APPLICATION OF AN ELECTRODIC TECHNIQUE TO STUDY TRANSPORT ACROSS MONOLAYERS

Monolayers have been extensively studied as a model for the natural biological membrane. The dropping mercury electrode has been developed by Pagano and Miller to study transport of species across lipid monolayers at the gas/water interface. New measurements show that this technique can be used to study the monolayer at the oil/water interface.     

木质素磺酸钠在TiO2/水界面的吸附特性

木质素磺酸钠在固体表面的吸附特性决定了其应用性能,利用红外和紫外分光光度仪,采用剩余质量分数法研究了温度、pH值、无机盐和氢键破坏剂脲对木质素磺酸钠在TiO₂/水界面吸附动力学和等温吸附性能的影响,初步探讨了其在固液界面的吸附作用机理。结果表明,该吸附为单层多点式吸附,随着温度升高和pH值减小,木质素磺酸钠在TiO₂/水界面的吸附速率常数和饱和吸附量均增大,而离子强度的增大和脲的加入却使吸附速率常数减小;木质素磺酸钠在TiO₂/水界面的吸附驱动力为静电、疏水和氢键作用,疏水作用力可显著增加其吸附量。     

The Interfacial Interactions of Glycine and Short Glycine Peptides in Model Membrane Systems

The interactions of amino acids and peptides at model membrane interfaces have considerable implications for biological functions, with the ability to act as chemical messengers, hormones, neurotransmitters, and even as antibiotics and anticancer agents. In this study, glycine and the short glycine peptides diglycine, triglycine, and tetraglycine are studied with regards to their interactions at the model membrane interface of Aerosol-OT (AOT) reverse micelles via ¹H NMR spectroscopy, dynamic light scattering (DLS), and Langmuir trough measurements. It was found that with the exception of monomeric glycine, the peptides prefer to associate between the interface and bulk water pool of the reverse micelle. Monomeric glycine, however, resides with the N-terminus in the ordered interstitial water (stern layer) and the C-terminus located in the bulk water pool of the reverse micelle.     

Study of the stability of the molten zone and the stresses induced during the growth of Al2O3-Y3Al5O12 eutectic composite by the laser floating zone technique

A series of analytical calculations is used to study both the effect of the thermal gradients and the stability of the molten zone in the laser floating zone growth of Al₂O₃-Y₃Al₅O₁₂ eutectic composite. The thermal gradients in the solidification interface have been calculated and the axial gradient compared with the experimental one of 4.5 × 10⁵ K/m. For these calculations the coefficients of heat transfer from the molten zone to the ambient at the solid-melt interface have been previously obtained. The thermal stresses generated by the high thermal gradients can induce crack formation during the cooling depending on the rod diameter. The theory predicts that it is possible to grow rods free of cracks up to R = 1.7 mm, at low rates (10 mm/h) in close agreement with the experimental critical radius of 1.6 mm.The dependence of the zone length on the input laser power used to carry out the growth is shown. The study of the floating zone profile allows determining the maximum stable zone length, verifying the stability criterion established by some authors.     

纳米Fe3O4稳定的Pickering型ASA乳液

利用纳米Fe₃O₄作为稳定剂和乳化剂来制备Pickering型ASA（alkenyl succinic anhydride） 施胶乳液,并研究了固体颗粒浓度、油水比、水分散相pH对乳液类型、稳定性、形态及施胶性能的影响。结果表明,纳米Fe₃O₄能够乳化制备均一稳定的Pickering型ASA乳液。乳液在室温下静置稳定,析出油相体积分数随固体颗粒用量的增加而增大,随油水比的增大而减小。油水比为2：1,水分散相浓度为0.1%（质量分数）时制备的ASA乳液稳定性最佳。固体颗粒部分吸附在油/水界面处,部分分散在分散相中,随分散相中固体颗粒浓度的增加,乳液稳定性变差。乳液静置分层之前,ASA发生部分水解。在放置1 h后用于纸页浆内施胶,随ASA乳液用量的增加,纸页表面接触角逐渐增大,且纸页表面粗糙度下降。在ASA的添加量为1.0%（质量分数）时,纸页表面接触角达到93.5°,纸页表面粗糙度为15.924 μm。     

Molecular dynamics simulations of ovalbumin adsorption at squalene/water interface

The adsorption of protein molecules to oil/water (O/W) interface is of critical importance for the product design in a wide range of technologies and industries such as biotechnology, food industry and pharmaceutical industry. In this work, with ovalbumin (OVA) as the model protein, the adsorption conformations at the O/W interface and the adsorption stability have been systematically studied via multiple simulation methods, including all-atom molecular dynamic (AAMD) simulations, coarse-grained molecular dynamic (CGMD) simulations and enhanced sampling methods. The computational results of AAMD and CGMD show that the hydrophobic tail of OVA tends to be folded under long time relaxation in aqueous phase, and multiple adsorption conformations can exist at the interface due to heterogeneous interactions raising from oil and water respectively. To further study the adsorption sites of the protein, the adsorption kinetics of OVA at the O/W interface is simulated using metadynamics method combined with CGMD simulations, and the result suggests the existence of multiple adsorption conformations of OVA at interface with the head-on conformation as the most stable one. In all, this work focuses on the adsorption behaviors of OVA at squalene/water interface, and provides a theoretical basis for further functionalization of the proteins in emulsion-based products and engineering.     

