Molecular simulations of surface forces and film rupture in oil/water/surfactant systems

We use dissipative particle dynamics (DPD) and molecular models to simulate interacting oil/water/surfactant interfaces. The system comprises sections of two emulsion droplets separated by a film. The film is in equilibrium with a continuous phase, in analogy with the surface force apparatus. This is achieved by combining DPD with a Monte Carlo scheme to simulate a muVT ensemble. The setup enables the computation of surface forces as a function of the distance between the two interfaces, as well as the detection of film rupture. We studied monolayers of nonionic model surfactants at different densities and compared oil-water-oil and water-oil-water emulsion films. Between surfactant monolayers facing each other tails-on (water-oil-water films), we observed repulsive forces due to the steric interaction between overlapping hydrophobic tails. The repulsion increases with surfactant density. Conversely, no such repulsion is observed between surfactant monolayers facing each other heads-on. Instead, the film ruptures, the monolayers merge, and a channel forms between the two droplet phases. Film rupture can also be induced in the water-oil-water films by forcing the interfaces together. The separation at rupture increases for oil-water-oil films and decreases for water-oil-water films when the surfactant density increases. The results are in qualitative agreement with existing theories of emulsion stability in creams, in particular with the channel nucleation theory based on the natural curvature of surfactants.     

Wetting in oil/water/surfactant systems

The behaviour of oils at aqueous interfaces is ubiquitous to many industrially and biologically relevant processes. In this review we consider modifications to the wetting properties of oils at the air/water, oil/water and solid/liquid interfaces in the presence of surfactants. First-order wetting transitions can be induced in a wide range of oils by varying the aqueous surfactant concentration, leading to the formation of mixed monolayers at the interface. In certain cases, these mixed monolayers display novel surface freezing behaviour, including the formation of unusual bilayer structures, which further modifies the properties of the interface. The effects of surfactant on line tension at the three-phase contact line and differences between the air/liquid and liquid/liquid interfaces are discussed.     

A study of oil droplet coalescence

Oil droplets dispersed in water can be readily studied when they are coated with surfactants, which lower their interfacial tension and enhance their stability. Pure oil droplets are more difficult to study because of their high interfacial tension, which facilitates coalescence and the adsorption of contaminants. In this study, we have characterised the surface charging properties of a water insoluble oil, bromododecane, which has a density close to water. The small density difference allows us to study relatively large drops of this oil and to analyse its coalescence behaviour. The results obtained with this simple, surfactant-free, system suggest that an additional attractive force, such as the long range hydrophobic interaction, might be required to explain oil droplet coalescence behaviour.     

Selective droplet coalescence using microfluidic systems

We report a microfluidic approach, which allows selective and controlled 1 : 1, 2 : 1 or 3 : 1 droplet fusion. A surfactant-stabilized droplet with an interfacial surfactant coverage, Γ, of >98% will fuse spontaneously with a second droplet when Γ of the latter droplet is <16%. However, when Γ of the second droplet is ~66%, the two droplets will not fuse, unless they have previously been brought into contact for critical time τ. Therefore, controlling the number of droplets in contact for time τ allows precise control over the number of fused droplets. We have demonstrated efficient (proportion of droplets coalesced p(c) = 1.0, n > 1000) and selective 1 : 1, 2 : 1 or 3 : 1 droplet fusion (proportion of correctly fused droplets p(s) > 0.99, n > 1000). Coalescence in this regime is induced by hydrodynamic flow causing interface separation and is efficient at different Ca numbers and using different dispersed phases, continuous phases and surfactants. However, when Γ of the second droplet is ~96% coalescence is no longer observed. Droplet-based microfluidic systems, in which each droplet functions as an independent microreactor, are proving a promising tool for a wide range of ultrahigh-throughput applications in biology and chemistry. The addition of new reagents to pre-formed droplets is critical to many of these applications and we believe the system described here is a simple and flexible method to do so, as well as a new tool to study interfacial stability phenomena.     

