Nonpolar Interface Research Articles

How Molecular Orientation Affects the Static Permittivity Profile of the Polar and Nonpolar Liquid-Liquid Interface.

The dielectric permittivity across the liquid-liquid interface presents an intrinsic response, with respect to the instantaneous interface reference. We hypothesize that dielectric responses across the nonpolar and polar liquid-liquid interfaces have different behaviors and underlying mechanisms. Molecular dynamics simulations were used to compare and contrast the dielectric response of a nonpolar (1,2-dichloroethane/water) and a polar (1-octanol/water) liquid-liquid interface system. We found that the enhanced dielectric permittivity at the nonpolar interface is attributed to the increased water dipole orientation and polarization density. In the case of the polar interface, strong association of the immiscible solvents inhibits the molecular dipole orientation, counteracting the effect from the enhanced surface water polarization density and resulting in a standard dielectric response. Detailed knowledge of the hydrogen bond networks and molecular dipole orientation with respect to the specific instantaneous interfacial and bulk regions reveals the effect of molecular proximity and the interaction with the opposing interfacial molecules on the mechanism of the dielectric permittivity response across the liquid-liquid interface phase boundary.

The polar and nonpolar interfaces influenced of magnetism in LaMnO3-based superlattices.

The interface of perovskite heterostructures has been shown to exhibit various electronic and magnetic phases such as two-dimensional electron gas, magnetism, superconductivity, and electronic phase separation. These rich phases are expected due to the strong interplay between spin, charge, and orbital degree of freedom at the interface. In this work, the polar and nonpolar interfaces are designed in LaMnO3-based (LMO) superlattices to investigate the difference in magnetic and transport properties. For the polar interface in a LMO/SrMnO3 superlattice, a novel robust ferromagnetism, exchange bias effect, vertical magnetization shift, and metallic behaviors coexist due to the polar catastrophe, which results in a double exchange coupling effect in the interface. For the nonpolar interface in a LMO/LaNiO3 superlattice, only the ferromagnetism and exchange bias effect characteristics exist due to the polar continuous interface. This is attributed to the charge transfer between Mn3+ and Ni3+ ions at the interface. Therefore, transition metal oxides exhibit various novel physical properties due to the strong correlation of d electrons and the polar and nonpolar interfaces. Our observations may provide an approach to further tune the properties using the selected polar and nonpolar oxide interfaces.

Erythrocyte Membrane Nanomechanical Rigidity Is Decreased in Obese Patients.

This work intends to describe the physical properties of red blood cell (RBC) membranes in obese adults. The hypothesis driving this research is that obesity, in addition to increasing the amount of body fat, will also modify the lipid composition of membranes in cells other than adipocytes. Forty-nine control volunteers (16 male, 33 female, BMI 21.8 ± 5.6 and 21.5 ± 4.2 kg/m2, respectively) and 52 obese subjects (16 male and 36 female, BMI 38.2± 11.0 and 40.7 ± 8.7 kg/m2, respectively) were examined. The two physical techniques applied were atomic force microscopy (AFM) in the force spectroscopy mode, which allows the micromechanical measurement of penetration forces, and fluorescence anisotropy of trimethylammonium diphenylhexatriene (TMA-DPH), which provides information on lipid order at the membrane polar–nonpolar interface. These techniques, in combination with lipidomic studies, revealed a decreased rigidity in the interfacial region of the RBC membranes of obese as compared to control patients, related to parallel changes in lipid composition. Lipidomic data show an increase in the cholesterol/phospholipid mole ratio and a decrease in sphingomyelin contents in obese membranes. ω-3 fatty acids (e.g., docosahexaenoic acid) appear to be less prevalent in obese patient RBCs, and this is the case for both the global fatty acid distribution and for the individual major lipids in the membrane phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS). Moreover, some ω-6 fatty acids (e.g., arachidonic acid) are increased in obese patient RBCs. The switch from ω-3 to ω-6 lipids in obese subjects could be a major factor explaining the higher interfacial fluidity in obese patient RBC membranes.

Interfacial Manganese‐Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle

AbstractMn‐doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton–dopant coupling. In this work, we report an immiscible bi‐phasic strategy for post‐synthetic Mn‐doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron‐donating oleylamine acts as a shuttle ligand to transport MnX2 through the water–hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton‐to‐dopant energy transfer process in doped NPls. Time‐resolved optical measurements offer a detailed insight into the exciton‐to‐dopant energy transfer process. This new approach for post‐synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar–nonpolar interface.

