Critical Point Of Phase Separation Research Articles

Motility-induced phase separation of soft active Brownian particles

Motility-induced phase separation (MIPS) is the hallmark of non-equilibrium phase transition in active matter. Here, by means of Brownian dynamics simulations, we determine the phase behavior and the critical point for phase separation induced by motility of a two-dimensional system of soft active Brownian particles, whose interaction is modeled by the generalized purely repulsive Weeks–Chandler–Andersen potential. We refer to this model as soft active Brownian particles. We determine and analyze the influence of particle softness on the MIPS and show that the liquid–gas coexistence region is wider, the softer the interparticle interactions becomes. Moreover, the critical value of the self-propulsion velocity at which diluted and dense phases start to coexist also increases; as a consequence, the softer the particle interaction is, the bigger self-propulsion velocities are needed in order to observe a MIPS.

Criticality of plasma membrane lipids reflects activation state of macrophage cells.

Signalling is of particular importance in immune cells, and upstream in the signalling pathway many membrane receptors are functional only as complexes, co-locating with particular lipid species. Work over the last 15 years has shown that plasma membrane lipid composition is close to a critical point of phase separation, with evidence that cells adapt their composition in ways that alter the proximity to this thermodynamic point. Macrophage cells are a key component of the innate immune system, are responsive to infections and regulate the local state of inflammation. We investigate changes in the plasma membrane’s proximity to the critical point as a response to stimulation by various pro- and anti-inflammatory agents. Pro-inflammatory (interferon γ, Kdo 2-Lipid A, lipopolysaccharide) perturbations induce an increase in the transition temperature of giant plasma membrane vesicles; anti-inflammatory interleukin 4 has the opposite effect. These changes recapitulate complex plasma membrane composition changes, and are consistent with lipid criticality playing a master regulatory role: being closer to critical conditions increases membrane protein activity.

Spin Dynamics and Andreev-Bashkin Effect in Mixtures of One-Dimensional Bose Gases.

We investigate the propagation of spin waves in two-component mixtures of one-dimensional Bose gases interacting through repulsive contact potentials. By using quantum MonteCarlo methods we calculate static ground-state properties, such as the spin susceptibility and the spin structure factor, as a function of the coupling strengths and we determine the critical parameters for phase separation. In homogeneous mixtures, results of the velocity of spin waves and of its softening close to the critical point of phase separation are obtained by means of hydrodynamic theory and a sum-rule approach. We quantify the nondissipative drag effect, resulting from the Andreev-Bashkin current-current interaction between the two components of the gas, and we show that in the regime of strong coupling it causes a significant suppression of the spin-wave velocity.

Active hole generation in a liquid droplet dissolving into a binary solvent.

In liquid-liquid dissolution, the critical point of phase separation is determined by the temperature. When the solvent consists of multi-components, in contrast, the mole fractions in the solvent also take on the role of control parameter. In this study an ionic liquid dissolves into a binary solvent composed of ethanol and water. It is found in this system that, near the critical point, a hole is spontaneously created in the droplet of the ionic liquid. The creation of the hole is initiated by a mutual interaction between the concentrations of the ionic liquid and the binary solvent via their affinity. A spatial inhomogeneity of the interfacial tension is induced through an amplification of fluctuation in the concentration due to an instability mechanism, and causes the Marangoni effect to create the hole. The hole moves inside the droplet and consequently leads to the motion of the droplet. The present system provides not only a new type of dissolution process but also a peculiar example of active matter realized in a liquid droplet.

Short chain chitosan solutions: self-assembly and aggregates disruption effects

Light scattering and microscopy studies show that short chain chitosans (oligochitosans) are partially associated in dilute solutions in the pH range below critical point of phase separation. The observed structural diversity of solutions (spherical and fibril-like aggregates, and molecularly dispersed systems – “true solution”) depends on purification procedure. It is shown that oligochitosan samples having molecular weights below 12 kg · mol−1 form aggregates of constant sizes and can be selected into a series of oligochitosans of a peculiar aggregation behavior. Comparing the results of different methods (light scattering, microscopy, and HP SEC) it is concluded that initial supramolecular structures occurring in oligochitosan solutions are disrupted under shear action in the course of HP SEC and viscometric measurements, and that the additional factor promoting structural rearrangments during filtration through a porous membrane is the interaction of aggregates with pore surfaces. The results discussed in terms of modern theories of polyelectrolyte solutions are indispensable for better understanding of the mode of oligochitosan antibacterial and antifungal activities and preparation of its injectable compositions.

