Emulsification of caraway essential oil in water by lecithin and beta-lactoglobulin: emulsion stability and properties of the formed oil-aqueous interface

The stability and droplet size of protein and lipid stabilised emulsions of caraway essential oil as well as the amount of protein on the emulsion droplets have been investigated. The amount of added protein (beta-lactoglobulin) and lipid (phosphatidylcholine from soybean (sb-PC)) were varied and the results compared with those obtained with emulsions of a purified olive oil. In general, emulsions with triglyceride oil proved to be more stable compared with those made with caraway essential oil as the dispersed phase. However, the stability of the emulsions can be improved considerably by adding sb-PC. An increase in the protein concentration also promoted emulsion stability. We will also present how ellipsometry can be used to study the adsorption of the lipid from the oil and the protein from the aqueous phase at the oil-water interface. Independently of the used concentration, close to monolayer coverage of sb-PC was observed at the caraway oil-aqueous interface. On the other hand, at the olive oil-aqueous interface, the presence of only a small amount of sb-PC lead to an exponential increase of the layer thickness with time beyond monolayer coverage. The amounts of beta-lactoglobulin adsorbed at the caraway oil-aqueous interface and at the olive oil-aqueous interface were similar, corresponding roughly to a protein monolayer coverage.     

Emulsification of caraway essential oil in water by lecithin and β-lactoglobulin: emulsion stability and properties of the formed oil–aqueous interface

The stability and droplet size of protein and lipid stabilised emulsions of caraway essential oil as well as the amount of protein on the emulsion droplets have been investigated. The amount of added protein (β-lactoglobulin) and lipid (phosphatidylcholine from soybean (sb-PC)) were varied and the results compared with those obtained with emulsions of a purified olive oil. In general, emulsions with triglyceride oil proved to be more stable compared with those made with caraway essential oil as the dispersed phase. However, the stability of the emulsions can be improved considerably by adding sb-PC. An increase in the protein concentration also promoted emulsion stability. We will also present how ellipsometry can be used to study the adsorption of the lipid from the oil and the protein from the aqueous phase at the oil–water interface. Independently of the used concentration, close to monolayer coverage of sb-PC was observed at the caraway oil–aqueous interface. On the other hand, at the olive oil–aqueous interface, the presence of only a small amount of sb-PC lead to an exponential increase of the layer thickness with time beyond monolayer coverage. The amounts of β-lactoglobulin adsorbed at the caraway oil–aqueous interface and at the olive oil–aqueous interface were similar, corresponding roughly to a protein monolayer coverage.     

Synergistic stabilization of emulsions by a mixture of surface-active nanoparticles and surfactant

The stability and rheology of tricaprylin oil-in-water emulsions containing a mixture of surface-active hydrophilic silica nanoparticles and pure nonionic surfactant molecules are reported and compared with those of emulsions stabilized by each emulsifier alone. The importance of the preparation protocol is highlighted. Addition of particles to a surfactant-stabilized emulsion results in the appearance of a small population of large drops due to coalescence, possibly by bridging of adsorbed particles. Addition of surfactant to a particle-stabilized emulsion surprisingly led to increased coalescence too, although the resistance to creaming increased mainly due to an increase in viscosity. Simultaneous emulsification of particles and surfactant led to synergistic stabilization at intermediate concentrations of surfactant; emulsions completely stable to both creaming and coalescence exist at low overall emulsifier concentration. Using the adsorption isotherm of surfactant on particles and the viscosity and optical density of aqueous particle dispersions, we show that the most stable emulsions are formed from dispersions of flocculated, partially hydrophobic particles. From equilibrium contact angle and oil-water interfacial tension measurements, the calculated free energy of adsorption E of a silica particle to the oil-water interface passes through a maximum with respect to surfactant concentration, in line with the emulsion stability optimum. This results from a competition between the influence of particle hydrophobicity and interfacial tension on the magnitude of E.     

