Oil and water famously don’t mix. Water molecules are polar—the shapes of their electron wavefunctions give them an uneven distribution of electric charge—and they energetically favor associating with other polar molecules to compensate. Nonpolar oil molecules don’t qualify.
The immiscibility can be overcome with the help of an amphiphilic substance, whose molecules have polar, hydrophilic heads and nonpolar, hydrophobic hydrocarbon tails. With their tails pointing in and heads pointing out, amphiphilic molecules—such as emulsifiers, detergents, and other surfactants—surround droplets of oil and disperse them into the water. Even with no oil around, amphiphilic molecules can arrange themselves in water into intricate, orderly structures to protect their own tails from the surrounding polar medium.
Biology makes use of that self-assembly capability all the time. Every cell in your body is enveloped by a membrane made of two layers of amphiphilic molecules. Mimicking biology’s powers of self-assembly is a goal of materials researchers who strive to make new engineered biointerfacing materials. (See, for example, the article by Simone Aleandri and Raffaele Mezzenga, Physics Today, July 2020, page 38.)
But structures assembled through hydrophobic interactions almost always require the presence of water for their continued existence. The amphiphilic molecules in a self-assembled bilayer are held to one another only through weak van der Waals interactions. When the water dries up, the structure falls apart.
Now MIT’s Julia Ortony, her graduate student Ty Christoff-Tempesta, and their colleagues have developed a new self-assembled nanomaterial inspired by Kevlar, the stuff of bulletproof vests. The nanoassemblies hold together even in a water-free environment, and they can be made into a dry, solid material.
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