One-phase Region Research Articles

Force density induced by preferential adsorption in a near-critical binary fluid mixture subject to the Soret effect.

We assume that two parallel plates are immersed in a binary fluid mixture lying in the one-phase region near the demixing critical point and that the surface of each plate attracts the mixture components differently via short-range interactions. It is known that the composition inhomogeneity caused by the difference can induce a force exerted on the plate at equilibrium. In the present study, we investigate how a temperature gradient imposed vertically on the plates changes the induced force by calculating the composition profile subject to the Soret effect. Numerically solving the derived differential equation, we show that a temperature gradient within the critical regime can change the force distinctly from its equilibrium value and can make the force direction opposite to the one at equilibrium.

PEG600 induced krill oil-based nanoemulsion system: ternary phase behaviour and cytotoxicity assessment

BackgroundEndogenous substances of krill oil (KO) are lipophilic in nature and have clinical significance viz. DHA/EPA, phospholipids and astaxanthin. To improve the nanodispersibility of endogenous substances of KO, a self-nanoemulsifying system (SNE) was developed.ResultsTernary phase behaviour of KO was explored in ethanol (ET); propylene glycol, (PG); and PEG600 using Tween80 and Tween20 as surfactants. PEG600 induced the self-nanoemulsification of KO and yielded one phase region (OPR); dilution lines (KO/Smix fraction containing PEG600) traversed across OPR, produced a fully dilutable nanoemulsion system. PEG600-based nanoformulations (NFs) of KO underwent phase transformation via percolation behaviour in nanostructure domains (86–207 nm). PEG600 induced ternary phase behaviour of KO as revealed from rheological data (higher eta values), refractive index (nonlinear) and conductivity (bimodal) patterns. Induced phase transformation could be an interaction between aqueous phase and KO/Tween20 in PEG600 environment; generating highly viscous domains of low electrical conductivity. NFs offered antioxidant activity over corresponding coarse systems (p < 0.01) as measured using DPPH method. Optimized NFs (F4 and F6) inhibited the growth of skin cancer cell line (A431) in the range of 100–500 × dilutions.ConclusionPhase behaviour of KO was induced by PEG600, transforming the dilution pattern via generation of one phase region; however, ethanol and propylene glycol as co-solvents did not. PEG600-based NFs of KO possessed antioxidant as well as cytotoxic to skin cancer cell lines (A431).

Unlocking the charge efficiency of γ’-V2O5 for Na-ion battery through a solution synthesis technique

In this study, the γ’-V2O5 cathode material was prepared through a solution synthesis technique leading to homogeneous, fine and porous particles 100–200 nm in size. This successful preparation allows to overcome the huge drawback of the microsized material in terms of charge efficiency and to take benefit of the attractive Na insertion properties of γ’-V2O5, i. e. a significant available capacity of 145 mAh g- 1, a high working potential of about 3.25 V vs. Na+/Na, an excellent charge efficiency, a high-rate capability and good cycle life. A detailed structural study upon Na insertion/extraction shows that the proposed nanosizing approach promotes a homogeneous Na solubility and solid solution behavior in a wider composition range (0.4 < x ≤ 1 in γ-NaxV2O5) compared to the results previously reported for solid-state synthesized γ’-V2O5. Furthermore, highly reversible structural changes are evidenced. Key kinetic parameters governing the Na insertion-extraction reaction are discussed thanks to an impedance spectroscopy study revealing a faster Na diffusivity in the one-phase region. The obtained results allow a comprehensive understanding of the enhanced performance exhibited by the present sub-micronic γ’-V2O5 material.

Dynamic Behavior of Poly(N-isopropylmethacrylamide) in Neat Water and in Water/Methanol Mixtures.

