Aqueous Solution Interface Research Articles

A self-assembled surfactant for efficient flotation of cassiterite: Experimental study and DFT calculation

Surfactants of different kinds are important in the flotation of metal oxide ores. In this paper, a combined organic surfactant was formed through self-assembly of salicylhydroxamic acid (SHA) and isopropanolamine dodecylbenzenesulfonate (DBIA). The effects of SHA and DBIA on the flotation behavior of cassiterite were investigated. Density functional theory (DFT) calculations were used to clarify the microscopic adsorption mechanism of SHA and DBIA on the cassiterite surface. The flotation performance of the combined surfactant was investigated to determine the optimum ratio. Subsequently, the self-assembly behavior of the surfactants in aqueous solutions was examined using surface tension tests and molecular dynamics simulations. Flotation experiments and zeta potential, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), contact angle, and adsorption capacity analyses were used to explore the flotation behavior and adsorption mechanism at the solid–liquid–gas boundary. The maximum flotation recovery of cassiterite were 45.73 % and 71.25 % when SHA and DBIA were used alone. By contrast, with the developed combined surfactant, the flotation recovery at the same reagent concentration was 84.92 %. The SHA/DBIA surfactant co-adsorbed at the aqueous solution interface, which decreased the surface tension of the solution to 4.943 × 10−4 mol/L and resulted in formation of a stable foam layer. These effects altered the surface properties of cassiterite and enhanced its surface hydrophobicity. Overall, this study provides valuable mechanistic insights into the enhanced chelation complexation of self-assembled surface collectors with metal oxides.

Electrocatalysis at vegetable oil water interface

Biphasic oxygen reduction and hydrogen evolution are studied for almost two decades, because of favourable overpotential decrease as compared to aqueous solution. Until now, polar solvents (ε > 7) were employed as organic phase in these studies. Here, we applied non polar vegetable oils (rapeseed, linen or sunflower) for biphasic H2O2 generation by oxygen reduction. This product was detected at oil|aqueous acid solution interface by scanning electrochemical microscopy, when electron donor – decamethylferrocene, was electrochemically recycled. Ejection of small fraction of decamethylferrocenium cation from oil to aqueous phase was also noticed.

Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

AbstractExploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent.

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Charge transfer reaction at gas-liquid interface of aqueous tetrabutylammonium iodide solution: influence of ions on dynamical response of solvent

Abstract Tetrabutylammonium iodide (TBAI) is known to create an electric double layer at the air-water interface. In the present study, we investigated the charge-transfer-to-solvent reaction from iodide ions to liquid water near the gas-liquid interface of an aqueous TBAI solution using extreme UV (EUV) time-resolved photoelectron spectroscopy. Use of EUV radiation ensured accurate measurements of electron kinetic energy distributions by minimizing spectral distortions caused by electron inelastic scattering in the liquid and the influence of the electron transmission efficiency through the gas-liquid interface. The spectra observed for photodetached electrons exhibited a rapid energy shift and a variation of the bandwidth in subpicoseconds, and the rates of these changes clearly depended on the TBAI concentration. The results indicate that the dynamical response of solvent water changes in the presence of a high density of ions.

Effects of polar fractions on interfacial and bulk properties at the oil/carbonated aqueous solution interface: Insight from molecular dynamics simulation

This study utilized molecular dynamics simulations to understand the liquid/liquid interfaces in a natural environment involving carbon dioxide and various acidic components. The aim is to unravel the impact of polar fractions with various chemical structures and functional groups, namely benzoic acid, decanoic acid, phenol, and decanol, on the interfacial phenomena of the oleic phase (decane) in contact with water or CO2-rich water at the molecular resolution. The findings indicate that carbon dioxide molecules enrich at the decane/water interface before dissolving into the oleic phase. When polar molecules are introduced to the oleic phase, there is a competition between carbon dioxide and acidic components to accumulate at the interface. The functional hydrocarbon fractions displace carbon dioxide at the interface and bridge the oleic and aqueous phases, reducing interfacial tension. Polar components change the charge distribution of water molecules in the biphasic system, signifying that the organized water layer at the interface with the non-aqueous phase has been disrupted. In contrast to the polar molecules, the carbon dioxide tends to diffuse into the oleic phase and changes the bulk properties like oil fluidity. Accordingly, the fluidity of the decane exhibited significant enhancement by CO2 diffusion throughout the bulk oil phase, which might lead to substantial viscosity reduction.

