Interfacial ion pairing at the interface between water and a room-temperature ionic liquid, N-tetradecylisoquinolinium bis(pentafluoroethylsulfonyl)imide

The specific interaction of N-tetradecylisoquinolinium (C(14)Iq+) with Cl- and Br- has been detected in the voltammetry of ion transfer and electrocapillarity at the interface between an aqueous solution (W) and a room-temperature ionic liquid (RTIL), N-tetradecylisoquinolinium bis(pentafluoroethylsulfonyl)imide ([C(14)Iq+][C(2)C(2)N-]). This specific interaction also makes the transfer of Cl- and Br- into [C(14)Iq+][C(2)C(2)N-] energetically more favorable in comparison with that of F- and SO(4)(2-). The width of the polarized potential window in ion-transfer voltammetry at the [C(14)Iq+][C(2)C(2)N-]|W interface is significantly narrower because of the transfer of anions from W to RTIL. The degree of affinity of the anion with C(14)Iq+ agrees with the Hofmeister series. Such an ion-pair formation of anions in W with cations in the RTIL is much weaker when the cation constituting the RTIL is a symmetric tetraheptylammonium ion.     

Potential-dependent adsorption and transfer of poly(diallyldialkylammonium) ions at the nitrobenzene|water interface

Electrochemically driven adsorption and partition of a series of poly(diallyldialkylammonium) ions (PDADAA(+): alkyl = methyl, ethyl, propyl, and butyl) at the nitrobenzene (NB)|water (W) interface have been studied using voltammetry and electrocapillary measurements. When the phase-boundary potential, Δφ, that is, the inner potential of the W phase referred to that of the NB phase, is negative, poly(diallyldimethylammonium) (PDADMA(+)) shows little surface activity. The scanning of Δφ in the positive direction induces, first, the adsorption of PDADMA(+) at the interface and, then, the desorption of adsorbed PDADMA(+) ions into the NB phase, followed by the diffusion-limited transfer of PDADMA(+) from W to NB. The elongation of the dialkyl chains gives the stronger surface activity of PDADAA(+) even when Δφ < 0. The PDADAA(+) polyions studied are only slightly more hydrophilic than the corresponding monomers. However, the polycationic character of PDADAA(+) renders the adsorption, desorption, and ion transfer strongly dependent on Δφ and gives rise to unusual, M-shaped electrocapillary curves. The interplay of adsorption-desorption and ion transfer of PDADAA(+) ions induces the electrochemical instability of the interface and the emulsion formation on the NB side of the interface.     

Voltammetric study of the transfer of fluoride ion at the nitrobenzene / water interface assisted by tetraphenylantimony.

The transfer of F- ion assisted by an organometallic complex cation tetraphenylantimony (TPhSb+) across the polarized nitrobenzene / water (NB / W) interface has been studied by means of ion-transfer voltammetry. A well-defined voltammetric wave was observed within the potential window at the NB / W interface when tetraphenylantimony tetrakis(4-chlorophenyl) borate and F- ion were present in NB and W, respectively. The voltammogram can be interpreted as being due to the reversible transfer of F- ion assisted by the formation of the TPhSbF complex through the coordination of F- to Sb atom in NB. The formal formation constant of TPhSbF in NB has been determined to be 10(1.95 +/- 0.2 M(-1). No voltammetric wave due to the TPhSb(+)-assisted transfer of other anions such as Cl-, Br, I-, NO3-, CH3COO- and H2PO4(-) ions has been observed within the potential window.     

A comparison of the ultraslow relaxation processes at the ionic liquid|water interface for three hydrophobic ionic liquids

The ultraslow relaxation, on the order of a few seconds or longer, of the structure of the electrical double layer in response to the change in the phase-boundary potential across the ionic liquid (IL)|water(W) interface, which was recently reported for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been confirmed in two new hydrophobic ionic liquids, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide and trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate. A comparison of the degree of the hysteresis in electrocapillary curves for these ILs with those for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide demonstrates that the degree of the hysteresis is not correlated with the viscosity of these ILs. The ultraslow relaxation of the electrical double layer seems to be a general feature of ILs at electrified interfaces.     

