The liquid–gas interface of oxygen–nitrogen solutions: 1. Surface tension

The capillary method of surface tension measurement has been used to measure the surface tension of oxygen–nitrogen solutions in the temperature range from 80 to 132 K. At temperatures below the nitrogen critical temperature (T_c = 126.2 K) the capillary constant and the surface tension of solutions are smaller than their additive values and vary linearly with the temperature. Experimental data are compared with the results of calculating the surface tension by the theories of Pinnes and Rowlinson.     

Liquid–liquid phase transfer of magnetite nanoparticles

The study presents first experimental results of the transfer of magnetite nanoparticles from an aqueous to a second non-miscible non-aqueous liquid phase. The transfer is based on the adsorption of macromolecular surfactants onto the particle surface at the liquid–liquid interface. For a successful direct phase transfer, it is essential to have cations, like ammonium ions, present in the aqueous phase as well as a threshold concentration of surfactant in the organic liquid phase. While penetrating the liquid–liquid interface, the particles are covered with the surfactant and therefore a partial de-agglomeration is initiated. Based on literature and experimental data a mechanism of surfactant adsorption is proposed. The competing adsorption of the surfactant molecules at the liquid–liquid interface leads to the formation of emulsions and therefore to a hindrance for particles passing the interface. Nevertheless a high efficiency of 100% yield can be reached using optimized process parameters for the phase transfer process.     

Ultrafiltration of triglyceride from biodiesel using the phase diagram of oil–FAME–MeOH

To continuously obtain biodiesel of high purity, a membrane separator integrated with liquid–liquid extraction for the oil–FAME–MeOH system is studied. The liquid–liquid phase equilibrium data for the oil–FAME–MeOH are determined experimentally and compared with the general prediction of the modified UNIFAC. The tie line test demonstrates that composition of the methanol-rich phase is free of TG at 20 °C. Using the continuous cross-flow ultrafiltration, the oil-rich phase can be rejected by the ceramic membranes while the methanol-rich phase permeates through the membranes. When the feed bulk composition is controlled within the two-phase zone, such as the oil:FAME:MeOH of 20:30:50 wt.%, the permeate is found to be free of oil while the obtained permeate flux is higher than 300 kg/m² h under the transmembrane pressure of 600 mmHg and the inlet flow rate of 300 ml/min at 20 °C. By contrast, it shows almost no separation when the inlet concentration of oil–FAME–MeOH locates on its boundary line or within the single-phase zone. The quantitative filtration tests show that the compositions in the two liquid phases and the operating parameters are also considered simultaneously to screen the origin oil and get the FAME product of high purity.     

Surface tension and tolman length of spherical particulate in contact with fluid

The structure, surface tension and Tolman length of particulate-fluid interfaces were studied theoretically. Within the framework of density functional theory, the nonlocal, modified fundamental measure theory and direct correlation function from the first-order mean spherical approximation were incorporated. The theory accurately predicted the structure of fluid and the particulate-vapor surface tensions. The predictions of surface tensions for particulate-liquid interface and particulate in supercritical fluid are also reasonable. Especially, Tolman lengths for particulate-fluid interfaces were investigated systematically. The correct prediction of surface tension from Tolman length indicates that our analysis is reliable. Furthermore, Tolman length as a function of spherical particulate diameter, particulate-fluid interaction energy, and the properties of the fluid is fully discussed.     

High-pressure vapor–liquid and vapor–liquid–liquid equilibria in the carbon dioxide + 1-heptanol system

Vapor–liquid equilibria (VLE) and vapor–liquid–liquid equilibria (VLLE) data for the carbon dioxide + 1-heptanol system were measured at 293.15, 303.15, 313.15, 333.15 and 353.15 K. Phase behavior measurements were made in a high-pressure visual cell with variable volume, based on the static-analytic method. The pressure range under investigation was between 0.58 and 14.02 MPa. The Soave–Redlich–Kwong (SRK)-EOS coupled with Huron–Vidal (HV) mixing rules and a reduced UNIQUAC model, was used in a semi-predictive approach, in order to represent the complex phase behavior (critical curve, LLV line, isothermal VLE, LLE, and VLLE) of the system. The topology of phase behavior is qualitatively correct predicted.     

