Interfacial Domains Research Articles

Fabrication of Janus Membranes for Desalination of Oil-Contaminated Saline Water.

Desalination of oil-contaminated saline water using membrane distillation requires hydrophobic membranes with underwater superoleophobic surfaces. For designing such membranes, the chemistry and morphology of the interfacial domains in contact with the contaminated water need to be adjusted such that a stable water layer, adhering to the surface, prevents oil droplets from wetting the membrane. In this article, we present an approach that relies on the controlled functionalization of the surface of polyvinylidene fluoride (PVDF) membranes; we adjust the surface topography of the membranes and introduce chemical heterogeneity to them. We show that the morphology of the PVDF surface can be altered by adjusting the composition of the nonsolvent bath used for the phase inversion process. Also, we render the surface of the membranes hydrophilic by using an alkaline chemical bath solution. The membrane morphology and effectiveness of our chemical treatment were confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR), and zeta potential measurements. A stable underwater contact angle, higher than 150°, was observed for both canola oil (ρ ≈ 0.913 g cm-3, γ ≈ 31.5 mN m-1) and hexane (ρ ≈ 0.655 g cm-3, γ ≈ 18 mN m-1). We evaluated the performance of both pristine and functionalized membranes in a laboratory-scale direct contact membrane distillation (DCMD) setup and desalinated a saline solution contaminated with 500 ppm canola oil. Our results show that oil does not wet the functionalized membrane during the desalination process. The average permeate flux and salt rejection values for the functionalized membranes were 45 ± 5 Lm-2h-1 ( Tfeed = 70 °C, Tdistillate = 20 °C) and 99.99%, respectively.

Tailoring exchange bias in ferro/antiferromagnetic FePt3 bilayers created by He+ beams

We report on artificial exchange bias created in a continuous epitaxial FePt3 film by introducing chemical disorder using a He+ beam, which features tailorable exchange bias strength through post-irradiation annealing. By design, the ferromagnetic (FM)/antiferromagnetic (AF) heterostructure exhibits stratified degrees of chemical order; however, the chemical composition and stoichiometry are invariant throughout the film volume. This uniquely allows for a consideration purely of the magnetic exchange across the FM/AF interface without the added hindrance of structural boundary parameters which inherently affect exchange bias quality. Annealing at 840 K results in the strongest exchange biased system, which displays a cross-sectional morphology of fine (<10 nm) domain structure composed of both of chemically ordered and chemically disordered domains. A magnetic model developed from fitting the characteristic polarised neutron reflectometry spectral features reveals that dual interactions can be attributed to the observed exchange bias: magnetic coupling at the FM/AF interface and also between neighbouring FM (chemically disordered) and AF (chemically ordered) domains within the nominally FM layer. Our results indicate that exchange bias is hindered at interfaces which are both chemically and magnetically perfect, while annealing can be used to balance the volume proportions of interfacial FM and AF domains to enhance the magnetic interface roughness for customisable exchange bias in mono-stoichiometric FM/AF heterostructures crafted by ion beams.

Revelation of Interfacial Energetics in Organic Multiheterojunctions.

Efficient charge generation via exciton dissociation in organic bulk heterojunctions necessitates donor–acceptor interfaces, e.g., between a conjugated polymer and a fullerene derivative. Furthermore, aggregation and corresponding structural order of polymer and fullerene domains result in energetic relaxations of molecular energy levels toward smaller energy gaps as compared to the situation for amorphous phases existing in homogeneously intermixed polymer:fullerene blends. Here it is shown that these molecular energy level shifts are reflected in interfacial charge transfer (CT) transitions and depending on the existence of disordered or ordered interfacial domains. It can be done so by systematically controlling the order at the donor–acceptor interface via ternary blending of semicrystalline and amorphous model polymers with a fullerene acceptor. These variations in interfacial domain order are probed with luminescence spectroscopy, yielding various transition energies due to activation of different recombination channels at the interface. Finally, it is shown that via this analysis the energy landscape at the organic heterojunction interface can be obtained.

Constructing Ionic Liquid-Filled Proton Transfer Channels within Nanocomposite Membrane by Using Functionalized Graphene Oxide.