Ethanol-in-oil emulsion (E/O) stabilized by polyglycerol polyricinoleate: A potential delivery system for ethanolic extract

This study evaluated how variations in polyglycerol polyricinoleate (PGPR) concentration and ethanol dispersed phase content affect the stability of ethanol-in-oil (E/O) emulsions. Results indicate that the stable 10?wt% E/O emulsions can be produced using 2?wt% PGPR. Increasing the ethanol dispersed phased content at constant PGPR concentration caused instability in emulsion. These emulsions remained stable to droplet flocculation and coalescence in the presence of Centella asiatica ethanol extract. PGPR does not greatly decrease the interfacial tension of the ethanol–oil interface. However, it adsorbed at the interface and stabilized the ethanol droplets in the emulsion via steric mechanism.     

Stabilization of carbon dioxide-in-water emulsions with silica nanoparticles

Stable carbon dioxide-in-water emulsions were formed with silica nanoparticles adsorbed at the interface. The emulsion stability and droplet size were characterized with optical microscopy, turbidimetry, and measurements of creaming rates. The increase in the emulsion stability as the silica particle hydrophilicity was decreased from 100% SiOH to 76% SiOH is described in terms of the contact angles and the resulting energies of attachment for the silica particles at the water-CO(2) interface. The emulsion stability also increased with an increase in the particle concentration, CO(2) density, and shear rate. The dominant destabilization mechanism was creaming, whereas flocculation, coalescence, and Ostwald ripening played only a minor role over the CO(2) densities investigated. The ability to stabilize these emulsions with solid particles at CO(2) densities as low as 0.739 g/mL is particularly relevant in practical applications, given the difficulty in stabilizing these emulsions with surfactants, because of the unusually weak solvation of the surfactant tails by CO(2).     

Conformational changes of α-lactalbumin adsorbed at oil-water interfaces: interplay between protein structure and emulsion stability

The conformation and structural dimensions of α-lactalbumin (α-La) both in solution and adsorbed at oil-water interfaces of emulsions were investigated using synchrotron radiation circular dichroism (SRCD) spectroscopy, front-face tryptophan fluorescence (FFTF) spectroscopy, and dual polarization interferometry (DPI). The near-UV SRCD and the FFTF results demonstrated that the hydrophobic environment of the aromatic residues located in the hydrophobic core of native α-La was significantly altered upon adsorption, indicating the unfolding of the hydrophobic core of α-La upon adsorption. The far-UV SRCD results showed that adsorption of α-La at oil-water interfaces created a new non-native secondary structure that was more stable to thermally induced conformational changes. Specifically, the α-helical conformation increased from 29.9% in solution to 45.8% at the tricaprylin-water interface and to 58.5% at the hexadecane-water interface. However, the β-sheet structure decreased from 18.0% in solution to less than 10% at both oil-water interfaces. The DPI study showed that adsorption of α-La to a hydrophobic C18-water surface caused a change in the dimensions of α-La from the native globule-like shape (2.5-3.7 nm) to a compact/dense layer approximately 1.1 nm thick. Analysis of the colloidal stability of α-La stabilized emulsions showed that these emulsions were physically stable against droplet flocculation at elevated temperatures both in the absence and in the presence of 120 mM NaCl. In the absence of salt, the thermal stability of emulsions was due to the strong electrostatic repulsion provided by the adsorbed α-La layer, which was formed after the adsorption and structural rearrangement. In the presence of salt, although the electrostatic repulsion was reduced via electrostatic screening, heating did not induce strong and permanent droplet flocculation. The thermal stability of α-La stabilized emulsions in the presence of salt is a combined effect of the electrostatic repulsion and the lack of covalent disulfide interchange reactions. This study reports new information on the secondary and tertiary structural changes of α-La upon adsorption to oil-water interfaces. It also presents new results on the physical stability of α-La stabilized emulsions during heating and at moderate ionic strength (120 mM NaCl). The results broaden our understanding of the factors controlling protein structural change at emulsion interfaces and how this affects emulsion stability.     

