Influence of interfacial characteristics on Ostwald ripening in hydrocarbon oil-in-water emulsions

The influence of the nature of the interfacial membrane on the kinetics of droplet growth in hydrocarbon oil-in-water emulsions was investigated. Droplet growth rates were determined by measuring changes in the droplet size distribution of 1 wt % n-tetradecane or n-octadecane oil-in-water emulsions using laser diffraction. The interfacial properties of the droplets were manipulated by coating them with either an SDS layer or with an SDS-chitosan layer using an electrostatic deposition method. The emulsion containing SDS-coated octadecane droplets did not exhibit droplet growth during storage for 400 h, which showed that it was stable to Ostwald ripening because of this oils extremely low water-solubility. The emulsion containing SDS-coated n-tetradecane droplets showed a considerable increase in mean droplet size with time, which was attributed to Ostwald ripening associated with this oils appreciable water-solubility. On the other hand, an emulsion containing SDS-chitosan coated n-tetradecane droplets was stable to droplet growth, which was attributed to the ability of the interfacial membrane to resist deformation because of its elastic modulus and thickness. This study shows that the stability of emulsion droplets to Ostwald ripening can be improved by using an electrostatic deposition method to form thick elastic membranes around the droplets.     

Impact of oil type on nanoemulsion formation and Ostwald ripening stability

The formation of stable transparent nanoemulsions poses two challenges: the ability to initially create an emulsion where the entire droplet size distribution is below 80 nm, and the subsequent stabilization of this emulsion against Ostwald ripening. The physical properties of the oil phase and the nature of the surfactant layer were found to have a considerable impact on nanoemulsion formation and stabilization. Nanoemulsions made with high viscosity oils, such as long chain triglycerides (LCT), were considerably larger ( D = 120 nm) than nanoemulsions prepared with low viscosity oils such as hexadecane ( D = 80 nm). The optimization of surfactant architecture, and differential viscosity eta D/eta C, has led to the formation of remarkably small nanoemulsions. With average sizes below 40 nm they are some of the smallest homogenized emulsions ever reported. What is more remarkable is that LCT nanoemulsions do not undergo Ostwald ripening and are physically stable for over 3 months. Ostwald ripening is prevented by the large molar volume of long chain triglyceride oils, which makes them insoluble in water thus providing a kinetic barrier to Ostwald ripening. Examination of the Ostwald ripening of mixed oil nanoemulsions found that the entropy gain associated with oil demixing provided a thermodynamic barrier to Ostwald ripening. Not only are the nanoemulsions created in this work some of the smallest reported, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insoluble triglyceride.     

Enhancement of Ostwald ripening by depletion flocculation

The time dependence of the dynamic mobility and the ultrasonic attenuation of octane and decane oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) was measured. The emulsions grew to larger droplets due to Ostwald ripening. The growth rate measured by attenuation depends on the surfactant concentration and the polydispersity of the emulsion. At surfactant concentrations below the critical micelle concentration (cmc) of SDS, the growth was linear with time and the rate was dependent on the polydispersity of the drops; the rate was several times faster than that predicted on the basis of a diffusion growth mechanism. Above the cmc, however, as the droplets grew in size there was a point at which the rate of growth increased, which corresponds to the droplet size at which depletion forces due to the surfactant micelles become significant. Under these conditions both the electroacoustic dynamic mobility and the acoustic attenuation spectra displayed characteristics of flocs: a large decrease in the phase lag at higher frequencies in the dynamic mobility spectrum and a decrease in the attenuation coefficient at low-megahertz frequencies with an increase at higher frequencies. This depletion flocculation enhancement in ripening rates in the presence of SDS micelles provides another, alternative, and self-consistent mechanism for the effect of surfactant micelles on Ostwald ripening.     

