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...     

Protein adsorption at liquid/liquid interface with low interfacial tension

Adsorption of proteins at the interface of two-liquid systems composed of aqueous ammonium sulfate solution and tert-butanol by phase separation was investigated by drop shape tensiometry. The change of interfacial tension with time and protein concentration as well as upon compression of the adsorbed layer were compared for bovine serum albumin, ovalbumin, β-lactoglobulin, lysozyme, trypsin and horse radish peroxidase. A correlation between the dilatational moduli of various protein films and the partitioning of proteins in the two-liquid system was found, which provides evidence to the role of emulsion stability in protein separation by three-phase partitioning.     

Selective silver ion transfer voltammetry at the polarised liquid/liquid interface

Transfer of silver ions across the water/1,2-dichloroethane interface was studied by cyclic voltammetry (CV). In the absence of added neutral ionophore, Ag+ transferred across the interface when the organic phase contained either tetraphenylborate or tetrakis(4-chloro)phenylborate anions, but this transfer was not possible in the presence of organic phase hexafluorophosphate or perchlorate anions. The ion transfer processes observed were independent of the nature of the organic phase cation. The CV in the presence of tetraphenylborate exhibited a shape consistent with an ion transfer followed by chemical reaction; the rate constant for the following chemical reaction was 0.016 s(-1). In the presence of tetrakis(4-chloro)phenylborate, a return peak equivalent in magnitude to the forward peak was observed, indicative of a simple ion transfer reaction uncomplicated by accompanying chemical reactions. The selectivity of the transfer was assessed with respect to other metal cations: no transfers for copper, cadmium, lead, bismuth, cobalt, nickel, palladium or zinc were observed. The selectivity of the transfer suggests this can form the basis of a selective voltammetric methodology for the determination of silver ions.     

Electron transfer mediated by glucose oxidase at the liquid/liquid interface

In order to establish an experimental basis for exploring the reactivity of membrane-bound redox enzymes using electrochemistry at an organic/aqueous interface, the reactivity of glucose oxidase adsorbed at the dichloroethane/water interface has been studied. Turnover of glucose in the aqueous phase mediated by dimethyl ferricenium electrogenerated in the organic phase was measured by measuring the feedback current caused by recycling the mediator as the generator electrode approached close to the interface from the organic side. An unexpected self-exchange reaction of the ferrocene at the interface was suppressed by adsorption of a surfactant. The interfacial enzyme reaction could be distinguished from reaction within the bulk of the aqueous phase. Reaction within a protein-surfactant film formed at the interface is conjectured.     

Observation of the marcus inverted region of electron transfer reactions at a liquid/liquid interface

Scanning electrochemical microscopy was used to probe the influence of a driving force on the heterogeneous electron transfer (ET) processes at the externally polarized water/1,2-dichloroethane interface. Being a part of the driving force, the Galvani potential difference at the interface, Deltaowphi, can be quantitatively controlled in a wide range, allowing the precise measurements of the rate constants of the ET reactions. Two opposite systems were chosen in this work. One was 5,10,15,20-tetraphenyl 21H,23H-porphyrin zinc (ZnPor, O)/Fe(CN)64- (W), and the other was TCNQ (O)/Fe(CN)63- (W). For both systems studied, the relations between the rate constant and the Deltaowphi were of parabolic shape; that is, the rate constants increased initially with the Deltaowphi until reaching a maximum and then decreased steadily as the Deltaowphi increased further. This is in accordance with the prediction of the Marcus theory. To our knowledge, this is the first report that the Marcus inverted region can be observed electrochemically at an unmodified liquid/liquid interface with only one redox couple at each phase. The effect of the concentrations of the organic supporting electrolyte has also been discussed in detail.     

Ultrafast excited-state electron transfer at an organic liquid/aqueous interface

Ultrafast excited-state electron transfer has been monitored at the liquid/liquid interface for the first time. Second harmonic generation (SHG) pump/probe measurements monitored the electron transfer (ET) occurring between photoexcited coumarin 314 (C314) acceptor and dimethylaniline (DMA) donor molecules. In the treatment of this problem, translational diffusion of solute molecules can be neglected since the donor DMA is one of the liquid phases of the interface. The dynamics of excited-state C314 at early times are characterized by two components with exponential time constants of 362 +/- 60 fs and 14 +/- 2 ps. The 362 fs decay is attributed to the solvation of the excited-state C314, and the 14 ps to the ET from donor to acceptor. We are able to provide conclusive evidence that the 14 ps component is the ET step by monitoring the formation of the radical DMA cation. The formation time is 16 ps in agreement with the 14 ps decay of C314*. The recombination dynamics of DMA+ plus C314- was determined to be 163 ps from the observation of the DMA+ SHG signal.     

