Emulsion Surface Research Articles

Flocculation Transitions of Weakly Charged Oil-in-Water Emulsions Stabilized by Different Surfactants

This study concerns dodecane-in-water emulsions stabilized primarily by a range of nonionic surfactants. The emulsions are stable with respect to coalescence and Ostwald ripening but unstable with respect to flocculation and creaming. Sharp and reversible transitions between a flocculated state and a nonflocculated state can be induced by the addition of small mole fractions of either the anionic surfactant sodium octadecyl sulfate or the cationic surfactant octadecyl trimethylammonium bromide and were measured as a function of NaCl concentration. The flocculation transitions are modeled in terms of three types of colloidal forces: a short-range repulsion modeled as a “hard wall”, van der Waals attraction, and electrostatic repulsion. The flocculation transition is taken to correspond to the ionic surfactant concentration for which the maximum in disjoining pressure equals the capillary pressure of the drops. Comparison of calculated and experimental results shows reasonable agreement for emulsions stabilized by n-decyl-β-d-glucoside and 1-O-n-decyl-d,l-xylitol. Emulsions stabilized by n-decyl-N-methylglucamide and n-dodecyl octaoxyethylene and n-hexadecyl 20-oxyethylene glycol ethers (C12E8 and C16E20) reveal a significant negative charge on the drop surfaces in the absence of added ionic surfactant. Additionally, we find that the surface charging of the surfactants with sugar headgroups by addition of charged surfactants is correctly predicted from the concentration of added ionic surfactant with the assumption that the ionic surfactant distributes equally between the emulsion surface monolayer and the micelles present in the continuous phase. For C12E8 and C16E20, the added ionic surfactant is found to distribute preferentially to the emulsion surfaces.

The Stabilization of Water-in-Hydrocarbon Emulsions by Asphaltenes and Resins

The role of asphaltenes and resins in stabilizing water-in-crude oil emulsions was investigated by measuring the interfacial composition and stability of model emulsions composed of water with mixtures of toluene, heptane, asphaltenes, resins, and native solids. The interfacial composition (mass surface coverage) was determined from a combination of emulsion surface area measurements and concentration measurements of both the continuous and the emulsion phases. The emulsion surface area was calculated from drop size distributions measured with optical microscopy. The concentrations were found from gravimetric analysis. The stability of the model emulsions was assessed from the amount of water resolved after heating and periodic centrifugation of the emulsions. Asphaltene surface coverage was found to increase with an increase in the asphaltene bulk concentration until a limiting surface coverage was achieved. Surprisingly, while asphaltenes always tend to stabilize these emulsions, the stability of the emulsions decreased as asphaltene surface coverage increased. This change in stability was attributed to a change in the asphaltene configuration on the interface. The addition of a good solvent was found to reduce both the amount of adsorbed asphaltenes and the emulsion stability. The addition of resins always destabilized model emulsions. It appears that resins act as a good solvent for the asphaltenes and, at sufficiently high concentrations, are able to replace asphaltenes on the interface. Naturally occurring solids that coprecipitate with asphaltenes had little or no effect on asphaltene adsorption but dramatically increased emulsion stability. The results suggest that the combination of asphaltenes and native solids causes the most stable emulsions.

Surface properties of Ascaris suum eggs: hydrophobic potential and Lewis acid–base interactions

The cell surface properties of Ascaris suum eggs were investigated to determine the hydrophobic properties of eggs of different origins. Hydrophobic potential measurements were based on adhesion to hydrocarbons. Freshly laid eggs with or without the external coat and eggs that had been stored for 3 months at −20°C were tested. The hydrophobic potential differed according to the hydrocarbon used (hexadecane or octane) and the eggs tested (stored or fresh). The hydrophobic potential with octane was 95% for fresh eggs and 72% for eggs stored at −20°C. Tests with Ascaris eggs with and without an external coat showed similar hydrophobic characteristics; the presence or absence of this external structure did not appear to influence hydrophobic interactions. Acidic interactions also appeared to play no part in the adhesion of eggs to the surface of octane emulsions.

