Phases In Binary Mixtures Research Articles

Caracterização da fusão peritética do cloreto de sódio di-hidratado por curvas de aquecimento

Binary mixture phase diagrams are normally obtained from thermal analysis involving freezing point curves. However, that approach is not always reliable and easy to follow to all kinds of mixtures in any proportion. In fact, even for a simple system, such as NaCl-H2O, this freezing methodology gives mixed results when one starts from a solid-solution system, due mostly to the formation of the NaCl.2H2O, which has an incongruent melting point, and the dependence of its solubility with the temperature. In this work we report a trustworthy, simple and cheap method involving heating curves to drawn the NaCl-H2O phase diagram.

Effective Hamiltonian for fluid membranes in the presence of long-ranged forces

If the constituent particles of fluid phases interact via long-ranged van der Waals forces, the effective Hamiltonian for interfaces between such fluid phases contains--in lateral Fourier space--nonanalytic terms approximately q4ln q. Similar nonanalytic terms characterize the effective Hamiltonian for two interacting interfaces which can emerge between the three possible coexisting fluid phases in binary liquid mixtures. This is in contrast with the structure of the phenomenological Helfrich Hamiltonian for membranes, which does not contain such nonanalytic terms. We show that under favorable conditions for the bulk densities characterizing a binary liquid mixture and for the long-ranged interparticle interactions, the corresponding effective Hamiltonian for a model fluid membrane does not exhibit such nonanalytic contributions. We discuss the properties of the resulting effective Hamiltonian, with a particular emphasis on the influence of the long range of the interactions on the coefficient of the bending rigidity.

Liquid Ordered Phase of Binary Mixtures Containing Dipalmitoylphosphatidylcholine and Sterols

The effect of cholesterol, desmosterol, stigmasterol, sitosterol, ergosterol, and androsterol on the phase behavior of aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) was studied to understand the role of the side chain in the formation of ordered phases of the type observed in membrane rafts. Thermotropic changes in the structure of mixed dispersions and transition enthalpies were examined by synchrotron X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The observations indicated that cholesterol was more efficient than phytosterols (stigmasterol and sitosterol) or ergosterol in its interaction with DPPC to form the liquid ordered phase ( L o). The L o induced by cholesterol or desmosterol was stable over a wide temperature range, whereas, the liquid ordered phase containing phytosterols or ergosterol was profoundly dependent on temperature, which should be distinguished as L o β and L o α , representing the phases below and above the main transition temperature. The characteristics in forming ordered structures of cholesterol and other sterols imply that the evolution may have selected cholesterol as the most efficient sterol for animals to form rafts in their cell membranes.

The phase behaviour of lyotropic liquid crystals in linear alkylbenzene sulphonate (LAS) systems

A commercial linear alkylbenzene sulphonate (LAS)/H 2O surfactant system was studied to develop understanding of the phase diagram and the important mechanisms determining the behaviour. This is an important area of understanding for formulations. Optical microscopy, differential scanning calorimetry, X-ray scattering and nuclear magnetic resonance were used to study samples between 20 and 75 wt% LAS over a temperature range of 5–80 °C. The system exhibits a broad two-phase (micellar/lamellar) region, which derives from the occurrence of an upper consolute loop within the lamellar phase for single isomer LAS/water mixtures. At low temperature the system appeared to behave as a Gibbs (binary mixture) phase rule material, but on heating unexpected behaviour was observed. There was a change in the swelling behaviour, giving rise to the existence of several concentrated lamellar phases co-existing with a single (more dilute) micellar phase. This was attributed to changes in head-group/counter-ion interactions as a function of composition and temperature. The behaviour is derived from the input of energy overcoming an attractive force between the bilayers, and the different isomers within the system having different properties leading to co-existing bilayers with different compositions. Such behaviour in formulations could result in slow changes with time, giving product instability.