Study on dynamic interfacial tension of 3-dodecyloxy-2-hydroxypropyl trimethyl ammonium bromide at liquid/liquid interface

Dynamic interfacial tension between aqueous solutions of 3-dodecyloxy-2-hydroxypropyl trimethyl ammonium bromide (R₁₂HTAB) and n-hexane were measured using the spinning drop method. The effects of the R₁₂HTAB concentration (the concentration below the CMC) and temperature on the dynamic interfacial tension have been investigated; the reason of the change of dynamic interfacial tension with time has been discussed. The effective diffusion coefficient, D_a, and the adsorption barrier, _a, have been obtained from the experimental data using the extended Word–Tordai equation. The results show that the dynamic interfacial tension becomes smaller while _a becomes higher with increasing R₁₂HTAB concentration in the bulk aqueous phase. D_a decreases from 5.56 × 10⁻¹² m⁻² s⁻¹ to 0.87 × 10⁻¹² m⁻² s⁻¹ while _a increases from 5.41 kJ mol⁻¹ to 7.74 kJ mol⁻¹ with the increase of concentration in the bulk solution of R₁₂HTAB from 0.5 × 10⁻³ mol dm⁻³ to 4 × 10⁻³ mol dm⁻³. Change of temperature affects the adsorption rate through altering D_a and _a. The value of D_a increases from 5.56 × 10⁻¹² m⁻² s⁻¹ to 13.98 × 10⁻¹² m⁻² s⁻¹ while that of _a decreases from 5.41 kJ mol⁻¹ to 5.07 kJ mol⁻¹ with temperature ascending from 303 K to 323 K. The adsorption of surfactant from the bulk phase into the interface follows a mixed diffusion–activation mechanism, which has been discussed in the light of interaction between surfactant molecules, diffusion and thermo-motion of molecules.     

Noise and ac impedance analysis of ion transfer kinetics at the micro liquid/liquid interface

Fluctuation analysis was utilized to determine the TEA ion transfer kinetics across the water/1,2-dichloroethane interface. The obtained data were compared with those derived from electrochemical impedance spectroscopy experiments using the same electrolytic cell. The apparent standard rate constants k_s determined by these two techniques have a similar value. The average value k_s = 0.37 cm s^− 1 is comparable with the previously reported value k_s = 0.2 cm s^− 1. The experimental approach utilizing a thick wall glass micro-capillary to fix the interface exhibits a very small stray capacitance value, proving this system to be suitable for determining the kinetics of the fast ion transfer across a liquid/liquid interface. Application of a method employing a small perturbation signal prevents polarization of the inner capillary surface by current flowing through the cell. The induced polarization of the capillary can affect ion concentration at the interface due to electroosmosis and thus make the kinetic data evaluation difficult or erroneous.     

Diffusion of isolated surface-active molecules at the air/water interface

Fluorescence correlation spectroscopy was applied to study the diffusion of isolated surface-active molecules at air/water interfaces. Rhodamine 6G was used as a surface-active fluorescent tracer. Results show that the diffusion coefficient of the Rhodamine 6G at the interface is about 2.5 times higher than in the bulk. Effects of Rhodamine 6G concentration and added SDS or CTAB surfactants have been studied. Diffusion of Rhodamine 6G at the interface is slowed down at surfactant concentration corresponding to a mean distance between molecules of 10 and 40 nm, indicating a long-range interaction.     

A new electrochemical method based on transfer at a liquid/liquid interface: Determination of barium and strontium

A new kind of polyoxyethylene ionophore is introduced to facilitate metal ion transfer across a liquid/liquid interface. The transfer of Ba(2+) and Sr(2+), facilitated by polyethylene glycol 400 across the interfaces of water/nitrobenzene and water/1,2-dichloroethane, has been studied in detail by cyclic voltammetry, and a new method for the determination of barium and strontium established. Both metals can be determined by cyclic voltammetry with good selectivity and reproducibility.     

