Aqueous Solution Interface Research Articles

Calculation and analysis of excess adsorption and surface tension of aqueous surfactant solutions under pool boiling conditions

In order to realize the theoretical calculation of surface tension of aqueous surfactant solution under pool boiling conditions, the relationships between surfactant adsorption and bubble behaviors were analyzed. The models of surfactant excess adsorption at liquid-vapor interface and surface tension of aqueous surfactant solution were developed. The parameters in developed models were determined in detail by experiments with ionic surfactant of sodium dodecyl sulfate (SDS) and non ionic surfactant of Triton X-114. Based on the established models, the local surfactant excess adsorption and surface tensions under pool boiling conditions were calculated and analyzed. The results show that bubble behaviors and surfactant adsorption generally take place simultaneously. In low heat flux range, the heat flux has little influence on the excess adsorption and surface tension, while in high heat flux range, the excess adsorption decreases and the surface tension increases significantly with the increasing heat flux. The bubble growth is so fast in high heat flux range that the surfactant adsorption at the liquid-vapor interface has no enough time to reach equilibrium, which leads to the decrease of excess adsorption and the increase of surface tension with the increasing heat flux.

Impact of Tetrabutylammonium on the Oxidation of Bromide by Ozone.

The reaction of ozone with sea-salt derived bromide is relevant for marine boundary layer atmospheric chemistry. The oxidation of bromide by ozone is enhanced at aqueous interfaces. Ocean surface water and sea spray aerosol are enriched in organic compounds, which may also have a significant effect on this reaction at the interface. Here, we assess the surface propensity of cationic tetrabutylammonium at the aqueous liquid–vapor interface by liquid microjet X-ray photoelectron spectroscopy (XPS) and the effect of this surfactant on ozone uptake to aqueous bromide solutions. The results clearly indicate that the positively charged nitrogen group in tetrabutylammonium (TBA), along with its surface activity, leads to an enhanced interfacial concentration of both bromide and the bromide ozonide reaction intermediate. In parallel, off-line kinetic experiments for the same system demonstrate a strongly enhanced ozone loss rate in the presence of TBA, which is attributed to an enhanced surface reaction rate. We used liquid jet XPS to obtain detailed chemical composition information from the aqueous-solution–vapor interface of mixed aqueous solutions containing bromide or bromide and chloride with and without TBA surfactant. Core level spectra of Br 3d, C 1s, Cl 2p, N 1s, and O 1s were used for this comparison. A model was developed to account for the attenuation of photoelectrons by the carbon-rich layer established by the TBA surfactant. We observed that the interfacial density of bromide is increased by an order of magnitude in solutions with TBA. The salting-out of TBA in the presence of 0.55 M sodium chloride is apparent. The increased interfacial bromide density can be rationalized by the association constants for bromide and chloride to form ion-pairs with TBA. Still, the interfacial reactivity is not increasing simply proportionally with the increasing interfacial bromide concentration in response to the presence of TBA. The steady state concentration of the bromide ozonide intermediate increases by a smaller degree, and the lifetime of the intermediate is 1 order of magnitude longer in the presence of TBA. Thus, the influence of cationic surfactants on the reactivity of bromide depends on the details of the complex environment at the interface.

Plasmon‐Enhanced, Self‐Traced Nanomotors on the Surface of Silicon

AbstractLight‐driven nanomotors have attracted much attention due to their potential applications. The movement of conventional nanomotors typically occurs in the solution phase, which limits their application fields. Utilizing visible light to drive nanomotors at the solid–liquid interface represents a grand challenge due to the large friction force between the nanomotor and the solid surface. Based on the attractive plasmon resonance of Au nanocrystals, for the first time, plasmon‐enhanced Au nanocrystal‐based nanomotors moving at the silicon–aqueous solution interface have been developed. Such nanomotors move with a clear trace engraved on the Si surface, representing an excellent and exceptional self‐traced nanomotor system. In addition, the nanomotor trace on the Si surface also provides a unique and promising approach to the fabrication of nanoscale Si patterns, which is central to many applications, including microelectronics, sensing, information storage, and optoelectronics.

Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions. II: Role of the Size and Charge Sign of the Counterions.

Understanding the role of the counterion species in surfactant solutions is a complicated task, made harder by the fact that, experimentally, it is not possible to vary independently bulk and surface quantities. Here, we perform molecular dynamics simulations at constant surface coverage of the liquid/vapor interface of lithium, sodium, potassium, rubidium, and cesium dodecyl sulfate aqueous solutions. We investigate the effect of counterion type and charge sign on the surface tension of the solution, analyzing the contribution of different species and moieties to the lateral pressure profile. The observed trends are qualitatively compatible with the Hofmeister series, with the notable exception of sodium. We point out a possible shortcoming of what is at the moment, in our experience, the most realistic nonpolarizable force field (CHARMM36) that includes the parametrization for the whole series of alkali counterions. In the artificial system where the counterion and surfactant charges are inverted in sign, the counterions become considerably harder. This charge inversion changes considerably the surface tension contributions of the counterions, surfactant headgroups, and water molecules, stressing the key role of the hardness of the counterions in this respect. However, the hydration free energy gain of the counterions, occurring upon charge inversion, is compensated by the concomitant free energy loss of the headgroups and water molecules, leading to a negligible change in the surface tension of the entire system.

SYNERGISTIC EFFECT OF A COMBINATION OF COLLECTORS IN ADSORPTION AT GAS-LIQUID INTERFACE

The promising nature of flotation with a combination of collecting agents is well known. The synergetic effect of combinations of collectors develops as hydrophobization of a mineral surface during adsorption and as reduction in surface tension at gas-liquid interface. The earlier proved mechanism of physisorption of an agent is based on the connection of its surface characteristics (surface pressure, spreading rate) and their collectability. It is assumed that the ratio of agents in their combination for flotation can be selected from the maximum synergistic effect achieved. This study focuses on adsorption at the gas-liquid interface in aqueous solutions of binary mixtures of sodium lauroyl sarcosinate and hexadecyltrimethylammonium bromide. The interaction parameters and the ratio of the components in the solution so that the maximum synergistic effect is achieved are determined.

Stability of a Monoethanolamine-CO2 Zwitterion at the Vapor/Liquid Water Interface: Implications for Low Partial Pressure Carbon Capture Technologies.

The need to chemically convert CO2 at the interface of aqueous amine solutions has become particularly relevant for the development and the broad distribution of cost-effective and near-future devices for direct air capture working at low (e.g., ambient) partial pressure. Here, we have determined the stability of a CO2-monoethanolamine zwitterion and its chemical conversion into carbamate at the vapor/liquid water interface by first-principles molecular dynamics simulations coupled with a recently introduced enhanced sampling technique. Contrary to the bulk water case, our results show that both the zwitterion and carbamate ions are poorly stable at the vapor/amine aqueous interface, further stating the differences between the homogeneous and heterogeneous CO2 chemical conversion. The design of novel and cost-effective capture systems, such as those offered by amine-based scrubbing solutions, working at low (e.g., ambient) CO2 partial pressure should explore the use of novel solvents, different from aqueous mixtures, to overcome the limits of the current absorbents.

<i>In-situ</i> Observation of Bubble Through Each Interface of a KCl Aqueous Solution / Hg and a Molten Salt / Sn by Controlling Applied Potential

In-situ observation of Ar bubble through each interface of a KCl aqueous solution / Hg and a molten salt / liquid Sn were conducted by controlling applied potential in order to simulate the behavior of bubble at interface of slag / metal. In a KCl aqueous solution, the rate of Ar bubble through each interface of a KCl aqueous solution / Hg became slow with applying −2.0 V at 298 K. This behavior caused by the stabilization of interface with deceasing the interfacial tension, and a lot of Hg droplet formed by rupture of bubble was drawn into Hg phase under the interface in seconds. In a LiCl-KCl eutectic melt, the interfacial tension of the molten salt / liquid Sn were measured by the in-site observation of contact angle with applying −2.0~0 V(vs. Ag+/Ag) at 793 K. The behavior of interfacial tension corresponded to typical electrocapillary curve. The interfacial tension at −0.75 V became highest, 310 (dyn/cm = mN/m), and the interfacial tension at −2.0, 0 V became lowest, 210 (dyn/cm = mN/m), and the Ar bubble through the interface became slowest with applying at −2.0 V. This behavior was same as that through the interface of KCl aqueous solution / Hg.