Molecular dynamics study of surfactant monolayers adsorbed at the oil/water and air/water interfaces

Atomistic molecular dynamics (MD) simulations have been carried out to investigate the physical properties of monolayers of monododecyl diethylene glycol (C(12)E(2)) surfactants adsorbed at the oil/water and air/water interfaces. The study shows that the surfactant molecules exhibit more extended conformations with a consequent increase of the thickness of the monolayer in the presence of the oil medium. It is noticed that the hydrocarbon tails of the surfactants are more vertically oriented at the oil/water interface. Interestingly, we notice that the presence of the oil medium has a strong influence in restricting both the translational and reorientational motions of the water molecules present in the hydration layer close to the surfactant headgroups.     

Novel thermo-responsive coloring phenomena in water/surfactant/oil emulsions

Emulsions comprising a dual-surfactant system of a long-chain amidoamine derivative and a quaternary ammonium salt developed an iridescent color at a specific temperature region. The emulsions underwent phase inversion on heating from an O/W emulsion to a W/O emulsion, passing through a periodical lamellar structure which developed a characteristic interference color. Interestingly, the color and the coloring temperature can be independently controlled by adjusting the concentration of surfactants, respectively.     

Nanoemulsification in the vicinity of phase inversion: Disruption of bicontinuous structures in oil/surfactant/water systems

Oil/surfactant/water systems may undergo phase inversion upon tuning the preferred curvature of the surfactant layer. The longstanding relationship between nanoemulsification and phase inversion is discussed in view of recent mechanistic advances. The name “phase inversion emulsification” is shown to result from a historical confusion. Both nanoemulsification and phase inversion are controlled by the properties of the surfactant layer but phase inversion is shown to be unnecessary to obtain nanoemulsions. Nanoemulsions can be obtained in the vicinity of phase inversion through the disruption of equilibrium bicontinuous networks. A first pathway involves a change of the interaction between the surfactant layer and water at a precise location in the parameter space and under shear. A non-equilibrium micellar solubilization of oil, named superswelling, leads to an ideal nanoemulsion after quenching. All the surfactant is used to cover the interfaces and none is wasted in the continuous phase. The sub-PIT (Phase Inversion Temperature) method falls within this category. A second pathway involves the addition of water to a water-deprived system. Oil phase separates within a bicontinuous sponge phase matrix at a precise location in the parameter space and leads to a nanoemulsion upon further addition of water. Larger droplets are obtained and some surfactant is wasted, which demonstrates that this pathway is different and less efficient, although easier to implement. It is shown that the identification of the two access states in the nanoemulsification pathways, the superswollen microemulsion and the separating sponge phase, is essential when using surfactant blends. On the contrary, phase inversion is not only irrelevant but also damaging to the success of the emulsification process.     

Optimum phase-behavior formulation of surfactant/oil/water systems for the determination of chromium in heavy crude oil and in bitumen-in-water emulsion

An “oil in water” formulation was optimized to determine chromium in heavy crude oil (HCO) and bitumen-in-water emulsion (Orimulsion-400®) samples by transversally heated electrothermal atomic absorption spectrometry (TH-ET AAS) using Zeeman effect background correction. The optimum proportion of the oil-water mixture ratio was 7:3 v/v (70 ml of oil as the internal phase) with a non-ionic surfactant concentration (Intan-100) in the emulsion of 0.2% w/w. Chromium was determined in different crude oil samples after dilution of the emulsions 1:9 v/v with a 0.2% w/w solution of surfactant in order to further reduce the viscosity from 100 to 1.6 cP and at the same time to bring the concentration of chromium within the working range of the ET AAS technique. The calibration graph was linear from 1.7 to 100 μg Cr l⁻¹. The sensitivity was of 0.0069 s l μg⁻¹, the characteristic mass (m_o) was of 5.7 pg per 0.0044 s and the detection limit (3σ) was of 0.52 μg l⁻¹. The relative standard deviation of the method, evaluated by replicate analyses of three crude oil samples varied in all cases between 1.5 and 2.6%. Recovery studies were performed on four Venezuelan crude oils, and the average chromium recovery values varied between 95.9-104.8, 90.6-107.6, 95.6-104.0 and 98.8-103.9% for the Cerro Negro, Crudo Hamaca and Boscán crude oils and for the Orimulsión®-400, respectively. The results obtained in this work for the Cerro Negro, Crudo Hamaca and Boscán crude oils and for the Orimulsión®-400 following the proposed procedure were of 0.448±0.008, 0.338±0.004 0.524±0.021 and 0.174±0.008 mg Cr l⁻¹, respectively, which were in good agreement with the values obtained by a tedious recommended standard procedure (respectively: 0.470±0.05, 0.335±0.080, 0.570±0.021 and 0.173±0.009 mg Cr l⁻¹).     

