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ABSTRACT: We present X-ray reflectivity and interfacial tension measurements of the electrified liquid/liquid interface between two immiscible electrolyte solutions for the purpose of understanding the dependence of interfacial ion distributions on the applied electric potential difference across the interface. The aqueous phase contains alkali-metal chlorides, including LiCl, NaCl, RbCl, or CsCl, and the organic phase is a 1,2-dichloroethane solution of bis(triphenylphosphor anylidene) ammonium tetrakis(pentafluorophenyl)borate (BTPPATPFB). Selected data for a subset of electric potential differences are analyzed to determine the potentials of mean force for Li+, Rb+, Cs+, BTPPA+ and TPFB-. These potentials of mean force are then used to analyze both X-ray reflectivity and interfacial tension data measured over a wide range of electric potential differences. Comparison of X-ray reflectivity data for strongly hydrated alkali-metal ions (Li+ and Na+), for which ion pairing to TPFB- ions across the interface is not expected, to data for weakly hydrated alkali-metal ions (Rb+ and Cs+) indicates that the Gibbs energy of adsorption due to ion pairing at the interface must be small (<1 kBT per ion pair) for both the CsCl and RbCl samples. This paper demonstrates the applicability of the Poisson-Boltzmann potential of mean force approach to the analysis of X-ray reflectivity measurements that probe the nanoscale ion distribution and the consequences of these underlying distributions for thermodynamic studies, such as interfacial tension measurements, that yield quantities related to the integrated ion distribution.
The Journal of Physical Chemistry B 04/2013; · 3.70 Impact Factor
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ABSTRACT: Ion distributions play a central role in various settings-from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength $$\Gamma $$ of ion-ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameter-free density functional theory that includes ion-ion correlations and ion-solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.
Proceedings of the National Academy of Sciences 11/2012; · 9.68 Impact Factor
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Danielle L Leiske,
Chad E Miller,
Liat Rosenfeld,
Colin Cerretani,
Alexander Ayzner,
Binhua Lin, Mati Meron,
Michelle Senchyna,
Howard A Ketelson,
David Meadows,
Sruthi Srinivasan,
Lyndon Jones,
Clayton J Radke,
Michael F Toney,
Gerald G Fuller
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ABSTRACT: Meibum is the primary component of the tear film lipid layer. Thought to play a role in tear film stabilization, understanding the physical properties of meibum and how they change with disease will be valuable in identifying dry eye treatment targets. Grazing incidence X-ray diffraction and X-ray reflectivity were applied to meibum films at an air-water interface to identify molecular organization. At room temperature, interfacial meibum films formed two coexisting scattering phases with rectangular lattices and next-nearest neighbor tilts, similar to the Ov phase previously identified in fatty acids. The intensity of the diffraction peaks increased with compression, although the lattice spacing and molecular tilt angle remained constant. Reflectivity measurements at surface pressures of 18 mN/m and above revealed multilayers with d-spacings of 50 Å, suggesting that vertical organization rather than lateral was predominantly affected by meibum-film compression.
Langmuir 07/2012; 28(32):11858-65. · 4.19 Impact Factor
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ABSTRACT: Floating monolayer mixtures of cationic dioctadecyldimethyldiammonium bromide and anionic lipids were used as variable templates for the biomimetic nucleation of calcium carbonate and studied using grazing incidence X-ray diffraction. Varying the ratio of constituents changes the monolayer charge, structure, and molecular tilt. The nucleating surface of calcite also changes as the mixture is varied, and at an 80:20 ratio the (012) face is seen under a floating monolayer template for the first time. Our results indicate that the average template lattice is the major controlling factor in the oriented nucleation of CaCO(3). This is in contrast to the current view that the orientation is controlled by the stereochemical matching of the terminal functional group and molecular tilt with respect to the carbonate groups in the crystal.
Langmuir 11/2011; 28(1):572-8. · 4.19 Impact Factor
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ABSTRACT: It has long been known that thiol-terminated molecules self-assemble as commensurate monolayers on Au(111) surfaces. By spreading floating octadecanethiol monolayers on aqueous solutions of chloroauric acid (HAuCl4) and using x rays to reduce the gold ions as well as to probe the structure, we have observed the nucleation of (111)-oriented Au nanoparticles at thiol surfaces. This process may be similar to the formation of biogenic gold by bacteria. The thiol monolayer acts as a "soft template," changing its structure as Au crystals form so that there is a sqrt[3]×sqrt[3] commensurate relationship.
