Planar Oil-water Interface Research Articles

Pickering Emulsions Stabilized by Polystyrene Particles Possessing Different Surface Groups.

Colloidal polystyrene (PS) latex particles in water can undergo interesting charge reversal in the presence of particular electrolytes. It is worth exploring the effect of charge reversal on the properties of Pickering emulsions they stabilize. Herein, emulsions stabilized by PS latex particles possessing different surface groups (sulfate, amidine, or carboxyl) were prepared in the presence of tetrapentylammonium bromide (TPeAB) or sodium thiocyanate (NaSCN) electrolytes. The effect of salt concentration on the charge of the particles and their colloid stability was measured. Emulsions were prepared from aqueous dispersions, and their type and stability were determined. The three-phase contact angle of particles at the planar oil–water interface was also measured using a gel trapping technique. It was found that the type of emulsion stabilized by latex particles is dominated by the hydrophobic PS portion on particle surfaces, although their surface charge is strongly affected by electrolyte addition. Preferred emulsions were always water-in-oil with dodecane, and charge reversal had little influence on the emulsion type and stability. However, transitional phase inversion of emulsions stabilized by carboxyl latex particles occurred on adding salt when the oil was a low-viscosity polydimethylsiloxane.

Experimental study on the falling and coalescence characteristics of droplets under alternating electric fields

Droplet–interface collision coalescence is an important pattern of phase separation, especially for removing water from water-in-oil emulsions in electrostatic dehydrators. Deformation, migration to the interface, and coalescence of droplets driven by electric fields are fundamental processes but have not been completely understood. In this work, we studied the influences of three typical alternating current or pulsed electric fields on these dynamic processes of a single water droplet falling to the planar oil-water interface. The results depicted that the falling droplet oscillated in both shape and trajectory in an electric field. Specifically, the deformation of droplets should be able to follow the change of electric fields, while the migration may lag behind that. The pattern of droplet deformation matched the electric field waveform, and the frequency had different multiple relationship with the electrical frequency in the electric field of different waveforms. Moreover, the increment of electrical frequency could reduce the degree of droplet deformation and extend the falling time required for droplets to reach the interface. When the droplet coalesced with the interface, secondary droplets were observed if the applied electric field strength exceeded a critical value (Ecrit), detrimental to dehydration efficiency. Ecrit varied from different electric field waveforms and increased with elevated electrical frequencies. Compared to sinusoidal AC and square electric fields, the values of Ecrit in a pulsed DC electric field were highest in the experiments which could be more effective in suppressing the formation of secondary droplets. These findings will be useful for designing high efficient electro-dehydration plants.

Pickering Emulsions of Hydrophilic Silica Particles and Symmetrical Organic Electrolytes.

At high pH, bare silica particles are not an effective Pickering emulsion stabilizer of nonpolar oils with water due to their high surface charge. One way to promote particle adsorption to the oil-water interface is to add salt to the aqueous phase, although particle flocculation normally ensues. In most cases, inorganic salts are added, while little attention has been paid to the use of organic salts. Here, we describe the effects of adding tetraalkylammonium salts (R4NX, X is an anion) to aqueous dispersions of silica nanoparticles at high pH and investigating the possibility of subsequently stabilizing octane-in-water (o/w) emulsions. The chain length of the R group is systematically increased from 1 (methyl) to 4 (butyl). Unlike inorganic electrolytes, the addition of these salts does not lead to particle flocculation in water, although the particle charge is reduced. No stable emulsion forms for the methyl analogue, but very stable o/w emulsions can be prepared with the other three members, with the minimum concentration of salt being required decreasing with R chain length to as low as 5 × 10-5 M. The three-phase oil-water-solid contact angle increases with salt concentration and R chain length, confirming the increase in particle hydrophobicity on addition of salt. We show that the butyl analogue behaves similarly to that of cetyltrimethylammonium bromide surfactant with respect to promoting silica particles to emulsion drop interfaces. Finally, we compare the arrangement of micrometer-sized silica particles at both curved droplet interfaces and at a planar oil-water interface at different concentrations of the most hydrophobic salt.

Effect of Surface Freezing on Stability of Oil-in-Water Emulsions.

