薄层法研究十二烷基苯磺酸钠对硝基苯/水界面电子转移的影响

用薄层法研究了阴离子表面活性剂十二烷基苯磺酸钠(SDBS)对硝基苯/水界面电子转移的影响. 实验结果表明, 随着水相中十二烷基苯磺酸钠浓度的增加, 有机相中十甲基二茂铁(DMFc)和水相中Fe(CN)₆^3-发生的界面双分子反应的阴极平台电流呈现递减趋势, 但是界面双分子反应速率常数却呈递增趋势. 这是由于阴离子表面活性剂十二烷基苯磺酸钠在硝基苯/水界面形成了修饰层, 影响了界面双电层结构. SDBS在液/液界面的吸附为Langmuir吸附.     

Molecular-dynamics simulation of the effect of ions on a liquid-liquid interface for a partially miscible mixture

Molecular-dynamics simulations were performed to model the effect of added salt ions on the liquid-liquid interface in a partially miscible system. Simulations of the interface between saturated phases of a model 1-hexanol+water system show a bilayer structure of 1-hexanol molecules at the interface with -OH heads of the first layer directed into the water phase and the opposite orientation for the second layer. The alignment of the polar -OH groups at the interface stabilizes a charge separation of sodium and chloride ions when salt is introduced into the aqueous phase, producing an electrical double layer. Chloride ions aggregate nearer the interface and sodium ions move toward the bulk water phase, consistent with the explanation that the -OH alignment presents a region of partial positive charges to which the hydrated chloride atoms are attracted. Ions near the interface were found to be less solvated than those in the bulk phase. An electric field was also applied to drive ions through the interface. Ions crossing the interface tended to shed water molecules as they entered the hexanol bilayer, leaving a trail of water molecules. Stabilization and facilitated transport of the ion by interactions with the second layer of hexanol molecules appeared to be an important step in the mechanism of sodium ion transport.     

Factors Affecting the Phase-Inversion Temperature of Fatty Alcohol Ethoxylates

Factors that affect the phase-inversion temperature (PIT) based on nonionic surfactant fatty alcohol ethoxylates were investigated. Phase-inversion process was continuously monitored by determining changes in conductivity with temperature. The influences of oil-to-water ratio, emulsifier concentration, emulsifier mixing ratio, sodium chloride, and oil types on the PIT of emulsions were investigated. Results showed that the PIT of the emulsions declined with increased oil-to-water ratio. Emulsifier concentration significantly affected the PIT temperature. High sodium chloride content suppressed phase inversion. A lower Brij72-to-Brij721 ratio corresponded to higher PIT. Different oils required different HLB numbers in the phase-inversion process.     

Fatty-acid monolayers at the nematic/water interface: phases and liquid-crystal alignment

The two-dimensional (2D) phases of fatty-acid monolayers (hexadecanoic, octadecanoic, eicosanoic, and docosanoic acids) have been studied at the interface of a nematic liquid crystal (LC) and water. When observed between crossed polarizers, the LC responds to monolayer structure owing to mesoscopic alignment of the LC by the adsorbed molecules. Similar to Langmuir monolayers at the air/water interface, the adsorbed monolayer at the nematic/water interface displays distinct thermodynamic phases. Observed are a 2D gas, isotropic liquid, and two condensed mesophases, each with a characteristic anchoring of the LC zenithal tilt and azimuth. By varying the monolayer temperature and surface concentration we observe reversible first-order phase transitions from vapor to liquid and from liquid to condensed. A temperature-dependent transition between two condensed phases appears to be a reversible swiveling transition in the tilt azimuth of the monolayer. Similar to monolayers at the air/water interface, the temperature of the gas/liquid/condensed triple-point temperature increased by about 10 degrees C for a two methylene group increase in chain length. However, the absolute value of the triple-point temperatures are depressed by about 40 degrees C compared to those of analogous monolayers at the air/water interface. We also observe a direct influence by the LC layer on the mesoscopic and macroscopic structure of the monolayer by analyzing the shapes and internal textures of gas domains in coexistence with a 2D liquid. An effective anisotropic line tension arises from elastic forces owing to deformation of the nematic director across phase boundaries. This results in the deformation of the domain from circular to elongated, with a distinct singularity. The LC elastic energy also gives rise to transition zones displaying mesoscopic realignment of the director tilt or azimuth between adjacent regions with a sudden change in anchoring.     

