Phase Behavior Of Systems Research Articles

Ordering Phenomena in Surfactant Systems: From Micellar Solutions to Gel and Crystalline Phases

Abstract The phase behavior of surfactant systems is investigated using alkyltrimethylammonium bromide surfactants with different alkyl chain lengths (dodecyl-, tetradecyl-, hexadecyl- and octadecyl) as model substances. The transitions between micellar solution, gel-phase and crystalline phase are characterized by the combination of Raman spectroscopy and small angle neutron scattering. It is demonstrated that in systems with small surfactant molecules the aqueous solution crystallizes directly on cooling to form hydrated crystals while an intermediate gel phase is observed in long chain systems. The cross-over is found for the hexadecyl-compound where the gel phase appears only at high concentrations.

High-Pressure Phase Behavior of Poly(l-lactic acid), Trichloromethane, and Carbon Dioxide Ternary Mixture Systems

In this study, the phase behavior of a poly(l-lactic acid) (M = 312 000, polydispersity 2.189)–trichloromethane–carbon dioxide ternary system was measured using a variable volume view cell at pressures up to 34.0 MPa and temperatures between (313.15 K and 363.15) K at an interval of 10 K. Bubble and cloud point pressures were determined as functions of the trichlomethane-to-carbon dioxide ratio, concentration of poly(l-lactic acid) (1.0, 2.0, and 3.0 wt %), and temperature. The hybrid equation of state for the supercritical CO2-polymer system combined with the van der Waals one-fluid mixing rule was used for correlation of the experimental results. The three binary parameters of this system were optimized by the simplex method algorithm.

The phase behaviour of mixed saturated and unsaturated monoglycerides in hydrophobic system

Lyotropic liquid crystals are formed on the dissolution of most surfactants in water as represented by regions on the phase diagram, and are more common than micellar solutions. Monoglycerides with saturated or unsaturated fatty acids are one of the most commonly used food surfactants. While there is considerable interest in the behaviour of monoglycerides mixed with oils (particularly triglycerides), limited information is available regarding the phase behaviour of such systems. We have examined the phase behaviour of mixed saturated and unsaturated monoglycerides1:1 (by weight) together with different oils systems. The main technique employed was X-ray diffraction, supplemented by optical microscopy, proton nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). Our results suggest that six phases (L2, α-gel phase, sub α- phase, β1 and oil crystallisation phase (β` for palm oil or β2 for miglyol oil or olive oil) were observed for G50/oil mixture. Furthermore, this paper proves that miglyol and palm oil incorporate into α-gel phase of G50. In contrast, olive oil dose not incorporate into α-gel phase of G50.

Electrolyte effect on the phase behavior of silica nanoparticles with lysozyme and bovine-serum-albumin proteins.

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) studies have been carried out to investigate the effect of an electrolyte on the phase behavior of anionic silica nanoparticles with two globular proteins-cationic lysozyme [molecular weight (MW) 14.7 kDa] and anionic bovine serum albumin (MW 66.4 kDa). The results are compared with our earlier published work on similar systems without any electrolyte [I. Yadav, S. Kumar, V. K. Aswal, and J. Kohlbrecher, Phys. Rev. E 89, 032304 (2014)]. Both the nanoparticle-protein systems transform to two phase at lower concentration of protein in the presence of an electrolyte. The autocorrelation function in DLS suggests that the diffusion coefficient (D) of a nanoparticle-protein system decreases in approaching two phase with the increase in protein concentration. This variation in D can be attributed to increase in attractive interaction and/or overall increase in the size. Further, these two contributions (interaction and structure) are determined from the SANS data. The changes in the phase behavior of nanoparticle-protein systems in the presence of an electrolyte are explained in terms of modifications in both the repulsive and attractive components of interaction between nanoparticles. In a two-phase system individual silica nanoparticles coexist along with their fractal aggregates.

Phase behaviour of inverse patchy colloids: effect of the model parameters

The phase behaviour of inverse patchy colloid systems composed of spherical particles with two oppositely charged patches at the poles is investigated by simulation-based thermodynamic integration schemes. The interaction between the particles is derived via a coarse-grained model characterized by three system parameters: the charge imbalance between the bare colloid and the patches, the patch surface extension and the particle interaction range. Starting from a set of parameters for which a stacking of parallel layers is thermodynamically stable, the effect of each of these three parameters on the phase diagram is studied. Our results show that the region of stability of the layered solid phase can be expanded by increasing the charge imbalance and/or by reducing the interaction range. A larger patch size, on the other hand, stabilizes the layered structure with respect to the competing face centered cubic solid at high pressures but destabilizes it with respect to the fluid phase at low pressures. The location of the liquid-vapour critical point in the temperature versus density plane is also investigated: while the charge imbalance and the patch size affect mainly the critical density, a change of the interaction range has a substantial impact also on the critical temperature.

