Steric stabilization emulsions

March GC, Napper DH. The thermodynamic limit to the flocculation stability of sterically stabilized emulsions. J Colloid Interface Sci 1977 61 383-387. 

The thermodynamic limit to the stability of sterically stabilized emulsions was investigated by March and Napper (1977). It would be expected that in this limit, both oil-in-water and water-in-oil emulsions would obey the same general rules as are observed for sterically stabilized latices (Phillips, 1975). 

The thermodynamic limit to the stability of sterically stabilized emulsions ... 

The above emulsion system is fairly simple, since it is likely that the nonyl phenyl and propylene oxide chain is on the oil side of the interface, whereas the poly(ethylene oxide) chain is on the aqueous side of the interface. The hydrodynamic thickness of the surfactants, given above, is much smaller than the droplet radius and hence these sterically stabilized emulsions may approximate hard-sphere dispersions very closely (with an effective radius = R + 8 ). This can be tested by fitting the data to the hard-sphere model suggested by Dougherty and Krieger (40,41). 

Because of this interaction, PVP has found appHcation in surfactant formulations, where it functions as a steric stabilizer for example to generate uniform particle-size polystyrene emulsions (110—112). In a variety of formulations, a surfactant s abiHty to emulsify is augmented by PVP s abiHty to stabilize coUoids stericaHy and to control rheology. 

Monosized polystyrene particles in the size range of 2-10 /am have been obtained by dispersion polymerization of styrene in polar solvents such as ethyl alcohol or mixtures of alcohol with water in the presence of a suitable steric stabilizer (59-62). Dispersion polymerization may be looked upon as a special type of precipitation polymerization and was originally meant to be an alternative to emulsion polymerization. The components of a dispersion polymerization include monomers, initiator, steric stabilizer, and the dispersion medium... 

In addition to adsorbing at mineral-oil interfaces, asphaltene molecules also adsorb at oil-water interfaces. Strong intermolecular dipole-dipole, hydrogen bonding, electron donor-acceptor and acid-base interactions cause the surface-adsorbed asphaltene molecules to form rigid skins" at oil-water interfaces (41 43). When water droplets are dispersed in an oil which contains asphaltene molecules, molecularly thick, viscous asphaltene films form around the water droplets, inhibit the drainage of intervening oil and sterically stabilize the water-inoil emulsion. 

A new class of amphiphilic, surface-active graft copolymers, hydrophobically modified hydroxyethyl celluloses (HM-HEC s), are comprised of a cellulose backbone with short polyethylene oxide (PEO) and grafted alkyl side chains. They are excellent steric stabilizers of 0/W emulsions. 

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). 

H. Sonntag, B. Unterberger, and S. Zimontkowski Experimental Investigation of the Steric Stabilization of Emulsions by (Poly)Vinyl Alcohol. Colloid Polym. Sci. 257, 286 (1979). 

Capek I. Sterically and electrosterically stabilized emulsion polymerization. Kinetics and preparation. Adv Colloid Interface Sci 2002 99 77-162. 

Electrostatic and non-electrostatic biopolymer complexes can also be used as effective steric stabilizers of double (multiple) emulsions. In this type of emulsion, the droplets of one liquid are dispersed within larger droplets of a second immiscible liquid (the dispersion medium for the smaller droplets of the first liquid). In practice, it is found that the so-called direct water-in-oil-in-water (W/O/W) double emulsions are more common than inverse oil-in-water-in-oil (O/W/O) emulsions (Grigoriev and Miller, 2009). In a specific example, some W/O/W double emulsions with polyglycerol polyricinoleate (PGPR) as the primary emulsifier and WPI-polysaccharide complexes as the secondary emulsifying agent were found to be efficient storage carriers for sustained release of entrapped vitamin Bi (Benichou et al., 2002). 

Therefore, two contributory factors may provide an explanation for more effective electrostatic / steric stabilization of the so-called mixed emulsions in comparison with the sequentially assembled biopolymer interfaces of the bilayer emulsions firstly, a greater hydrophilicity of the adsorbed protein-polysaccharide complexes, caused by the larger net negative charge, and, secondly, a more bulky architecture of the normal complexes as compared to the interface complexes. 

In principle, an emulsion contained within an emulsion droplet, as shown schematically in Figure 7.24, can be regarded as an excellent reservoir for "active matter . Benichou et al. (2004) have suggested that complexes of proteins with polysaccharides can be employed to improve the steric stabilization of such a double ("duplex ) emulsion by forming a thick... 

In accord with experiments on emulsions (Husband et al., 1997), the molecular configurations deduced from SCF calculations have demonstrated the crucial role of the cluster ( blob ) of 5 charged phosphoserine residues in p-casein in maintaining the steric stabilizing layer, whilst also preventing interfacial precipitation (multilayers). The mobility of this blob was demonstrated experimentally by P NMR measurements on P-casein-stabilized emulsions (ter Beek et al., 1996). It was inferred that, when the effective charge on the blob is reduced (by dephosphorylation) or screened (by salt addition), the macromolecular spring relaxes... 

A series of amphiphilic diblock macromonomers were successfully used as steric stabilizers in the emulsion polymerization styrene [98]. Copolymerization led to the formation of polymer latexes of high colloidal stability. These... 

Steric stabilization of poly(2-ethyl hexyl acrylate) (PEHA) emulsions in C02 was studied with static and dynamic light scattering (DLS) (O Neill et al., 1997 Yates et al., 1997). These emulsions were stabilized with PFOA-based surfactants. Figure 8.9 shows the average droplet size (hydrodynamic radius) measured by DLS for a liquid PEHA emulsion in C02 stabilized... 

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