Phase Behavior Of Aqueous Mixtures Research Articles

Phase Behavior of Aqueous Mixtures of Sodium Alginate with Fish Gelatin: Effects of pH and Ionic Strength.

The phase behavior of aqueous mixtures of fish gelatin (FG) and sodium alginate (SA) and complex coacervation phenomena depending on pH, ionic strength, and cation type (Na+, Ca2+) were studied by turbidimetric acid titration, UV spectrophotometry, dynamic light scattering, transmission electron microscopy and scanning electron microscopy for different mass ratios of sodium alginate and gelatin (Z = 0.01-1.00). The boundary pH values determining the formation and dissociation of SA-FG complexes were measured, and we found that the formation of soluble SA-FG complexes occurs in the transition from neutral (pHc) to acidic (pHφ1) conditions. Insoluble complexes formed below pHφ1 separate into distinct phases, and the phenomenon of complex coacervation is thus observed. Formation of the highest number of insoluble SA-FG complexes, based on the value of the absorption maximum, is observed at рHopt and results from strong electrostatic interactions. Then, visible aggregation occurs, and dissociation of the complexes is observed when the next boundary, pHφ2, is reached. As Z increases in the range of SA-FG mass ratios from 0.01 to 1.00, the boundary values of рНc, рHφ1, рHopt, and рHφ2 become more acidic, shifting from 7.0 to 4.6, from 6.8 to 4.3, from 6.6 to 2.8, and from 6.0 to 2.7, respectively. An increase in ionic strength leads to suppression of the electrostatic interaction between the FG and SA molecules, and no complex coacervation is observed at NaCl and CaCl2 concentrations of 50 to 200 mM.

Retraction notice to: Phase behaviour of aqueous mixtures of acetic acid with isomers of xylene [J. Chem. Thermodyn. 67 (2013) 148–152

Retraction notice to: Phase behaviour of aqueous mixtures of acetic acid with isomers of xylene [J. Chem. Thermodyn. 67 (2013) 148–152

Hydrogen-Bonding Polarizable Intermolecular Potential Model for Water.

A polarizable intermolecular potential model with short-range directional hydrogen-bonding interactions was developed for water. The model has a rigid geometry, with bond lengths and angles set to experimental gas-phase values. Dispersion interactions are represented by the Buckingham potential assigned to the oxygen atom, whereas electrostatic interactions are modeled by Gaussian charges. Polarization is handled by a Drude oscillator site, using a negative Gaussian charge attached to the oxygen atom by a harmonic spring. An explicit hydrogen-bonding term is included in the model to account for the effects of charge transfer. The model parameters were optimized to density, configurational energy, pair correlation function, and the dielectric constant of water under ambient conditions, as well as the minimum gas-phase dimer energy. Molecular dynamics and Gibbs ensemble Monte Carlo simulations were performed to evaluate the new model with respect to the thermodynamic and transport properties over a wide range of temperature and pressure conditions. Good agreement between model predictions and experimental data was found for most of the properties studied. The new model yields better performance relative to the majority of existing models and outperforms the BK3 model, which is one of the best polarizable models, for vapor-liquid equilibrium properties, whereas the new model is not better than the BK3 model for representation of other properties. The model can be efficiently simulated with the thermalized Drude oscillator algorithm, resulting in computational costs only 3 times higher than those of the nonpolarizable TIP4P/2005 model, whereas having significantly improved properties. Because it involves only a single Drude oscillator site, the new model is significantly faster than polarizable models with multiple sites. With the explicit inclusion of hydrogen-bond interactions, the model may provide a better description of the phase behavior of aqueous mixtures.

Magnetically Alignable Bicelles with Unprecedented Stability Using Tunable Surfactants Derived from Cholic Acid.