The diamond/aqueous electrolyte interface: an impedance investigation

We have investigated the electrochemical interface between diamond electrodes and aqueous electrolytes using electrochemical techniques such as cyclic voltammetry and ac impedance spectroscopy. High-quality CVD-grown boron-doped polycrystalline diamond electrodes and IIa single crystalline natural diamond electrodes have been used in this study. In the case of hydrogen-terminated diamond electrodes, the electrochemical interface is dominated by the electrochemical double layer. Frequency-dependent impedance spectroscopy reveals a potential regime in which the contribution of ion adsorption becomes relevant. We have conducted experiments to evaluate the effect of pH and ionic strength on the double layer. Our results suggest that only ions resulting from water auto-dissociation, i.e., hydroxide and hydronium ions, are responsible for ion adsorption and, thus, able to modify the charge at the double layer. In contrast, no effect of the adsorption of several dissolved ions (such as Na+, K+, Cl-) has been observed On the basis of the electrochemical characterization of H-terminated diamond surfaces, we also discuss the phenomenon of the surface conductivity in diamond, as well as the pH sensitivity of the diamond surface. The influence of the O2/OH- and H2/H3O+ redox couples on the origin of the surface conductivity is discussed.     

Electrochemical instability in the transfer of cationic surfactant across the 1,2-dichloroethane/water interface

The electrochemical instability has been shown to appear in the transfer of cationic surfactant ions across the 1,2-dichloroethane/water interface. Cyclic voltammograms possess all fundamental characteristics that are predicted by the theory of electrochemical instability: the presence of the instability window, that is, the potential range where the interface becomes unstable, the location of the instability window around the standard ion transfer potential of surface-active ions, and the dependence of the width of the instability window on the concentration of the surfactant ions. Electrocapillary measurements clearly demonstrate that the interface becomes unstable, while the interfacial tension is positive, being higher than 20 mN m(-1). The electrocapillary curve exhibits the discontinuities at both ends of the instability window, indicating the similarity between the electrochemical instability and the phase transitions induced by the temperature, pressure, and chemical potential. The results from voltammetry and interfacial tension measurements for cationic surfactants support the idea that the electrochemical instability, so far reported in the transfer of anionic surfactants across the liquid/liquid interface, is one of intrinsic properties of the two-phase systems where the partition of surface-active ions takes place.     

Electrocapillary at contact: potential-dependent adhesion between a gold electrode and a mica surface

Using the electrochemical surface forces apparatus, we investigated adhesion (from pull-off measurements) between gold and mica as the potential of the gold surface was changed externally. Measurements were performed at different concentrations of KClO(4) in a potential window where the gold electrode is ideally polarizable. At applied potentials where the gold-mica interactions are repulsive, we obtain double layer forces that are predictable by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability but deviate from the theory at short range. At applied potentials where the gold-mica interactions are attractive, we observed a very strong dependence of adhesion on the applied potential, a result that cannot be directly related to DLVO theory. We show, however, that an approach based on electrocapillary thermodynamics can be employed to model the potential dependence of adhesion seen in our measurements. This electrocapillary approach presents evidence of charging at the gold-mica interface and stresses the relation between the charge within and outside of the contact area.     

Li-KCl熔融界面上离子扩散的分子动力学研究

采用分子动力学(MD)方法研究了熔融Li(电极)-KCl(电解质)界面上离子的扩散行为。熔融界面上离子的扩散动力学通过离子质心的均方位移(MSD)和速度自相关函数(VACF)进行研究,扩散系数由MSD(t)函数线性区间的斜率和VACF(t)函数积分得到。模拟结果表明,在熔融的Li-KCl界面上,Li+离子在浓度梯度的驱动下穿过界面发生定向迁移,导致双电层的形成和外电路上电流的输出。Li+离子的扩散系数比K+和Cl-离子的大7～8倍,说明在界面上Li+离子是主要的载荷子,热电池的电荷传输机制主要与Li+离子的扩散运动有关。由Nernst-Einstein公式对电导率进行估算,由扩散到KCl层中的Li+离子产生的电导率约为0.4 S.cm-2,对应的电流密度估算值为3.27×105A.cm-2。     

