Phase Transitions In Mixtures Research Articles

Phase transitions in mixtures based on poly(lactic acid) and two polymeric nucleants processed under a fast or slow cooling from the melt

Mixtures based on two different poly(lactic acid)s (PLA) matrices and two distinct nucleating agents, incorporated at several contents, were prepared by extrusion. Furthermore, two thermal treatments were applied during the processing of films from the melt: a quenching (Q) and a slow (S) cooling. Crystalline characteristics of the resultant polymeric systems were mainly triggered by the PLA matrix used and by the cooling rate applied, turning out less important the nature of the nucleant. Thus, stereocomplex crystals were developed almost exclusively in the Q samples while coexistence of homo and stereocomplex crystallites was noticed when slow cooling was imposed. The type of homocrystallites present in the specimens was also dependent on the cooling rate applied during film processing. In the Q systems, crystals were grown on heating through a cold crystallization, as observed by differential scanning calorimetry; while they were developed along the cooling stage in the S materials. Influence of these structural characteristics on mechanical response was also evaluated by microhardness measurements.

Electrical interactions in mixtures of ferroelectric powders

The results of studies of phase transitions in mixtures of C6H16NBr (DIPAB) powders with PbTiO3 and Pb(Zr0.52Ti0.48)O3 (PZT) by the method of differential thermal analysis (DTA) are presented. The measurements were carried out in the heating and cooling mode in the range from 300 to 440 K. It was found that electrical interactions between ferroelectric particles led to the appearance of additional phase transitions for DIPAB in powder mixtures. Interaction effects in mixtures of DIPAB and PbTiO3 as well as DIPAB and PZT powders depend on the magnitude of the dipole moments of the particles.

Электрические взаимодействия в смесях сегнетоэлектрических порошков хлорида диизопропиламмония и титаната свинца

The results of studies of phase transitions in mixtures of C6H16NCl (DIPAC) powders with PbTiO3 by the method of differential thermal analysis (DTA) are pre-sented. The measurements were carried out in the heating and cooling mode in the tem-perature range from 300 to 440 K. It was found that electrical interactions between ferroelectric particles lead to the appearance of additional phase transitions for DIPAC in powder mixtures.

Thermodynamics of a Hierarchical Mixture of Cubes

We investigate a toy model for phase transitions in mixtures of incompressible droplets. The model consists of non-overlapping hypercubes in {mathbb {Z}}^d of sidelengths 2^j, jin {mathbb {N}}_0. Cubes belong to an admissible set {mathbb {B}} such that if two cubes overlap, then one is contained in the other. Cubes of sidelength 2^j have activity z_j and density rho _j. We prove explicit formulas for the pressure and entropy, prove a van-der-Waals type equation of state, and invert the density-activity relations. In addition we explore phase transitions for parameter-dependent activities z_j(mu ) = exp ( 2^{dj} mu - E_j). We prove a sufficient criterion for absence of phase transition, show that constant energies E_jequiv lambda lead to a continuous phase transition, and prove a necessary and sufficient condition for the existence of a first-order phase transition.

Radionuclide 188Re‐Loaded Photothermal Hydrogel for Cancer Theranostics

AbstractHerein, an injectable photothermal hydrogel system containing a therapeutic radionuclide 188Re is studied for combined radioisotope therapy and photothermal therapy (PTT) of cancer. A dopamine‐conjugated poly(α,β‐aspartic acid) copolymer (PDAEA) is used to trigger a sol–gel phase transition in mixture with Fe3+ ions, rapidly forming a gel by simply mixing PDAEA and FeCl3 phosphate buffer saline solutions. The injectable hydrogel exhibits strong near‐infrared light absorbance and can efficiently convert light into a heating effect for local PTT treatment. The obtained hydrogel possesses a porous 3D microstructure, and can be utilized for radionuclide loading. After the Na188ReO4 loading, the hydrogel is intratumorally injected into the tumor of mice bearing 4T1 murine breast cancer cells for studying the tumor retention and therapeutic efficiency. In vivo results show that Na188ReO4‐loaded hydrogel exhibits significantly longer time in the tumor sites than that of free Na188ReO4. The tumor growth of mice treated with Na188ReO4‐loaded hydrogel under near‐infrared radiation is significantly inhibited compared with control groups. Therefore, the results show that the developed strategy using an injectable and biocompatible hydrogel may promote the applications of radioisotope therapy and photothermal therapy for cancer.

