Steric emulsions, stabilizing

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Figure 2.22. (a) Disjoining pressure vs. thickness isotherm for an emulsion film stabilized by 0.1% BSA, ionic strength of 10 mol/1 NaCl, oil phase = hexadecane. The dots are the experimental data, dashed line is the double-layer contribution to the total disjoining pressure, and the solid line is the best fit done supposing additivity of the double-layer and steric forces, (b) Force vs. distance profiles for ferrofluid emulsions stabilized with mixed BSA-Tween-20 adsorption layers. The total concentration of the Tween-20 is kept constant = 5CMC, pH = 5.8. (Adopted from [78].)... 

In a previous paper 8), was inferred for com oil-in-water as well as toluene-in-water emulsions stabilize by bovine serum albumin (BSA). The effects of pH, ionic strength and BSA concentration on Hmax were investigated. Comparison of experimental maximum disjoining pressure witii predicted Ilmax indicated that steric interaction is the predominant mechanism of stabilization in such systems. 

For electrostatically or sterically interacting drops, emulsion viscosity will be higher when droplets are smaller. The viscosity will also be higher when the droplet sizes are relatively homogeneous, that is, when the drop size distribution is narrow rather than wide. The nature of the emulsifier can influence not just emulsion stability but also the size distribution, mean droplet size, and therefore the viscosity. To describe the effect of emulsifiers on emulsion viscosity Sherman [215] has suggested a modification of the Richardson Equation to the following form ... 

Non-dairy creams (cream alternatives) are O/W emulsions stabilized by milk proteins. A relatively thick adsorption layer provides stability, mostly by steric stabilization and partly by electrostatic stabilization [829]. Figure 13.3 shows an example of a soybean-oil and milk-protein emulsion stabilized by fat globules and protein membranes. Stabilizers, such as hydrocolloid polysaccharides, are added to increase the continuous phase viscosity and reduce the extent of creaming. They must be stable enough to have a useful shelf-life but de-stabilize in a specific way when they are... 

Certain comb-type silicone surfactants have been shown to stabilize emulsions in the presence of salts, alcohol and organic solvents that normally cause failure of emulsions stabilized using conventional hydrocarbon surfactants and a study by Wang et al. [66,67] investigated the cause of this stability. Interaction forces due to silicone surfactants at an interface were measured using AFM. Steric repulsion provided by the SPE molecules persisted up to an 80% or higher ethanol level, much higher than for conventional hydrocarbon surfactants. Nonionic hydrocarbon surfactants lose their surface activity and ability to form micelles in... 

Stability in plasma is an important requirement for IV emulsions as flocculated droplets may result in lung embolism. It was found that tocol-based emulsions stabilized by sodium deoxycholate/lecithins flocculated strongly when mixed with mouse, rat, and sheep plasma and serum, whereas soya oil-based emulsions with the same emulsifiers did not [123], It was hypothesized that this effect was caused by the adsorption of plasma proteins onto the tocol droplets (opsonization). Indeed, the steric stabilization of emulsions by incorporation of emulsifiers like poloxamer 188 or PEGylated phospholipids such as PEG5000PE proved to be effective in the stabilization of tocol-based emulsions in plasma. Conversely, in vitro studies were... 

Steric Hindrance. Another form of stabilization is relatively independent of ionic strength the oil droplets are prevented from making contact by simple steric hindrance. This may take two forms, either an immobilized water layer at the interface or a solid interfacial film. Emulsion stabilization by proteins, gums, and polyoxyethylene derivatives occurs by the first mechanism. Hydrophobic parts of the stabilizers adsorb at the oil surface, but adjacent large hydrophilic segments are hydrated and form an immobilized layer on the order of 10-100 nm thick (Figure 9). As mentioned, these hydrated segments frequently interact to cause flocculation, while coalescence of the oil drops themselves is prevented. Such emulsions are frequently used as carriers for oil-soluble flavors, essences, and colorants. 

The presence of surfactants, either natural or added, promotes emulsion stability by the reduction of interfacial tension and the formation of highly rigid films on the surface of the droplets. This reduction of interfacial tension can increase the maximum, M, in Figure 4 significantly through charge stabilization or steric stabilization (J5). Because the nature and shape of the interaction energy curve determine the stability of OAV (and other types) of emulsions, any process, parameter, or phenomenon that affects the shape of this curve will ultimately control emulsion stability. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

The important thickness and high monomer and charge densities encountered in polyelectrolyte brushes are expected to provide emulsion droplets with large steric and electrostatic repulsions. As a consequence, diblock polyelectrolyte surfactants are suitably designed to be effective emulsion stabilizers. Pincus [212] has proposed a description of polyelectrolyte brushes using an approach similar to that of Alexander and de Gennes for neutral brushes [75,213,214], The latter model is thus first presented before developing that of polyelectrolyte brushes. 

A polysaccharide can be added as a component in a protein system to produce a protein-polysaccharide composite structure. Tolstoguzov (2003) reviewed the main function of protein and polysaccharide in protein-polysaccharide food formulation. Generally, polysaccharides have less surface activity in comparison to proteins. This inferiority is related to their low flexibility and monotonic repetition of the monomer units in the backbone. The low surface activity of polysaccharides results in their inability to form a primary adsorbed layer in the system. The nature of interactions between polysaccharides and adsorbed proteins, as well as their influence on colloid stability, can either stabilize or destabilize the emulsions. Attractive protein-polysaccharide interactions can enhance the emulsion stability by forming a thicker and stronger steric-stabilizing layer. In contrast, the attractive interactions... 

Emulsion droplets are normally stabilized by enhancing the mechanical strength of the interfacial film formed around the oil droplets (38). by steric stabilizaticm effects, and/or by the presence of charged surfactants which create an electrostatic barrier. The stabilizing factor of the latter is the electrostatic repulsimi of similarly charged droplets. The emulsion stability can be considerably improved with the use of mixed emulsifying agents (39). 

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