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During the solvent extraction of metal ions from an aqueous to an organic phase, organic-soluble extractants selectively target aqueous-soluble ions for transport into the organic phase. In the case of extractants that are also soluble in the aqueous phase, our recent studies of lanthanide ion-extractant complexes at the surface of aqueous solutions have suggested that ion-extractant complexation in the aqueous phase can hinder the solvent extraction process. Here, we investigate a similar phenomenon relevant to the separation of Co(II) and Ni(II) with supporting experiments that probe the separation of Fe(III) and Ni(II). X-ray fluorescence near total reflection and tensiometry are used to characterize the adsorption behavior of Co(II) and Ni(II) at the surface of aqueous solutions containing water-soluble extractants, either bis(2-ethylhexyl) phosphoric acid (HDEHP) or 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP). Consistent with our earlier studies of lanthanides, we observe a comparable adsorption behavior of Co(II) and Ni(II) at the surfaces of both HDEHP and HEHEHP aqueous solutions in spite of the known preference for Co(II) under solvent extraction conditions. Comparison experiments that utilized the water-insoluble extractant di-hexadecyl phosphoric acid (DHDP), confined to a monolayer on the water surface, reveal that Co(II) is preferentially adsorbed to the surface, as expected. This preference for Co(II) is also supported by molecular dynamics simulations of the potential of mean force for the ions interacting with the soluble extractants in water. These results highlight the possibility that complexation of extractants and ions in the aqueous phase can hinder the desired selectivity in the solvent extraction of critical elements.

Solvent extraction of trivalent rare earth metal ions by organophosphorus extractants proceeds via binding of phosphoric acid headgroups to the metal ion. Water molecules in the tightly bound first hydration shell of the metal ions must be displaced by oxygen atoms from phosphoric acid headgroups. Here, we use classical molecular dynamics simulations to explore the event in which a fully hydrated Er3+ binds to its first phosphoric acid headgroup. Approach of the headgroup into the region between the first and second hydration shells leads to a fast ejection of a water molecule that is accompanied by reordering of the hydration water molecules, including discretization of their angular positions and collective rotation about the metal ion. The water molecule ejected from the first shell is located diametrically opposite from the binding oxygen. Headgroup binding places a headgroup oxygen closer to Er3+ than its first hydration shell and creates a loosely bound water that subsequently exchanges between the first shell and its environment. This second exchange of water also occurs at discrete angular positions. This geometrical aspect of binding may be of relevance to understanding the binding and transport of ion–extractant complexes that are expected to occur at the organic–aqueous liquid–liquid interface used in solvent extraction processes.

Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from -450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted.

Solvent extraction is used widely for chemical separations and environmental remediation. Although the kinetics and efficiency of this process rely upon the formation of ion–extractant complexes, it has proven challenging to identify the location of ion–extractant complexation within the solution and its impact on the separation. Here, we use tensiometry and X-ray scattering to characterize the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2-ethylhexyl) phosphoric acid (HDEHP), in the absence of a coexisting organic solvent. These studies restrict ion–extractant interactions to the aqueous phase and its liquid–vapor interface, allowing us to explore the consequences that one or the other is the location of ion–extractant complexation. Unexpectedly, we find that light lanthanides preferentially occupy the liquid–vapor interface. This contradicts our expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in solvent extraction processes. These results reveal the antagonistic role played by ion–extractant complexation within the aqueous phase and clarify the advantages of complexation at the interface. Extractants in common use are often soluble in water, in addition to their organic phase solubility, and similar effects to those described here are expected to be relevant to a variety of separations processes., Observations of lanthanide ions and HDEHP extractants at the aqueous−vapor interface suggest an unexpected role for aqueous complexation and speciation in the process of liquid−liquid extraction.

Thiol ligands bound to the metallic core of nanoparticles determine their interactions with the environment and self-assembly. Recent studies suggest that equilibrium between bound and free thiols alters the ligand coverage of the core. Here, X-ray scattering and MD simulations investigate water-supported monolayers of gold-core nanoparticles as a function of the core-ligand coverage that is varied in experiments by adjusting the concentration of total thiols (sum of free and bound thiols). Simulations demonstrate that the presence of free thiols produces a nearly symmetrical coating of ligands on the core. X-ray measurements show that above a critical value of core-ligand coverage the nanoparticle core rises above the water surface, the edge-to-edge distance between neighboring nanoparticles increases, and the nanoparticle coverage of the surface decreases. These results demonstrate the important role of free thiols: they regulate the organization of bound thiols on the core and the interactions of nanoparticles with their surroundings.

