Detection of a diffusive two-dimensional gas of amphiphiles by lateral force microscopy

Sparsely adsorbed amphiphiles with high surface mobility play a central role in surfactant spreading and in the nucleation and growth of self-assembled monolayers. Here we show that lateral force microscopy can directly visualize a gas phase of adsorbed long-chain alcohols and fatty acids. The two-dimensional (2D) gas originates from the edge evaporation of dense monolayer domains, transferred to a mica surface by microcontact printing. Monolayer corrals act as 2D containers, eventually saturating the enclosed area with the trapped gas phase. Scratching a small hole in the corral allows the gas to leak out of its container, and monitoring this transport process provides a rough estimate of the surface diffusion constant. Our results suggest that friction measurement and mapping can detect amphiphile densities down to 1% of a monolayer, making this technique useful in studying the early stages of monolayer formation.