Interfacial Manganese-Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle.

Mn‐doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton–dopant coupling. In this work, we report an immiscible bi‐phasic strategy for post‐synthetic Mn‐doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron‐donating oleylamine acts as a shuttle ligand to transport MnX2 through the water–hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton‐to‐dopant energy transfer process in doped NPls. Time‐resolved optical measurements offer a detailed insight into the exciton‐to‐dopant energy transfer process. This new approach for post‐synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar–nonpolar interface.

New Insight into the Lewis Basic Sites in Metal-Organic Framework-Doped Hole Transport Materials for Efficient and Stable Perovskite Solar Cells.

Currently, Spiro-OMeTAD is the most widely used hole transport material (HTM) in the best-performing perovskite solar cells (PSCs), resulting from its suitable energy level and facile processing. However, the intrinsic properties of organic molecules, such as low conductivity and a nonpolar contact interface, will limit the power conversion efficiency (PCE) and stability of Spiro-OMeTAD-based PSCs. Chemical doping could be an effective strategy to ameliorate the performance of Spiro-OMeTAD, and most of the dopants are designed for controllably oxidizing Spiro-OMeTAD. In this work, a highly stable metal-organic framework {[Zn(Hcbob)]·(solvent)}n (Zn-CBOB) with rod topology and Lewis basic sites is assembled and employed as a dopant for the hole transport layer. It is found that Zn-CBOB not only controllably oxidizes Spiro-OMeTAD and improves the conductivity of the HTM but also passivates the surface traps of the perovskite film by coordinating with Pb2+. The Zn-CBOB-doped PSCs achieved a remarkable PCE of 20.64%. In addition, the hydrophobicity of Zn-CBOB can prevent water from destroying the perovskite layer, which helps elevate the stability of PSCs.

Nonpolar Interface Composition in Cetyltrimethylammonium Bromide Reverse Micellar Environments to Control Size and Induce Anisotropy on Gold Nanoparticles

AbstractWe study the composition effect of non‐polar organic media in cetyltrimethylammonium bromide (CTAB) reverse micelles (RMs) on the interface properties and their use as nanoreactors for gold nanoparticles (AuNPs) synthesis. We evaluate how the molecular structure of aliphatic (n‐hexane) and aromatic (toluene) organic solvent influences the environments and interactions at the interface of n‐hexane/1‐butanol/CTAB and toluene/1‐butanol/CTAB RMs, as a key factor on AuNPs finals properties. Data show that the intermicellar exchange rate is affected by changing the organic solvent, and these facts influence AuNPs size, polydispersity, and morphology. FTIR and 1H NMR results suggest that in n‐hexane a rigid and compact micellar interface favors the formation of smaller AuNPs, while in toluene a more fluent micellar interface enhances the formation of larger and dispersed AuNPs. This specific interaction also affects the CTAB counterion (Br −) availability at the interface, which appears to be crucial to induce anisotropy of the AuNPs obtained.

Electrolyte gate controlled metal-insulator transitions of the CaZrO3/SrTiO3 heterointerface

Two-dimensional electron gas (2DEG) at a complex oxide interface shows an extraordinary spectrum of intriguing phenomena and functionality. Another oxide 2DEG was recently created via strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3 (CZO/STO). Herein, we report an effective way to tune the CZO/STO interface via ionic liquid (IL) electrolyte gating. An unexpected metal-insulator transition of the interfacial 2DEG occurs readily with the immersion of the sample in an IL even before the gate voltage is applied. This suggests the presence of intrinsic polarization of CZO, which could act as a negative bias. The carrier density is found to be suppressed and shows a temperature-independent behavior after electrolyte gating which also resulted in higher electron mobility. These results suggest that the oxygen vacancies are annihilated via oxygen electromigration to the interface induced by electrolyte gating. The effective tunability by IL gating shed more light on the mechanism of electrolyte gating on the buried heterointerface.

Extreme Reconfigurable Nanoelectronics at the CaZrO3 /SrTiO3 Interface.