Adsorption of Hydrophilic Amine-Based Protic Ionic Liquids on Iron-Based Substrates.

We synthesized hydrophilic amine-based protic ionic liquids (PILs) with hydroxy groups in their cations and anions, and characterized their adsorption at a solid (iron-based substrate) / aqueous solution interface. The IL samples employed in this study were triethanolamine lactate, diethanolamine lactate, and monoethanolamine lactate. Quartz crystal microbalance with dissipation monitoring (QCM-D) measurements revealed that the adsorption mass of the hydrophilic PILs was larger than that of the comparative materials, including a non-IL sample (1,2,6-hexanetriol) and an OH-free sample in the cations (triethylamine lactate). Additionally, an increase in the number of hydroxy groups in the cations resulted in an increased adsorption mass. Force curve measurements by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements proved the high adsorption density of the hydrophilic PILs on the iron-based substrate. A decreased kinetic friction coefficient was also observed in the hydrophilic PIL systems. Moreover, hydrophilic PILs are expected to have potential applications as water-soluble lubricants and additives for metal surface treatments.

Tuning CO2 Capture at the Gas/Amine Solution Interface by Changing the Solvent Polarity.

Carbon dioxide scrubbing by aqueous amine solution is considered as a promising technology for post-combustion CO2 capture, while mitigating climate change. The lack of physicochemical details for this process, especially at the interface between the gas and the condensed phase, limits our capability in designing novel and more cost-effective scrubbing systems. Here, we present classical and first-principles molecular dynamics results on CO2 capture at the gas/amine solution interfaces using solvents of different polarities. Even if it is apolar, carbon dioxide is absorbed at the gas/monoethanolamine (MEA) aqueous solution interface, forming stable and interfacial [CO2·MEA] complexes, which are the first reaction intermediate toward the chemical conversion of CO2 to carbamate ions. We report that the stability of the interfacial [CO2·MEA] precomplex depends on the nature and polarity of the solution, as well as on the conformer population of MEA. By changing the polarity of the solvent, using chloroform, we observed a shift in the interfacial MEA population toward conformers that form more stable [CO2·MEA] complexes and, at the same time, a further stabilization of the complex induced by the solvent environment. Thus, while lowering the polarity of the solvent could decrease the solubility of MEA, at the same time, it favors conformers that are more prone to CO2 capture and mineralization. The results presented here offer a theoretical framework that helps in designing novel and more cost-effective solvents for CO2 scrubbing systems, while shedding further light on the intrinsic reaction mechanisms of interfacial environments in general.

Surface Affinity of Alkali and Halide Ions in Their Aqueous Solution: Insight from Intrinsic Density Analysis.

The surface tension of all aqueous alkali halide solutions is higher than that of pure water. According to the Gibbs adsorption equation, this indicates a net depletion of these ions in the interfacial region. However, simulations and experiments show that large, soft ions, such as I-, can accumulate at the liquid/vapor interface. The presence of a loose hydration shell is usually considered to be the reason for this behavior. In this work, we perform computer simulations to characterize the liquid-vapor interface of aqueous alkali chloride and sodium halide solutions systematically, considering all ions from Li+ to Cs+ and from F- to I-. Using computational methods for the removal of surface fluctuations, we analyze the structure of the interface at a dramatically enhanced resolution, showing that the positive excess originates in the very first molecular layer and that the next 3-4 layers account for the net negative excess. With the help of a fictitious system with charge-inverted ion pairs, we also show that it is not possible to rationalize the surface affinity of ions in solutions in terms of the properties of anions and cations separately. Moreover, the surface excess is generally dominated by the smaller of the two ions.

