Dynamics near a liquid-liquid phase transition in a non-tetrahedral liquid: the case of gallium

We use molecular simulation to analyze liquid dynamics in the vicinity of the liquid-liquid phase transition (LLPT) recently discovered in the modified embedded-atom model for elemental gallium. For this purpose we analyze the diffusive behavior in terms of the mean-squared displacement and self-intermediate scattering functions for two systems obtained by cooling the stable liquid through the LLPT at different cooling rates. The results show a pronounced heterogeneity of the dynamics upon the onset of the LLPT. Furthermore, it is found that this heterogeneity is closely correlated to the structural properties of the 9-fold coordinated high-density and 8-fold coordinated low-density liquid forms involved in the transition, showing a mixture of domains with very different diffusion time scales. The dynamics of the low-density liquid is found to be much more sluggish than that of the high-density form. Analysis of the energetics suggests that the origin of this difference is rooted in the fact that the cohesion in the former is significantly stronger than that in the latter.     

Communication: probable scenario of the liquid-liquid phase transition of SnI4

We have shown from in situ synchrotron x-ray diffraction measurements that there are two thermodynamically stable liquid forms of SnI(4), depending on the pressure. Based on the liquid-liquid critical point scenario, our recent measurements suggest that the second critical point, if it exists, may be located in a region close to the point at which the melting curve of the crystalline phase abruptly breaks. This region is, unlike that of water, experimentally accessible with relative ease.     

Effect of confinement on the liquid-liquid phase transition of supercooled water

We report on an observation of the phase transition between two liquid phases of supercooled confined water in simulations. The temperature of the liquid-liquid transition of water at zero pressure slightly decreases due to confinement in the hydrophobic pore. The hydrophilic confinement affects this temperature in the opposite direction and shifts the critical point of the liquid-liquid transition to a higher pressure. As a result, in a strongly hydrophilic pore the liquid-liquid phase transition becomes continuous at zero pressure, indicating the shift of its critical point from negative to a positive pressure. These findings indicate that experimental studies of water confined in the pores of various hydrophobicity/hydrophilicity may clarify the location of the liquid-liquid critical point of bulk water.     

A reactive force field simulation of liquid-liquid phase transitions in phosphorus

A force field model of phosphorus has been developed based on density functional (DF) computations and experimental results, covering low energy forms of local tetrahedral symmetry and more compact (simple cubic) structures that arise with increasing pressure. Rules tailored to DF data for the addition, deletion, and exchange of covalent bonds allow the system to adapt the bonding configuration to the thermodynamic state. Monte Carlo simulations in the N-P-T ensemble show that the molecular (P(4)) liquid phase, stable at low pressure P and relatively low temperature T, transforms to a polymeric (gel) state on increasing either P or T. These phase changes are observed in recent experiments at similar thermodynamic conditions, as shown by the close agreement of computed and measured structure factors in the molecular and polymer phases. The polymeric phase obtained by increasing pressure has a dominant simple cubic character, while the polymer obtained by raising T at moderate pressure is tetrahedral. Comparison with DF results suggests that the latter is a semiconductor, while the cubic form is metallic. The simulations show that the T-induced polymerization is due to the entropy of the configuration of covalent bonds, as in the polymerization transition in sulfur. The transition observed with increasing P is the continuation at high T of the black P to arsenic (A17) structure observed in the solid state, and also corresponds to a semiconductor to metal transition.     

The putative liquid-liquid transition is a liquid-solid transition in atomistic models of water

We use numerical simulation to examine the possibility of a reversible liquid-liquid transition in supercooled water and related systems. In particular, for two atomistic models of water, we have computed free energies as functions of multiple order parameters, where one is density and another distinguishes crystal from liquid. For a range of temperatures and pressures, separate free energy basins for liquid and crystal are found, conditions of phase coexistence between these phases are demonstrated, and time scales for equilibration are determined. We find that at no range of temperatures and pressures is there more than a single liquid basin, even at conditions where amorphous behavior is unstable with respect to the crystal. We find a similar result for a related model of silicon. This result excludes the possibility of the proposed liquid-liquid critical point for the models we have studied. Further, we argue that behaviors others have attributed to a liquid-liquid transition in water and related systems are in fact reflections of transitions between liquid and crystal.     

Solution crystallisation via a submerged liquid-liquid phase boundary: oiling out

In many situations the process of crystallisation from solution is known to occur via metastable crystalline states (polymorphs or solvates). Here we present what we believe to be a novel example of small molecule crystallisation in which the initial separation of a solute rich liquid phase precedes the crystallisation event. We believe this occurs because a submerged liquid-liquid phase boundary is accessible within the metastable zone of the crystal nucleation process.     