Complex oxide heterostructures have fascinating emergent properties that originate from the properties of the bulk constituents as well as from dimensional confinement. The conductive behavior of the polar/nonpolar LaAlO3 /SrTiO3 interface can be reversibly switched using conductive atomic force microscopy (c-AFM) lithography, enabling a wide range of devices and physics to be explored. Here, extreme nanoscale control over the CaZrO3 /SrTiO3 (CZO/STO) interface, which is formed from two materials that are both nonpolar, is reported. Nanowires with measured widths as narrow as 1.2 nm are realized at the CZO/STO interface at room temperature by c-AFM lithography. These ultrathin nanostructures have spatial dimensions at room temperature that are comparable to single-walled carbon nanotubes, and hold great promise for alternative oxide-based nanoelectronics, as well as offer new opportunities to investigate the electronic structure of the complex oxide interfaces. The cryogenic properties of devices constructed from quasi-1D channels, tunnel barriers, and planar gates exhibit gate-tunable superconductivity, quantum oscillations, electron pairing outside of the superconducting regime, and quasi-ballistic transport. This newly demonstrated ability to control the metal-insulator transition at nonpolar oxide interface greatly expands the class of materials whose behavior can be patterned and reconfigured at extreme nanoscale dimensions.

The spatial range of protein hydration.

Proteins interact with their aqueous surroundings, thereby modifying the physical properties of the solvent. The extent of this perturbation has been investigated by numerous methods in the past half-century, but a consensus has still not emerged regarding the spatial range of the perturbation. To a large extent, the disparate views found in the current literature can be traced to the lack of a rigorous definition of the perturbation range. Stating that a particular solvent property differs from its bulk value at a certain distance from the protein is not particularly helpful since such findings depend on the sensitivity and precision of the technique used to probe the system. What is needed is a well-defined decay length, an intrinsic property of the protein in a dilute aqueous solution, that specifies the length scale on which a given physical property approaches its bulk-water value. Based on molecular dynamics simulations of four small globular proteins, we present such an analysis of the structural and dynamic properties of the hydrogen-bonded solvent network. The results demonstrate unequivocally that the solvent perturbation is short-ranged, with all investigated properties having exponential decay lengths of less than one hydration shell. The short range of the perturbation is a consequence of the high energy density of bulk water, rendering this solvent highly resistant to structural perturbations. The electric field from the protein, which under certain conditions can be long-ranged, induces a weak alignment of water dipoles, which, however, is merely the linear dielectric response of bulk water and, therefore, should not be thought of as a structural perturbation. By decomposing the first hydration shell into polarity-based subsets, we find that the hydration structure of the nonpolar parts of the protein surface is similar to that of small nonpolar solutes. For all four examined proteins, the mean number of water-water hydrogen bonds in the nonpolar subset is within 1% of the value in bulk water, suggesting that the fragmentation and topography of the nonpolar protein-water interface has evolved to minimize the propensity for protein aggregation by reducing the unfavorable free energy of hydrophobic hydration.

Influence of interface structure on photoelectric properties of InGaN light-emitting diodes

The influence of interface structure on the photoelectric properties of InGaN light-emitting diodes grown on (0001) polar, (11–22) semi-polar, and (11–20) nonpolar planes is studied. Nonpolar and semi-polar interface structures are characterized by reduced polarization-related electric fields and suppressed QCSE. The energy bands of the structures grown on these orientations are undistorted, and QWs have more rectangular shapes. Consequently, these structures do not undergo separation of the electron and hole wavefunctions typical of c-plane structures. The simulation results suggest that potoelectric properties and efficiency drop is markedly improved when the polar interface structure is replaced by nonpolar or semi-polar interface structures.

Interfacial B-site atomic configuration in polar (111) and non-polar (001) SrIrO3/SrTiO3 heterostructures

The precise control of interfacial atomic arrangement in ABO3 perovskite heterostructures is paramount, particularly in cases where the subsequent electronic properties of the material exhibit geometrical preferences along polar crystallographic directions that feature inevitably complex surface reconstructions. Here, we present the B-site interfacial structure in polar (111) and non-polar (001) SrIrO3/SrTiO3 interfaces. The heterostructures were examined using scanning transmission electron microscopy and synchrotron-based coherent Bragg rod analysis. Our results reveal the preference of B-site intermixing across the (111) interface due to the polarity-compensated SrTiO3 substrate surface prior to growth. By comparison, the intermixing at the non-polar (001) interface is negligible. This finding suggests that the intermixing may be necessary to mitigate epitaxy along heavily reconstructed and non-stoichiometric (111) perovskite surfaces. Furthermore, this preferential B-site configuration could allow the geometric design of the interfacial perovskite structure and chemistry to selectively engineer the correlated electronic states of the B-site d-orbital.

Correction to Creation of High Mobility Two-Dimensional Electron Gases via Strain Induced Polarization at an Otherwise Nonpolar Complex Oxide Interface.

