Aqueous Solution Interface Research Articles

Large-area release and transfer of ultrathin, freestanding nanocrystalline ceria films

Methods for fully releasing ultrathin ceria films onto an aqueous solution interface, and transferring the film onto arbitrary substrates are presented. The area of the released and transferred film is virtually unlimited, and the release procedure completely avoids etching steps which can be slow and contaminating. Transfer of the ceria film from one substrate to another substrate takes <10min. Furthermore, the procedure is most-likely applicable to a wide array of sample configurations. The possibility of applying the procedure to other materials and film dimensions is discussed. This method will be extremely useful for research and development of thin oxide films and membranes, and has potential for larger scale production.

How Does the P7C3-Series of Neuroprotective Small Molecules Prevent Membrane Disruption?

Molecular dynamics (MD) simulations are conducted to suggest a mechanism of action for the aminopropyl dibromocarbazole derivative (P7C3) small molecule, which protects neurons from apoptotic cell death. At first, the influence of embedded Aβ42 stacks on the structure of membrane is studied. Then, the effect of P7C3 molecules on the Aβ42 fibril enriched membrane and Aβ42 fibril depleted membrane (when Aβ42 fibrils are originally dissolved in the aqueous phase) are evaluated. Also, the formation of an amyloid ion channel in the Aβ42 enriched membrane is examined by calculating deuterium order parameter, density profile, and surface thickness. For Aβ42 in the fully inserted state, ion channel-like structures are formed. The presence of P7C3 molecules in this case just postpones membrane destruction but could not prevent pore formation. In contrast, when both Aβ42 and P7C3 molecules are embedded in the aqueous solution, the P7C3 molecules are self-assembled at membrane/ionic aqueous solution interface and prevent the precipitation and deposition of Aβ42 fibrils into the membrane.

Rate Theory of Ion Pairing at the Water Liquid–Vapor Interface

There is overwhelming evidence that certain ions are present near the liquid–vapor interface of aqueous salt solutions. Despite their importance in many chemical reactive phenomena, how ion–ion interactions are affected by interfaces and their influence on kinetic processes is not well understood. We carried out molecular simulations to examine the thermodynamics and kinetics of small alkali halide ions in the bulk and near the water liquid–vapor interface. We calculated dissociation rates using classical transition-state theory and corrected them with transmission coefficients determined by the reactive flux method and Grote–Hynes theory. Our results show that in addition to affecting the free energy of ions in solution, the interfacial environments significantly influence the kinetics of ion pairing. Results on the relaxation time obtained using the reactive flux method and Grote–Hynes theory present an unequivocal picture that the interface suppresses ion dissociation.

Electric fields within clay materials: How to affect the adsorption of metal ions

Electric fields exist ubiquitously in chemical and biological systems while how to affect the interfacial adsorption processes remain elusive. Here, molecular dynamics simulations are used to understand at a molecular level the adsorption of metal ions at the interface of aqueous solutions and clay materials that are generally endowed with strong electric fields. In absence of electric fields, even Cs+, one of the strongest adsorbed metal ions, is facile to detach from solvated clay surfaces. Electric fields are critical to construct stable inner-sphere complexes and enhance pronouncedly the adsorption strengths of both inner- and outer-sphere complexes. Heavy metal ions such as Pb2+ that are exclusively outer-sphere adsorbed are driven inner-sphere by electric fields, causing the adsorption strengths to surpass Cs+ and explaining partially the serious pollution to clay systems. Adsorption quantities of inner-sphere alkali ions increase with electric fields and in the case of Pb2+, are closely correlated with the intensities of electric fields. Previous models fail to account for Hofmeister effects occurring at solvated clay surfaces, and forceful supports are given to polarization effects that correctly interpret Hofmeister series (Cs+>Na+) and respond towards the change of electric fields.

X-Ray Reflectivity Analysis of SiO2 Nanochannels Filled with Water and Ions: A New Method for the Determination of the Spatial Distribution of Ions Inside Confined Media

Chemical reactions occurring at the material–aqueous solution interface are controlled by an interfacial layer of a few nm where conceptual models such as the Electrical Double or Triple Layer models can be applied. These models describing the spatial distribution of ions in term of perpendicular distance from the planar surface and ignoring topography or structure parallel to the surface are not validated in confined media. In order to investigate the critical dimensions of these models, our first approach was to use a model system consisting of two parallel plane surfaces of SiO2 spaced of 5nm (nanochannels) filled with salt solutions XCl2 (X = Ca2+, Mg2+, Ba2+). These filled nanochannels were characterized using hard X-Ray reflectivity for the determination of electron density profiles perpendicular to the surface. From these results, the surface densities of adsorbed ions at SiO2 surface were calculated and the solution density inside the nanochannels was determined. This method opens new perspectives to a better understanding of water and ion distribution inside nanoconfined media.

