Preparation of a stable double emulsion (W1/O/W2): role of the interfacial films on the stability of the system.

This paper presents new protocols enabling preparation of W1/O/W2 double emulsions: one, using soybean oil as the O phase, that yields edible emulsions with industrial applications, and a second that yields emulsions with a previously unattainable concentration 15% (w/w) of surfactants in the external phase (the 15% target was chosen to meet the typical industry standard). Preparation of a stable W1/O emulsion was found to be critical for the stability of the system as a whole. Of the various low HLB primary surfactants tested, only cethyl dimethicone copolyol (Abil EM90), A-B-A block copolymer (Arlacel P135), and polyglycerol ester of ricinoleic acid (Grinstead PGR-90) yielded a stable W/O emulsion. Investigation of the surface properties of those surfactants using the monolayer technique found two significant similarities: (1) stable, compressible, and reversibly expandable monolayers; and (2) high elasticity and surface potential. The high degree of elasticity of the interfacial film between W1 and O makes it highly resilient under stress; its failure to break contributes to the stability of the emulsion. The high surface potential values observed suggest that the surfactant molecules lie flat at the O/W interfaces. In particular, in the case of PGR-90, the hydroxyl (-OH) groups on the fatty acid chains serve as anchors at the O/W interfaces and are responsible for the high surface potential. The long-term stability of the double emulsion requires a balance between the Laplace and osmotic pressures (between W1 droplets in O and between W1 droplets and the external aqueous phase W2). The presence of a thickener in the outer phase is necessary in order to reach a viscosity ratio (preferably approximately 1) between the W1/O and W2 phases, allowing dispersion of the viscous primary emulsion into the W2 aqueous phase. The thickener, which also serves as a dispersant and consequently prevents phase separation due to its thixotropic properties, must be compatible with the surfactants. Finally, the interactions between the low and high HLB emulsifiers at the O/W2 interface should not destabilize the films. It was observed that such destructive interaction for the system could be prevented by the use of two high HLB surfactants in the outer aqueous phase: an amphoteric surfactant, Betaine, and an anionic surfactant, sodium lauryl ether sulfate. The combination of such pairs of surfactants was found to contribute to the films' stability.

Concentrated emulsions of perflubron in aqueous media.

Flocculation of o/w emulsions consisting of a perfluorochemical (PFC) emulsified by either phospholipids or decaglyceroldioleate (10-2-O) was assessed both by direct observation and through photon correlation spectroscopy (PCS) and viscoelasticity measurements in unsteady oscillatory flow. Flocculation gives rise to emulsion instability but can be prevented (a) by the addition of a negatively charged surfactant to either phosphatidylcholine (PC) or 10-2-O, respectively the zwitterionic phospholipid and the nonionic surfactant used as primary emulsifiers, or (b) by using a saccharide solution as the continuous phase. The study indicates that both electrostatic (Coulombic) repulsive forces and hydration (steric) forces play a role in preventing flocculation. Various minor components of the egg yolk phospholipids (EYP) used in commercial emulsion preparation probably stabilize the emulsion by increasing both electrostatic and hydration repulsion.

Perfluorooctyl bromide dispersions in aqueous media for biomedical applications.

In studying perfluorooctyl bromide (PFOB) dispersions in aqueous media, we have used two types of surfactant: egg yolk phospholipids (EYP) and polyglycerol esters (PGE). Our interest in these dispersions arises from their potential biomedical applications as imaging solutions and oxygen-carrying solutions (i.e., blood substitutes). For EYP systems, we have identified the dispersion structure as consisting of (a) PFOB droplets (250-nm diameter) stabilized by a phospholipid monolayer adsorbed irreversibly at the o/w interface and (b) small empty phospholipid vesicles. With both surfactants commercial preparations yielded stable systems, while purified samples, being non-dispersible, could not be made to act as emulsifiers. In both cases, minor components in the commercial surfactant were found to be necessary for the formation of a stable dispersion, enabling the transport of the pure surfactant to the PFOB/water interface.