Interfacial Behavior Research Articles

Interfacial behaviour of human bile and its substitution for in vitro lipolysis studies

This study examined the interfacial evolution of individual bile salts (BSs) and their blends with phosphatidylcholine (BS/PC) to simulate the complex behaviour of human bile (HB) during lipolysis at the triglyceride/water interface. Using adsorption and desorption cycles, mimicking exposure to small intestinal fluids, we demonstrate that the interfacial behaviour of HB can be replicated using simple mixtures of BSs and PC. Interfacial tension (IFT) measurements after lipolysis and desorption showed no significant differences (P > 0.05) between HB samples and BS/PC mixtures across the total BS concentrations analysed (2.23–7.81 mM). However, individual BSs without PC yielded significantly different IFT results (P < 0.01) compared to HB, highlighting the importance of phospholipids. Dilatation rheology further emphasised the need for accurate phospholipid representation in bile models. Our results suggest that phospholipids in HB and BS/PC systems enhance resistance to desorption, potentially affecting lipolysis. This is important, as current in vitro digestion models often replicate only intestinal BS concentrations to mimic the behaviour HB in the intestinal lumen. Furthermore, the specific composition of BSs in HB appears less critical than the overall BS and phospholipid content, suggesting that the kinetics of triglyceride digestion is influenced by the combined luminal concentrations of these components. These findings have significant implications for understanding the role of bile in digestion and offer insights for designing more accurate in vitro models to study food emulsions and lipid-based delivery systems.

Unlocking the Mechanism of Unique Interfacial Behavior and Highly Efficient Oil Displacement of Amphiphilic Carbon Nanosheets

Unlocking the Mechanism of Unique Interfacial Behavior and Highly Efficient Oil Displacement of Amphiphilic Carbon Nanosheets

Axial behavior variation of 72-story concrete-filled steel tubes with different joint connections due to interface debonding considering long-term deformation of concrete core

Long-term concrete deformation including shrinkage and creep occurring seriously threats the longevity, safety and serviceability of concrete-filled steel tube (CFST) structures. In this study, numerical simulation on the variation of axial behavior of 72-story CFST structures with different joint connection details due to interface debonding considering shrinkage and creep of concrete core is carried out. Three connection details including connections with inner horizonal diaphragms only, two-direction through-beams only, and inner horizontal diaphragms combined with two-direction through-beam, are considered, respectively. Vertical loading from floors is treated as centralized vertical force applied on steel tube directly. The shrinkage of concrete core is simulated by decreasing concrete temperature. A cohesive constitutive law and a Coulomb friction model are employed to describe the interface behavior in tangential direction before and after debonding, respectively. The interaction between concrete core and steel tube in normal direction is treated as a hard contact. The concrete plastic-damage (CPD) constitutive model is used for concrete core and a plastic constitutive law is employed for the steel tube. The initiation of the interface debonding between steel tube, H-shaped steel beam flanges, inner diaphragms and concrete, the variation of the force transfer path, stress redistribution, steel tube yielding and the final failure are described in detail. Results show that the axial load-carrying capacity of each CFST structure has a decrease of 26-28% when compared with that determined by the current design code where the confinement effect is considered, and a decrease of 9-10% when compared with that determined by the current design code where the confinement effect of steel tube is not considered. Moreover, the axial stiffness decreases to 76-78% of their theoretical values when debonding is not considered. Numerical results show that the interface debonding negatively affects the long-term axial load-carrying capacity and stiffness obviously, which has not been considered in current design codes. Some comments on the design of CFST members are proposed based on the numerical simulation results.