Effect of thermal treatment, ionic strength, and pH on the short-term and long-term coalescence stability of beta-lactoglobulin emulsions

We present experimental results about the effects of thermal treatment, ionic strength, and pH on the protein adsorption and coalescence stability of freshly prepared (2 h after emulsification) and 6-day-stored emulsions, stabilized by the globular protein beta-lactoglobulin (BLG). In all emulsions studied, the volume fraction of the dispersed soybean oil is 30% and the mean drop diameter is d(32) approximately 40 microm. The protein concentration, C(BLG), is varied between 0.02 and 0.1 wt %, the electrolyte concentration, C(EL), between 1.5 mM and 1 M, and pH between 4.0 and 7.0. The emulsion heating is performed at 85 degrees C, which is above the denaturing temperature of BLG. The results show that, at C(BLG) > or = 0.04 wt %, C(EL) > or = 150 mM, and pH > or = 6.2, the heating leads to higher protein adsorption and to irreversible attachment of the adsorbed molecules, which results in enhanced steric repulsion between the protein adsorption multilayers and to higher emulsion stability. At low electrolyte concentration, C(EL) < or = 10 mM, the emulsion stability is determined by electrostatic interactions and is not affected significantly by the emulsion heating. The latter result is explained by electrostatic repulsion between the adsorbed protein molecules, which keeps them separated from each other and thus precludes the formation of disulfide covalent bonds in the protein adsorption layer. The coalescence stability of heated and nonheated emulsions is practically the same and does not depend on C(EL), when pH is around the isoelectric point (IEP) of the protein molecules. This is explained with the adsorption of uncharged BLG molecules, in compact conformation, which stores the reactive sulfhydryl groups hidden inside the molecule interior, thus preventing the formation of covalent intermolecular bonds upon heating. We studied also the effect of storage time on the stability of heated and nonheated emulsions. The stability of nonheated emulsions (C(BLG) = 0.1 wt %, C(EL) > or = 150 mM, and pH = 6.2) significantly decreases after 1 day of storage (aging effect). In contrast, no aging effect is observed after emulsion heating. FTIR spectra of heated and nonheated, fresh and aged emulsions suggest that the aging effect is caused by slow conformational changes of the protein molecules in the adsorption layer, accompanied with partial loss of the ordered secondary structure of the protein and with the formation of lateral noncovalent bonds (H-bonds and hydrophobic interactions) between the adsorbed molecules. After thermal treatment of the BLG emulsions, the molecules preserve their original secondary structure upon storage, which eliminates the aging effect.     

Effects of electrolyte concentration and pH on the coalescence stability of beta-lactoglobulin emulsions: experiment and interpretation

Experimental results are presented about the effects of ionic strength and pH on the mean drop-size after emulsification and on the coalescence stability of emulsions, stabilized by a globular protein beta-lactoglobulin (BLG). The mean drop-size is determined by optical microscopy, whereas the coalescence stability is characterized by centrifugation. In parallel experiments, the zeta-potential and protein adsorption on drop surface are determined. The experiments are performed at two different BLG concentrations, 0.02 and 0.1 wt%. The electrolyte concentration in the aqueous phase, C(EL), is varied between 1.5 mM and 1 M, and pH is varied between 4.0 and 7.0. The experiments show that the mean drop-size after emulsification depends slightly on C(EL), at fixed protein concentration and natural pH = 6.2. When pH is varied, the mean drop-size passes through a maximum at fixed protein and electrolyte concentrations. A monolayer protein adsorption is registered in the studied ranges of C(EL) and pH at low BLG concentration of 0.02 wt%. In contrast, a protein multilayer is formed at higher BLG concentration, 0.1 wt%, above a certain electrolyte concentration (C(EL) > 100 mM, natural pH). The experimental results for the emulsion coalescence stability are analyzed by considering the surface forces acting between the emulsion drops. The electrostatic, van der Waals, and steric interactions are taken into account to calculate the barriers in the disjoining pressure isotherm at the various experimental conditions studied. The comparison of the theoretically calculated and the experimentally determined coalescence barriers shows that three qualitatively different cases can be distinguished. (1) Electrostatically stabilized emulsions, with monolayer protein adsorption, whose stability can be described by the DLVO theory. (2) Sterically stabilized emulsions, in which the drop-drop repulsion is created mainly by overlapping protein adsorption multilayers. A simple theoretical model is shown to describe emulsion stability in these systems. (3) Sterically stabilized emulsions with a monolayer adsorption on drop surface.     

