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J. Chem. Phys. 111, 9761 (1999); http://dx.doi.org/10.1063/1.480312 (17 pages)
Structure of poly(γ-benzyl-L-glutamate) monolayers at the gas–water interface: A Brewster angle microscopy and x-ray scattering study
(Received 14 June 1999; accepted 9 September 1999)
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Add to FacebookThis 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 α-helical poly(γ-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> ∼ 21 Å2/monomer, surface pressure Π = 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 Å2/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in Π that is accompanied by a decrease in the interhelix distance. Further compression results in a plateau of constant Π in the Π-A isotherm ( ∼ 11.5<A< ∼ 18.5 Å2/monomer, Π ∼ 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 ∼15 interhelix distances, implying short-range order. © 1999 American Institute of Physics.
© 1999 American Institute of Physics
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