Periodic protein adsorption at the gold/biotin aqueous solution interface: evidence of kinetics with time delay.

An oscillatory molecular adsorption pattern of the protein neutravidin from aqueous solution onto gold, in presence of a pre-deposited self assembled mono-molecular biotin film, is reported. Real time surface Plasmon resonance sensing was utilized for evaluation of the adsorption kinetics. Two different fractions were identified: in the initial phase, protein molecules attach irreversibly onto the Biotin ligands beneath towards the jamming limit, forming a neutravidin-biotin fraction. Afterwards, the growth rate exhibits distinct, albeit damped adsorption-desorption oscillations over an extended time span, assigned to a quasi reversibly bound fraction. These findings agree with, and firstly confirm a previously published model, proposing macro-molecular adsorption with time delay. The non-linear dynamic model is applicable to and also resembles non-damped oscillatory binding features of the hetero-catalytic oxidation of carbon monoxide molecules on platinum in the gas phase. An associated surface residence time can be linked to the dynamics and time scale required for self-organization.

Octadecyl-viologen Photooxidation in Surface Films: Macroscopic Contraction of Langmuir Monolayer by UV Irradiation.

The effects of UV radiation on a viologen derivative, octadecylviologen (OV), in Langmuir monolayers at the air-aqueous solution interface and in Langmuir-Blodgett (LB) films have been investigated. Langmuir monolayers suffer a sharp contraction after UV irradiation, clearly visible by the drop in surface pressure or the loss of surface area observed in the surface pressure-area isotherms. The UV-vis reflection measurements reveal a deep change in the OV monolayer caused by a photochemical reaction, which suggests the pyridones formation as photoreaction products. LB films (Z type), before and after being irradiated with UV light, have been studied by using UV-vis absorption and infrared and X-ray photoelectron spectroscopy. The results confirm that after the photodegradation of the viologen films, the presence of oxygen results in the appearance of pyridones as reaction products. This article demonstrates that, in the absence of catalysts, the photooxidation of viologen surface films occurs only under a particular molecular organization imposed by the air-water interface.

Importance of Protons and Specifically Adsorbing Ions on Changing Capacitance, Space Charge Potential Inside the Solid, and the Interfacial Potential at the TiO2 Aqueous Solution Interface

This paper reports on the effect of protons and specifically adsorbing ions on the capacitance in aqueous systems employing TiO2 porous thin-film electrodes. The influence of surface potential on capacitance was studied through zeta potential (ζ) measurements, open circuit potential (OCP) studies and charge-discharge testing. Both ζ and OCP varies as a function of pH and phosphate adsorption. The slopes of the OCP versus pH curve were close to but lower than the Nernstian value as often reported in the literature. The shift of OCP-OCPpztc (OCP evaluated at the Point of Zero Total Charge) with phosphate adsorption implies that the space charge (charge developed inside the solid oxide material itself) potential, varies with phosphate concentration. We show that the electrochemical capacitance increases with the deposition of TiO2 thin-films on the platinized Ti electrode that directly relates to its ζ with the lowest values in capacitance occurring near the isoelectric pH (pH (I) or pHiep), in the absence of phosphate. We also demonstrate that capacitance can be improved with phosphate adsorption over the pH range of 5 to 9 for aqueous based systems. As expected, capacitance depends on OCP, the voltage range referenced to the OCP, and the rates of adsorbing and desorbing ions. In this context, it would appear that it is easier to electro-adsorb than desorb protons in these porous oxide thin-film electrodes. Capacitance extracted from cyclic voltammetry (CV) conducted over a large voltage range from −0.5 to +0.5V resulted in a high pseudocapacitance. However, when cycled continuously (5-25 scans) pseudocapacitance decreased from 110 to 65F/g but electrical double layer (EDL) capacitance (OCP to OCP+0.1V) increased in the process, depending upon pHiep, between 100-150%. Lastly, the highest value of capacitance (encompassing both Pseudo and EDL capacitance) was 155F/g and obtained over a voltage range of 1V at a scan rate of 5mV/s for a pH of 6.

