Structure of Poly(\(\gamma\)-Benzyl-L-Glutamate) Monolayers at the Gas–Water Interface: A Brewster Angle Microscopy and X-Ray Scattering Study

Abstract

This paper reports Brewster angle microscopy (BAM), x-ray specular reflectivity (XR), grazing incidence diffraction (GID) and off-specular diffuse scattering (XOSDS) measurements of Langmuir monolayers formed on water by both mono- and polydisperse samples of \(\alpha\)-helical poly(\(\gamma\)-benzyl L-glutamate) (PBLG) as a function of area/monomer A. The microscopic behavior does not exhibit any discernible effects due to differing dispersity. At low surface densities (A > \(\sim\) 21 Å\(^2\)/monomer, surface pressure \(\Pi\) = 0), BAM images reveal partial surface coverage by solidlike monolayer islands. GID measurements show an interhelix peak corresponding to a local parallel alignment of rodlike PBLG molecules, indicating their tendency to aggregate laterally without external pressure. Compression to A < 21 Å\(^2\)/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in \(\Pi\) that is accompanied by a decrease in the interhelix distance. Further compression results in a plateau of constant \(\Pi\) in the \(\Pi\)-A isotherm ( \(\sim\) 11.5 < A < \(\sim\) 18.5 Å\(^2\)/monomer, \(\Pi\) \(\sim\) 9 dyn/cm), which has previously been attributed to a first-order monolayer–bilayer transition. The interfacial electron density profiles determined by the XR measurements on both sides of the coexistence plateau provide direct evidence for this transition. On the basis of x-ray scattering results, the film on the high-density side of the plateau is shown to consist of a newly formed incomplete and incommensurate second layer that sits on top of and has lower average density than a homogeneous first layer. GID measurements indicate that the second layer can be characterized by larger interhelix d-spacing than the first layer, while XOSDS measurements on the bilayer suggest that the second layer is microscopically inhomogeneous. For both mono- and bilayers, the analysis of observed GID peak widths indicates that the extent of lateral positional correlations between parallel PBLG rods ranges from a few to no more than \(\sim\)15 interhelix distances, implying short-range order.