Penetration of alkane molecules into the adsorbed film of a cationic surfactant gives rise to a surface freezing transition at the alkane-water interface upon cooling. In this paper, we show that surface freezing of hexadecyltrimethylammonium chloride (CTAC) at the tetradecane-water interface stabilizes oil-in-water (OW) emulsions. For concentrations of CTAC near the critical micelle concentration, an OW emulsion coalesced readily above the surface freezing transition whereas the OW emulsion was stable in the surface frozen state. There was a discontinuous change in the stability of the OW emulsion at a temperature very close to the surface phase transition temperature as determined by interfacial tensiometry and ellipsometry on a planar oil-water interface. The mechanical elasticity of the surface frozen layer opposes film drainage and density fluctuations that could lead to rupture and is the most likely cause of the enhanced emulsion stability.

Molecular characterization of water and surfactant AOT at nanoemulsion surfaces.

Nanoemulsions and microemulsions are environments where oil and water can be solubilized in one another to provide a unique platform for many different biological and industrial applications. Nanoemulsions, unlike microemulsions, have seen little work done to characterize molecular interactions at their surfaces. This study provides a detailed investigation of the near-surface molecular structure of regular (oil in water) and reverse (water in oil) nanoemulsions stabilized with the surfactant dioctyl sodium sulfosuccinate (AOT). Vibrational sum-frequency scattering spectroscopy (VSFSS) is used to measure the vibrational spectroscopy of these AOT stabilized regular and reverse nanoemulsions. Complementary studies of AOT adsorbed at the planar oil-water interface are conducted with vibrational sum-frequency spectroscopy (VSFS). Jointly, these give comparative insights into the orientation of interfacial water and the molecular characterization of the hydrophobic and hydrophilic regions of AOT at the different oil-water interfaces. Whereas the polar region of AOT and surrounding interfacial water molecules display nearly identical behavior at both the planar and droplet interface, there is a clear difference in hydrophobic chain ordering even when possible surface concentration differences are taken into account. This chain ordering is found to be invariant as the nanodroplets grow by Ostwald ripening and also with substitution of different counterions (Na:AOT, K:AOT, and Mg:AOT) that consequently also result in different sized nanoparticles. The results paint a compelling picture of surfactant assembly at these relatively large nanoemulsion surfaces and allow for an important comparison of AOT at smaller micellar (curved) and planar oil-water interfaces.

Coalescence stability of water-in-oil drops: Effects of drop size and surfactant concentration

We study the effects of (1) drop size; (2) surfactant chain-length and concentration; and (3) viscosity of the oil phase on the stability of water drops, pressed by gravity towards planar oil-water interface. The experimental results show that at low surfactant concentrations (around and below the CMC) the drop lifetime is controlled by the drainage time of the oily film, viz. by the time for reaching the critical film thickness at which this film ruptures. The small drops coalesce before the formation of a planar film with the large interface and, as a consequence, their lifetime rapidly decreases with the increase of drop diameter. In contrast, the bigger drops coalesce after formation of a plane-parallel film and their lifetime increases with the drop diameter. Therefore, the drop lifetime passes through a minimum when varying the drop diameter. The obtained results at low surfactant concentrations and for small drops are described well by theoretical models, available in the literature. The lifetime of the respective larger drops is shorter, as compared to the theoretical predictions, due to the faster thinning of the oil films which are with uneven thickness. The increase of surfactant concentration above the CMC leads to a significant increase in the stability of the small drops. Interestingly, the stability of the intermediate in size drops remains low for all surfactants and oils studied. At higher surfactant concentrations, the stability of the large water drops increases significantly when Span 80 is used, while it remains rather low for drops stabilized by Span 20. The results obtained with single water drops are in a good agreement with the stability of the respective batch water-in-oil emulsions.

