The double layer and ion adsorption at the interface between two immiscible solutions: Part III. Long chain alkyl-trimethylammonium halides at partition equilibrium between water and nitrobenzene

The surface excesses of alkyltrimethylammonium halides have been evaluated from interfacial surface tension measurements carried out at the partition equilibrium between water and nitrobenzene.The model considered is constituted by two space charge layers on either side of a central compact layer which contains alkyltrimethylammonium ions arranged as in an adsorbed monolayer.The value of the electrical potential at the junction plane between the two phases depends on the degree of dissociation within the inner layer and on the number of specifically adsorbed ions. If one assumes complete dissociation it is possible to establish the variation of the adsorption isotherms with the chemical and electrical parameters and thus determine the value of the free energy of adsorption with the number of CH₂ groups in the alkyl chain.The results obtained do not indicate an important effect due to the counter ion, which would tend to substantiate the hypothesis of a total dissociation within the central layer.Furthermore the fact is stressed that the knowledge of the isotherm depends on the magnitude of the potential drop within the compact layer, the value of which cannot be determined by purely thermodynamical measurements.     

The double layer and ion adsorption at the interface between two non-miscible solutions: Part II. Electrocapillary behaviour of some water-nitrobenzene systems

The model of two diffuse layers separated by an inner compact layer proposed in a previous paper [1] for the liquid-liquid interface is tested again on the water-nitrobenzene interface. The sodium bromide and the tetraalkylammonium tetraphenylborate (R⁺R′^?) maintain the ionic strength constant respectively in the aqueous and organic phase and RBr is added to the aqueous phase at different concentrations (R⁺-tetrabutyl, tetrapropyl, tetraethylammonium). Interfacial tensions and potential differences between the two phases, measured for various concentrations of R⁺, lead to an electrocapillary curve comparable to the curve obtained with the classical mercury-aqueous solution system. Furthermore, the dipolar potential drop λ is estimated as very nearly zero.     

A potentiostatic study of hydrophobic ion transfer across lipid bilayer membranes: Part II. Transport of tetraphenylborate ions at high concentration in the aqueous solution

The transfer of tetraphenylborate (TPB^?) ions across black lipid membranes in aqueous solutions ranging from 10^?5M to 10^?3M, has been studied under potentiostatic conditions. A theoretical treatment of the problem accounts for the two time constants given and for the diffusion current for prolonged periods of time if one assumes that the adsorption-desorption transfer currents are large. This is tantamount to considering that these two processes are highly reversible and that the dipolar nature of the drop in potential between the closest approach and adsorption planes determines the adsorption. The resistances corresponding to the translocation and adsorption-desorption processes have thus also been evaluated. Using these results and those given by surface potential measurements on monolayers of glycerylmono-oleate, the number of adsorbed ions has been calculated. This has led to an explanation of the well known conductance maximum encountered when the concentration of the tetraphenylborate ions varies.     

Anti-Reflective Coatings for CRTs by Sol-Gel Process

Two types of anti-reflective coatings composed of nano-particles were developed for cathode ray tubes (CRTs). The anti-reflective and anti-static coating is composed of two layers. An outer SiO₂ layer is formed over a porous inner layer composed of titanium oxynitride (TiO _x N_y), antimony-doped tin oxide (ATO) and SiO₂. To control the reflection of the film, a porous structure is formed using a mixed sol composed of TiO _x N _y -ATO particles and hydrolyzed-polymerized tetraethoxy-silane (TEOS). The resulting double layered coating is shown to consist in a nanocomposite pseudo three-layer structure. The antireflective electromagnetic-wave-shielding coating is also composed of two layers. An outer SiO₂ layer is formed over an electric-conductive inner layer composed of silver colloids, TiO _x N _y nano particles. Silver colloids are used to obtain a film having low surface resistivity and TiO _x N _y nano-particles contained in the inner layer enhance the durability of the film. To reduce the plasma-resonance absorption caused by silver colloids, silver ions are added to the outer layer solution. The silver ions diffuse into the inner layer from the outer layer when the film is cured, touch to the surface of silver colloids, suppress the silver colloid growth and reduce the specific absorption of the film. These coatings are successfully applied to the panel glass for CRTs on an industrial scale.     