Double layer structure and adsorption/desorption hysteresis of neat ionic liquid on Pt electrode surface — an in-situ IR-visible sum-frequency generation spectroscopic study

The electrochemical interface between a polycrystalline Pt electrode and the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]) has been studied by in-situ IR-visible sum-frequency generation (SFG) spectroscopy. Potential dependent adsorption/desorption processes of OTf anions has been monitored within the electrochemical window. SFG results indicate that the ions form a double layer structure at the interface. Significant adsorption/desorption hysteresis has been observed for the anions on the Pt surface.     

The equation of state for an interface during the adsorption of organic substances on an electrode

The state of the interface between a metal and a solution of an electrolyte containing a neutral surfactant was investigated using a method alternative to the traditional thermodynamic approach. The method was based on the concept that there was a stability limit of a surfactant on an electrode, and the corresponding state could be described in terms of the catastrophe theory. The surface pressure was approximated by the Whitney polynomial in powers of the de Donder parameter (completeness of adsorption) with the coefficients depending on the chemical potential and polarization of the interface. The equation of state and the equation for the stability limit were obtained from the condition of zero first and second derivatives. These equations correctly described the results of electrocapillary measurements in the spirit of the law of corresponding states. The correlation between surface pressure maxima and critical stability potentials predicted by the theory was substantiated by the electrocapillary measurements data provided that the inflexions of surface pressure curves calculated from the electrocapillary data were related to the limiting stability at which the competing forces are balanced during the adsorption of surfactants. A simple equation for surface pressure was suggested in the form of a function of the state of thermodynamic parameters and completeness of adsorption. This function described the state of a surfactant at the interface. Equilibrium equations were derived for the state of a surfactant and the spinodal.     

Cyclic voltammetry of ion transfer across a room temperature ionic liquid membrane supported by a microporous filter

Cyclic voltammetry is used to study the transfer of a series of cations and anions across a room-temperature ionic liquid (RTIL) membrane composed of tridodecylmethylammonium cation (TDMA⁺) and tetrakis(pentafluorophenyl)borate anion (TPFPB⁻), and supported by a thin (∼112 μm) microporous filter. Essential advantage of the thin membrane system is the substantial reduction of the ohmic potential drop, which is compensated in voltammetric measurements. Reversible partition of TPFPB⁻ allows fixing the potential difference at one membrane interface and polarizing the other membrane interface in a defined way. It is shown that the polarized potential window for the interface between an aqueous electrolyte solution and RTIL attains the value of ca. 0.7 V at the ambient temperature of 25 ± 2 °C. Tetraphenylarsonium tetraphenylborate hypothesis is used for the first time to estimate the standard Gibbs energies of ion transfer from water to RTIL and to establish the scale of the absolute potential differences. A linear Gibbs energy relationship is found for the ion transfer from water to RTIL and o-dichlorobenzene.     

Adsorption and inhibition effects of tribenzylamine in ethanol+water mixtures on a mercury electrode

The adsorption properties of tribenzylamine on mercury were studied using electrocapillary and C-E curves measurements in ethanol+water mixtures in various supporting electrolytes. With the increase of ethanol content a sudden decrease of the differential capacity to a minimum value was observed followed at negative potentials by an abrupt increase of the capacity to the value of the supporting electrolyte. These transition potentials were dependent on the percentage of ethanol and on the concentration of tribenzylamine.The observed behaviour was attributed to a two-dimensional association of the adsorbate in the double layer. The quantitative evaluation of the results fulfilled the theoretical relations for association in the film. Another support for this idea was obtained from the inhibition effect of tribenzylamine on several fast charge transfer reactions at the DME. The potential of the sudden decrease of the limiting diffusion controlled current was very near to the positive transition potential at the C-E curves. The inhibition effect decreased with ethanol content.     

