure synthetic phospholipid is a binary system, and the Phase Rule can be applied to it in a straightforward manner. Non-isothermal transitions and invariant three-phase reactions of the eutectic, peritectic and polytectic type can be encountered in lipid-water systems, like in other binary mixtures. The concentration dependence of the enthalpy of the isothermal three-phase transition yields a well-known Tammann triangle. For the volume change at such transition an analog of the Tammann triangle can be drafted which can be useful in interpretation of dilatometric and densitometric data for heterogeneous binary systems. The literature thermodynamic data for the aqueous lipids are analyzed on the basis of the phase diagram of the DPPC-water system. The measured values of the transition enthalpy and volume change at the pre- and main transitions refer to invariant phase reactions. The measured heat capacities and thermal coefficients of expansion actually characterize the biphasic lipid-water mixtures and may reflect phenomena specific for heterogeneous binary systems only. A reinterpretation of the literature DSC data for the DPPC-water system at temperatures below zero is suggested. Experience gained in the study of equilibrium phase diagram, kinetic and colloidal aspects of phase behaviour of dioctadecyldimethylammonium chloride-water system indicates that the common procedures of the sample preparation of lipid-water mixtures may lead to the states which are non-equilibrium in two respects: the system contains a metastable phase, and the phase is dispersed to such an extent that an irreversible colloidal structure is formed. Definitive phase studies of the phase behaviour of phospholipid-water systems in the whole concentration range are urgently needed.

Erratum: Isothermal glass transitions in supercooled and overcompressed liquids [J. Chem. Phys. 100, 2250 (1994)

Erratum: Isothermal glass transitions in supercooled and overcompressed liquids [J. Chem. Phys. <b>100</b>, 2250 (1994)

Photomechanically induced ferroelectricity in smectic liquid crystals

Abstract We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm−2) UV light. In the case of a sample initially in the S*c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P s) and changes in electro-optic switching characteristics. If the sample is illuminated in the SA phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the SA phase, a reversible, isothermal transition to a biphasic S*c/isotropic state occurs. In this case, the P s is seen to rise from zero in the SA phase to a finite value(∼2 nC cm−2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye conce...

Isothermal glass transitions in supercooled and overcompressed liquids

We report the first calorimetric characterization of isothermal glass transitions obtained by changing the pressure at constant temperature. This isothermal glass transition is described in the case of m-fluoroaniline, a so-called fragile liquid. The method employed is based on Maxwell’s equations and involves measurement of the heat of compression of a sample enclosed in a hydrostatic pressure cell under isothermal and reversible conditions. A discontinuity of the thermodynamic quantity αV, where α is the expansivity and V is the molar volume, defines the glass transition pressure Pg at which ergodicity is broken on the time scale of the experiments (300–1000 s) which depends on the pressure step employed. The step in αV is found initially to decrease rapidly with increasing pressure. The slope (∂Pg/∂T)τ seems smaller than that determined by the usual isobaric methods, implying some distinction in the degrees of freedom involved and associated differences in defining the values of Tg. Because of the high sensitivity and stability of the calorimeter, we can also observe annealing effects in the overcompressed glass at a pressure above Pg; the slow dynamic processes associated are called entropic relaxation because it is the entropy which is measured directly. An approximate relaxation function and a model for the compression data are discussed. Keeping in mind the overall pattern of ‘‘strong and fragile’’ liquid behavior, isothermal pressure variations appear to offer a useful tool to connect these extremes and may constrain the glass transition of a molecular liquid in a particular range of the P–T diagram. The concept of Kauzmann pressure PK is introduced, but it is noted that PK must become less well defined the further from ambient pressure it lies due to the decrease in ΔαV which is observed with increasing pressure.