Five novel surfactants were prepared by modifying the three hydroxy groups of sodium cholate with triethylene glycol chains endcapped with an amide (SC-C1 , SC-n C4 , and SC-n C5 ) or a carbamoyl group (SC-On C4 and SC-Ot C4 ). The phase behavior of aqueous mixtures of these surfactants with 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) was systematically studied by 31 P NMR spectroscopy. The surfactants endcapped with carbamate groups (SC-On C4 and SC-Ot C4 ) formed magnetically alignable bicelles over unprecedentedly wide ranges of conditions, in terms of temperature (from 21-23 to >90 °C), lipid/surfactant ratio (from 5 to 8), total lipid content (5-20 wt %), and lipid type [DMPC, 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine (DLPC), or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC)]. In conjunction with appropriate phospholipids, the carbamate-endcapped surfactants afforded unique bicelles, characterized by exceptional thermal stabilities (from 0 to >90 °C), biomimetic lipid compositions (DMPC/POPC=25:75 to 50:50), and extremely large 2 H quadrupole splittings (up to 71 Hz).

The development of unlike induced association-site models to study the phase behaviour of aqueous mixtures comprising acetone, alkanes and alkyl carboxylic acids with the SAFT-γ Mie group contribution methodology

Providing accurate predictions of the thermodynamic properties of highly polar and hydrogen bonding compounds and their mixtures is challenging from a theoretical perspective. The combination of an equation of state (EoS) based on the statistical associating fluid theory (SAFT) with a group contribution (GC) methodology offers both accuracy and predictive capability for the thermodynamic properties of mixtures. In our current work, the SAFT-γ Mie equation of state is used to capture the underlying complexity of systems in which specific interactions (e.g. hydrogen bonding, dipolar interactions, chemical association) play an important role, by incorporating highly versatile association-site schemes to model mixtures in which unlike induced association interactions occur; this is done by assigning to the functional groups a number of association sites that are inactive in the pure fluid, but become active in certain mixtures. We refer to this type of association mechanism as “unlike induced” association and to the sites involved in this interaction as “unlike induced” association sites. The concept of unlike induced association sites is applied here to develop reliable SAFT-γ Mie group contribution models to describe the properties of acetone, alkyl carboxylic acids, and their mixtures with water and n-alkanes. The parameter table of available SAFT-γ Mie models is expanded to incorporate the corresponding group interaction parameters for acetone, which is treated as a molecular group, the carboxyl group COOH, and their unlike interaction group parameters with water, and the methyl CH3, methanediyl CH2, and methanetriyl CH alkyl groups. In particular, one unlike induced site is used with the acetone model to mediate hydrogen bonding of the acetone oxygen in mixtures containing hydrogen bond donors, and two pairs of unlike induced sites are included on the COOH group to mediate hydrogen bond formation in mixtures of carboxylic acids and hydrogen bond donors. The models developed allow for the successful description of the complex fluid-phase behaviour of the relevant binary and ternary mixtures, including accurate predictions of systems which have not been used to determine the model parameters.

RETRACTED: Phase behaviour of aqueous mixtures of acetic acid with isomers of xylene

RETRACTED: Phase behaviour of aqueous mixtures of acetic acid with isomers of xylene

Simultaneous prediction of vapour–liquid and liquid–liquid equilibria (VLE and LLE) of aqueous mixtures with the SAFT-γ group contribution approach

Group contribution (GC) methodologies have been combined with the statistical associating fluid theory (SAFT), to couple the predictive capabilities of GC methods with the accuracy of the SAFT description of complex fluids and fluid mixtures. An example of such an procedure is the SAFT-γ group contribution approach [A. Lymperiadis, C.S. Adjiman, A. Galindo, G. Jackson, J. Chem. Phys., 127 (2007) 234903], which was developed based on the SAFT-VR equation of state and employs a heteronuclear molecular model of fused segments which represent the various chemical groups. In this work we examine the predictive capabilities of the method focusing on the fluid phase behaviour of aqueous mixtures. Within SAFT-γ it is often possible to obtain information for both the like and unlike group interactions from pure component fluid thermophysical data alone. When this is not possible, e.g., in the case of water where the whole molecule represents a distinct chemical “group”, the interactions can be obtained, in the usual activity coefficient GC fashion, from a minimal amount of experimental fluid phase equilibrium data of the appropriate mixtures. The group like and unlike parameters can then be successfully transferred to represent the thermophysical properties and fluid phase behaviour of a wide range of mixtures in a predictive manner. The binary systems investigated in this work are aqueous solutions of alkanes and alkanols. The successful description of the highly non-ideal fluid phase behaviour of systems of this kind with transferable interaction parameters within the SAFT-γ approach and the simultaneous prediction of both vapour–liquid and liquid–liquid equilibria using the same parameters are the key contributions of our work.