Proton Uptake at the Barite–Aqueous Solution Interface: A Combined Potentiometric, Electrophoretic Mobility, and Surface Complexation Modeling Investigation

Proton Uptake at the Barite–Aqueous Solution Interface: A Combined Potentiometric, Electrophoretic Mobility, and Surface Complexation Modeling Investigation

Preparation of Highly Conductive Phthalocyaninato-Cobalt Iodide at the Interface between Aqueous KI Solution and Organic Solvent and Catalytic Properties for Electrochemical Reduction of CO2

Abstract For the industrial practical use of cobalt phthalocyanine (CoPc) as a catalyst for CO2 reduction, the effect of improving conductivity of CoPc crystal on the catalytic activity was investigated. Focusing on (phthalocyaninato) cobalt iodide (CoPcI), which is a highly conductive charge-transfer complex, a simple method for synthesizing CoPcI by simply mixing at an interface between aqueous KI solution and CH2Cl2 solution containing CF3COOH and CoPc was developed by applying the CoPc crystal phase transformation method. The X-ray diffraction pattern of the obtained powder at the interface of CoPc organic and potassium iodide aqueous solutions revealed that the powder is composed of one-dimensional columns of CoPc and a linear array of triiodide ions (I3−). UV-vis absorption spectra showed that iodide ions (I−) change to triiodide ions (I3−) at the interface between aqueous KI solution and organic solvent under acidic conditions. The catalytic property of CoPcI was investigated by polarization measurement and electrochemical impedance spectroscopy using a gas diffusion carbon electrode. CoPcI showed high catalytic activity for CO2 reduction with high CO production selectivity.

Corn Oil-Water Separation: Interfacial Behavior of Extended Surfactant and Glutelin.

The purification and collection of various products from oil/water mixtures are routine procedures. However, the presence of emulsifiers that can displace other surface active components in the mixtures can significantly influence the efficiency of such procedures. Previously, we investigated interfacial mechanisms of zein protein-induced emulsification and the opposing surfactant-induced demulsification related to corn oil refinement. In this paper, we further investigated a different class of protein, glutelin, inside corn and proved that glutelin acts as an oil/water emulsifier in an acidic water environment. Furthermore, an extended surfactant's protein disordering and removal ability was tested and compared with a conventional surfactant. An extended surfactant contains a poly(propylene oxide) or poly(propylene oxide)-poly(ethylene oxide) chain inserted between the hydrophilic head and the hydrophobic tail. In this study, a nonlinear optical spectroscopic technique, sum frequency generation (SFG) vibration spectroscopy, was used to study the behavior of glutelin and extended as well as regular surfactants at the corn oil/water or aqueous solution interface. In most cases, the conventional surfactant shows better protein disordering or removal ability than the extended surfactant. However, with the addition of heat and salt to an extended surfactant solution, the experiment resulted in a substantial increase in the extended surfactant's protein disorder or removal ability.

Aliquots of MIL-140 and Graphene in Smart PNIPAM Mixed Hydrogels: A Nanoenvironment for a More Eco-Friendly Treatment of NaCl and Humic Acid Mixtures by Membrane Distillation.

