A comparative study of Langmuir surfactant films: Grazing incidence x-ray off-specular scattering vs. x-ray specular reflectivity

Abstract

Surface monolayers assembled on a liquid sub-phase represent a class of systems that is of great interest for studies of phase transitions in quasi-2D systems, chemical self-assembly, surfactant behavior, and biologically relevant monolayers and membranes. X-ray scattering is ideal for studying structural, dynamic, and mechanical properties of these surface monolayers at nanoscale due to the penetrating ability and short wavelength of x-rays. We show here that grazing incidence x-ray off-specular scattering (GIXOS) provides rapid access to in-plane and out-of-plane nanoscale structure, surface fluctuating modes, and potentially bending stiffness. We show that analysis of GIXOS data is highly sensitive to resolution effects. We further present detailed analysis of GIXOS from phospholipid 1,2-dipalmitoyl-phosphatidyl-choline C40H80NO8P (DPPC) and obtain quantitative, angstrom-resolution details of electron density profile normal to the surface that is comparable to those that are obtained from specular x-ray reflectivity measurements. We compare these GIXOS results to x-ray reflectivity measurements performed on the same samples. While electron density and main structural characteristics (such as monolayer thickness) obtained by GIXOS agree with x-ray reflectivity results, the interfaces of GIXOS-derived density profiles are found to be systematically sharper than those obtained with x-ray reflectivity. The possible reasons for these differences are discussed.