Effect of zwitterionic betaine surfactant on interfacial behavior of bovine serum albumin (BSA)

The interfacial adsorption and denaturation of proteins is a significant concern for the loss of protein structure and function during protein processing or storage. Surfactants can be used to prevent interfacial damage to proteins. Zwitterionic surfactants have shown intriguing effects on stability and activity of proteins. The aim of this research was to study the effect of cocamidopropyl betaine (CAPB), a non-denaturing zwitterionic carboxy betaine containing surfactant, on the air/water interfacial adsorption of a model protein bovine serum albumin (BSA). Dynamic and equilibrium surface tensions were measured using the pendant drop method. The results of surface tension measurements were coupled with a thermodynamic analysis, based on the interface and bulk chemical potentials, to estimate the surface coverage of the components and to calculate surface adsorption. Our results indicated that CAPB can effectively associate with the protein at different concentrations, even at concentrations lower than the critical micelle concentration (CMC). Furthermore, the addition of the protein significantly increased the CMC of the CAPB form 45 ppm to 407 ppm. Most importantly, CAPB prevented the adsorption of the protein at the interface. The interface of aqueous solution of CAPB+BSA was also compared with the Triton X-100 + BSA. Results of this study will help us to understand surfactant+protein interactions at interfaces.

Direct observation of water-mediated single-proton transport between hBN surface defects.

Aqueous proton transport at interfaces is ubiquitous and crucial for a number of fields, ranging from cellular transport and signalling, to catalysis and membrane science. However, due to their light mass, small size and high chemical reactivity, uncovering the surface transport of single protons at room temperature and in an aqueous environment has so far remained out-of-reach of conventional atomic-scale surface science techniques, such as scanning tunnelling microscopy. Here, we use single-molecule localization microscopy to resolve optically the transport of individual excess protons at the interface of hexagonal boron nitride crystals and aqueous solutions at room temperature. Single excess proton trajectories are revealed by the successive protonation and activation of optically active defects at the surface of the crystal. Our observations demonstrate, at the single-molecule scale, that the solid/water interface provides a preferential pathway for lateral proton transport, with broad implications for molecular charge transport at liquid interfaces.

Structure of the Electrical Double Layer Revisited: Electrode Capacitance in Aqueous Solutions.

The structure of the electrical double layer at the interface of planar electrodes and aqueous solutions is investigated. Electrical impedance spectroscopy is used to measure the impedance of aqueous solutions of sodium chloride and two different surfactants over a wide range of concentrations. The electrode capacitance is directly inferred from the admittance spectra as well as by regression of the impedance spectra to an equivalent circuit. It is found that the electrode capacitance remains on the same order of magnitude over the entire range of investigated concentrations. This is contradictive to the predictions of the Gouy-Chapman-Stern theory which predicts that, at low concentrations, the electrode capacitance should be determined by the diffuse layer. It is concluded that the Stern layer capacitance always dominates the electrode capacitance, even at very low concentrations, and the establishment of a diffuse layer capacitance requires an ionic strength of around 1 mM.

A Coarse-Grained Model for Microbial Lipopeptide Surfactin and Its Application in Self-Assembly.

Biosurfactants exhibit outstanding interfacial properties and unique biological activities that fairly related to their self-assembly in solutions and at interfaces. Computational simulations provide structural details of biosurfactant aggregates at the molecular level relevant to thermodynamic properties, but the understanding of kinetics of self-assembly remains limited due to lower simulation efficiency. In this work, a coarse-grained model has been developed for microbial lipopeptide surfactin, and surfactin monolayer at the octane/water interface and micelle in aqueous solution were studied using molecular dynamics simulations. Interaction parameters were optimized and validated by comparing with results obtained from experiments and atomistic molecular dynamics simulations. In particular, self-assembly of surfactin in aqueous solution was studied using the optimized parameters. Results showed that coarse-grained simulations well reproduced structural properties of surfactin monolayer and micelle and the molecular behavior such as surfactin orientation and conformation. Self-assembly features of surfactin in different stages have been captured, and the aggregation numbers of dominant clusters were in accordance with experimental data. This report suggested that the present coarse-grained model and interaction parameters allowed surfactin simulations over longer timescales and larger systems, which provide insights into characterizing both the kinetics of surfactin self-assembly and the adsorption of surfactin onto varying interfaces.