A study on the liquid–liquid equilibria of 1-alkyl-3-methylimidazolium hexafluorophosphate with ethanol and alkanes

This work demonstrates the ability of the 1-alkyl-3-methylimidazolium hexafluorophosphate to act as an extraction solvent in petrochemical processes for the removal of alkanes from their azeotropic mixture with ethanol. LLE (liquid–liquid equilibrium) of the ternary systems hexane + ethanol + 1-hexyl-3-methylimidazolium hexafluorophosphate (HMIM PF₆) or 1-octyl-3-methylimidazolium hexafluorophosphate (OMIM PF₆) and heptane + ethanol + OMIM PF₆ are carried out at 298.15 K and atmospheric pressure. Experimental liquid–liquid data are correlated by using different equations. The solute distribution ratio and the selectivity, determined from tie-line data, suggest the efficiency of the ILs used as solvents. A comparison with other IL, in terms of solvent capacity, is included. The liquid–liquid extraction process is simulated by using conventional software and the obtained results are shown.     

Detachment of colloids from a solid surface by a moving air-water interface

Colloid attachment to liquid–gas interfaces is an important process used in industrial applications to separate suspended colloids from the fluid phase. Moving gas bubbles can also be used to remove colloidal dust from surfaces. Similarly, moving liquid–gas interfaces lead to colloid mobilization in the natural subsurface environment, such as in soils and sediments. The objective of this study was to quantify the effect of moving air–water interfaces on the detachment of colloids deposited on an air-dried glass surface, as a function of colloidal properties and interface velocity. We selected four types of polystyrene colloids (positive and negative surface charge, hydrophilic and hydrophobic). The colloids were deposited on clean microscope glass slides using a flow-through deposition chamber. Air–water interfaces were passed over the colloid-deposited glass slides, and we varied the number of passages and the interface velocity. The amounts of colloids deposited on the glass slides were visualized using confocal laser scanning microscopy and quantified by image analysis. Our results showed that colloids attached under unfavorable conditions were removed in significantly greater amounts than those attached under favorable conditions. Hydrophobic colloids were detached more than hydrophilic colloids. The effect of the air–water interface on colloid removal was most pronounced for the first two passages of the air–water interface. Subsequent passages of air–water interfaces over the colloid-deposited glass slides did not cause significant additional colloid removal. Increasing interface velocity led to decreased colloid removal. The force balances, calculated from theory, supported the experimental findings, and highlight the dominance of detachment forces (surface tension forces) over the attachment forces (DLVO forces).     

Liquid–liquid equilibria of aromatics removal from middle distillate using NMP

We investigated the removal of aromatics from a typical Kuwait middle distillate using liquid–liquid extraction. A multicomponent system; dodecane + hexadecane + 1,3,5-trimethyl benzene (mesitylene) + butylbenzene; was used to represent the middle distillate mixture. N-methyl-2-pyrrolidone (NMP) was used as a solvent. The liquid–liquid equilibria (LLE) of the mixture was studied over a temperature range of 293–323 K and at different solvent to feed ratios (1.35–1.97). The selectivity and distribution coefficient of the solvent were correlated successfully using the universal quasi-chemical (UNIQUAC) model for the pseudo-ternary system (paraffins (1) + aromatics (2) + NMP (3)) at the studied temperature range. The UNIQUAC model fit the experimental data with an average percentage root mean square deviation (rmds) of 0.2980%.     

On the curvature dependence of the interfacial tension in a symmetric three-component interface

We consider a symmetric interface between two polymers A(N) and B(N) in a common monomeric solvent S using the mean-field Scheutjens-Fleer self-consistent field theory and focus on the curvature dependence of the interfacial tension. In multi-component systems there is not one unique scenario to curve such an interface. We elaborate on this by keeping either the chemical potential of the solvent or the bulk concentration of the solvent fixed, that is we focus on the semi-grand canonical ensemble case. Following Helfrich, we expand the surface tension as a Taylor series in the curvature parameters and find that there is a non-zero linear dependence of the interfacial tension on the mean curvature in both cases. This implies a finite Tolman length. In a thermodynamic analysis we prove that the non-zero Tolman length is related to the adsorption of solvent at the interface. Similar, but not the same, correlations between the solvent adsorption and the Tolman length are found in the two scenarios. This result indicates that one should be careful with symmetry arguments in a Helfrich analysis, in particular for systems that have a finite interfacial tension: one not only should consider the structural symmetry of the interface, but also consider the constraints that are enforced upon imposing the curvature. The volume fraction of solvent, the chain length N as well as the interaction parameter chi(AB) in the system can be used to take the system in the direction of the critical point. The usual critical behavior is found. Both the width of the interface and the Tolman length diverge, whereas the density difference between the two phases, adsorbed amount of solvent at the interface, interfacial tension, spontaneous curvature, mean bending modulus as well as the Gaussian bending modulus vanish upon approach of the critical point.     

