Surfactants multiple emulsions

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

Slow release rates and remarkable long shelf-life (months) were obtained compared to typical multiple emulsions stabilized by two short surfactants (SMO and polyoxyethylene (20) sorbitan monolaurate). Finally, the long lifetime of the emulsions allowed study via diffusing wave spectroscopy (DWS) of the interactions between the droplets and the globule surface [37],... 

N. Jager-Lezer, I. Terrisse, E. Bruneau, S. Tokgoz, L. Eerreira, D. Clausse, M. SeiUer,d and J.L. Grossiord Influence of Lipophihc Surfactant on the Release Kinetics of Water-Soluble Molecules Entrapped in a W/O/W Multiple Emulsion. J. Controlled Release 45, 1 (1997). 

Y. Sela, Y. Magdassi, and N. Garti Polymeric Surfactants Based on PolysUoxanes-Graft-Poly(Oxyethylene) for Stabilization of Multiple Emulsions. Colloids Surfaces 83, 143 (1993). 

A dispersion of liquid-in-gas-in-liquid in which a droplet of liquid is surrounded by a thin layer of gas that in turn is surrounded by bulk liquid. Example In an air-aqueous surfactant solution system this dispersion would be designated as water-in-air-in-water, or W/A/W, in fluid film terminology. A liquid-liquid analogy can be drawn with the structures of multiple emulsions. See also Fluid Film. 

No systematic studies of the use of silicone surfactants as emulsifiers have yet been published. Silicone polyoxyalkylene copolymers with relatively high molecular weight and a high proportion of silicone are effective water-in-silicone oil emulsifiers and a recent study of these copolymers suggests that they stabilize emulsions by a solid-particle mechanism [68]. This type of silicone surfactant has been used to prepare transparent water-in-oil emulsions (often with an active ingredient in the internal phase) for use as deodorants or antiperspirants as well as cosmetics and other personal care products. Their use as drug delivery vehicles has also been claimed. These copolymers can also be used to prepare multiple emulsions not requiring a two-pot process. 

The examples given here involve lung surfactant replacement compositions and surfactant systems used for 2D protein crystallization. Other potential systems include direct, reverse and multiple emulsions for drug and gene delivery, as well as micro- and nano-sized gas bubbles for O2 delivery and diagnosis [3, 4]... 

In the other procedure, Rojas et al. [215] optimized the encapsulation of BLG within PLGA microparticles prepared by the multiple emulsion solvent evaporation method. The role of the pH of the external phase and the introduction of the surfactant tween 20, in the modulation of the entrapment and release of BLG from microparticles were studied. Better encapsulation of BLG was noticed on decreasing the pH of the external phase. Addition of tween 20 increased the encapsulation efficiency of BLG and considerably reduced the burst release effect. 

Oil-in-water emulsions lend themselves readily to the delivery of oils and oil-soluble bioactives. The surfactant or biopolymer provides a means of isolating and protecting the lipophihc cores. Many types of materials with emulsifying capacity have been used to encapsulate oils and oil-soluble bioactives in single and multiple emulsion systems. Multilayered interfaces have also been used to improve the robustness of microcapsules. 

Polyoxyethylene alkyl ethers are nonionic surfactants widely used in topical pharmaceutical formulations and cosmetics, primarily as emulsifying agents for water-in-oil and oil-in-water emulsions and the stabilization of microemulsions and multiple emulsions. 

The chapter has dealt with the stability and stabilisation of colloidal systems and covered topics such as their formation and aggregation. If the particle size of a colloidal particle determines its properties (such as viscosity or fate in the body), then maintenance of that particle size throughout the lifetime of the product is important. The emphasis in the section on stability is understandable. Various forms of emulsions, microemulsions and multiple emulsions have also been discussed, while other chapters deal with other important colloidal systems, such as protein and polymer micro- and nanospheres and phospholipid and surfactant vesicles. 

The current state OF THE ART of various aspects of macro- and microemulsions is reflected in this volume. The symposium upon which this volume is based was organized in six sessions emphasizing major areas of research. Major topics discussed include a review of macro- and microemulsions, enhanced oil recovery, reactions in microemulsions, multiple emulsions, viscoelastic properties of surfactant solutions, liquid crystalline phases in emulsions and thin films, photochemical reactions, and kinetics of microemulsions. 