The double layer at the interface between two immiscible electrolyte solutions: Part II. Structure of the water/nitrobenzene interface in the presence of 1:1 and 2:2 electrolytes

From fast galvanostatic pulse measurements at 25°C the capacitance of the water/nitrobenzene interface was evaluated as a function of the interfacial potential difference Δ_o^w? for systems consisting of NaBr, LiCl or MgSO₄ in water and tetrabutylammonium tetraphenylborate, tetraphenylarsonium tetraphenylborate or tetraphenylarsonium dicarbollylcobaltate in nitrobenzene. The modified Verwey—Niessen model, in which an inner layer of solvent molecules separates two space-charge regions (the diffuse double layer), describes the structure of the water/nitrobenzene interface well at electrolyte concentrations above ca. 0.02 mol dm^?3, provided that the ions are allowed to penetrate into the inner layer over some distance. For all the systems studied the zero-charge potential difference was found at Δ^w_o?_pzc ≈ 0 on the basis of the standard potential difference Δ^w_o?⁰_TMA + = 0.035 V for tetramethylammonium cation which was used as a reference ion. At zero surface charge a comparison was made with the theoretical capacitance calculated using the mean spherical approximation for a model consisting of two ion and dipole mixtures facing each other. The effect of ion penetration on the interfacial capacitance was estimated from the solution of the linearized Poisson-Boltzmann equation for a triple dielectric model with a continuous distribution of the point ions. The concentration-independent inner layer potential difference and capacitance can only be inferred from the capacitance data if the ion size effect is taken into account. A non-iterative procedure based on the hypernetted-chain equation was used for the evaluation of the potential drop across the diffuse double layer. The extend of the penetration into the inner layer appears to be a function of ion solvation, e.g. the more hydrated ion the less extensive ion penetration is likely.     

Impact of imidazolium-based ionic liquid surfactant additions on dilational rheology properties of different protein adsorption layer

The interfacial behavior of β-casein and BSA solutions have been investigated in the presence of imidazolium-based ionic liquid surfactant ([C₁₄mim]Br) at the decane/water interface with the oscillating the drop and interfacial tension relaxation measurements. Both the electrostatic and the hydrophobic interaction between protein and [C₁₄mim]Br played crucial roles as [C₁₄mim]Br concentration increases. Furthermore, it was found that the dilational rheology parameters provided information of the adsorbed layers structure, and the dynamics properties of the adsorbed layers depend on the bulk [C₁₄mim]Br concentration. Moreover, with the concentration of [C₁₄mim]Br increasing, β-casein in the interfacial layer was subject to conformational changes where it gave space to [C₁₄mim]Br molecules in the form of co-adsorb; for BSA/[C₁₄mim]Br solutions, the globule protein BSA deformed and then co-adsorb with [C₁₄mim]Br molecules at the decane/water interface. These results will contribute to elucidation of the influence of the surfactant on the different structure proteins and the wide applications of protein/surfactant systems in practice.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part IV. Carbon Tetrachloride electroreduction in the presence of Cl−, Br−, SCN−, and N3−

The rate constant for CCl₄ electroreduction on mercury, once corrected for diffuselayer effects according to Frumkin, still depends on the charge density q_i due to specifically abdsorbed supporting ions. Thus, in the presence of the adsorbed anions Cl^?, Br^?, SCN^? and N₃^?, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with |q_i|. The slopes of the various Φ vs. q_i plots are in fairly good agreement with the theoretical treatment of ref. 9, which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring adsorbed electroinactive ions within the compact layer. An analogous behaviour is observed in the reduction of CBr₄.     

Study of ion adsorption at passive iron by using the impedance and photoadmittance methods

Intensity-modulated photocurrent (photoadmittance) and electrochemical impedance of anodicoxidized iron electrode in neutral nitrate solutions and in the presence of Ba²⁺, Ca²⁺, Cl^?, and C₆H₅COO^? (benzoate) are studied. It is shown that the ion adsorption at passive iron affects but slightly the system’s impedance; by contrast, it affects the photocurrent value significantly: when adsorbing, the anions increase the photoeffect, while the cations decrease it. These effects are associated with the potential drop redistribution in the Helmholtz layer and the film. The dissimilar changes of the generation current in the presence of similarly charged ions at their equal concentration evidence their different adsorption activity. The correlation between the generation current and surface-active ion concentration in solution is found. The photoelectrochemical spectroscopy allows evaluating qualitatively the surface-active ion adsorption at the passive iron and judging on the ion adsorption by the dependence of the generation current on the ion concentration.     