Modeling phase separation and droplet size of W/O emulsions with oregano essential oil as a function of its formulation and homogenization conditions

Oregano essential oil emulsions (W/O) were prepared using different emulsifiers’ blend concentrations of Tween 80/Span 20, to study their phase separation during storage and to optimize the homogenization processing parameters by minimizing the droplet size of emulsions. Phase separation followed a second-order kinetic model, and relationships between the kinetic parameters and the blend concentrations of emulsifiers were established for determining the best emulsion formulations. The instability mechanism of emulsions demonstrated to be Ostwald ripening; therefore, by means of surface response methodology, mechanical homogenization parameters (11,700?rpm for 12 minutes at 1°C) were specified in order to minimize the droplet size of emulsions (1.02?±?0.12?µm).     

Degradation of kinetically-stable o/w emulsions

This article summarizes the studies on the degradation of the thermodynamically unstable o/w (nano)emulsion--a dispersion of one liquid in another, where each liquid is immiscible, or poorly miscible in the other. Emulsions are unstable exhibiting flocculation, coalescence, creaming and degradation. The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanisms: first coagulation possibly followed by coalescence and second by Ostwald ripening. Coalescence is often considered as the most important destabilization mechanism leading to coursing of dispersions and can be prevented by a careful choice of stabilizers. The molecular diffusion of solubilizate (Ostwald ripening), however, will continuously occur as soon as curved interfaces are present. Mass transfers in emulsion may be driven not only by differences in droplet curvatures, but also by differences in their compositions. This is observed when two or more chemically different oils are emulsified separately and the resulting emulsions are mixed. Compositional ripening involves the exchange of oil molecules between emulsion droplets with different compositions. The stability of the electrostatically- and sterically-stabilized dispersions can be controlled by the charge of the electrical double layer and the thickness of the droplet surface layer formed by non-ionic emulsifier. In spite of the similarities between electrostatically- and sterically-stabilized emulsions, there are large differences in the partitioning of molecules of ionic and non-ionic emulsifiers between the oil and water phases and the thickness of the interfacial layers at the droplet surface. The thin interfacial layer (the electrical double layer) at the surface of electrostatically stabilized droplets does not create any steric barrier for mass transfer. This may not be true for the thick interfacial layer formed by non-ionic emulsifier. The interactive sterically-stabilized oil droplets, however, can favor the transfer of materials within the intermediate agglomerates. The stability of electrosterically-stabilized emulsion is controlled by the ratio of the thickness of the non-ionic emulsifier adsorption layer (delta) to the thickness of the electrical double layer (kappa(-1)) around the oil droplets (delta/(kappa(-1))) = (deltakappa). The monomer droplet degradation can be somewhat depressed by transformation of coarse emulsions to nano-emulsion (miniemulsion) by intensive homogenization and by the addition of a surface active agent (coemulsifier) or/and a water-insoluble compound (hydrophobe). The addition of hydrophobe (hexadecane) to the dispersed phase significantly retards the rate of ripening. A long chain alcohol (coemulsifier) resulted in a marked improvement in stability, as well, which was attributed to a specific interaction between alcohol and emulsifier and to the alcohols tendency to concentrate at the o/w interface to form stronger interfacial film. The rate of ripening, according to the Lifshitz-Slyozov-Wagner (LSW) model, is directly proportional to the solubility of the dispersed phase in the dispersion medium. The increased polarity of the dispersed phase (oil) decreases the stability of the emulsion. The molar volume of solubilizate is a further parameter, which influences the stability of emulsion or the transfer of materials through the aqueous phase. The interparticle interaction is expected to favor the transfer of solubilizate located at the interfacial layer. The kinetics of solubilization of non-polar oils by ionic micelles is strongly related to the aqueous solubility of the oil phase (the diffusion approach), whilst their solubilization into non-ionic micelles can be contributed by interparticle collisions.     