Dissipative particle dynamics simulation study on the binary mixture phase separation coupled with polymerization

The influence of polymerization on the phase separation of binary immiscible mixtures has been investigated by the dissipative particle dynamics simulations in two dimensions. During polymerization, the bulk viscosity increases, which consequently slows down the spinodal decomposition process. The domain size growth is monitored in the simulations. The absence of 23 exponent for inertial hydrodynamic mechanism clearly reflects the suppressing effect of polymerization on the phase separation. Due to the increasing viscosity, the individual phase may be trapped in a metastable stage instead of the lamellar morphology identified for symmetric mixtures. Moreover, the polymerization induced phase separation in the binary miscible mixture has been studied. The domain growth is strongly dependent on the polymerization probability, which is naturally related to the activation energy for polymerization. The observed complex phase separation behavior is attributed to the interplay between the increasing thermodynamic driving force for phase separation and the increasing viscosity that suppresses phase separation as the polymerization proceeds.

Smectic, nematic, and isotropic phases in binary mixtures of thin and thick hard spherocylinders

A second-virial Onsager theory, based on Parsons-Lee rescaling and suitably extended to deal with multicomponent systems and smectic phases, has been used to calculate the phase diagram of a collection of binary mixtures of thin and thick hard spherocylinders. In particular, two types of phase diagrams are investigated. First, a number of binary mixtures where the two components have the same total length have been considered; in addition, the phase diagram of a binary mixture where the two components have the same volume has been calculated. For the particles of one of the two components, the length of the cylindrical part and the diameter have always been set equal to 5 and 1, respectively. Spherocylinders of the same total length and different diameter tend to demix considerably as soon as the diameter ratio deviates from unity. This happens especially at high pressures, when at least the phase richer in the thicker component is smectic. In the case where the two components have equal volumes, demixing is further increased due to the disparity not only in particle diameter but also in particle lengths. The incorporation of inhomogeneous layered phases is seen to alter significantly the phase diagrams calculated if only homogeneous phases are allowed, since transitions to a smectic phase often preempt those to a nematic or an isotropic phase. The apparent versatility of the recent experimental techniques suggests that the phase diagram features predicted by the theory might be also observed in real systems.

Frustrated Smectic Phase Appearing as Transitional State between Single-Layer and Antiferroelectric Bilayer Smectic Phases in Binary Mixtures of Dimeric Compounds

Twin dimers, mOAM5AMOm, form characteristic smectic phases, single-layer SmCAs and antiferroelectric bilayer SmCAb phases. Structural transformation from SmCAs to SmCAb was examined in two series of binary mixtures of 8OAM5AMO8/4OAM5AMO4 and 8OAM5AMO8/16OAM5AMO16. In this transformation, the frustrated SmCAf with density modulation along the layer is formed and the size of modulation increases from 50 to 250 Å with variations in mixture contents from an edge near SmCAs to another edge near SmCAb. From polymorphisms shown by some mixtures, it was found that SmCAs and SmCAb are located at high-temperature and low-temperature edges, respectively, and that the SmCAf phase is set between them.

Dielectric study of nematic and induced smectic-A phases in binary mixtures of pentyloxy cynobiphenyl with three p-azoxybenzenes

Static dielectric properties of 18 binary mixtures of 4,4′-pentyloxy cyanobiphenyl (5OCB) with 4,4′-bis(alkoxy)azoxybenzenes (nOAB) for n = 5, 6 and 7 have been studied in detail. Seventeen mixtures show induced smectic-A (SmA) phase; a few mixtures exhibit nematic phase as well. Pure 5OCB has a large positive dielectric anisotropy while nOAB members have small but negative anisotropy. However, all the mixtures exhibit positive anisotropy. In the mixtures, except those with x 5OCB ≥ 0.85, the values of ε∥, ε⊥ and increase smoothly with temperature. The values of εiso are found to be higher than the values within the mesophases, with a jump at T NI or T SI. Effective values of the dipole moments in the mixtures are found to be less than the values observed in the pure compounds. This suggests that anti-parallel homo-dimers of 5OCB exist in the mixtures as observed in the pure compound. Moreover, at the onset of SmA phase, values of are found to decrease from those in the nematic phase. This implies that additional anti-ferroelectric type of ordering is developed in the induced SmA phase. In all the mixtures the anisotropy parameter Δε decreases with concentration, becomes minimum at x 5OCB = 0.5, and then increases quite fast. Kirkwood correlation factors have also been calculated to find the nature and extent of the molecular associations.

