Lab Guide: Gaby Shemer, room 202 Multi-Disciplinary research Bldg., Tel:640-9018

Introduction

A Langmuir monolayer is a monomolecular film formed at the air-water interface, usually
composed of amphiphilic molecules. As an ideal two-dimensional system and a model of biomembranes, it has been studied for more than a century. In Langmuir films, unlike the bulk state, the 2D pressure (or density) is easily controlled by moving the barrier and the surface pressure (tension) can be measured by the Wilhelmy's balance.

When molecules are first spread on the water surface they are very loosely pack and form a gas phase. This is seen at large molecular areas and very low surface pressures P. As the barriers are closed there is a transition at which the pressure suddenly starts to increase more rapidly. This change in compressibility signifies the onset of the liquid phase where there is no long range order but the molecules are not as free to move about as in the gas phase. As the barriers are even closed further, the nearly incompressible solid phase transition is noted by an onset of yet another change in slope. At low molecular areas and high pressures, this solid phase typically has a 2D lattice with hexagonal arrangement of molecules and an extrapolated molecular area of A₀. 

Compressing the layer further would lead "collapse" of the monolayer at Pc. This final phase, not shown, often looks like the aftermath of the collision of tectonic plates -- a catostrophic situation best avoided. It is interesting to note that a 2D surface pressure of 25mN/m on a molecule which is 25 Å long is akin to 100 atmospheres of pressure!