AC calorimetry at the first order phase transition point

A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C₂₀H₄₂). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.

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Zusammenfassung Es wird ein Modell für die AC-Kalorimetrie (ACC) von Phasenumwandlungen erster Ordnung vorgeschlagen. Das Modell wird mit den Ergebnissen von ACC in Nähe des Schmelzpunktes des n-Paraffins C20H42 verglichen. Die beobachtete Frequenzabhängigkeit von ACC stimmt mit dem Modell überein. Im Phasenumwandlungspunkt kann eine harmonische Komponente der Temperaturmodulation mit einer Frequenz festgestellt werden, die doppelt so hoch wie die Heizfrequenz ist. Es wird gezeigt, daß die harmonische Komponente anhand des vorgeschlagenen Modelles erklärt werden kann.

     

Shear-induced parallel-to-perpendicular orientation transition in the amphiphilic lamellar phase: a nonequilibrium molecular-dynamics simulation study

The present work is devoted to a study of the shear-induced parallel-to-perpendicular orientation transition in the lamellar system by the large-scale nonequilibrium molecular-dynamics (NEMD) simulation. An effective generic model-A2B2 tetramer for amphiphilies is used. The NEMD simulation produces unambiguous evidence that undulation instability along the vorticity direction sets in well above a critical shear rate and grows in magnitude as the shear rate is further increased. At a certain high shear rate, the coherent undulation instability grows so large that defects are nucleated and the global lamellar monodomain breaks into several aligned lamellar domains. Subsequently layers in these domains rotate into the perpendicular orientation with the rotation of chains towards the y direction, merge into a global perpendicular-aligned lamellar monodomain, and organize into a perfect well-aligned perpendicular lamellar phase by the migration and annihilation of edge dislocations and disclinations. The macroscopic observable viscosity as a function of time or shear rate is correlated with the structural response such as the mesoscopic domain morphology and the microscopic chain conformation. The onset of undulation instability concurs with the start-up of shear-thinning behavior. During the orientation transformation at the high shear rate, the complex time-dependent thixotropic behavior is observed. The smaller viscosity in the perpendicular lamellar phase gives an energetic reason for the shear-induced orientation transition.     

Interplay between two phase transitions: crystallization and liquid-liquid phase separation in a polyolefin blend

The interplay between liquid-liquid phase separation (LLPS) and crystallization at several compositions in statistical copolymer blends of poly(ethyleneco-hexene) and poly(ethylene-cobutene) has been examined by optical microscopy (OM), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). The phase contrast optical microscopy shows interconnected bicontinuous structures for deeply quenched LLPS, characteristic of spinodal decomposition. After a second quench to a temperature below the melting point, an overwhelming change in crystallization kinetics has been clearly observed, which is caused by the increase of the nucleation rate assisted by concentration fluctuations due to the spontaneous spinodal LLPS. We propose a new mechanism of "fluctuation assisted nucleation" in the crystallization process for such interactive process in a blend system. The experimental results from OM, AFM, and DSC measurements at various conditions are all consistent with the fluctuation assisted nucleation model.     

Unsuccessful search for a liquid-liquid transition in compression isotherms of polystyrene

In a search for a liquid-liquid transition in the melt of polystyrene, we have measured and analyzed a number of compression isotherms to 1800 kg/cm² in a relevant temperature range (1.2T_g < T < 1.4T_g). Volume data were recorded at pressure intervals of 10 kg/cm², i.e., at pressure intervals 5-20 times smaller than used in previous work. For the analysis the data were fitted to the Tait equation. Deviations between fits and data are small, typically 0.0004 cm³/g, but nonrandom. From the absence of any systematic shifts of the nonrandom deviation patterns with temperature, we conclude that the deviations are not manifestations of a pressure-dependent liquid-liquid transition, and that there is, in fact, no evidence for the existence of such a transition in our data.     

Molecular dynamics simulation of nanoparticle self-assembly at a liquid-liquid interface

We have used molecular dynamics simulations to investigate the in situ self-assembly of modified hydrocarbon nanoparticles (mean diameter of 1.2 nm) at a water-trichloroethylene (TCE) interface. The nanoparticles were first distributed randomly in the water phase. The MD simulation shows the in situ formation of nanoparticle clusters and the migration of both single particles and clusters from the water phase to the trichloroethylene phase, possibly due to the hydrophobic nature of the nanoparticles. Eventually, the single nanoparticles or clusters equilibrate at the water-TCE interface, and the surrounding liquid molecules pack randomly when in contact with the nanoparticle surfaces. In addition, the simulations show that the water-TCE interfacial thickness analyzed from density profiles is influenced by the presence of nanoparticles either near or in contact with the interface but is independent of the number of nanoparticles present. The nanoparticles, water molecules, and TCE molecules all exhibit diffusion anisotropy.     