Binary Phase Diagram of Triphenylene Derivatives: The Role of Hydrogen Bonds

ABSTRACT In this study, the binary phase diagram of two triphenylene derivatives is investigated by using differential scanning calorimetry (DSC), optical polarizing microscopy (OPM), and wide-angle x-ray scattering (WAXS). The triphenylene derivatives studied are based on hexa-alklyoxy-triphenylene. One compound in the binary mixture HAT6 has six symmetrical hexyloxy tails each containing six carbon atoms. The other compound, HAT6-C10UC6, has one asymmetrical tail bearing a urea functionality at a specific distance from the aromatic core. The introduction of this asymmetrical H-bonding unit into the structure results in a complete disruption of liquid crystallinity while a “pseudo re-entrant” phenomenon is observed in the binary mixture phase diagram.

Molecular alignment and thermal stability of liquid-crystalline phases in binary mixtures of electron donor and acceptor

The molecular alignment and thermal stability of the mixtures DA composed of two different kinds of Schiff base liquid-crystalline materials, D with an electron-donating –N(CH3)2 unit and A with an electron-withdrawing–NO2 unit, were investigated by using DSC, POM, XRD, and FT-IR measurements. The mesophases of the mixtures DA were more stable and had wider temperature ranges than those of the individual compounds D and A. The mixture DA55 (D:A=50:50mol%) on cooling exhibited the most stable liquid-crystalline phases affording the highest TSI (119°C), the widest temperature range of the smectic phase (46.4°C) and the highest values of ΔSSI (26.1JK−1mol−1) among the mixtures DA examined. Based on the analysis of the XRD measurements and the temperature dependence on the FT-IR spectra, the molecular packing of the mixture DA55 was proposed to form a bilayer structure in the smectic A phase, in which the heading group of the compounds D and A overlapped partially. The induction of the smectic phase and the enhanced thermal stability for the mixtures DA would be caused by the intermolecular interaction between the electron donor and acceptor mesogens.

Modelling the phase behaviour and excess properties of alkane + perfluoroalkane binary mixtures with the SAFT–VR approach

The phase and volumetric behaviour of binary mixtures of n-alkanes and n-perfluoroalkanes at subcritical conditions have been studied using the SAFT–VR equation of state. In previous work [C. McCabe, A. Galindo, A. Gil-Villegas, G. Jackson, Predicting the high-pressure phase equilibria of binary mixtures of perfluoro-n-alkanes plus n-alkanes using the SAFT–VR approach, J. Phys. Chem. B 102 (1998) 8060–8069] the SAFT–VR equation was successfully used to predict the phase behaviour of binary mixtures of low molecular weight n-perfluoroalkanes + n-alkanes. A transferable binary interaction parameter was proposed which enabled the accurate prediction of the critical lines for all the systems studied. In this work we focus on the low pressure liquid–liquid immiscibility displayed by binary mixtures of longer chains (C5C8). New transferable binary interaction parameters for these mixtures are proposed, based on the reproduction of the UCST and excess volumes for the n-hexane + n-perfluorohexane system. With these parameters, predictions are then made for the vapour–liquid equilibria (VLE), liquid–liquid equilibria (LLE) and excess volumes of other binary mixtures without any additional fitting to experimental data. The resulting predictions from the SAFT–VR equation are in good agreement with the available experimental data.

Orientational order in binary mixtures of hard Gaussian overlap molecules.

Based on a standard constant-pressure Monte Carlo molecular simulation, we have studied liquid crystal phases of binary mixtures of nonspherical molecules. The components of the mixtures are two types of hard Gaussian overlap (HGO) molecules. The first type of molecule has a small molecularelongation parameter (short HGO molecules) and cannot form stable liquid crystal phase in the bulk by themselves. The second type of molecule has a large elongation parameter (long HGO molecules) and can form a liquid crystal phase easily. In the mixtures, the short HGO molecules can form an orientationally ordered phase because the long HGO molecules form confining surfaces to induce the alignment of the short molecules. We also study the isotropic-nematic phase transition in different mixtures composed of short and long HGO molecules with different elongations and concentrations. The obtained result implies that small anisotropic molecules can show liquid crystal behavior.

