Multipurpose x-ray reflectometer optimized for the characterization of organic surface films on aqueous subphases

Abstract

A reflectometer based on a conventional sealed x-ray source for the study of molecular organic surface films (such as Langmuir monolayers) has been devised, which outperforms similar instruments using rotating anode generators and approaches even most advanced experimental stations attached to third generation synchrotron sources. Reflectivities of ∼5×10−9 are thus becoming available while the measurement of a full reflectivity curve takes approximately 4 h to complete. The instrument is operated under full digital control, permitting the automated recording of measurement programs. In an example of its performance we demonstrate that dipalmitoylphosphatidylglycerol monolayers on electrolytic subphases may be characterized to a level of detail which until recently was not even available at synchrotron sources. While conventional box models of lipid monolayers are inadequate for modeling experimental data at the high momentum transfer that has become accessible with the new instrument, a recently developed modeling technique based on volume-restricted distribution functions [Schalke et al., Biochim. Biophys. Acta 1464, 113 (2000)] enables studies of ion binding to the phospholipid in submolecular detail.