硅电极/溶液界面开路电位-时间谱和原子力显微镜在化学镀Ag中的应用研究

用开路电位-时间谱技术,表征了在硅（100）表面化学镀银的硅电极/溶液界 面吸附态。所得结果与原子力显微镜在纳米尺寸上的面结构信息分析结果作了对比 。同时也将该结果与循环伏安法（CV）结果作了比较。证明当硅电极表面具有单层 吸附Ag~+离子、表面单层吸附Ag~+离子发生沉积反应、Ag~+离子发生本体沉积时的 开路电位-时间曲线有完全不同的特征。     

Interfacial tension and electrocapillary measurements of the room temperature ionic liquid/aqueous interface

The surface and aqueous interfacial tensions for a series of water-immiscible room-temperature ionic liquids (RTILs) have been measured. The RTILs used in this study were based on 1-alkyl-3-methylimidazolium cations (Cnmim, n=6, 8, 10, and 12) and bis(perfluoromethylsulfonyl)imide (BMSI) and bis(perfluoroethylsulfonyl)imide (BETI) anions. It was found that the surface tensions of the RTILs increased with an increasing cation chain length similar to the behavior of n-alkanes. Interfacial tensions of the RTILs with aqueous solutions, however, were found to decrease with the cation chain length, which has been attributed to the increased surface activity of the longer chain cations. We have also demonstrated the first use of electrocapillary measurements to study the polarizable RTIL/aqueous interfaces. From the electrocapillary data, the potential of zero charge (PZC) for these RTIL/aqueous interfaces was determined, as well as the relative surface excess charge and capacitance. The PZC was found to be dependent upon the structure of the anions and cations with PZC values ranging from -357 mV for C6mimBETI and -161 mV for C10mimBMSI. The electrocapillary results also show that the cations of the RTIL are becoming increasingly surface-active as the alkyl chain on the cation is lengthened, thereby modulating the interfacial potential.     

Adsorption behavior of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) at the mercury-electrolyte solution interface as a function of electrode potential and time

The dependence of the differential capacitance (C) of the electrode double layer of a hanging mercury drop electrode in bis (2-ethylhexyl) sodium sulfosuccinate (AOT) solutions on electrode potential (E) and time is measured using three-dimensional phase sensitive ac voltammetry. This methodology, possessing a very wide time window that permits a detailed study of the adsorption phenomena, is based on the reconstruction of C vs E curves, sampled after many phase-sensitive ac chronoamperometric experiments. The shape of these curves allows an estimation of the structure of the layer of AOT molecules absorbed at the electrode surface. AOT molecules form micelles in bulk solutions and they also associate in the charged interface under the strong influence of the electric field into surface aggregates which depend on their concentration and applied potential. The presence of AOT micelles in the bulk solution can be linked with the appearance of a surface film at potentials more negative than those corresponding to a condensed film linked with a capacitance value slightly higher than that normally attributed to a compact layer. The whole phenomenon is proved to be very dependant upon time.     

Regression analysis of electrocapillary curves of mercury electrode in solutions of secondary butyl alcohol

A regression technique is used for analyzing electrocapillary curves (ECC) in secondary butanol solutions, which were published earlier. It was shown that, in contrast to similar analysis of electrocapillary curves for a number of other compounds, if we want to describe the adsorption of 2-BuOH within the framework of the model of two parallel capacitors, the linear dependence of the parameter of intermolecular interaction on the electrode potential should be taken into account. This result stems from the very high accuracy of interface tension measurements in the system under study. It is found that, in terms of the Frumkin isotherm and the model of two parallel capacitors, the interpretation of data on the adsorption of 2-BuOH has a much simpler physical meaning as compared with the strictly thermodynamic approach used earlier.     

Ion-transfer voltammetry at a polarized room-temperature molten salt/water interface.