Methylcellulose-induced stability changes in protein-based emulsions

The emulsifying and oil-in-water stabilizing properties of methylcellulose (MeC) were investigated in bovine serum albumin (BSA)-based emulsions. The creaming stability, flocculation, surface concentration of BSA and MeC and droplet size were determined. Results obtained showed modifications of creaming rates that were related to MeC concentrations in the continuous and dispersed phases. Viscosity effects on creaming and changes in average droplet size (d₄₃) relating to droplet coverage were identified and delineated. Studies performed on macroscopic oil–water and air–water interfaces were used to identify interfacial structuring and composition. A good agreement was found between droplet surface composition and the resistance to coalescence of emulsion droplets. Emulsions that demonstrated a more rigid-like adsorbed interfacial layer were more stable with respect to coalescence. This study involving model emulsion systems provides a new insight into the stability of industrial preparations containing mixtures of proteins and polysaccharides.     

Flocculation and Coalescence of Oil-in-Water Poly(dimethylsiloxane) Emulsions

The stability of poly(dimethylsiloxane) (PDMS) oil-in-water emulsions has been investigated in the presence of added NaCl as well as in the presence of added surfactant. The emulsions were prepared using a combination of nonionic (C(x)E(y), x and y represent the number of methylene (C) and ethylene oxide (E) groups, respectively) and cationic (quarternary alkylammonium) surfactants. The droplets were observed to exhibit weak flocculation in the presence of high NaCl concentration (1 M). Phase separation and optical microscopic observations revealed that the principal mechanism for emulsion destabilization at high salt concentration was coalescence, which was accelerated at elevated temperature (50 degrees C). The effective coalescence rate for diluted emulsions was investigated using photon correlation spectroscopy. The small effective Hamaker constant for PDMS is the primary reason for the slow rate of coalescence observed for the emulsions at neutral pH in the presence of NaCl. The stability of PDMS emulsions to flocculation is qualitatively similar to that reported for low Hamaker constant dispersions (e.g., microgel particles). Addition of cationic surfactants (cetyltrimethylammonium chloride and dodecyl dimethylbenzylammonium chloride) to the negatively charged droplets after preparation was shown to decrease the emulsion stability once the surfactant concentration exceeded the CMC. Electrophoretic mobility measurements showed that added cationic surfactant changed the sign of the droplet charge from negative to positive at concentrations well below the CMC. Charged micelles of the same sign as the droplets are electrostatically excluded from close approach to the droplet surface within a distance (varepsilon) which results in depletion flocculation. Copyright 2000 Academic Press.     

Adsorption behaviour of lactoferrin in oil-in-water emulsions as influenced by interactions with beta-lactoglobulin

Oil-in-water emulsions (pH 7.0 or pH 3.0) containing 30 wt% soya oil and various concentrations of lactoferrin were made in a two-stage valve homogenizer. The average droplet size (d32), the surface protein coverage (mg/m2) and composition, and the zeta-potential of the emulsions were determined. The value of d32 decreased with increasing lactoferrin concentration up to 1%, and then was almost independent of lactoferrin concentration beyond 1% at both pH 7.0 and pH 3.0. The surface protein coverage of the emulsions made at pH 7.0 increased almost linearly with increasing lactoferrin concentration from 0.3 to 3%, but increased only slightly in emulsions made at pH 3.0 at lactoferrin concentrations >1%. The surface protein coverage of the emulsions made at pH 3.0 was lower than that of the emulsions made at pH 7.0 at a given protein concentration. The emulsion droplets had a strong positive charge at both pH 7.0 and pH 3.0, indicating that stable cationic emulsion droplets could be formed by lactoferrin alone. When emulsions were formed with a mixture of lactoferrin and beta-lactoglobulin (beta-lg) (1:1 by weight), the charge of the emulsion droplets was neutralized at pH 7.0 suggesting the formation of electrostatic complexes between the two proteins. The composition of the droplet surface layer showed that both proteins were adsorbed, presumably as complexes, from the aqueous phase at pH 7.0 in equal proportions, whereas competitive adsorption occurred between lactoferrin and beta-lg at pH 3.0. At this pH, beta-lg was adsorbed in preference to lactoferrin at low protein concentrations (1%), whereas lactoferrin appeared to be adsorbed in preference to beta-lg at high protein concentrations.     

