Anomalous Reflectivity Research Articles

Thermodynamics, Interfacial Structure, and pH Hysteresis of Rb+ and Sr2+ Adsorption at the Muscovite (001)−Solution Interface

The coverage and average height of adsorbed Rb+ and Sr2+ at the muscovite (001)-solution interface were measured with resonant anomalous X-ray reflectivity (RAXR) as a function of cation concentration (10-8 < [Sr2+] < 10(-1) m, 10-6 < [Rb+] < 10(-1) m at pH 5.5 and 3.5) and pH (1.5 to 5.5 at [Me(n+)] = 10(-3) m) without background electrolyte. At pH 5.5, Rb+ uptake approximately follows a Langmuir isotherm with deltaG(Rb)(o) = -23.5 +/- 4.0 kJ x mol(-1) and a saturation coverage of Tmax = 0.94 +/- 0.06 Rb+ per unit cell area, Auc = 46.72 A2, compensating the nominal surface charge density (1 e-/Auc). The Sr2+ isotherm has a saturation coverage of 0.47 +/- 0.05 Sr2+/Auc that also compensates the muscovite's charge, but the adsorption edge is both more abrupt and shifted significantly to lower concentration than that for Rb+. The uptake of Sr2+ is consistent with a Frumkin isotherm with an intrinsic adsorption constant, deltaG(Sr)(o) = -28.8 +/- 6.0 kJ x mol(-1) and a correlation energy, gamma(Sr) = -7.2 +/- 3.7 kJ x mol(-1). The average height of each adsorbed cation, corresponding to inner-sphere dominant Rb+ and coexisting inner- and outer-sphere Sr2+ distributions, was independent of ion coverage at pH 5.5. At pH 3.5, the adsorption edges of both ions shift to higher cation concentration, indicating competition with hydronium, and the shifts are accompanied by an irreversible reduction in the saturation coverage. The inner-sphere dominant mode of Rb+ adsorption did not change at pH 3.5, while that of Sr2+ changed to an outer-sphere dominant distribution. Hysteresis in both the amount and height of the adsorbed ion was observed as a function of the direction in which pH was changed, indicating that the intrinsic surface charge density decreased after reaction with acidic solutions. These results suggest new and unexpected interrelationships among the distribution of adsorbed ions, competitive adsorption of hydronium, and surface charge density at the mineral-solution interface.

Adsorption of Rb + and Sr 2+ at the orthoclase (0 0 1)–solution interface

Adsorption of Rb + and Sr 2+ at the orthoclase (0 0 1)–solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb + adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K + site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å 2). These results are consistent with an ion-exchange reaction in which Rb + occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO 3) 2 solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr 2+ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr 2+ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb +, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr 2+ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr 2+ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr 2+ on orthoclase. Differences in the partitioning of Sr 2+ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson–Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.

Phasing of resonant anomalous X-ray reflectivity spectra and direct Fourier synthesis of element-specific partial structures at buried interfaces

A formalism for model-independent determination of element-specific partial structures at buried interfaces using the phase-dependent behavior of resonant anomalous X-ray reflectivity (RAXR) data is described. Each RAXR spectrum (i.e.reflectivityversusenergy at a fixed momentum transfer near the absorption edge of interest) is uniquely constrained by the amplitude and phase of the resonant partial structure factor with pre-determined non-resonant total structure factor and anomalous dispersion corrections of the resonant species. The element-specific partial density distribution is then imaged by discrete Fourier synthesis with the partial structure factor. The utility of this approach is demonstrated in the comparison of Rb+and Sr2+distributions at muscovite (001)–aqueous solution interfaces derived by model-independent and model-dependent approaches. This imaging method is useful for rapid determination of complex buried interfacial structures where element-specific atomic distributions are poorly constrained by conventional X-ray reflectivity analysis.

Resonant anomalous X-ray reflectivity as a probe of ion adsorption at solid–liquid interfaces

We discuss new opportunities to understand processes at the solid–liquid interface using resonant anomalous X-ray reflectivity (RAXR). This approach is illustrated by determination of element-specific density profiles at mica surfaces in aqueous electrolyte solutions containing Rb + and Sr 2+. The total interfacial electron density profile is determined by specular reflectivity (i.e., reflected intensity vs. momentum transfer, q, at an energy, E, far from any characteristic absorption edge). RAXR spectra (i.e., intensity vs. E at fixed q) reveal element-specific ion distributions. Key differences in the interaction of Rb + and Sr 2+ with mica are observed using resonant anomalous X-ray reflectivity: Rb + adsorbs in a partially hydrated state, but Sr 2+ adsorbs in both fully and partially hydrated states.