Effect of adsorption of catanionic surfactant mixtures on wettability of quartz surface

The adsorption behaviors of dodecyl trimethyl ammonium bromide (C12TAB) and the catanionic mixtures of C12TAB with sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDDS) and sodium dodecanoate (DAS) on quartz surface have been investigated. Physicochemical parameters such as the critical micelle concentration (CMC), surface tension, contact angle, adhesional tension and work of adhesion have been estimated. At the interface of quartz and aqueous solution of catanionic mixture, there is a competition between anionic surfactants and quartz surface in attracting cationic surfactants. SDDS and DAS fail to inhibit the adsorption of C12TAB because of the smaller size and less negative charge of the head-groups, even the amount of anionic surfactants well surpasses that of C12TAB. Instead the anionic surfactants are dragged by cationic surfactants to quartz surface and form a mixed monolayer on the quartz surface. The insert of anionic surfactants reduces the repulsion between the cationic molecules and results in closer packing of molecules in adsorption layers. Thus, SDDS-C12TAB and DAS-C12TAB have displayed stronger hydrophobic modification at quartz surface than pure C12TAB. However, the electrostatic attraction between excessive SDS and C12TAB is strong enough to constrain C12TAB and decreases the adsorption on quartz surface. Only a small amount of catanionic complexes with the structure like zwitterionic surfactant adsorb on quartz through the interaction between the positive sites of surfactant complexes and negative sites of quartz. Over CMC, bilayers are formed on quartz surface in the systems of SDDS-C12TAB and DAS-C12TAB, while micelle structures are adsorbed on quartz surface for SDS-C12TAB.

Light-Induced Reaction of Benzene with Carbonates

We found an ultraviolet (UV)-light induced formation of biphenyl and sodium benzoate from benzene and sodium carbonate. The reaction happens in the interface of benzene and aqueous solution at the room temperature. After 5 h of UV-light exposure, 11.4% of initial amount of 4.4 g (5.0 mL) benzene are converted to biphenyl and sodium benzoate, which are distributed in benzene and aqueous solution, respectively. Using density function theory (DFT) and time dependent DFT, we have investigated the mechanism of this light-induced reaction, and found that the sodium carbonate is not only a reactant for the formation of sodium benzoate, but also a catalyst for the formation of biphenyl.

In situ Raman spectroscopic investigation of flux-controlled crystal growth under high pressure: A case study of carbon dioxide hydrate growth in aqueous solution

We developed a high-pressure in-situ non-invasive technique to measure the solute concentration distribution in the solution around a growing crystal, in order to study the effect of increasing flux to the crystal surface on the growth process. Growth rates of carbon dioxide hydrate in pure water in a capillary high-pressure optical cell were determined at 20 and 40MPa and 275.15, 278.15, 280.15, and 283.15K. During the growth of the hydrate, CO2 was supplied through diffusion in the aqueous phase from the liquid CO2–aqueous solution interface, and the dissolved CO2 concentrations along the diffusion path were monitored by Raman spectroscopy. We demonstrated that the hydrate growth rates could change a lot under constant driving force, such as under the same degrees of super-saturation, super-cooling or over-pressurization of the system, and that the growth rates depend on the mass transfer flux to the surface of the hydrate crystal; with the increasing flux to the crystal surface, the rate-dominant process changes from “diffusion controlled” to “interfacial reaction controlled” after the diffusion flux reaches a critical (maximum) value. The interfacial reaction coefficient kr is estimated to be 1.31×10−6m/s at 20MPa and 280.15K, and the molar volume and hydration number of CO2 hydrate vary from 21.7 to 22.0cm3/mol and 7.09 to 7.67, respectively. These observations and measurements could be useful for the design of crystal growth.

Two-Dimensional Growth Rate Control of l-Phenylalanine Crystal by Laser Trapping in Unsaturated Aqueous Solution

The growth rate control of single l-phenylalanine plate-like anhydrous crystal is successfully demonstrated by laser trapping at an air/solution interface of the unsaturated aqueous solution. Focusing a continuous-wave near-infrared laser beam into the interface generates single l-phenylalanine crystal at the focal spot even under unsaturated condition. Subsequently, the plane area of the generated crystal becomes larger linearly with time under continued laser irradiation into the crystal central part. Two-dimensional crystal growth rate defined as a slope of the temporal change in the crystal plane area strongly depends on initial solution concentration as well as irradiation time until single crystal formation is confirmed by eye under a microscope. When the laser power is decreased after the crystallization, the growth rate is slowed down accordingly. Thus, the two-dimensional growth rate is arbitrarily controlled by tuning the laser power. As the critical phenomenon underlying the crystal growth, we ...