Butane isomer transport properties of 6FDA–DAM and MFI–6FDA–DAM mixed matrix membranes

For the first time, we report the C4s’ transport properties: solubility, diffusivity, permeability in 6FDA–DAM polymer, one of most permeable glassy polymers with significant nC4 vs. iC4 selectivity. An nC4 permeability of 3.7 Barrer, and nC4/iC4 ideal selectivity of 21 was found in pure 6FDA–DAM polymer membrane. Mixed matrix films were successfully fabricated using a 6FDA–DAM as the matrix, with up to 35 wt% loading of MFI, modified by a two-step Grignard treatment (GT) that produced Mg(OH)₂ whiskers on the surface of the MFI particles. The permeability of nC4 more than doubled; however, the selectivity for the C4s remained the same. Permeation of mixed matrix films with impermeable GT-uncalcined-MFI agree with Maxwell modeling of films with an impermeable solid loading, thereby supporting the existence of a defect-free interface between MFI particles and the polymer matrix. This indicates that the MFI is too permeable to optimally match the properties of the 6FDA–DAM, which is one of the most permeable selective matrix polymers available. It appears unlikely that any currently known, adequately selective glassy polymer can match the high permeability of nC4 in MFI to enable development of promising composite membrane for the C4s separation based on MFI. Therefore a smaller pore size zeolite is required for a better match.     

Preparation and properties of mechanically alloyed and electrochemically etched porous Ti–6Al–4V

Formation of porous Ti–6Al–4V nanostructure biomaterial was described. The alloy was prepared by mechanical alloying followed by pressing, sintering and subsequent anodic electrochemical etching in 1 M H₃PO₄ + 2% HF electrolyte at 10 V for 30 min. Mechanically alloyed Ti–6Al–4V has nanostructure with grain size of about 35 nm and large grain boundaries volume fraction, which essentially improve etching process. The electrolyte penetrates sintered compacts through the grain boundaries, resulting in effective material removing and pores formation. The pore diameter reaches up to 60 μm, which is very attractive for strong bonding with bone. The anodization of the microcrystalline alloy ingot results in selective etching, revealing of the two-phase structure with relatively flat surface. The corrosion properties were investigated in Ringer’s solution. Mechanically alloyed samples shows worse corrosion resistance than the bulk microcrystalline alloy ingot, but electrochemical etching results in improving corrosion resistance.     

High-pressure phase behaviour of the binary system {CO2 + cis-decalin} from (292.75 to 373.75) K

The phase behaviour of the {CO₂ (1) + cis-decalin (2)} binary system has been experimentally studied at temperatures ranging from (292.75 to 373.75) K. Saturation pressures, ranging from (15.9 to 490.5) bar, were obtained using a variable volume high-pressure cell by visual observation of phase transitions at constant overall composition. For this system, no literature data are available and the results obtained in this study reveal the occurrence of vapor–liquid, liquid–liquid, and vapor–liquid–liquid phase transitions in the investigated temperature range. A total of 133 experimental points are reported including bubble points, dew points, liquid–liquid phase equilibria, and coordinates of the three-phase line. The experimental data can be reasonably predicted by the PPR78 model in which the temperature-dependent binary interaction parameter is calculated by a group contribution method.     