Herein, nanocomposite membranes are fabricated based on functionalized graphene oxides (FGOs) and sulfonated poly(ether ether ketone) (SPEEK), followed by being impregnated with imidazole-type ionic liquid (IL). The functional groups (acidic group or basic group) on FGOs generate strong interfacial interactions with SPEEK chains and then adjust their motion and stacking. As a result, the nanocomposite membranes possess tunable interfacial domains as determined by its free volume characteristic, which provides regulated location for IL storage. The stored ILs act as hopping sites for water-free proton conduction along the FGO-constructed interfacial channels. The microstructure at SPEEK-FGO interface governs the IL uptake and distribution in nanocomposite membrane. Different from GO and vinyl imidazole functionalized GO (VGO), the presence of acidic (-SO3H) groups confers the p-styrenesulfonic acid functionalized GO (SGO) incorporated nanocomposite membrane loose interface and strong electrostatic attraction with imidazole-type IL, imparting an enhanced IL uptake and anhydrous proton conductivity. Nanocomposite membrane containing 7.5% SGO attains the maximum IL uptake of 73.7% and hence the anhydrous conductivity of 21.9 mS cm(-1) at 150 °C, more than 30 times that of SPEEK control membrane (0.69 mS cm(-1)). In addition, SGOs generate electrostatic attractions to the ILs confined within SGO-SPEEK interface, affording the nanocomposite membrane enhanced IL retention ability.

Relationships between carbon fiber type and interfacial domain in ZrB2-based ceramics

Different kinds of commercial carbon chopped fibers were added to a ZrB2 ceramic doped with Si3N4 as sintering additive in order to study the different interfacial domains between fiber and matrix. The composites were hot-pressed at 1700°C and their final relative densities were higher than 95%. Depending on the fiber type, different interfaces formed with the matrix, resulting in different extent of fiber pullout. Fibers reacted with the additive and were surrounded by SiC and ZrC phases, weakly bonded to the fiber itself. Only high modulus fiber maintained a rounded shape and no reaction products formed around it. The fracture toughness values did not vary among the various composites, but the work of fracture notably increased for higher modulus fiber. The room-temperature strength varied from about 250–400MPa and the strength in air at 1500°C decreased to 110–270MPa depending on the interface formed around the fiber.

Deformation on segment-structure of electrostrictive polyurethane/polyaniline blends

Blends of elastic polyurethane (PU) and conductive polyaniline (PANI) were assessed for such improved electromechanical properties that are promising for further application in actuators, sensors and energy conversion. For the experiments, electrostrictive polyurethane blends were prepared with varied PANI contents. These blends were prepared with two alternative solvents, namely N,N-dimethylformamide (DMF) and 1-methyl-2-pyrrolidone (NMP), and were fabricated by solution casting. The morphology along with electrical and mechanical properties, and specifically the electrostrictive coefficient were studied. The electrostrictive behavior was determined from electric field induced strain, observed with a photonic displacement apparatus in the film thickness direction. The results suggest that the PANI component in NMP solvent effectively enhanced the electrostriction of these PU/PANI polymer blends. The solvent effects during film forming on the film's electrostriction were likely related to the hard interfacial domains between the dispersed PANI and the PU matrix that could increase the electrostriction coefficient.

Influence of surface concentration on poly(vinyl alcohol) behavior at the water-vacuum interface: a molecular dynamics simulation study.

Poly(vinyl alcohol) (PVA) is an amphiphilic macromolecule with surfactant activity. The peculiar behavior of this polymer at the water-air interface is at the basis of its use as material for hydrated microdevices, films, and nanofibers. This work aims to investigate the behavior of PVA and water within the surface domain of highly diluted aqueous solutions by means of atomistic molecular dynamics simulations. Monodisperse atactic oligomers of 30 residues were distributed within water slabs in a vacuum box and allowed to diffuse toward the surface. After equilibration, structural features and dynamical properties of polymer chains and water in the interfacial domains were analyzed as a function of PVA surface concentration at 293 K. Surface pressure values obtained from simulations are in agreement with experimental values at corresponding polymer specific surface areas. In the explored concentration range of 6-34 μmol of residues/m(2), the chains display a transition between two states. At lower surface concentrations, elongated, quite rigid structures are adsorbed on the surface, whereas partially submerged globular aggregates, locally covered by thin water layers, are formed at higher surface concentrations. At PVA concentrations higher than about 20 μmol of residues/m(2), the percolation of chain aggregates over the interface plane produces a surface-confined polymer network with stable pores filled by water molecules. A substantial slowing of polymer and water dynamics in the interfacial domains is highlighted by the mean squared displacement time behavior of terminal residues and the interaction time of PVA-water hydrogen bonding. The diffusion coefficient of water and lifetime of hydrogen bonds between solvent molecules are halved and doubled, respectively, at the interface with the highest polymer concentration. The attenuation of water and polymer mobility concur to stabilize PVA hydrated networks in contact with air.

