X-ray reflectivity study of monolayers of amphiphilics at the air-water interface

Monolayers of amphiphilic molecules spread at the air-water interface have been studied using X-ray reflectivity, a technique which allows an independent determination of their thicknesses and densities (i.e. structural parameters), as well as their roughnesses (due to thermally excited capillary waves). The phase diagrams of C₁₅, C₂₁ and C₂₉ fatty acids and of the phospholipid L- alpha -dipalmitoylphosphatidylcholine (DPPC) have been investigated and their phase transitions characterized. Evidence for the liquid-expanded-liquid-condensed transition is given by an abrupt increase in the thickness of the aliphatic medium, and the structure is characteristic of the liquid-condensed phase below and above the triple point. A strong decrease in the roughness is observed at the transition to the solid state. This striking feature is attributed to the bending rigidity of the monolayer in the solid phase, whose value has been determined for different lengths of the aliphatic chains.