Biopolymer nanoparticles designed for polyunsaturated fatty acid vehiculization: Protein–polysaccharide ratio study

Information about the design of biopolymer nanoparticles (BNPs) for polyunsaturated fatty acid (PUFA) vehiculization is provided. Linoleic acid (LA) was used as a model PUFA. The binding ability of LA to β-lactoglobulin (BLG) was applied for obtaining BLG–LA complexes. BLG–LA complex formation was monitored by fluorimetry and it was observed that a moderate heat treatment (60°C, 10min) enhanced BLG–LA complexation. Obtaining BNPs involved the electrostatic deposition of high methoxyl pectin (HMP) onto the BLG–LA complex surface. The phase behavior of biopolymer systems was discussed at different Prot:HMP ratio (RProt:HMP, wt.%) levels (1:1–6:1). Absorbance at 600nm, particle size, and ζ potential were analyzed at pH 4.0. At 1:1–2:1 RProt:HMP, BNPs showed appreciable turbidity, a nanometric diameter (337–364nm), and a negative ζ potential. Finally, intrinsic and extrinsic fluorimetry was used for examining the HMP protective role at the LA binding site. At 2:1 RProt:HMP, HMP cover could promote significant LA protection in BNPs.

Extended law of corresponding states for protein solutions.

The so-called extended law of corresponding states, as proposed by Noro and Frenkel [J. Chem. Phys. 113, 2941 (2000)], involves a mapping of the phase behaviors of systems with short-range attractive interactions. While it has already extensively been applied to various model potentials, here we test its applicability to protein solutions with their complex interactions. We successfully map their experimentally determined metastable gas-liquid binodals, as available in the literature, to the binodals of short-range square-well fluids, as determined by previous as well as new Monte Carlo simulations. This is achieved by representing the binodals as a function of the temperature scaled with the critical temperature (or as a function of the reduced second virial coefficient) and the concentration scaled by the cube of an effective particle diameter, where the scalings take into account the attractive and repulsive contributions to the interaction potential, respectively. The scaled binodals of the protein solutions coincide with simulation data of the adhesive hard-sphere fluid. Furthermore, once the repulsive contributions are taken into account by the effective particle diameter, the temperature dependence of the reduced second virial coefficients follows a master curve that corresponds to a linear temperature dependence of the depth of the square-well potential. We moreover demonstrate that, based on this approach and cloud-point measurements only, second virial coefficients can be estimated, which we show to agree with values determined by light scattering or by Derjaguin-Landau-Verwey-Overbeek (DLVO)-based calculations.

Effects of functional groups on the solubilities of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide

The phase behavior of the binary systems containing CO2 and various hybrid polyhedral oligomeric silsesquioxanes (POSS) with different functional groups were investigated. A high-pressure view cell was used to measure the dew-points or cloud-points at temperatures between 308 and 323K, up to 30MPa. Among the studied POSS structures, methacryl, isooctyl and octaisobutyl POSS were observed to form homogenous solutions with CO2, while octamethyl POSS was found to be insoluble in the supercritical fluid. The solubilities of both methacryl and isooctyl POSS in scCO2 decrease with increasing temperature, while isooctyl POSS exhibits nearly an order of magnitude higher solubility than methacryl POSS at both temperatures. The octaisobutyl POSS-CO2 binary system exhibits a crossover pressure, below which the solubility of the component decreases with increasing temperature. Above this pressure, as the temperature is increased, the solubility of POSS in scCO2 increases. These results show that the functional groups of POSS affect its solubility in scCO2 and the phase behavior of the POSS-CO2 binary system significantly. The scCO2-soluble POSS can be implemented in environmentally benign material processing applications using the supercritical fluid.