Thermotropic phase behavior in aqueous mixtures of dioctadecyldimethylammonium bromide and alkyltrimethylammonium bromide surfactant series studied by differential scanning calorimetry

The effect of the micelle-forming surfactant series alkyltrimethylammonium bromide (C n TAB, n = 12, 14, 16 and 18) on the thermotropic phase behavior of dioctadecyldimethylammonium bromide (DODAB) vesicles in water was investigated by differential scanning calorimetry at constant 5.0 mM total surfactant concentration and varying individual surfactant concentrations. The pre-, post- and main transition temperatures ( T s, T p and T m), melting enthalpy (Δ H) and peak width of the main transition (Δ T 1/2) are reported as a function of the surfactant molar fraction. No clear dependence of these parameters on the C n TAB chain length was found. At 5 mM, neat DODAB in water exhibits two transition temperatures, T s = 32.1 and T m = 42.7 °C, as obtained from the DSC upscans, but not a clear T p. For every n, except n = 12, T s vanishes as C n TAB concentration increases and approaches CMC. T m behaves differently for different n, the longer C 14TAB and C 16TAB decrease, while C 18TAB increases T m with increasing concentration. The data indicate that changes in T m, T s, T p and Δ H of the transition are related not only to the extent of C n TAB affinity to DODAB but also to the surfactant chain length. Accordingly, C 18TAB yields a more compact bilayer, thus increasing T m, while C 14TAB and C 16TAB yield a less organized bilayer and reduce T m. C 12TAB does not much affect T s and T m, although it yields T p ≈ 51.6 °C.

Phase Behavior of Aqueous Mixtures of 2-Phenylbenzimidazole-5-sulfonic Acid and Cetyltrimethylammonium Bromide: Hydrogels, Vesicles, Tubules, and Ribbons

We studied the phase behavior and aggregation in mixed aqueous solutions of the anionic UV-absorber 2-phenylbenzimidazole-5-sulfonic acid sodium salt, PhBSA (Na salt), and the cationic surfactant cetyltrimethylammonium bromide, CTAB. The mixtures of the two components behave similarly to catanionic surfactant mixtures. The samples on the PhBSA-rich side have low viscosity and are turbid. The turbidity, due to uni- and multilamellar vesicles (SUVs and MLVs), increases with the mole ratio of CTAB. The interbilayer distance inside the MLV changes with the mole ratio of the two components from a few 10 nm for the 7:3 (molar ratio of PhBSA, Na salt, to CTAB) system to practically zero for the 5:5 mixture. The latter mixture forms a precipitate within less than 1 h. With the exception of the 5:5 mixture, all samples on the PhBSA-rich side are stable for many days. After that period, within one more day, the turbid vesicle phases are transformed into more or less clear hydrogels. We found that the gelation is due to the formation of very long stiff tubules about 14 nm in diameter, which is independent of the mixing ratio of the samples. The hydrogels and the tubules melt around 45 degrees C. On the CTAB-rich side, the 4:6 sample behaves like the 6:4 sample, whereas at 3:7 a precipitate was found to form shortly after mixing. At still smaller PhBSA (Na salt) to CTAB ratios, only clear, viscoelastic solutions are found that do not change with time. We determined the micellar structures in the samples by cryo-TEM and by SAXS. The rheological properties of the hydrogels and of the viscoelastic samples were characterized by oscillating rheological measurements. DSC measurements indicated that the tubules are in a semicrystalline state and melt at around 45 degrees C. The semicrystalline bilayer of the tubules seems to have a 1:1 composition of PhBSA to CTAB. The excess PhBSA seems to be adsorbed on the tubules. It is assumed that the stiffness of the bilayer of the vesicles and the stiffness of the tubules are due to the stiffness of the PhBSA molecule.