We investigate the collective dynamics of thermoresponsive polymer poly(N-isopropylmethacrylamide) (PNIPMAM) in aqueous solution and in water/methanol mixtures in the one-phase region. In neat water, the polymer concentration c is varied in a wide range around the overlap concentration c*, that is estimated at 23 g L-1. Using dynamic light scattering (DLS), two decays ("modes") are consistently observed in the intensity autocorrelation functions for c = 2-150 g L-1 with relaxation rates which are proportional to the square of the momentum transfer. Below c*, these are attributed to the diffusion of single chains and to clusters from PNIPMAM that are formed due to hydrophobic interactions. Above c*, they are assigned to the diffusion of the chain segments between overlap points and to long-range concentration fluctuations. From the temperature-dependent behavior of the overall scattering intensities and the dynamic correlation lengths of the fast mode, the critical temperatures and the scaling exponents are determined. The latter are significantly lower than the static values predicted by mean-field theory, which may be related to the presence of the large-scale inhomogeneities. The effect of the cosolvent methanol on the dynamics is investigated for polymer solutions having c = 30 g L-1 and methanol volume fractions in the solvent mixtures of up to 60 vol %. The phase diagram was established by differential scanning calorimetry. The slow mode detected by DLS becomes significantly weaker as methanol is added, i.e., the solutions become more homogeneous. Beyond the minimum of the coexistence line, which is located at 40-50 vol % of methanol, the dynamics is qualitatively different from the one at lower methanol contents. Thus, going from the water-rich to the methanol-rich side of the miscibility gap, the change of interaction of the PNIPMAM chains with the two solvents has a severe effect on the collective dynamics.

Thermo-osmosis of a near-critical binary fluid mixture: A general formulation and universal flow direction.

We consider a binary fluid mixture, which lies in the one-phase region near the demixing critical point, and study its transport through a capillary tube linking two large reservoirs. We assume that short-range interactions cause preferential adsorption of one component onto the tube's wall. The adsorption layer can become much thicker than the molecular size, which enables us to apply hydrodynamics based on a coarse-grained free-energy functional. For transport processes induced by gradients of the pressure, composition, and temperature along a cylindrical tube, we obtain the formulas of the Onsager coefficients to extend our previous results on isothermal transport, assuming the critical composition in the middle of each reservoir in the reference equilibrium state. Among the processes, we focus on thermo-osmosis-mass flow due to a temperature gradient. We explicitly derive a formula for the thermal force density, which is nonvanishing in the adsorption layer and causes thermo-osmosis. This formula for a near-critical binary fluid mixture is an extension of the conventional formula for a one-component fluid, expressed in terms of local excess enthalpy. We predict that the direction of thermo-osmotic flow of a mixture near the upper (lower) consolute point is the same as (opposite to) that of the temperature gradient, irrespective of which component is adsorbed on the wall. Our procedure would also be applied to dynamics of a soft material, whose mesoscopic inhomogeneity can be described by a coarse-grained free-energy functional.

Thermoresponsive Polymers under Pressure with a Focus on Poly(N-isopropylacrylamide) (PNIPAM).

Pressure is a key variable in the phase behavior of responsive polymers, both for applications and from a fundamental point of view. In this feature article, we review recent developments, particularly applications of neutron techniques such as small-angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS), across the temperature-pressure phase diagram. These are complemented by kinetic SANS experiments following pressure jumps. In the prototype system poly(N-isopropylacrylamide) (PNIPAM), QENS revealed the pressure-dependent characteristics of hydration water around the lower critical solution temperature transition. The size, water content, and inner structure of the mesoglobules formed in the two-phase region depend strongly on pressure, as shown by SANS. Beside these changes at the phase transition, the mesoglobule formation at low pressure is determined by kinetic factors, namely the formation of a polymer-rich, rigid shell, which hampers further growth by coalescence. At high pressure, in contrast, the growth proceeds by diffusion-limited coalescence without any kinetic hindrance. The disintegration of the mesoglobules evolves either via chain release from their surface or via swelling, depending on the osmotic pressure of the water. Moreover, we report on the profound influence of pressure on the cononsolvency effect. In the temperature-pressure frame, the one-phase region is hugely expanded upon the addition of the cosolvent methanol. SANS experiments unveil the enthalpic and entropic contributions to the effective Flory-Huggins interaction parameter between the segments and the solvent mixture. QENS experiments demonstrate an increase in polymer associated water with pressure, whereas methanol is released. Correspondingly, the solvent phase becomes enriched in methanol, providing a mechanism for the breakdown of cononsolvency at a high pressure. Finally, we outline future opportunities for high-pressure studies of thermoresponsive polymers, with a focus on neutron methods.

