Emulsions Double Up

Salad dressing aficionados know that it only takes a little surfactant and a good shake to coax oil and vinegar into a harmonious mix. But it usually requires multiple surfactants to construct a more complex double emulsion, wherein a drop of water is encased in a larger droplet of oil that's surrounded by a sea of water. These water-in-oil-in-water emulsions can simultaneously carry both hydrophobic and hydrophilic cargo, making them attractive for food, cosmetic, and drug delivery applications. Timothy J. Deming, Thomas G. Mason, and colleagues at the University of California, Los Angeles, have now developed a way to make these double emulsions with just one surfactant—a block copolypeptide (Nature 2008, 455, 85). The researchers prepared this surfactant by connecting a series of hydrophilic L-lysine residues with a shorter chain of hydrophobic leucine residues. Because poly(L-leucine) adopts a helical structure that's not very soluble in nonpolar solvents, Deming and Mason's team used both leucine enantiomers. This racemic structure has a flexible conformation that's more soluble in hydrophobic media. The team also made double droplets as small as 10 nm in diameter; these structures proved to be remarkably stable, resisting phase separation for up to nine months.