Whether it's a glass of water or a bathtub, or the surface of a lake or ocean, interesting things happen where water meets air. A new study from chemical engineers at Rensselaer addresses fundamental questions about water, salt, and their interactions near an air-water interface and may help development of new self-assembling biomolecular materials.
“Air-water interfaces are everywhere. What takes place in their vicinity, within 1 or 2 nanometers of the interface, is fascinating. There is still much to learn about this space,” says Shekhar Garde, the Elaine S. and Jack S. Parker Chaired Professor in Engineering and recently named dean of the School of Engineering, who led the study, which was supported by the National Science Foundation.
Garde, along with chemical and biological engineering graduate students Vasudevan Venkateshwaran and Srivathsan Vembanur, used molecular dynamics simulations—powered by the Rensselaer supercomputing center, the Center for Computational Innovations (CCI)—to study how ions interact with each other when near the air-water interface.
They found that oppositely charged ions attract each other much more strongly near an interface than when in bulk water. More surprising, Garde says, was the finding that similarly charged ions (two positive ions, or two negative ions), which are expected to repel each other strongly, do not do so when near an airwater interface, and may even attract each other when drawn slightly out of the interface. This behavior arises from the complex interplay of the structure of the water molecules, deformation of the surface, and the capillary waves along the surface of the water, he says.
“In essence, we found that ion-ion interactions become more ‘sticky’ near the air-water interface,” Garde says. These findings have interesting implications for the structure and assembly of biological molecules at interfaces, which is a topic of significant interest. In bulk water, proteins fold into different functional structures as a result of hydrophobic and hydrophilic interactions.
At an airwater interface, however, hydrophobic interactions become weaker and hydrophilic/ionic interactions become stronger, resulting in proteins unfolding, aggregating, or assembling into significantly different structures. In this study, Garde and the graduate students demonstrated how a peptide (a piece of a protein) that forms a helix shape at the interface, changes dramatically into a hairpin turn shape when its ends are charged. This ability to switch from one kind of structure to another by charging the ends demonstrates the way in which ion-ion interaction can influence protein structure, Garde says.