Theoretical analysis of the decay of shear strength of adhesion in metal/epoxy/metal joints in an aqueous environment

A theory for the decay of shear adhesion strength has been applied to lead-alloy/ epoxy/lead-alloy joints in water at 65°C, 75°C and 85°C. Theoretical values for the retention of wet shear strength over time coincided with experimental data. From experiments on wet shear adhesion strength and water absorption, it has been clarified that the following two cases exist: (1) in adhesive system A, the diffusion coefficient of water is greater at the interface than in the adhesive; (2) in adhesive system B, the diffusion coefficient of water is lower at the interface than in the adhesive, and water at the interface does not immediately contribute to adhesion failure — ie, there is a time lag between contact with water and bond breakage.     

The molecular mechanism for destabilization of foams by organic ions

The effect of tetraalkylammonium ions on the destabilization of foam has been studied by measuring the half-life of foam (τ_1/2), area/molecule at the air/water interface, surface viscosity and critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS). The area/molecule of SDS in organic salt solutions, calculated from the Gibbs isotherm, increases as the size of organic ion increases, and surface viscosity of the film decreases with the size of organic ions. The interaction of tetraalkylammonium ions with SDS decreases the CMC of the solution, and hence the concentration of SDS monomers decreases. The CMC of SDS decreases with the increase in the size and concentration of organic ions. The decrease in the CMC, increase in the area/molecule of SDS at the air/water interface and the decrease in surface viscosity by tetraalkylammonium ions all work to decrease the foam stability. The results indicate that the change in intermolecular distance between surfactant molecules in the adsorbed film by organic ions can significantly influence the surface viscosity and foam stability. The foam destabilizing efficiency of tetra-alkylammonium ions was superior or equivalent to that of tributyl phosphate and 2-ethyl hexanol, which are used in many antifoaming formulations.     

Surface activity of sodium salts ofalpha-sulfo fatty esters. The oil/water interface

Adsorption studies at the heptane/water and the benzene/water interfaces have been conducted for four α-sulfo fatty esters: sodium hexyl α-sulfopelargonate, sodium heptyl α-sulfopelar-gonate, sodium methyl α-sulfomyristate, and sodium methyl α-sulfopalmitate. The results were compared with those at the air/water (A/W) interface (3). Surfactant solutions with constant counter-ion concentrations of 0.01 and 0.04 M Na⁺ were used. Interfacial tensions were measured by using a drop-volume method and were found to be in the range 1 to 25 dynes/cm. The critical micelle concentration (CMC) values of, these esters were in the range 8.1 to 0.14 mM at the A/W interface (3); they were decreased by about 10% in the presence of heptane (7.2 to 0.11 mM) and by 30–40% in the presence of benzene (5.5 to 0.08 mM). This decrease in CMC was attributed to the water solubility of the hydrocarbon; the greater the solubility, the larger was the decrease in CMC. The co-areas at the A/W interface were in the range 44 to 59 Å^2/ for all the esters (3). At the heptane/water interface they were in the range 49 to 66 Å^2/ and at the benzene/water interface in the range 59 to 75 Å^2/, i.e., an increase of about 10% was obtained in the presence of heptane and about 30–40% in the presence of benzene. The surface films at the O/W interface were more expanded than at the A/W interface, as expected, owing to the lack of lateral cohesion between the hydrocarbon chains at the O/W interface.     

Studies on adsorption of a copolymer at air/water-and at oil/water-interfaces

The interfacial properties of a synthetic copolymer have been investigated, both at air/water (A/W) and at oil/water (O/W) interfaces, utilizing modified WILHELMY plate method. The number average molecular weight and the thermodynamic parameters have been evaluated. The increased flexibility of the macromolecular chain at oil/water interface has been observed.     

Effect of Water on Adsorption Behavior at Polymer/γ-Fe2O3 Interface

The adsorption behavior of a hydroxylated polymer onto γ-Fe₂O₃ particles in a tetrahydrofuran/water solvent system was investigated. The saturation adsorption of the polymer decreased with an increase in the water content of the solvent system, and the degree of dispersion of the magnetic particles in the paint also decreased. It was shown that the water located not at the interface between the particles and the solution but in the solution. Water interacted with the polymer and increased the radius of gyration of the polymer in solution. Thus, the polymer did not interact with the particles, and the particles were prevented from having a high dispersion.     