Aggregation of hydrophilic/hydrophobic montmorillonites at oil–water interface

This study aimed to investigate aggregation of montmorillonites in a planar oil–water interface under gravity-driven motion. For observations, clay particles were added in the upper oil phase or in the lower water phase and their interfacial adsorption and desorption behavior was observed during sedimentation. Hydrophilic clay particles (natural Na-montmorillonite, NMt) initially dispersed in the upper oil phase settled to the bottom of the water phase after occupying the oil–water interface. Hydrophobic particles (organically modified montmorillonite, MMt) accumulated just above the interface without settling into the water phase. When NMt and MMt were dispersed together in the upper oil phase, NMt no longer settled in the water phase; the clays accumulated at the interface and rapidly aggregated together. The presence of MMt significantly delayed transfer of NMt to the water phase, and NMt underwent swelling by limited trans-diffused water molecules that increased the amount of interaction. The aggregation of montmorillonites at the interface was enhanced by the association of hydrophilic and hydrophobic interaction when clays with different surface properties existed together. It caused build-up of the complex interface and resulted in significant emulsion stability.

Modeling the Assembly of Polymer-Grafted Nanoparticles at Oil-Water Interfaces.

Using dissipative particle dynamics (DPD), I model the interfacial adsorption and self-assembly of polymer-grafted nanoparticles at a planar oil-water interface. The amphiphilic core-shell nanoparticles irreversibly adsorb to the interface and create a monolayer covering the interface. The polymer chains of the adsorbed nanoparticles are significantly deformed by surface tension to conform to the interface. I quantitatively characterize the properties of the particle-laden interface and the structure of the monolayer in detail at different surface coverages. I observe that the monolayer of particles grafted with long polymer chains undergoes an intriguing liquid-crystalline-amorphous phase transition in which the relationship between the monolayer structure and the surface tension/pressure of the interface is elucidated. Moreover, my results indicate that the amorphous state at high surface coverage is induced by the anisotropic distribution of the randomly grafted chains on each particle core, which leads to noncircular in-plane morphology formed under excluded volume effects. These studies provide a fundamental understanding of the interfacial behavior of polymer-grafted nanoparticles for achieving complete control of the adsorption and subsequent self-assembly.

Diglycerol esters of fatty acids promote severe coalescence between protein-stabilized oil droplets by emulsifier–protein competitive interactions

Diglycerol ester of mono-oleic acid (DO), a bacteriostatic emulsifier causes severe coalescence of oil droplets stabilized by milk proteins and phase separation between oil and aqueous phase under agitating conditions. In order to clarify the destabilizing mechanism of the emulsifier, physicochemical and colloidal properties of the emulsifier were compared to similar emulsifiers without destabilizing effects. DO, that is dispersible both in oil and water, adsorbed to the oil–water interface to reduce the interfacial tension, and migrated from oil phase to aqueous phase and vice versa in a plane interface system. Experiments performed in an emulsion system revealed that DO had little ability to emulsify food-grade oil, but displaced milk proteins from the oil droplet surface. These data indicate that DO with little emulsifying ability predominantly occupies the oil droplet surfaces via emulsifier–protein competitive interactions to promote severe coalescence of emulsion oil droplets probably because DO actively migrates between oil and aqueous phase and it favors to form the planar oil–water interface.

Direct Measurement of Contact Angles of Silica Particles in Relation to Double Inversion of Pickering Emulsions

In an alkane-water system containing submicrometer silica particles at high pH, double emulsion inversion from oil-in-water (o/w) to water-in-oil (w/o) to oil-in-water can be effected by increasing the concentration of a dichain cationic surfactant in water. The contact angle θ of the particles at the planar oil-water interface has been measured directly using freeze-fracture shadow-casting cryo-scanning electron microscopy, enabling single-particle measurements of high accuracy. θ passes through a maximum with respect to surfactant concentration. It is shown that particles undergo a hydrophilic-hydrophobic-hydrophilic transition corresponding closely to the o/w-w/o-o/w transformation observed in emulsions. These results unequivocally link the single-particle contact angles to the type of particle-stabilized emulsion, confirming macroscopic emulsion inversion on the microscopic level.

Geometrically and chemically anisotropic particles at an oil–water interface

We present the behaviour of particles with chemical and geometrical anisotropy at a planar oil–water interface. We find that Janus cylinders with a small aspect ratio adopt an upright configuration, whereas the particles with a large aspect ratio exhibit both the upright and tilted configurations, which can be explained by the presence of two minima in the attachment energy profile. Such unique configurations significantly affect their assembly structure and lateral interactions. In particular, we observe strong capillary attractions between two tilted Janus cylinders and show that the scaling behaviour of the interaction depends on the lateral alignments of two cylinders. Consequently, this capillarity leads to a variety of assembled structures, which we attribute to the quasi-quadrupolar interface deformation observed around each particle.