Correction: Large Deviations in Quantum Lattice Systems: One-Phase Region

Correction: Large Deviations in Quantum Lattice Systems: One-Phase Region

Purification and Fractionation of Lignin via ALPHA: Liquid-Liquid Equilibrium for the Lignin-Acetic Acid-Water System.

In order to effectively practice the Aqueous Lignin Purification with Hot Agents (ALPHA) process for lignin purification and fractionation, the temperatures and feed compositions where regions of liquid-liquid equilibrium (LLE) exist must be identified. To this end, pseudo-ternary phase diagrams for the lignin-acetic acid-water system were mapped out at 45-95 °C and various solvent: feed lignin mass ratios (S:F). For a given temperature, the accompanying SL (solid-liquid), SLL (solid-liquid-liquid), and one-phase regions were also located. For the first time, ALPHA using acetic acid (AcOH)-water solutions was applied to a lignin recovered via the commercial LignoBoost process. In addition to determining tie-line compositions for the two regions of LLE that were discovered, the distribution of lignin and key impurities (the latter can negatively impact lignin performance for materials applications) between the two liquid phases wasalso measured. As a representative example, lignin isolated in thelignin-rich phase was reduced 7x in metals and 4x in polysaccharides by using ALPHA with a feed solvent composition of 50-55% AcOH and an S:F of 6:1, with said lignin being obtained at a yield of 50-70% of the feed lignin and having a molecular weight triple that of the feed.

Phase diagram in cyclopentanone + water + secondary butyl alcohol and light scattering investigations near the LCST

We report light scattering investigations and study of phase diagram in the system cyclopentanone (CP) + water (W) + secondary butyl alcohol (sBA), a more promising system for exploring the Quadruple Critical Point (QCP) as compared to methyl ethyl ketone (MEK) + W + sBA. Unlike in MEK + W + sBA [A. P. Naseer and P. K. M. Unni, J. Mol. Liq., 296, 111,776 (2019)], the diameter of the tunnel D≡TU-TL decreases from 66.626 K at XCP≡Xketone = 0.1000 to 24.926 K at Xketone = 0.2500 without addition of a fourth component. Our studies indicate that addition of only 0.0018 weight fraction of KSCN (i.e., 5% of the concentration of KSCN required in case of the system MEK + W + sBA), is sufficient to split the tunnel-like portion into two paraboloids with an intervening one-phase region in between. The tunnel-like feature in the phase diagram of CP + W + sBA occurs for 0.1000≤XCP≤0.2000 and for XCP>0.2000, the tunnel is open towards higher water weight fractions, though the critical line of UCSTs and LCSTs is continuous. Light scattering investigations show Fisher renormalization of the susceptibility exponent γ for 0.1750≤Xketone≡XCP≤0.2500 with a near complete renormalization observed for Xketone≡XCP = 0.2500 and normal Ising exponents observed for the region 0.1750>Xketone≡XCP>0.2500. Similarly, Ising-like behavior is observed at both low and high XCP weight-fractions while a tendency for mean-field behavior is observed for Xketone = 0.2500. Both these features appear generic to ternary systems involving (ketone + water + secondary butyl alcohol). The correlation length amplitude of critical concentration fluctuation ξ0 starts from a value of 6.5Å for XCP = 0.15 and saturates at 8.5Å in the range 0.175≤XCP≤0.350.