The wettability of polytetrafluoroethylene by aqueous solution of cetylpyridinium bromide and propanol mixtures

Measurements of advancing contact angles (θ) were carried out for aqueous solutions of cetylpyridinium bromide (CPBr) and propanol mixtures at constant CPBr concentration equal to 1 × 10⁻⁵, 1 × 10⁻⁴, 6 × 10⁻⁴, 1 × 10⁻³ M, respectively, on polytetrafluoroethylene (PTFE). The obtained results indicate that the wettability of PTFE by aqueous solutions of these mixtures depends on their composition and concentration. In contrast to Zisman, there is no linear dependence between the cos θ and surface tension of aqueous solutions of CPBr and propanol mixtures (γ_LV), but a linear relationship exists between the adhesion tension and the surface tension of aqueous solutions of CPBr and propanol mixtures which have a slope equal to −1, and between cos θ and the reciprocal of the surface tension of solution. The slope equal to −1 and the intercept on the cos θ axis close to −1 suggest that adsorption of CPBr and propanol mixtures and the orientation of their molecules at aqueous solution–air and PTFE–aqueous solution interfaces are the same. This also suggests that the work of solution adhesion to the PTFE surface does not depend on the concentration of propanol and CPBr. Extrapolation of the straight line to the point corresponding to the surface tension of solution, which completely spreads over the PTFE surface, gives the value of the critical surface tension of PTFE wetting equal to 24.84 mN/m. This value is higher than PTFE surface tension (20.24 mN/m) and the values of the critical surface tension of PTFE wetting determined by other investigators from the contact angle of nonpolar liquids (e.g. n-alkanes). The differences between the value of the critical surface tension obtained here and those which can be found in the literature were discussed on the basis of the simple thermodynamic rules. Using the measured values of the contact angles and Young equation the PTFE–aqueous solution interfacial tension was determined. The values of PTFE–aqueous solution interfacial tension were also calculated from Miller and co-workers equation in which the correction coefficient of nonideality of the surface monolayer was introduced. From comparison of the obtained values it appears that good agreement exists between the values of PTFE–solution interfacial tension calculated on the basis of Young and Miller and co-workers equations in the whole range of propanol concentration.     

Effect of temperature on the density and surface tension of aqueous solutions of HMT

In this study, a systematic study of the effect of the temperature on the density and surface tension of HMT (hexamethylentetramine) in water was developed. The density and surface tension were determined at temperatures of 288.15, 293.15, 298.15, 303.15, and 308.15 K. Precise data of surface tension have not been reported previously in literature. From the density measurements, the apparent molar and partial molar volumes were calculated. The apparent molar volume decreases with concentration, the molar partial volume increases with temperature. The surface tension of the aqueous solutions of HMT decreases with concentration. The excess surface concentration was calculated, the values increase with concentration, indicating that the amount of HMT that goes to the interface gas liquid increases at higher concentrations of HMT.     

Gradient theory computation of the radius-dependent surface tension and nucleation rate for n-nonane clusters

The Van der Waals-Cahn-Hilliard gradient theory (GT) is applied to determine the structure and the work of formation of clusters in supersaturated n-nonane vapor. The results are analyzed as functions of the difference of pressures of the liquid phase and vapor phase in chemical equilibrium, which is a measure for the supersaturation. The surface tension as a function of pressure difference shows first a weak maximum and then decreases monotonically. The computed Tolman length is in agreement with earlier results [L. Granasy, J. Chem. Phys. 109, 9660 (1998)] obtained with a different equation of state. A method based on the Gibbs adsorption equation is developed to check the consistency of GT results (or other simulation techniques providing the work of formation and excess number of molecules), and to enable an efficient interpolation. A cluster model is devised based on the density profile of the planar phase interface. Using this model we analyze the dependency of the surface tension on the pressure difference. We find three major contributions: (i) the effect of asymmetry of the density profile resulting into a linear increase of the surface tension, (ii) the effect of finite thickness of the phase interface resulting into a negative quadratic term, and (iii) the effect of buildup of a low-density tail of the density profile, also contributing as a negative quadratic term. Contributions (i)-(iii) fully explain the dependency of the surface tension on the pressure difference, including the range relevant to nucleation experiments. Contributions (i) and (ii) can be predicted from the planar density profile. The work of formation of noncritical clusters is derived and the nucleation rate is computed. The computed nucleation rates are closer to the experimental nucleation rate results than the classical Becker-D?ring theory, and also the dependence on supersaturation is better predicted.     