Autonomous Self-Assembly of Ionic Nanoparticles into Hexagonally Close-Packed Lattices at a Planar Oil-Water Interface

Let's get charged! Positively charged nanoparticles (NPs) spontaneously self-assemble into hexagonally close-packed lattices at a planar CH(2)Cl(2)-water interface. The self-assembly process is fully autonomous and occurs without any external manipulation.

Coalescence of particle-laden drops with a planar oil–water interface

The coalescence mechanism of a particle-laden drop resting at an oil–water interface has been studied. Two mechanisms for drop coalescence are observed; (i) complete coalescence, in which the drop experiences total coalescence in one event, and (ii) partial coalescence, where a drop is observed to separate during coalescence, producing a smaller secondary drop that rebounds and comes to rest at the planar oil–water interface. For particle-laden drops of approximately 4mm in diameter, we show the critical condition for partial to complete coalescence to be dependent on the particle concentration, and the interparticle interaction energy. Colloidal silica spheres dispersed in 10−4M KNO3 electrolyte solution are highly charged and remain dispersed in the drop. By increasing the solids concentration, we measure the transition from partial to complete coalescence at 20wt.%. However, this critical condition can be reduced by increasing the interparticle interaction energy. In 1M KNO3 electrolyte solution, the particle surface charge is sufficiently screened such that particle clusters readily form in the water drop. With particle clustering, transition from partial to complete coalescence is measured at 8wt.% solids.

Mixed-Monolayer-Protected Gold Nanoparticles for Emulsion Stabilization

Nanometer-sized gold nanoparticles have been prepared and surface-modified in order to stabilize alkane-in-water emulsions. A mixed hexane-undecanol ligand layer at the surface of the nanoparticles allowed us to tune their wettability and thus the adsorption at the oil-water interface. Oil droplets of the stable emulsions have been evidenced by confocal fluorescence microscopy, freeze-fracture transmission electron microscopy, and dynamic light scattering. Prepared emulsions were stable during performed cooling-heating cycles, in which the temperature stability of the emulsions has been studied by means of dynamic light scattering. The interfacial structure of the oil droplets was investigated by small-angle X-ray scattering. The obtained area per nanoparticle at the oil droplet interface was 30 nm(2). The investigation of the nanoparticle adsorption at the curved interface of the emulsion droplets is in agreement with our previous study at a planar oil-water interface, in which the nanoparticles started to interact with each other at about the same area per particle.

Spontaneous charging and crystallization of water droplets in oil

We study the spontaneous charging and the crystallization of spherical micron-sized water droplets dispersed in oil by numerically solving, within a Poisson-Boltzmann theory in the geometry of a spherical cell, for the density profiles of the cations and anions in the system. We take into account screening, ionic Born self-energy differences between oil and water, and partitioning of ions over the two media. We find that the surface charge density of the droplet as induced by the ion partitioning is significantly affected by the droplet curvature and by the finite density of the droplets. We also find that the salt concentration and the dielectric constant regime in which crystallization of the water droplets is predicted is enhanced substantially compared to results based on the planar oil-water interface, thereby improving quantitative agreement with recent experiments.

Stability of additive-free water-in-oil emulsions

We calculate ion distributions near a planar oil–water interface within nonlinearPoisson–Boltzmann theory, taking into account the Born self-energy of the ions in the twomedia. For unequal self-energies of cations and anions a spontaneous charge separation isfound, such that the water and oil phases become oppositely charged in slabs with a typicalthickness of the Debye screening length in the two media. From the analyticalsolutions, the corresponding interfacial charge density and the contribution to theinterfacial tension is derived, together with an estimate for the Yukawa potentialbetween two spherical water droplets in oil. The parameter regime is explored wherethe plasma coupling parameter exceeds the crystallization threshold, i.e. wherethe droplets are expected to form crystalline structures due to a strong Yukawarepulsion, as recently observed experimentally. Extensions of the theory that wediscuss briefly, include numerical calculations on spherical water droplets in oil, andanalytical calculations of the linear PB-equation for a finite oil–water interfacialwidth.