Analysis of UV Assisted Phase Transitions in Mixtures of Liquid Crystals with Photochromic Compounds by Photopyroelectric Calorimetry

In this work, an upgraded photopyroelectric setup has been used to investigate phase transitions in liquid crystals dispersed with photochromic molecules. In particular, we have studied the thermal diffusivity before, during, and after the photo-induced conformational changes of the dispersed photochromic molecules. Upon sample irradiation, an opposite shift of the phase transition temperatures were observed in the investigated 8CB–NP and 8CB–7AB mixtures due to the different changes in the shape undergone by the dispersed active molecules.

Nematic to smectic-A phase transition in mixture of liquid crystal and nonmesogenic impurities: existence of tricritical point

ABSTRACTExperiments on the mixture of liquid crystals and nonmesogenic impurities showed the significant role of nonmesogenic impurities on the nematic–smectic-A phase transition. Using both Flory–Huggins theory of isotropic mixing and Landau–de Gennes theory, we present a phenomenological theory that discusses the role of such impurities on the nematic–smectic-A phase transition in a mixture of smectic liquid crystal and nonmesogenic impurities. We discuss the impact of nonmesogenic impurities on the order parameters, Frank elastic constants (splay and bend) and transition temperature of the nematic–smectic-A phase transition. Our theoretical results show that there exists a tricritical point for which the second-order nematic–smectic-A phase transition becomes first order at a tricritical point by increasing the concentration of nonmesogenic solute. We find a remarkable agreement between theoretical and experimental results.

Effect of nanoparticles on the [formula omitted] rotator phase transitions of alkanes

Experimental studies have shown that nanoparticles play an important role on the rotator phase transitions of n-alkanes. A phenomenological model for predicting the RII-RI-RV phase transitions in mixtures of alkanes and nanoparticles has been proposed by combining Flory-Huggins free energy of isotropic mixing and Landau free energy. The impact of nanoparticles on the RII-RI-RV phase transitions and their transition temperatures is discussed by means of phenomenological theory. The possibility of the tricritical behavior of the RI-RV phase transition in the mixtures of alkanes and nanoparticles is discussed. The theoretical predictions are in good qualitative agreement with available experimental results.

Field-induced oblique helicoidal cholesteric phases in mixtures of chiral and achiral liquid crystalline molecules

ABSTRACTWe present a mean field theory to describe phase transitions in mixtures of chiral and achiral liquid crystalline molecules in the presence of an external field. When a longitudinal external field is applied parallel to the pitch axis of a chiral nematic phase (N*), the director of the N* phase can lead to an oblique heliconical cholesteric () state with a twist–bend distortion, in which a bend elastic constant is smaller than a twist elastic constant . We predict that the exists between N* and nematic (N) phases and can be controlled by temperature and concentration. We find various phase transitions: the first-order (isotropic) and the second-order , transitions, on the temperature–concentration plane. We also predict the pitch and the phase diagrams, depending on temperature and concentration under the external field.

Interplay of Noncovalent Interactions in Ionic Liquid/Sodium Bis(2-ethylhexyl) Sulfosuccinate Mixtures: From Lamellar to Bicontinuous Cubic Liquid Crystalline Phase.

Phase transitions in mixtures of imidazolium based ionic liquid ([C12mim]Br) and anionic double tail surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), were studied using a multitechnique approach. The system was primarily chosen for its expected ability to form a variety of lamellar and nonlamellar liquid crystalline phases which can transform into each other via different mechanisms. Depending on the bulk composition and total surfactant concentration, mixed micelles, coacervates, and lamellar and inverse bicontinuous cubic liquid crystalline phase were observed. Along with electrostatic attractions and geometric packing constraints, additional noncovalent interactions (hydrogen bonding, π-π stacking) enhanced attractive interactions and stabilized low curvature aggregates. At stoichiometric conditions, coexistence of coacervates and vesicles was found at lower, while bicontinuous cubic phase and vesicles were present at higher total surfactant concentrations. The phase transitions from a dispersed lamellar to inverse cubic bicontinuous phase occur as a consequence of charge shielding and closer packing of oppositely charged headgroups followed by a change in bilayer curvature. Transition is continuous with both phases coexisting over a relatively broad range of concentrations and very likely involves a sponge-like phase as a structural intermediate. To the best of our knowledge, this type of phase transition has not been observed before in surface active ionic liquid/surfactant mixtures.