The evolution of equilibrium poly(styrene sulfonate) and dodecyl trimethylammonium bromide supramolecular structure in dilute aqueous solution with increasing surfactant binding

HypothesisIn the last 20 years, it has been demonstrated that oppositely charged polyelectrolyte-surfactant (PE-S) mixtures are prone to forming kinetically arrested non-equilibrium aggregates, which are present in the prepared mixtures from rather low surfactant-to-polymer-repeat-unit ratios. Practically, this means that the PE-S mixtures used for the structural investigations of the formed PE-S complexes are typically a mixture of the primary PE-S complexes and large non-equilibrium aggregates of close to charge-neutral complexes. ExperimentsIn this work, we present a unique approach that allows the preparation of PE-S mixtures in the equilibrium one-phase region (surfactant binding β, is typically below 80%) without forming non-equilibrium aggregates. We used this method to prepare equilibrium, non-aggregated complexes of sodium poly(styrene sulfonate) (NaPSS, Mw = 17 kDa) and dodecyltrimethylammonium bromide (DTAB) (β = 10 – 70%) both in water and in an inert electrolyte (100 mM NaCl). The evolution of the complex structure was monitored by small-angle X-ray scattering (SAXS) as a function of increasing surfactant binding (β), and the measured scattering data were fitted by suitable structural models on an absolute scale where concentrations, compositions, and scattering contrasts calculated from molecular properties are used as restraints. FindingsWe could show that at low binding (β < 30%), the system is a mixture of bare polyelectrolyte coils and NaPSS-DTAB complexes containing a closed surfactant associates of low aggregation number wrapped by the polyelectrolyte chain. Once all polymer chains are occupied by a micelle-like surfactant aggregate, the aggregation number increases linearly with increasing surfactant chemical potential. Using the structural insight provided by the SAXS measurements, we could fit the experimental binding isotherm data with a physically coherent, simple thermodynamic model. Finally, we also compared the stoichiometric NaPSS-DTAB precipitate's structure with the equilibrium complexes' structure.

Thermodynamics of Highly Interacting Blend PCHMA/dPS by TOF-SANS.

We investigate the thermodynamics of a highly interacting blend of poly(cyclohexyl methacrylate)/deuterated poly(styrene) (PCHMA/dPS) with small-angle neutron scattering (SANS). This system is experimentally challenging due to the proximity of the blend phase boundary (>200 °C) and degradation temperatures. To achieve the large wavenumber q-range and flux required for kinetic experiments, we employ a SANS diffractometer in time-of-flight (TOF) mode at a reactor source and ancillary microscopy, calorimetry, and thermal gravimetric analysis. Isothermal SANS data are well described by random-phase approximation (RPA), yielding the second derivative of the free energy of mixing (G″), the effective interaction (χ̅) parameter, and extrapolated spinodal temperatures. Instead of the Cahn-Hilliard-Cook (CHC) framework, temperature (T)-jump experiments within the one-phase region are found to be well described by the RPA at all temperatures away from the glass transition temperature, providing effectively near-equilibrium results. We employ CHC theory to estimate the blend mobility and G″(T) conditions where such an approximation holds. TOF-SANS is then used to precisely resolve G″(T) and χ̅(T) during T-jumps in intervals of a few seconds and overall timescales of a few minutes. PCHMA/dPS emerges as a highly interacting partially miscible blend, with a steep dependence of G″(T) [mol/cm3] = -0.00228 + 1.1821/T [K], which we benchmark against previously reported highly interacting lower critical solution temperature (LCST) polymer blends.

The pressure-induced crystal structure transformations in the high-pressure annealed Bi1− xTbxFeO3 compounds (x = 0.05, 0.1, and 0.3)

The substitution of Bi by rare-earth ions is one of the common approaches for improving the electrical, magnetic, and multiferroic properties of the most studied multiferroic material BiFeO3. In this work, Bi1−xTbxFeO3 compounds with x = 0.05, 0.1, and 0.3 were synthesized using a two-step process: standard solid-state synthesis and high-pressure annealing. The obtained samples were studied by means of x-ray diffraction at normal pressure and neutron powder diffraction at high pressure. It was shown that high-pressure annealing could increase the Tb solubility limit to 10 at. %. It is proposed that the maximum solubility limit is even higher and could be achieved with high-pressure annealing in bulk samples. The transition from the R3c phase to the Pnma phase for the compounds with x = 0.05, 0.1 occurs through a two-phase region and starts at P≈4.4 and 1.7 GPa, respectively. The Pnma phase is stable in the compound with x = 0.3 up to P≈3.2 GPa. The values of Fe magnetic moments decrease with an increase in the Tb concentration or with external pressure for the compounds with x=0.05,0.3 in one-phase regions. The results will help to optimize the synthesis of multiferroic materials with improved magnetoelectric coupling for use in technological applications.