Effects of carbohydrate headgroups on the stability of induced cubic phases in binary mixtures of glycolipids

This paper is part in a series of papers, investigating the influence of carbohydrate headgroups on the mesogenic properties of glycolipids. While previous papers focussed on the synthesis and mesogenic properties of the pure compounds, we will discuss here our results obtained with binary mixtures. Mixtures of compounds, one forming a lamellar phase and the other one a columnar phase in their pure state, displayed always an induced cubic phase. The stability of this induced cubic phase depends significantly on the structure of the carbohydrate headgroup of both components. Thus it was possible to derive structure–property relationships by comparison of the phase diagrams that have been obtained, if the carbohydrate headgroup of one component was changed systematically. We observed an interesting effect of galactose headgroups which might be of great biological importance. Furthermore, the observed kind of kinetic of the S A→cub transition might also be of great biological relevance.

Re-entrant ferroelectric phases in binary mixtures of ferroelectric and antiferroelectric homologues of a series with three chiral centers

In binary mixtures composed of homologues from a series derived from 4-alkoxybiphenyl- 4′-carboxylic acid exhibiting ferroelectric (FE) and antiferroelectric (AF) phases, a re-entrant FE phase may be induced in a certain concentration region, which is miscible with the SmC* phase. Dielectric spectroscopy study revealed a softening of the antiphase mode in the AF phase, which is responsible for the appearance of the re-entrant phase. The helix twist inversion occurs at the low temperature limit of the (AF) phase so that the helix handedness of the upper and lower temperature FE phases have opposite sense. The values of spontaneous polarization and the spontaneous tilt angle evolve continuously with the concentration, while dielectric strength exhibits a maximum for a certain concentration of homologues. All phase transitions existing in the binary mixtures are sharp and divide homogeneous phases in contrast to results reported in E. Gorecka et al. [Phys. Rev. Lett. 81 2946 (1998)], where a frustrated cluster phase exists. The reason is in different behavior of interlayer interactions at the FE-AF phase transition.

Induction of B1, B6 and Biaxial Smectic A Phases in Binary Mixtures of Compounds with Rod-Like and Bent-Core Molecules

We have studied a binary mixture of compounds, one of which has rod-like molecules and the other bent-core molecules. The rod-like molecules are biphilic in nature with a long alkoxy chain attached to an aromatic moeity only at one end, and exhibit the bilayer SmA2 phase, even though they do not have the highly polar cyano or nitro groups. The compound with bent-core molecules exhibit the B2 phase which has polarized layers with tilted molecules. Three liquid crystalline phases are induced in the binary mixtures, depending on the concentration: the 2-dimensionally ordered B1 phase for compositions with 15 to 63 mol% of the rod-like molecules and the B6 phase for 63 to 87 mol%. The sequence obtained on increasing the concentration of rod-like molecules is the same as that seen on shortening the chain length of the BC molecules. Between 87 to 95.5 mol% of the rod-like molecules, a new biaxial smectic A2 (SmA2b ) phase is induced. On the basis of several observations, we argue that it corresponds to a structure in which the BC molecules are reoriented with their arrow directions pointed along the layer normal. The SmA2 to SmA2b transition corresponds to an orientational transition of the bent-core molecules in the anisotropic SmA2 background medium.

Effect of cholesterol on the formation of an interdigitated gel phase in lysophosphatidylcholine and phosphatidylcholine binary mixtures.

We previously reported that 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) forms an interdigitated gel phase in the presence of 1-palmitoyl-sn-glycero-3-phosphocholine (16:0LPC) at concentrations below 30 mol%. In the present investigation, fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH), X-ray diffraction, and differential scanning calorimetry (DSC) were used to investigate the effect of cholesterol on the phase behavior of 16:0LPC/DPPC binary mixtures. At 25 degrees C, 30 mol% 16:0LPC significantly decreases the DPH fluorescence intensity during the transition of DPPC from the L(beta') phase to the L(betaI) phase. However, the addition of cholesterol to 16:0LPC/DPPC mixtures results in a substantial increase in fluorescence intensity. The changes in DPH fluorescence intensity reflect the probe's redistribution from an orientation parallel to the acyl chain to the center of the bilayer, suggesting a bilayer structure transition from interdigitation to noninterdigitation. The normal repeat period of small angle X-ray diffraction patterns can be restored and a reflection appears at 0.42 nm with a broad shoulder around 0.41 nm in wide angle X-ray diffraction patterns when 10 mol% cholesterol is incorporated into 30 mol% 16:0LPC/DPPC vesicles, indicating that the mixtures are in the gel phase (L(beta')). Moreover, DSC results demonstrate that 10 mol% cholesterol is sufficient to significantly decrease the main enthalpy, cooperativity and lipid chain melting of 30 mol% 16:0LPC/DPPC binary mixtures, which are L(betaI), indicating that the transition of the interdigitated phase is more sensitive to cholesterol than that of the noninterdigitated phase. Our data imply that the interdigitated gel phase induced by 16:0LPC is prevented in the presence of 10 mol% cholesterol, but unlike ethanol, an increasing concentration of 16:0LPC is not able to restore the interdigitation structure of the lipid mixtures.