薄层循环伏安法研究硝基苯/水界面上离子诱导的电子转移反应

应用薄层循环伏安法研究了硝基苯/水两相界面间,且有共同离子四丁基铵TBA+存在于两相中,在有机相中的四氰化二甲基苯醌(TCNQ)与水相中的K4Fe(CN)6之间发生的反向电子转移反应。在直径为0.64cm的裂解石墨电极上用2μL硝基苯溶液使之自然扩散在电极表面形成薄层的有机相,并以此作为工作电极。对电极为铂丝(0.5mm),参比电极为Ag/AgCl电极,均置于总体积为2mL的水相中。由于共同离子TBA+的诱导,在硝基苯/水界面间,在已氧化的TCNQ+阳离子(在有机相中)与[Fe(CN)6]4-阴离子(在水相中)之间发生了反向电子转移反应。试验证明:在一定条件下,通过改变两相中共同离子的浓度,可使一些不能发生的两相界面的电子转移反应得以发生;这类电子转移反应系受界面电位差所控制。此外,还测得了在恒定的共同离子浓度比值的条件下,此两相界面电子转移反应的表观速率常数(k)为0.135cm.s-1.mol-1。     

Optimization of the atmospheric pressure chemical ionization liquid chromatography mass spectrometry interface

Use of optimized instrument parameters that result from statistical experimentation revealed that the sensitivity of atmospheric pressure chemical ionization (APCI) liquid chromatography-mass spectrometry (LC/MS) is greater than the sensitivity of an optimized Thermabeam? LC/MS interface by about 3 orders of magnitude, when tested on aromatic compounds. APCI is one of the few LC/MS techniques in which the chromatogram is directly comparable with liquid chromatographs that use ultraviolet detection. The optimum instrument parameters for a Finnigan SSQ-7000 APCI LC/MS interface were found at low flow rates (e. g., 0. 1 mL/min), relatively low capillary heat (e. g., 225 °C), and high sheath-gas pressure (e. g., 60 lb/in²). The optimization was achieved by monitoring the responses of sensitivity, fragmentation, and cluster ion formation. The fine tuning for high sensitivity calls for a high percentage of water in the mobile phase. In contrast, a high percentage of organic content in the mobile phase is required to obtain abundant protonated molecular ions with respect to fragmentation and clustering. This is an important consideration for analyses of unknowns.     

Reversible Photoresponsive Molecular Alignment of Liquid Crystals at Fluid Interfaces with Persistent Stability

This work demonstrates a noninvasive approach to control alignment of liquid crystals persistently and reversibly at fluid interfaces by using a photoresponsive azobenzene‐based surfactant dissolved in an ionic liquid (IL), ethylammonium nitrate (EAN). As the first report on the orientational behavior of LCs at the IL/LC interface, our study also expands current understanding of alignment control of LCs at the aqueous/LC interface by adding electrolytes into aqueous solutions. The threshold concentration for switching the optical responses of LCs can be changed just by simply manipulating the ratio of EAN to H₂O. This work will inspire fundamental studies and novel applications of using the LC‐based imaging technique to investigate various chemical and biological events in ILs.     

Brewster angle microscopy of calcium oxalate monohydrate precipitation at phospholipid monolayer phase boundaries

The precipitation of calcium oxalate monohydrate (COM) at phospholipid monolayers confined to the air/water interface is observed in situ with the aid of Brewster angle microscopy. COM crystals appear as bright objects that are easily identified and quantified to assess the effects of different conditions on crystallization. Crystal precipitation was monitored at monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in liquid condensed (LC) and liquid expanded (LE) phases. Within the LC phase, higher pressures reduce the incidence of crystallization at the interface, implying that within this phase precipitation is enhanced by higher compressibility or fluidity of the monolayer. Precipitation at biphasic LC/LE and LE/gas (G) monolayers was also studied. COM appears preferentially at phase boundaries of the DPPC LC/LE and LE/G monolayers. However, when an LC/LE phase boundary is created by two different phospholipids that are phase segregated, such as DPPC and 1,2-dimyristoyl-sn-glycero-3-phosphocholine, crystal formation occurs away from the interface within the DPPC LC phase. It is suggested that COM growth at phase boundaries is preferred only when there is molecular exchange between the phases.     