Phase behavior of binary mixture of 1-dodecyl-3-methylimidazolium bromide and water revealed by differential scanning calorimetry and polarized optical microscopy

Phase behavior of aqueous mixture of imidazolium ionic liquid, 1-dodecyl-3-methylimidazolium bromide, was investigated by means of differential scanning calorimetry and polarized optical microscopy. The mixture forms two types of lyotropic liquid-crystalline gels, one is composed of lamellar phase and the other is of hexagonal phase. T – X phase diagram of the mixture was constructed, which defines the regions of various phases appearing in this mixture.

Phase behavior of gemini surfactant hexylene-1,6-bis(dodecyldimethylammonium bromide) and polyelectrolyte NaPAA

The phase behavior of aqueous mixtures of gemini surfactant hexylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) and oppositely charged polyelectrolyte sodium polyacrylate (NaPAA) has been studied experimentally. Compared to the mixtures of the traditional surfactant dodecyltrimethylammonium bromide (DTAB) and NaPAA, the gel phase region in the 12-6-12/NaPAA solution is larger. Element analysis reveals that NaPAA in the gel phase tends to replace the counterions of surfactant micelle and to release its own counterions. Spherical aggregates are observed in either top or bottom gel phase as detected by transmission electron microscopy. The addition of sodium bromide (NaBr) leads to a decrease in the gel phase region and the occurrence of a new cream phase.

Added Surfactant Can Change the Phase Behavior of Aqueous Polymer−Particle Mixtures

The phase behavior of aqueous mixtures of the "clouding" polymer ethyl(hydroxyethyl)cellulose (EHEC) mixed with colloidal particles and surfactants has been studied. These types of mixtures are important in many technical formulations. Two types of particles, polystyrene latex and silica, and two types of EHEC, nonmodified EHEC (N-EHEC) and hydrophobically modified EHEC (HM-EHEC), were studied. The EHECs adsorb to both kinds of particles. Both the amount and the type of added surfactant were seen to dramatically influence the partitioning of the particles between the EHEC-rich and EHEC-poor phases of phase-separated mixtures (above the cloud point temperature). Surfactants that are known not to associate with the EHEC backbone, that is, nonionic surfactants and short-chain cationic surfactants, changed the interaction between EHEC and the colloidal particles from attraction to repulsion above a specific surfactant concentration, resulting in a change in the partitioning of the particles from the EHEC-rich to the EHEC-poor phase. No such particle inversion was observed for ionic surfactants that bind to the EHEC backbone. An analysis considering both the binding of surfactant to EHEC and the competitive adsorption of surfactant to the particle surfaces could rationalize all observations, including the large variations observed, among the studied mixtures, in the surfactant concentration required for particle inversion.

Phase Separation of Aqueous Mixtures of Poly(ethylene oxide) and Dextran

The phase behavior of aqueous mixtures of poly(ethylene oxide) (PEO) and dextran is studied as a function of the polymer concentration, the PEO molar mass, and temperature. The molar mass distributions of the two polymers in the coexisting phases are measured. From the temperature dependence we conclude that the phase separation between PEO and dextran is partially caused by sterical interactions. From the equilibrium phase volumes of the phase-separated mixture and the shape of the temperature−composition phase diagram of PEO and dextran, we conclude that also a decrease of the solvent quality of water for PEO at increasing temperatures is involved. It is suggested that the characteristics of the PEO−water interaction can affect the degree of fractionation. This suggestion is based on the observation that the degree of fractionation is not a simple exponential function of the molar mass. The phase behavior of the mixture PEO/dextran is compared to the previously studied phase behavior of the aqueous mixture of gelatin and dextran.