Isothermal Switching of Phases in Lyotropic Liquid Crystalline Surfactant Systems via Photoreactions of Solubilized Molecules

AbstractLight induced phase transitions were found in lyotropic liquid crystalline systems of aqueous cetyltrimethyl‐ammonium bromide (CTAB) and of Aerosol‐OT(AOT)/decane/water: the isothermal transitions isotropic → nematic and isotropic → hexagonal in CTAB systems were triggered by the photocyclization of a solubilized naphthyl vinyl ether, the transition isotropic → inverse‐hexagonal in the ternary AOT system was induced by the trans‐cis photoisomerization of a solubilized stilbazolium salt. The transition time (between starting irradiations and detecting anisotropic phases) was found to increase with the difference between irradiation temperature and transition temperature in the non irradiated case; transition times were shorter by two orders of magnitude in the AOT‐system as compared to CTAB. The position of isotropic/liquid crystalline phase borders produced by partial irradiation of layers of aqueous CTAB were more stable in hexagonal as compared to nematic systems.

A model of polymorphic transformations in densely packed structures at arbitrary temperatures

An approach is described which expands the concept of the ANNNI model, within which a mathematically correct description of polymorphic transformations under load at finite temperatures is possible. The unique feature of the approach is its consideration of all possible configurations in the final block of densely packed planes for arbitrary interplane interaction. The proposed method has a number of advantages over traditional ones. The technique is illustrated by calculations of isothermal transitions 2H-3C and 9R-3C under load.

Approximate model of melting of semitransparent bodies in pulse irradiation

An approximate model of melting of semitransparent media with formation of an isothermal transition zone is constructed which makes it possible to establish possible regimes of melting and to analytically calculate the dynamics of the process under the action of pulse radiation flux.

Strain-induced liquid-like structures in glassy polymers

Experimental evidence verifying strain-induced isothermal structural transition of glassy polymers into liquid-like structures is presented for five amorphous polymers: polymethyl methacrylate, polystyrene, polycarbonate, polyvinyl chloride and styrene-acrylonitrile co-polymer. This transition presumably accelerates the relaxation and results in the non-linear viscoelasticity in glassy polymers. The evidence was derived by an application of the Eyring equation to the steady state of plastic flow in the polymers. In this analysis, a novel method of applying the Eyring equation was used, which provided numerical values of the activation enthalpy, ΔH, activation entropy, ΔS, and activation volume, v a, for each polymer as variables depending on strain-rate and temperature. Comparison of a unique functional relation between the rate factors ΔH and ΔS for the glass with a relation of ΔH vs. ΔS derived from the relaxation or dynamic viscoelastic data of the melt showed a good agreement for each polymer. Since the rate factors ΔH and ΔS are governed in quantity by the intermolecular interaction in the polymer system, this agreement led to a verification of the structural change of glassy polymers into liquid-like structures.

A dynamical model for phospholipid—calcium binding

A model for an isothermal gel-liquid crystalline transition induced by ionic binding is proposed. A Ginsburg-Landau functional was used to describe the long-range order that spontaneously arises during the transition. By calculation of the corresponding chemical potential we obtain the mass current of phospholipids in gel-phase described by an order parameter. In the conservation of mass equation the kinetics of the phospholipids-calcium interaction is introduced, together with the flux divergency. A circular membrane is considered for the analysis, so that the model can be studied in polar coordinates. A solution approximated to first order shows an heterogeneous distribution of domains of phospholipids in gel and liquid crystalline phases. These spatial domains have been detected experimentally by diverse methods in vesicles and cellular membranes. Spatial heterogeneities may cause destabilization of the membrane in the boundaries between domains. This may explain the enhanced vesicle fusion observed in the presence of Ca2+.

A dynamical model for phase transitions of lipids induced by calcium ions

Lipid membranes exist essentially in two different phases. A phase transition can be triggered off either by changing the temperature or, isothermally, by varying an external factor such as ionic concentration, pH, organic solvents, etc. Since the isothermal transition may be induced at physiological temperature, it may play an important regulatory role in diverse cellular functions. Based on the Landau-Ginsburg theory, the thermotropic transitions of lipids has been described by a number of models. In the present work a dynamical model for an isothermal phase transition of phospholipids induced by ionic binding is proposed. The properties of the model show that by ionic binding, phospholipids may form spatial heterogeneous distributions of lipids in fluid and crystalline phases. This heterogeneity possibly being the cause of membrane instabilities, which favour enhanced vesicle fusions observed in the presence of Ca2+.