Ostwald ripening of oil-in-water emulsions stabilized by phenoxy-substituted dextrans

The stability of oil-in-water emulsions prepared using dextran, a natural polysaccharide, hydrophobically substituted with phenoxy groups, was studied. The evolution of the emulsion droplet size was investigated as a function of polymer concentration (Cp=0.2 to 1% w/w in a water phase) and the degree of phenoxy substitution (tau=4.2 to 15.7%). For the highest tau values, emulsions, which presented submicrometer droplets, were stable over more than 4 months at room temperature. The most substituted polymers clearly showed a better efficiency to lower the surface tension at the oil/water interface. DexP did not induce real viscosification of the continuous phase. The linearity of the particle volume variation with time, and the invariability of the volume distribution function, proved that Ostwald ripening was the main destabilization mechanism of the phenoxy dextran emulsions. The nature of the oil dispersed phase drastically affected the behavior of emulsions. While the emulsions prepared with n-dodecane presented a particle growth with time, only few size variations occurred when n-hexadecane was used. Furthermore, small ratios of n-hexadecane in n-dodecane phase reduced the particle growth due to the lower solubility and lower diffusion coefficient in water of n-hexadecane, which acted as a ripening inhibitor.     

Solubilization in monodisperse emulsions

The kinetics of oil solubilization into micelles from nearly monodisperse alkane-in-water emulsion droplets was investigated. Emulsions containing either hexadecane or tetradecane oils were fractionated to be narrowly distributed, using a method developed by Bibette [J. Bibette, J. Colloid Interface Sci. 147 (1991) 474]. These monodisperse emulsions were mixed with SDS or Tween 20 aqueous micellar solutions of various concentrations. Time-dependent solubilization was monitored using light scattering and a decrease in average droplet size over time was observed, in contrast to what has been observed previously with polydisperse emulsions. The rate at which the droplet size decreased was found to be independent of the initial droplet size. Turbidity measurements were also used to track the solubilization kinetics, and a population balance analysis used on both types of measurements to extract effective mass transfer coefficients. The dependence of these transfer coefficients on droplet size, alkane type, surfactant type and concentration provide insights into plausible mechanisms of emulsion droplet solubilization within micellar solutions.     

Ostwald ripening of water-in-hydrocarbon emulsions

The purposes of this study are to determine Ostwald ripening rates in water-in-oil (W/O) emulsions and evaluate the potential application of the LSW theory to W/O emulsions. Water-in-oil emulsions were prepared by mixing water and hydrocarbon phases containing Span 83 and homogenizing. n-Heptane, n-decane, n-dodecane, and n-tetradecane were used to obtain a range of solubilities. A linear increase in the cube of the droplet size with time was observed (within the initial period of 1-2 h after emulsion preparation), supporting the LSW theory. Based on this linear relationship, Ostwald ripening rates were determined to be 3.0 x 10(-24), 2.3 x 10(-24), 1.8 x 10(-24), and 5.8 x 10(-25) m3 s(-1) for water-in-heptane, water-in-decane, water-in-dodecane, and water-in-tetradecane emulsions, respectively. These values are in agreement with theoretical predictions calculated using the LSW equation. It was observed that the ripening process gradually slowed, resulting in deviations from the LSW theory. This was attributed to the effect of the interfacial surfactant film through which the dispersed material has to diffuse during Ostwald ripening. This effect is not taken into consideration by the LSW theory. The results showed that Ostwald ripening of W/O emulsions was less sensitive to the nature of oil used and slower compared to O/W emulsions consisting of the same hydrocarbons.     

Oil-in-water Pickering emulsion destabilisation at low particle concentrations

Droplet evolution in unstable, dilute oil-in-water Pickering emulsions was characterised using a combination of light scattering, confocal microscopy and rheology. Emulsions were formed at concentrations of silanised fumed silica particles that are not sufficient to prevent destabilisation. The key result is that destabilisation initially occurs via a combination of droplet flocculation and permeation. Close contact between the drops enhances oil transfer from smaller drops to the larger ones. The large drops swell over time until the attached particle density is insufficient to protect the drops against coalescence. Examination of the emulsion microstructure revealed the relationship between drop stability and the structural characteristics of the aggregates formed due to coagulation of the silica particles in the emulsions. The implications of these results for controlling Pickering emulsion stability are discussed.     