A tribo-dynamics model of ball screws based on flexible body element

Dynamic actuation accuracy of ball screws is dramatically affected by screw flexible deformation and the behaviors of ball-groove interface such as friction, stiffness and damping. However, few dynamics studies have considered the screw flexible deformation and rubbing interface behaviors in ball screws. In this study, a dynamics model is developed by including both the screw flexible deformation and lubrication interface behavior. Discretization of the screw/nut is completed using the 6 degree-of-freedom Timoshenko beam element. The tribological properties of ball-groove interface are solved by the mixed lubrication model. Comparison with experimentally tested nut acceleration frequencies verifies the correctness of theoretical model. On this basis, the effects of nut motion, nut position and rough surface on the dynamic response are analyzed. With the increase of time, the screw axial displacement decreases with a certain slope and is accompanied by periodic vibrations. The centers of axis trajectories at different nut positions are far apart. From Hertz, smooth surface to rough surface, the interface stiffness coefficient decreases and the nut vibration amplitude grows. The novel tribo-dynamics model can provide a theoretical basis for fault diagnosis and performance optimization of ball screws.

Effect of cellulose nanocrystals on the emulsion stability and rheological properties of microalgal Pickering emulsions

This study investigates the effect of cellulose nanocrystals (CNCs) to Pickering emulsions prepared with microalgal particles (Spirulina sp. (SPI), Chlorella sp. HS2 (CLO)). The microalgae particles show a weak interfacial localization and Pickering behavior on the O/W emulsion depending on the size (avg. drop size ∼5.39 μm with SPI and 22.15 μm with CLO), resulting in a different stabilization effect. When CNC is additionally mixed with the Pickering emulsions including large microalgae particles (CLO), CNC replaces microalgae particles and localizes at the interface, enhancing strong emulsion stabilization. For the Pickering emulsions including small microalgae (SPI), CNC localizes at the continuous phase, forming a network structure regardless of the concentration. This interfacial localization behavior of CNC against microalgae particles is reflected in the rheological behavior of the Pickering emulsion. Depending on the location of CNC, the emulsions exhibit the two-step yielding behavior, mainly attributed to the CNC network in the continuous phase. The complex role of particles in the emulsion system is more sensitively reflected in the large amplitude oscillatory shear (LAOS) region, characterized using the sequence of physical process (SPP) rheological analysis. The maximum elasticity (Emax) in SPP analysis, which indicates the recovery of the deformed structure, exhibits a significant difference, discriminating structural characteristics of CNC dispersion incorporated with microalgae particles. Emulsion with CLO-CNC has lower Emax than the SPI-CNC case because CNC particles disperse at the interface and the continuous phase. Then the distance between CNC particles is longer, resulting in a weak network structure throughout the emulsion. Due to a weak network of CNC, the emulsion is more vulnerable to coalescence compared to the SPI-CNC system. Therefore, this study suggests that CNC particles added to the Pickering emulsion with microalgae compete to localize at the interface and give coalescence suppression effects to the emulsion. Also, for the Pickering emulsion system composed of multi-particles, rheological analysis including SPP analysis successfully indicates structural characteristics and flow-induced stabilization of Pickering emulsions with multi-particles that microscopic characterization could not detect.

Investigation on post-fire bond behavior between section steel and concrete in SRCFST columns

This paper presents an experimental and numerical investigation on post-fire bond behavior between section steel and concrete in steel-reinforced concrete-filled steel tubular (SRCFST) columns. The pushout tests on 22 post-fire SRCFST specimens were conducted, and test parameters included the fire exposure time, cross-sectional dimension of section steel, section steel-concrete interface length, and the loading ways. The effect of different parameters on the load-slip curves were analyzed. The results shown that the fire exposure time has an obvious influence on the bond strength of the interface. As the fire exposure time increases from 30min to 60min and 90min, the average ultimate bond strength of post-fire specimens decreases by 48.3% and 69.5%, respectively. Based on the experimental results, the simplified full-range bond strength-slip model of the section steel-concrete interface in post-fire SRCFST columns was proposed. The finite element analysis model on the bond behavior of the section steel-concrete interface of post-fire SRCFST columns was finally developed and calibrated, and was used to analyze the stress distribution and variation of the section steel-concrete interface.