Unfolding-refolding behaviour of chicken egg white ovomucoid and its correlation with the three domain structure of the protein

The urea and heat-induced unfolding-refolding behaviours of chicken egg white ovomucoid and it four fragments representing domains I, II + III, I ÷ II and III were systematically investigated in 0.06 M sodium phosphate buffer (pH 7.0) by difference spectral measurements. The effect of temperature on ovomucoid and its fragments was also studied in 0.05 M sodium acetate buffer (pH 5.0) and in presence of 2 M urea at pH 7.0. Intrinsic viscosity data showed that ovomucoid and its different fragments did not lose any significant amount of their structure under mild acidic conditions (pH 4.6). Difference spectral results showed extensive disruption of the native structure by urea or temperature. Isothermal transitions showed single-step for domain I, domain I + II and domain III, and two-step having one stable intermediate, for ovomucoid and its fragment representing domain II + III. However, the presence of intermediate was not detected when the transitions were studied with temperature at pH 7.0. Strikingly, the single-step thermal transitions of ovomucoid and its fragment representing domain II + III, became two-step when measured either at pH 5.0 or in presence of 2 M urea at pH 7.0. Analysis of the equilibrium data on urea and heat denaturation showed that the second transition observed with ovomucoid or domain II + III represent the unfolding of domain III. The kinetic results of ovomucoid and its fragments indicate that the protein unfolds with three kinetic phases. A comparison of three rate constants for the unfolding of intact ovomucoid with that of its various fragments revealed that domain I, II and III of the protein correspond to the three kinetic phases having rate constants 0.456, 0.120 and 0.054 min −1, respectively. These data have led us to conclude: (i) the unusual stability of ovomucoid towards various denaturants, including temperature, is due to its domain III, (ii) initiation of the folding of the ovomucoid molecule starts from its NH 2-terminal region which probably provides the nucleation site for the formation of the subsequent structure and (iii) domains I and II have greater mutual recognition between them as compared to the recognition either of them have with domain III.

PARTIAL HYSTERESIS CYCLES IN SHAPE-MEMORY ALLOYS : EXPERIMENTS AND MODELLING

This work presents a collection of experimental pseudoelastic hysteresis cycles associated with the isothermal stress-induced martensitic transition of a Cu-Zn-Al single crystal. It is found that stress-strain trajectories retain memory of loading and unloading points, as observed for polycrystals. A particular version of the Preisach model for hysteresis is used to predict partial loops, using a limited number of experimental trajectories as input data and no adjustable parameters. Comparison between measured and predicted trajectories reveal that energy dissipation in the forward transition is different than in the reverse transition ; this is confirmed by a distinct evolution with transformed fraction of the energy losses computed from two different (loading and unloading) kinds of partial cycles.

Sorption and dilation properties of poly(p‐phenylene sulfide) under high‐pressure carbon dioxide

AbstractSorption and dilation properties of the system poly(p‐phenylene sulfide) (PPS) and carbon dioxide have been investigated at pressures up to 50 atm over the temperature range 25–85°C. The shapes of the sorption isotherms are in agreement with those predicted by the dual‐mode theory. Analysis of the sorption and dilation isotherms confirms the occurrence of an isothermal glass transition. This occurrence and its corresponding disappearance in the range investigated is discussed. Values of the partial molar volume of CO2 sorbed in PPS are calculated below and above the isothermal transition. Compared with sorption by other polymers, these values were about the same in the glassy state, but were smaller in the rubbery state. Under 50 atm CO2 at 85°C crystallization of amorphous PPS was observed. The crystallinity estimated from the solubility before and after crystallization is about 20%.