Fabrication of Stable Oleofoams with Sorbitan Ester Surfactants.

Sorbitan esters have been extensively used as surfactants to stabilize emulsions in many fields. However, the preparation of an oleofoam with sorbitan ester alone has not been reported. Here, we apply a novel protocol to fabricate stable oleofoams of high air volume fraction from mixtures of vegetable oil and sorbitan ester. To incorporate more air bubbles into the oil matrix, aeration is first carried out in the one-phase region at high temperatures, during which the highest over-run can reach 280%. Due to foam instability at high temperatures, the foam is then submitted to rapid cooling, followed by storage at low temperatures. For high-melting sorbitan monostearate, the resulting foams containing many crystal-encased air bubbles are ultrastable to drainage, coarsening, and coalescence for several months. On the contrary, the cooled foams with low-melting sorbitan monooleate go through a gradual decay lasting for more than 1 month. We highlight the importance of hydrogen bond formation between surfactant and oil in enhancing foam stability. The generic nature of the above findings is demonstrated by preparing oil foams with various vegetable oils and sorbitan monooleate.

Off-critical wetting layer divergence at the liquid/vapor interface of binary liquid mixtures.

Surface wetting phenomena impact chemistry, physics, biology, and engineering. The wetting behaviors of partially miscible binary liquid systems are especially complex. Here, we report evidence of universal behavior in the divergence of wetting layer growth at liquid-vapor interfaces of the cyclohexane + aniline, hexane + o-toluidine, and methanol + carbon disulfide systems. Layer growth on the micron scale was followed using visible light scattering from stirred samples. The layer thicknesses were found to diverge with decreasing temperature when coexistence was approached from the one-phase region, but only for solutions richer in the higher density/higher surface tension component. The onset of divergence was <1K above the bulk coexistence temperature; nearer the critical composition, the onset temperature was the critical temperature itself. All three systems showed identical divergent wetting properties after variable normalization. In contrast, no divergent wetting layer formation was seen in the benzene + 1,2-propanediol or water + phenol systems. The mathematical signof the Hamaker constant correlates with the contrasting behaviors. Collectively, these results have implications for theoretical descriptions of adsorption layer growth and crossover behavior, for measurements of complete wetting temperatures, and for practical applications.

Experimental and theoretical studies of solvent bubble nucleation and liberation processes in different heavy crude oil−solvent systems

