Interest in nano-emulsions has been developing for about 20 years now, mainly for nano-particle preparation. Not until recent years did direct applications of nano-emulsions in consumer products develops, mainly in pharmaceutical/drug, personal care, health care, agrochemical, film coating, cosmetic, consumable, carbon nano-tubes, and oil industries. Surfactants play major roles to ease the formation of nano-emulsions by lowering the interfacial tension; the Laplace pressure is reduced and hence the stress needed to break up a drop is reduced. In addition, their self-assembling amphiphilic nature also make surfactants very useful for applications used in many new technology areas.
Nano-emulsions are attractive in various application fields due to the following advantages:
– The very small droplet size causes a large reduction in gravitational force, so Brownian motion may be sufficient to overcome gravity. This means that no creaming or sedimentation occurs on storage.
– The small droplet size also prevents their coalescence, since these droplets are non-deformable and hence surface fluctuations are prevented.
– The significant surfactant film thickness (relative to droplet radius) prevents any disruption of the liquid film between the droplets.
– The large surface area of the emulsion system allows rapid penetration of actives. Due to their small size, nano-emulsions can penetrate through the rough skin surface and this enhances penetration of actives.
– The fluidity of the transparent nature of the system, as well as the absence of any thickeners may give them a pleasant aesthetic character and skin feel.
– The small size of the droplets allows them to deposit uniformly on substrates; wetting, spreading and penetration may be also enhanced because of the low surface tension of the whole system and the low interfacial tension of the O/W droplets.
– Nano-emulsions can be applied for delivery of fragrant or active ingredients, which may be incorporated in many personal care, food, and medical products. For example, this could be applied in perfumes, lubricants, cutting oils and corrosion inhibitors.
– Nano-emulsions may be applied as a substitute for liposomes and vesicles and it is possible in some cases to build lamellar liquid crystalline phases around the nano- emulsion droplets.