Sorption and dilation in poly(ethyl methacrylate)–carbon dioxide system

AbstractSorption and dilation in the system poly(ethyl methacrylate) (PEMA) and carbon dioxide are reported for pressures up to 50 atm over the temperature range 15–85°C. The sorption isotherms were obtained gravimetrically. The dilation accompanying sorption was measured directly with a cathetometer. At low temperatures the sorption and dilation isotherms were concave toward the pressure axis in the low‐pressure region and turned to convex with increasing pressure. As the experimental temperature approached and exceeded the glass transition temperature of 61°C, both isotherms became convex or linear over the whole range of pressure. Partial molar volumes of CO2 in PEMA were obtained from sorption and dilation data, which were described well by the extended dual‐mode sorption and dilation models developed recently. The temperature dependence of the dual‐mode parameters and the isothermal glass transition are discussed.

Study of pressure-induced transitions of polyisoprene by laser Raman spectroscopy

This paper reports isothermal pressure-induced glass transitions and β transitions of polyisoprene in the pressure range 0–2 GPa studied by laser Raman spectroscopy at room temperature. The values obtained are supported by experimental results of piston-cylinder isothermal compression. Elementary discussions on the mechanism of pressure-induced transitions are also given.

Cholesterol affects divalent cation-induced fusion and isothermal phase transitions of phospholipid membranes

The influence of cholesterol on divalent cation-induced fusion and isothermal phase transitions of large unilamellar vesicles composed of phosphatidylserine (PS) was investigated. Vesicle fusion was monitored by the terbium/dipicolinic acid assay for the intermixing of internal aqueous contents, in the temperature range 10–40°C. The fusogenic activity of the cations decreases in the sequence Ca 2+ > Ba 2+ > Sr 2+ ⪢ Mg 2+ for cholesterol concentrations in the range 20–40 mol%, and at all temperatures. Increasing the cholesterol concentration decreases the initial rate of fusion in the presence of Ca 2+ and Ba 2+ at 25°C, reaching about 50% of the rate for pure PS at a mole fraction of 0.4. From 10 to 25°C, Mg 2+ is ineffective in causing fusion at all cholesterol concentrations. However, at 30°C, Mg 2+-induced fusion is observed with vesicles containing cholesterol. At 40°C, Mg 2+ induces slow fusion of pure PS vesicles, which is enhanced by the presence of cholesterol. Increasing the temperature also causes a monotonic increase in the rate of fusion induced by Ca 2+, Ba 2+ and Sr 2+. The enhancement of the effect of cholesterol at high temperatures suggests that changes in hydrogen bonding and interbilayer hydration forces may be involved in the modulation of fusion by cholesterol. The phase behavior of PS/cholesterol membranes in the presence of Na + and divalent cations was studied by differential scanning calorimetry. The temperature of the gel-liquid crystalline transition ( T m) in Na + is lowered as the cholesterol content is increased, and the endotherm is broadened. Addition of divalent cations shifts the T m upward, with a sequence of effectiveness Ba 2+ > Sr 2+ > Mg 2+. The T m of these complexes decreases as the cholesterol content is increased. Although the transition is not detectable for cholesterol concentrations of 40 and 50 mol% in the presence of Na +, Sr 2+ or Mg 2+, the addition of Ba 2+ reveals endotherms with T m progressively lower than that observed at 30 mol%. Although the presence of cholesterol appears to induce an isothermal gel-liquid crystalline transition by decreasing the T m, this change in membrane fluidity does not enhance the rate of fusion, but rather decreases it. The effect of cholesterol on the fusion of PS/phosphatidylethanolamine (PE) vesicles was investigated by utilizing a resonance energy transfer assay for lipid mixing. The initial rate of fusion of PS/PE and PS/PE/cholesterol vesicles is saturated at high Mg 2+ concentrations. With Ca 2+, saturation is not observed for cholesterol-containing vesicles. The highest rate of fusion for both Ca 2+- and Mg 2+-induced fusion is observed with vesicles containing 30 mol% cholesterol.