The discovery of two-dimensional electron gases (2DEGs) in SrTiO3-based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3. Remarkably, this heterointerface is atomically sharp and exhibits a high electron mobility exceeding 60 000 cm2 V–1 s–1 at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.

Study of simple microparticles formation of limonene in modified starch using PGSS – Particles from gas-saturated suspensions

Supercritical CO2 was studied for impregnation or encapsulation of essential oils in modified starches via Particle from Gas Saturated Solutions or Suspensions (PGSS). Modified starches were choose as function of its low cost. The advantage of this method over conventional encapsulation that use modified starch via spray drier refers to the low temperatures used and absence of water in the process. Modified starch presents hydrophobic elements and this molecules present amphiphilic character. Usually it is employed in the encapsulation of essences as wall material with excellent volatiles retention due to its polar and nonpolar interface. Considering its hydrophobic characteristics, interactions between the modified starch and supercritical CO2 occurred, resulting in two different structural interactions of the limonene and modified starch in the PGSS (50°C and 60°C at 100bar and 120bar). When hydrous ethanol was used in the suspension, impregnation occurred and, when anhydrous ethanol was used, encapsulation occurred. Analysis of particle morphology via scanning electron and confocal microscopy, thermo-oxidative characterization by differential scanning calorimetry and determination of microencapsulated limonene via gas chromatography coupled to mass spectrometry indicated limonene microencapsulation and impregnation occurred despite the highly solubility of limonene in supercritical CO2. The retention of limonene by Purity Gum Ultra® was 86% when encapsulated and, 53% when impregnated, similar values to those obtained in conventional microencapsulation methods via a spray drier or via PGSS-drying.

Highly Confined Electronic and Ionic Conduction in Oxide Heterostructures

The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. In this talk I will present our recent results both on ionic and electronic conductivity at different heterostructures systems. In the first part of my talk I will show some of our resent results that we demonstrated the possibility of stabilizing δ-Bi2O3 using highly coherent interfaces of alternating layers. Remarkably, an exceptionally high chemical stability in reducing conditions and redox cycles at high temperature, usually unattainable for Bi2O3-based materials, is achieved[1]. These confined heterostructures provide a playground not only for new high ionic conductivity phenomena that are sufficiently stable but also uncover a large variety of possible technological perspectives. At the second part, I will discuss and show our recent results of high mobile samples realized by, interface confined redox reactions [2] , strain induced polarization [3] and modulation doping at complex oxide interfaces. This collection of samples offers unique opportunities for a wide range of rich world of mesoscopic physics. [1] S. Sanne et al. “Enhancement of the chemical stability in confined δ-Bi2O3”. Nature Materials (2015) doi:10.1038/nmat4266 [2] Y. Z. Chen et al. “A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al2O3/SrTiO3”. Nature Commun. 4, 1371 (2013) [3] Y. Z. Chen et al. “Creation of High Mobility Two-Dimensional Electron Gases via Strain Induced Polarization at an Otherwise Nonpolar Complex Oxide Interface” Nano Letters. 3774-3778 (2015) 10.1021/nl504622w

Creation of high mobility two-dimensional electron gases via strain induced polarization at an otherwise nonpolar complex oxide interface.

The discovery of two-dimensional electron gases (2DEGs) in SrTiO3-based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3. Remarkably, this heterointerface is atomically sharp and exhibits a high electron mobility exceeding 60,000 cm(2) V(-1) s(-1) at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.

Role of Charges of the Surface-grafted Polymer Chains for Aqueous Lubrication at a Nonpolar Interface

Charged polymer chains, i.e., polyelectrolytes, are known to show superior aqueous lubricating properties compared to those of neutral polymer chains, especially in brush conformation. This is primarily because of the incorporation of a large amount of counterions within the polymer layers and the consequently increased osmotic pressure. However, this effect is active only when the polymer chains remain immobilized even under tribostress, which is not realistic for high-contact pressure tribological applications, especially when they are irreversibly immobilized on tribopair surfaces. In contrast, with free polymers, which can be included as surface-active additives in the base lubricant (water), long-term lubricating performance based on "self-healing" properties is readily expected. In order to assess whether the superior aqueous lubricating properties of polyelectrolyte chains are valid for free polymers too, this study reviews recent studies on the tribological properties of many charged biopolymer and synthetic copolymers at a nonpolar, hydrophobic interface. In contrast to the irreversibly immobilized polyelectrolyte chains, free polyelectrolyte chains show inferior aqueous lubricating properties compared to their neutral counterparts owing to charge accumulation and the consequently impeded surface adsorption on the nonpolar surface. Nevertheless, bovine submaxillary mucin (BSM), a representative biopolymer, shows a sufficiently effective surface adsorption and aqueous lubricating capabilities even at neutral pH without losing the polyanionic characteristics.