Multicriticality and interaction-induced first-order phase transitions in mixtures of ultracold bosons in an optical lattice

We study a critical behavior of mixtures of two species of ultracold bosons trapped in an optical lattice. Using mean-field approximation, we determine the ground-state phase diagram of the system for a wide range of parameters. The introduction of interactions between different species of atoms strongly renormalizes the phase diagram. It can alter the critical behavior modifying multicriticality of crossing points and order of phase transitions in their vicinity between mixed and superfluid states. For selected values of model parameters, the behavior of the system falls out of the $XY$ model universality class, which usually is a hallmark of superfluid phase transition. We supplement our analysis with analytical calculations to explain the observed scenario.

Modeling phase transitions in mixtures of β-γ lens crystallins.

We analyze the experimentally determined phase diagram of a γD-βB1 crystallin mixture. Proteins are described as dumbbells decorated with attractive sites to allow inter-particle interaction. We use thermodynamic perturbation theory to calculate the free energy of such mixtures and, by applying equilibrium conditions, also the compositions and concentrations of the co-existing phases. Initially we fit the Tcloudversus packing fraction η measurements for a pure (x2 = 0) γD solution in 0.1 M phosphate buffer at pH = 7.0. Another piece of experimental data, used to fix the model parameters, is the isotherm x2vs. η at T = 268.5 K, at the same pH and salt content. We use the conventional Lorentz-Berthelot mixing rules to describe cross interactions. This enables us to determine: (i) model parameters for pure βB1 crystallin protein and to calculate; (ii) complete equilibrium surface (Tcloud-x2-η) for the crystallin mixtures. (iii) We present the results for several isotherms, including the tie-lines, as also the temperature-packing fraction curves. Good agreement with the available experimental data is obtained. An interesting result of these calculations is evidence of the coexistence of three phases. This domain appears for the region of temperatures just out of the experimental range studied so far. The input parameters, leading good description of experimental data, revealed a large difference between the numbers of the attractive sites for γD and βB1 proteins. This interesting result may be related to the fact that γD has a more than nine times smaller quadrupole moment than its partner in the mixture.

Role of cholesterol on the transfection barriers of cationic lipid/DNA complexes

Most lipid formulations need cholesterol for efficient transfection, but the precise motivation remains unclear. Here, we have investigated the effect of cholesterol on the transfection efficiency (TE) of cationic liposomes made of 1,2-dioleoyl-3-trimethylammonium-propane and dioleoylphosphocholine in Chinese hamster ovary cells. The transfection mechanisms of cholesterol-containing lipoplexes have been investigated by TE, synchrotron small angle X-ray scattering, and laser scanning confocal microscopy experiments. We prove that cholesterol-containing lipoplexes enter the cells using different endocytosis pathways. Formulations with high cholesterol content efficiently escape from endosomes and exhibit a lamellar-nonlamellar phase transition in mixture with biomembrane mimicking lipid formulations. This might explain both the DNA release ability and the high transfection efficiency. These studies highlight the enrichment in cholesterol as a decisive factor for transfection and will contribute to the rational design of lipid nanocarriers with superior TE.

Evidence from adiabatic scanning calorimetry for the Halperin-Lubensky-Ma effect at the N-SmA phase transitions in mixtures of 7OCB+heptane with an injected SmA phase

The high-resolution adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A transition (N-SmA in binary mixtures of the non-smectogenic liquid crystal heptyloxycyanobiphenyl (7OCB) and heptane, exhibiting a so-called injected smectic A phase. With the exception of a mixture with the lowest heptane mole fraction for which only an upper limit of 0.2 ± 0.2 J kg(-1) for a possible latent heat could be obtained, for all other mixtures finite latent heats were obtained. The mole fraction dependence of the latent heat could be well fitted with a crossover function consistent with a mean-field free energy expression with a non-zero cubic term arising from the Halperin-Lubensky-Ma (HLM) coupling between the SmA order parameter and the orientational director fluctuations. The mole fraction dependence of the heat capacity effective critical exponents is similar to that observed in mixtures of the two liquid crystals octyloxycyanobiphenyl (8OCB) and nonylcyanobiphenyl (9OCB). The thermal behavior observed along the N-SmA phase transition line yields further strong evidence for the HLM coupling effect.