In this paper, solvent bubble nucleation and liberation processes in the heavy crude oil−CO2 systems, heavy crude oil−CH4 systems and heavy crude oil−C3H8 systems were experimentally studied and theoretically analyzed. First, two respective series of tests were conducted for different heavy crude oil−solvent systems. The first series included eleven conventional isothermal constant-composition-expansion (CCE) tests and the other series consisted of three new isothermal constant-composition-expansion & compression (CCEC) tests. Second, the amount of the evolved gas (i.e., the dispersed gas and free gas) in each pressure reduction step was determined from the measured CCE test data to study the solvent bubble nucleation process in each heavy crude oil−solvent system. A new quantity named the bubble nucleation index (BNI) was introduced and used to represent the solvent bubble nucleation strength. Third, the respective amounts of the dispersed gas and free gas in each pressure reduction step were obtained from the measured CCEC test data to examine the solvent bubble liberation processes in three heavy crude oil−solvent systems. A second new quantity named the bubble liberation index (BLI) was defined and applied to represent the solvent bubble liberation strength. It was found from the CCE and CCEC tests that the measured Pcell−vt data for each heavy crude oil−solvent system had three distinct regions. Region I was the one-phase region. Region II was the foamy-oil region, in which the solvent bubble nucleation started and the solvent bubbles were dispersed in the heavy oil. Region III was the two-phase region, in which the free-gas phase was formed and started to dominate the total compressibility of the heavy crude oil−solvent system. In addition, the solvent supersaturation vs. reduced pressure data indicated that the heavy crude oil−CH4 system had the lowest solvent supersaturation at the same reduced pressure in comparison with the heavy crude oil−CO2 or C3H8 system. Thus CH4 was easier to be nucleated from the heavy oil in comparison with CO2 or C3H8. Moreover, the BNI vs. solvent supersaturation data showed that the BNI of the heavy crude oil−CH4 or C3H8 system was slowly increased at lower solvent supersaturations but quickly increased at higher solvent supersaturations, whereas the BNI of the heavy crude oil−CO2 system was almost linearly increased with solvent supersaturation. Furthermore, the BLI vs. reduced pressure data revealed that in comparison with C3H8/CO2, CH4 was the most difficult solvent to be liberated from the heavy oil once its bubbles were nucleated. A large amount of CH4 bubbles could be trapped in the heavy oil to induce the strongest and most stable foamy oil in comparison with C3H8 and CO2. The above experimental findings help to better understand the foamy-oil strengths and stabilities in different heavy crude oil−solvent systems and determine the most suitable solvent to optimize a solvent-based enhanced oil recovery (EOR) process in a heavy oil reservoir.

Cononsolvency of the responsive polymer poly(N-isopropylacrylamide) in water/methanol mixtures: a dynamic light scattering study of the effect of pressure on the collective dynamics

The collective dynamics of 25 wt% poly(N-isopropylacrylamide) (PNIPAM) solutions in water or an 80:20 v/v water/methanol mixture are investigated in the one-phase region in dependence on pressure and temperature using dynamic light scattering. Throughout, two dynamic modes are observed, the fast one corresponding to the relaxation of the chain segments within the polymer blobs and the slow one to the relaxation of the blobs. A pressure scan in the one-phase region on an aqueous solution at 34.0 °C, i.e., slightly below the maximum of the coexistence line, reveals that the dynamic correlation length of the fast mode increases when the left and the right branch of the coexistence line are approached. Thus, the chains are rather swollen far away from the coexistence line, but contracted near the phase transition. Temperature scans of solutions in neat H2O or in H2O/CD3OD at 0.1, 130, and 200 MPa reveal that the dynamic correlation length of the fast mode shows critical behavior. However, the critical exponents are significantly larger than the value predicted by mean-field theory for the static correlation length, ν = 0.5, and the exponent is significantly larger for the solution in the H2O/CD3OD mixture than in neat H2O.

Late-stage coarsening of oil droplets of excess oil in microemulsions following a temperature quench

Abstract An oil-in-water microemulsion of the system C12E5/water/ n-decane with a fixed surfactant-to-oil volume fraction ratio φs/φo of 0.85 is studied. This clear microemulsion is temperature-quenched from a clear one-phase region (26 °C) with spherical microemulsion droplets to an unstable region (13 °C) where the excess oil phase is in equilibrium with the microemulsion droplets. Using a high-resolution version of a laboratory small-angle X-ray scattering (SAXS) instrument, the evolution of the excess oil and its growth as a function of time is studied. Unlike coarsening in alloys and polymer blends, a maximum in the intensity at a finite scattering vector is not observed. This clearly indicates that the droplet of the excess oil is formed homogeneously and that there is no depletion zone around the formed droplets. The scattering curves are quantitatively analysed as a function of time. The microemulsion droplets are modeled as polydisperse core-shell ellipsoidal particles, using molecular constraints and the oil droplets are modeled as polydisperse spheres. The average radius of the oil droplets increases with a scaling exponent in agreement with that of coarsening of precipitates in alloys and phase separations in polymer blends, suggesting some degree of universality of coarsening processes.