Aqueous Phase Behavior of Hexaethylene Glycol Dodecyl Ether Studied by Differential Scanning Calorimetry, Fourier Transform Infrared Spectroscopy, and 13C NMR Spectroscopy

The phase behavior of aqueous mixtures of a nonionic surfactant, hexaethylene glycol dodecyl ether (C12E6), was investigated by means of differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy, and 13C nuclear magnetic resonance (13C NMR) spectroscopy. The T−X phase diagram of this system was constructed from the DSC results. Comparison of the phase diagram with those previously obtained for C12E7 and C12E8 demonstrates that with the decrease in the polyoxyethylene (POE) chain length, the region of Lα phase expands toward higher temperature, whereas the H1 region shrinks toward lower temperature. This behavior is well explained in terms of the change in the critical packing parameter of the surfactant molecules associated with the change in the POE chain length. FT-IR results indicate that the ordered conformational structure of both the hydrocarbon and POE chains is kept after the transformation from solid to mesophases. The order-to-disorder transformation of the conformational structure of the chains takes place rather gradually within mesophases with the increase in temperature. This behavior of the chain melting associated with the temperature rise is common to aqueous mixtures of POE-type nonionic surfactants. 13C NMR line widths depend strongly on the phase structure formed by this mixture system: quite broad signals are observed for H1 and Lα phases, whereas the signals for V1 phase are rather sharp. According to the two-step model for NMR relaxation in aqueous surfactant systems, the line broadening observed in H1 and Lα phases is attributed to the anisometry of the molecular assemblies in these phases.

FT-IR and ESR Spin-Label Studies of Mesomorphic Phases Formed in Aqueous Mixtures of Heptaethylene Glycol Dodecyl Ether

The phase behavior of aqueous mixtures of heptaethylene glycol dodecyl ether (C12E7) was studied by the use of Fourier transform infrared spectroscopy and electron spin resonance spin-label techniques, stressing the conformational structure of the surfactant molecules and the dynamic aspects of the molecular assemblies in various phases assumed by this mixture system. When the mixture transforms from solid to mesophases, the hydrogen bonds between terminal OH groups of polyoxyethylene (POE) chains in the surfactant molecules and also between the POE chain and water molecules are mostly broken, whereas the conformational structure of the alkyl and POE chains remains still highly ordered. The order−disorder transformation of the chains is induced by the temperature rise in the mesomorphic phases. The microviscosity of the V1 phase reported by a spin probe was the lowest among the three mesophases assumed by this mixture system, although the bulk viscosity of the V1 phase is higher than those of the other two phases, H1 and Lα. This suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer constituting the bicontinuous network structure in the V1 phase. No definite correlation was found between the conformational structure of the surfactant molecule and the order parameter derived from the spin-label study. This implies that the dynamic properties of the surfactant molecular assemblies are determined mostly by the molecular packing in the assemblies and are rather insensitive to the conformational structure of the constituent surfactant molecules.

Viscosity, microstructure and phase behavior of aqueous mixtures of commercial milk protein products and xanthan gum

The behavior of commercial milk protein/xanthan mixtures was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate and whey protein isolate were considered. For the xanthan concentrations used, up to 1wt%, the viscosity of the mixtures was dominated by the viscosity of xanthan. Mixtures of xanthan with skim milk powder or milk protein concentrate showed phase separation, as seen by confocal micrographs, and phase diagrams have been established for these two systems. No visible phase separation was observed in the case of mixtures of sodium caseinate or whey protein isolate systems. However, mixtures of sodium caseinate and xanthan, under certain conditions, showed formation of ‘thread-like’ xanthan-rich regions by confocal microscopy. We believe that the phase separation occurring in milk protein concentrate/xanthan or skim milk powder/xanthan mixtures was a result of depletion flocculation of casein micelles by the xanthan macromolecules, but thermodynamic incompatibility was likely to occur in sodium caseinate/xanthan mixtures.