Effect of an external magnetic field on a critical point for phase separation in a dusty plasma

The effect of an external magnetic field on a critical point for phase transitions in a dusty plasma is investigated. It is shown that the ambient magnetic field increases the effective hard core radius of dust particles, which, in turn, would affect a critical point in terms of the Coulomb coupling parameter and the ratio between the inter-dust grain spacing and the dusty plasma Debye radius. The present result may be useful in understanding the phenomenon of liquid–vapor phase transitions in laboratory dusty plasmas that are held in an external magnetic field.

A study on the liquid-phase oxidation of toluene in ionic liquids

The effect and mechanism of the hydrophilic ionic liquids (ILs) with cobalt naphthenate as catalyst on the liquid-phase oxidation of toluene were studied systematically. Five ILs with different hydrophilicities were employed as reaction media. The results showed that toluene conversion increased with increasing hydrophilicity of ILs. The toluene conversion and the corresponding selectivity of benzaldehyde reached 19.6% and 19.5%, respectively, in 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4). The solubilities of toluene, benzaldehyde, benzyl alcohol and benzoic acid in [Emim]BF4 were further measured at temperatures from 299.2K to 343.2K. The correlations between solubility and temperature were developed based on the critical point of phase separation or ideal solution model. The low solubility of toluene in [Emim]BF4 indicated a heterogeneous oxidation mechanism for toluene. The improvement of toluene conversion and product selectivity in [Emim]BF4 was attributed to the interactions between reaction and extraction separation. The sensitivity study demonstrated that the increase in oxygen pressure was beneficial to the reaction. A much lower reaction temperature of 130°C was achieved in [Emim]BF4 instead of a typical 160–170°C for current commercial plants. The high conversion, low reaction temperature and high product selectivity for the liquid-phase oxidation of toluene in [Emim]BF4 makes it a green and efficient hydrocarbon oxidation process.

Pairing imbalance in the BCS-BEC crossover of an inhomogeneous three-component Fermi gas in two dimensions

We in this paper investigate the phase diagram associated with the BCS-BEC crossover of a three-component ultracold superfluid-Fermi-gas of different chemical-potentials and equal masses in two dimensions. The gap order parameter and number densities are found analytically by using the functional path-integral method. The balance of paring will be broken in the free space due to the unequal chemical-potentials. We obtain the same particle number-density and condensed fraction in the BCS superfluid phase as that in a recent paper (Phys. Rev. A 83, 033630), while the Sarma phase of coexistence of normal and superfluid Fermi gases is the characteristics of inhomogeneous system. The minimum ratio of BCS superfluid phase becomes 1/3 in the BCS limit corresponding to the zero-ratio in the two-component system in which the critical point of phase separation is {\epsilon}B/{\epsilon}F = 2 but becomes 3 in the three-component case.

Structure formation and phase-separation behaviour of aqueous casein–alginate emulsions in the presence of strong polyelectrolyte

Rheo-small angle light scattering (SALS), optical microscopy (OM), rheology, phase composition analysis, and environment scanning electron microscopy (ESEM) were applied to characterize the effect of strong polyelectrolyte on the structure formation and phase-separation behaviour in aqueous sodium caseinate–sodium alginate (W–SC–SA) water-in-water emulsion with the droplet morphology in the concentration ranges both near and far from the critical point (CP). Addition of 0.04–1.5 wt% of dextran sulfate sodium salt (DSS) to the emulsion placed on the phase diagram close to the CP leads at pH 7.0 and ionic strength ( I) = 0.002 to a considerable decrease in the compatibility of SC with SA, disappearance of the droplet morphology, change in solvent distribution between coexisting phases, and a dramatic increase in moduli (G′ and G″) and viscosity, due to formation of water soluble intermacromolecular SC–DSS associates with the pronounced network structure. The maximal relative changes in rheological properties take place at the 10/1 “molar” ratio SC/DSS, and at 6–10 wt% content of SC in the water-in-water emulsion. Beta casein interacts with DSS in less degree than other casein fractions. Near the critical point of phase separation, when the concentration of SC is not so high (6 wt%), the complexes are formed and stabilized via electrostatic interactions, but at higher SC concentrations the contribution of secondary hydrogen bonds and hydrophobic interactions becomes obvious at a higher ionic strength ( I = 1.13). Addition of DSS to the water-in-water emulsion placed on the phase diagram far from the CP leads to reinforcement of the SC–DSS network, partial inclusion of SA in the SC–DSS network, and as a consequence, increases in cosolubility of SC with SA. The molecular origin of the unusually strong interaction of SC with DSS at a high concentration range on the “wrong side” of pH values, far from isoelectric point of caseins is discussed. Experimental observations suggest that use of small amount of sulfate polysaccharide in aqueous water-in-water biopolymer emulsions is a promising tool for regulation their structure, phase behaviour, and rheological properties.