Studies of dynamic surface tension of polyoxyethylene alkylphenols at the air–water interface

This paper presents the study of dynamic surface tension of polyoxyethylene alkylphenol surfactants (Igepals) at the air–solution interface. The experimental investigation of the surface tension dynamics are carried out using a pendant drop method for two of the representative alkylphenols (Igepal-630 and Igepal-720) nonionic surfactants. The general trend of the dynamic surface tension for the two surfactants appears to be similar. However, the absolute surface activities are different. Between the two poloxyethylene alkylphenol surfactants, it was found that Igepal-C0-630 has a higher surface activity and a lower critical micelle concentration (CMC) value. This agreed well with their reported hydrophile–lipophile balance (HLB). The equilibrium adsorption parameters for these surfactant systems have been estimation using two different methods and are in good agreement. The theoretical model developed for the surface tension dynamics based on the Statistical Rate Theory (SRT) in our earlier (J. Colloid Interface Sci., 286, 2005, 14–27) work satisfactorily predicted the experimental results for the present systems.     

Enhanced catalytic reaction at an air–liquid–solid triphase interface

Gaseous reactant involved heterogeneous catalysis is critical to the development of clean energy, environmental management, health monitoring, and chemical synthesis. However, in traditional heterogeneous catalysis with liquid–solid diphase reaction interfaces, the low solubility and slow transport of gaseous reactants strongly restrict the reaction efficiency. In this minireview, we summarize recent advances in tackling these drawbacks by designing catalytic systems with an air–liquid–solid triphase joint interface. At the triphase interface, abundant gaseous reactants can directly transport from the air phase to the reaction centre to overcome the limitations of low solubility and slow transport of the dissolved gas in liquid–solid diphase reaction systems. By constructing a triphase interface, the efficiency and/or selectivity of photocatalytic reactions, enzymatic reactions, and (photo)electrochemical reactions with consumption of gaseous reactants oxygen, carbon dioxide, and nitrogen are significantly improved.

Gaseous reactant involved liquid–solid diphase interface reactions can be significantly enhanced using rationally designed and constructed air–liquid–solid triphase systems.     

The influence of calcination on the size of nanocrystals, porous structure and acid–base properties of mesoporous anatase used as catalytic support

Mesoporous anatase was prepared following sol–gel and using urea as template. The influence of calcination temperature on the phase stability, nanocrystal/aggregate size, pore size distribution and specific surface area as well as on the acid–base behavior in aqueous solutions was studied using X-ray diffraction, laser-Raman and diffuse reflectance spectroscopies, scanning electron microscopy and laser scattering as well as N₂ adsorption–desorption isotherms and potentiometric mass titrations.The crystal structure was kept constant upon calcination over the whole temperature range, 200–500 °C. In this range anatase is constituted from primary nanocrystals. These are assembled into larger, rather spherical, clusters of about 30–40 nm and then into aggregates of various sizes (0.2–0.3 μm and 2–100 μm) with a distribution centered at about 12 μm. Increase of the calcination temperature caused an increase in the size of the primary nanocrystals from 8.1 nm at 200 °C to 17.1 nm at 500 °C, whereas calcination does not influence the morphology at micro-scale. Moreover, increase of the calcination temperature from 200 °C to 500 °C brings about a shift in the mean pore diameter from 47 nm to 91 nm accompanied by a decrease in the specific surface area and pore volume. The above effects were related with the aforementioned increase in the size of the primary nanocrystals. The value of pzc and the values of surface charge determined at various pH do not practically depend on the calcination temperature. The absence of pore space confinement effects was explained in terms of the structure and size of the interface development between the anatase surface and the electrolytic solution.     

Liquid–liquid equilibria for pseudo-ternary systems: (Sulfolane + 2-ethoxyethanol) + octane + toluene at 293.15 K

Liquid–liquid equilibrium data, both binodal and tie lines are presented for the pseudo-ternary systems: {(sulfolane + 2-ethoxyethano) (1) + octane (2) + toluene (3)} at 293.15 K. The experimental liquid–liquid equilibrium data have been correlated using NRTL and UNIQUAC models, and the binary interaction parameters of these components have been presented. The correlated tie lines have been compared with the experimental data. The comparisons indicate that both NRTL and UNIQUAC models satisfactorily correlated the equilibrium compositions. The tie-line data of the studied systems also were correlated using the Hand method.