Influence of xanthan gum on the formation and stability of sodium caseinate oil-in-water emulsions

Studies have been made of the changes in droplet sizes, surface coverage and creaming stability of emulsions formed with 30% (w/w) soya oil, and aqueous solution containing 1 or 3% (w/w) sodium caseinate and varying concentrations of xanthan gum. Addition of xanthan prior to homogenization had no significant effect on average emulsion droplet size and surface protein concentration in all emulsions studied. However, addition of low levels of xanthan (≤0.2 wt%) caused flocculation of droplets that resulted in a large decrease in creaming stability and visual phase separation. At higher xanthan concentrations, the creaming stability improved, apparently due to the formation of network of flocculated droplets. It was found that emulsions formed with 3% sodium caseinate in the absence of xanthan showed extensive flocculation that resulted in very low creaming stability. The presence of xanthan in these emulsions increased the creaming stability, although the emulsion droplets were still flocculated. It appears that creaming stability of emulsions made with mixtures of sodium caseinate and xanthan was more closely related to the structure and rheology of the emulsion itself rather than to the rheology of the aqueous phase.

Quantitative analysis of surface-located triacylglycerol in intact emulsion particles.

The amount of triacylglycerol (TG) in the surface monolayer of intact phospholipid-stabilized emulsions was determined using (13)C nuclear magnetic resonance ((13)C NMR). (13)C NMR spectra of emulsions composed of bulk long-chain or medium-chain TG were prepared with [(13)C(3)]-carbonyl-enriched triolein, tripalmitin, or trioctanoin, and were analyzed and compared with NMR spectra of phosphatidylcholine vesicles with and without added TG. Identification of carbonyl peaks intermediate between those of phosphatidylcholine carbonyls and bulk TG confirmed the presence of surface TG in each emulsion. The surface of emulsions contained 2.2 mol % tripalmitin and 1.4 mol % triolein, but significantly more medium-chain TG, 9.1 mol % trioctanoin, as predicted by measurements of TG in phospholipid vesicles. Thus, medium-chain TGs are more accessible than long-chain TGs to enzymes or pro-oxidants in the continuous phase of phospholipid-stabilized emulsion systems. The quantitative determination of surface-located TG in intact particles will advance the understanding of emulsion colloidal properties, physicochemical stability, and metabolic behavior.

On the Viscosity of Dilute Emulsions

The slow motion of a liquid droplet in a shear flow in the presence of surfactants is studied. The effects of the interfacial viscosity, Gibbs elasticity, surface diffusion and bulk diffusion of surfactants in both phases are taken into account. The analytical solution of the problem for small Reynolds and Peclet numbers gives a simple criterion for estimation of the tangential mobility of the droplet interface. By applying the standard procedure for averaging of the stress tensor flux at an arbitrary surface of the dilute emulsion, an analytical formula for the viscosity of emulsions in the presence of surfactants is derived. The result is a natural generalization of the well-known formula of Einstein for the viscosity of monodisperse dilute suspensions and of the expressions derived by Taylor and Oldroyd for the viscosity of monodisperse dilute emulsions taking into account the Marangoni effect.

Modulation of apolipoprotein E-mediated plasma clearance and cell uptake of emulsion particles by cholesteryl ester.

Cholesteryl ester, along with triglyceride (TG), is the major core component of plasma lipoproteins. We investigated the effect of core composition on the physical state and metabolic behavior of lipid emulsions, as model particles of lipoproteins. Fluorescence studies using 1,6-diphenylhexatriene analogs showed that although cholesteryl oleate (CO) significantly decreased core mobility, the surface rigidity of phosphatidylcholine (PC) monolayers was independent of core composition. When intravenously injected into rats, the increased amount of core CO tended to retard TG emulsion removal from plasma, and the initial clearance rate was correlated with the amount of apolipoprotein E (apoE) bound from plasma. In addition, PC liposomes with a similar emulsion particle size showed negligible binding of apoE and were cleared at a slower rate compared to all emulsions. Furthermore, the effect of CO on the binding behavior of apoE to the emulsion surface and the emulsion uptake by hepatocytes was assessed in vitro. Replacing core TG with CO was found to decrease the apoE binding capacity to emulsions markedly without changing the binding affinity and thereby to reduce the cell uptake of emulsion particles by HepG2 cells. These results indicate that the physical state of core lipids, which can be modulated by CO content, plays a role in emulsion metabolism through the alteration in apoE binding.

