Stability of nanoemulsions

Wang, L., Tahor, R., Eastoe, J., Li, X., Heenan, R.K. and Dong, J. (2009) Formation and stability of nanoemulsions with mixed ionic—nonionic surfactants. Physical Chemistry Chemical Physics, 11, 9772-9778. 

The stability of nanoemulsions prepared using isohexadecane was assessed by following the droplet size as a function of time. Plots of versus time for four... 

For understanding the drug release profile Physical and chemical stability of nanoemulsions... 

Chausson, M., Fluchere, A.S., Landreau, E., Aguni, Y., Chevalier, Y., Hamaide, T., Abdul-Malak, N., Bonnet, 1. (2008) Block copolymers of the type poly (caprolactone)-b-poly (ethylene oxide) for the preparation and stabilization of nanoemulsions. Int. J. Pharm.,362,153-162. 

This section, which is by no means exhaustive, will deal with the following topics (i) Surfactants used in cosmetic formulations, (il) Interaction forces between particles or droplets in a dispersion and their combination, (iil) Description of stability in terms of the interaction forces, (iv) Self-assembly structures and their role in stabilization, skin feel, moisturization and delivery of actives, (v) Use of polymeric surfactants for stabilization of nanoemulsions, multiple emulsions, liposomes and vesicles. 

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. 

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. 

Stability of nanoemulsions - The role of adsorbed layer thickness... 

To produce a stable film + Tig > and this is the driving force for prevention of coalescence which can be achieved by two mechanisms eind their combination (i) Increased repulsion both electrostatic and steric. (il) Deimpening of the fluctuation by enhancing the Gibbs elasticity. In general, smaller droplets are less susceptible to surface fluctuations and hence coalescence is reduced. This explains the high stability of nanoemulsions. 

TP. Tadros, J.E. Izquierdo, C. Solans, Formation and stability of nanoemulsions. Advances in Colloids and Interface Science, 108 303-318,2004. 

It may seem that due to the different mechanisms and factors influencing the stability of nanoemulsions it is not possible to know a priori which will be the behavior of a defined system. However, the growing knowledge about a great variety of systems and a wise use of this knowledge would allow the formulation of nano-emulsions with enough stability for the applications intended. 

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