In situ measurement of water at the organic coating/substrate interface

In situ and quantitative information on the water layer at the organic coating/substrate interface is crucial for understanding and preventing the failure of organic coating systems. A technique, based on a two-layer model derived rigorously from internal reflection theory, has been developed for measuring in situ the thickness and amount of the water layer at the organic coating/substrate interface. The technique gives new insight into the processes by which water degrades the coating/substrate bonds. In this technique, a transparent or an opaque organic coating of sufficient thickness is applied to an internal reflection element (IRE) with or without a thin metallic film, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectra are taken automatically at specified time intervals without disturbing the specimens or the instrument. Water uptake in the coating and FTIR-MIR spectra of water on the coating-free substrate are also used for the analysis. Examples of clear and pigmented coatings on untreated and treated substrate surfaces are given to demonstrate the technique. Results of water accumulation at the coating/iron interface with and without applied electrical potentials are given. In addition to measuring water at the coating/substrate interface, the technique provides a means for studying the transport of water through a coating adhered to a substrate. Information on water at the interface and its transport properties through coatings applied to a substrate is valuable for interpreting corrosion, blistering and delamination of organic coating systems, and for developing models for use in predicting the serivce lives of protective coatings.     

Weathering of fibre reinforced plastics. Progress of dbonding detected in model systems by using fibres as light pipes

The progress of interfacial debonding caused by the diffusion of boiling water to glass fibre/polyester resin interfaces has been followed by shining a He-Ne laser beam along fibres and recording the changes in intensity of the emergent beam. Compared with measurements of electical conductivity arising from the presence of diffused water at the interface, it is concluded that the laser beam method provides more certain evidence of debonding. The glass/resin interface has also been investigated in partially cured composites and in filled composites, during exposure to boiling water.     

Hydrogen bonding at the water/quasi-freestanding graphene interface

Vibrational spectroscopy has been used to study the interaction of water with quasi-freestanding graphene grown on Pt(1 1 1). A sharp O–H (O–D) vibrational band centered at 457 (337) meV is a direct evidence of the existence of non-H- (non-D-) bonded water molecules at the water/graphene interface. This finding is expected to play a significant role in understanding the behavior of water at hydrophobic surfaces. Such finding is in agreement with results reported for water adsorbed on other hydrophobic surfaces and with the behavior of water confined in carbon nanotubes and between graphene sheets.     

Quantifying Water at the Organic Film/Hydroxylated Substrate Interface

A method, based on Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectroscopy, for determining the amount and thickness of water at an organic film/hydroxylated substrate interface has been developed. The analysis uses a two-layered model, which takes into account: 1) water at the organic film/hydroxylated substrate interface, 2) water taken up by the organic film within the penetration depth of the evanescent wave and 3) change of the penetration depth as water displaces the organic film from the substrate. Experimentally, the method requires the application of an organic film, transparent or opaque, of sufficient thickness on a hydroxylated internal reflection element, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, FTIR-MIR spectra are taken automatically at specified time intervals without disturbing the specimen or the instrument. Water uptake in the organic films and FTIR-MIR spectra of water on the substrates are also obtained and used for the analysis. Results of examples of three organic films: a clear epoxy, an unmodified asphalt, and a pigmented ester, on a hydroxylated SiO₂-Si substrate were presented to demonstrate the method. The water layer at the interface for the ester and asphalt specimens was found to be much thicker than that for the epoxy, and this was attributed to the presence of a water-sensitive layer accumulated at the interface for the formers. The method should be equally applicable to studies of organic and inorganic compounds at the organic film/hydroxylated substrate interface and their transport rates through films adhered to a substrate.     

Oxygen permeation studies on alternative proton exchange membranes designed for elevated temperature operation

Kinetic and mass transport properties were investigated for the oxygen reduction reaction in Nafion 117 and a sulfonated poly (arylene ether sulfone) membrane (SPES-40, 40% sulfonated groups/repeat unit) under 1 atm oxygen pressure, 100% relative humidity in a temperature range of 303-343 K using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)/proton exchange membrane (PEM) interface. Oxygen reduction kinetics was found to be similar for both Nafion® 117 and SPES-40 membrane at the Pt microelectrode interface. The temperature dependence of O₂ permeation parameters showed same trends for both the membranes studied, there was an increase in D and a concomitant decrease in C. Despite lower equivalent weight and hence higher water content SPES-40 exhibited relatively close values of D with Nafion® 117. The results are discussed in the context of their different microstructures. Values of C showed a closer relationship to water content and the percent volume of aqueous phase in the respective membranes. The values of overall oxygen permeability were significantly higher in Nafion® 117, with a higher positive slope in its variation with temperature.     