Behaviour of formula emulsions containing hydrolysed whey protein and various lecithins

Formula emulsion systems are used as enteral, sports and health products. In some formulas addition of hydrolysed protein is necessary to guarantee ease of digestion and hypoallergenicity. In the low fat emulsion model an increase in the content of lecithin (phospholipid mixture) was required, in consideration of the advice of the Food and Nutrition Board (USA) for choline supplementation. The individual and interactive effects of whey protein isolate (WPI) or hydrolysate (WPH) (3.7 and 4.9% w/w), unmodified deoiled or hydrolysed lecithin (0.48 or 0.7% w/w) and carbohydrate in the form of maltodextrin with dextrose equivalent (DE) 18.5 or glucose syrup with DE 34 (11% w/w) on the properties of formula emulsions with 4% v/w sunflower oil, were investigated using a full factorial design. The emulsions were characterised by particle size distribution, coalescence stability, creaming rate, and also surface protein and lecithin concentration. WPI-containing emulsions proved to be stable against coalescence and showed only little creaming after 1 and 7 days standing. There was a significant increase in the mean droplet size and a significant deterioration of coalescence and creaming stability when WPH instead of WPI was used as the protein source, due to the lower number of large peptides and lower surface activity of the WPH. Increasing the WPH concentration led to an increase in oil droplet size and further deterioration of the stability of the emulsions. The starch hydrolysate and lecithin also significantly influenced the emulsion properties. Their influence was less strong when the emulsion contained WPI. Under the conditions used WPH-based emulsions were more stable, in terms of creaming and coalescence, when a low level of protein was used in conjunction with hydrolysed lecithin and glucose syrup. Oil droplets in emulsions containing unmodified lecithin in either the continuous or disperse phase and WPH in the continuous phase were very sensitive to coalescence. The addition of starch hydrolysates (DE 18.5) induced intensive flocculation and phase separation in these emulsions.     

Effect of cationic polyacrylamide adsorption kinetics and ionic strength on precipitated calcium carbonate flocculation

The effect of polymer adsorption kinetics and ionic strength on the dynamics of particle flocculation was quantified using a model system consisting of precipitated calcium carbonate (PCC) and cationic polyacrylamide (CPAM) at a low shear rate. All early flocculations detectable by a photodispersion analyzer (PDA) happened in nonequilibrium polymer adsorption regimes. We observed discrepancies in flocculation rates with the surface coverage theory, which is based on a simple monolayer adsorption model, in both early and late flocculation stages. For instance, the same amount of adsorbed CPAM reached at different polymer doses demonstrated different flocculating capabilities. This highlighted the importance of polymer adsorption kinetics upon flocculation. The transient conformation of the adsorbed CPAM during the kinetic process sometimes even superceded the adsorbed amount in the determination of PCC flocculation. Both antagonistic and synergetic effects of increased ionic strength on the CPAM-induced PCC aggregation were observed during early flocculation. However, late-stage PCC flocculation shared some similarities, irrespective of polymer dose and ionic strength. Despite the decreased amount of adsorbed polymer from the increased ionic strength, the combination of CPAM and salt, at certain concentrations, demonstrated a synergy to promote PCC aggregation more efficiently than the same amount of the respective components.     

Effect of adding anionic surfactant on the stability of Pickering emulsions

We investigated the structure and stability of dodecane-in-water emulsions stabilised by partially hydrophobised silica particles after dilution of the emulsions in solutions of sodium dodecyl sulfate and sodium chloride. The emulsions were stable to coalescence on dilution in salt solutions, but did cream over time. The rate and extent of creaming gradually decreased as the salt concentration in the diluted emulsion increased. Dilution in low concentrations of the anionic surfactant did not affect the emulsion stability to coalescence or alter the creaming behaviour of the emulsion. At surfactant concentrations above the critical micelle concentration, however, the rate and extent of creaming and flocculation of the drops were enhanced.     