Antiplasticization and the elastic properties of glass-forming polymer liquids

We investigate the effect of antiplasticizer additives on long-wavelength thermodynamic properties relating to the efficiency of molecular packing (density ρ and isothermal compressibility κT) and material stiffness (bulk modulus K, shear modulus G, and Poisson ratio ν) of model glass-forming polymer melts. Variations in the stiffness (G and molecular force constant km from the Debye–Waller factor 〈u2〉) and density ρ at a molecular scale are also considered. We find that antiplasticizer additives cause significant changes in the long-wavelength properties that are associated with an enhanced packing efficiency in the glass state, such as a decrease in κT and an increase in K, G, and ν. Moreover, values of the local elastic moduli in the glass state follow a nearly universal (approximately log-normal) distribution and a Fourier transform of the elastic constant fluctuations in space reveals a general −2 wave vector q scaling, as in order parameter fluctuations in critical fluids and magnetic systems near their critical point for phase separation and ordering, respectively. No discernible large-scale fluctuations in the density were observed, however, indicating that the local elastic constant fluctuations do not reflect density (free volume) fluctuations, as often assumed. These observations provide essential insights into how varying the fragility of glass formation, through the addition of antiplasticizer additives, alters the local mechanical properties of polymer melts and other glass-forming liquids.

PHASE SEPARATION OF POLY(AMIC ACID-CO-IMIDE) SOLUTION

Poly(amic acid-co-imide) (PA-I) is an intermediate for preparation of polyimide from polyamic acid (PAA). Phase separation does not appear for the solution until a certain imidization degree. The experimental results of a rotation viscometer and FT-IR analysis showed that the critical point of phase separation (CPPS) was related to the imidization methods (thermal or chemical imidization) and the backbone structures (flexible ODPA-ODA or semi-rigid PMDA-ODA). Phase separation time is shorted with the increase of the initial PAA concentration, acetic anhydride amount, or temperature. When phase separation occurs, the solution viscosity increases, while the imidization degree is almost constant as the initial PAA concentration rises. The greater the chain mobility, the higher the imidization degree at phase separation point. At CPPS, higher imidization degree and lower solution viscosity are obtained in the thermal imidization than in the chemical imidization; ODPA-ODA system exhibits higher imidization degree and better solubility than that of PMDA-ODA system.

Statics and dynamics of colloid-polymer mixtures near their critical point of phase separation: A computer simulation study of a continuous Asakura–Oosawa model

We propose a new coarse-grained model for the description of liquid-vapor phase separation of colloid-polymer mixtures. The hard-sphere repulsion between colloids, and between colloids and polymers, which is used in the well-known Asakura-Oosawa (AO) model, is replaced with Weeks-Chandler-Andersen potentials. Similarly, a soft potential of height comparable to thermal energy is used for the polymer-polymer interaction, rather than treating polymers as ideal gas particles. It is shown by grand-canonical Monte Carlo simulations that this model leads to a coexistence curve that almost coincides with that of the AO model and that the Ising critical behavior of static quantities is reproduced. Then the main advantage of the model is exploited-its suitability for Molecular Dynamics simulations-to study the dynamics of mean square displacements of the particles, transport coefficients such as the self-diffusion and interdiffusion coefficients, and dynamic structure factors. While the self-diffusion of polymers increases slightly when the critical point is approached, the self-diffusion of colloids decreases and at criticality the colloid self-diffusion coefficient is about a factor of 10 smaller than that of the polymers. Critical slowing down of interdiffusion is observed, which is qualitatively similar to symmetric binary Lennard-Jones mixtures, for which no dynamic asymmetry of self-diffusion coefficients occurs.

Diverging curvature correction to the interfacial tension in polymer solutions

Application of polymer scaling to the problem of Tolman's length, a curvature correction coefficient in the interfacial tension, shows that Tolman's length in polymer solutions may become as large as half of the thickness of the interface. Tolman's length depends on the degree of polymerization N and the distance to the critical point of phase separation, Delta T. In the "critical" regime (N 1/2|Delta T|<<1) Tolman's length diverges upon approach to the critical temperature as approximately N 0.348|Delta T|-0.304. In the "polymer" regime (N 1/2|Delta T|>> 1) Tolman's length does not depend on N , but diverges more strongly, as approximately |Delta T|-1, proportional to the thickness of the interface.

Critical opalescence points to thermodynamic instability: relevance to small-angle X-ray scattering of resorcinol–formaldehyde gel formation at low pH

During the formation at low pH of resorcinol–formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a non-monotonic intensity variation as a function of reaction time. The data are analyzed in terms of scattering by statistical fluctuations of polymer concentration, the amplitude of which is maximal close to the critical point for phase separation between polymer and solvent. The data do not carry any morphological information, but they unambiguously show that the driving force of the gel formation is a thermodynamic instability of the polymerizing solution.