Numerous applications of liquid crystals rely on control of molecular orientation at an interface. However, little is known about the precise molecular structure of such interfaces. In this work, synchrotron X-ray reflectivity measurements, accompanied by large-scale atomistic molecular dynamics simulations, are used for the first time to reconstruct the air-liquid crystal interface of a nematic material, namely, 4-pentyl-4'-cyanobiphenyl (5CB). The results are compared to those for 4-octyl-4'-cyanobiphenyl (8CB) which, in addition to adopting isotropic and nematic states, can also form a smectic phase. Our findings indicate that the air interface imprints a highly ordered structure into the material; such a local structure then propagates well into the bulk of the liquid crystal, particularly for nematic and smectic phases.

The intrinsic overexpression of secretory phospholipase A2 (sPLA2) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA2 (from bee venom) with the DPPC monolayer exhibited Ca2+ dependence. DPPC molecules at the interface without Ca2+ retained a monolayer organization; upon adsorption of sPLA2 to the monolayer the packing became tighter. In contrast, sPLA2-catalyzed degradation of DPPC occurred in the presence of Ca2+, leading to disruption of the ordered monolayer structure of DPPC. The interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA2. A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.

Electrochemistry at ITIES, which has been extensively studied for the oil (O)/water(W) interface, has been extended to the liquid/liquid interfaces of ionic liquids (ILs), namely, to the IL/W interface [1] and even to the IL/O interface [2,3]. One of several characteristics of ILs, compared with conventional molecular liquids like W and O, is the formation of ionic multilayers [4-6] at the interfaces due to the excluded volume and local electrostatic interactions between ions. Although such a multilayering behavior of ILs had been expected to appear in the electric double layer (EDL) at the IL/W interface, it had been challenging to detect the ionic distribution at the IL/W interface. In the present paper, we will introduce our recent study on the ionic distribution at the IL/W interface, revealed by x-ray reflectometry (XR) using the liquid surface reflectometer at NSF’s ChemMatCARS Sector 15-ID at Advanced Photon Source in Chicago [7]. We designed an ionic liquid that has both high hydrophobicity and high electron-density contrast between the cation and anion, which enabled us to widely polarize the IL/W interface and sensitively probe the cation-rich and anion-rich layers by XR, respectively. We observed the growth of alternately charged cation-rich and anion-rich layers along with a polarity reversal of the layers as the potential changed sign. We quantitatively compared the XR results with the BSK model [8], a phenomenological EDL model in ILs that takes into account both excluded volume and overscreening effects, by combining it with the Gouy-Chapman-Stern model for EDL on the W side of the IL/W interface. Our XR data reveal that the excess charge beyond the first ionic layer decays more rapidly than the model prediction, suggesting that the BSK model needs an improvement with which the layering periodicity and decay length are separately evaluated. References T. Kakiuchi and N. Nishi, Electrochemistry, 74 (2006) 942. N.E.R. Cousens and A.R. Kucernak, Electrochem. Commun., 31 (2013) 63. Y. Kuroyama, N. Nishi, T. Sakka, submitted. N. Nishi, Y. Yasui, T. Uruga, H. Tanida, T. Yamada, S. Nakayama, H. Matsuoka, T. Kakiuchi, J. Chem. Phys., 132 (2010) 164705. N. Nishi, T. Uruga, H. Tanida, T. Kakiuchi, Langmuir, 27 (2011) 7531. N. Nishi, T. Uruga, H. Tanida, J. Electroanal. Chem., 759 (2015) 129. S. Katakura, K. Amano, T. Sakka, W. Bu, B. Lin, M.L. Schlossman, N. Nishi, J. Phys. Chem. B, 124 (2020) 6412. M.Z. Bazant, B.D. Storey, A.A. Kornyshev, Phys. Rev. Lett., 106 (2011) 046102.

Previous X-ray scattering measurements demonstrated the formation of a voltage-tunable 2-dimensional lattice of nanoparticles situated at the liquid-liquid interface between two immiscible electrolyte solutions (Nano Letters 2014, 14, 6816−6822). The nanoparticles had Au cores coated with trimethylammonium terminated ligands and the ITIES consisted of NaCl in water and BTPPATPFB in 1,2-dichloroethane. Here, we present additional measurements on this system which illustrate a transition that occurs at more negative potentials. The transition represents a breakup of the 2D monolayer lattice into a disordered sub-monolayer of nanoparticles that coexists with small nanoparticle clusters in the vicinity of the interface. Grazing Incidence Small Angle X-ray Scattering (GISAXS) provides evidence for the breakup of the lattice and the appearance of clusters. X-ray reflectivity (XR) characterizes the relative location along the interfacial normal of the different types of nanoparticle assemblies.