Voltammetric study of the transfer of polyammonium ions at nitrobenzene / water interface.

The transfer of polyammonium ions, poly[(dimethylimino)-1,6-hexanediyl] (n = 140, n being the degree of polymerization) ion and poly[(dimethylimino)(2-oxo-1,2-ethanediyl)imino-alpha,omega-alkanediylimino(1-oxo-1,2-ethanediyl)(dimethylimino)-alpha',omega'-alkanediyl] ([-N+ (CH3)2CH2CONH(CH2)x NHCOCH2N+ (CH3)2(CH2)y-]n, x = 2, 3, 4, or 6, y = 3 or 6, and n = 30-130) ions, at a polarizable nitrobenzene / water interface has been studied by normal pulse voltammetry and cyclic voltammetry. Despite the polydispersity of the preparations, by normal pulse voltammetry, an S-shaped current-potential curve with a well-defined limiting current, and, by cyclic voltammetry, a pair of anodic and cathodic peak currents due to the transfer of polyammonium ions across the interface were observed within the potential window. The voltammetric behavior is described. Also, the effect of ion-pair formation of the polyammonium ions with supporting electrolyte anions in nitrobenzene- and water-phases on the half-wave or midpoint potential of the ion-transfer, and the relation between the structure of the polyammonium ions and the transfer potentials are discussed.     

A simple operational relationship between drop-time and interfacial tension at a charged mercury/aqueous solution interface

This paper describes a simple operational relationship between the drop-time of a dropping mercury electrode and the interfacial tension at a charged mercury/aqueous solution interface. An apparatus and technique for measuring drop-times is reported, and examples are given of the fit of experimental drop-times to literature values of interfacial tensions. The operational relationship is independent of temperature in the range 293-313 K.     

Dynamic interfacial tension at the oil/surfactant-water interface

We have used dynamic interfacial tension measurements to understand the structure of the ordered monolayer at the hexadecane/water interface induced by the presence of surfactant molecules. No abrupt changes in the interfacial tension (gamma) are observed during the expansion and contraction cycle below the interfacial ordering temperature (Ti) as observed for alkanes in contact with air. The lack of an abrupt change in gamma and the magnitude of this change during the expansion process indicate that the ordered phase may not be crystalline. The change in the interfacial tension is due to an increase in contact between water and hexadecane molecules and the disordering of the interfacial ordered layer. At low surfactant concentrations, the recovery of the interfacial tension is slower below Ti, suggesting that there is a critical surfactant concentration necessary to nucleate an ordered phase at the interface.     

Switchable self-assembly of a bioinspired alkyl catechol at a solid/liquid interface: competitive interfacial, noncovalent, and solvent interactions

The large tendency of catechol rings to adsorb on surfaces has been studied by STM experiments with molecular resolution combined with molecular-dynamics simulations. The strong adhesion is due to interactions with the surface and solvent effects. Moreover, the thermodynamic control over the differential adsorption of 1 and the nonanoic solvent molecules has been used to induce a new temperature-induced switchable interconversion. Two different phases that differ in their crystal packing and the presence of solvent molecules coexist upon an increase or decrease in the temperature. These results open new insight into the behavior of catechol molecules on surfaces and 2D molecular suprastructures.     

SECM study of solute partitioning and electron transfer at the ionic liquid/water interface

Molecular partitioning and electron-transfer kinetics have been studied at the ionic liquid/water (IL/water) interface by scanning electrochemical microscopy (SECM). The ionic liquid C8mimC1C1N is immiscible with water and forms a nonpolarizable interface when in contact with it. Partitioning of ferrocene (Fc) across the IL/water interface was studied by SECM and found to be kinetically fast with a partition coefficient CIL/CW of 2400:1. The partition coefficient value was measured by SECM under quasi-steady-state conditions without waiting for complete solute equilibration. To investigate the kinetics of the electron transfer (ET) between aqueous ferricyanide and Fc dissolved in IL, a new approach to the analysis of the SECM current-distance curves was developed to separate the contributions of Fc partitioning and the ET reaction to the tip current. Several combinations of different aqueous and nonaqueous redox species were investigated; however, only the Fc/Fe(CN)63- system behaved according to the Butler-Volmer formalism over the entire accessible potential range.     