Electrically driven motion of an air bubble on hemispherical oil/water interface by three-phase boundary reactions

Our electrochemical cell consisted of a ferrocene-included hemispherical nitrobenzene (NB) droplet on the glassy carbon (GC) electrode which was immersed in the aqueous solution including sodium sulfate and sodium dodecyl sulfate (SDS). When an air bubble was injected near the boundary between the oil and the aqueous phase, it stayed at the top of the hemisphere on the boundary so that the lower half of the bubble was put in oil and the other half was in water. From the force balance of surface tension and buoyancy of the bubble, the bubble took an energetic minimum at the interface. It sank into the oil phase when ferrocene in the oil was electrochemically oxidized through the GC electrode by the three-phase boundary reaction. The electrochemical reduction caused the bubble to move back toward the aqueous phase. The motion of the bubble was synchronized with the redox reaction of ferrocene. The potential step oxidation showed such a rapid response that the motion could not be attributed to diffusion of ferricenium ion from the three-phase boundary to the bubble. Our idea of explaining the rapidity was the translational motion of the SDS layer along the boundary, which was driven by the difference in the surface concentration of SDS caused by the electrochemical generation of the ferricenium ion. The motion of the SDS layer was demonstrated by the shrinkage of the oil layer spread on the water surface when SDS solution was dropped on the oil layer. The spreading velocity was close to the velocity of propagating the oxidation of ferrocene to the bubble.     

23Na, 29Si, and 13C MAS NMR investigation of glass-forming reactions between Na2CO3 and SiO2

The glass-forming reactions between sodium carbonate (Na2CO3) and silica (SiO2) have been investigated by 23Na, 29Si, and 13C magic-angle spinning (MAS) NMR spectroscopy. The multinuclear MAS NMR approach identifies and quantifies reaction products and intermediates, both glassy and crystalline. A series of powdered batches of initial composition Na2CO3.xSiO2 (x = 1, 2) corresponding to a sodium metasilicate (Na2SiO3) and sodium disilicate (Na2Si2O5) stoichiometry were investigated after periods of isothermal and nonisothermal heat treatments at different temperatures. Analysis of the 23Na quadrupolar coupling parameters has identified the early reaction product in all cases as crystalline Na2SiO3. In the nonisothermal experiment, this reaction is preceded by an early silica-rich melt phase formed around 850 degrees C. The early reactions are controlled by solid-state Na+ diffusion across the reaction zone in the grain interface layer. Crystalline Na2SiO3 precipitates in the interface layer, increasing its thickness between the Na2CO3 and the SiO2 grains and slowing down the rate of Na+ migration. This creates a secondary phase, which is temperature dependent. At low temperatures, where Na+ migration is impaired, the production of Na2SiO3 ceases and silica-richer phases are precipitated. In the case of the sodium disilicate batch, where excess SiO2 is present, a secondary reaction of Na2SiO3 with SiO2 forming a glassy phase is observed. A transient carbon-bearing phase has been identified by 13C NMR as a NaCO3- complex loosely bound to bridging oxygens in the silicate network at the SiO2 grain surface.     

Influence of simple electrolytes on the orientational ordering of thermotropic liquid crystals at aqueous interfaces

We report orientational anchoring transitions at aqueous interfaces of a water-immiscible, thermotropic liquid crystal (LC; nematic phase of 4'-pentyl-4-cyanobiphenyl (5CB)) that are induced by changes in pH and the addition of simple electrolytes (NaCl) to the aqueous phase. Whereas measurements of the zeta potential on the aqueous side of the interface of LC-in-water emulsions prepared with 5CB confirm pH-dependent formation of an electrical double layer extending into the aqueous phase, quantification of the orientational ordering of the LC leads to the proposition that an electrical double layer is also formed on the LC-side of the interface with an internal electric field that drives the LC anchoring transition. Further support for this conclusion is obtained from measurements of the dependence of LC ordering on pH and ionic strength, as well as a simple model based on the Poisson-Boltzmann equation from which we calculate the contribution of an electrical double layer to the orientational anchoring energy of the LC. Overall, the results presented herein provide new fundamental insights into ionic phenomena at LC-aqueous interfaces, and expand the range of solutes known to cause orientational anchoring transitions at LC-aqueous interfaces beyond previously examined amphiphilic adsorbates.     

