Emulsions multilayer formation

When a biopolymer mixture is either close to phase separation or lies in the composition space of liquid-liquid coexistence (see Figure 7.6a), the effect of thermodynamically unfavourable interactions is to induce biopolymer multilayer formation at the oil-water interface, as observed for the case of legumin + dextran (Dickinson and Semenova, 1992 Tsapkina et al, 1992). Figure 7.6b shows that there are three concentration regions describing the protein adsorption onto the emulsion droplets. The first one (Cprotein< 0.6 wt%) corresponds to incomplete saturation of the protein adsorption layer. The second concentration region (0.6 wt% < 6 proiem < 6 wt%) represents protein monolayer adsorption (T 2 mg m 2). And the third region (Cprotein > 6 wt%) relates to formation of adsorbed protein multilayers on the emulsion droplets. 

Studies on PEM are of interest because of the versatility of multilayer formation process with respect to variety of support materials, combination with other assembly techniques and possibility of incorporation of different functional species [1-4]. PEM are applied in chemical and biochemical sensing or preparation of new biomaterials. When colloidal particles or emulsion droplets are used as cores for polyelectrolyte deposition one can obtain hollow micro- and nanocapsules. Such capsules have potential as drug delivery systems [5] capable of sustained release [6,7], microreactors [8,9] or catalytic systems [8-10]. Detailed summaries on the properties and application of PEM prepared by LbL technique can be found in [1,11]. 

As outlined in the introduction, a wide range of literature exists showing the beneficial effects of multilayer formation in liquid emulsions. Since the bUayer formation is based on electrostatic interactions, the stability of the interfacial film is... 

Pidgeon, C., Hunt, A.H., and Dittrich, K. (1986) Formation of multilayered vesicles from water/organic-solvent (W/O) emulsions theory and practice. Pharm. Res. 3, 23-34. 

McClements, 2006 Anal et al., 2008). Different combinations of proteins and polysaccharides (e.g., P-lactoglobulin + pectin, carrageenan or alginate casein + pectin) have been investigated within the context of multilayer emulsion stabilization (Guzey and McClements, 2006). It seems that the main technical challenge associated with the utilization such complex formation for layer-by-layer emulsion stabilization is the avoidance of bridging flocculation (McClements, 2005, 2006). 

Guzey, D., McClements, D.J. (2006). Formation, stability and properties of multilayer emulsions for application in the food industry. Advances in Colloid and Interface Science, 128-130,227-248. 

Our understanding of the influence of competitive adsorption on emulsion stability is less secure. Recent work has identified several marked differences between the adsorbed layer properties atair/water and oil/water interfaces (e.g., multilayer versus monolayer formation). Advancing our knowledge of the stabilization of emulsions by protein merits further investigation, since emulsions comprise a major sector of processed foods. If competitive adsorption of surfactants influences the stability of protein emulsions in a similar manner to foams, use of the strategies outlined above may be appropriate for controlling destabilization. 

The stracture of the multilayered emulsions may be preserved during spraydrying, enabling the delivery of emulsions with multilayered interfaces in a powder format. Spray-dried tuna oil powders made from emulsions containing oil droplets with lecithin-chitosan membranes, with added com symp showed good oil retention and water dispersibility (Klinkesom et al. 2006). 

The better understanding of the mechanisms of stability incomplex dermatological emulsions stabilized by surfactants and amphiphiles has enabled the development of a rapid microscopic method for evaluation of potential emulsifiers. The method is based on the observation that good emulsifier blends that stabilize emulsions by the formation of multilayers of stable gel phase also swell spontaneously in water at ambient temperature and this process can be observed microscopically. Mixtures that do not form gel phase or form metastable gels only after a heating and cooling cycle cannot be observed to swell spontaneously at ambient temperature. ... 

It is presumed that the limitation (no more than 11% by weight before gelation) of the concentratability of the AA(H) emulsions is the result of the crystallinity of the fatty tertiary amine, combined with the formation of the multilayered vesicle structures. This may be analogous with the dihydrogented tallow dimethyl ammonium chloride dispersions or the phospholipid dispersions where the dispersed phase consists of multiwalled vesicles and the liquid crystalline phase is partially preserved in aqueous dispersions [14],... 

Nano-emulsion droplets are generally stabilized by surfactants. Although it is considered that surfactant molecules are adsorbed at the oil-water interface in the form of monolayers, other surfactant self-organizing structures such as multilayers may play an important role in nano-emulsion stability. In this context, the results of studies of the relation between nano-emulsion formation, stability, and phase behavior are very illustrative [14-16,47]. 

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