Origin of Interfacial Nanoscopic Gaseous Domains and Formation of Dense Gas Layer at Hydrophobic Solid–Water Interface

Interfacial gas enrichment (IGE) covering the entire area of hydrophobic solid-water interface has recently been detected by atomic force microscopy (AFM) and hypothesized to be responsible for the unexpected stability and anomalous contact angle of gaseous nanobubbles and the significant change from DLVO to non-DLVO forces. In this paper, we provide further proof of the existence of IGE in the form of a dense gas layer (DGL) by molecular dynamic simulation. Nitrogen gas adsorption at the water-graphite interface is investigated using molecular dynamic simulation at 300 K and 1 atm normal pressure. The results show that a DGL with a density equivalent to a gas at pressure of 500 atm is formed and equilibrated with a normal pressure of 1 atm. By varying the number of gas molecules in the system, we observe several types of dense gas domains: aggregates, cylindrical caps, and DGLs. Spherical cap gas domains form during the simulation but are unstable and always revert to another type of gas domain. Furthermore, the calculated surface potential of the DGL-water interface, -17.5 mV, is significantly closer to 0 than the surface potential, -65 mV, of normal gas bubble-water interface. This result supports our previously stated hypothesis that the change in surface potential causes the switch from repulsion to attraction for an AFM tip when the graphite surface is covered by an IGE layer. The change in surface potential comes from the structure change of water molecules at the DGL-water interface as compared with the normal gas-water interface. In addition, the contact angle of the cylindrical cap high density nitrogen gas domains is 141°. This contact angle is far greater than 85° observed for water on graphite at ambient conditions and much closer to the 150° contact angle observed for nanobubbles in experiments.

Hydration Shell Parameters of Aqueous Alcohols: THz Excess Absorption and Packing Density

Solvation in water requires minimizing the perturbations in its hydrogen bonded network. Hence solutes distort water molecular motions in a surrounding domain, forming a molecule-specific hydration shell. The properties of those hydration shells impact the structure and function of the solubilized molecules, both at the single molecule and at higher order levels. The size of the hydration shell and the picoseconds time-scale water dynamics retardation are revealed by terahertz (THz) absorption coefficient measurements. Room-temperature absorption coefficient at f = 0.28 [THz] is measured as a function of alcohol concentration in aqueous methanol, ethanol, 1,2-propanol, and 1-butanol solutions. Highly diluted alcohol measurements and enhanced overall measurement accuracy are achieved with a THz absorption measurement technique of nL-volume liquids in a capillary tube. In the absorption analysis, bulk and interfacial molecular domains of water and alcohol are considered. THz ideal and excess absorption coefficients are defined in accordance with thermodynamics mixing formulations. The parameter extraction method is developed based on a THz excess absorption model and hydrated solute molecule packing density representation. First, the hydration shell size is deduced from the hydrated solute packing densities at two specific THz excess absorption nonlinearity points: at infinite alcohol dilution (IAD) and at the THz excess absorption extremum (EAE). Consequently, interfacial water and alcohol molecular domain absorptions are deduced from the THz excess absorption model. The hydration shell sizes obtained at the THz excess absorption extremum are in excellent agreement with other reports. The hydration shells of methanol, ethanol, 1- and 2-propanol consist of 13.97, 22.94, 22.99, and 31.10 water molecules, respectively. The hydration shell water absorption is on average 0.774 ± 0.028 times the bulk water absorption. The hydration shell parameters might shed light on hydration dynamics of biomolecules.