Phase Behavior of Aqueous Mixtures of Some Polyethylene Glycol Decyl Ethers Revealed by DSC and FT-IR Measurements

Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) were used to investigate the phase behavior of aqueous binary mixtures of three polyoxyethylene type nonionic surfactants, penta-, hexa-, and octaethylene glycol decyl ethers (C10E5, C10E6, and C10E8, respectively) in the temperature range −40–70°C. Many thermotropic transitions among various phases assumed by these mixture systems, including mesomorphic phases, were detected in DSC thermograms, from which theT–Xphase diagrams were constructed with the aid of polarized optical microscopic observation to characterize the mesomorphic phases. It was revealed by FT-IR measurements that the phase compounds are formed in a solid phase between these surfactants and water molecules; the compositions of the compounds for the three surfactant species are expressed as C10E5· 10H2O, C10E6· 12H2O, and C10E8· 16H2O, respectively. The stoichiometry of the phase compounds clearly demonstrates that just two water molecules are bound per oxyethylene unit of polyoxyethylene (POE) chain of the surfactants in the solid phase, probably due to the hydrogen bonding. It was also found that the phase state of the mixture is reflected in the wavenumber of the absorption maximum associated with the OH stretching vibration, νOH, although no significant difference was appreciable in νOHamong mesomorphic phases. The νOHdecreased in the orderL(liquid phase) >H1(normal hexagonal phase) ≈V1(normal cubic phase) ≈Lα(lamellar phase) > solid phase, which indicates that the hydrogen bonding between water and the POE chain of the surfactants becomes stronger in the sequence liquid < mesophase < solid.

Phase behavior of aqueous gelatin/oligosaccharide mixtures

The phase behavior of aqueous mixtures of gelatin and oligosaccharides above their gelation temperature is investigated experimentally, and rationalized according to a simple multicomponent Flory-Huggins model. When the gelatin is only weakly charged, entropic considerations dominate and it is found that the cloud point curve of the mixtures is extremely sensitive to the molecular weight distribution of the oligosaccharide. Even very small quantities of long-chain oligosaccharides present in an otherwise short-chain oligosaccharide population can radically reduce the compatibility. Added salt does not significantly affect the phase diagram, although a strong effect on the kinetics of phase separation is seen. Lowering the pH increases the electrostatic charge on the gelatin and strongly enhances the compatibility. Because the kinetics of gelation and phase separation are different, gelation can freeze in nonequilibrium states. Therefore, all phase diagrams were determined well above the gelation temperature (about 37°C). © 1997 John Wiley & Sons, Inc. Biopoly 41: 607–622, 1997

Phase Behavior of Aqueous Mixtures of Cetyltrimethylammonium Bromide (CTAB) and Sodium Octyl Sulfate (SOS)

The phase behavior and aggregate morphology of mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored with cryotransmission electron microscopy, quasielastic light scattering, and surface tensiometry. Differences in the lengths of the two hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phases), and vesicles form spontaneously over a wide range of compositions in both CTAB-rich and SOS-rich solutions. Bilayer properties of the vesicles depend on the ratio of CTAB to SOS, with CTAB-rich bilayers stiffer than SOS-rich ones. We observe two modes of microstructural transition between micelles and vesicles. The first transition, between rodlike micelles and vesicles, is first order, and so there is macroscopic phase separation. This transition occurs in CTAB-rich solutions and in SOS-rich solutions at higher surfactant concentrations. In the second transition mode, mixtures...

A new CSLM-based method for determination of the phase behaviour of aqueous mixtures of biopolymers

On mixing different types of high molecular weight (bio)polymers in an aqueous solution, phase separation often occurs. In some cases, the occurrence of phase separation may be readily observed, because due to density differences the heavier of the two phases is accumulated at the bottom of the vessel in which the mixture is contained. By using classical techniques, the composition of the two phases may then be determined. In the case where the density differences are not so large, and the viscosity of the system is high, the two phases remain intimately mixed. An alternative route to determine the phase behaviour of these systems might be a microscopic technique (Confocal Scanning Laser Microscopy, CSLM), using the fluorescence intensity of labelled biopolymers to quantify their concentration and phase volume in the system. Experiments were performed with several mixtures of sodium alginate, labelled with fluorescein, and sodium caseinate, fluorescently labelled with Texas Red. The viscosity of the mixtures studied was low enough to allow bulk phase separation of the phases by using an ultracentrifuge. Results of the phase volumes, and the composition of the phases, obtained independently by applying the two different methods (CSLM, or analysis of the separate phases after centrifugation) were compared and found to be in reasonable agreement.