During solvent extraction, amphiphilic extractants assist the transport of metal ions across the liquid–liquid interface between an aqueous ionic solution and an organic solvent. Investigations of the role of the interface in ion transport challenge our ability to probe fast molecular processes at liquid–liquid interfaces on nanometer-length scales. Recent development of a thermal switch for solvent extraction has addressed this challenge, which has led to the characterization by X-ray surface scattering of interfacial intermediate states in the extraction process. Here, we review and extend these earlier results. We find that trivalent rare earth ions, Y(III) and Er(III), combine with bis(hexadecyl) phosphoric acid (DHDP) extractants to form inverted bilayer structures at the interface; these appear to be condensed phases of small ion–extractant complexes. The stability of this unconventional interfacial structure is verified by molecular dynamics simulations. The ion–extractant complexes at the interface are an intermediate state in the extraction process, characterizing the moment at which ions have been transported across the aqueous–organic interface, but have not yet been dispersed in the organic phase. In contrast, divalent Sr(II) forms an ion–extractant complex with DHDP that leaves it exposed to the water phase; this result implies that a second process that transports Sr(II) across the interface has yet to be observed. Calculations demonstrate that the budding of reverse micelles formed from interfacial Sr(II) ion–extractant complexes could transport Sr(II) across the interface. Our results suggest a connection between the observed interfacial structures and the extraction mechanism, which ultimately affects the extraction selectivity and kinetics.

Grazing-incidence (GI) X-ray absorption spectroscopy (XAS) under conditions of total external reflection is used to explore the coordination environment of the trivalent erbium ion, Er(3+), at an electrolyte-vapor interface. A parallel study of the bulk aqueous electrolyte (1 M ErCl3 in HCl at pH = 1.54) shows that the Er(3+) ions have a simple hydration shell with an average Er-OH2 bond distance of 2.33(1) Å, consistent with previous descriptions of the aquated cation, [Er(OH2)8](3+). No other correlations are observed in the electrolyte EXAFS (extended X-ray absorption fine structure) data acquired at room temperature. In contrast, the coordination of the Er(3+) ions at the electrolyte-helium interface, as interrogated by use of electron-yield detection, reveal correlations beyond the Er-OH2 hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 6-7 O atoms at 2.36(1) Å and a second one of 3 Cl atoms at 2.89(2) Å, suggesting the formation of a neutral [(H2O)6-7ErCl3] entity at the surface of the electrolyte. The presence of a third, distant peak in the Fourier transform data is attributed to Er-Er correlations (in possible combination with contributions from distant Er-O and Er-Cl interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) Å, confirming the near-surface-enrichment of Er(3+) as revealed previously by use of X-ray reflectivity measurements (J. Phys. Chem. C 2013, 117, 19082). Here, the strong associations between the Er-aqua-chloro entities at the electrolyte-vapor interface are shown to be consistent with the formation of domains of polynuclear cluster motifs, such as would arise through hydrolysis reactions of the aquated Er(3+) cations. The local structural results and the calculated surface coverage are of relevance to understand the myriad reactions involved in the hydrometallurgical process of solvent extraction (SX) for metal purification, which involves the transfer of a selected metal ion, like Er, across an interface from an aqueous electrolyte to an organic phase.

The interface between two immiscible electrolyte solutions consisting of alkali chlorides in water and the organic electrolyte BTPPATPFB in 1,2-dichloroethane is characterized with X-ray reflectivity, interfacial tension and impedance spectroscopy measurements over a range of applied voltage between the bulk solutions. X-ray reflectivity probes the interfacial ion distribution on the sub-nanometer length scale, whereas interfacial tension and impedance spectroscopy characterize quantities such as interfacial excess charge and differential capacitance that represent integrations over the interfacial ion distribution. Predictions of interfacial ion distributions by the recently introduced PB-PMF method, which combines Poisson's equation with ion potentials of mean force, provide excellent agreement, within one to two experimental standard deviations, with both X-ray reflectivity and interfacial tension measurements. However, the agreement with the differential capacitance measured by impedance spectroscopy, and modeled by the Randles equivalent circuit, is not as good. Values of measured and calculated differential capacitance can deviate by as much as 20% for applied electric potential differences larger than approximately +/- 100 mV. These comparisons indicate that our understanding of the ion distributions that underlie these measurements is adequate, but that further understanding of the modeling of impedance spectroscopy data is required for quantitative agreement at larger applied electric potential differences. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.

The dynamic nature of lipid membranes presents significant challenges with respect to understanding the molecular basis of protein/membrane interactions. Consequently, there is relatively little known about the structural mechanisms by which membrane-binding proteins might distinguish subtle variations in lipid membrane composition and/or structure. We have previously developed a multidisciplinary approach that combines molecular dynamics simulation with interfacial x-ray scattering experiments to produce an atomistic model for phosphatidylserine recognition by the immune receptor Tim4. However, this approach requires a previously determined protein crystal structure in a membrane-bound conformation. Tim1, a Tim4 homolog with distinct differences in both immunological function and sensitivity to membrane composition, was crystalized in a closed-loop conformation that is unlikely to support membrane binding. Here we have used a previously described highly mobile membrane mimetic membrane in combination with a conventional lipid bilayer model to generate a membrane-bound configuration of Tim1 in silico. This refined structure provided a significantly improved fit of experimental x-ray reflectivity data. Moreover, the coupling of the x-ray reflectivity analysis with both highly mobile membrane mimetic membranes and conventional lipid bilayer molecular dynamics simulations yielded a dynamic model of phosphatidylserine membrane recognition by Tim1 with atomic-level detail. In addition to providing, to our knowledge, new insights into the molecular mechanisms that distinguish the various Tim receptors, these results demonstrate that in silico membrane-binding simulations can remove the requirement that the existing crystal structure be in the membrane-bound conformation for effective x-ray reflectivity analysis. Consequently, this refined methodology has the potential for much broader applicability with respect to defining the atomistic details of membrane-binding proteins.