Ion distributions at charged aqueous surfaces by near-resonance X-ray spectroscopy

Near-resonance X-ray spectroscopy is used to determine ion distributions and their local environment at charged aqueous interfaces. Energy scans at fixed momentum-transfers under specular reflectivity conditions near the L(III) Cs(+) resonance reveal the formation of a diffuse Gouy-Chapman layer at a charged surface formed by a Langmuir monolayer (dihexadecyl phosphate) spread on CsI solution. The energy scans exhibit a periodic dependence on photon momentum-transfer (Q(z)) with a line-shape that consists of a Q(z)-dependent linear combination of the dispersive f'(E) and absorptive f''(E) fine-structure corrections. The results in the Born approximation are discussed and more quantitatively by using the dynamical method numerically (i.e. recursive or matrix methods to calculate the reflection of electromagnetic waves from stratified media). The ion distributions obtained from the analysis of the spectroscopy are in excellent agreement with those obtained from anomalous reflectivity measurements, providing further confirmation to the validity of the renormalized surface-charge Poisson-Boltzmann theory for monovalent ions. The fine structures of f'(E) and f''(E) obtained in the process differ significantly from the multi-electron photoexcitation spectra of the isolated ion, revealing the local environment of a Cs(+) ion in the solution at the interface. The comparison with similar X-ray absorption fine structures suggests that the Cs(+) ion is surrounded by a shell of eight O atoms.

Direct observations of ion adsorption and interfacial hydration with resonant anomalous X-ray reflectivity

Direct observations of ion adsorption and interfacial hydration with resonant anomalous X-ray reflectivity

Charge Inversion at Minute Electrolyte Concentrations

Anionic dimyristoylphosphatidic acid monolayers spread on LaCl3 solutions reveal strong cation adsorption and a sharp transition to surface overcharging at unexpectedly low bulk salt concentrations. We determine the surface accumulation of La3+ with anomalous x-ray reflectivity and find that La3+ compensates the lipid surface charge by forming a Stern layer with approximately 1 La3+ ion per 3 lipids below a critical bulk concentration, ct approximately 500 nM. Above ct, the surface concentration of La3+ increases to a saturation level with approximately 1 La3+ per lipid, thus implying that the total electric charge of the La3+ exceeds the surface charge. This overcharge is observed at approximately 4 orders of magnitude lower concentration than predicted in ion-ion correlation theories. We suggest that transverse electrostatic correlations between mobile ions and surface charges (interfacial Bjerrum pairing) may contribute to the charge inversion.

Hydration and Distribution of Ions at the Mica-Water Interface

Molecular-scale structures of mica surfaces in electrolyte solutions reveal how ion and interfacial hydration control cation adsorption. Key differences are obtained for Rb+and Sr2+ using resonant anomalous x-ray reflectivity: Rb+ adsorbs in a partially hydrated state and incompletely compensates the surface charge, but Sr2+ adsorbs in both fully and partially hydrated states while achieving full charge compensation. These differences are driven by balancing the energy cost of disrupting ion and interface hydration with the electrostatic attraction between the cation and charged surface.

How Accurate Is Poisson−Boltzmann Theory for Monovalent Ions near Highly Charged Interfaces?

Surface sensitive synchrotron X-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion (Cs+) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H3O+ to the phosphate group, leading to proton-transfer back to the phosphate group and to a reduced surface charge. The increase of Cs+ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating possible deviations from Poisson-Boltzmann theory.

Monovalent counterion distributions at highly charged water interfaces: Proton-transfer and Poisson-Boltzmann theory

Surface sensitive synchrotron-x-ray scattering studies reveal the distributions of monovalent ions next to highly charged interfaces. A lipid phosphate (dihexadecyl hydrogen phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion distributions (Cs+) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H3O+ at the interface leads to proton transfer back to the phosphate group by a high contact potential, whereas high salt concentrations lower the contact potential resulting in proton release and increased surface charge density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.

