Water-dodecane Interface Research Articles

Using Faraday Waves to Measure Interfacial Tension.

We use Faraday waves to measure interfacial tension σ between two immiscible fluids, with an interest in (ultra)low values of σ. The waves are excited by vertically oscillating the container in which the fluids reside. Using linear stability theory, we map out the accessible range of interfacial tensions. The smallest value (σmin ≈ 5 × 10–4 N/m) is limited by the joint influence of gravity and viscous dissipation. A further limitation is posed by the greatest accelerations that can be realized in a laboratory. We perform experiments on a water–dodecane interface with an increasing concentration of a surfactant in the water layer that decreases the interfacial tension into the ultralow domain [σ = (10–6 N/m)]. Surprisingly, the smallest measured wavelength is larger by a factor of 2 than that predicted for vanishing σ. We hypothesize the effect of transport of the surfactant in the fluid flow associated with the waves.

Insights into the behavior of six rationally designed peptides based on Escherichia coli's OmpA at the water-dodecane interface.

The Escherichia coli’s membrane protein OmpA has been identified as a potential biosurfactant due to their amphiphilic nature, and their capacity to stabilize emulsions of dodecane in water. In this study, the influence of surfactant type, concentration, preservation time and droplet size on the crystallization of n-dodecane and water, in oil-in-water emulsions stabilized with six rationally designed Escherichia coli’s OmpA-based peptides was investigated. A differential scanning calorimetry (DSC) protocol was established using emulsions stabilized with Tween 20® and Tween 80®. A relationship between the surfactant concentration and the crystallization temperatures of n-dodecane and water was observed, where the crystallization temperatures seem to be dependent on the preservation time. A deconvolution analysis shows that the peak morphology possibly depends on the interactions at the interface because the enthalpic contributions of each Gaussian peak remained similar in emulsions stabilized with the same peptide. Adsorption results show that the main driver for adsorption and thus stabilization of emulsions is polar interactions (e.g. H-bonding) through the hydrophilic parts of the peptides. Those peptides with a preponderance of polar interaction groups distribution (i.e. NH2, COOH, imidazole) showed the highest interfacial activity under favorable pH conditions. This suggests that custom-made peptides whose hydrophilic/hydrophobic regions can be fine-tuned depending on the application can be easily produced with the additional advantage of their biodegradable nature.

Adsorption of Sodium Dodecyl Sulfate at Water-Dodecane Interface in Relation to the Oil in Water Emulsion Properties.

The control of the behavior of oil in water emulsions requires deeper investigations on the adsorption properties of the emulsion stabilizers at the interfaces, which are fundamental to explain the (de)stabilization mechanisms. In this work, we present an extensive study on oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) below its critical micellar concentration. Dynamic tensiometry, dilational rheology, and electrical conductivity measurements are used to investigate the adsorption properties at the droplet interface, whereas the aging of the respective emulsions was investigated by monitoring the macroscopic thickness of the emulsion layer, by microimaging and dynamic light scattering (DLS) analysis, to get information on the drop size distribution. In addition, the droplet coalescence is investigated by a microscopy setup. The results of this multitechnique study allow deriving a coherent scenario where the adsorption properties of this ionic surfactant relate to those of the emulsion, such as, for example, the prevention of droplet coalescence and the presence of other mechanisms, such as Ostwald ripening, responsible for the emulsion aging.

Electrokinetic motion of a spherical polystyrene particle at a liquid-fluid interface

Electrokinetic movement of spherical polystyrene particles at different liquid-fluid interfaces was experimentally investigated in this paper. A novel method was developed to place the particles rightly at the interfaces formed in a large plastic container. The velocity was measured by an optical microscope. The experimental results show that the particles (3, 5 and 10μm in diameter) at the water-air interface, water-dodecane interface and NaCl solution-air interface move in the opposite direction of the applied electric field. The magnitude of the particles’ velocity increases linearly with the increase in the applied electric field. Moreover, for particles of the same size, the electrokinetic velocity at the liquid-fluid interfaces is larger than particles’ electrophoretic velocity in the bulk liquid phase. Under the same electric field, however, the electrokinetic velocity of smaller particles at the liquid-fluid interfaces is larger than that of larger particles. Such results are attributed to the surface charges at the liquid-fluid interface and the particle-liquid interface.

Peptides design based on transmembrane Escherichia coli’s OmpA protein through molecular dynamics simulations in water–dodecane interfaces

Recent research efforts have focused on the production of environmentally nonthreatening products, including identifying biosurfactants that can replace conventional surfactants. In order to utilize biosurfactants in different industries such as cosmetic, food or petroleum, it is necessary to understand the underpinnings behind the interactions that could take place for biosurfactants which display potential for interface activity. This work aimed to use molecular dynamics simulations to understand the interactions of rationally obtained peptide sequences from the original sequence of the OmpA gene in Escherichia coli, based on the free energy change (ΔG) during peptide insertion at the water–dodecane interface. Seventeen OmpA–based peptide sequences were selected and analyzed based on their hydropathy index profiles. We found that free energy change due to Columbic interactions and SASA (ΔGCoul/SASA), total free energy change and MW (ΔG/MW), and free energy change due to Coulombic and van der Waals interactions (ΔGCoul/ΔGvdW) ratios could provide a better understating in the contribution of the free energy decrease at the interface. The results indicated that the peptide sequences GKNHDTGVSPVFA and THENQLGAGAFG display biosurfactant potential based on low ΔG per square nanometer, high ΔGCoul/ΔGvdW ratio, clearly defined moieties along its hydrophobic surface and sequence, and the presence of charged residues in the polar head. Clearly defined moieties and SASA were determinant for electrostatic interactions between oil–water interfaces. Experimental validations exhibited that the emulsions prepared remained stable between 3 and 27h, respectively. Even though the peptide GKNHDTGVSPVFA displays strong interactions at the interface, stabilization times showed that the peptide THENQLGAGAFG exhibited the best performance suggesting that the stability can be better described by kinetic rather than thermodynamic criteria once the emulsion is formed.