The problem of water scarcity is already serious and risks becoming dramatic in terms of human health as well as environmental safety. Recovery of freshwater by means of eco-friendly technologies is an urgent matter. Membrane distillation (MD) is an accredited green operation for water purification, but a viable and sustainable solution to the problem needs to be concerned with every step of the process, including managed amounts of materials, membrane fabrication procedures, and cleaning practices. Once it is established that MD technology is sustainable, a good strategy would also be concerned with the choice of managing low amounts of functional materials for membrane manufacturing. These materials are to be rearranged in interfaces so as to generate nanoenvironments wherein local events, conceived to be crucial for the success and sustainability of the separation, can take place without endangering the ecosystem. In this work, discrete and random supramolecular complexes based on smart poly(N-isopropyl acrylamide) (PNIPAM) mixed hydrogels with aliquots of ZrO(O2C-C10H6-CO2) (MIL-140) and graphene have been produced on a polyvinylidene fluoride (PVDF) sublayer and have been proven to enhance the performance of PVDF membranes for MD operations. Two-dimensional materials have been adhered to the membrane surface through combined wet solvent (WS) and layer-by-layer (LbL) spray deposition without requiring further subnanometer-scale size adjustment. The creation of a dual responsive nanoenvironment has enabled the cooperative events needed for water purification. According to the MD's rules, a permanent hydrophobic state of the hydrogels together with a great ability of 2D materials to assist water vapor diffusion through the membranes has been targeted. The chance to switch the density of charge at the membrane-aqueous solution interface has further allowed for the choice of greener and more efficient self-cleaning procedures with a full recovery of the permeation properties of the engineered membranes. The experimental evidence of this work confirms the suitability of the proposed approach to obtain distinct effects on a future production of reusable water from hypersaline streams under somewhat soft working conditions and in full respect to environmental sustainability.

Zooming into the Inner Helmholtz Plane of Pt(111)-Aqueous Solution Interfaces: Chemisorbed Water and Partially Charged Ions.

The double layer on transition metals, i.e., platinum, features chemical metal-solvent interactions and partially charged chemisorbed ions. Chemically adsorbed solvent molecules and ions are situated closer to the metal surface than electrostatically adsorbed ions. This effect is described tersely by the concept of an inner Helmholtz plane (IHP) in classical double layer models. The IHP concept is extended here in three aspects. First, a refined statistical treatment of solvent (water) molecules considers a continuous spectrum of orientational polarizable states, rather than a few representative states, and non-electrostatic, chemical metal-solvent interactions. Second, chemisorbed ions are partially charged, rather than being electroneutral or having integral charges as in the solution bulk, with the coverage determined by a generalized, energetically distributed adsorption isotherm. The surface dipole moment induced by partially charged, chemisorbed ions is considered. Third, considering different locations and properties of chemisorbed ions and solvent molecules, the IHP is divided into two planes, namely, an AIP (adsorbed ion plane) and ASP (adsorbed solvent plane). The model is used to study how the partially charged AIP and polarizable ASP lead to intriguing double-layer capacitance curves that are different from what the conventional Gouy-Chapman-Stern model describes. The model provides an alternative interpretation for recent capacitance data of Pt(111)-aqueous solution interfaces calculated from cyclic voltammetry. This revisit brings forth questions regarding the existence of a pure double-layer region at realistic Pt(111). The implications, limitations, and possible experimental confirmation of the present model are discussed.

Evidence of defect-induced broadband light emission from 2D Ag-Bi double perovskites grown at liquid-liquid interfaces.

Controlling the light emission spectra of low-dimensional hybrid organic-inorganic materials remains an important goal toward the implementation of these materials into real-world optoelectronic devices. In this study, we present evidence that the self-assembly of two-dimensional (2D) silver bismuth iodide double perovskite derivatives at the interface of aqueous and organic solutions leads to the formation of defects capable of modulating the light emission spectra of these materials. Through an analysis of the structural parameters used to explain the photoluminescence (PL) spectra of 2D perovskites, we show the light spectra emitted by (4-ammonium methyl)piperidinium (4-AMP) and (3-ammonium methyl)pyridinium (3-AMPy)-spaced AgBiI8 double perovskites formed through interfacial solution-phase chemistry differ qualitatively and quantitatively from thin film samples. We use previous results to propose the differences observed in the PL spectra of different material morphologies stem from equatorial iodide vacancy formation driven by the kinetics of self-assembly at the liquid-liquid interface. These results show the generality of these chemical physics principles in the formation of defect sites in solution-processed semiconducting nanomaterials, which could help enable their broad use in optoelectronic technologies.