Complex interactions that determine ionic ordering in the bulk of electrolyte solutions are modified by surface-region inhomogeneities. We present results from an investigation of surface-ionic profiles that provide insights into the underlying physical chemistry in this region. X-ray reflectivity measurements from the liquid surfaces of aqueous ErCl3 solutions reveal in unprecedented detail a nonmonotonic electron density profile, which is interpreted in terms of a nonmonotonic surface distribution of cations (Er3+) and their relationship to the bulk. The combination of a heavy, multivalent Er3+ and a lighter, monovalent anion (Cl–) results in a significant cation depletion layer at the surface followed by a subsurface region of notably enhanced Er3+. Studying a series of solutions as a function of solute concentration reveals marked changes in Er3+ distribution, the most notable of which are the depletion layer thickness variation from 7.8 A at 0.2 M to 5.5 A at 1.0 M and the damped, oscillatory, cation...

Adsorption of diaza-18-crown-6 ether substituted with two tetracosane (–C24H49) alkyl chains (ACE-24) was investigated at the liquid–liquid interface. X-ray reflectivity measurements determined the structure of a close-packed monolayer at the hexane–water interface, which is consistent with conclusions drawn indirectly from earlier interfacial tension measurements on similar molecules. These data provide further insights into the role of interfacial processes involving azacrown ethers in ion separation techniques such as the permeation liquid membrane.

Surfactants have their primary utility, both scientific and industrial, at the liquid-liquid interface. We review recent X-ray surface scattering experiments that probe the molecular ordering and phase behavior of surfactants at the water-oil interface. The presence of the oil modifies the interfacial ordering in a manner that cannot be understood simply from analogies with studies of Langmuir monolayers of surfactants at the water-vapor interface or from the traditional view that the solvent is fully mixed with the interfacial surfactants. These studies explored the role of chain flexibility and head group interactions on the ordering of long-chain alkanols and alkanoic acids. Small changes in the surfactant may produce large changes in the interfacial ordering. The interfacial monolayer can be spatially homogeneous or inhomogeneous. Investigators have observed interfacial phase transitions as a function of temperature between homogenous phases, as well as between homogeneous and inhomogeneous phases. Finally, varying the solvent chain length can alter the fundamental character of the phase transitions and lead to the formation of multilayer interfacial structures.

Synchrotron radiation X-ray scattering is a useful tool for structural characterization of liquid interfaces. Specular reflectivity provides precise measurement of the interfacial widths and of the ordering of surfactants adsorbed to these interfaces. Diffuse scattering gives information on phase transitions and domain formation in surfactant monolayers and on interfacial fluctuations confined by and coupled across fluidic films.

X-ray reflectivity was used to study the interaction of the PX domain of p40(phox) protein (p40(phox)-PX) with a Langmuir monolayer of a mixture of SOPC (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine), SOPS (1-stearoyl-2-oleoyl-sn-glycero-3-phosphoserine), and DPPtdIns(3)P (1,2-dipalmitoylphosphatidylinositol 3-phosphate) lipids supported on a buffered aqueous solution. The reflectivity is analyzed in terms of the known crystallographic structure of the p40(phox)-PX domain and a slab model that represents the lipid layer, yielding an electron density profile of the lipid layer and bound PX domains. This analysis determines the angular orientation and penetration depth of the p40(phox)-PX domain bound to the SOPC/SOPS/DPPtdIns(3)P monolayer. The best fit orientation is characterized by the following angles: theta = 30 +/- 10 degrees and phi = 140 +/- 30 degrees. These angles describe rotations, about axes in a coordinate system fixed to the domain, that are required to orient the domain with respect to the lipid layer at the interface. The protein penetrated into the lipid layer by 9 +/- 2 A, indicating that the protein penetrated into the headgroup region, but not deeply into the hydrocarbon region of the monolayer. In this analysis, polar Tyr(94) and hydrophobic Val(95) penetrated deepest into the lipid monolayer. The backbone of these residues was approximately 5 A above the headgroup-buffer interface, i.e., at the level of the SOPC/SOPS lipid phosphates. Positively charged Lys(92) and Lys(98) were also near the SOPC/SOPS lipid phosphates. This position of the protein allows for a favorable electrostatic contribution to binding.

Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head- and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH(2)OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH(3)(CH(2))(29)OH) and triacontanoic acid (CH(3)(CH(2))(28)COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.

