Simple Interpretative Model for the Anomalous Behavior of the Excess Surface Area in Mixed Systems with Large Composition Fluctuations: A Theoretical Analysis and an Experimental Investigation of Mixed Phospholipid/Omega-3 Fatty Acid Langmuir-Blodgett Films

We derive an elementary phenomenological lattice hole model to describe the complex behavior of the excess surface area as a function of the composition in two-component monolayers. The model accounts for the coupling between the composition fluctuations and the density (holes) concentration in mixed films. When the composition fluctuations are large and the hole-composition coupling parameters are strong, our model predicts the occurrence of a double maximum (or minimum) in the plot of the excess surface area A(E) against the film composition. The theoretical results are compared with excess surface area measurements of Langmuir-Blodgett films of mixed phospholipid/omega-3 fatty acids performed at different temperature, pH, and pressure.