Phase behavior and characterization of the poly(methyl methacrylate-co-octafluoropentyl methacrylate) [P(MMA-co-OFPMA)] by supercritical dispersion polymerization

Various industries have been moving away from the use of organic solvents because of environmental regulations on safety hazards. To reduce the consumption of organic solvent to the minimum, a number of studies have been carried out using supercritical carbon dioxide (scCO2) as reaction medium to synthesize a variety of polymers in a heterogeneous reaction system. In this study, poly(methyl methacrylate-co-2,2,3,3,4,4,5,5-octafluoropentyl methacrylate) [P(MMA-co-OFPMA)] as a fluoric copolymer was prepared using supercritical dispersion polymerization in scCO2. The characterization for the prepared P(MMA-co-OFPMA) was investigated using the particle size and shape, the molecular weight (Mw), and thermal analysis. In addition, Pressure isotherms for phase behavior of binary and ternary systems between P(MMA-co-OFPMA) and association liquid fluid as well as the influence of quality of association solvents, weight average molecular weight (Mw), and cosolvents were measured.The experimental data of high pressure phase behavior for the P(MMA-co-OFPMA) in supercritical CHF3, CH2F2 and CHClF2 are presented for binary and ternary mixtures at temperatures ranging from 334K to 454K and pressures up to 194MPa. Changes of the pressure–temperature (p–T) slope of cloud–point curves for the P(MMA-co-OFPMA) [25:1]+CH2F2+methyl methacrylate (MMA) (or CHClF2) systems were measured and compared, along with cosolvents MMA (or CHClF2) mass fraction of 0.0∼30.0wt%. The phase behavior data for the P(MMA-co-OFPMA)+CH2F2+0.0∼95.0wt% CHClF2 systems are reported as a change in the p–T curve from upper critical solution temperature (UCST) region to lower critical solution temperature (LCST) region as the CHClF2 mass fraction increases. The cloud–point curves for the P(MMA-co-OFPMA)+CHF3 and +CHClF2 mixtures were measured for a LCST region accordingly as the AIBN (1.0, 2.0, 3.0 and 4.0wt%) and mole ratio (MMA:OFPMA=20:1, 22:1, 25:1 and 30:1) increases.

Mitigating crystallization of saturated FAMEs (fatty acid methyl esters) in biodiesel: 2. The phase behavior of 2-stearoyl diolein–methyl stearate binary system

The phase behavior of a model binary system made of OSO (2-stearoyl diolein) and MeS (methyl stearate) was investigated with differential scanning calorimetry and X-ray diffraction. The study is part of a series of investigations of unconventional additives such as TAGs (triacylglycerols) and dimers of TAGs with a demonstrated potential to significantly alter the crystallization of biodiesel. The TAG (triacylglycerol) was found to be effective in depressing the crystallization onset of the FAME (fatty acid methyl ester) significantly even at low concentration. OSO was shown to affect the crystallization of the mixtures strongly, and to dramatically alter their polymorphism. The system's phase diagram involved marked transformation lines including eutectics and solid–solid transitions. The molecular interactions were evaluated using a simple thermodynamic model. A mechanism for disruption of crystallization was proposed to be dependent on the peculiar geometry of OSO: the “straight” stearic acid participates easily in the lamellar packing of the equally “straight” FAME, whilst its kinked oleic acids effectively halt additional saturated FAMEs from participating due to steric hindrances. The findings of the study indicate that judicious loadings of TAGs which would target biodiesel's saturated FAMEs will have a substantial beneficial effect on the low temperature performance of the fuel.

Phase Behavior and Structure of Systems Based on Mixtures of n‐Hentriacontane and Melissic Acid

AbstractCandelilla wax (CW) organogelation is a promising method for providing structure to solid‐like fat‐based foods, cosmetics, and drug delivery systems. The main component, n‐hentriacontane (C31), defines, to a large extent, the thermal properties of CW. CW is a complex mixture of n‐alkanes, fatty acids, triterpenic alcohols, and fatty alcohols. In this initial examination of the role of CW components on the phase behavior and structure, CW and binary mixtures of C31 and melissic acid (C30) were studied in bulk and in solutions of vegetable oil. Binary mixtures of C31 and C30 presented a predominantly monotectic phase behavior in which the melting temperature of C31 remained constant independently of the mixture composition. Crystal packing of the pure components and mixtures had an orthorhombic subcell, and the microstructures of the mixtures were extremely different from that of the pure components. In solutions with less than 50 % C31, the order–disorder transition observed in bulk C31 was absent. Organogels of C31, C30, or the mixture were softer than those of CW. The increased strength of CW organogels is associated with the relatively small crystal size of this system.

Phase behavior of binary mixture systems of saturated-unsaturated mixed-acid triacylglycerols: effects of glycerol structures and chain-chain interactions.