Effect of thermal treatment on interfacial properties of β-lactoglobulin

The changes in the secondary conformation and surface hydrophobicity of β-lactoglobulin subjected to different thermal treatments were characterized at pH values of 7, 5.5 and 4 using circular dichroism (CD) and hydrophobic dye binding. Heating resulted in a decrease in α-helix content with a corresponding increase in random coil at all pH values, this change being more pronounced for small heating times. Heating also resulted in an increase in surface hydrophobicity as a result of partial denaturation, this increase being more pronounced at pH 4. Thermal treatment resulted in a shift of the spread monolayer isotherm at air–water interface to smaller area per molecule due to increased flexibility and more loop formation. Thermal treatment led to an increase in interfacial shear elasticity and viscosity of adsorbed β-lactoglobulin layer at pH 5.5 and 7. Interfacial shear elasticity, shear viscosity, stability of β-lactoglobulin stabilized emulsion and average coalescence time of a single droplet at a planar oil–water interface with adsorbed protein layer exhibited a maximum for protein subjected to 15 min heat treatment at pH 7. At pH 5.5, the interfacial shear rheological properties and average single drop coalescence time were maximum for 15 min heat treatment whereas emulsion stability was maximum for 5 min heat treatment. At pH 7, thermal treatment was found to enhance foam stability. Analysis of thin film drainage indicated that interfacial shear rheological properties do not influence thin film drainage.

Formation of giant colloidosomes by transfer of pendant water drops coated with latex particles through an oil–water interface

We have devised a new technique for producing and studying giant pendant colloidosomes based on transferring particle-coated pendant water drops in oil through a planar oil–water interface.

Lattice-Boltzmann study of spontaneous emulsification

We study the dynamics of spontaneous emulsification of an initially planar oil-water interface when surfactants are added. The thermodynamic properties of the ternary oil-water-surfactant system are modeled by a Ginzburg-Landau-type free energy. The lattice Boltzmann method is used to solve the dynamic equations. The dynamics is found to be governed by a complicated interplay of convection and diffusion as the two relevant transport mechanisms. As long as the interface is almost flat, we find the interfacial area to grow first exponentially and then linearly in time. Later finger-like structures form which grow with a constant velocity. The tip velocity is found to increase roughly linearly with the mobility of the amphiphile, and to decrease as ν−1/2 with the solvent viscosity ν.

Effect of high-methoxy pectin on properties of casein-stabilized emulsions

We report on the effect of high-methoxy pectin on the stability and rheological properties of fine sunflower oil-in-water emulsions prepared with αs1-casein, β-casein or sodium caseinate. The aqueous phase was buffered at pH 7.0 or 5.5 and the ionic strength was adjusted with sodium chloride in the range 0.01–0.2 M. Average emulsion droplet sizes were found to be slightly larger at the lower pH and/or with pectin present during emulsification. Analysis of the serum phase after centrifugation indicated that some pectin becomes incorporated into the interfacial layer at pH 5.5 but not at pH 7.0. This was also supported by electrophoretic mobility measurements on protein-coated emulsion droplets and surface shear viscometry of adsorbed layers at the planar oil–water interface. A low pectin concentration (∼0.1wt%) was found to give rapid serum separation of moderately dilute emulsions (11vol% oil, 0.6wt% protein) and highly pseudoplastic rheological behaviour of concentrated emulsions (40vol% oil, 2 wt% protein). We attribute this to reversible depletion flocculation of protein-coated droplets by non-adsorbed pectin. At ionic strength below 0.1 M, the initial average droplet sizes, the creaming behaviour, and the rheology were found to be similar for emulsions made with either of the individual caseins (αs1 and β) or with sodium caseinate. At higher ionic strength, however, whereas emulsions containing β-casein or sodium caseinate were stable, the corresponding αs1-casein emulsions exhibited irreversible salt-induced flocculation which was not inhibited by the presence of the pectin.