Dipalmitoyllecithin Monolayers at the Air/Water Interface: Measurements of the Response to Shear as a Function of Surface Density and pH

The time-dependent response to a shearing stress of a monolayer of dipalmitoylphosphatidylcholine (lecithin), which had been adsorbed on the surface of water, has been measured as a function of both pH and surface density using recently designed instrumentation. The substrate was buffered to the pH values of 4.5, 6, and 7.5 with a sodium phosphate/citric acid solution (Mcllvaine's buffer). At low surface density the monolayers are fluid. As the density increases the monolayers became viscoelastic; i.e., they begin to display an instantaneous or impact shear modulus, μ(∞), that decays with a characteristic time, τ. Within the experimental uncertainty, the static modulus, μ(0), vanishes, which implies that the films are not solidlike.