Elucidation of the methylene blue adsorption mechanism at the interface of powder pumice and aqueous solution by electro-kinetic and static contact angle measurements

Elucidation of the methylene blue adsorption mechanism at the interface of powder pumice and aqueous solution by electro-kinetic and static contact angle measurements

Ion and Particle Size Effects on the Surface Reactivity of Anatase Nanoparticle–Aqueous Electrolyte Interfaces: Experimental, Density Functional Theory, and Surface Complexation Modeling Studies

At the nanoscale, particle size affects the surface reactivity of anatase–water interfaces. Here, we investigate the effect of electrolyte media and particle size on the primary charging behavior of anatase nanoparticles. Macroscopic experiments, potentiometric titrations, were used to quantitatively evaluate surface charge of a suite of monodisperse nanometer sized (4, 20, and 40 nm) anatase samples in five aqueous electrolyte solutions. The electrolyte media included alkaline chloride solutions (LiCl, NaCl, KCl, and RCl) and Na-Trifluoromethanesulfonate (NaTr). Titrations were completed at 25 °C, as a function of pH (3–11) and ionic strength (from 0.005 to 0.3 m). At the molecular scale, density functional theory (DFT) simulations were used to evaluate the most stable cation surface species on the predominant (101) anatase surface. In all electrolyte media, primary charging increased with increasing particle size. At high ionic strength, the development of negative surface charge followed reverse lyotropic behavior: charge density increased in the order RbCl < KCl < NaCl < LiCl. Positive surface charge was greater in NaCl than in NaTr media. From the DFT simulations, all cations formed inner-sphere surface species, but the most stable coordination geometry varied. The specific inner-sphere adsorption geometries are dependent on the ionic radius. The experimental data were described using surface complexation modeling (SCM), constrained by the DFT results. The SCM used the charge distribution (CD) and multisite (MUSIC) models, with a two-layer (inner- and outer-Helmholtz planes) description of the electric double layer. Subtle charging differences between the smallest and larger anatase particles were the same in each electrolyte media. These results further our understanding of solid–aqueous solution interface reactivity of nanoparticles.

Mechanisms of ionic liquids on the enhancement of interfacial transport of lithium ions in crown ether system

Lithium and lithium isotopes are important for clean energy development. By introducing ionic liquid, the extraction performance of crown ethers for lithium ions can be significantly improved. However, the enhancement mechanism of ionic liquids in the system is not clear, which directly limits the selection of suitable ionic liquids for various applications. In this work, the synergy mechanisms of different ionic liquids with crown ether during the extraction process were investigated by molecular dynamics simulations and verified by experimental results. There were two molecular mechanisms to capture lithium ions for ionic liquids in the crown ether system: co-extraction and assisted extraction. It was generally considered that the cation exchange mechanism was responsible for the extraction of lithium ions in the crown ether-ionic liquids system. Here, a novel perspective was revealed that one [Li·H2O]+ complexed with crown ether and then the chelate cation coordinated with the anion of the ionic liquid to form a neutral molecule based on the simulation of the whole extraction process with molecular dynamics that favored the transport of Li+ from aqueous solution to organic solution. In the system without ionic liquids, Li+-crown ether chelates were distributed only at the interface and did not enter the organic phase which is supported by very low extraction efficiency. It was attributed that charge equilibrium cannot be achieved when Li+-crown ether chelates enter into organic solution in the absence of ionic liquids. Overall, a new insight into the interfacial transport mechanisms of lithium ions was proposed, in which the anion of ionic liquids played a key role in capturing lithium ions at the interface of aqueous solution and organic solution.

Oxygen Nanobubbles for Lake Restoration—Where Are We at? A Review of a New-Generation Approach to Managing Lake Eutrophication

Nutrient enrichment of lakes from anthropogenic activities is a significant and increasing issue globally, impairing the health, biodiversity and service provisioning from lakes, with impacts on cultural, recreational, economic and aesthetic values. Internal nutrient loads from lakebed sediment releases are a primary cause of lake eutrophication and have necessitated geoengineering methods to mitigate releases and speed up recovery from eutrophication. Our objective in this review was to evaluate the use of oxygen nanobubbles as a geoengineering technology to remediate low oxygen conditions at the lake sediment/water interface, as a precursor to alleviating eutrophication linked to high internal nutrient loads. Oxygen nanobubbles (NBs) are bubbles < 1000 nm formed at the interface of solid surfaces and aqueous solutions. These bubbles have higher density than water, persist for longer and facilitate greater oxygen solubility than larger bubbles. Methods have been developed to enable NB formation at the surface of carrier materials, which are then used in conjunction with modified local soils (MLSs), to ‘floc, lock and oxygenate’ to strip nutrients from the water column, locking them in lakebed sediments and oxygenating the sediments to prevent re-release of nutrients. Most studies of NBs for lake restoration have thus far only demonstrated their potential for this purpose, using short-term, small-scale core incubations conducted mainly in laboratory settings. Work is required to (1) address scalability, including procurement and cost, (2) extend laboratory incubation studies to large outdoor enclosures and pond/lake trials, (3) examine longevity of the effects in the natural environment, including potential for MLSs to smother benthos and/or have toxic effects, and (4) extend to a range of lake environments and MLS types. Legal, cultural and social acceptance of the technology is another prerequisite of applications in the natural environment and requires individualised analysis. Until these issues are addressed in a systematic way that addresses scalability and recommends suitable carrier materials and MLSs, NBs may continue to remain largely untried as a geoengineering method to address lake eutrophication.