We systematically examined the phase behavior of binary mixtures of mixed-acid triacylglycerols (TAGs) containing palmitic and oleic acid moieties 1,3-dioleoyl-2-palmitoyl-glycerol (OPO), 1,2-dipalmitoyl-3-oleoyl-rac-glycerol (PPO), and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP), which are widely present in natural fats and are employed in the food, pharmaceutical, and cosmetic industries. Differential scanning calorimetry and X-ray diffraction methods were applied to observe the mixing behavior of PPO/OPO, OOP/OPO, and PPO/OOP under metastable and stable conditions. The results led to three conclusions: (1) Eutectic behavior was observed in PPO/OPO. (2) Molecular compound (MC) crystals were formed in the mixtures of OOP/OPO and PPO/OOP. (3) However, the MC crystals occurred only under metastable conditions and tended to separate into component TAGs to form eutectic mixture systems after 17 months of incubation. These results were contrary to those of previous studies on 1,3-dipalmitoyl-2-oleoyl glycerol (POP)/OPO and POP/PPO in which the MC crystals were thermodynamically stable. We determined that specific molecular interactions may cause this different phase behavior (stability of POP/OPO and POP/PPO MC crystals and metastability of OOP/OPO and PPO/OOP MC crystals). All results confirm the significant effects of molecular structures of glycerol groups, interactions of fatty acid chains, and polymorphism of the component TAGs on the mixing behavior of mixed-acid TAGs.

Phase Behavior of Aqueous Biphasic Systems Composed of Ionic Liquids and Organic Salts

This study evaluated the effects of ionic liquids (ILs) on the formation of aqueous biphasic systems (ABS) with organic salts. Phase diagrams, tie lines, and respective tie line lengths were determined at 298 K and atmospheric pressure. Using imidazolium-based ILs, the effects of the IL anions and salt anions, as well as the addition of dipolar aprotic solvents (DAS) on ABS formation were investigated. In general, the IL with hydrogen bond basicity ranging between 0.38 and 0.60 formed ABS with organic salts. However, those below this range were insoluble, and those above this range did not undergo phase separation. The salt anion capacity to induce ABS increased with increased valence and decreased Gibbs free energy of hydration. Moreover, the biphasic area for the IL-based ABS reduced with increased DAS concentration.

Ternary phase behavior of phytosterol ethoxylate, water, and imidazolium-based ionic liquid systems – Lyotropic liquid crystal formation over a wide range of compositions

Abstract Lyotropic liquid crystal formation from ternary mixtures of nonionic surfactants [phytosterol ethoxylates (BPS-n)], water, and room-temperature ionic liquids [1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)] was evaluated using optical and polarized microscopic observations, small-angle X-ray scattering (SAXS), and rheological measurements. In this study, the number of POE units was varied (BPS-n, n = 10, 20, 30) to form lyotropic liquid crystalline phases over a wide composition range for all of the systems. The major liquid crystalline phases formed for BPS-10, -20, and -30 were lamellar, hexagonal, and cubic phases, respectively. The differences could be explained by the size of the hydrophilic moieties (i.e., a decrease in critical packing parameters through an increase in the number of surfactant POE units). In addition, structural parameters were calculated from SAXS data for the hexagonal liquid crystalline phase with 60 wt% BPS-20. The area occupied by the hydrophilic moieties in the hexagonally packed cylinder micelle decreased with increasing ratio of water to ionic liquid, which indicated that the surfactant molecules were packed more closely in the hexagonal liquid crystalline phase at greater water concentrations. Rheology measurements indicated that ternary mixtures exhibited more elastic behavior at greater water concentrations, which also supports the change in structural parameters suggested by SAXS measurements.

Electronic Band Structures of La2/3Pb1/3Mn2/3(Fe,Co,Ni)1/3O3

Studies were performed to investigate the influence of nanoparticles with magnetic core Fe3O4 on biomembrane model systems, based on DMPC. NP's were synthetized using standard methods [1,2] and characterized by TEM, magnetic (SQUID) and Raman spectroscopies. Samples with 10% DMPC and different concentrations of NP's were examined by ATF-FTIR to establish the impact of NP's on phase behavior of model phospholipids systems. NP's with magnetic core have attractive properties which can be used in biomedical application (for example MRI), so also were done measurements to define the nanotoxicity on model systems at physiological temperature of human body.

Mechanical instability at finite temperature.

