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 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.
Highlights
The results show that the numbers of molecules per unit area in the two layers of a bilayer are not the same. ͑ii The in-plane arrangement of these PBLG rods parallel to the water surface was probed by the Grazing incidence diffraction (GID) technique
The observed microscopic behavior showed no significant dependence on sample dispersity
͑I AϾϳ21 Å2/monomer: As soon as being spread on water surface, PBLG rods lie down flat on water surface, self-aggregate laterally, and align themselves parallel to their immediate neighbors to form solidlike 2D islands. Compression in this regime (⌸ϭ0) only reduces areas of bare water surface coexisting with monolayer islands and results in no structural changes at the intermolecular level
Summary
Structure of poly„␥-benzyl-L-glutamate... monolayers at the gas–water interface: A Brewster angle microscopy and x-ray scattering study. Tirrellc) Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003
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