Consistent Pressure–Area Isotherms via X-ray Reflectometry and Molecular Simulation

Surface-pressure–area-per-molecule (π–A) isotherms provide useful mechanical, molecular, and affinity information about surfactant monolayers, especially lipid monolayers at air–water interface. However, significant discrepancies are seen between the reported experimental π–A curves even for the same lipid composition. We believe a major source of these differences may lie in the estimation of the lipid amount at the interface, and the consequent estimation of A. In this work, we propose a simple method to harmonize the area-per-lipid estimation by combining X-ray reflectometry and molecular simulations. First, an experimentally measured X-ray reflectivity profile is compared against reflectivity profiles calculated from simulations done at various fixed areas per lipid. From the A of the best fitting simulation, a harmonization factor for each reported experimental π–A isotherm can then be obtained. We show that this process leads to much improved consistency among experimental isotherms obtained from different sources.