Stability of water/toluene interfaces saturated with adsorbed naphthenic acids—A molecular dynamics study

Stability of water/toluene interfaces saturated with adsorbed naphthenic acids with various sizes at room temperature was studied by molecular dynamics simulation. In direct comparison with our recent results on similar systems involving n-heptane as the oil phase, it has been found that toluene significantly enhances the stability of the water/oil interface, as suggested by the considerably reduced diffusion coefficients of the interface-bound naphthenic acids. The oil phase dependence may be rationalized in terms of the oil-naphthenic acid interactions, both electronically and sterically, and the intermolecular interactions between the molecules in the oil phase. Consistent with our previous studies, mobility of naphthenic acid has been found to be dependent on its size, with naphthenic acid featuring a longer carbon chain in between the carboxylic and cycloaliphatic groups showing lower mobility, leading to a more stable interface. However, such size dependence is much more pronounced for the water/toluene than the water/n-heptane interface. This may be understood in terms of the structural influence of toluene on the compactness of the liquid crystalline mesophase formed by naphthenic acids adsorbed at the interface.     

Relationship Between Interfacial Water Layer Adhesion Loss of Silicon/Glass Fiber-Epoxy Systems: A Quantitative Study

Water at the polymer/substrate interface is often the major cause of adhesion loss in coatings, adhesives, and fiber-reinforced polymer composites. This study critically assesses the relationship between the interfacial water layer and the adhesion loss in epoxy/siliceous substrate systems. Both untreated and silane-treated Si substrates and untreated and silane-treated E-glass fibers were used. Thickness of the interfacial water layer was measured on epoxy/Si systems by Fourier transform infrared-multiple total internal reflection (FTIR-MTIR) spectroscopy. Adhesion loss of epoxy/Si systems and epoxy/E-glass fiber composites was measured by peel adhesion and short-beam shear tests, respectively. Little water accumulation at the epoxy/Si substrate interface was observed for silane-treated Si substrates, but about 10 monolayers of water accumulated at the interface between the epoxy and the untreated Si substrate following 100 h of exposure at 24 °C. More than 70% of the initial epoxy/untreated Si system peel strength was lost within 75 h of exposure, compared with 20% loss after 600 h for the silane-treated Si samples. Shear strength loss in composites made with untreated E-glass fiber was nearly twice that of composites fabricated with silane-treated fiber after 6 months of immersion in 60 °C water. Further, the silane-treated composites remained transparent, but the untreated fiber composites became opaque after water exposure. Evidence from FTIR-MTIR spectroscopy, adhesion loss, and visual observation strongly indicated that a water layer at the polymer/substrate interface is mostly responsible for the adhesion loss of epoxy/untreated siliceous substrate systems and epoxy/untreated glass fiber composites and that FTIR-MTIR is a viable technique to reliably and conveniently assess the adhesion loss attributable to water sorption at the interface.     

Dilational Properties of an Anionic Gemini Surfactant with a Hydrophobic Spacer

The dilational properties of the anionic gemini surfactant, ethanediyl‐1,2‐bis(sodium N‐decanoyl‐β‐alaninate), and the comparable conventional surfactant were investigated via the oscillation barrier technique. The oscillation frequency was varied between 0.0033 and 0.1 Hz. The dilational moduli of the gemini surfactant at low surfactant concentration show less dependence on the frequency at the air/water interface compared with the decane/water interface. The interface layer is basically elastic. The dilational moduli at the air/water interface are remarkably higher than those at the decane/water interface. The dilational moduli of the gemini surfactant show two maxima with increasing concentration, originating from the reorientation and compression of the surfactant molecules adsorbed at the interfaces. A possible schematic diagram of the adsorption of the gemini surfactant at the air/water and decane/water interfaces is proposed.     

Surface activity and premicellar aggregation of some novel diquaternary gemini surfactants

A series of novel cationic gemini surfactants, C _n H _2n+1 N⁺(CH₃)₂CH₂CHOHCHOHCH₂N⁺(CH₃)₂C _n H _2n+1 ·2Br⁻, have been synthesized, and their surface properties were investigated in water, 0.1 N NaCl, and 0.1 N NaBr at 25°C. From surface tension-log molar concentration plots, the pC₂₀, critical micelle concentration (CMC), and γ_CMC values have been determined, and the area/molecule at the aqueous solution/air interface was calculated. When the number of carbon atoms in the alkyl (hydrophobic) chains is above a certain number, which depends upon the molecular environment, the surface activity of the compounds is less than expected. This appears to be due to formation of small, soluble aggregates below the CMC. Equilibrium constants calculated for this aggregation indicate that a series of oligomers are formed.