FRAGMENTATION OF PRIMARY FLOCS IN EMULSIONS AND THE SUBSEQUENT REDUCTION OF COALESCENCE

In existing theories emulsion desiabilization is considered as the combined processes of irreversible flocculation and coalescence of dispersed droplets. This approach can be justified when the potential pit characterizing the energy of droplet interaction is sufficiently deep, i.e. excluding small droplet dimensions, strong electrosiatic repulsion and low electrolyte concentrations. For smaller droplet dimensions and stronger electrostatic repulsions the emulsion instability must be considered as a combined process of reversible flocculation and coalescence. In this paper a mathematical model that couples the kinetics of flocculation, coalescence and floe fragmentation is developed in order to quantify the kinetic instability of emulsions with charged submicron droplets. The characteristic limes for flocculation (Smoluchowski's time τ_c) for coalescence (coalescence time τ_c) and for disaggregation (doublet lifetimeτ_d) are considered model parameters. The mathematical model applies to the case when and τ_d<< τ_c, which corresponds to a situation with a small multiplet concentration compared to the concentration of doublets and a singlet-doublet quasi-equilibrium. It is established that at singlet-doublet quasi-equilibrium the rate of the decline in the total droplet concentration is described by second order kinetics in distinction to the exponential time dependence valid for coalescence at irreversible flocculation. The double disintegration reduces the entire coalescence rate, expressed as τ_sm/ τ_d. This reduction is very large at small values of Td. The mathematical model presented can hased on the spontaneous disintegration of doublets predict changes in emulsion stability for model systems and also for technologically important emulsions.     

Discussion of the Drop Rest Phenomenon at Millimeter Scale and Coalescence of Droplets at Micrometer Scale

Adsorption of surfactants at water-oil interfaces is of great importance in the coalescence of drops and stability of emulsions. In this work, we have studied the adsorption of nonionic surfactants Span 80 at water-oil interfaces and its influence on the drop rest phenomenon and W/O emulsion stability in a pulsed DC electrical field. The variation of interfacial tension with the concentration of surfactant was studied and the data were fitted using a surface equation of state derived from the Langmuir adsorption isotherm. A stochastic model for coalescence was used to fit the coalescence time distributions. The significance of the model parameters was discussed. The stability of the emulsion was evaluated by conductivity methods. The researches in this article indicated that both of the rest time distribution of the drops at the interface and stability of the emulsion in the electrical field was significantly affected by surfactant concentration.     

Hydrophobicity Enhances the Formation of Protein-Stabilized Foams

Screening proteins for their potential use in foam applications is very laborious and time consuming. It would be beneficial if the foam properties could be predicted based on their molecular properties, but this is currently not possible. For protein-stabilized emulsions, a model was recently introduced to predict the emulsion properties from the protein molecular properties. Since the fundamental mechanisms for foam and emulsion formation are very similar, it is of interest to determine whether the link to molecular properties defined in that model is also applicable to foams. This study aims to link the exposed hydrophobicity with the foam ability and foam stability, using lysozyme variants with altered hydrophobicity, obtained from controlled heat treatment (77 °C for 0–120 min). To establish this link, the molecular characteristics, interfacial properties, and foam ability and stability (at different concentrations) were analysed. The increasing hydrophobicity resulted in an increased adsorption rate constant, and for concentrations in the protein-poor regime, the increasing hydrophobicity enhanced foam ability (i.e., interfacial area created). At higher relative exposed hydrophobicity (i.e., ~2–5 times higher than native lysozyme), the adsorption rate constant and foam ability became independent of hydrophobicity. The foam stability (i.e., foam collapse) was affected by the initial foam structure. In the protein-rich regime—with nearly identical foam structure—the hydrophobicity did not affect the foam stability. The link between exposed hydrophobicity and foam ability confirms the similarity between protein-stabilized foams and emulsions, and thereby indicates that the model proposed for emulsions can be used to predict foam properties in the future.     