Anomalous Behavior of Coercivity in Ferromagnet/Antiferromagnet Bilayers

The dependence of exchange-bias field (HEB) and coercivity (HC) on the ferromagnetic (FM) layer thickness in CoFe/IrMn and CoFe/FeMn bilayers was investigated. The coercivity of the CoFe/IrMn bilayers is very sensitive to the IrMn (110) structure. Through inserting a thin antiferromagnetic (AFM) FeMn layer into the CoFe/IrMn interface, an obvious change in the coercivity behavior is observed. Our results show that the interfacial magnetic domains play a prominent effect on the exchange-bias field, but a negligible influence on the coercivity behavior, suggesting that the enhanced coercivity in FM/AFM exchange biased system comes from the irreversible spin rotation in the AFM layer near interface.

Hierarchical simulation of microcrystalline PECVD silicon film growth and structure

We have designed and implemented a hierarchical simulation methodology capable of addressing the growth rate and microstructural features of thin silicon films deposited through PECVD (Plasma Enhanced Chemical Vapor Deposition). Our main objective is to elucidate the microscopic mechanisms as well as the interplay between atomic level and macroscopic design parameters associated with the development of nano- or micro-scale crystalline regions in the grown film. The ultimate goal is to use multi-scale modeling as a design tool for tackling the issue of local crystallization and its dependence on operating variables. At the heart of our simulation approach is a very efficient, large-scale kinetic Monte Carlo (kMC) algorithm which allows generating samples of representative Si films based on a validated chemistry model. In a second step, the generated film is subjected to an atomistic simulation study which restores the molecular details lost or ignored in the kMC model. The atomistic simulations are computationally very demanding; they are, however, an important ingredient of our work: we use it to back-map the coarse grained model employed in the kMC simulations to an all-atom model which is further relaxed through detailed NPT molecular dynamics (MD) or Monte Carlo simulations. This tunes local structure thus also important morphological details associated with the presence of crystalline and amorphous regions (and the intervening interfacial domains) in the grown film.The kMC algorithm is based on a carefully chosen set of reacting or active radicals (species) in the gas phase impinging the film and a detailed set of surface reactions. Inputs for species fluxes are taken from a well-tuned plasma fluid model that includes a detailed gas phase chemistry reaction scheme. The growth mechanism consists of various surface kinetic events including radical-surface and adsorbed radical-radical interactions, radical-surface diffusion, and surface dissociation reactions. The very fast surface diffusion is decoupled from the rest of the kMC events and is treated deterministically in our work. For a three-dimensional Si(001)-(2x1):H crystalline lattice, our kMC algorithm allows us to simulate film growth over several seconds, resulting in thickness on the order of tens of nanometers. In the following pages we provide more details about the implementation of our kMC algorithm along with validation results.

Features of structure formation and electrophysical properties of poly(vinylidene fluoride) crystalline ferroelectric polymers

AbstractThe influence of intramolecular dipole–dipole interaction changes on structure formation peculiarities and some electrophysical properties were investigated with example of copolymers of vinylidene fluoride with tetrafluoroethylene and hexafluoropropylene with different compositions. The decrease of such dipole–dipole interactions in vinylidene fluoride/tetrafluoroethylene copolymers leads to an increase of the a and b parameters of the ferroelectric phase lattice and were accompanied by a shift of the Curie point to lower temperatures. The presence of peak‐halo at angles near 2θ = 18° were attributed to a paraelectric phase localized in the interfacial domains at the crystal–amorphous phase boundaries. Similar peak‐halos for vinylidene fluoride/hexafluoropropylene copolymers crystallizing into the nonpolar α phase were associated with the presence of an antiferroelectric phase formed by the chains in the planar zigzag conformation. The temperature range where dielectric anomalies were detected was characterized by conformational changes at which the decrease in planar zigzag conformation isomers took place. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Bipolar resistance switching property of Al-Ag/La0.7Ca0.3MnO3/Pt sandwiches

Bipolar resistance switching was investigated on La0.7Ca0.3MnO3 (LCMO) thin film with Al-Ag alloy top electrode (TE) including different Ag contents. The switching capability of Al-Ag/LCMO/Pt was greatly improved in Al-33%Ag TE structure compared to in Al-50%Ag TE structure. Switching times of faster than 100 ns and rewrite cycles of more than 400 were obtained while maintaining a ratio of resistance change larger than 1000%. The mechanism of resistance switching was explained by a model with interfacial nanostructured domains composed of Ag and insulator AlOx matrix, as previously proposed for Al-50%Ag/LCMO/Pt structure.

