X-ray reflectivity analysis of thin complex Langmuir-Blodgett films

Using specular x ray reflectivity, we have examined the vertical structure of charge coupled polyelectrolyte (PDADMAC)-lipid (DPPA) films on the water surface and on a silicon substrate, prepared by means of the Langmuir-Blod gett technique. The complicated structure of these films, characterized by electron density variations on the sub nanometre scale and low density cont rast between sub layers, causes failure of the standard box model fitting m ethod-it yields poor matching to data and/or ambiguous density profiles. We show that this problem can be overcome utilizing a different, model free f itting method, which yields perfect fits and less ambiguous electron densit y profiles for all investigated films. The ambiguity of results can be furt her reduced when the sets of possible density profiles obtained for several similar samples are compared to each other. Discussing the model free fitt ing method in detail along with general questions of reflectivity data eval uation, we aim to give practical instructions for the structure analysis of thin organic films. The obtained density profiles, being in good agreement with the structure m odel based on previous diffraction experiments, reveal some new details: Th e DPPA's phosphate head groups penetrate inside the PDADMAC helix rather th an being bound onto its surface. The structure remains almost unchanged aft er transferring one layer onto Si wafer, but subsequently transferred layer s exhibit decreasing order.