Probing Outer-Sphere Adsorption of Aqueous Metal Complexes at the Oxide-Water Interface with Resonant Anomalous X-Ray Reflectivity

Resonant anomalous x-ray reflectivity near the Pt L(III) edge simultaneously revealed the geometric and spectroscopic structures of Pt(NH(3))(4)(2+) ions adsorbed at the quartz(100)-water interface. The derived Pt geometric subprofile shows two discrete "outer-sphere" adsorbed layers, and the interface-specific x-ray absorption edge profile exhibits a significant white-line enhancement compared to the bulk-solution species.

Chemical depth profile of passive oxide on stainless steel

The chemical depth profile of the passive oxides formed on a stainless-steel surface (type 430, Fe–16.3%Cr) in pH 8.4 borate solution was obtained quantitatively by in situ anomalous x-ray reflectivity. The passive film consists of a Cr oxide inner layer and a Fe∕Cr oxide outer layer. The absence of Fe oxide underneath Cr oxide indicates that Cr oxide provides the passivity in stainless steel by inhibiting oxygen anion diffusion. The oxide grows by limited metallic cation transfer. The passivity breaks down in transpassive regime due to the oxidation of the Cr oxide to a soluble oxide.

SARS E protein in phospholipid bilayers: an anomalous X-ray reflectivity study

We report on an anomalous X-ray reflectivity study to locate a labelled residue of a membrane protein with respect to the lipid bilayer. From such experiments, important constraints on the protein or peptide conformation can be derived. Specifically, our aim is to localize an iodine-labelled phenylalanine in the SARS E protein, incorporated in DMPC phospholipid bilayers, which are deposited in the form of thick multilamellar stacks on silicon surfaces. Here, we discuss the experimental aspects and the difficulties associated with the Fourier synthesis analysis that gives the electron density profile of the membranes.

Oscillatory behavior of5dmagnetic moments inFe∕Wmultilayers

We used x-ray resonant magnetic scattering (XRMS) across the W ${L}_{3}$ absorption edge in combination with x-ray magnetic circular dichroism and anomalous x-ray reflectivity to obtain the magnetic polarization profile in a $\mathrm{Fe}(30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{W}(11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ multilayer. This extends the use of XRMS to the $5d$ transition metals for the study of induced magnetization. Analysis of our experimental results shows that there is a strong oscillatory behavior of the magnetization in the W layer. This tendency is in good agreement with the recent theoretical predictions by Tyer et al. [Phys. Rev. B 67, 104409 (2003)].

Fluid flow in compressive tectonic settings: Implications for midcrustal seismic reflectors and downward fluid migration

Beneath the brittle‐ductile transition of the Earth's crust, the dilational deformation necessary to create fluid pathways requires fluid pressure that is near to rock confining pressure. Although the deformation may be brittle, it is rate limited by the ductile compaction process necessary to maintain elevated fluid pressure; thus the direction of fluid expulsion is dictated by the mean stress gradient. The paradox posed by the conditions required to maintain high fluid pressure simultaneously with lower crustal rock strength can be explained by a model whereby fluids are localized within self‐propagating hydraulic domains. Such domains would behave as weak inclusions imbedded within adjacent fluid‐poor rocks. Because the mean stress gradient in a weak inclusion depends on its orientation with respect to far‐field stress, the direction of fluid flow in such domains is sensitive to tectonic forcing. In compressional tectonic settings, this model implies that fluid flow may be directed downward to a depth of tectonically induced neutral buoyancy. In combination with dynamic propagation of the brittle‐ductile transition, this phenomenon provides a mechanism by which upper crustal fluids may be swept into the lower crust. The depth of neutral buoyancy would also act as a barrier to upward fluid flow within vertically oriented structural features that are normally the most favorable means of accommodating fluid expulsion. Elementary analysis based on the seismogenic zone depth and experimental rheological constraints indicates that tectonically induced buoyancy would cause fluids to accumulate in an approximately kilometer thick horizon 2–4 km below the brittle‐ductile transition, an explanation for anomalous midcrustal seismic reflectivity.