Emulsions co-stabilized by Polygonatum sibiricum saponin and soy protein isolate: Physical stability, interaction mechanism, interface behavior

This study investigates the interaction mechanism between Polygonatum sibiricum saponin (PSS) and soy protein isolate (SPI) to enhance the stability of oil-in-water emulsions, laying the foundation for an efficient PSS delivery system. Initially, we evaluated the stability of oil-in-water emulsions with various PSS concentrations (0.1%, 0.2%, 0.3%, 0.5%, and 1% w/w). Results indicated that PSS enhances emulsion stability by increasing absolute zeta potential, reducing average particle size, and improving resistance to external factors. Notably, concentrations of 0.3%, 0.5%, and 1% (w/w) PSS show significant improvements in emulsion stability (p < 0.05). Subsequently, we elucidated the mechanism by which PSS enhances stability. Our findings reveal that PSS interacts with SPI, inducing structural modifications in SPI and expanding its hydrophobic regions. Additionally, PSS forms SPI-PSS complexes through hydrogen bonding with aromatic amino acid residues in SPI, thereby reducing interfacial tension and maintaining stable viscoelasticity. Comparative analysis of Lissajous plots identifies the optimal PSS to SPI ratio as 0.3% (w/w) each, resulting in a resilient interface film formed by numerous SPI-PSS complexes capable of withstanding compression or extension. Overall, our study underscores the efficacy of PSS in navigating the emulsion system and collaborating with SPI to stabilize emulsions.

Evaluation on the diffusion-fusion properties and interfacial mechanical behavior of virgin and aged asphalt

To investigate the regeneration mechanisms of rejuvenated asphalt mixtures, molecular simulation techniques were employed to research virgin and aged asphalt's fusion characteristics and interface mechanical properties. The blended zone was identified based on relative density distribution, and a molecular-scale calculation method for the Degree of binder Blended (DOB) between virgin and aged asphalt was proposed. Finally, the influence of DOB on the mechanical properties of the asphalt-aggregate interface was evaluated through interface pull-out tests. The results revealed that the proposed molecular-scale DOB calculation method effectively assessed the blending effectiveness between virgin and aged asphalt. Higher degrees of asphalt aging made it more challenging for aged asphalt to fuse with virgin asphalt, while elevated temperatures facilitated diffusion fusion between them. Under tensile loading, initial cracks were primarily initiated from the rejuvenated asphalt's blended zone. This region represented the weakest area within the interface system and was prone to fracture under load. The increase in DOB significantly improved the interfacial properties of recycled asphalt, with the increase in DOB the interfacial tensile strength increased by 4%–14% and the fracture energy increased by 10%–19%. However, this enhancement effect diminished with an increased degree of asphalt aging. At the molecular scale, increasing DOB improved the interface strength and deformation resistance of rejuvenated asphalt. The molecular simulation results provide theoretical guidance for evaluating the fusion degree of aged asphalt in RAP materials and determining the utilization efficiency of aged asphalt.

Effect of Hofmeister Anions Series on the Langmuir Film of Tetronic 90R4 and Tetronic 701 Block Copolymers.

The air-water interfacial behavior of Tetronic 90R4 and Tetronic 701 was studied in the presence of sodium salts with different anions namely , , , , Cl-, Br-, , I-, and SCN-. Their presence in the subphase altered the arrangement of both tetronic molecules at the air-water interface. The limiting mean molecular area of the Langmuir film for both tetronics was found to be ion specific; it increased following the series < < < < Cl-< Br-< < I-< SCN-, which was found to be aligned with the Hofmeister series of anions. Furthermore, the study explored the effects of the hydration enthalpy, free energy, viscosity BJD coefficient, and polarizability of these anions on the interfacial behavior of tetronics. The Langmuir-Blodgett film morphology was also examined in the presence of these salt species using SEM. Morphologies were explained considering kosmotropic and chaotropic nature of these anions.