13C and 15N NMR evidence for peripheral intercalation of uniformly labeled fusogenic peptides incorporated in a biomimetic membrane

Membrane fusion requires drastic and transient changes of bilayer curvature and here we have studied the interaction of three de novo designed synthetic hydrophobic peptides with a biomimetic three-lipid mixture by solid state NMR. An experimental approach is presented for screening of peptide–lipid interactions and their aggregation, and their embedding in a biomimetic membrane system using established proton-decoupled 13C, 15N and proton spin diffusion heteronuclear 1H− 13C correlation NMR methods at high magnetic field. Experiments are presented for a set of de-novo designed fusion peptides in interaction with their lipid environment. The data provide additional support for the transmembrane model for the least fusogenic peptide, L16, while the peripheral intercalation model is preferred for the fusogenic peptides LV16 and LV16G8P9. This contributes to converging evidence that peripheral intercalation is both necessary and sufficient to trigger the fusion process for a lipid mixture close to a critical point for phase separation across the bilayer.

Conditions for extreme sensitivity of protein diffusion in membranes to cell environments

We study protein diffusion in multicomponent lipid membranes close to a rigid substrate separated by a layer of viscous fluid. The large-distance, long-time asymptotics for Brownian motion are calculated by using a nonlinear stochastic Navier-Stokes equation including the effect of friction with the substrate. The advective nonlinearity, neglected in previous treatments, gives only a small correction to the renormalized viscosity and diffusion coefficient at room temperature. We find, however, that in realistic multicomponent lipid mixtures, close to a critical point for phase separation, protein diffusion acquires a strong power-law dependence on temperature and the distance to the substrate H, making it much more sensitive to cell environment, unlike the logarithmic dependence on H and very small thermal correction away from the critical point.

Competition of mesoscales and crossover to theta-point tricriticality in near-critical polymer solutions

The approach to asymptotic critical behavior in polymer solutions is governed by a competition between the correlation length of critical fluctuations diverging at the critical point of phase separation and an additional mesoscopic length scale, the radius of gyration. In this paper we present a theory for crossover between two universal regimes: a regime with Ising (fluctuation-induced) asymptotic critical behavior, where the correlation length prevails, and a mean-field tricritical regime with theta-point behavior controlled by the mesoscopic polymer chain. The theory yields a universal scaled description of existing experimental phase-equilibria data and is in excellent agreement with our light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 2 x 10(5) up to 11.4 x 10(6). The experiments demonstrate unambiguously that crossover to theta-point tricriticality is controlled by a competition of the two mesoscales. The critical amplitudes deduced from our experiments depend on the polymer molecular weight as predicted by de Gennes [Phys. Lett. 26A, 313 (1968)]. Experimental evidence for the presence of logarithmic corrections to mean-field tricritical theta-point behavior in the molecular-weight dependence of the critical parameters is also presented.

Rotational behavior of the fluorescent probe molecules near the critical point of phase separation

Rotational behavior of fluorescein sodium salt (FL) dissolved in a critical binary liquid mixture of 2,6-lutidine+water (LW) was investigated by the steady-state and time-resolved fluorescence depolarization measurements. It was found that FL molecule motion is slowed down near the critical point of LW. Two different rotational motion modes of FL molecule were observed.

Scaling of demixing curves and crossover from critical to tricritical behavior in polymer solutions

In this paper we show that the virial expansion up to third order for the osmotic pressure of a dilute polymer solution, including first-order perturbative corrections to the virial coefficients, allows for a scaling description of phase-separation data for polymer solutions in reduced variables. This scaling description provides a method to estimate the Θ-temperature, where demixing occurs in the limit of vanishing polymer volume fraction φ and infinite chain length N, without explicit assumptions concerning the chain-length dependence of the critical parameters Tc and φc. The scaling incorporates three limiting regimes: The Ising limit asymptotically close to the critical point of phase separation, the pure-solvent limit, and the tricritical limit for the polymer-rich phase asymptotically close to the theta point. We incorporate the effects of critical and tricritical fluctuations on the coexistence-curve scaling by using renormalization-group methods. We present a detailed comparison with experimental and simulation data for coexistence curves and compare our estimates for the Θ-temperatures of several systems with those obtained from different extrapolation schemes.

Scaling and crossover to tricriticality in polymer solutions

We propose a scaling description of phase separation of polymer solutions. The scaling incorporates three universal limiting regimes: the Ising limit asymptotically close to the critical point of phase separation, the “ideal gas” limit for the pure solvent phase, and the tricritical limit for the polymer-rich phase asymptotically close to the theta point. We have also developed a phenomenological crossover theory based on the near-tricritical-point Landau expansion renormalized by fluctuations. This theory validates the proposed scaled representation of experimental data and crossover to tricriticality.