Inhibition of lipoprotein lipase activity by sphingomyelin: role of membrane surface structure

We have recently shown that sphingomyelin (SM) strongly inhibits lipoprotein lipase (LPL)-mediated lipolysis in monolayers and emulsion particles. To further evaluate how SM modulates LPL activity on the emulsion surface, the relationship between membrane surface structure and LPL activity was investigated. We measured fluorescence anisotropy of 1-palmitoyl-2-[3-(diphenylhexatrienyl)propionyl]- sn-3-phosphatidylcholine, probing surface acyl chain fluidity, and fluorescence lifetime of N-(5-dimethylaminonaphthalene-1-sulfonyl)dipalmitoylphosphatidylethanolamine in H 2O and D 2O buffer, assessing the degree of hydration in the head group region. The results revealed that incorporation of egg SM into triolein-egg phosphatidylcholine emulsions markedly increased acyl chain order and decreased head group hydration of the surface monolayers. In contrast, cholesterol was shown to increase head group hydration despite a strong increase in acyl chain order. The close correlation between the apparent K m values of LPL and the degree of head group hydration indicated that LPL interacts with the head group region rather than with the hydrophobic interior of the surface monolayers. However, apparent V max did not show a simple correlation with any surface structure, and the finding in which SM had no effect on apparent V max of medium-chain triglyceride emulsions suggested that the hydrophobic interaction between acyl chains of SM and triglyceride at the emulsion surface is important for determining the apparent V max. These results showed conclusively that SM inhibits LPL activity mainly by changing the emulsion surface structure and not by a specific interaction between SM and LPL.

エマルション表面膜構造に依存するリポ蛋白質リパーゼ酵素反応

The hydrolysis of triglycerides (TG) by lipoprotein lipase (LPL) is a crucial process in the metabolism of TG-rich lipoproteins and artificial lipid emulsions injected intravenously. In this study, we found that sphingomyelin (SM) at the emulsion surface inhibits LPL-mediated lipolysis both in vivo and in vitro. Incorporation of SM into the emulsion surface caused an increase in the apparent Michaelis-Menten constant (Km (app)) and a decrease in the apparent maximal lipolysis rate (Vmax (app)). SM was also found to affect factors which may be related to the kinetic parameters; that is, SM increased TG solubility in surface layers and decreased apoC-II binding to the emulsion surface. Interestingly, cholesterol (Chol) did not affect the lipolysis rates although it decreased TG solubility and apoC-II binding. These results indicated that neither TG solubility at the surface layer nor amount of apoC-II binding are determining factors in LPL-mediated lipolysis under physiological conditions. Furthermore, on the basis of kinetic studies, we showed that SM inhibits lipolysis by decreasing both the binding affinity for emulsions and the catalytic activity of LPL.The mechanism by which SM at the emulsion surface inhibits lipolysis was also discussed. SM strongly increased head group packing probably due to the high capacity of forming hydrogen bonds, whereas Chol had little effect on the head group structure of the emulsion surface. Decreasing the head group mobility by SM could inhibit the insertion of the binding region of LPL protein, resulting in increases in Km (app). In addition, SM stabilizes TG in the surface layer and retards the transfer of TG from the lipid particle surface to the catalytic pocket of LPL, resulting in decreases in LPL catalytic activity and Vmax (app). Our results suggested that head group packing significantly affects LPL binding to the lipid surface and that TG stability in the surface layer is important for the LPL catalytic activity.From these results, the content of SM in the lipoprotein surface is presumed to play an important role in controlling LPL-mediated lipolysis by the mechanism described above. Artificial lipid emulsions are used as drug carriers, and the control of TG hydrolysis of lipid carriers is important for development of better drug delivery systems. The lipolysis activity can be modulated by surface lipid properties, i.e., the surface lipid composition.

Biological Emulsions

Lipid emulsions consisting of triglyceride cores and surface monolayers of phosphatidylcholine (PC) are physical models for plasma lipoproteins. Despite similar surface compositions, emulsion surface monolayers are more rigid in the acyl chain region but more hydrated in the interfacial region compared to bilayers. These features appear to be attributed to surface-core interactions in emulsions in which acyl chains of surface PC interdigitate to some extent with those of core lipid. Emulsion particles show quite different behavior from bilayer vesicles with respect to physico-chemical stability and physiological metabolism, indicating emulsion surface-core interactions to be importantly involved in physico-chemical and biological functions of lipid emulsions.