Effect of compositional variability due to thermal instability on critical phenomena in n-hexane + IL mixture

Isochoric heat capacity of binary n-hexane + IL (1-octyl-3-methylimidazolium chloride, abbreviated as [C8C1im]Cl) along the fixed isochore of 255.13 kg∙m−3 as a function of temperature near the critical point of pure solvent (n-hexane) have been measured. The measurements were performed for the selected concentration of 0.0089 mol fraction of IL in the three-, two-, and one-phase regions over a temperature range from (308 to 527) K at pressures to 4 MPa. The measurements were performed using a high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter. The combined expanded uncertainty of the density (ρ), temperature (T), concentration (x), and isochoric heat capacity (CV) measurements at the 0.95 confidence level with a coverage factor of k = 2 is estimated to be 0.0015, 15 mK, 0.0001, and 0.03, respectively. The possibility of the chemical reactions occurring during isochoric heat capacity measurements at high temperatures has been discussed. The onset of the effects of thermal decomposition on the isochoric heat capacity of binary n-hexane + [C8C1im]Cl mixture was found at temperatures above approximately 420 K. The addition of n-hexane into the IL made little change in thermal stability of IL. The liquid–vapor phase transition parameters (TS=509.88 ± 0.05 K, ρS=255.13 kg∙m−3, and PS = 3.1175 MPa) have been measured using the technique based on isochoric heat capacity abrupt behavior. Addition of the IL (0.0089 mol fraction) increases the phase transition temperature of pure n-hexane by 2.17 K, while the phase transition pressure increases by 0.0734 MPa.

A novel strategy to fabricate stable oil foams with sucrose ester surfactant

HypothesisCan a mixture of sucrose ester surfactant in vegetable oil be aerated to yield stable oleofoams? Is foaming achievable from one-phase molecular solutions and/or two-phase crystal dispersions? Does cooling a foam after formation induce surfactant crystallisation and enhance foam stability? ExperimentsConcentrating on extra virgin olive oil, we first study the effect of aeration temperature and surfactant concentration on foamability and foam stability of mixtures cooled from a one-phase oil solution. Based on this, we introduce a strategy to increase foam stability by rapidly cooling foam prepared at high temperature which induces surfactant crystallisation in situ. Differential scanning calorimetry, X-ray diffraction, infra-red spectroscopy, surface tension and rheology are used to elucidate the mechanisms. FindingsUnlike previous reports, both foamability and foam stability decrease upon decreasing the aeration temperature into the two-phase region containing surfactant crystals. At high temperature in the one-phase region, substantial foaming is achieved (over-run 170%) within minutes of whipping but foams ultimately collapse within a week. We show that surfactant molecules are surface-active at high temperature and that hydrogen bonds form between surfactant and oil molecules. Cooling these foams substantially increases foam stability due to both interfacial and bulk surfactant crystallisation. The generic nature of our findings is demonstrated for a range of vegetable oil foams with a maximum over-run of 330% and the absence of drainage, coalescence and disproportionation being achievable.

Phase separation of binary mixtures induced by soft centrifugal fields.

We use the model system ethanol-dodecane to demonstrate that giant critical fluctuations induced by easily accessible weak centrifugal fields as low as 2000g can be observed above the miscibility gap of a binary liquid mixture. Moreover, several degrees above the phase transition, i.e. in the one-phase region, strong gradients of ethanol concentration occur upon centrifugation. In this case, the standard interpretation of sedimentation equilibrium in the analytical ultracentrifuge (AUC) yields an apparent molar mass of ethanol three orders of magnitude higher than the real value. Notably, these composition gradients have no influence on the distribution gradient of solutes such as dyes like Nile red. The thick opaque interphase formed upon centrifugation does not appear as the commonly observed sharp meniscus, but as a turbidity zone, similar to critical opalescence. This layer is a few millimeters thick and separates two fluids with low compositional gradients. All these effects can be qualitatively understood and explained using the Flory-Huggins solution model coupled to classical density functional theory (DFT). In this domain hetero-phase fluctuations can be triggered by gravity even far from the critical point. Taking into account Jean Perrin's approach to external fields in colloids, a self-consistent definition of the Flory effective volume and an explicit calculation of the total free energy per unit volume is possible.