Self-depletion flocculation of tetralin oil-in-water emulsions

Oil-in-water emulsions of slightly soluble oils such as tetralin prepared by high-pressure homogenization and stabilized by sodium dodecyl sulfate undergo depletion flocculation induced by an initially polydisperse droplet size distribution. The smaller droplets flocculate the larger ones; the flocculation can be reversed by gentle sonication. After aging, the flocs disappear because the smaller droplets dissolve through Ostwald ripening. These effects were observed by electroacoustic measurements, supplemented by light scattering.     

Langevin-dynamics simulation of flocculation in water-in-oil emulsions

Aggregation of internal phase droplets in water-in-oil emulsions has been simulated by the Langevin-dynamics method. At initial stages, the process rate obeys the Smoluchowski equations. The concentration ranges have been determined in which percolation clusters are formed. It has been shown that the examined emulsions may be divided into three groups, which enable one to predict their sedimentation stability. Percolation clusters are not formed in emulsions with internal-phase fractions of ≤0.1; therefore, their sedimentation rate is high. In the range of internal-phase fractions from 0.1 to 0.3–0.4, the percolation clusters are formed within a few fractions of a second; however, the rate of subsequent droplet addition to the clusters is low. The flocculation of internal-phase droplets results in the formation of a weak network structure followed by its densification and the separation of a dispersion medium with time. In emulsions with internalphase fractions of >0.3–0.4, almost all droplets unite into percolation clusters within a few fractions of a second. This structure is slowly densified; therefore, the rate of dispersion medium separation is low. The results obtained have been confirmed by experimental data on the sedimentation stability of the water-in-oil emulsions.     

Enhanced stabilization of emulsions due to surfactant-induced nanoparticle flocculation

We have shown recently (Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626) that, for mixtures of negatively charged silica nanoparticles and cationic surfactant, oil-in-water emulsions are most stable to creaming and coalescence at conditions of maximum flocculation of particles by surfactant in aqueous dispersions alone. Here, we extend the idea using positively charged silica particles in mixtures with anionic surfactant.     

Ultrasonic attenuation spectroscopy study of flocculation in protein stabilized emulsions

The extent and kinetics of droplet flocculation in emulsions was studied using ultrasonic attenuation spectroscopy. Flocculation in 10 wt.% soybean oil-in-water emulsions, stabilized by whey protein isolate (0.75 wt.%), was controlled by adjusting the pH (between 3 and 7) to alter the electrostatic interactions between the droplets. Droplet flocculation was then monitored by measuring the ultrasonic attenuation spectra (1–150 MHz) and by using laser light scattering. Extensive droplet flocculation was observed in the emulsions around the isoelectric point of the proteins (pH 3.5–5.5). Flocculation caused an appreciable change in the ultrasonic attenuation spectra, which was in good qualitative agreement with a theory recently developed to describe the ultrasonic properties of flocculated emulsions. Our results indicate that ultrasonic spectroscopy is a powerful tool for monitoring both the extent and kinetics of flocculation in concentrated emulsions in situ.     

Coalescence coupled with either coagulation or flocculation in dilute emulsions

In current theories [R.G.P. Borwankar, L.A. Lobo, D.T.G. Wasan, Colloids Surf. 69 (1992) 35; K.D. Danov, N.D. Denkov, D.N. Petsev, R. Borwankar, Langmuir 9 (1993) 1731; S.S. Dukhin, J. Sjöblom, J. Dispers. Sci. Technol. 19 (1998) 311], emulsion destabilization is considered as the combined processes of droplet coagulation/flocculation and coalescence. Irreversible aggregation is usually called coagulation, and the term flocculation is used for reversible aggregation. These theories have different conditions of applicability which are not specified. An approximate criterion for discrimination between coalescence coupled with either coagulation or flocculation in dilute emulsions is proposed. Such discrimination is made possible by comparing the characteristic time for coagulation (Smoluchowski time) with that for floc fragmentation (droplet doublet lifetime, τ_d). As droplet dimension and electrolyte concentration decrease, and the Stern potential increases, τ_d is reduced and flocculation takes place. The quantity τ_d enables one to specify the conditions for the two separate cases. The important role of retarded van der Waals forces, screening length and hydration forces in emulsion coalescence is demonstrated.     