Physical Review Letters 09/2011; 107(11):115503. · 7.37 Impact Factor
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ABSTRACT: Interactions between amphiphilic block copolymers and lipids are of medical interest for applications such as drug delivery and the restoration of damaged cell membranes. A series of monodisperse poly(ethylene oxide)-poly(butylene oxide) (EOBO) block copolymers were obtained with two ratios of hydrophilic/hydrophobic block lengths. We have explored the surface activity of EOBO at a clean interface and under 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers as a simple cell membrane model. At the same subphase concentration, EOBO achieved higher equilibrium surface pressures under DPPC compared to a bare interface, and the surface activity was improved with longer poly(butylene oxide) blocks. Further investigation of the DPPC/EOBO monolayers showed that combined films exhibited similar surface rheology compared to pure DPPC at the same surface pressures. DPPC/EOBO phase separation was observed in fluorescently doped monolayers, and within the liquid-expanded liquid-condensed coexistence region for DPPC, EOBO did not drastically alter the liquid-condensed domain shapes. Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) quantitatively confirmed that the lattice spacings and tilt of DPPC in lipid-rich regions of the monolayer were nearly equivalent to those of a pure DPPC monolayer at the same surface pressures.
Langmuir 08/2011; 27(18):11444-50. · 4.19 Impact Factor
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ABSTRACT: Lipid interfaces, ranging from cell membranes to thin surfactant layers that stabilize lung alveoli, are integral to living systems. Such interfaces are often subjected to mechanical forces, and because of their membrane-like geometry, they can easily deform by bending into localized folds. In this work, we explore the role of small molecules (i.e., glycerol) on the mechanical stability of model lung surfactant monolayers. We demonstrate that the presence of glycerol increases local monolayer bending stiffness by orders of magnitude. Our x-ray and neutron reflectivity measurements indicate that water is preferentially depleted, or glycerol is preferentially enriched, at the lipid headgroup/solvent interface, and that this glycerol-enriched layer extends O(10Å) beneath the monolayer with an adsorption free energy of -2.5 to -4.6 kJ/mol. The dramatic change in membrane bending stiffness in the presence of the sugar adlayer is understood in terms of two models: 1), lipid antiplasticization by glycerol; and 2), a continuum mechanical model of the viscous adlayer.
Biophysical Journal 07/2011; 101(1):118-27. · 3.65 Impact Factor
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ABSTRACT: 2-Hydroxyacids display complex monolayer phase behavior due to the additional hydrogen bonding afforded by the presence of the second hydroxy group. The placement of this group at the position α to the carboxylic acid functionality also introduces the possibility of chelation, a utility important in crystallization including biomineralization. Biomineralization, like many biological processes, is inherently a nonequilibrium process. The nonequilibrium monolayer phase behavior of 2-hydroxyoctadecanoic acid was investigated on each of pure water, calcium chloride, sodium bicarbonate and calcium carbonate crystallizing subphases as a precursor study to a model calcium carbonate biomineralizing system, each at a pH of ∼6. The role of the bicarbonate co-ion in manipulating the monolayer structure was determined by comparison with monolayer phase behavior on a sodium chloride subphase. Monolayer phase behavior was probed using surface pressure/area isotherms, surface potential, Brewster angle microscopy, and synchrotron-based grazing incidence X-ray diffraction and X-ray reflectivity. Complex phase behavior was observed for all but the sodium chloride subphase with hydrogen bonding, electrostatic and steric effects defining the symmetry of the monolayer. On a pure water subphase hydrogen bonding dominates with three phases coexisting at low pressures. Introduction of calcium ions into the aqueous subphase ensures strong cation binding to the surfactant head groups through chelation. The monolayer becomes very unstable in the presence of bicarbonate ions within the subphase due to short-range hydrogen bonding interactions between the monolayer and bicarbonate ions facilitated by the sodium cation enhancing surfactant solubility. The combined effects of electrostatics and hydrogen bonding are observed on the calcium carbonate crystallizing subphase.