On the interfacial properties of micrometre–sized water droplets in crude oil

The remarkable stability of water–in–crude oil emulsions is due to the presence of a complex adsorbed layer at the surfaces of the dispersed droplets. Except for its role as a steric barrier, little is known about the in situ properties of this interfacial structure. In this study, new insights into the adsorbed layer are provided by direct micrometre–scale measurements. At low crude content in the bulk where, according to interfacial tension isotherms, there should be little or no surfactants on the droplet surface, the adsorbed layer displays pronounced rigidity and is capable of preventing coalescence and coagulation of the droplets. Such interfaces are highly dissipative and can be well described by the Boussinesq–Scriven model. As the supply of surface active materials in the bulk (i.e. the crude content) increases , the adsorbed layer transforms from a rigid structure to a fluid interface. This fluid layer continues to inhibit coalescence, although signs of weak interdroplet adhesion begin to appear. Under area compression, the fluid interface will discharge micrometre–sized emulsion droplets into the oil phase via a ‘budding’ mechanism.

Milk fat microencapsulation using whey proteins

Abstract The effects of homogenisation conditions (pressure and number of passes) and emulsion composition (fat, whey protein, lactose, salts and water) on the properties of emulsions and microencapsulated milk fat powders were measured. The emulsions were assessed by fat globule diameter and size stability over time. The properties of the powders measured were particle diameter, reconstituted emulsion fat globule diameter, free fat, surface fat and fat oxidation. As expected, increasing homogenisation pressure reduced the fat globule diameter and increasing the number of homogenisation passes reduced the diameter of the largest globules. Increasing fat and salts reduced fat globule diameter stability after 2 homogenisation passes but the reduction in stability was less after 4 passes. Increasing the lactose : whey protein concentrate ratio reduced free fat and fat globule aggregation after powder reconstitution, but not the surface fat. The higher level of fat increased the surface fat on powder particles and the level of oxidation during storage.

Progress in understanding emulsion metastability and surface forces

We provide an overview of the most recent advances in the field of surfactant and polymer-stabilized emulsions. The article is focused on inter-droplet forces, monolayer adhesion, aggregation of emulsion droplets, emulsion gels and finally destruction through coalescence and Ostwald ripening.

Some effects of heat on the competitive adsorption of caseins and whey proteins in oil-in-water emulsions

Abstract It is common industrially to subject food emulsions to heat treatments of different severity. This may be important when both caseins and whey proteins are present in the emulsion system, because the whey α-lactalbumin and β-lactoglobulin denature when they are heated at temperatures greater than about 70°C and may interact with κ-casein and αs2-caseins. At room temperature and neutral pH, caseins and whey proteins do not exchange readily between the interfaces of emulsion droplets and the bulk solution. Even at elevated temperatures (85°C), caseins cannot displace whey proteins adsorbed to the emulsion surface, nor do they interact with the adsorbed whey proteins. The caseins may however co-adsorb if the layer of adsorbed whey proteins is not saturated. Conversely, when whey proteins are added to an emulsion stabilized by caseinate, heating of the mixture causes the whey proteins to adsorb to the interface and to displace the αs1- and β-casein fractions, despite the fact that the latter are generally considered more hydrophobic. This is evidence that processing operations may profoundly change the structure and perhaps the properties of food emulsions stabilized by proteins.

The molecular structure of the surface of commercial cow's milk

The structure of the top 100 A of the air/liquid interface of commercial cow's milk has been determined by X-ray reflectometry. The excellent reflectivity from the surface of this dilute emulsion indicates the accessibility of other emulsion structures to the reflectivity method. The surface structure has been studied as a function of fat content (0.1–8.6 wt.%), aging and heating at temperatures up to 37°C. The interfacial structure is dominated by the presence of β-casein in this system, with a two layer model consisting of a more dense top layer, 18 A thick, and a less dense second layer, 29 A thick. Removal of the fat component by skimming hardly changes the surface density profile for fresh milk but clear differences are observed (presumably in the protein distribution) for aged and heated samples.

Recognition of Lipid Particle by Plasma Apolipoproteins.

Binding of plasma apolipoproteins to surface membrane of lipid particles causes the strong positive curvature stain. Although apolipoproteins can hardly bind to egg phosphatidylcholine planar surface in bilayers, many apolipoproteins bind to the emulsion surface with significantly higher capacity compared to bilayers. This marked difference in apolipoprotein binding between bilayers and emulsions can be explained by the difference in mechanisms by which the apolipoprotein-induced membrane curvature stress is relaxed. The binding capacity of apolipoprotein A-I increases with increasing spontaneous negative curvature in bilayers, whereas in emulsions, the contribution of core lipids to the relief of curvature stress appears to be the key event for apolipoprotein binding.

Preparation of Gadolinium-Containing Emulsions Stabilized with Phosphatidylcholine-Surfactant Mixtures for Neutron-Capture Therapy.