Effect of intermetallic compounds on the mechanical properties of Cu/Al clad sheets with an SS304 interlayer

Annealing treatments were conducted on Cu/Al clad sheets incorporating a 304 stainless steel (SS304) interlayer to investigate the impact of intermetallic compounds (IMCs) on the mechanical properties of these clad sheets with an interfacial layer. Additionally, the mechanisms of interfacial strengthening and crack propagation behavior were examined. It was found that the best overall performance was achieved for samples annealed at 200 °C, showing tensile strength, fracture elongation, and peeling strength values of 219 MPa, 7.19 %, and 19.6 N/mm, respectively. For the rolled clad sheet, cracks typically initiated at the edges of the SS304 fragments, accompanied by minor residual holes and stress concentrations. Post-annealing, the Cu and Al layers underwent varying degrees of recovery and recrystallization, and element diffusion increased, leading to improvements in elongation and interfacial bonding strength of the clad sheets. As the thickness of the IMCs grew, the interfacial crack propagation path shifted to the Al2Cu/AlCu interface within the IMC layer, significantly reducing the bonding strength.

Unlocking the Potential of Food Waste: A Review of Multifunctional Pectins.

This review comprehensively explores the multifunctional applications of pectins derived from food waste and by-products, emphasizing their role as versatile biomaterials in the medical-related sectors. Pectins, known for their polyelectrolytic nature and ability to form hydrogels, influence the chemical composition, sensory properties, and overall acceptability of food and pharmaceutical products. The study presents an in-depth analysis of molecular parameters and structural features of pectins, such as the degree of esterification (DE), monosaccharide composition, galacturonic acid (GalA) content, and relative amounts of homogalacturonan (HG) and rhamnogalacturonan I (RG-I), which are critical for their technofunctional properties and biological activity. Emphasis is placed on pectins obtained from various waste sources, including fruits, vegetables, herbs, and nuts. The review also highlights the importance of structure-function relationships, especially with respect to the interfacial properties and rheological behavior of pectin solutions and gels. Biological applications, including antioxidant, immunomodulatory, anticancer, and antimicrobial activities, are also discussed, positioning pectins as promising biomaterials for various functional and therapeutic applications. Recalled pectins can also support the growth of probiotic bacteria, thus increasing the health benefits of the final product. This detailed review highlights the potential of using pectins from food waste to develop advanced and sustainable biopolymer-based products.

The Application of Machine Learning Algorithms to Bond Strength between Steel Rebars and Concrete Using Bayesian Optimization.

The purpose of this study is to estimate the bond strength between steel rebars and concrete using machine learning (ML) algorithms with Bayesian optimization (BO). It is important to conduct beam tests to determine the bond strength since it is affected by stress fields. A machine learning approach for bond strength based on 401 beam tests with six impact factors is presented in this paper. The model is composed of three standard algorithms, including random forest (RF), support vector regression (SVR), and extreme gradient boosting (XGBoost), combined with the BO technique. Compared to empirical models, BO-XGB`oost was found to be the most accurate method, with values of R2, MAE, and RMSE of 0.87, 0.897 MPa, and 1.516 MPa for the test set. The development of a simplified model that contains three input variables (diameter of the rebar, yield strength of reinforcement, concrete compressive strength) has been proposed to make it more convenient to apply. According to this prediction, the Shapley additive explanation (SHAP) can help explain why the ML-based model predicts the particular outcome it does. By utilizing machine learning algorithms to predict complex interfacial mechanical behavior, it is possible to improve the accuracy of the model.

Long-term effects including shrinkage and interfacial creep on bond behavior of concrete-filled steel tubes (CFST): Experiment and design