Langmuir 03/2011; 27(8):4430-8. · 4.19 Impact Factor
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Angewandte Chemie International Edition 11/2010; 49(45):8462-5. · 13.45 Impact Factor
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ABSTRACT: Micro-focusing optical devices at synchrotron beamlines usually have a limited acceptance, but more flux can be intercepted if such optics are used to focus secondary sources created by the primary optics. Flux throughput can be maximized by placing the secondary focusing optics close to or exactly at the secondary source position. However, standard methods of beamline optics analysis, such as the lens equation or matching the mirror surface to an ellipse, work poorly when the source-to-optics distance is very short. In this paper the general characteristics of the focusing of beams with Gaussian profiles by a ;thin lens' are analysed under the paraxial approximation in phase space, concluding that the focusing of a beam with a short source-to-optics distance is distinct from imaging the source; slope errors are successfully included in all the formulas so that they can be used to calculate beamline focusing with good accuracy. A method is also introduced to use the thin-lens result to analyse the micro-focusing produced by an elliptically bent trapezoid-shaped Kirkpatrick-Baez mirror. The results of this analysis are in good agreement with ray-tracing simulations and are confirmed by the experimental results of the secondary focusing at the 18-ID Bio-CAT beamline (at the APS). The result of secondary focusing carried out at 18-ID using a single-bounce capillary can also be explained using this phase-space analysis. A discussion of the secondary focusing results is presented at the end of this paper.
Journal of Synchrotron Radiation 09/2010; 17(5):644-52. · 2.73 Impact Factor
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ABSTRACT: The mechanical response to compression of a self-assembled gold nanoparticle monolayer and trilayer at the air-liquid interface is examined. Analysis of the film's buckling morphology under compression reveals an anomalously low bending rigidity for both the monolayer and the trilayer, in contrast with continuum elastic plates. We attribute this to the spherical geometry of the nanoparticles and poor coupling between layers, respectively. The elastic energy of the trilayers is first delocalized in wrinkles and then localized into folds, as predicted by linear and nonlinear elastic theory for an inextensible thin film supported on a fluid.
Physical Review Letters 07/2010; 105(5):058301. · 7.37 Impact Factor
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ABSTRACT: X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a nonmonotonic ion-specific potential of mean force are in good agreement with the data.
The Journal of chemical physics 05/2010; 132(17):171101. · 3.09 Impact Factor
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ABSTRACT: The IMCA-CAT bending-magnet beamline was upgraded with a collimating mirror in order to achieve the energy resolution required to conduct high-quality multi- and single-wavelength anomalous diffraction (MAD/SAD) experiments without sacrificing beamline flux throughput. Following the upgrade, the bending-magnet beamline achieves a flux of 8 x 10(11) photons s(-1) at 1 A wavelength, at a beamline aperture of 1.5 mrad (horizontal) x 86 microrad (vertical), with energy resolution (limited mostly by the intrinsic resolution of the monochromator optics) deltaE/E = 1.5 x 10(-4) (at 10 kV). The beamline operates in a dynamic range of 7.5-17.5 keV and delivers to the sample focused beam of size (FWHM) 240 microm (horizontally) x 160 microm (vertically). The performance of the 17-BM beamline optics and its deviation from ideally shaped optics is evaluated in the context of the requirements imposed by the needs of protein crystallography experiments. An assessment of flux losses is given in relation to the (geometric) properties of major beamline components.
Journal of Synchrotron Radiation 10/2009; 16(Pt 5):647-57. · 2.73 Impact Factor
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ABSTRACT: We report the results of studies of the pair correlation function and equation of state of a quasi-one-dimensional colloid suspension, focusing attention on the behavior in the density range near close packing. Our data show that, despite deviations from true one-dimensional geometry, the colloid fluid is well described as a hard rod Tonks fluid. In our experimental realization, the colloid suspension does not wet the confining walls, one consequence of which is a surface tension induced weak attractive interaction between the particles. The reality of this interaction is confirmed after correction of the raw experimental data for overlap of the optical images of particles that are nearly in contact and by an alternative particle location algorithm based on edge location.
The Journal of Physical Chemistry B 08/2009; 113(42):13742-51. · 3.70 Impact Factor
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ABSTRACT: Langmuir monolayers of straight-chain and branched hexadecanol and eicosanol mixtures were previously studied using surface pressure- area isotherms, Brewster angle microscopy, and interfacial rheology. In this paper, we investigate the structure of these fatty alcohol mixtures using these previous results together with X-ray diffraction and reflectivity measurements, which provide a better understanding of the structure of the monolayer in terms of the phase segregation and location of branched chains. For eicosanol below 25 mN/m, the branched chains are incorporated into the monolayer, yet they are phase-separated from the straight chains. At higher surface pressures, the branched chains are expelled from the monolayer and presumably form micelles or some other aggregate in the subphase. In contrast, the hexadecanol branched chains are not present in the monolayer at any surface pressure. These behaviors are interpreted with the help of the X-ray measurements and density profiles, and are explained in terms of straight-chain flexibility. We will discuss the effect of the monolayer structure on the surface shear viscosity. These studies provide a deeper understanding of the structure and behavior of amphiphilic mixtures, and will ultimately aid in developing models for lipids, micelle formation, and other important biological functions.