Amphiphilic polysaccharides: useful tools for the preparation of nanoparticles with controlled surface characteristics

Polymeric surfactants obtained by hydrophobic modification of dextran are used as stabilizers for oil-in-water emulsions. The kinetics of interfacial tension decrease is studied as a function of polymer structural characteristics (degree of hydrophobic substitution) and at various polymer concentrations. Several hydrocarbon oils, either aliphatic (octane, decane, dodecane, and hexadecane) or aromatic (styrene), are tested. Kinetics exhibits the same general trends no matter which oil or polymer is considered. The emulsifying properties of the polymeric surfactants are illustrated by the preparation of oil-in-water emulsions. The droplet size at the preparation is correlated to the amount of oil and to the polymer concentration in the aqueous phase. For low polymer/oil ratios, it is shown that the droplet size is limited by the initial amount of polymer. On the contrary, for high polymer/oil ratios, the droplet size seems to level down, indicating that other parameters become predominant. Emulsion aging occurs by Ostwald ripening, and it is demonstrated that the theoretical equation of Lifshitz, Slyozov, and Wagner (LSW) correctly describes the experimental results. The nature of the oil has important effects on emulsion aging, as described by the LSW equation. The aging of emulsions containing oil mixtures is quantitatively described on the basis of the results with pure oils. The influence of polymer chemical structure can be conveniently correlated to interfacial tension results through the LSW equation. On the contrary, the influence of oil volume fraction seems to be overestimated by the usual correction factor, k(phi). The effect of temperature on emulsion aging is finally examined. Miniemulsions stabilized with dextran derivatives are used for the radical polymerization of styrene. Following this procedure, polysaccharide-covered polystyrene nanoparticles are prepared and characterized (size and surface coverage). The size of the particles is directly correlated to that of the initial droplets for styrene volume fractions around 10%. On the contrary, for initial styrene volume fractions around 20%, particles exhibit a larger size than the initial droplets, indicating that coalescence processes take place during polymerization. The amount of dextran at the surface of the particles is determined and compared to the adsorbed amounts resulting from emulsion preparation.     

Influence of oil polarity on droplet growth in oil-in-water emulsions stabilized by a weakly adsorbing biopolymer or a nonionic surfactant

The influence of oil type (n-hexadecane, 1-decanol, n-decane), droplet composition (hexadecane:decanol), and emulsifier type (Tween 20, gum arabic) on droplet growth in oil-in-water emulsions was studied. Droplet size distributions of emulsions were measured over time (0-120 h) by laser diffraction and ultrasonic spectroscopy. Emulsions containing oil molecules of low polarity and low water solubility (hexadecane) were stable to droplet growth, irrespective of the emulsifier used to stabilize the droplets. Emulsions containing oil molecules of low polarity and relatively high water solubility (decane) were stable to coalescence, but unstable to Ostwald ripening, irrespective of emulsifier. Droplet growth in emulsions containing oil molecules of relatively high polarity and high water solubility (decanol) depended on emulsifier type. Decanol droplets stabilized by Tween 20 were stable to droplet growth in concentrated emulsions but unstable when the emulsions were diluted. Decanol droplets stabilized by gum arabic exhibited rapid and extensive droplet growth, probably due to a combination of Ostwald ripening and coalescence. We proposed that coalescence was caused by the relatively low interfacial tension at the decanol-water boundary, which meant that the gum arabic did not absorb strongly to the droplet surfaces and therefore did not prevent the droplets from coming into close proximity.     