Concrete-filled steel tubes (CFST) are widely used in significant projects due to their high performance stemming from full utilization of combined material advantages. The interfacial bond behavior is the foundation of their high performance, but will deteriorate during the long-term service. Due to the difficulties on carrying out long-term experiments and decoupling the multi-constitutive component bond strength including chemical adhesion, micro-interlocking and friction, there is still a lack of research on long-term interface constitutive models, restricting the safe design of CFST structures throughout their full-lifecycle. Here, push-out tests were carried out on CFST specimens considering long-term effects spanning 850 days including concrete age and duration of long-term interfacial load. The steel plate-concrete specimens, reversed push-out tests and interfacial normal pressure calculation were used to decouple the components of bond stress. the CFST interface database considering long-term effects was established and the mechanism of long-term effects have been revealed. As the duration increases, the bond stress decreases at the beginning, and thereafter stabilizes where the friction component dominates. Compared to that without interfacial load, long-term interfacial load accelerates the damage to chemical adhesion and micro-interlocking at initial stage, but the difference of their effects on bond stress is not significant in the later stage. On this basis, a long-term interfacial constitutive model for its full-lifecycle safe design and a simplified numerical method for long-term effects on bond behavior have been proposed.

Enhanced O/W emulsifying properties of pea proteins via deamidation: Insights into interfacial behavior

This study examines the effects of protein glutaminase modification on the interfacial properties and emulsion stability of pea protein isolates (PPI). Emulsions were prepared using native (NPPI) and deamidated PPI (DPPI) at concentrations from 0.5 wt% to 3.6 wt%. The stability of these emulsions was evaluated by examining droplet size distribution, flocculation index, ζ-potential, and CLSM. DPPI demonstrated superior emulsifying ability and stability, requiring only 2.0 wt% to prevent flocculation compared to NPPI's 3.6 wt%. Interfacial properties, such as protein coverage, composition, thickness, tension, and rheology, were characterized. Large Amplitude Oscillatory Dilatation analysis showed minimal differences between NPPI and DPPI-stabilized interfaces at 1 wt%. However, at 3.6 wt%, NPPI interfaces demonstrated abrupt intra-cycle yielding and viscous behavior, whereas DPPI interfaces exhibited gradual softening and a higher maximum linear strain. Additionally, DPPI showed higher interfacial protein coverage and lower interfacial tension. NPPI formed dense, brittle films prone to rupture under dynamic deformation, leading to poor stability. Deamidation of PPI unfolded the protein structure, exposing hydrophobic groups and increasing carboxyl groups, which reduced aggregation. This resulted in a uniform, extensible, and elastic interfacial film resistant to large deformations. Thus, DPPI-stabilized emulsions demonstrated superior stability, showcasing their potential for industrial applications.

Influence of hydrophobic groups in polyglycerol ester emulsifier at oil-polyol interface: Interfacial adsorption behavior and its relationship with emulsification effect

This study investigated the variance in adsorption behavior of polyglycerol emulsifiers at the oil- polyol interface and its consequential impact on emulsion properties and stability. The critical micelle concentration (CMCIFT), saturation adsorption capacity (Γ∞), and Langmuir constant (κ) of the emulsifier were determined through measurements of dynamic interfacial tension between glycerol and mineral oil with varying hydrophobic chains. Results indicated that the CMCIFT exhibits an increase corresponding to the rising number of hydrophobic chains. Emulsifiers with differing initial particle sizes were synthesized, and the actual emulsifying area (SA), theoretical covering area (ST), and emulsifying index (EI) at the interface were calculated. Variations in coverage rate led to disparate adsorption behavior and arrangement of the emulsifier at the interface, thereby impacting emulsion stability. This study establishes a theoretical foundation for predicting the stability trend of polyol/oil (P/O) emulsions.

Overlooked challenges of interfacial chemistry upon developing high energy density silicon anodes for lithium-ion batteries

Evaluation of silicon (Si) anode performance by the assembled Si||Li half-cells is the primary approach in the development of high-energy-density lithium-ion batteries (LIBs). However, most studies focus solely on the variations of Si anode, the stability of electrolyte on the lithium (Li)-metal counter electrode has been overlooked. Herein, we discovered that the acquired cell performance not only depends on the Li+ (de-)solvation behaviors on the Si anode surface but also was affected significantly by the lithiation overpotential caused by the side reactions on the Li electrode. It is significant to identify this point, as these influences of electrolyte decomposition on the Li electrode have been previously regarded as an integral part of side reactions on the Si anode. We proposed a new perspective of the electrolyte solvation structure and electrode interfacial model to unravel the interfacial behaviors on the Si and Li electrodes respectively. The identified differences in the Li+ solvation and (de-)solvation behaviors not only provide reasons for the varied electrolyte stability in different electrolytes but also interpret the superior performance in tetrahydrofuran (THF)-based electrolytes. This study underscores the importance of understanding electrolyte behavior at the interfaces of individual electrodes to discern the reliability of electrode performance and also introduce a novel principle for designing superior electrolytes for high-energy-density LIBs.