Adsorption at the liquid/liquid interface in mixed systems with hydrophobic extractants and modifiers. 2. Dynamic interfacial tension at the hydrocarbon/water interface in binary mixed systems

The dynamic interfacial tension for binary mixtures of hydrophobic metal ion extractants and a modifier were measured by using the drop volume technique. Four types of equimolar mixtures were considered: two chelating extractants: 2-hydroxy-5-nonylacetophenone oxime (HNAF) and beta-diketone (1-phenyldecan-1,3-dion), two solvating extractants: trioctylphosphine oxide (TOPO) and tributyl phosphate (TBP), chelating and solvating extractants TOPO and beta-diketone, and the chelating extractant HNAF and the modifier (decanol). With the aid of the Ward and Torday equation the values of the diffusion coefficients of individual compounds and their equimolar mixtures were estimated. It was found that in the case of two types of investigated mixtures, i.e., HNAF + beta-diketone and HNAF + decanol the compound HNAF that was dominant in the mixed adsorbed monolayer and the more interfacially active also determined the kinetics of adsorption in mixed systems. In contrary to the mixture of two chelating reagents, in the case of a mixture of two solvating extractants the mixed system behaves like the less active, though dominant at the interface, reagent TBP. The same effect was observed in both of the considered diluents (toluene and octane).     

Adsorption at the liquid/liquid interface in mixed systems with hydrophobic extractants and modifiers 1. Study of equilibrium interfacial tension at the hydrocarbon/water interface in binary mixed systems

Equilibrium interfacial tension at the liquid/liquid interfaces for two chelating metal ion extractants, 2-hydroxy-5-nonylacetophenone oxime (HNAF) and 1-phenyldecane-1,3-dion (beta-diketone), two solvating extractants, trioctylphosphine oxide (TOPO) and tributyl phosphate (TBP), and a modifier, decanol, were obtained with a drop volume tensiometer. Moreover, four equimolar binary mixtures of extractant/extractant and extractant/modifier type were considered. The composition of the mixed adsorbed monolayer and the molecular interaction parameters beta were determined by the Rosen equation. It was found that in all the studied systems coadsorption exists; however, synergism in the reduction of interfacial tension was not observed. The obtained results indicate that in the case of three mixtures considered the composition of a mixed monolayer at the hydrocarbon/water interface was quite different from that in the bulk organic phase. Only for the TOPO/beta-diketone mixture were the compositions at the interface and in the bulk organic phase similar. The obtained results indicate that it is impossible to predict the composition of a mixed monolayer by taking into account the interfacial activity of individual components of the mixture. In some cases the compound shows lower interfacial activity (smaller efficiency and effectiveness of adsorption) and occupies a dominant position at the interface, regardless of the type of hydrocarbon used as the organic diluent.     

Dynamic interfacial properties of poly(ethylene glycol)-modified ferritin at the solid/liquid interface

Poly(ethylene glycol)-modified ferritins (PEG-ferritins) with various molecular weights were synthesized by the grafting method, and their dynamic interfacial properties at the solid/liquid interface were investigated. The number of PEG grafted to ferritins was controlled by the amount of 1,1'-carbonyldiimidazole-modified PEG adding to the reaction solution. The adsorption kinetics and energy dissipation of PEG-ferritins onto bare Si substrate and amino-modified Si substrate were investigated with a quartz crystal microbalance (QCM) in 10 mM bis-Tris/HCl buffer (pH 5.8), while their morphologies were characterized by scanning electron microscopy (SEM). The adsorption dynamics of PEG-ferritins onto amino-modified Si substrate were quite different from those of unmodified ferritin, which can be reasonably interpreted by the desorption capability of PEG-ferritins on the surface attributed to amphiphilicity and the high-chain mobility of PEG chains.     

Self-assembly of ordered 3D Pd nanospheres at a liquid/liquid interface

A novel route for the self-assembly of nanoparticles to nanospheres at a liquid/liquid interface has been developed to prepare palladium nanospheres. It has proved that the interface offers an excellent site and plays a key role in the self-assembly of nanoparticles to nanospheres. The palladium nanospheres are characterized by electron microscopy, energy-dispersive X-ray analysis, UV-visible spectroscopy, X-ray diffraction spectroscopy, and X-ray photoelectron spectroscopy. The mechanism of the self-assembly process is also proposed.