Effect of SDBS on interfacial electron transfer at the liquid/liquid interface by thin layer method

The effect of an adsorbed anionic surfactant sodium dodecyl benzene sulfonate(SDBS) on electron transfer(ET) reaction between ferricyanide aqueous solution and decamethylferrocene(DMFc) located on the adjacent organic phase was investigated for the first time by thin layer method.The adsorption of SDBS at the interface resulted in a decay in the cathodic plateau current of bimolecular reaction with increasing concentrations of SDBS in aqueous phase.However,the rate constant of electron transfer(k_(et)) i...     

Determination of Monensin by Voltammetry at the Interface between Two Immiscible Electrolyte Solutions

Abstract

The coccidiostat monensin can be determined by voltammetry at the interface between two immiscible electrolyte solutions (ITIES) in the concentration range between 0.05 mM to 3 mM. The peaks obtained with cyclic voltammetry at the water/nitrobenzene interface correspond to the transfer of sodium ions from the aqueous to the nonaqueous phase facilitated by complexation with monensin functioning as an ionophore and are proportional to its concentrations. The method was applied to the extracts from Streptomyces cultures.     

Phase behavior of amphiphiles at liquid crystals/water interface: A coarse-grained molecular dynamics study

We present a coarse-grained molecular dynamics simulation study of phase behavior of amphiphilic monolayers at the liquid crystal （LC）/water interface. The results revealed that LCs at interface can influence the lateral ordering of amphiphiles. Particularly, the amphiphile tails along with perpendicularly penetrated LCs between tails undergo a two-dimension phase transition from liquid-expanded into a liquid-condensed phase as their area density at interface reaches 0.93. While, the liquid-condensed phase of the monolayer never appears at oil/water interface with isotropic shape oil particles. These findings reveal the penetration of anisotropic LC can promote ordered lateral organization of amphiphiles. Moreover, we find the phase transition point is shifted to lower surface coverage of amphiphiles when the LCs have larger affinity to the amphiphile tails.     

Local high-density distributions of phospholipids induced by the nucleation and growth of smectic liquid crystals at the interface

Amphiphilic molecules adsorbed at the interface could control the orientation of liquid crystals (LCs) while LCs in turn could influence the distributions of amphiphilic molecules. The studies on the interactions between liquid crystals and amphiphilic molecules at the interface are important for the development of molecular sensors. In this paper, we demonstrate that the development of smectic LC ordering from isotropic at the LC/water interface could induce local high-density distributions of amphiphilic phospholipids. Mixtures of liquid crystals and phospholipids in chloroform are first emulsified in water. By fluorescently labeling the phospholipids adsorbed at the interface, their distributions are visualized under fluorescent confocal microscope. Interestingly, local high-density distributions of phospholipids showing a high fluorescent intensity are observed on the surface of LC droplets. Investigations on the correlation between phospholipid density, surface tension and smectic LC ordering suggest that when domains of smectic LC layers nucleate and grow from isotropic at the LC/water interface as chloroform slowly evaporates at room temperature, phospholipids transition from liquid-expanded to liquid-condensed phases in response to the smectic ordering, which induces a higher surface tension at the interface. The results will provide an important insight into the interactions between liquid crystals and amphiphilic molecules at the interface.     

Stability of emulsions of water in mineral oils containing asphaltenes

Emulsions of water in mineral oils are stable if the oil phase contains asphaltenes which are near the condition of incipient flocculation. This condition is determined by the composition of the oil phase and by the nature of the asphaltenes. High aromaticity of the oil phase and the presence of deflocculants prevent flocculation of asphaltenes; the deflocculants may be interfacially active agents or asphaltene-like compounds with better solubility in the oil phase. Conditions of incipient flocculation of asphaltenes correlate very well with a considerable increase of rheological resistance of the interface between the oil phase and distilled water, determined according to the torsion oscillation method. Stabilization of the water-in-oil emulsions is therefore caused by the build-up of a coherent layer of asphaltenes in the water-oil interface in these cases. Deflocculants of asphaltenes in the oil phase destroy their stabilizing effect; however, the deflocculants themselves may stabilize the water-in-oil emulsions by adsorption on the water-oil interface and then the correlation between the condition of asphaltenes and emulsion stability does not hold, nor is the interfacial viscosity perceptibly increased. Under borderline conditions of emulsion stability a few percent of sodium chloride in the water phase counteracts the build-up of a stabilizing layer of asphaltenes in the water-oil interface and so do higher p_H values of a buffered water phase. At low p_H-values emulsion stability does not correlate with interfacial resistance. It can be concluded that asphaltenes stabilize water-in-oil emulsions if they accumulate on the water-oil interface. This interfacial layer may show a coherence, which is an indication of the presence of asphaltenes rather than a condition for stability of the emulsions.     