Mean-Field Approximation to the Hydrophobic Hydration in the Liquid–Vapor Interface of Water

A mean-field approximation to the solvation of nonpolar solutes in the liquid-vapor interface of aqueous solutions is proposed. It is first remarked with a numerical illustration that the solvation of a methane-like solute in bulk liquid water is accurately described by the mean-field theory of liquids, the main idea of which is that the probability (Pcav) of finding a cavity in the solvent that can accommodate the solute molecule and the attractive interaction energy (uatt) that the solute would feel if it is inserted in such a cavity are both functions of the solvent density alone. It is then assumed that the basic idea is still valid in the liquid-vapor interface, but Pcav and uatt are separately functions of different coarse-grained local densities, not functions of a common local density. Validity of the assumptions is confirmed for the solvation of the methane-like particle in the interface of model water at temperatures between 253 and 613 K. With the mean-field approximation extended to the inhomogeneous system the local solubility profiles across the interface at various temperatures are calculated from Pcav and uatt obtained at a single temperature. The predicted profiles are in excellent agreement with those obtained by the direct calculation of the excess chemical potential over an interfacial region where the solvent local density varies most rapidly.

Adsorption layer structure in the system of the ionic block polyamino acid copolymers/SiO2 particles

Abstract The solution pH impact on the adsorption process of the block copolymers on the colloidal silica surface was investigated. All polymeric chains consist of the poly(ethylene glycol) (PEG) fragment and the suitable ionic polyamino acid segment (poly- l -lysine or poly- l -aspartic acid). PEG is a biocompatible water-soluble polymer, which is widely used in the biopharmaceutical industry because of its excellent water solubility, chain mobility, and non-immunogenicity. When it comes to polyamino acid block, it is also highly biocompatible, non-toxic, and economic with versatile functional groups such as carboxyl or amino groups. The analyzed substances are characterized by different polymer chain structures but have similar average molecular weight. Adsorption amount was measured with a static method using a UV–vis spectrophotometer. Based on the values obtained from the electrokinetic measurements (the surface charge density and the zeta potential), the interactions between the block copolymer macromolecules and the solid surface were explained. As a result, it was possible to determine the mechanism of compounds binding at the silicon dioxide–aqueous solution interface. It is worth emphasizing that both studied macromolecular compounds and the adsorbent are broadly applied in many branches of industry as well as in medicine.

The development of a cholesterol biosensor using a liquid crystal/aqueous interface in a SDS-included β-cyclodextrin aqueous solution

Sodium dodecyl sulphate (SDS) including β-cyclodextrin (β-CD) (β-CDSDS) was used to detect cholesterol at the 4-cyano-4′-pentylbiphenyl (5CB)/aqueous interface in transmission electron microscopy (TEM) grid cells. The β-CD acts as a host for SDS (guest). The guest SDS enclosed within the β-CD cavity was replaced with cholesterol by injecting cholesterol solution into the TEM cell at concentrations greater than 3 μM. The replacement of SDS with cholesterol was confirmed by pH measurement and high performance liquid chromatography (HPLC). The SDS excluded from the β-CD altered the planar orientation of the 5CB confined within the TEM grid cell to a homeotropic orientation. This planar-to-homeotropic transition was observed using a polarized optical microscope under crossed polarizers. This convenient TEM grid cell provides a new method for the selective detection of cholesterol without immobilization of the detecting receptors (enzyme, antibody, or aptamer) or the use of sophisticated instruments.

Adsorbed film of n-tetradecylphosphocholine at the tetradecane/water interface studied by interfacial tensiometry and X-ray reflection

Abstract The adsorbed film of n-tetradecylphosphocholine (C14PC) at the tetradecane (C14)/aqueous solution (W) interface was studied by interfacial tensiometry and X-ray reflection (XR). The interfacial tension γ was measured as a function of molality m C 14 PC and temperature T under atmospheric pressure. The interfacial density of C14PC Γ C 14 PC H increases with increasing m C 14 PC and converges into 3 μmol m−2 (first saturation) at m C 14 PC ≈ 0.08 mmol kg − 1 and into 4.2 μmol m−2 (second saturation) at the critical micelle concentration (CMC). The electron density profile determined by XR indicated that C14PC molecules form monolayer with relatively loose packing at the first saturation. In the second saturation, on the other hand, the molecules form bilayer in which charge separated phosphocholine groups take upside-down arrangement to interact attractively between neighbors. The positive entropy changes associated with adsorption both from monomeric and from micellar states result from that the contribution of dehydration around C14PC molecules, especially around PC groups, by the adsorption from aqueous solution overcomes that of a comparatively ordered molecular orientation at the interface. By comparing the results at the C14/W interface with those at the Air (A)/W one, it was shown that in the monolayer and bilayer states, hydrophobic chains of C14PC molecules are more ordered at the C14/W than at the A/W interface because of attractive interaction between C14PC and C14 molecules. This leads to smaller value of the partial molar entropy and energy changes associated with the adsorption at the C14/W than at the A/W interface. Furthermore the entropy of micelle formation changes from positive to negative with increasing T, suggesting that the entropy gain due to the dehydration exceeds the entropy loss by the aggregation at low temperatures, while the gain is less than the loss at high temperature.

