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 utilize 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 — despite being unable to fully resolve all source reproducibility issues — leads to significantly improved consistency among experimental isotherms obtained from different sources. • Problem: In a Langmuir trough, the real packing density may differ from the estimated value. • Solution: XRR data + MD simulations restore the real packing density. • Considering real packing density improves the precision of the π – A isotherm. • Harmonized π – A isotherms align also better with MD-simulated ones.