Coalescence-induced coalescence and dimensional crossover during the phase separation in ternary surfactant/polymer/water mixtures

We studied the separation process in the ternary mixtures of nonionic surfactant (C(12)E(6), hexaethylene glycol monododecyl ether), polymer (PEG = poly(ethylene glycol)), and water. The separation process of PEG/water rich domains from the surfactant rich matrix was observed by the optical microscopy. From the morphological analysis, we determined the size of the domains as a function of time. On this basis we identified a dominating mechanisms of domains growth, that is the coalescence-induced coalescence mechanism. The coalescence (collision) event of two droplets induces a flow or a change of concentration distribution around droplets which pushes other droplets together inducing further growth. We also observed the evaporation-condensation (Lifshitz-Slyozov) mechanism of growth, but it did not affect the growth of large domains appreciably. We determined two regimes of the coalescence-induced coalescence associated with the dimensionality of the system. When the domains were smaller or comparable in size to the sample thickness we observe a three-dimensional growth. When the domains became larger than the sample thickness, a two-dimensional growth was observed. In the first regime, the size of the domains, L(t), grew linearly with t, while in the second regime, L(t) approximately t(0.3). In the binary, surfactant/water system, water domains grew by the geometrical coalescence-induced coalescence as L(t) approximately t in three dimensions.     

An empirical boundary potential for water droplet simulations

An empirical modified boundary potential has been derived to correct the structural perturbations arising from the presence of the vacuum boundary in the simulation of spherical TIP4P water systems. The potential is parameterized for a 12.0-Å sphere of TIP4P water and gives improved number density and orientational sampling behavior. It is also transferable to both larger and smaller simulation systems with only a moderate degradation in performance. Free-energy calculations have been conducted for the perturbation of a TIP4P water molecule to methane under aqueous conditions, and the modified boundary potential gives results consistent with those from simulations using periodic boundary conditions. However, simple half-harmonic boundary potentials give unsatisfactory number density, orientational sampling, and free-energy results. Moreover, use of the modified boundary potential results in a negligible increase in simulation time. It is envisaged that the modified boundary potential will find use in free-energy perturbation calculations on proteins with a solvent sphere centered on the active site. © 1995 by John Wiley & Sons, Inc.     

Studies of synergism/antagonism for lowering dynamic interfacial tensions in surfactant/alkali/acidic oil systems. 1. Synergism/Antagonism in surfactant/model oil systems

Experimental studies are conducted in order to elucidate the mechanisms responsible for synergism/antagonism for lowering dynamic interfacial tension in model oil/surfactant/brine systems. A well-defined model oil is selected for controlled design of experiments, thus enhancing verification of known and unknown mechanisms. The systems examined contain model oils and two petroleum sulfonate solutions. The influence of additives in oil phase, such as carboxylic acids with different chain length, n-octadecanol, and oil soluble surfactant SP-60, on the equivalent alkane carbon number (EACN) values has been examined. The interfacial tensions of different model oils with different EACN values against surfactant solutions with different n(min) values have also been obtained. We find that antagonism has been observed when EACN/n(min) value is far from unity by adding organic components, while synergism has been observed when EACN/n(min) value is close to unity. The results present here suggest that organic additives in oil phase controlled interfacial tension by changing the partition of surfactants in oil phase, aqueous phase, and interface.     