Monitoring of flocculation and creaming of sodium-caseinate-stabilized emulsions using diffusing-wave spectroscopy

Diffusing-wave spectroscopy (DWS) has been used to study the stability of sodium-caseinate-stabilized emulsions. The emulsions underwent creaming as a result of depletion flocculation when excess sodium caseinate was added. The creaming process was monitored over a 3-h period and each autocorrelation function was collected for 2 min to ensure adequate signal-to-noise ratio. The temporal variation of average particle size times the coefficient of viscosity of the continuous phase was derived from the backscattering measurements, and the variation of the scattering mean free path length with time was found from the backscattering and transmission measurements. It was confirmed that the creaming process was delayed at high oil concentrations, presumably due to the formation of oil droplet networks.     

Critical flocculation density of dilute water-in-CO2 emulsions stabilized with block copolymers

The critical flocculation density (CFD), that is, the CO(2) density below which flocculation occurs, was studied for dilute water-in-CO(2) (W/C) miniemulsions stabilized with poly(1,1-dihydroperfluorooctyl methacrylate)-b-poly(ethylene oxide) (PFOMA-b-PEO) surfactants. The CFD, which was measured by turbidimetry, decreased as the PFOMA molecular weight was increased, the average droplet size was decreased, the surfactant loading was increased, and the temperature was increased. A simple model, which addressed both the van der Waals attraction between droplets and osmotic solvent-tail interactions, was in good qualitative agreement with the experimentally observed trends for the CFD and predicted a decrease in emulsion stability as the CO(2) density was lowered toward the theta density for PFOMA in bulk CO(2).     

Influence of iota-carrageenan on droplet flocculation of beta-lactoglobulin-stabilized oil-in-water emulsions during thermal processing

The influence of thermal processing on droplet flocculation in oil-in-water emulsions stabilized by either beta-lactoglobulin (primary emulsions) or beta-lactoglobulin-iota-carrageenan (secondary emulsions) at pH 6 has been investigated. In the absence of salt, the zeta-potential of the primary emulsion was less negative (-40 mV) than that of the secondary emulsion (-55 mV) due to adsorption of anionic iota-carrageenan to the anionic beta-Lg-coated droplet surfaces. The zeta-potential and mean diameter (d(43) approximately 0.3 microm) of droplets in primary and secondary emulsions did not change after storage at temperatures ranging from 30 to 90 degrees C. In the presence of 150 mM NaCl, the zeta-potential of the primary emulsion was much less negative (-27 mV) than that of the secondary emulsion (-50 mV), suggesting that the latter was less influenced by electrostatic screening effects. The zeta-potential of the primary emulsions did not change after storage at elevated temperatures (30-90 degrees C). The zeta-potential of the secondary emulsions became less negative, and the aqueous phase iota-carrageenan concentration increased at storage temperatures exceeding 50 degrees C, indicating iota-carrageenan desorbed from the beta-Lg-coated droplets. In the primary emulsions, appreciable droplet flocculation (d(43) approximately 8 microm) occurred at temperatures below the thermal denaturation temperature (T(m)) of the adsorbed proteins due to surface denaturation, while more extensive flocculation (d(43) > 20 microm) occurred above T(m) due to thermal denaturation. In the secondary emulsions, the extent of droplet flocculation below T(m) was reduced substantially (d(43) approximately 0.8 microm), which was attributed to the ability of adsorbed carrageenan to increase droplet-droplet repulsion. However, extensive droplet flocculation was observed above T(m) because carrageenan desorbed from the droplet surfaces. Differential scanning calorimetry showed that iota-carrageenan and beta-Lg interacted strongly in aqueous solutions containing 0 mM NaCl, but not in those containing 150 mM NaCl, presumably because salt weakened the electrostatic attraction between the molecules.     

Morphological transformations of native petroleum emulsions. I. Viscosity studies

Emulsions of water in as-recovered native crude oils of diverse geographical origin evidently possess some common morphological features. At low volume fractions varphi of water, the viscosity behavior of emulsions is governed by the presence of flocculated clusters of water droplets, whereas characteristic tight gels, composed of visually monodisperse small droplets, are responsible for the viscosity anomaly at varphi approximately 0.4-0.5. Once formed, small-droplet gel domains apparently retain their structural integrity at higher varphi, incorporating/stabilizing new portions of water as larger-sized droplets. The maximum hold-up of disperse water evidently is the close-packing limit of varphi approximately 0.74. At higher water contents (up to varphi approximately 0.83), no inversion to O/W morphology takes place, but additional water emerges as a separate phase. The onset of stratified flow (W/O emulsion gel + free water) is the cause of the observed viscosity decrease, contrary to the conventional interpretation of the viscosity maximum as a reliable indicator of the emulsion inversion point.     

