Stable ridges in a collapsing monolayer

Abstract

The nature of monolayer collapse has long interested physical chemists studying surface phenomena^1–3, and is also of interest to biologists seeking cell-membrane models⁴, since monolayers of fatty-acid type molecules are often used as models for each half of the biomembrane bilayers, and double layers or bilayers may well form on monolayer collapse. Furthermore thin-film properties of fatty acids are similar in many ways to those of cholesterol, lecithins, and other important components of biomembranes. There is little direct experimental evidence to support proposed collapse mechanisms, however. In this note a remarkable type of monolayer collapse is described. Pressure–area isotherms for 2-hydroxytetracosanoic (cerebron) acid rise steeply and bend over sharply at a high pressure without the subsequent falloff in pressure expected for such rigid films. Electron micrographs of the collapsing film clearly show many folds or ridges of unusual stability. The compressed monolayer thus provides a model for membranes and thicker films under compression.