An explicit dynamics framework suited to highly non-smooth interface behaviors

Dynamic systems, and in particular mechanical structures, may be subjected to non-smooth loadings such as impacts or shocks. Moreover, their behavior itself may exhibit more or less non-smooth evolutions, as when fracture occurs. Therefore, robust simulation models are of interest to capture such behaviors. A particular focus is made herein on time-stepping explicit dynamics schemes to allow efficient simulations, and non-smoothness is embedded within the discrete resolution model, so that robust simulations can be obtained, with a minimum number of numerical parameters. The original contributions of this article lie in the way the non-smooth behavior is formulated to be embedded in an explicit dynamics framework. This study focuses on the solver for dynamics with non-smooth interface behavior, rather than on the behavior models themselves. The applications concern non-smooth interface behaviors at macroscopic scale, between displacement jump on the 2D interface surface with no thickness, and interfacial force distributions acting on the bodies apart the interface. The proposed test cases which can serve as benchmarks for simulation codes, concern in a first step contact and perfectly plastic interface behavior (for illustrative purpose, on a 0D example). The last numerical test deals with contact, friction, fracture and adhesion for an extrinsic perfectly brittle interface behavior, to exemplify the feasibility on a full 3D finite element model.

Transglutaminase–mucin binding dynamics in gastrointestinal mucus: Interfacial behaviour, thermodynamics and gelation mechanism

Transglutaminase, an enzyme present in epithelial cells and as a residual component in processed foods, is hypothesised to interact with gastrointestinal mucus, impacting its structure and function. To test the physical validity of this phenomenon, this study investigates the binding kinetics and thermodynamics between porcine gastric mucin (PGM) and microbial transglumatinse (TGM) in a model gastrointestinal mucus, followed by the rheological analysis of the resulting systems. At pH 7, TGM exhibited pronounced binding with the PGM interface, characterised by a high surface density (55.34 ± 1.86 µg/m−2) and a low dissociation constant (KD ∼ 4.03 ± 0.10 μM) as determined by surface plasma resonance (SPR). Conversely, at pH 3, TGM showed weak adhesion onto PGM, resulting in a less stable binding, reflected by a lower surface density (10.94 ± 0.67 µg/m−2) and a higher dissociation constant (KD ∼ 6.24 ± 0.22 μM). Regardless of the nature of interaction, the Hill coefficients (nH ≥ 1) indicated that the binding sites were markedly denser at pH 7 (1383.38 ± 46.47 pmol/m−2), than at pH 3 (273.68 ± 16.84 pmol/m−2). Fluorimetry analysis suggested temperature-dependent dynamic binding between PGM and TGM. The resulting Benesi – Hildebrand plots illustrated a linear correlation between PGM and increasing TGM concentration, suggesting a single-step interaction mechanism. The calculated thermodynamic parameters indicated spontaneous interaction between PGM and TGM (ΔG < 0) via endothermic (entropic) interactions (ΔH > 0). Notably, hydrophobic forces played a significant role in the network stabilisation of PGM−TGM complex (ΔS > 0). Rheometry analysis elucidates that the interaction maxima within TGM−PGM systems substantially elevate both shear viscosity (η), and the melting point (Tm), shifting from 6.75 ± 0.28 to 70.94 ± 2.21 (×10−3) Pa s (at 1 s−1), and 36.80 ± 0.80 to 48.20 ± 0.11 °C, respectively. Furthermore, the coexistence of these two macromolecular species results in a 3- to 4-fold dramatic increase in the viscoelastic moduli of the binary complex. These findings build a strong physicochemical basis for the interaction between TGM and PGM, with profound effects on the rhological behaviour of the latter; it also highlights the need to examine the biochemical/enzymatic aspects of said interactions, and their potential effects on mucosa and human physiology.