Direct observation of single-molecule generation at a solid-liquid interface

Direct observation of single-molecule generation from a chemical reaction was achieved at a solid-liquid interface. The reaction between fluorescamine and immobilized N'-(3-trimethoxysilylpropyl)diethylenetriamine (DETA) was studied at the single-molecule level. Time-lapse fluorescence images of single-molecule products, excited by the evanescent field generated at a quartz-liquid interface, were recorded to follow the chemical reaction to its completion. The reactions were restricted to the approximately 1 nm thick layer nearest to the interface. Analysis of the photoelectron intensity of the fluorescent product of the reaction and its distribution shows that the reaction kinetics goes through a transition from zeroth-order to first-order as the reaction proceeds. This approach offered a novel means to study single-molecule reactions at the solid-liquid interface. It also enabled the investigation of reaction kinetics and chemical mapping of surface heterogeneity at the single-molecule level.     

Phenomenological kinetics of the thermal decomposition of sodium hydrogencarbonate

Aiming to find rigorous understanding and novel features for their potential applications, the physico-geometrical kinetics of the thermal decomposition of sodium hydrogencarbonate (SHC) was investigated by focusing on the phenomenological events taking place on a single crystalline particle during the course of the reaction. The overall kinetics evaluated by systematic measurements of the kinetic rate data by thermogravimetry under carefully controlled conditions were interpreted in association with the morphological studies on the precursory reaction, mechanism of surface reaction, structure of the surface product layer, diffusion path of evolved gases, crystal growth of the solid product, and so on. The precursory reaction was identified as the decomposition of impurity, taking place at the boundary between the surface of the SHC crystal and the adhesive small SHC particles deposited on the surface. In flowing dry N(2), the thermal decomposition of SHC proceeds by two-dimensional shrinkage of the reaction interface controlled by chemical reaction with the apparent activation energy of about 100 kJ mol(-1), after rapid completion of the surface reaction and formation of porous surface product layer. Atmospheric CO(2) and water vapor influence differently on the overall kinetics of the thermal decomposition of SHC. Added gas phase of CO(2) slightly inhibits the overall rate because of the increasing contribution of the surface reaction. Under higher water vapor pressure, the physico-geometrical mechanism of the surface reaction changes drastically, indicating the preliminary reformation of reactant surface and the formation of needle crystals of solid product on the surface. The mechanistic change and extended contribution of the surface reaction result in the deceleration of the surface reaction and acceleration of the established reaction.     

Time-of-flight secondary ion mass spectroscopy analysis of Na adatoms interacting with water-ice film

The origins of a slow reaction rate between the sodium adatoms and the water-ice film have been investigated by analyzing the surface composition using time-of-flight secondary ion mass spectroscopy in the temperature range of 13-230 K. An unhydrated NaOH layer is formed at the water-Na interface at 13 K which is followed by the growth of the metallic Na layer, whereas domains of both NaOH and unreacted Na are created only in the multilayer regime at 100 K. The NaOH layer plays a role as a separator between the water and Na layers, and its poor solubility in water is responsible for the small reaction rate of Na on glassy water. The solubility of NaOH in the deeply supercooled liquid water is low as well, but the mobile water molecules diffusing to the surface react with the Na adatoms, thereby quenching the growth of the metallic Na overlayer.     

Alkylation of pentaerythritol by phase-transfer catalysis : 2. Crucial effect of the aqueous sodium hydroxide solution

The mechanistic aspects of the alkylation of pentaerythritol (2-2'-bis(hydroxymethyl)-l,3-propanediol) (PE) by phase-transfer catalysis have been investigated. The dramatic effect of an excess and renewal of sodium hydroxide solutions on the one hand, and the solubility of the PT catalyst in the organic phase on the other, argue that the selectivity of this reaction appears to be controlled almost completely by the possible protonation of the sodium form of the alkoxide anion of PE (4). This protonation depends on the assumption that water molecules are present in the PT medium, and is based upon the observation that this PT medium is actually a three liquid-layer system. As the unusual layer (catalyst layer) may be readily hydrated, the concentrated sodium hydroxyde solution can be no longer considered as an unquestionable desiccant.