Evaluation of Generalized Born Model Accuracy for Absolute Binding Free Energy Calculations.

Generalized Born (GB) implicit solvent models are widely used in molecular dynamics simulations to evaluate the interactions of biomolecular complexes. The continuum treatment of the solvent results in significant computational savings in comparison to an explicit solvent representation. It is, however, not clear how accurately the GB approach reproduces the absolute free energies of biomolecular binding. On the basis of induced dissociation by means of umbrella sampling simulations, the absolute binding free energies of small proline-rich peptide ligands and a protein receptor were calculated. Comparative simulations according to the same protocol were performed by employing an explicit solvent model and various GB-type implicit solvent models in combination with a nonpolar surface tension term. The peptide ligands differed in a key residue at the peptide-protein interface, including either a nonpolar, a neutral polar, a positively charged, or a negatively charged group. For the peptides with a neutral polar or nonpolar interface residue, very good agreement between the explicit solvent and GB implicit solvent results was found. Deviations in the main separation free energy contributions are smaller than 1 kcal/mol. In contrast, for peptides with a charged interface residue, significant deviations of 2-4 kcal/mol were observed. The results indicate that recent GB models can compete with explicit solvent representations in total binding free energy calculations as long as no charged residues are present at the binding interface.

The structure of latherin, a surfactant allergen protein from horse sweat and saliva.

Latherin is a highly surface-active allergen protein found in the sweat and saliva of horses and other equids. Its surfactant activity is intrinsic to the protein in its native form, and is manifest without associated lipids or glycosylation. Latherin probably functions as a wetting agent in evaporative cooling in horses, but it may also assist in mastication of fibrous food as well as inhibition of microbial biofilms. It is a member of the PLUNC family of proteins abundant in the oral cavity and saliva of mammals, one of which has also been shown to be a surfactant and capable of disrupting microbial biofilms. How these proteins work as surfactants while remaining soluble and cell membrane-compatible is not known. Nor have their structures previously been reported. We have used protein nuclear magnetic resonance spectroscopy to determine the conformation and dynamics of latherin in aqueous solution. The protein is a monomer in solution with a slightly curved cylindrical structure exhibiting a ‘super-roll’ motif comprising a four-stranded anti-parallel β-sheet and two opposing α-helices which twist along the long axis of the cylinder. One end of the molecule has prominent, flexible loops that contain a number of apolar amino acid side chains. This, together with previous biophysical observations, leads us to a plausible mechanism for surfactant activity in which the molecule is first localized to the non-polar interface via these loops, and then unfolds and flattens to expose its hydrophobic interior to the air or non-polar surface. Intrinsically surface-active proteins are relatively rare in nature, and this is the first structure of such a protein from mammals to be reported. Both its conformation and proposed method of action are different from other, non-mammalian surfactant proteins investigated so far.

Forces acting on dielectric colloidal spheres at a water/nonpolar fluid interface in an external electric field. 2. Charged particles

Here, we calculate the electric forces acting on charged dielectric colloidal particles, which are attached to the interface between a nonpolar fluid (air and oil) and water in the presence of applied uniform external electric field, E0, directed normal to the interface. Electric charges are present on the particle–nonpolar fluid interface. The solution to the problem represents a superposition of the solutions of two simpler problems: (i) charged particle in the absence of external field and (ii) uncharged particle in the presence of external field. Because the external field can be directed upward or downward, it enhances or opposes the effect of the particle surface charges. As a result, the vertical (electrodipping) force vs. E0 may have a maximum or minimum and can be positive or negative depending on the particle contact angle and dielectric constant. In contrast, the lateral electric force between two identical charged floating particles is always positive (repulsive), but it can vary by many orders of magnitude with E0. This is because at a certain value of E0, the net dipolar moment of the particle becomes zero. Then, the interparticle force is governed by the octupolar moment, which leads to a much weaker and short-range repulsion. In the vicinity of this special value of E0, the interparticle repulsion is very sensitive to the variations in the external field. These effects can be used for a fine control of the lattice spacing in non-densely packed interfacial colloidal crystals of regular hexagonal packing for producing lithographic masks with various applications in nanotechnology.