Resonance Energy Transfer in Polymer Nanodomains

The properties of nanostructured polymer materials are often determined by their interfacial domains, which are typically 1−10 nm thick. This range matches the length scale of Forster resonance energy transfer (FRET). While the use of FRET in polymers has been pioneered by Morawetz in the 1980s, only recently the technique was extended to obtain quantitative detailed information on nanostructured materials. This review describes a new tool that can be used to analyze FRET in nanostructures and characterize their morphology and component distribution. We describe the application of this tool to a few examples of polymer systems containing nanodomains and heterogeneous distributions of the components, from block copolymer films and micelles to polymer nanoparticles.

Computer simulations of catanionic surfactants adsorbed at air/water interfaces. II. Full coverage

We extend our previous molecular dynamics experiments [Rodriguez et al., J. Phys. Chem. B 109, 24427 (2005)] to the analysis of the adsorption of catanionic surfactants at water/air interfaces, at a surfactant coverage close to that of the saturated monolayer: 30.3 A(2) per headgroup. The mixture of surfactants investigated corresponds to equal amounts of dodecytrimethylammonium (DTA) and dodecylsulfate (DS). The structure of the interface is analyzed in terms of the local densities and orientational correlations of all relevant interfacial species. In accordance with experimental evidence, the DTA headgroups penetrate deeper into the aqueous substrate than the DS ones, although the average positions of all headgroups, with respect to the interface, lie in positions somewhat more external than the ones observed at lower coverages. Average tail tilts are close to 45 degrees. The characteristics of the headgroup-water substrate correlations are also analyzed using a tessellation procedure of the interface. The density and polarization responses of the interfacial domains closest to the DS headgroups are enhanced, compared to those adjacent to the DTA detergents. Dynamical aspects related to the diffusion and to the orientational correlations of different water layers in close contact with the surfactant are also investigated.

Coating of negatively charged liposomes by polylysine: Drug release study

The present work describes surface coating of carboxyfluorescein(CF)-loaded liposomes with poly- l-lysine (PLL) and liposome membrane permeability. The vesicles were prepared from synthetic or natural lipids. Interaction between PLL and the liposomes leads to the formation of complexes — either single PLL-coated vesicles or vesicle aggregates. Formation of the complexes is strongly affected by PLL/lipid molar ratio and the molecular mass of the PLL chains. Liposome permeability depends strongly on the lipid phase state — vesicles in the solid state retained the entrapped dye for a long time, but continuous CF release was registered for “fluid” vesicles. Crossing the transition temperature leads to intensive dye leakage because of the appearance of leaky interfacial domains between the coexisting solid and liquid phases and also because of a reversible change in the vesicle size upon the solid–liquid state phase transition. PLL coverage does not cause permeabilization of “solid” liposomes, but increases the permeability of “fluid” vesicles. At the same time, the results of differential scanning calorimetry and vesicle fusion suggest that PLL adsorption occurs exclusively on the vesicular surface and that the lipidic organization is not significantly disturbed. Moreover, PLL does not prevent lipid exchange between vesicles induced by temperature change.

Surface-Enhanced Raman Spectroscopy of Dodecanethiol-Bound Silver Nanoparticles at the Liquid/Liquid Interface

Adsorption and domain formation of dodecanethiol (DT)-bound silver nanoparticles (SNPs) at the cyclohexane/water interface were studied by means of total internal reflection (TIR) light scattering microscopy and TIR surface-enhanced Raman scattering (SERS). By the TIR light scattering microscopy, the extent of the interfacial adsorption and domain formation of SNP was observed, which was produced by the reaction between citrate-reduced SNPs in the aqueous phase and DT in the cyclohexane phase. The Raman spectra of DT on SNP showed that the relative intensity ratios of gauche to trans conformers in the nu(C-S) band region decreased with the increase of the initial concentration of DT, suggesting the change from the liquidlike structure to the solidlike structure of the DT. The residue of the negative charges on the SNPs at the interface was detected by the resonance SERS (SERRS) peaks of the adsorbed cationic porphyrin, 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine (TMPyP). The efficiency of the interfacial SNPs domains as a SERS substrate for TMPyP strongly depended on the adsorption state of the DT.