Many physical systems including lattices near structural phase transitions, glasses, jammed solids and biopolymer gels have coordination numbers placing them at the edge of mechanical instability. Their properties are determined by an interplay between soft mechanical modes and thermal fluctuations. Here we report our investigation of the mechanical instability in a lattice model at finite temperature T. The model we used is a square lattice with a φ(4) potential between next-nearest-neighbour sites, whose quadratic coefficient κ can be tuned from positive to negative. Using analytical techniques and simulations, we obtain a phase diagram characterizing a first-order transition between the square and the rhombic phase and different regimes of elasticity, as well as an 'order-by-disorder' effect that favours the rhombic over other zigzagging configurations. We expect our study to provide a framework for the investigation of finite-T mechanical and phase behaviour of other systems with a large number of floppy modes.

Investigation on the phase behaviors of aqueous surfactant two-phase systems in a mixture of N-dodecyl-N-methylpiperidinium bromide (C12MDB) and sodium dodecyl sulfate (SDS)

Abstract To study the phase behavior and formation mechanism of an aqueous surfactant two-phase system (ASTP), which was composed of a cationic surfactant N-dodecyl-N-methylpiperidinium bromide (C12MDB) and an anionic surfactant sodium dodecyl sulfate (SDS), an investigation was conducted. The phase diagram for C12MDB/SDS aqueous solutions was mapped, indicating that the ASTP forms between the homogeneous and heterogeneous phase regions in a narrow concentration range. By measurements of surface tension, transmission electron microscopy, dynamic light scattering, and zeta potential, the formation mechanism of the ASTP was proposed, which is attributed to the aggregation of vesicles. Different aggregation behaviors of vesicles lead to phase separation and the formation of the ASTP.

Solution properties and phase behavior of a combination flooding system consisting of hydrophobically amphoteric polyacrylamide, alkyl polyglycoside and n-alcohol at high salinities

Application of polymer/surfactant (SP) combination flooding technique is attracting considerable interest in enhanced oil recovery (EOR).

Crude Oil Fouling: Fluid Dynamics, Reactions and Phase Change

In the present study, the fluid dynamics and phase behavior of crude-oil fouling in a closed-end heat-exchanger is studied. The deposition process associated with fouling is assumed to be due to two routes: asphaltene precipitation, and a two-step chemical reaction. The SAFT-γ Mie theory is employed to describe the phase behavior of an asphaltene-containing crude oil system, which comprises pseudo-components (C13 ∼ C20+). The predicted phase equilibrium constants are used to quantify the asphaltene precipitation rate. A computational fluid dynamics framework is then used to simulate the fouling process, accounting for the multiphase flow dynamics, heat transfer, and the two deposition routes. Fouling is simulated due to the two routes individually and in concert. In the latter case, it is found that the interaction of the two routes is due to the fouling layer adhering to the heat- exchanger walls, which influences heat transfer from the hot walls to the cooler oil in the bulk. The delicate interplay between heat transfer and fluid dynamics, which accompanies the flow, leads to enhancement and suppression of chemical reaction- and precipitation-driven fouling, respectively, and an overall rise in the fouling rate.

Influence of macromolecular precipitants on phase behavior of monoclonal antibodies.

For the successful application of protein crystallization as a downstream step, a profound knowledge of protein phase behavior in solutions is needed. Therefore, a systematic screening was conducted to analyze the influence of macromolecular precipitants in the form of polyethylene glycol (PEG). First, the influence of molecular weight and concentration of PEG at different pH-values were investigated and analyzed in three-dimensional (3-D) phase diagrams to find appropriate conditions in terms of a fast kinetic and crystal size for downstream processing. In comparison to the use of salts as precipitant, PEG was more suitable to obtain compact 3-D crystals over a broad range of conditions, whereby the molecular weight of PEG is, besides the pH-value, the most important parameter. Second, osmotic second virial coefficients as parameters for protein interactions are experimentally determined with static light scattering to gain a deep insight view in the phase behavior on a molecular basis. The PEG-protein solutions were analyzed as a pseudo-one-compartment system. As the precipitant is also a macromolecule, the new approach of analyzing cross-interactions between the protein and the macromolecule PEG in form of the osmotic second cross-virial coefficient (B23 ) was applied. Both parameters help to understand the protein phase behavior. However, a predictive description of protein phase behavior for systems consisting of monoclonal antibodies and PEG as precipitant is not possible, as kinetic phenomena and concentration dependencies were not taken into account.