Ionic Conductivity of Block Copolymer Electrolytes in the Vicinity of Order−Disorder and Order−Order Transitions

Order−order and order−disorder phase transitions in mixtures of poly(styrene-block-ethylene oxide) (SEO) copolymers and lithium bis(trifluoromethylsulfonimide) (LiTFSI), a common lithium salt used in polymer electrolytes, were studied using a combination of small-angle X-ray scattering (SAXS), birefringence, and ac impedance spectroscopy. The SEO/LiTFSI mixtures exhibit lamellar, hexagonally packed cylinders, and gyroid microphases. The molecular weight of the blocks and the salt concentration was adjusted to obtain order−order and order−disorder transition temperatures within the available experimental window. The ionic conductivities of the mixtures, normalized by the ionic conductivity of a 20 kg/mol homopolymer PEO sample at the salt concentration and temperature of interest, were independent of temperature, in spite of the presence of the above-mentioned phase transitions.

Phase transitions in solutions of hydroxypropoyl cellulose and cyanoethyl cellulose

LC phase transitions in mixtures of cyanoethyl cellulose and hydroxypropyl cellulose with different solvents have been studied by the cloud point and polarization microscopy methods and polarization photoelectric measurements. It has been found that, as the solvent polarity is decreased and the molecular mass of a polymer is increased, boundary curves delimiting the regions of isotropic and anisotropic solutions shift to the region of lower concentrations.

Impact of Water on the Miscibility of DAB-dendr-(NH2)64 and Benzene

The miscibility of the fifth-generation 1,4-diaminobutane poly(propyleneimine) (DAB-PPI) dendrimer and benzene was investigated. The liquid−liquid phase transitions in mixtures containing from 0.02 to 0.26 weight fraction of the dendrimer were determined. It was shown that the miscibility of the dendrimer and benzene depend to a great extent on the water content in the system. The binary system dry dendrimer + dry benzene does not show a liquid−liquid phase split. The addition of small amounts of water (as small as 0.06 wt %) to the system induces complex phase behavior. The liquid−liquid phase transitions in the ternary systems DAB-PPI dendrimer + benzene + water (0.06 wt %) are accompanied by unusual changes in the properties of the coexisting phases. The influence of water on the 1H NMR spectra of the fifth-generation DAB-PPI dendrimer dissolved in benzene was also investigated.

New design of fiber-optic reflectometer for determining the phase boundary of multicomponent fluid mixtures at high pressures and high temperatures

A dynamic synthetic method based on an optic fiber sensor has been developed to measure phase boundaries of multicomponent fluid at high temperatures >300°C and pressures >30MPa. The breakthrough has been the design of the equilibrium cell containing the optic fiber, which gives highly reproducible signals for the phase transition. We demonstrate that this method can clearly distinguish between dew points and bubble points in the phase transitions of mixtures. Overall, the method is characterized by speed, simplicity, high pressures, and high temperatures.

Nematic Phase Transitions in Mixtures of Thin and Thick Colloidal Rods

We report experimental measurements of the phase behavior of mixtures of thin (charged semiflexible fd virus) and thick (fd-PEG, fd virus covalently coated with polyethylene glycol) rods with diameter ratio varying from 3.7 to 1.1. The phase diagrams of the rod mixtures reveal isotropic-nematic, isotropic-nematic-nematic, and nematic-nematic coexisting phases with increasing concentration. In stark contrast to predictions from earlier theoretical work, we observe a nematic-nematic coexistence region bound by a lower critical point. Moreover, we show that a rescaled Onsager-type theory for binary hard-rod mixtures qualitatively describes the observed phase behavior.

Molecular Theory on Binary Mixtures of Hard Rods in an External Field

We study isotropic–nematic (Iso–N) phase transition in mixtures of hard rods of different lengths L A and L B in an external field by using the symmetry breaking potential method with the mean field approximation. We discuss effect of the external field on a triple point and phase co-existence regions. Iso+N+N triple point, which appears on phase diagram in the pressure–composition plane for mixtures showing N+N co-existence, is moved to shorter rod side on the phase diagram by the external field. We find for the case of L A / L B =4 that, depending on pressure, three types appear for phase diagrams in the external field-composition plane. One type shows Iso+N co-existence region that has upper critical solution field, another shows Iso+N and N+N co-existence region critical solution field of which is higher than critical field of monodisperse shorter rods, and the other shows Iso+N+N triple point.