Monitoring the simultaneous ostwald ripening and solubilization of emulsions

The simultaneous Ostwald ripening of an emulsion and the solubilization of its oil droplets by added micellar surfactant solutions are monitored by measurements of time-averaged scattered intensities. A simple computer simulation model for the interpretation of the measurements is presented. Experimental data are analyzed with this model using one single parameter: an effective ratio of oil to surfactant molecules involved in the withdrawal of oil from the Ostwald ripening process by the added micelles. The fitted value of this parameter appears to be more than twice the one that can be predicted from the equilibrium solubilization of oil by the surfactant micelles, indicating that more oil is involved in the nonequilibrium exchange of oil and surfactant between micelles and droplets.     

Effect of the type of the oil phase on stability of highly concentrated water-in-oil emulsions

The water-in-oil high internal phase emulsions were the subject of the study. The emulsions consisted of a super-cooled aqueous solution of inorganic salt as a dispersed phase and industrial grade oil as a continuous phase. The influence of the industrial grade oil type on a water-in-oil high internal phase emulsion stability was investigated. The stability of emulsions was considered in terms of the crystallization of the dispersed phase droplets (that are super-cooled aqueous salt solution) during ageing. The oils were divided into groups: one that highlighted the effect of oil/aqueous phase interfacial tension and another that investigated the effect of oil viscosity on the emulsion rheological properties and shelf-life. For a given set of experimental conditions the influence of oil viscosity for the emulsion stability as well as the oil/aqueous interfacial tension plays an important role. Within the frames of our experiment it was found that there are oil types characterized by optimal parameters: oil/aqueous phase interfacial tension being in the region of 19–24 mN/m and viscosity close to 3 mPa s; such oils produced the most stable high internal phase emulsions. It was assumed that the oil with optimal parameters kept the critical micelle concentration and surfactant diffusion rate at optimal levels allowing the formation of a strong emulsifier layer at the interface and at the same time creating enough emulsifier micelles in the inter-droplet layer to prevent the droplet crystallization.     

Emulsion Ripening through Molecular Exchange at Droplet Contacts

Two coarsening mechanisms of emulsions are well established: droplet coalescence (fusion of two droplets) and Ostwald ripening (molecular exchange through the continuous phase). Here a third mechanism is identified, contact ripening, which operates through molecular exchange upon droplets collisions. A contrast manipulated small‐angle neutron scattering experiment was performed to isolate contact ripening from coalescence and Ostwald ripening. A kinetic study was conducted, using dynamic light scattering and monodisperse nanoemulsions, to obtain the exchange key parameters. Decreasing the concentration or adding ionic repulsions between droplets hinders contact ripening by decreasing the collision frequency. Using long surfactant chains and well‐hydrated heads inhibits contact ripening by hindering fluctuations in the film. Contact ripening can be controlled by these parameters, which is essential for both emulsion formulation and delivery of hydrophobic ingredients.     

NMR on emulsions: characterisation of liquid dispersed systems

Pulsed field gradient NMR (PFG-NMR) is an important method for the characterisation of emulsions. Apart from its application in quality control and process development, especially high-field NMR methods can be applied to investigate emulsions properties on the molecular level. Meanwhile, complex emulsion structures such as double emulsions have been developed and require analytical tools especially for the determination of droplet size distributions. This contribution provides an overview on the possibilities and methods of PFG-NMR referring to measurement, data processing and interpretation of droplet size distributions. Comparison of techniques and measurements on double emulsions are presented.     