Langmuir 01/2009; 24(24):14005-14. · 4.19 Impact Factor
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ABSTRACT: We describe an integrated Brewster angle microscope (BAM), Langmuir trough, and grazing incidence x-ray diffraction assembly. The integration of these three techniques allows for the direct observation of radiative beam damage to a lipid monolayer at the air-water interface. Although beam damage has been seen in x-ray measurements, it has not been directly observed in situ at the micron scale. Using this integrated assembly, we examined the effects of radiative beam damage on Langmuir monolayers of 1,2-dimyristoyl-sn-glycero-3-[phospho-L-serine] (DMPS), 1:1 DMPS:1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 1:1 DMPS:1,2-dioleoyl-sn-glycero-3-phosphocholine held at a constant surface pressure. For constant surface pressure experiments, we observed a marked decrease in the surface area of the film upon exposure to the beam due to photodissociation. For a condensed lipid film, a change in refractive index of the film was observed post-beam-exposure, indicating areas of damage. For DMPS in an oxygenated environment, the Bragg peak intensity decreased with beam exposure. In mixed monolayer systems, with saturated and unsaturated lipids, an increase in the number of small saturated lipid domains was seen as the unsaturated lipid was preferentially damaged and lost from the monolayer. We show that BAM is a highly effective technique for in situ observation of the effects of radiative damage at the air/water interface during a synchrotron experiment.
Review of Scientific Instruments 11/2007; 78(10):103705. · 1.37 Impact Factor
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ABSTRACT: X-ray reflectivity studies reveal atomic-level surface-segregation at the free surface of the eutectic Au71Sn29 liquid alloy. The surface-segregation extends up to three layers, in which the top layer is almost a pure monolayer of Sn, the second layer is almost a pure monolayer of Au and the third layer appears to be slightly enhanced in Au. Although the surface-segregation concentration profiles can be qualitatively accounted for by the theories of Defay-Prigogine and Strohl-King, they cannot satisfactorily account for the measured surface tension. PACS numbers: 61.20.-p, 61.10.-i, 68.03.-g
03/2007;
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ABSTRACT: A two dimensional crystalline layer is found at the surface of the liquid eutectic Au$_{82}$Si$_{18}$ alloy above its melting point $T_M=359 ^{\circ}$C. Underlying this crystalline layer we find a layered structure, 6-7 atomic layers thick. This surface layer undergoes a first-order solid-solid phase transition occurring at $371 ^{\circ}$C. The crystalline phase observed for T$>$371 $^{\circ}$C is stable up to at least 430 $^{\circ}$C. Grazing Incidence X-ray Diffraction data at T$>$371 $^{\circ}$C imply lateral order comprising two coexisting phases of different oblique unit cells, in stark contrast with the single phase with a rectangular unit cell found for low-temperature crystalline phase $359 ^{\circ}$C$<T<371 ^{\circ}$C. Comment: 10 pages, 8 figures
02/2007;
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ABSTRACT: The assembly of nanoparticles into large, two-dimensional structures provides a route for the exploration of collective phenomena among mesoscopic building blocks. We characterize the structure of Langmuir monolayers of dodecanethiol-ligated gold nanoparticles with in situ optical microscopy and X-ray scattering. The interparticle spacing increases with thiol concentration and does not depend on surface pressure. The correlation lengths of the Langmuir monolayer crystalline domains are on the order of five to six particle diameters. Further compression of the monolayers causes wrinkling; however, we find that wrinkled monolayers with excess thiol can relax to an unwrinkled state following a reduction of surface pressure. A theoretical model based on van der Waals attraction and tunable steric repulsion is adopted to explain this reversibility.
The Journal of Physical Chemistry B 01/2007; 110(48):24522-9. · 3.70 Impact Factor
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ABSTRACT: X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloy's melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.
Science 08/2006; 313(5783):77-80. · 31.20 Impact Factor