Self-assembly of porous polyaniline microspheres via template-free interfacial method for high-performance electromagnetic absorption property

Self-assembly porous polyaniline (PANI) microspheres were synthesized via template-free interfacial method at the interface of gallic acid aqueous solution and carbon tetrachloride. The morphology of PANI can be controlled by changing the content of gallic acid. The obtained porous PANI microspheres showed good electromagnetic (EM) wave absorption performance. The maximum reflection loss value of which is −26.5 dB and the effective absorption bandwidth (reflection loss <−10 dB) is about 4.1 GHz. This study is mainly focused on analyzing EM loss based on complex permittivity as the magnetic loss of PANI is neglectable. Further EM analysis indicated that porous PANI microspheres have better impedance matching and attenuation constant, which attributes to the good EM wave absorption. This work can act as a reference for designing PANI microspheres as EM wave absorber by the template-free process.

Specific counter-cation effect on the molecular orientation of thiocyanate anions at the aqueous solution interface.

Understanding the interfacial structure of aqueous electrolyte solutions is important and relevant to a wide range of systems, ranging from atmospheric aerosols to electrochemistry, and biological environments. Though significant efforts have been made to unravel the interfacial structure of water molecules, the structure and dynamics of ions at the interface have not yet been fully elucidated. Here, the interfacial structure of the aqueous solution was investigated directly by monitoring the thiocyanate (SCN-) anions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The molecular orientation of the SCN- anions and their adsorption behavior at the air/water interface were systematically determined by quantitative polarization analysis. The transition dipole of the CN stretching of the SCN- anion is oriented around 44° from the surface normal of the NaSCN aqueous solution surface and remained unchanged with the bulk concentration varying from 1 mol kg-1 to 13 mol kg-1. The free energy of adsorption of SCN- anions at the air/water interface was determined to be -1.53 ± 0.04 kcal mol-1. Furthermore, a new SFG peak positioned at 2080 cm-1 in the ppp polarization combination was observed at the air/15.0 mol kg-1 NaSCN aqueous solution interface for the first time. Concentration-dependent SFG analysis and density functional theory (DFT) calculation further revealed that the SCN- anions form an ion clustering structure at the air/water interface. The subtle and specific Na+ and K+ counter-cation effects on the interfacial structure of the SCN- anions at the aqueous solution interface were also observed, which showed that ion cooperativity plays an important role in affecting the interfacial structure of ions at the air/water interface. The results are expected to yield significant insights into the understanding of the structure of aqueous solution surfaces and the molecular level mechanism of the cationic Hofmeister effect.

Transition effect of under- and over-potentially deposited hydrogen and negative resistance at a poly-Rh/alkaline aqueous solution interface

Negative resistance (NR) phenomena due to the tunneling and inter-valley scattering effects in semiconductors have been applied in microwave power sources. An NR phenomenon due to the transition between under- and over-potentially deposited hydrogen (UPD H and OPD H) at noble metal/acidic and alkaline aqueous solution interfaces has not been studied based on the Frumkin adsorption isotherms, which are not easily determined using conventional methods. We determine the Frumkin adsorption isotherms of UPD H and OPD H for the hydrogen evolution reaction (HER) at a poly-Rh/0.1 M LiOH aqueous solution interface using the phase-shift method and electrochemical impedance spectroscopy. The NR phenomenon due to the transition effect of UPD H and OPD H is analyzed using the Frumkin adsorption isotherms and square wave voltammetry. The NR characteristic due to the transition effect of UPD H and OPD H at the poly-Rh interface is similar to that due to the two valley model theory or the inter-valley scattering effect in III−V compound semiconductors. The NR phenomenon due to the transition between UPD H and OPD H is a unique feature of the poly-Rh interface. We hope that the NR phenomenon for the HER at the poly-Rh interface will be studied for its potential and application as a new power source.