X‐ray reflectivity studies of surfactants at the water‐oil interface yield structural information with sub‐nanometer resolution. In this presentation, we reviewed recent X‐ray reflectivity measurements of the interface between water and a hexane solution of the hydrocarbon alkanol CH3(CH2)19OH and fluorocarbon alkanol CF3(CF2)7(CH2)2OH. The mixed system exhibits three monolayer phases, two of which are similar to single surfactant phases. A transition from a liquid monolayer to a solid monolayer occurs with increasing temperature. This unusual phase transition and the qualitative features of the phase diagram are predicted by an appropriate superposition of the behavior of the two single surfactant systems.

Synchrotron X-ray reflectivity is used to study ion distributions at the liquid-liquid interface between a nitrobenzene solution of tetrabutylammonium tetraphenylborate (TBATPB) and a water solution of tetrabutylammonium bromide (TBABr). The concentration of TBABr is varied to alter the ion distribution. Our X-ray measurements are inconsistent with several commonly used theories of ion distributions, including Gouy-Chapman, modified Verwey-Niessen, and the MPB5 version of the Poisson-Boltzmann equation. These structural measurements are described well by ion distributions predicted by a version of the Poisson-Boltzmann equation that explicitly includes a free energy profile for ion transfer across the interface when this profile is described by a simple analytic form or by a potential of mean force from molecular dynamics simulations. These X-ray measurements from the liquid-liquid interface provide evidence for the importance of interfacial liquid structure in determining interfacial ion distributions.

Long-period X-ray standing wave fluorescence (XSW) and X-ray reflectivity techniques are employed to probe the conformation of a Br-poly(ethylene glycol) (PEG)-peptide adsorbate at the hydrated interface of a polystyrene substrate. The Br atom on this Br-PEG-peptide construct serves as a marker atom allowing determination by XSW of its position and distribution with respect to the adsorption surface with angstrom resolution. Adsorption occurs on native or ion-beam-modified polystyrene films that are spin-coated onto a Si substrate and display either nonpolar or polar surfaces, respectively. A compact, oriented monolayer of Br-PEG-peptide can be formed with the peptide end adsorbed onto the polar surface and the PEG end terminating with the Br tag extending into the aqueous phase. The 108-141 {angstrom} distance of the Br atom from the polystyrene surface in this oriented monolayer is similar to the estimated {approx}150 {angstrom} length of the extended Br-PEG-peptide. This Br-polystyrene distance depends on adsorption time and surface properties prior to adsorption. Incomplete multilayers form on the polar surface after sufficient adsorption time elapses. By contrast, adsorption onto the nonpolar surface is submonolayer, patchy, and highly disordered with an isotropic Br distribution. Overall, this combination of X-ray surface scattering techniques with a novel sample preparation strategymore » has several advantages as a real space probe of adsorbed or covalently bound biomolecules at the liquid-solid interface.« less

Synchrotron X-ray reflectivity is used to study the electron density as a function of depth through the bulk water/2-heptanone interface. The measured interfacial width of 7.0 ± 0.2 Å is comparable to the value calculated from capillary wave theory (7.3 Å) using the measured interfacial tension of 12.6 mN/m. This result is consistent with capillary wave theory and molecular dynamics simulations that describe a molecularly sharp interface roughened by thermal fluctuations. Keywords: Liquid–liquid, Interface, Width, X-ray reflectivity

Liquid–liquid interfaces play an important role in many chemical and biological systems in addition to being interesting model systems to study the statistical physics of interfaces and membranes. Water–oil interfaces are a model for the interaction of water with a hydrophobic molecular environment, important for protein folding and the formation of structures in complex fluids. Biological membranes exist at aqueous–aqueous interfaces and provide a dynamic platform for important cell processes. Recent advances in X-ray scattering measurements of liquid–liquid interfaces allow for the study of ordering and fluctuations on the molecular length scale. Several fundamental issues can be addressed, including (a) ordering of solvents (bulk liquids), surfactants, and electrolytes at liquid–liquid interfaces, and the influence of the solvents on surfactant ordering, and (b) the existence of monolayer domains at these interfaces, the issue of domain equilibrium (creation and annihilation of domains), and phase transitions in these domain phases. Studies of water–oil interfaces that address the first of these issues will be presented.

The interface between water and mixed surfactant solutions of CH3(CH2)19OH and CF3(CF2)7(CH2)2OH in hexane was studied with interfacial tension and X-ray reflectivity measurements. Measurements of the tension as a function of temperature for a range of total bulk surfactant concentrations and for three different values of the molal ratio of fluorinated to total surfactant concentration (0.25, 0.28, and 0.5) determined that the interface can be in three different monolayer phases. The interfacial excess entropy determined for these phases suggests that two of the phases are condensed single surfactant monolayers of CH3(CH2)19OH and CF3(CF2)7(CH2)2OH. By studying four different compositions as a function of temperature, X-ray reflectivity was used to determine the structure of these monolayers in all three phases at the liquid−liquid interface. The X-ray reflectivity measurements were analyzed with a layer model to determine the electron density and thickness of the headgroup and tailgroup layers. The refle...