Tetraoctylammonium cation forms a room-temperature molten salt (RTMS) with 2,4,6-trinitrophenolate anion. The RTMS is immiscible with water (W) and forms a stable RTMS/W interface. It has been shown that the RTMS/W interface can be electrochemically polarized. A well-defined voltammetric wave due to the transfer of thiocyanate ion across the RTMS/W interface was observed within the potential window. This is the first example of a polarized RTMS/W interface.     

Solvation of sodium chloride in the 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid: a molecular dynamics study

We report molecular dynamics studies on the solvation of sodium chloride in the 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid ([BMI][Tf2N] IL). We first consider the potential of mean force for dissociating a single Na+Cl- ion pair, showing that the latter prefers to be undissociated rather than dissociated (by ca. 9 kcal/mol), with a free energy barrier of ca. 5 kcal/mol (at d approximately 5.2 A) for the association process. The preference for Na+Cl- association is also observed from a 100 ns molecular dynamics simulation of a concentrated solution, where the Na+Cl- ions tend to form oligomers and microcrystals in the IL. Conversely, the simulation of Na13Cl14- and Na14Cl13+ cubic microcrystals (with, respectively, Cl- and Na+ at the vertices) does not lead to dissolution in the IL. Among these, Na14Cl13+ is found to be better solvated than Na13Cl14-, mainly due to the stronger Na+...Tf2N- interactions as compared to the Cl-...BMI+ interactions at the vertices of the cube. We finally consider the solid/liquid interface between the 100 face of NaCl and the IL, revealing that, in spite of its polar nature, the crystal surface is solvated by the less polar IL components (CF3(Tf2N) and butyl(BMI) groups) rather than by the polar ones (O(Tf2N) and imidazolium(BMI) ring). Specific ordering at the interface is described for both Tf2N- anions and BMI+ cations. In the first IL layer, the ions are rather parallel to the surface, whereas in the second "layer" they are more perpendicular. A similar IL structure is found at the surface of the all-neutral Na0Cl0 solid analogue, confirming that the solvation of the crystal is rather "apolar", due to the mismatch between the IL and the crystal ions. Several comparisons with water, methanol, or different BMI+-based ILs as solvents are presented, allowing us to better understand the specificity of the ionic liquid-NaCl interactions.     

Cyclic voltammetry of ion transfer across the polarized interface between the organic molten salt and the aqueous solution

A molten salt, or ionic liquid, composed of tetrahexylammonium bis(perfluoroethylsulfonyl)imide forms with an aqueous solution a polarized interface where the phase-boundary potential can be controlled externally. The available potential window of about 300 mV at 40 °C enables us to apply various electrochemical techniques for studying the structure and charge transfer reactions at the molten salt–water interface. Cyclic voltammetry of the transfer of moderately hydrophobic ions, such as 1-octyl-3-methylimidazolium and hexafluorophosphate ions, across the interface exemplifies the potentiality of this new electrochemical interface. This new type of polarized interface would facilitates electrochemical studies of molten salt–water two-phase systems that have been studied as an environmentally benign alternative of organic solvent–water two-phase systems for liquid–liquid extraction and two-phase organic synthesis.     

Description of the Hg/[H<Subscript>2</Subscript>O + <Emphasis Type="Italic">mc</Emphasis> KCl + (1 − <Emphasis Type="Italic">m</Emphasis>)<Emphasis Type="Italic">c</Emphasis> KF] system in terms of the modified model of electrical double layer

Specific adsorption of anion Cl⁻ at the Hg/H₂O interface from mixed solutions mc KCl + (1 − m)c KF is analyzed in terms of a recently put forward modified model of electrical double layer. The calculations showed that the modified model of electrical double layer not only adequately gives the form of experiment curves of differential capacitance but also allows explaining the deviation from linearity of the dependence of the adsorption energy on the electrode charge. Of importance is a decrease in the dense layer capacitance when passing from KF to KCl under the condition of constant surface coverage with the Cl⁻ ions. The results of calculations in terms of the modified model of electrical double layer are compared with those obtained in terms of the Sanz-Gonzalez model.