Preparation of water-in-diesel oil nano-emulsion using nonionic surfactants with enhanced stability and flow properties

Formation of water-in-diesel oil (w/o) nano-emulsion has been achieved by a low-energy emulsification method by stabilizing a new combination of nonionic sorbitan esters surfactants, that is PEG20-sorbitan monostearate and sorbitan monooleate in mixed proportions. Different combinations of the surfactants (T6?+?S8) have been tested and the best possible combination of mixed surfactants is found at a surfactants ratio of 35:65 (wt/wt) for T6:S8 at hydrophile–lipophile balance (HLB)?=?8.01, which resulted in smaller droplet size of 44.87?nm. A phase diagram study is performed to identify the zones of formation of transparent, translucent, and opaque emulsions (44?nm??27?m³?·?s^?1. Comparison of Ostwald ripening rate with other sets of surfactants obtained by different authors showed the lowest rate among them, indicative of enhanced stability. A rheological study of the tested set of nano-emulsions depicts the Newtonian behavior (1.0371?≤?n?≤?1.0826) over a wider range of shear rates (10–1000?s^?1) at different temperatures (25–40°C).     

Effect of Ionic Strength of Dispersed Phase on Ostwald Ripening in Water-in-Oil Emulsions

Water-in-oil emulsions with a low electrolyte content in the internal phase are unstable with respect to Ostwald ripening. The main components of the total pressure acting on the surface of internal phase droplets are considered. The equilibrium values of the diameters of dispersed phase droplets are calculated. The dependences of the difference in the osmotic and Laplace pressures on the droplet size and electrolyte concentration in the droplets are obtained. It is shown that, at the electrolyte concentration below the critical value, the number of droplets in emulsion decreases. If the concentration is above the critical value, water diffuses from small to large droplets, but their number remains unchanged. The change in NaCl concentration in the droplets of internal phase of polydisperse emulsion during the Ostwald ripening is calculated. The results of calculations correlate with the experimental data on the stability of emulsions with respect to coalescence and sedimentation.     

Water Transport by Nanodispersion Droplets in a Water-in-Oil Emulsion

The mechanisms of water transport through an organic dispersion medium are considered for an emulsion during Ostwald ripening and for a three-phase system upon a contact of a water-in-oil emulsion with an external aqueous phase. Electron microscopy shows a formation of nanodispersion droplets during the diffusion of water through the organic phase of water-in-oil emulsions. The experimental water diffusion coefficient during Ostwald ripening in emulsions is 40 times smaller than the calculated molecular diffusion coefficient. The experimental diffusion coefficients are determined for rhodamine C, which solubilizes in the surfactant micelles, and for ethyl alcohol, a cosurfactant, which reduces the interfacial tension in the emulsion and promotes the formation of nanodispersion droplets. The experimental diffusion coefficients of rhodamine C and ethanol are three orders of magnitude smaller than the calculated values. The ratio between the numbers of rhodamine C and water molecules diffusing through the organic phase is 1 : 10000. The nanodispersion droplets are shown to make the main contribution to the water transport in the organic dispersion medium of the emulsions. Water can also be transported by single surfactant molecules, but this mechanism is not the predominant one.     

Stability of Water/Crude Oil Systems Correlated to the Physicochemical Properties of the Oil Phase

A characterization of 30 crude oils has been performed to determine the relative level of influence that individual parameters have over the overall stability of w/o emulsions. The crude oils have been analyzed with respect to bulk and interfacial properties and the characteristics of their w/o emulsions. The parameters include compositional properties, acidity, spectroscopic signatures in the infrared and near‐infrared region, density, viscosity, molecular weight, interfacial tension, dilational relaxation, droplet size distribution, and stability to gravitationally and electrically induced separation. As expected, a strong covariance between several physicochemical properties was found. Near‐infrared spectroscopy proved to be an effective tool for crude oil analysis. In particular, we have showed the importance of the hydrodynamic resistance to electrically‐induced separation (static) in heavy crude oil‐water emulsions. A rough estimate of the drag forces and dielectrophoretic forces seemed to capture the difference between the 30 crude oils. Given enough time, water‐in‐heavy oil emulsions could be destabilized even at very low electric field magnitude (d.c.). When droplets approach each other in an inhomogeneous electric field, strong dielectrophoretic forces disintegrate the films and result in coalescence. The relative contribution from film stability to the overall emulsion stability may therefore be very different in a gravitational field compared to that in an electrical field.