Proton-Coupled Electron Transfer of Coenzyme Q in Unbuffered Solution by Pore Confined In Situ Liquid ToF-SIMS

Proton-coupled electron transfer (PCET) process of coenzyme Q in buffered solutions, which is a well-defined overall 2 e−, 2 H+ process, has been systematically studied, while that in unbuffered aqueous solutions is still too complicated to be fully understood, primarily due to the uncontrolled local proton concentration at the electrode-electrolyte interface. Herein, time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with a microfluidic electrochemical reactor, namely pore confined in situ liquid ToF-SIMS analysis, was adopted to monitor the PCET process of coenzyme Q0 (CoQ0) at the electrode-electrolyte interface in unbuffered aqueous solution. Evolutions of CoQ0 and related intermediates during the electrochemical reaction were measured in real-time, which provided direct molecular evidences for the PCET process. The direct observation of CoQ0H2 and hydrated CoQ0 dianion implied that the reduction of CoQ0 in unbuffered electrolyte was not a simple overall 2 e−, 2 H+ procedure. Moreover, the identification of the CoQ0H2 dimer and the quinone-hydroquinone complex demonstrated the further transformation of CoQ0 and CoQ0H2 by hydrogen bonding interaction or π-interaction. These results provided a full picture of the mechanism for the PCET process of CoQ0 in unbuffered aqueous solution, which could contribute to the comprehensive understanding of the electrochemical reactions of coenzyme Q.

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ∼240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Synthesis, crystal structure and Hirshfeld surface analysis of 2-(perfluoro-phen-yl)acetamide in comparison with some related compounds.

The mol-ecular and crystal structures of the title compound, C8H4F5NO, were examined by single-crystal X-ray diffraction and Hirshfeld surface analysis. The title compound was synthesized by a new method at the interface of aqueous solutions of LiOH and penta-fluoro-phenyl-aceto-nitrile. In the crystal, hydrogen bonds and π-halogen inter-actions connect the mol-ecules into double layers. Analysis of the Hirshfeld surface showed that the most important contributions to the crystal packing are made by F⋯F (30.4%), C⋯F/F⋯C (22.9%), O⋯H/H⋯O (14.9%), H⋯F/F⋯H (14.0%) and H⋯H (10.2%) contacts. The Hirshfeld surfaces of analogues of the title compound were compared and the effect of perfluorination on the crystal packing was shown.

Electrostatically self-assembled filamentous sodium alginate/ε-polylysine fiber with antibacterial, bioadhesion and biocompatible in suturing wound

In the work, a novel filamentou sodium alginate (SA) /ε-polylysine (PL) fiber with excellent mechanical properties and controllable sizes is prepared in an efficient and environmentally friendly manner via continuous pulling of an electrostatically assembled SA/PL composites at the contact interface of aqueous solutions of cationic polyelectrolyte ε-PL and anionic natural polysaccharide SA. The SA/PL fiber exhibits good antibacterial activity, low cytotoxicity, anti-hemolysis, bioadhesion, and environmental friendliness due to its natural raw materials and green preparation process. In vivo experiments have shown that the SA/PL fiber can promote the healing and repair of skin wounds on the backs of mice via resistance to pathogen infection, reduction of inflammation, and anti-allogeneic allergy of the wound. In summary, these results demonstrate that the SA/PL fiber is a green and biosafe multifunctional natural polymer material, with potential applications in suturing wound.