Structural characterization of epitaxial Fe/Cr multilayers using anomalous X-ray and neutron reflectivity

The interface structure of epitaxial Fe/Cr multilayers was studied using anomalous X-ray and neutron reflectivity. The analysis of X-ray reflectivity at three different energies provided a reliable information about the interface roughnesses. It is found that the Cr-on-Fe interface is more diffused as compared to the Fe-on-Cr interface and that the roughness exhibits a significant increase with increasing depth. The magnetic roughness, as determined from neutron reflectivity, is lower than the geometrical roughness, in conformity with the behavior of a number of magnetic thin films and multilayers.

Anomalous x-ray reflectivity characterization of ion distribution at biomimetic membranes.

Anomalous x-ray reflectivity measurements provides detailed information on ion binding to biomembrane surfaces. Using a monochromatic beam tuned to various x-ray energies at the Argonne National Laboratory Advanced Photon Source and utilizing a newly commissioned x-ray liquid surfaces reflectometer, measurements at and away from ion absorption edges allow determination of the distribution of these ions as they accumulate near lipid membranes. As a model, the interaction of Ba2+ ions with DMPA- (1,2-dimyristoyl-sn-glycero-3-phosphatidic acid) monolayers at the aqueous surface is studied. We find an unexpectedly large concentration of barium at the interface, approximately 1.5 per DMPA-, forming a Stern layer of bound ions and a cloud of less densely bound ions near the lipid headgroups. This result can be understood only if one assumes that bound cations are partially speciated, e.g., as BaOH+.

Growth of Fe4N epitaxial layers displaying anomalous light reflectivity modulated by an external magnetic field

Cubic Fe4N epitaxial layers have been grown on a MgO (100) substrate by atmospheric pressure halide vapor-phase epitaxy using FeCl3 and NH3 as the starting materials. The full width at half maximum of the X-ray (200) diffraction peak for the Fe4N epitaxial layer was about 0.05°. Transmission electron diffraction measurements revealed a diffraction pattern similar to that of a single crystal with cubic structure. The saturation magnetization and coercive force of an Fe4N epitaxial layer deposited at 600 °C were 182.0 emu g−1 and 30 Oe, respectively. It was found, for the first time, that the epitaxial layer of Fe4N displayed anomalous light reflectivity modulated by an external magnetic field.

Anomalous reflection of water surface during laser ablation

The temporal behaviour of KrF laser pulses reflected from a water surface was analysed over a broad range of laser fluences and absorption coefficients. We observed that the surface reflectivity strongly changes when the volume energy density exceeds a value of 1000 J/cm 3. The change in surface reflectivity, with increasing volume energy, depends on whether we have an air/water interface, where the reflectivity is increased, or a glass/water interface, where the reflectivity is decreased. We estimate the refraction index of water, which decreases from n=1.38 to ca. 1.1 for open water surface with the increase in volume energy. The mechanism of anomalous reflectivity is discussed and we suggest that this is due to heating of the absorbing solution above the water critical point, which is followed by a process of thermal expansion of the superheated layer.

Widespread fluid expulsion on a translational continental margin: Mud volcanoes, fault zones, headless canyons, and organic-rich substrate in Monterey Bay, California

Remotely operated vehicle (ROV)-based mapping of tectonic features, zones of anomalous reflectivity, and geomorphic targets in Monterey Bay, California, demonstrates the regional abundance of fluid expulsion along the active transform margin between the Pacific and North American plates. Cold seeps—extant communities characterized by chemosynthetic bivalves, bacterial mats, and rare tubeworms—are the surface manifestations of present-day fluid expulsion of sulfide- and methane-rich fluids, whereas slabs, veins, and chimneys of authigenic carbonate represent regions of either dormant methane-rich fluid expulsion, or areas where the present rate of flow is too low to support chemosynthetic fauna. We have found both active and dormant fluid seepage along fault zones, at the surface expression of mud volcanoes, on organic-rich or permeable substrate, and within headless canyons across a wide range of depths within Monterey Bay. The fluid egress at these sites may be driven by a combination of (1) pore-space reduction caused by rapid sedimentation and/or tectonic compaction related to residual Pacific–North America compression, and (2) increased buoyancy due to a decrease in pore-fluid density related to diagenesis and/or catagenesis at depth. Although provocative, the relationship between topographically driven aquifer discharge and sea-floor fluid expulsion remains speculative for Monterey Bay. The widespread distribution of fluid expulsion features controlled by a variety of conduits in Monterey Bay implies that cold seeps may be common features on translational margins.