Viscosity of concentrated suspensions: influence of cluster formation

Dispersed particles can form clusters even at low concentrations. Colloidal and hydrodynamic forces are responsible for this phenomenon and these forces determine both structure and size of clusters. We assume that the viscosity of a concentrated suspension is completely determined by cluster size distribution, regardless if clusters form under the action of colloidal, hydrodynamic interactions or applied shear rates. Based on this assumption an equation, which describes dependency of viscosity on a concentration of dispersed particles taking into account cluster formation, is deduced. Under special restrictions the deduced dependency coincides with the well-known Dougherty-Krieger's equation except for a clear physical meaning of parameters entered. Our consideration shows that Dougherty-Krieger's equation has deeper physical background than it has been supposed earlier. Experimental verification of the suggested model shows a good agreement with the theory predictions and proves a presence of clusters even at low concentrations of dispersed particles.     

Coalescence coupling with flocculation in dilute emulsions within the primary and/or secondary minimum

The simplest singlet–doublet emulsions (SDE) exhibit singlet–doublet quasi-equilibrium (s.d.e.) and slow coalescence between doublets. Orthokinetic coagulation and creaming may be eliminated using low-density contrast emulsions. The investigation of the evolution in time of the total number of droplet in such emulsions at SDE was recommended as a standard method to determine the characteristic times of the elementary acts of coalescence (τ_c) and doublet fragmentation (τ_d) [J. Disp. Sci. Technol. 19 (1998) 311]. Further improvement of this method and its automation requires discrimination between singlets and doublets in the bulk, which is possible for droplets of approximately 5 μm in size. Simultaneously, the droplet dimension must not be too large as the fragmentation time increases rapidly with the droplet dimension, while it has to be very small in compare to the Smoluchowski time (τ_sm) to preserve the condition for SDE. It is shown here that this controversy in the demand to the droplet size can be satisfied by a simultaneous decrease in electrolyte concentration, increase in surface potential and decrease in droplet volume fraction. The calculation of the fragmentation time in a wide range of electrolyte concentration, surface potential and particle radius shows that τ_d may be very small in compare to τ_sm (increases at small volume fractions) at such large mini-emulsion droplet dimension as it is necessary for a singlet and doublet discrimination. The second experimental difficulty is the discrimination between doublets in the primary (PD) and secondary minimum (SD) using video microscopy, while there is a huge difference in their kinetic behavior in respect to fragmentation and coalescence. The analysis of the domains for PD only, for SD only, and for a coexistence of SD and PD shows that the characterization of mini-emulsions has to be accomplished using experimental conditions corresponding to the domain of SD only.     

Comparison of droplet flocculation in hexadecane oil-in-water emulsions stabilized by beta-lactoglobulin at pH 3 and 7

The influence of surface and thermal denaturation of adsorbed beta-lactoglobulin (beta-Lg) on the flocculation of hydrocarbon oil droplets was measured at pH 3 and compared with that at pH 7. Oil-in-water emulsions (5 wt % n-hexadecane, 0.5 wt % beta-Lg, pH 3.0) were prepared that contained different levels of salt (0-150 mM NaCl) added immediately after homogenization. The mean particle diameter (d43) and particle size distribution of diluted emulsions were measured by laser diffraction when they were either (i) stored at 30 degrees C for 48 h or (ii) subjected to different thermal treatments (30-95 degrees C for 20 min). In the absence of salt, little droplet flocculation was observed at pH 3 or 7 because of the strong electrostatic repulsion between the droplets. In the presence of 150 mM NaCl, a progressive increase in mean particle size with time was observed in pH 7 emulsions during storage at 30 degrees C, but no significant change in mean particle diameter with time (d43 approximately 1.4 +/- 0.2 microm) was observed in the pH 3 emulsions. Droplet aggregation became more extensive in pH 7 emulsions containing salt (added before thermal processing) when they were heated above 70 degrees C, which was attributed to thermal denaturation of adsorbed beta-Lg leading to interdroplet disulfide bond formation. In contrast, the mean particle size decreased and the creaming stability improved when pH 3 emulsions were heated above 70 degrees C. These results suggest that the droplets in the pH 3 emulsions were weakly flocculated at temperatures below the thermal denaturation temperature of beta-Lg (T < 70 degrees C) but that flocs did not form so readily above this temperature, which was attributed to a reduction in droplet surface hydrophobicity due to protein conformational changes. The most likely explanation for the difference in behavior of the emulsions is that disulfide bond formation occurs much more readily at pH 7 than at pH 3.     