Size and Shape-Dependent Solubility of CuO Nanostructures.

In our theoretical study, the enhanced solubility of CuO nanoparticles in water saturated by air is predicted based on a simple thermodynamic model. CuO is considered in the form of nanoparticles with various shapes. The interfacial energy of a solid CuO/dilute aqueous solution interface was assessed by applying the average CuO surface energy and contact angle of a sessile drop of water. The equilibrium CuO solubility was calculated using Gibbs energy minimization technique. For the smallest spherical nanoparticles considered in this work (r = 2 nm), the solubility is significantly higher than the solubility of bulk material. In the case of cylindrical nanoparticles, the solubility increase is even more considerable. The CuO spherical nanoparticles solubility was also calculated using the Ostwald–Freundlich equation which is known to overestimate the solubility as discussed in this contribution.

Probing the Interfacial Interactions of Monoclonal and Bispecific Antibodies at the Silicone Oil-Aqueous Solution Interface by Using Sum Frequency Generation Vibrational Spectroscopy.

Silicone oil has been widely utilized in the pharmaceutical industry especially as a lubricant coating commonly used in syringes for the smooth delivery of drugs. Protein structure perturbation and aggregation have been reported upon protein contacting silicone oil by using indirect methods and ex-situ techniques. The conclusions derived from such indirect and ex-situ methods may not truly reflect the exact nature of the protein-silicone oil interfacial interactions. Recently, we have successfully demonstrated that sum frequency generation (SFG) vibrational spectroscopy can be used as a powerful and direct method of studying the fusion protein-silicone oil interfacial interactions in situ and in real time. In this article, we studied monoclonal and bispecific antibody interactions with the silicone oil surface by using SFG spectroscopy. Being structurally and functionally different in the nature of fusion proteins and antibodies, this study is important in enhancing our current understanding of protein-silicone oil interfacial interactions. Both types of antibodies investigated here readily and strongly adsorb onto the silicone oil surface and remain stable at least for 10 h. SFG spectra in the amide I region for monoclonal and bispecific antibodies centered at 1660 and 1665 cm-1, respectively, suggest the difference in their molecular structures. The absence of the antibody signals in the amide I region of time-dependent and static SFG spectra obtained for preadsorbed antibodies onto silicone oil after contacting polysorbate 80 (PS-80) surfactant suggests that PS-80 can effectively remove both types of antibodies from the silicone oil surface. This study demonstrated the feasibility of using SFG spectroscopy as a powerful tool for probing the antibody-interfacial interactions in situ and in real time.

Selective flotation-spectrophotometric determination of chromium using Rhodamine B in Egyptian ilmenite phosphate solution

ABSTRACT The method is sensitive and it is the novelty of the work for flotation-spectrophotometric determination of Cr(III). This method is based on the formation of ion associate complex between Cr(III), KI and Rhodamine B (RhB). The formed complex is floated in the interface of aqueous solution and 1,2-dichloroethane via vigorous shaking. By discarding the aqueous solution and 1,2-dichloroethane, the floated ion associate [(CrI4)−] [(RhB)+] complex on the wall of a separating funnel was then dissolved in a small volume of methanol solvent. By studying different affecting parameters, it was found that the maximum absorbance of the formed ion associate complex was 0.675 at λmax 560 nm. The molar absorptivity of the colour is 9.51 × 105 L mol−1 cm−1 at λmax 560 nm and the colour was stable for 1 h. The calibration graph was linear in the concentration range of 0.1–100 mg L−1 of Cr(III). The low limit of detection (LOD) was 0.1 mg L−1. The relative standard deviations (RSD) at 0.1 and 100 mg L−1 of Cr(III) were 4.7% and 7.2% (n = 10), respectively. The method was successfully applied for the determination of Cr(III) in different Egyptian ilmenite phosphate solution.