Flocculation and stabilization of colloidal silica by the adsorption of poly-diallyl-dimethyl-ammoniumchloride (PDADMAC) and of copolymers of DADMAC with N-methyl-N-vinyl- acetamide (NMVA)

 The stabilization and flocculation behavior of colloidal silica-particles with cationic polyelectrolytes (PE) is investigated. The zetapotentials, diffusion coefficients and flocculation rate constants of silica particles have been measured as a function of the adsorbed amount of cationic polyelectrolytes poly(diallyl-dimethyl-ammoniumchloride) (PDADMAC) of different molar masses and of statistic copolymers of DADMAC and N-methyl-N-vinyl-acetamide (NMVA) of various compositions at different salt concentrations and pH-values. Very fast flocculation due to van der Waals attraction occurs if the zetapotential is small. At low ionic strength this condition occurs just below the plateau of the adsorption isotherms where the surface charges are screened by adsorbed polycations. Additionally with high molecular polycations slow mosaic flocculation is observed at lower PE concentrations. At high ionic strength fast flocculation takes place at low macroion concentration due to the screening of the surface charges by adsorbed polycations and salt ions. At medium concentrations of polycations below plateau adorption slow bridging flocculation is observed. At plateau adsorption the suspensions become stabilized up to high ionic strength. At low salt concentration charge reversal at full coverage with polycations results in electrostatic repulsion. At high ionic strength the particles are stabilized sterically due to the osmotic repulsion of the long adsorbed PE tails. Therefore macroions of high molar mass are necessary to stabilize the suspension at high ionic strength. Received: 27 January 1998 Accepted: 23 March 1988     

Stability of Emulsions Containing Both Sodium Caseinate and Tween 20

The creaming and rheology of oil-in-water emulsions (30 vol% n-tetradecane, pH 6.8) stabilized by a mixture of commercial sodium caseinate and the non-ionic emulsifier polyoxyethylene sorbitan monolaurate (Tween 20) has been investigated at 21 degrees C. The presence of sufficient Tween 20 to displace most of the protein from the emulsion droplet surface leads to greatly enhanced emulsion creaming (and strongly non-Newtonian rheology) which is indicative of depletion flocculation by nonadsorbed surface-active material (protein and emulsifier). In emulsions containing a constant amount of surface-active material, the replacement of a very small fraction of Tween 20 by caseinate in a stable pure Tween 20 emulsion leads to enhanced creaming for a small fraction of the droplets, and this fraction increases with increasing replacement of emulsifier by protein. This behavior is probably due to depletion flocculation, although an alternative bridging mechanism is also a possibility. The overall stability of these sets of emulsions can be represented in terms of a global stability diagram containing regions of bridging flocculation and coalescence (low content of surface-active material), stability (intermediate content), and depletion flocculation (high content). Copyright 1999 Academic Press.     

Adsorption of polyelectrolytes on colloidal latex particles, electrostatic interactions and stability behaviour

The stabilization and flocculation behaviour of colloidal latex particles covered with cationic polyelectrolytes (PE) is studied with photon correlation spectroscopy and zetapotential measurements. Diffusion coefficients, flocculation rate constants and zetapotentials have been determined as a function of adsorbed amount of cationic poly-(diallyl-dimethyl-ammoniumchloride) (PDADMAC) of different molar masses and of statistic copolymers of DADMAC and N-methyl-N-vinyl-acetamide (NMVA) of various compositions in water and at high ionic strength. Flocculation by van der Waals attraction can be observed if the zetapotential is low. This occurs, if the surface charge is screened by the oppositely charged cations. Furthermore, in the case of adsorption of high molecular polycations mosaic flocculation occurs if the adsorbed amount is low. At high ionic strength, flocculation takes place if the adsorbed amount is below the adsorption plateau. If the adsorption plateau is reached the suspensions become stabilized. In water the charge reversal at full coverage leads to electrosteric stabilization both with low and high molar mass polycations. At high ionic strength only polycations with high molar mass are able to stabilize the suspension. If a certain molar mass of the polycation is exceeded, steric stabilization of the suspension occurs due to the formation of long adsorbed PE tails and their osmotic repulsion. The layer thicknesses are determined as a function of the molar mass. Received: 4 July 2000/Accepted: 18 August 2000