The influence of surfactant on the alkali-thickening of acrylic emulsions

Colloid and Polymer Science -     

Influence of free protein on flocculation stability of beta-lactoglobulin stabilized oil-in-water emulsions at neutral pH and ambient temperature

The influence of protein concentration and order of addition relative to homogenization (before or after) on the extent of droplet flocculation in oil-in-water emulsions stabilized by a globular protein was examined using laser diffraction. n-Hexadecane (10 wt%) oil-in-water emulsions (pH 7, 150 mM NaCl) stabilized by beta-lactoglobulin (beta-Lg) were prepared by three methods: (1) 4 mg/mL beta-Lg added before homogenization; (2)10 mg/mL beta-Lg added before homogenization; (3) 4 mg/mL beta-Lg added before homogenization and 6 mg/mL beta-Lg added after homogenization. Emulsion 1 contained little nonadsorbed protein (<3%) and underwent extremely rapid and extensive droplet flocculation immediately after homogenization. Emulsion 2 contained a significant fraction of nonadsorbed beta-Lg and exhibited relatively slow droplet flocculation for some hours after homogenization. Measurements on Emulsion 3 showed that the extremely rapid particle growth observed in Emulsion 1 could be arrested by adding native beta-Lg immediately after homogenization. The extent of particle growth in the three types of emulsions was highly dependent on the time that the salt was added to the emulsions, i.e., after 0 or 24 h aging. We postulate that the observed differences are due to changes in droplet surface hydrophobicity caused by differences in the packing or conformation of adsorbed proteins. Our data suggest that history effects have a strong influence on the flocculation stability of protein-stabilized emulsions, which has important implications for the formulation and production of protein stabilized oil-in-water emulsions.     

Microstructure of beta-lactoglobulin-stabilized emulsions containing non-ionic surfactant and excess free protein: influence of heating

The influence of the non-ionic surfactant Tween 20 on the microstructure of beta-lactoglobulin-stabilized emulsions with substantial excess free protein present was investigated via confocal microscopy. The separate distributions of oil droplets and protein were determined using two different fluorescent dyes. In the emulsion at ambient temperature the excess protein and protein-coated oil droplets were associated together in a reversibly flocculated state. The pore-size distribution of the initial flocculated emulsion was found to depend on the surfactant/protein ratio R, and at higher values of R the system became more inhomogeneous due to areas of local phase separation. Evidence for competitive displacement of protein from the oil-water interface by surfactant was obtained only on heating (from 25 to 85 degrees C) during the process of formation of a heat-set emulsion gel. By measuring fluorescence intensities of the protein dye inside and outside of the oil-droplet-rich areas, we have been able to quantify the evolving protein distribution during the thermal processing. The results are discussed in relation to previous work on the competitive adsorption of proteins and surfactants in emulsions and the effect of emulsion droplets on the rheology of heat-set protein gels.     

Dielectric spectroscopy on W/O emulsions under influence of shear forces

The dielectric properties of concentrated w/o-emulsions have been investigated, both at rest and during shear. The volume fraction water ranged from 0.50 to 0.95. The time domain dielectric spectroscopy techniques (TDS) was used to record the dielectric spectra, which covered the frequency region from 25 MHz to 2 GHz. In order to simultaneously record rheological and dielectric data a modified viscometer of the coaxial cylinder type was applied.A close connection between the viscosity and the dielectric properties of w/o emulsions is demonstrated. The very large effects of shear both on the static permittivity and the dielectric relaxation time for the emulsion can partially be ascribed to the degree of flocculation in the system. At high shear rates, at which the emulsions are expected to have a low degree of flocculation, the observed dielectric properties differ from those expected from a theoretical model for spherical emulsion droplets.     

