Nanoemulsions in cosmetics

Nanoemulsions are transparent or translucent systems in the size range 20-200 nm [35]. Whether the system is transparent or translucent depends on the droplet size, the volume fraction of the oil and the refractive index difference between the droplets and the medium. Nanoemulsions having diameters 50 nm appear transparent when the oil volume fraction is 0.2 and the refractive index difference between the droplets and the medium is not large. With increasing droplet diameter and oil volume fraction the system may appear translucent, and at higher oil volume fractions the system may become turbid. 

Nanoemulsions are only kinetically stable. They have to be distinguished from microemulsions (that cover the size range 5-50 nm) which are mostly transparent and thermodynamically stable. The long-term physical stability of nanoemulsions (with no apparent flocculation or coalescence) makes them unique and they are sometimes referred to as approaching thermodynamic stability . The inherently high colloid stability of nanoemulsions can be well understood from a consideration of their steric stabilization (when using nonionic surfactants and/or polymers) and how this is affected by the ratio of the adsorbed layer thickness to droplet radius as will be discussed below. 

Unless adequately prepared (to control the droplet size distribution) and stabilized against Ostwald ripening (that occurs when the oil has some finite solubility in the continuous medium), nanoemulsions may show an increasing droplet size and an initially transparent system may become turbid on storage. 

The inherently high colloid stability of nanoemulsions when using polymeric surfactants is due to their steric stabilization. The mechanism of steric stabilization was discussed above. As shown in Fig. 1.3 (a), the energy-distance curve shows a shallow attractive minimum at separation distance comparable to twice the adsorbed layer thickness 28. This minimum decreases in magnitude as the ratio between adsorbed layer thickness to droplet size increases. With nanoemulsions the ratio of adsorbed layer thickness to droplet radius (8/R) is relatively large (0.1 0.2) when compared with macroemulsions. This is schematically illustrated in Fig. 1.28 which shows the reduction in with increasing 8/R. 

G is very small when 5/R 0.1 Can approach thermodynamic stability 

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