Buckled Surfactant Monolayers

Folding and rippling in surfactant layers

Surfactant molecules have one polar end and one non-polar end. They form a monolayer at the interface between a polar substance and a nonpolar one. Monolayers can be found on the surface of soapy water, paints and oil droplets in milk. A monolayer is simply a one molecule thick "coating" that separates the two substances and reduces surface tension. There is a monolayer on our lungs that makes it easier for us to breathe. The less surface tension there is the less energy we need to breathe.

Figure 2.

Sometimes a monolayer will separate into two phases; this is a biphasic monolayer. University of Chicago Materials Center Professors Ka Yee Lee and Tom Witten have observed these phases in a monolayer of biological surfactants (phospholipids) and have hypothesized that each phase change contorts the monolayer to form a mesa up to a few nanometers high (1 nanometer = 10-9 meters). In Figure 1 each red mesa represents a different domain of one of the phases in a biphasic monolayer.

A biphasic monolayer exhibits even stranger properties when its lateral tension is decreased. The mesa walls can become unstable by folding (Figure 2) or rippling (Figure 3). The unusual thing is that these instabilities occur while lateral tension is still positive. This is like a piece of paper that starts to crumple while you're still pulling on either end.

Figure 3.

The mesa topography and its instabilities have far reaching consequences on the structure and stability of heterogeneous films. In particular, understanding what controls the folding phenomenon could have a number of important medical ramifications. Premature infants and victims of various diseases can die for lack of surfactant coating on their lungs. The buckling effect observed allows the surfactant in our lungs to withstand regular contraction and expansion. Understanding how these biphasic monolayers work could make it easier to create a synthetic replacement.

by Seth B. Darling, Adam Kalafarski
created 03/02

References:

  1. Topography and instability of monolayers near domain boundaries H. Diamant, T. A. Witten, C. Ege, A. Gopal and K. Y. C. Lee Phys. Rev. E 63, 061602 (2001)
  2. Unstable topography of biphasic surfactant monolayers H. Diamant, T. A. Witten, A. Gopal and K. Y. C. Lee Europhys. Lett. 52, 171 (2000)