Monolayer X-ray reflectometry at the air–water interface

X-ray reflectometry (XR) of the air–liquid interface has been restricted because of instrumental limitations. We have newly constructed a high-performance XR apparatus which can accommodate a Langmuir–Blodgett trough for air–water interface study. Since this apparatus has a large dynamic range, XR measurements can be performed for monolayers on a water surface at controlled surface pressure. The XR profiles for the lipids distearoylphosphatidylcholine (DSPC) and dibehenoylphosphatidylcholine showed clear Kiessig fringes. The total thickness of the DSPC monolayer changed from 25.5 to 27.1 Å with increasing surface pressure from 10 to 40 mN/m. A systematic XR study for monolayers on water surfaces is possible with this instrument.