One of the main drawbacks to the commercial development of multiple emulsions is their inherent instability. The intention of this paper is to review studies on the stability and mechanism of breakdown of multiple systems and attempts to minimise such instability, for example, by appropriate choice of surfactant, polymerisable surfactants or gelation of the aqueous or oily phases. 

Multiple emulsions which will possess some degree of stability may be prepared by using pairs of surfactants, one of which will stabilize a w/o emulsion (lipophilic) and this forms the basis for the intentional preparation of multiple systems. 

Apart from the fact that the use of the HLB system is limited as it is based on the observation of creaming or separation of the emulsions, as an index of instability the HLB system also neglects the effects of surfactant concentration on stability (26) and of course it is irrelevant to the particular problems with multiple emulsion systems. Nevertheless, it provides a useful approach to the choice of optimal surfactant system. In general, in a w/o/w emulsion, the optimal HLB value of the primary surfactant will be in the range 2-7 and in the range 6-16 for the secondary surfactant. Equilibration of the systems after mixing will undoubtedly result in the transfer of surfactant between the aqueous and nonaqueous components. Saturation of the phases with the two surfactants used should prevent instability during this equilibration. 

Figure 2. Change in droplet diameter of multiple emulsions as a function of the concentration of secondary emulsifier (il) and of the calculated weighted or apparent HLB of the surfactant system. Hatched regions represent boundaries for inversion. Reprinted with permission from Ref. 28. Copyright 1979, Academic Press.

The presence of liquid crystal structures at both the w-o and o-w interfaces in multiple emulsions has been investigated by Kavaliunas and Frank (31). Microscopic examination of w/o/w emulsions between crossed polarizers revealed the presence of liquid crystal phases at both inner (w-o) and outer (o-w) interfaces in a w/o/w system composed of water, p-xylene and nonylphenol diethylene glycol ether. Liquid crystalline phases were also detected in o/w/o emulsions at both interfaces. The presence of these liquid crystal structures was found to improve the stability of the emulsions markedly. Matsumoto (32, 33) have concluded that the oil layers in w/o/w systems are likely to be composed of or contain,at least in proximity to the aqueous phase,multilamellar layers of the lipophilic surfactant used in the formulation this is postulated in part to explain the rate of volume flux of water through the oily layer. 

The nature of entrapped materials may have a bearing on the stability of the system. Due to the nature of the multiple emulsion, the middle phase may act as an osmotic reservoir, thus virtually all additions to this phase will set up osmotic gradients. This might include high concentrations of surfactant. To this end polymeric microspheres have been used as the internal reservoir when osmotic transfer of water will not compromise stability. 

Figure 7. Three possible interactions (.attractive and repulsive) between phases in multiple emulsions are shown in the upper diagram, while below those arrangements of the aqueous (W), surfactant (s and Sg) and oil phases are shown which must be taken into account in calculation of attractive and repulsive forces in these interactions. ...

Knowledge of surfactant equilibration and interactions will probably lead to improved formulations of multiple emulsions. Failing this the use of polymerisable surfactants can lead to obvious strengthening of interfacial barriers and allow control of stability and drug release. Nonetheless further detailed work on both w/o/w and o/w/o systems is justified. 

Silva Chunha, A. Grossiord, J.L. Seiller, M. The formulations and industrial applications of multiple emulsions an area of fast development. In New Products and Applications in Surfactant Technology, Karsa, D.R., Ed. CRC Press LLC Boca Raton, FL, 1998 Vol. 1, 205-226. 

Several classes of formulations of disperse systems are encountered in the chemical industry, including suspensions, emulsions, suspoemulsions (mixtures of suspensions and emulsions), nanoemulsions, multiple emulsions, microemulsions, latexes, pigment formulations, and ceramics. For the rational preparation of these multiphase systems it is necessary to understand the interaction forces that occur between the particles or droplets. Control of the long-term physical stability of these formulations requires the application of various surfactants and dispersants. It is also necessary to assess and predict the stability of these systems, and this requires the application of various physical techniques. 

---