Fluorocarbon films were grown on polystyrene in vacuum from 25- to 100-eV mass-selected C3F5+ ion beams. The films were analyzed by X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray reflectivity after exposure to the atmosphere for 4−8 weeks. The X-ray reflectivity indicates films that range from ∼30 to 60-A thick. The thinner films form at lower ion energies, where the ion penetration depth and efficiency of film formation are lowest. X-ray reflectivity estimates air−fluorocarbon film roughness values of ∼6 A for 25- and 50-eV films but ∼20 A for the 100-eV films. The fluorocarbon−polystyrene-buried interface displays similar roughness and trends with ion energy. The AFM roughness trends are similar, but the absolute AFM roughnesses are only ∼1/4 of the X-ray reflectivity values. This discrepancy is attributed to tip effects and the method of determining roughness by AFM. The AFM images and power spectral densities of the 100-eV films displayed quasi-periodic cones spaced 300−700 A apa...

We discuss results from the first experiments on a new liquid surface/interface X-ray spectrometer recently commissioned by ChemMatCARS at Sector 15 of the Advanced Photon Source. These experiments include studies of liquid/liquid interfaces, monolayers supported on the water surface, and liquid metal surfaces.

The formation of thin aqueous films on top of an aqueous subphase is demonstrated. The films form through a complex spreading process that results in the coexistence of macroscopic lenses and films that are several nanometers thick. Aqueous biphase solutions of poly(ethylene glycol), potassium phosphates, and water are used to form these films. Synchrotron X-ray scattering is used to characterize the structure of the thin film and to probe the adsorption of proteins to the film. X-ray reflectivity measures the layer thickness (4-5 nm) and the roughness of the two interfaces of the film. Surface and interfacial tension measurements, combined with the X-ray measurements, indicate that the films are a thin layer of the bulk solution rather than a monolayer of PEG molecules. The film can be described by an excess free energy with a short range piece (due primarily to capillary wave entropic repulsion) and a long-range van der Waals interaction. Biomolecules, such as proteins, can be trapped at the aqueous-aqueous interface or in the thin film. This idea is demonstrated by an X-ray reflectivity study of ferritin proteins that form a 2-dimensional array at the interface. It is shown that the electron density interfacial profile of the ferritin trapped in this thin film is consistent with the known crystal structure of ferritin. In the absence of the thin film, ferritin does not adsorb to the interface.

Hyperthermal polyatomic ion beams can be used to fabricate thin film nanostructures with controlled morphology. Several experiments are described in which mass-selected and non-mass-selected polyatomic ion beams are used to create nanometer thick films with controlled surface and buried interface morphologies. Fluorocarbon and thiophenic films are grown on silicon wafers and/or polystyrene from 5 to 200 eV C3F5+ or C4H4S+ ions, respectively. X-ray photoelectron spectroscopy, atomic force microscopy, X-ray reflectivity, and scanning electron microscopy are utilized to analyze the morphology and chemistry of these films. Polyatomic ions are found to control film morphology on the nanoscale through variation of the incident ion energy, ion structure and/or substrate.

Our view of molecular ordering in Langmuir monolayers at the water−vapor interface influences our understanding of molecular ordering at other interfaces, including liquid−liquid interfaces for which structural information is scarce. We present a comparative study of a monolayer of a long-chain alkanol at the water−vapor and water−hexane interfaces using X-ray reflectivity to highlight significant differences between these two interfaces. The molecules in the Langmuir monolayer form an ordered phase of nearly rigid rods. In contrast, at the water−hexane interface, the triacontanol molecules form a condensed phase with progressive disordering of the chain from the −CH2OH to the −CH3 group. Surprisingly, at the water−hexane interface, the density in the headgroup region is 10−15% greater than either bulk water or the ordered headgroup region found at the water−vapor interface. It is conjectured that this higher density is a result of water penetration into the headgroup region of the disordered monolayer.

Major recent advances include the development of new experimental techniques that enabled the first precise measurements of interfacial widths at water–oil interfaces and of the ordering of surfactants adsorbed to these interfaces, studies of phase transitions and domain formation in surfactant monolayers, and studies of interfacial fluctuations confined by and coupled across thin liquid films.

X-ray off-specular diffuse scattering is used to directly probe the statistical distribution of domains in monolayers of F(CF2)10(CH2)2OH adsorbed at the hexane-water interface. These domains appear very near a transition from a monolayer solid to gas phase. The domains have a nearly constant radius of ≈ 1.5 μm and a nearest-neighbor separation that varies from 2 to 8 μm with temperature. Evidence that the domains are of equilibrium size suggests that the domain line tension, λt ≈ 3 × 10−11 N, can be determined.