The influence of surfactants on latex flocculation with poly(diallyldimethyl ammonium chloride)

Aqueous latex was flocculated by mixtures of poly(diallyldimethyl ammonium chloride), PDADMAC, and anionic surfactants. Sodium dodecyl sulfate, (SDS), and Aerosol OT influenced flocculation whereas nonionic Tergitol NP-10 did not. The flocculation domains were correlated with properties of the polymer-surfactant complexFlocculation was never observed above the CMC of the corresponding surfactant solution without polymer or latex. At SDS concentrations greater than 10^–3.6 M the flocculation boundary corresponded to the first appearance of insoluble polymer-surfactant complex which was characterized by dynamic light scattering and microelectrophoresis. Under these conditions latex (diameter 570 nm) and dispersed polymer-surfactant complex particles (diameters between 30 and 2 000 nm) displayed simultaneous homo and heteroflocculation. The boundaries of the flocculation domains at low surfactant concentration were determined by the ratio of polymer to latex and by the net electrostatic charge of the soluble polymer-surfactant complex. On the other hand, the mechanisms controlling flocculation boundaries in the dispersed polymer-surfactant domain require further clarification.     

The influence of internal phase viscosity on the viscosity of concentrated water-in-oil emulsions

Summary The influence of the viscosity of the internal phase on the viscosity of concentrated water-in-oil emulsions has been investigated and found to be negligible. It is indicated that thechemical nature of the dispersed medium may be of importance, however, with particular reference to its relationship to the stabilising agent, and an example of this is given. In liquid suspensions the ratio of the viscosities of the two phases is of importance in the transmission of viscous effects from the continuous to the disperse medium. This is not found to be the case for the emulsions examined, and the significance of the rigid structure of the interfacial film in this respect is discussed.

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Zusammenfassung Der Einflu? der Viskosit?t der inneren Phase auf die Viskosit?t von konzentriertem Wasser-in-?lemulsion wurde untersucht. Er ist vernachl?ssigbar. Dagegen ist die chemische Struktur des dispergierten Mediums wichtig, besonders im Zusammenhang mit dem Stabilisator, wie ein Beispiel zeigt. Da in flüssigen Suspensionen das Verh?ltnis der Viskosit?ten der beiden Phasen für die Berechnung der Viskosit?t der Dispersion aus denen der beiden Medien eine Rolle spielen sollte und dies bei den untersuchten Emulsionen nicht der Fall war, wird eine Wirkung von festen Grenzfl?chenfilmen für eine Erkl?rung diskutiert.

     

Physicochemical behavior of oil-in-water emulsions: influence of milk protein mixtures, glycerol ester mixtures and fat characteristics

Different emulsions based on two protein mixtures (skim milk powder (SMP) and functional dairy proteins (FDP)), two mono-di-glyceride mixtures (MDG) (saturated and partially unsaturated), three fats (hydrogenated and refined coconut oils and refined palm oil) were studied to investigate the interactions occurring between the oil phase, low molecular weight emulsifiers and proteins. Immediately following the emulsification process, high diameters of fat globules were obtained in FDP-based systems, relevant of an aggregation phenomenon. At this stage, the fat globule size characteristics were dependent on the emulsifier and fat types present in the formulation. In contrast, SMP-based emulsions were characterized by low proportions of aggregated particles regardless the formulations. Ageing (24 h at 4 °C) promoted disaggregation in FDP formulations, while SMP emulsions were well stabilized. Just after the homogenization step, less proteins were required to stabilize the globule interface in FDP systems as compared to SMP ones. Only with SMP, the amount of protein load at the fat globule surface was influenced by the oil nature and/or by the emulsifier type. A competitive adsorption of caseins, over whey proteins, was demonstrated in the case of FDP. The ageing period promoted a displacement of the proteins adsorbed at the oil droplet interface, suggesting a disruption of the interfacial protein interactions. This disruption was more marked with SMP than with FDP and, in both cases, was more or less influenced by the emulsifier and oil phase natures. The variations of the viscosity and rheological parameters (elastic and viscous moduli) were not dependent on one specific component of the formulation.