Formation of single-phase microemulsions in toluene/water/nonionic surfactant systems

Mixtures of toluene and water from 5 to 50% oil fraction and 5 to 25% surfactant by weight were studied. Winsor Type IV microemulsions were formed in numerous cases. Review of partial ternary phase diagrams for these systems indicated the area of single-phase microemulsion with toluene could be maximized at an hydrophilic-lipophilic balance (HLB) of approximately 14.5. Select single-phase samples were further analyzed by surface tension and dynamic light scattering techniques, which allowed a detailed characterization of the solution equilibrium thermodynamics and size stability. Particle sizes averaged approximately 5 nm and were nearly constant over a wide variety of conditions and for 6-18 months. When benzyl alcohol was used instead of toluene, the optimum HLB for the formation of single-phase systems was found to have a lower limit of 17. Particle sizes in these systems were <30 nm but showed greater variability. The decrease in particle size as surfactant concentration increased was determined to be associated with changes in ethlyene oxide chain conformation. The increase in particle size due to swelling with increased oil concentration was used to determine the surfactant surface area in the oil phase. A detailed comparison of alkylamine ethoxylate to octyl- and nonylphenol ethoxylate surfactants in terms of micelle thermodynamics, size, and stability indicate that the alkylamine-based surfactants are potential candidates for the replacement of nonylphenol-based surfactants in some systems with a more polar oil phase like benzyl alcohol.     

Phase diagrams of surfactant/water/synthetic perfume ternary systems

The phase diagrams of hexadecyl polyoxyethylene ether (C₁₆POE₁₀)/water/synthetic perfume, and sodium dodecyl sulfate (SDS)/water/synthetic perfume ternary systems were prepared. The synthetic perfumes used are, d-limonene, -hexylcinnamaldehyde, -ionone, benzyl acetate, linalool, and eugenol. In a series of C₁₆POE₁₀/water/synthetic perfume ternary systems, as the hydrophilicity of synthetic perfume increases, the regions of normal and inverse micellar solution phases were found to be extended, while that of the lamellar liquid crystal phase was reduced. Moreover, every region of normal micellar solutions, inverse micellar solutions, and lamellar liquid crystal phases in SDS/water/synthetic perfume ternary systems was found to be smaller than those in C₁₆POE₁₀/water/synthetic perfume systems.     

Studies of synergism/antagonism for lowering dynamic interfacial tension in surfactant/alkali/acidic oil systems. 3. Synergism/antagonism in surfactant/alkali/acidic model oil systems

Experimental studies have been conducted to elucidate the mechanisms responsible for synergism/antagonism for lowering dynamic interfacial tension (IFT) in surfactant/alkali/hydrocarbon and surfactant/alkali/acidic model oil systems. Dynamic IFTs between hydrocarbon/acidic model oil and alkali/surfactant solutions were measured. We learned from our experimental results that alkali has the function of decreasing n(min) values of surfactant solutions. The synergism/antagonism for lowering the stable values of dynamic IFTs in surfactant/alkali/hydrocarbon and surfactant/alkali/acidic model oil systems depends on factors that can change the EACN/n(min) value, such as the oleic acid in the oil phase and the n(min) values of surfactant and alkali. A new explanation with respect to EACN/n(min) values is provided.     

Polyelectrolyte/surfactant mixtures in the bulk and at water/oil interfaces

Stabilization of emulsions by mixed polyelectrolyte/surfactant systems is a prominent example for the application in modern technologies. The formation of complexes between the polymers and the surfactants depends on the type of surfactant (ionic, non-ionic) and the mixing ratio. The surface activity (hydrophilic–lipophilic balance) of the resulting complexes is an important quantity for its efficiency in stabilizing emulsions. The interfacial adsorption properties observed at liquid/oil interfaces are more or less equivalent to those observed at the aqueous solution/air interface, however, the corresponding interfacial dilational and shear rheology parameters differ quite significantly. The interfacial properties are directly linked to bulk properties, which support the picture for the complex formation of polyelectrolyte/surfactant mixtures, which is the result of electrostatic and hydrophobic interactions. For long alkyl chain surfactants the interfacial behavior is strongly influenced by hydrophobic interactions while the complex formation with short chain surfactants is mainly governed by electrostatic interactions.     