Lipid binding and associating proteins are necessary components of cell signaling pathways historically overlooked for more amenably characterized protein-protein interactions. As peripheral membrane binding proteins attract more attention, reliable structural methods are needed to elucidate the protein-lipid interactions that facilitate their function. Traditional methods such as crystallography or NMR have produced structures of many peripheral membrane binding proteins in isolation, bound to a single lipid, or in a lipid cubic phase but not in complex with full lipid membranes. X-ray reflectivity provides structural characterization of lipid monolayer associated proteins assuming a known structure of the desired protein has already been obtained. Depending on the experimental conditions of the given structure, it is possible this structure is representative of the membrane associated structure. In our studies of three members of the T-cell Immunoglobulin Mucin (TIM) family of proteins, involved in the recognition of the apoptotic cellular signal phosphatidylserine (PtdSer) in lipid membranes, the crystal structure was only representative for TIM4 and not TIM1 or TIM3. TIM1 was crystallized without PtdSer in a closed conformation that cannot represent the PtdSer bound state and TIM3 has much lower affinity resulting in a weak x-ray reflectivity signal. We developed data analysis methods employing molecular dynamics to refine the structures of TIM1 and TIM3 to better represent their membrane bound conformations. The newly obtained structures provide much improved fits of the data and highlight protein-lipid interactions that can explain the differences in binding affinity between the TIM protein family members.

Experiments and computer simulations provide a new perspective that strong correlations of counterions with charged nanoparticles can influence the localization of nanoparticles at liquid-liquid interfaces and support the formation of voltage-tunable nanoparticle arrays. We show that ion condensation onto charged nanoparticles facilitates their transport from the aqueous-side of an interface between two immiscible electrolyte solutions to the organic-side, but contiguous to the interface. Counterion condensation onto the highly charged nanoparticles overcomes the electrostatic barrier presented by the low permittivity organic material, thus providing a mechanism to transport charged nanoparticles into organic phases with implications for the distribution of nanoparticles throughout the environment and within living organisms. After transport, the nanoparticles assemble into a two-dimensional (2D) nearly close-packed array on the organic side of the interface. Voltage-tunable counterion-mediated interactions between the nanoparticles are used to control the lattice spacing of the 2D array. Tunable nanoparticle arrays self-assembled at liquid interfaces are applicable to the development of electro-variable optical devices and active elements that control the physical and chemical properties of liquid interfaces on the nanoscale.

The electric potential difference across cell membranes, known as the membrane potential, plays an important role in the activation of many biological processes. To investigate the effect of the membrane potential on the molecular ordering of lipids within a biomimetic membrane, a self-assembled monolayer of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) lipids at an electrified 1,2-dichloroethane/water interface is studied with X-ray reflectivity and interfacial tension. Measurements over a range of electric potential differences, -150 to +130 mV, that encompass the range of typical biomembrane potentials demonstrate a nearly constant and stable structure whose lipid interfacial density is comparable to that found in other biomimetic membrane systems. Measurements at higher positive potentials, up to 330 mV, illustrate a monotonic decrease in the lipid interfacial density and accompanying variations in the interfacial configuration of the lipid. Molecular dynamics simulations, designed to mimic the experimental conditions, show that the measured changes in lipid configuration are due primarily to the variation in area per lipid with increasing applied electric field. Rotation of the SOPC dipole moment by the torque from the applied electric field appears to be negligible, except at the highest measured potentials. The simulations confirm in atomistic detail the measured potential-dependent characteristics of SOPC monolayers. Our hybrid study sheds light on phospholipid monolayer stability under different membrane potentials, which is important for understanding membrane processes. This study also illustrates the use of X-ray surface scattering to probe the ordering of surfactant monolayers at an electrified aqueous-organic liquid-liquid interface.

The interfacial behavior of a model solvent extraction liquid-liquid system, consisting of solutions of dihexadecyl phosphate (DHDP) in dodecane and SrCl2 in water, was studied to determine the structure of the interfacial ion-extractant complex and its variation with pH. Previous experiments on a similar extraction system with ErCl3 demonstrated that the kinetics of the extraction process could be greatly retarded by cooling through an adsorption transition, thus providing a method to immobilize ion-extractant complexes at the interface and further characterize them with X-ray interface-sensitive techniques. Here, we use this same method to study the SrCl2 system. X-ray reflectivity and fluorescence near total reflection measured the molecular-scale interfacial structure above and below the adsorption transition for a range of pH. Below the transition, DHDP molecules form a homogeneous monolayer at the interface with Sr(2+) coverage increasing from zero to saturation (one Sr(2+) per two DHDP) within a narrow range of pH. Experimental values of Sr(2+) interfacial density determined from fluorescence measurements are larger than those from reflectivity measurements. Although both techniques probe Sr(2+) bound to DHDP, only the fluorescence provides adequate sensitivity to Sr(2+) in the diffuse double layer. A Stern equation determines the Sr(2+) binding constant from the reflectivity measurements and the additional Sr(2+) measured in the diffuse double layer is accounted for by Gouy-Chapman theory. Above the transition temperature, a dilute concentration of DHDP-Sr complexes resides at the interface, even for temperatures far above the transition. A comparison is made of the structure of the interfacial ion-extractant complex for this divalent metal ion to recent results on trivalent Er(3+) metal ions, which provides insight into the role of metal ion charge on the structure of interfacial ion-extractant complexes, as well as implications for extraction of these two differently charged ions.

