Figure 1

(Color online) Schematic representation of the interferometric approach to x-ray reflectivity to determine the profile structure of an ultrathin Langmuir film of an amphiphile at the liquid-gas interface. The close proximity of the multilayer reference structure without contacting the otherwise unperturbed Langmuir monolayer of the amphiphile at the liquid-gas interface allows for the critical interference of x rays reflected by the underlying reference structure with those reflected by the amphiphile.Reuse & Permissions

Figure 2

(Color online) Photograph of the experimental setup used for the interferometric x-ray reflectivity measurements, viewed through the kapton window of the trough canister. The rod supporting the reference structure appears in the foreground separated from the Langmuir monolayer of the AP0-RuPZn amphiphile by the adjacent fixed Teflon barrier. The reference multilayer structure is on the upper surface of the silicon wafer visible in the aqueous subphase on the other side of the fixed barrier. The moving Teflon barrier employed to compress the monolayer of the amphiphile is visible just beyond the reference structure, with a large expanse of only the aqueous subphase behind the moving barrier following compression.Reuse & Permissions

Figure 3

(Color online) (a) Schematic representation of AP0-RuPZn peptide-chromophore complex. The straight $\alpha$ helices are shown in the ribbon representation, wherein the lower and upper halves representing the hydrophilic and hydrophobic domains, respectively, of the amphiphilic 4-helix bundle AP0. The RuPZn chromophore is shown in the center of the bundle using stick representation, with its zinc and ruthenium atoms shown as dark-colored spheres. (b) A typical pressure-area isotherm for a Langmuir monolayer of the AP0-RuPZn amphiphile. Note the steepness of the isotherm in the higher surface pressure regime, namely, in the vicinity of $39\text{mN}{\text{m}}^{-1}$ employed in these experiments.Reuse & Permissions

Figure 4

(a) Fresnel-normalized specular x-ray reflectivity from a Langmuir monolayer of the AP0-RuPZn amphiphile upon compression at three selected surface pressures corresponding to the average area/helix values indicated. Note the expanded ordinate scale compared with that in the subsequent Figs. 5a, 6a. (b) Corresponding monolayer electron-density profiles calculated from the data shown in (a). Note that at the higher surface pressure of $39\text{mN}{\text{m}}^{-1}$, corresponding to an average area/helix in the plane of the interface of $100{\text{Å}}^2$, the excess electron density arising from the amphiphile at the water-helium interface is relatively uniform over about $50\text{Å}$. At this high surface pressure, the bundle axis has become perpendicular to the interface, the helices extending about $60\text{Å}$ along the profile axis allowing for interfacial roughness at both the peptide-helium interface at $z=0\text{Å}$ and the peptide-water interface at $z=-60\text{Å}$. Also note the expanded ordinate scale compared with that in the subsequent Figs. 5d, 6d.Reuse & Permissions

Figure 5

X-ray reflectivity results (measured at $22017\text{eV}$) from a Langmuir monolayer of the AP0-RuPZn amphiphile obtained using noninterferometric and interferometric approaches shown along with those for the Si-Ni-Si reference structure alone that possessed a macroscopically hydrophobic upper surface [dashed lines, Si-Ni-Si reference structure in He; continuous lines, Si-Ni-Si reference structure with adjacent Langmuir monolayer (interferometric case); and dotted lines, Langmuir monolayer of the amphiphile only]. (a) Fresnel-normalized x-ray reflectivity data shown on a linear scale, (b) Fresnel-normalized x-ray reflectivity data shown on a semilogarithmic scale, (c) Patterson (autocorrelation) functions, and (d) electron-density profiles. See text for further description and discussion.Reuse & Permissions

Figure 6

X-ray reflectivity results (measured at 9559 eV) from a Langmuir monolayer of the AP0-RuPZn amphiphile obtained using noninterferometric and interferometric approaches shown along with those for the Si-Ni-Si reference structure alone that possessed a macroscopically hydrophilic upper surface [dashed lines, Si-Ni-Si reference structure in He; continuous lines, Si-Ni-Si reference structure with adjacent Langmuir monolayer (interferometric case); and dotted lines, Langmuir monolayer of the amphiphile only]. (a) Fresnel-normalized x-ray reflectivity data shown on a linear scale, (b) Fresnel-normalized x-ray reflectivity data shown on a semilogarithmic scale, (c) Patterson (autocorrelation) functions, and (d) electron-density profiles. See text for further description and discussion.Reuse & Permissions

Figure 7

Difference electron-density profiles for the interferometry case at the water-gas interface, namely, the difference between the electron-density profile for the multilayer substrate with the adjacent Langmuir monolayer of the amphiphile [i.e., continuous lines in Figs. 5d, 6d] and the profile for the multilayer substrate itself [i.e., dashed lines in Figs. 5d, 6d]. Here, the continuous-line difference profile is for the substrate exhibiting a macroscopically hydrophobic surface, while the dashed-line difference profile is for the substrate exhibiting a hydrophilic surface. The intervening water layer on the proximal side (with respect to the multilayer substrate) of the Langmuir monolayer of the AP0-RuPZn amphiphile for the latter hydrophilic surface is shown with crosshatching. For both these continuous- and dashed-line difference profiles, refer to the ordinate scale on the left. The electron-density profile (dotted-line) for noninterferometry case, namely, the electron-density profile of the amphiphile at the water-gas interface at comparable surface pressure [i.e., the continuous-line profile from Fig. 4b] is presented for reference. For this dotted-line profile, refer to the ordinate scale on the right. The left-side and right-side ordinate scales are different in order to superimpose the intervening water layer electron density for the interferometric case with the hydrophilic substrate surface and that of the bulk water subphase for the noninterferometric case. See results and discussion section for further details. All three electron-density profiles have been aligned along the profile $z$ axis to make their respective peptide-helium interfaces coincide.Reuse & Permissions

Figure 8

Resonance specular x-ray reflectivity from a Langmuir monolayer of the AP0-RuPZn amphiphile about the $\text{Ru}K$ absorption edge obtained using the interferometric approach described in this work. (a) Fresnel-normalized reflectivity at the three x-ray energies, namely, the edge energy of $22117\text{eV}$ and $22117\text{eV}\pm 100\text{eV}$. Energy-dependent resonance effects are apparent. These are more apparent in the differences shown in (b).Reuse & Permissions