First-principles determination of hybrid bilayer membrane structure by phase-sensitive neutron reflectometry

The application of a new, phase-sensitive neutron reflectometry method to r eveal the compositional depth profiles of biomimetic membranes is reported. Determination of the complex reflection amplitude allows the related scatt ering length density (SLD) profile to be obtained by a first-principles inv ersion without the need for fitting or adjustable parameters. The SLD profi le so obtained is unique for most membranes and can therefore be directly c ompared with the SLD profile corresponding to the chemical compositional pr ofile of the film, as predicted, for example, by a molecular dynamics simul ation. Knowledge of the real part of the reflection amplitude, in addition to enabling the inversion, makes it possible to assign a spatial resolution to the profile for a given range of wavevector transfer over which the ref lectivity data are collected. Furthermore, the imaginary part of the reflec tion amplitude can be used as a sensitive diagnostic tool for recognizing t he existence of certain in-plane inhomogeneities in the sample. Measurement s demonstrating the practical realization of this phase-sensitive technique were performed on a hybrid bilayer membrane (self-assembled monolayer of t hiahexa (ethylene oxide) alkane on gold and a phospholipid layer) in intima te contact with an aqueous reservoir. Analysis of the experimental results shows that accurate compositional depth profiles can now be obtained with a spatial resolution in the subnanometer range, primarily limited by the bac kground originating from the reservoir and the roughness of the film's supp orting substrate.