Effects of Solubilization on the Structure and Function of the Sensory Rhodopsin II/Transducer Complex

Lipid-protein interactions are known to play a crucial role in structure and physiological activity of integral membrane proteins. However, current technology for membrane protein purification necessitates extraction from the membrane into detergent micelles. Also, due to experimental protocols, most of the data available for membrane proteins is obtained using detergent-solubilized samples. Stable solubilization of membrane proteins is therefore an important issue in biotechnology as well as in biochemistry and structural biology. An understanding of solubilization effects on structural and functional properties of specific proteins is of utmost relevance for the evaluation and interpretation of experimental results. In this study, a comparison of structural and kinetic data obtained for the archaebacterial photoreceptor/transducer complex from Natronomonas pharaonis (NpSRII/NpHtrII) in detergent-solubilized and lipid-reconstituted states is presented. Laser flash photolysis, fluorescence spectroscopy, and electron paramagnetic resonance spectroscopy data reveal considerable influence of solubilization on the photocycle kinetics of the receptor protein and on the structure of the transducer protein. Especially the protein-membrane proximal region and the protein-protein interfacial domains are sensitive towards non-native conditions. These data demonstrate that relevance of biochemical and structural information obtained from solubilized membrane proteins or membrane protein complexes has to be evaluated carefully.

Adaptation and optimization of the emulsification-diffusion technique to prepare lipidic nanospheres

In this study, the emulsification-diffusion method traditionally used to prepare polymeric nanoparticles was adapted to obtain lipidic nanospheres (LN) using four model lipids. The method consists of dissolving the lipid in a partially water-miscible solvent (previously saturated with water) at room temperature or at controlled temperature depending on lipid solubility. This organic phase is emulsified in an aqueous solution of a stabilizing agent (saturated with solvent) by conventional stirring at the same temperature used to dissolve the lipid. This oil-in-water emulsion is then diluted with an excess of water at controlled temperature in order to provoke the diffusion from the internal phase into the external phase thereby causing lipid aggregation in the form of LN. This new approach for the preparation of LN has clear advantages over the existing methods, namely: (i) it is efficient and versatile; (ii) easy implementation and scaling up (with no need of high energy sources); (iii) high reproducibility and narrow size distribution; (iv) less physical stress (i.e., long exposure to high temperatures and to mechanical dispersion); (v) it is not necessary to dissolve the drug in the melted lipid. The selection of the water-miscible solvent and the stabilizers are critical parameters to obtain lipidic particles in the nanometric range. In general, solvents with high water miscibility and stabilizers able to form stable emulsions are preferred. The results demonstrated that it was possible to reduce the particle size by increasing the process temperature, the stirring rate, the amount of stabilizer, and by lowering the amount of lipid. Control of the preparative variables allowed to obtain LN with diameters under 100 nm. It was found that the influence of preparative parameters was associated with a mechanism based on a physicochemical instability. In this sense, it is suggested that the rapid solvent diffusion produces regions of local supersaturation near the interface, and LN are formed due to the ensuing interfacial phase transformations and lipid aggregation that occur in these interfacial domains. In terms of stability, only poly(vinyl alcohol) (PVAL) was able to preserve the physical stability of the dispersion for long periods after preparation. This effect was attributed to the ability of PVAL chains to form a strongly attached layer on the nanoparticle surface with an excellent repulsion effect.

Magnetization and polarized neutron reflectivity experiments on patterned exchange bias structures

Patterned Co/CoO thin film structures have been investigated by magnetization and polarized neutron reflectivity measurements in order to study the influence of finite size and shape anisotropy effects on the magnetization reversal. An anomaly was found in the upper branch of the hysteresis loops, probably caused by incomplete bias in the patterned structures. The asymmetry in magnetization reversal mechanism commonly found in the two branches of the hysteresis loops of unpatterned Co/CoO layers is altered in the patterned structures, consistent with the existence of interfacial domains.