Interactions of Milk Fat and Milk Fat Fractions with Confectionery Fats

The objectives of this study were to provide a better understanding of the effects of triacylglycerol (TAG) and non-TAG compononents (minor lipids) of milk fat on phase and crystallization behavior of binary mixtures palm kernel oil (PKO) and the physical properties of corresponding compound coatings. Binary mixtures of a fractionatedd PKO with the different milk fats were examined for melting profiles, crystallization kinetics, and crystalline microstructures, and polymorphic changes during storage. Compound coatings were made with equivalent binary fat mixtures and measured for hardness and bloom formation. Milk fat and milk fat fractions affected crystallization rates of fractionated PKO, depending on the melting point of the fat. High-melting components resulted in more rapid crystallization, whereas the original milk fat and low-melting components inhibited crystallization. The crystal structure (e.g., number, size, shape) of the PKO crystals was influenced significantly by the addition of milk fat fractions and was influenced by the presence or absence of the minor lipids in milk fat. Milk fat and milk fat fractions had a softening effect on fractionated PKO, which was apparent in the binary mixtures as well as the compound coatings. In general, as the solide fat content (at 25°C) of the binary mixtures increased, the hardness of the respective coatings increased. This also was related to an increased rate of bloom formation during storage.

Effect of an interface with solid on the component distribution in separated phases of binary polymer mixtures

Using the data on glass transition temperatures and phase diagrams of the unfilled and filled poly(vinyl acetate)–poly(methyl methacrylate) mixtures (1:1), the compositions of phases obtained after phase separation were calculated. Thermodynamic parameters of interaction between components in the unfilled and filled systems has been calculated. It was established that introduction of a filler increases the thermodynamic stability of the mixture (effect of compatibilization) and changes the compositions of separated phases. The observed effects were explained by the specific interaction of the mixture components with the active groups at the interface with solid and by the surface segregation of one of the components.

Interlayer interactions in enantiomeric anticlinic liquid crystalline mixtures

The interlayer interaction coefficient U in the anticlinic phase of enantiomeric binary mixtures was determined by measuring the threshold electric field for the onset of solitary waves. U was found to increase with decreasing temperature in the anticlinic phase for a given enantiomer excess X. The ratio U(X,straight theta)/U(X=1,straight theta), where straight theta is the polar angle, was found to be significantly smaller for mixtures with low enantiomer excess than for the optically pure material. The observed behavior is analyzed using a mean-field model for dipole-dipole interactions in adjacent layers.

COMPUTER SIMULATION OF PROFILES OF INTERFACES BETWEEN COEXISTING PHASES: DO WE UNDERSTAND THEIR FINITE SIZE EFFECTS?

Interfaces between coexisting phases are very common in condensed matter physics, and thus many simulations attempt to characterize their properties, in particular, the interfacial tension and the interfacial profile. However, while theory usually deals with the "intrinsic profile", the latter is not a straightforward output of a simulation: The actual profile (observed in simulations and/or experiments!) is broadened by lateral fluctuations. Therefore, in the usual simulation geometry of L × L × L (in three dimensions), where one chooses suitable boundary conditions to stabilize one or two interfaces of (minimal) area L × L, the profile (and in particular the interfacial width) depends on both linear dimensions L and D (parallel and perpendicular to the interface). Choosing recent simulations of interfaces between coexisting phases of unmixed binary polymer mixtures as an example, we show that this interfacial broadening is not a small correction, but has pronounced effects; for a reliable data analysis, it is (unfortunately!) necessary to vary L and D over a wide range. We present counterexamples to the widespread belief that for small linear dimensions, the intrinsic profile is straightforwardly recovered and speculate about conditions where this belief may be valid.