Temperature- and pH-Dependent Shattering: Insoluble Fatty Ammonium Phosphate Films at Water-Oil Interfaces.

We study the films formed by tetradecylamine (TDA) at the water-dodecane interface in the presence of hydrogen phosphate ions. Using Fourier transform infrared spectroscopy (FTIR), interfacial shear rheology, confocal fluorescence microscopy, cryo-scanning electron microscopy (cryo-SEM), and small-angle neutron scattering (SANS), we find that between pH 5 and 8 tetradecylammonium cations bind to hydrogen phosphate anions to form needle-shaped crystallites of tetradecylammonium hydrogen phosphate (TAHP). These crystallites self-assemble into films with a range of morphologies; below pH 7, they form brittle, continuous sheets, and at pH 8, they form lace-like networks that deform plastically under shear. They are also temperature-responsive: when the system is heated, the film thins and its rheological moduli drop. We find that the temperature response is caused by dissolution of the film in to the bulk fluid phases. Finally, we show that these films can be used to stabilize temperature-responsive water-in-oil emulsions with potential applications in controlled release of active molecules.

Second harmonic generation monitoring of nitric acid extraction by a monoamide at the water–dodecane interface

The interface dynamic properties of a monoamide extractant with potential for application to the front end of the nuclear cycle and to waste treatment are examined by second harmonic generation. The results are compared with bulk nitric ion titration and surface pressure measurements. SH static studies show the extractant reaching the interface and accurately match the IFT measurements. The main feature of the SH dynamic studies is a chaotic fluctuation period, strongly related to intense extraction. Fluctuations are a signature of the interface behaviour during the extraction process. Vertical development of the interface, often called protrusion, remains the most probable origin of the measured fluctuation. Additionally, interfacial measurements show a non-monotonic lag time during extraction, probably related to cooperative effects not observed in the bulk at the working concentration. Such mutual behaviour could be a supplementary prerequisite for the ion transfer across this liquid-liquid interface.

A study of dilational rheological properties of polymers at interfaces

Viscoelastic properties of two polymers, partially hydrolyzed polyacrylamide and partially hydrolyzed modified polyacrylamide, widely used in chemical flooding in the petroleum industry, were investigated at three interfaces, water–air, water–dodecane, and water–crude oil, by means of a dilational method provided by I.T. Concept, France, at 85 °C. Polymer solutions were prepared in brine with 10,000 mg/l sodium chloride and 2000 mg/l calcium chloride. It has been shown that the viscoelastic modulus increases with the increment of polymer concentration in the range of 0–1500 mg/l at the water–air interface. Each polymer shows different viscoelatic behavior at different interfaces. Generally speaking, values of the viscoelastic modulus ( E), the real part ( E′), and the imaginary part ( E″) at the crude oil–water interface for each polymer are lower than at the air–water or water–dodecane interface. The two polymers display different interfacial properties at the same interface. Polymer No. 2 gives more viscous interfaces than polymer No. 1. All the information obtained from this paper will be helpful in understanding the interfacial rheology of ultra-high-molecular-weight polymer solutions.

Rational determination of transfer free energies of small drugs across the water-oil interface.

The application of statistical simulations to the estimation of transfer free energies of pharmacologically relevant organic molecules is reported. Large-scale molecular dynamics simulations have been carried out on a series of four solutes, viz. antipyrine, caffeine, ganciclovir, and alpha-D-glucose, at the water-dodecane interface as a model of a biological water-membrane interfacial system. Agreement with experimentally determined partition coefficients is remarkable, demonstrating that free energy calculations, when executed with appropriate protocols, have reached the maturity to predict thermodynamic quantities of interest to the pharmaceutical world. The computational effort that warrants accurate, converged free energies remains, however, in large measure, incompatible with the high-throughput exploration of large sets of pharmacologically active drugs sought by industrial settings. Compared to the cost-effective, fast estimation of simple partition coefficients, the present free energy calculations, nevertheless, offer a far more detailed information about the underlying energetics of the system when the solute is translocated across the water-dodecane interface, which can be valuable in the context of de novo drug design.

A quasi-periodic time dependence of interfacial tension in an oil-water-surfactant system

A strong fluctuation of interfacial tension (IFT), that is quasi-periodic in time, has been observed for the water-dodecane interface, with 8-phenylhexadecane sodium sulfonate (“Texas no. 1”) surfactant. This fluctuation was observed with sessile but not pendant drops, and in a massive cell. It ceased when vibrations were introduced. The periodic effect was probably caused by the growth of the interfacial layer, from a monolayer of surfactant, into a layer of liquid crystal (LC) material of appreciable thickness, which can be considered to be a degenerate (or underdeveloped) middle phase. Growth would occur by capture of upward-drifting liquid crystal particles from the aqueous phase. The IFT on the aqueous side of the middle phase is probably relatively high, and controls the overall drop shape. The IFT on the oil side of the middle phase is low, and falls with time, to a value so low that, when the layer attains sufficient thickness, droplets can be thrown off by small random vibrations. With a vibration-insulated cell, vibrations of sufficient amplitude come along only at widely separated intervals. When the excess liquid crystal material is thrown off, the layer thickness decreases to a small enough value that the oil-LC layer boundary prevents the water-LC layer boundary from achieving its maximum IFT. A brief discussion is given of the relation between the phenomenon we have observed and the disjoining pressure.