Surfactants, lamellar, interaction with

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

Emulsifiers are necessary to allow water and lipids to combine. A surfactant is an amphiphilic molecule that has affinities for fats as well as water and that can be incorporated into lamellar lipid structures (e.g. cell walls). Surfactants increase the fluidity of the lipid structures by partitioning into the lipid membranes, as their lateral interactions with the membrane-forming lipids reduce the force of their attractive interaction. The mobility of the membrane lipids increases considerably in a similar manner to when a liquid crystal is converted into a gel. Finally, lipids can be seen to micellize or simply dissolve. Membranes lose their relative impermeability. See Figure 5.16. 

Most liquid crystal structures are based on lamellar or bent layers containing surfactant molecules in such an arrangement that their polar groups strongly interact with each other, thus forming some kind of solidlike crystal pattern, while the organization at the level of the hydrophobic tail, which is due to the weaker London interactions, is much looser and more liquidlike, from which the term liquid crystal originates. In some ways a microemulsion can be considered a disordered or molten state of a liquid crystal. 

Therefore, to understand the behavior of food emulsions, we need to know as much as possible about these types of emulsifiers, because fliey may not behave exactly similarly to classical small-molecule emulsifiers. For example, phospholipid molecules can interact with each other to form lamellar phases or vesicles they may interact with neutral lipids to form a mono- or multi-layer around the lipid droplets, or they may interact with proteins which are either adsorbed or free in solution. Any or all of these interactions may occur in one food emulsion. The properties of the emulsion system depend on which behavior pattern predominates. Unfortunately for those who have to formulate food emulsions, it is rarely possible to consider the emulsion simply as oil coated with one or a mixture of surfactants. Almost always there are other components whose properties need to be considered along with those of the emulsion droplets themselves. For example, various metal salts may be included in the formulation (e.g. Ca " is nearly always present in food products derived from milk ingredients), and there may also be hydrocolloids present to increase the viscosity or yield stress of the continuous phase to delay or prevent creaming of the emulsion. In addition, it is very often the case, in emulsions formulated using proteins, that some of the protein is free in solution, having either not adsorbed at all or been displaced by other surfactants. Any of these materials (especially the metal salts and the proteins) may interact with the molecules... 

Pluronics, also known as poloxamers, are a class of synthetic block copolymers which consist of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO), arranged in an A-B-A triblock structure, thus giving PEO-PPO-PEO (Fig. 11.7) (Batrakova and Kabanov 2008). They can be found either as liquids, pastes or solids (Ruel-Gariepy and Leroux 2004). Due to their amphiphilic characteristics (presence of hydrophobic and hydrophilic components), pluronics possess surfactant properties which allow them to interact with hydrophobic surfaces and biological membranes (Batrakova and Kabanov 2008). Being amphiphilic also results in the ability of the individual block copolymers, known as unimers, to combine and form micelles in aqueous solutions. When the concentration of the block copolymers is below that of the critical micelle concentration (CMC), the unimers remain as molecular solutions in water. However, as the block copolymer concentration is increased above the CMC, the unimers will self-assemble and form micelles, which can take on spherical, rod-shaped or lamellar geometries. Their shapes depend on the length and concentration of the block copolymers (i.e. EO and PO), and the temperature (Kabanov et al. 2002). Micelles usually have a hydrophobie eore, in this case the PO chains, and a hydrophilic shell, the EO ehains. 

Interaction of Water-Soluble Polymers with Dilute Lamellar Surfactants... 

One of the main areas of interest of cosmetic formulations is their interaction with the skin. The top layer of the skin, which is the main barrier to water loss, is the stratum corneum, which protects the body from chemical and biological attack. This layer is very thin, approximately 30 [Jim, and it consists of 10% by weight of lipids that are organized in a bilayer structure (lamellar liquid crystalline) which at high water content is soft and transparent. When a cosmetic formulation is apphed to the skin, it interacts with the stratum corneum and it is essential to maintain the hquid-hke nature of the bilayer and prevent any crystallization of the lipids. This happens when the water content is reduced below a certain level. Any surfactant that causes disruption of the stratum corneum must be avoided and this is usually accompanied by skin irritation and the skin will feel dry. ... 

Let us now turn to a consideration of polar molecules in an aqueous environment where ionization exists and local charge densities are allowed. The lamellar surfactant phases (discussed above) provide a nice example where electrostatic interactions play an important role. Indeed, the problem of the structure of a charged surface in contact with a fluid which contains the neutralizing counterions (and possibly dissolved salts, acids, bases, etc.) has... 

A general conclusion of this subseaion is that in order to obtain small droplet-sized nano-emulsions with low polydispersity through the PIT method, the mean feature to take into account is to choose an adequate composition in which, at the HLB temperature, flat structures (bicoutinuous uucroemulsion and/or lamellar liquid crystal) are formed without an excess of oil. The excess of water does not influence the nano-emulsions properties, but acts as a simple dilution medium, probably because when enough water is contained that assures the complete hydration of the PEO chains at the final tanpera-ture of emulsions, the water in excess does not interact with surfactants, nor with oil. 

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