Experimental study of foam stability and interfacial behaviour of cellulose nanocrystals-enhanced C22-tailed zwitterionic betaine surfactant

The intrinsic thermodynamic instability of foam systems significantly constrains their effectiveness in enhancing oil and gas recovery from unconventional reservoirs. This study investigated the foam properties and interfacial rheological characteristics of the EDAB/CNC/APG system, analyzing the influence of various factors including temperature, pH, and inorganic salts. The results indicated that the optimal concentration ratio for CNC-surfactant system consists of 0.3 % EDAB, 1.5 % CNC and 0.5 % APG by mass fraction. In this system, the addition of CNC forms a three-dimensional network structure at the gas–liquid continuous interface, significantly extending the drainage half-life of the system. At elevated temperatures, the intense molecular thermal motion disrupts the interactions between CNC and EDAB. This disruption leads to a decrease in surface tension and the viscoelastic modulus of interfacial expansion, resulting in an increase of foam volume but a decline in foam drainage half-life. Foam performance and interfacial rheological characteristics are optimal under neutral pH conditions and manifest higher stability in alkaline conditions compared to acidic conditions. In highly mineralized reservoirs (≥5.0 % salt concentration), inorganic salt ions shield the attraction between CNC and oppositely charged surfactants. This shielding weakens the cross-linking within CNC-surfactant systems, altering the structure of the liquid film interface and causing a sharp decrease in foam half-life. The C22-tailed zwitterionic betaine surfactant EDAB, enhanced by CNC, provides a novel, environmentally friendly, efficient, and safe approach to the exploitation of complex, low-permeability oil and gas reservoirs.

Hydrophilicity Dependent Distribution of Water at Ionic Liquids/Metal Interface Monitored by Electrochemical SERS.

The understanding of the interfacial processes is critically important for extending the practical application of ionic liquids, particularly for the role of interfacial water. In the electrochemical system based on ionic liquid electrolytes, small amounts of water at the interface generate a significant change in the electrochemical behaviors of ionic liquids. Therefore, the investigation on the interfacial behavior of water is highly desired in ionic liquids with different anions, water content, and hydrophilicity. Herein, based on the probe strategy, in situ surface enhanced Raman spectroscopy (SERS) combined with electrochemical control (EC-SERS) was developed to investigate the influence of hydrophilicity/hydrophobicity of ionic liquids on the interfacial water. The water-sensitive transformation reaction of 4,4'-dimercaptoazobenzene (DMAB) to para-aminothiophenol (PATP) was employed as a probe reaction for investigating the behavior of interfacial water. The changes of relative SERS intensities of DMAB to PATP served as an indication of the quantity variation of interfacial water. The results show that the transformation reaction efficiencies were critically dependent on the additional water contents, potential, and hydrophilicity of ionic liquids. With a very low molar fraction of additional water (Xw = 0.01), transformation efficiency of DMAB (the amount of interfacial water) followed the sequence of [BMIm]BF4 < [BMIm]PF6 < [BMIm]Tf2N. It was in agreement with the hydrophobicity order of the ionic liquids. With the increase in additional water content, the potential for the full transformation was positively moved, and the efficiency increased significantly. The stronger hydrophobicity allowed more water molecules to migrate to the interface, which was attributed to the difference in interactions between water and the anions of ionic liquids. It demonstrated that the small amount of water tended to gather at the interface in hydrophobic ionic liquids. Compared to traditional cyclic voltammetry, the EC-SERS technique combined with probe reactions is more sensitive to interfacial water. It is anticipated to develop as a promising tool for the investigating water-related issues at interfaces and to provide guidance to screen ionic liquids for practical application.