Microcomputer simulation of the countercurrent liquid-liquid distribution

The simulation of the countercurrent liquid-liquid distribution by using a microcomputer is described. The computer program that is proposed for this simulation has been made in BASIC language and its characteristics are discussed. According to the wide possibility of calculation that this program presents, several simulations of different solute mixtures have been performed.     

Activity and behavior of imidazolium salts as a phase transfer catalyst for a liquid-liquid phase system

The structure-activity relationship and behavior of N,N′-dialkylimidazolium salts as a phase transfer and/or ion-exchange catalyst in a liquid-liquid phase system were investigated for the reactions such as β-elimination reaction of alkyl halides, nucleophilic epoxidation of α,β-unsaturated carbonyl compounds, alkylation of active methylenes, and nucleophilic substitution reaction.     

Atomistic simulation of discotic liquid crystals: transition from isotropic to columnar phase example

Molecular dynamics simulations at atomistic level have been performed on a metal-porphyrazine complex. Starting from an isotropic state, the system was cooled until transition from isotropic to columnar phase was observed; no nematic phase was encountered. Many tools were utilized to follow the system evolution: order parameter, g(r), g(||)(r(||)), g(c)(r(||)), g(perpendicular)(r(perpendicular)), g(2)(r), also density and energy changes. Very long runs were required to get reliable results, times greater than 40 ns of simulation. The structure of columnar phase was analyzed and the organization of molecules in the columns was investigated, along with the role of conformation of side chains. We found that in columnar phase the molecules are tilted versus the column axis and the conformation of side chains changes during the phase transition to allow this kind of organization; moreover the direction of columns axes is different from that of the director.     

Ab initio molecular dynamics simulation of pressure-induced phase transition in MgS

Pressure-induced phase transition in MgS is studied using a constant pressure ab initio molecular dynamics method, and a solid evidence of existence of its high-pressure phase is provided. As predicted by total energy calculations, MgS undergoes a structural phase transformation from the rocksalt structure to a CsCl-type structure under hydrostatic pressure. The transformation mechanism is characterized, and two intermediate phases having P4/nmm and P2₁/m symmetries for the rocksalt-to-CsCl-type phase transformation of MgS are proposed, which is different from the previously proposed mechanisms. We also study this phase transition using the total energy calculations. Our predicted transition parameters and bulk properties are in good agreement with the earlier first principle simulations.     

Perfluorocarbon-based liquid-liquid extraction for separation of transition metal ions.

We report a liquid-liquid extraction system based on perfluorocarbon (fluorous-solvent extraction) that allows selective extraction of metal ions from aqueous and organic phases to a perfluorocarbon phase (FC-72) with perfluorinated beta-diketone (1,1,1,5,5,6,6,6-octafluoro-2,4-hexanedione), which can be followed by backward extraction using 1 M nitric acid.     

Phase-ordering kinetics of the liquid-liquid transition in single-component molecular liquids

Recently it has been revealed that even a single-component liquid can have more than two liquid states. The transition between these liquid states is called the "liquid-liquid transition." Most known liquid-liquid transitions occur at temperatures and pressures which are difficult to access experimentally, so the physical nature of the transition, particularly the kinetics, has remained elusive. However, the recent discovery of liquid-liquid transitions in molecular liquids opens up a possibility to study the kinetics in detail. Here, we report the first phase field simulation on the kinetics of a liquid-liquid transition and its direct comparison with experimental results of the molecular liquids. Both nucleation-growth-type and spinodal-decomposition-type liquid-liquid transformation observed experimentally are well reproduced by numerical simulation based on a two-order-parameter model of liquid that regards the liquid-liquid transition as the cooperative formation of locally favored structures. Thus, phase field calculations may allow us to predict the kinetics of liquid-liquid transitions and the resulting spatiotemporal change of various physical properties of the liquid, such as density and refractive index.     

Design and application of a new phase separator for flow injection liquid-liquid extraction

A new versatile phase separator for flow injection solvent extraction has been designed and its analytical performance investigated. Manifolds for various instrumentation and the corresponding operating procedures have been established. Experimental results show that the newly designed phase separator possesses several advantages over the existing ones, such as long term stability, robustness, sufficient phase separation and integral in situ collection of the solvent extracts at different phase flow rate ratios. The detection limits for cobalt, gold, iodide, iron, lead, nickel, nitrate and tungsten were 1.4, 0.45, 70, 1.9, 160, 0.9, 137 and 23 ng ml⁻¹, and the relative standard deviations 2.5, 1.3, 2.2, 2.9, 4.8, 3.6, 1.4 and 3.3%, respectively.