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The binary phase behaviour of two potentially polymerisable quaternary ammonium surfactants in water has been investigated. Allyldodecyldimethylammonium bromide (ADAB) a single-chain surfactant displays a conventional phase progression upon increasing concentration. Whereas the doublechain analogue allyldidodecylmethylammonium bromide (ADDAB) forms two lamellar liquid crystalline phases built from surfactant bilayers, which transform via a first order phase transition. The formation of two distinct lamellar phases and their coexistence has been evidenced by optical microscopy, small-angle x-ray scattering and D₂O deuterium quadrupolar nuclear magnetic resonance spectroscopy. The lamellar phase formed at higher surfactant compositions is a normal lamellar phase (typeL ) consisting of bilayers which are on average parallel and flat. The lower compositional lamellar phase (typeL ) in contrast may not be comprised of planar bilayers but rather aggregates having a high degree of curvature in comparison to those of theL phase. The presence of the allyl polymerisable moiety in the head group position of these surfactants has the effect of reducing the rigidity of the surfactant and increasing its solubility in comparison to nonpolymerisable analogues. Polymerisation of the surfactants was attempted by using thermal and photochemical initiation in isotropic and self-assembled systems. Polymerisation occurred to approximately 30% for DADB but did not occur for ADDAB. Where polymerisation did occur the polymer was incorporated into the monomer matrix by interweaving between the surfactant aggregates. The polymers had a molecular wieght not greater than 8000 Daltons, independent of the monomer concentration of the original solution and type of polymerisation.     

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The phase behaviour and polymerisation of a quaternary ammonium surfactant containing an ethylmethacrylate polymerisable moiety within the head group region has been investigated. The addition of this large flexible hydrophilic moiety alters significantly the inherent surface activity of the surfactant and the surfactant may be compared with the class of non-ionic polyethyleneoxide surfactants. Polymerisation of both self-assembled and non self-assembled states went to near completion with the resulting polymer being completely insoluble in water.     

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The self-assembly behaviour of the polymerisable surfactant -undecenyltrimethylammonium bromide (-UTAB) both before and after polymerisation has been investigated. In addition polymerisation of the liquid crystalline phases formed by this surfactant in aqueous solution has been studied. Introduction of the carbon-carbon double bond at the end of the hydrocarbon chain increases the rigidity of the paraffinic chains such that the Krafft curve is shifted to higher temperatures compared with that of dodecyltrimethylammonium bromide, a nonpolymerisable analogue. Both the polymerised and non-polymerised forms have been observed to have the same phase progression, with the polymer being more soluble in water such that the liquid crystalline phases formed at high surfactant concentration are accessible at room temperature. Polymerisation of the liquid crystalline phases of -UTAB indicate that polymerisation proceeds to approximately 40% (in comparison) with 80% in a non-aggregated form) and that the original monomeric matrix is undisturbed upon partial polymerisation.     

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The polymerisation of a polymerisable fatty acid surfactant (sodium 10-undecenoate) has been studied in both its self-assembled and non self-assembled forms. Polymerisation in non self-assembled solution was achieved to near completion. The polymerisation produces a surface active polymer. The self-assembling behaviour of this pre-polymerised form differs markedly from that observed for the monomeric surfactant [1]. A lamellar phase only is formed in the polymeric phase diagram with no hexagonal or lamellar gel phases being observed. Polymerisation in the different self-assembled forms of sodium 10-undecenoate reached a limit of approximately 30% only, i.e., the surfactant aggregates act to inhibit the polymerisation. The nature of the hydrocarbon chain was found to play a critical role in determining the effect that polymerisation had on the underlying geometry of the surfactant molecules. When the chains are in a fluid-like state (as for the micellar and hexagonal phases) the original monomeric matrix remains largely unchanged. Whereas partial polymerisation of the lamellar gel phase results in a phase transformation.In addition the hydrolysis of the fatty acid soap at low concentrations (close to the critical micelle concentration) has been investigated. Hydrolysis was shown to produce both the parent fatty acid and an acid soap dimer. The presence of these species greatly affects the solution behaviour in this region of the phase diagram shifting the critical micelle concentration to very high concentrations of sodium 10-undecenoate (ca. 0.4 M).     

Redox-photosensitized reaction of indene using photosensitive surfactant in emulsion: dependence on oil droplet size and surfactant charge

A redox-photosensitized reaction of indene 2 using a photosensitive surfactant 1a in an oil-in-water emulsion proceeded efficiently to give alcohol 3 as a major product and is strongly influenced by the oil droplet size and surfactant charge.