Credit: © 2008 Nature

Double emulsions — for example ones where water droplets are dispersed in oil droplets, which are dispersed in a continuous aqueous phase — have advantages over single emulsions for encapsulating both polar and non-polar cargos. However, their preparation is more difficult, requiring mixtures of surfactants for stabilization, and emulsions where both the inner and outer droplets are on the nanoscale have not been demonstrated. Now, with a simple process using single-component block copolypeptides to stabilize water-in-oil-in-water emulsions it is possible to mass-produce double nanoemulsions1.

The problem with using only one surfactant is that a high ratio of hydrophilic to hydrophobic residues favours stabilization of oil droplets in water, whereas the inner water–oil interface is best stabilized by a surfactant with a low hydrophilic content. Jarrod Hanson and colleagues at the University of California, Los Angeles used a copolypeptide in which the hydrophobic segments interact through hydrogen bonding. The unstable inner droplets are therefore prevented from merging with the outer phase because the hydrophobic segments at the inner interface form hydrogen bonds with those at the outer interface.

The emulsions were shown to encapsulate hydrophilic quantum dots and hydrophobic molecules simultaneously for time periods of at least three months. The double nanoemulsions could prove useful in applications such as drug delivery, foods and cosmetics.