Effect of alkane chain length and counterion on the freezing transition of cationic surfactant adsorbed film at alkane mixture - water interfaces.

Penetration of alkane molecules into the adsorbed film gives rise to a surface freezing transition of cationic surfactant at the alkane-water interface. To examine the effect of the alkane chain length and counterion on the surface freezing, we employed interfacial tensiometry and ellipsometry to study the interface of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride aqueous solutions against dodecane, tetradecane, hexadecane, and their mixtures. Applying theoretical equations to the experimental results obtained, we found that the alkane molecules that have the same chain length as the surfactant adsorb preferentially into the surface freezing film. Furthermore, we demonstrated that the freezing transition temperature of cationic surfactant adsorbed film was independent of the kind of counterion.

Effect of a double‐structured microporous polymer support on the catalytic activity, stability and aggregation behavior of immobilized enzymes

AbstractA double‐structured microporous polymer composite support consisting of a microporous skeleton for protection of immobilized enzymes and a gel‐like polymer bearing different functionalities has been synthesized for immobilization of various enzymes. Among three approaches based on two types of interactions (physical and chemical), we found that under our experimental conditions the covalent bonding between amine moieties of enzymes and epoxide groups of the support was the best, with enzyme leaching being reduced to as low as zero for nine enzymes and catalytic activities increased by factors of 1–4 for two lipases. The improvement in catalytic activity could be attributed to the amphiphilic soft gel which might create a kind of lipid–water interface or microenvironment in aqueous solution, favoring the interfacial activation of certain lipases. Furthermore, the thermal and operational stability, and reusability of all the enzymes could be enhanced to varying degrees. We also found that enzyme aggregates of different sizes could be formed on the spherical support surface at quite low enzyme concentrations, which might also influence the final catalytic activities of the immobilized enzymes. © 2015 Society of Chemical Industry

Structure-charge relationship - the case of hematite (001).

We present a multidisciplinary study on the hematite (001)-aqueous solution interface, in particular the relationship between surface structure (studied via surface diffraction in a humid atmosphere) and the macroscopic charging (studied via surface- and zeta-potential measurements in electrolyte solutions as a function of pH). Upon aging in water changes in the surface structure are observed, that are accompanied by drastic changes in the zeta-potential. Surprisingly the surface potential is not accordingly affected. We interpret our results by increasing hydration of the surface with time and enhanced reactivity of singly-coordinated hydroxyl groups that cause the isoelectric point of the surface to shift to values that are reminiscent of those typically reported for hematite particles. In its initial stages after preparation the hematite surface is very flat and only weakly hydrated. Our model links the entailing weak water structure with the observed low isoelectric point reminiscent of hydrophobic surfaces. The absence of an aging effect on the surface potential vs. pH curves is interpreted as domination of the surface potential by the doubly coordinated hydroxyls, which are present on both surfaces.

Reorientation of the "free OH" group in the top-most layer of air/water interface of sodium fluoride aqueous solution probed with sum-frequency generation vibrational spectroscopy.

Many experimental and theoretical studies have established the specific anion, as well as cation, effects on the hydrogen-bond structures at the air/water interface of electrolyte solutions. However, the ion effects on the top-most layer of the air/water interface, which is signified by the non-hydrogen-bonded so-called "free OH" group, have not been explicitly discussed or studied. In this report, we present the measurement of changes of the orientational angle of the "free OH" group at the air/water interface of the sodium fluoride (NaF) solutions at different concentrations using the interface selective sum-frequency generation vibrational spectroscopy (SFG-VS) in the ssp and ppp polarizations. The polarization dependent SFG-VS results show that the average tilt angle of the "free OH" changes from about 36.6° ± 0.5° to 44.1° ± 0.6° as the NaF concentration increases from 0 to 0.94 M (nearly saturated). Such tilt angle change is around the axis of the other O-H group of the same water molecule at the top-most layer at the air/water interface that is hydrogen-bonded to the water molecules below the top-most layer. These results provide quantitative molecular details of the ion effects of the NaF salt on the structure of the water molecules at the top-most layer of the air/water interface, even though both the Na(+) cation and the F(-) anion are believed to be among the most excluded ions from the air/water interface.