Density modulated micro-separated phases (microphases) occur at 2D liquid interfaces in the form of alternating regions of high and low density domains. Brewster angle microscopy (BAM) images demonstrate the existence of microphases in cluster, stripe, and mosaic morphologies at the buried interface between hexane and water with fluoro-alkanol surfactant dissolved in the bulk hexane. At high temperature, the surfactant assembles at the interface in a 2D gaseous state. As the system is cooled additional surfactants condense onto the interface, which undergoes a 2D gas–solid phase transition. Microphase structure is observed within a few degrees of this transition in the form of clusters and labyrinthine stripes. Microphases have been observed previously in a number of other systems; nevertheless, we demonstrate that adsorption transitions at the liquid–liquid interface provide a convenient way to observe a full sequence of temperature-dependent 2D phases, from gas to cluster to stripe to mosaic to inverted stripe phases, as well as coexistence between some of these microphases. Cracking and fracture of the clusters reveal that they are a solid microphase. Theories of microphases often predict a single length scale for cluster and stripe phases as a result of the competition between an attractive and a repulsive interaction. Our observation that two characteristic length scales are required to describe clusters whose diameter is much larger than the stripe period, combined with the solid nature of the clusters, suggests that a long-range elastic interaction is relevant. These results complement earlier X-ray measurements on the same system.

Recognition of phosphatidylserine (PS) lipids exposed on the extracellular leaflet of plasma membranes is implicated in both apoptotic cell removal and immune regulation. The PS receptor T cell immunoglobulin and mucin-domain-containing molecule 4 (Tim4) regulates T-cell immunity via phagocytosis of both apoptotic (high PS exposure) and nonapoptotic (intermediate PS exposure) activated T cells. The latter population must be removed at lower efficiency to sensitively control immune tolerance and memory cell population size, but the molecular basis for how Tim4 achieves this sensitivity is unknown. Using a combination of interfacial X-ray scattering, molecular dynamics simulations, and membrane binding assays, we demonstrate how Tim4 recognizes PS in the context of a lipid bilayer. Our data reveal that in addition to the known Ca(2+)-coordinated, single-PS binding pocket, Tim4 has four weaker sites of potential ionic interactions with PS lipids. This organization makes Tim4 sensitive to PS surface concentration in a manner capable of supporting differential recognition on the basis of PS exposure level. The structurally homologous, but functionally distinct, Tim1 and Tim3 are significantly less sensitive to PS surface density, likely reflecting the differences in immunological function between the Tim proteins. These results establish the potential for lipid membrane parameters, such as PS surface density, to play a critical role in facilitating selective recognition of PS-exposing cells. Furthermore, our multidisciplinary approach overcomes the difficulties associated with characterizing dynamic protein/membrane systems to reveal the molecular mechanisms underlying Tim4's recognition properties, and thereby provides an approach capable of providing atomic-level detail to uncover the nuances of protein/membrane interactions.

X-ray surface scattering and interfacial tension measurements are used to study the solid-to-gas phase transition in soluble monolayers of F(CF2)8(CH2)2OH and F(CF2)10(CH2)2OH adsorbed at the water−hexane interface. X-ray reflectivity and diffuse scattering measurements determine the molecular ordering, the presence of domains, and the interfacial coverage of solid domains as a function of temperature. The temperature-dependent coverage can be analyzed by a functional form consistent with a critical transition proposed by theory.

We present our recent progress in using synchrotron x-ray surface scattering to study several different aspects of ordering at liquid–liquid interfaces. (1) The interfacial width at the water–alkane interface has been measured for a series of different chain length alkanes. The variation of interfacial width with the carbon number can be described by combining the capillary wave prediction for the width with a contribution from the intrinsic structure. (2) Under appropriate conditions, a surfactant monolayer forms at the interface between water and a hexane solution of a fluorinated surfactant. Reflectivity measurements that probe the electron density profile normal to the interface provide information on the surfactant ordering. This monolayer undergoes a solid to gas transition as a function of temperature. Diffuse scattering near the transition reveals the presence of islands. (3) Equilibrium interfaces between two aqueous phases containing polyethylene glycol and potassium phosphate salts can be studied. We present studies of conformal capillary wave fluctuations between two interfaces of a thin film of this biphase system. We also demonstrate that ferritin can be trapped and studied at this aqueous–aqueous interface.