Gadolinium-containing lipid emulsions for neutron-capture therapy were designed to fulfill the following requirements : particle size smaller than 100 nm; gadolinium content as high as possible; surface of the emulsions modified with hydrophilic moieties to provide prolonged circulation in the blood. Emulsions containing soybean oil, water, Gd-diethylenetriaminepentaacetic acid-distearylamide (Gd-DTPA-SA), as an amphiphilic drug, and hydrogenated egg yolk phosphatidylcholine (HEPC), as an emulsifier, in a weight ratio of 7.36 : 92 : 1 : 2 were prepared without co-surfactants by the thin-layer hydration method using a bath-type sonicator. The mean particle size of the emulsions was 280.7 nm. In order to make the droplet size of the emulsions smaller than 100 nm, as well as to modify the emulsion surfaces, a co-surfactant, Tween[○!R] 80, HCO[○!R]-60, Pluronic[○!R] F68, polyoxyethylene alkyl ether (Brij[○!R]) or polyoxyethylene alkyl ester (Myrj[○!R]), was introduced into the standard system. Tween 80, HCO-60, Brij 76, 78 and 700 were effective in reducing the particle size to below 100 nm when the co-surfactant weight ratio (CWR), defined as co-surfactant/(HEPC+Gd-DTPA-SA) (w/w), was larger than 0.67; the particle size with Tween 80 and HCO-60 was reduced to 52.7 and 74.7 nm, respectively, at a CWR of 1.0 (w/w). In order to increase the gadolinium content, the weight ratio of Gd-DTPA-SA to HEPC was increased from 1 : 2 of the standard-Gd formulation to 2 : 1 of the high-Gd formulation. The measured particle size of the HCO-60 high-Gd emulsions was 78.7 nm when the CWR was 1.0 (w/w). In this case, the calculated gadolinium content reached 3.0 mg Gd/ml. These results indicate that HCO-60 is an effective co-surfactant not only in terms of particle size reduction but also with respect to gadolinium enrichment.

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

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

Surface composition regulates clearance from plasma and triolein lipolysis of lipid emulsions.

Sphingomyelin (SM) and cholesterol (Chol) are major surface lipid constituents of plasma lipoproteins. We investigated the effects of SM and Chol on the plasma clearance of lipid emulsions as a model for lipoprotein particles in rats. The presence of Chol facilitated the removal of emulsion particles from plasma, whereas SM delayed particle removal. Preinjection of lactoferrin, an inhibitor of the apolipoprotein E (apoE) receptor, revealed that the differences in clearance of emulsions were due to the differences in affinity for the apoE receptor. Measurement of apolipoprotein binding suggested that the balance of apoE and apoC (apoC-II and apoC-III) bound to emulsions caused the difference in plasma clearance of emulsion particles. That is to say, SM in the emulsion surface decreased binding of apoE, which led to a longer circulation of emulsion particles in plasma. Chol, on the other hand, decreased the ratio of apoC to apoE, which may have promoted emulsion uptake through the apoE receptor. We also examined in vitro lipolysis using immobilized lipoprotein lipase (LPL) in a heparin affinity column. Lipolysis rates were significantly reduced by the incorporation of SM into the emulsion surface, but not by the incorporation of Chol, indicating that SM in the lipoprotein surface is an important lipid component regulating LPL-mediated lipolysis. Our results suggest that the presence of SM and Chol in the lipoprotein surface plays an important role in the circulation behavior and LPL-mediated lipolysis of lipid emulsions through their effect on the selectivity of plasma protein binding.

Effect of amount of emulsifier added in the second stage on morphology of composite latex particles

In this article, PBA/P(MMA–crosslinking agent)-composite particle latexes were prepared by semicontinuous seeded emulsion polymerization. To determine the seed emulsion's saturating capacity of an emulsifier, a mathematical model was built to simulate the changes of the seed PBA emulsion's surface tension with the amount of emulsifier added dropwise. The effects of the emulsifier amount added in the second stage and the addition method on the morphology of the composite particles were studied. The results were shown as follows: If the amount of emulsifier added in one batch to the seed emulsion in the second stage was less than or equal to the saturating capacity of emulsifier of the seed emulsion (Cs), the morphology of the particles was “core–shell”; otherwise, a few particles were of a core–shell structure. However, if shell materials were preemulsified and added dropwise at an appropriate rate, the latex particles were still of a core–shell structure, even when the amount of emulsifier added to the seed emulsion was greater than the Cs. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 969–975, 1998