Examination of the role of ion size in determining double layer properties on the basis of a generalized mean spherical approximation

A new model for the diffuse double layer which accounts for the effects of ion size and solution permittivity is described. It is then used to estimate the potential drop across the diffuse layer at negative charge densities for the cases that Li⁺ and Cs⁺ are the electrolyte cations. The potential drop in the Li⁺ system is considerably smaller than that in the Cs⁺ system at 1 M, and both values are smaller than the value predicted by the Gouy–Chapman model. As the electrolyte concentration decreases these differences become smaller so that at 0.01 M, the present model predicts that the diffuse layer potential drop is approximately 90% of the Gouy–Chapman estimate. The results of the model are used to examine the differences in inner layer structure at mercury electrodes with Li⁺ and Cs⁺ ions at the outer Helmholtz plane, and to reconsider the question of the specific adsorption of Cs⁺ at negative-charge densities.     

Polypyrrole Bilayers: Charge Trapping and Application for Amperometric Ions Sensing

Bilayers composed of polypyrrole: doped by perchlorate ions (PPy(ClO₄) – anion exchanging inner layer) and by dodecyl sulfate ions (PPy(DS) – cation exchanging outer layer) are very effective charge trapping systems that are usually not observed for other bilayers comprised of polypyrrole. Chronopotentiometric experiments carried out for oxidation and reduction showed that the trapping effect in the inner layer resulted from different ion exchange properties of the component polymers, leading to a low permeability of the reduced outer layer towards anions. Estimated diffusion coefficients of Cl^? anions in the oxidized and reduced PPy(DS) are in the range of 10^?9 and lower than 10^?10 cm² s^?1, respectively. The presence of the outer layer limiting the ion transfer was found to be beneficial to improve the signal resolution in amperometric mode of ion sensing within wide KCl concentration range, from 10^?5 M up to 3 M. The influence of experimental conditions (film thickness, response time) on optimization of this novel kind of polymeric bilayer ion sensors was studied.     

A discussion on ion conductivity at cation exchange membrane/solution interface

By Gouy–Chapman–Stern–Grahame (CGSG) model, the electric double layer at ion exchange membrane/solution interface consists of two parts: the Stern layer and the diffusion layer. The ions in Stern layer are compacted and considered to be immobile. The relation of diffusion layer mean conductivity K with outer Stern layer potential φ₀, the boundary potential φ_δ and the electrolyte concentration C₀ is educed for symmetric electrolyte system. The results show that K is higher than that of the bulk solution and is greatly influenced by φ₀, φ_δ and C₀.The examination of PE01 cation exchange membrane/solution interface resistance R_e measured by ac impedance technique, shows that R_e value decreases quickly as the KCl electrolyte concentration rises. The effect of electrolyte concentration on the resistance of EDL can be explained by the electrical interactions between ions and charged groups of the membrane. Since the membrane/solution interface resistance is much higher than that of bulk solution, therefore, a further analysis based on the theory developed in this study proves that the ion transfer resistance R_e of membrane–solution interface predominantly occurs at Stern layer as a result of static electrical interaction.     

Double-layer structure and mechanism of electrode reactions: I. Nitromethane reduction on mercury from aqueous solutions

Kinetic measurements of CH₃NO₂ electroreduction in aqueous solutions of pH values ranging from 1 to 9 were carried out with the polarographic technique under different experimental conditions. In particular, different concentrations of the supporting electrolytes NaClO₄ and NaCl, which are nonspecifically adsorbed at the potentials investigated, or else of the specifically adsorbed electrolyte KI, were employed to change the potential _d at the outer Helmholtz plane and/or the charge density q_i at the inner Helmholtz plane. The effects of these changes between pH 4 and 9 are consistent with a rate-determining protonation step $C{H}_{3}N{O}_2^{?}\stackrel{(+H^{+})}{\to }C{H}_{3}N{O}_{2}H$ taking place within the compact layer. As pH is gradually decreased from 4 to 1, double-layer effects are satisfactorily interpreted by assuming that the control of the overall electrode process passes from the above protonation step to the electron-transfer step CH₃NO₂+e→CH₃NO₂^?, again occurring within the compact layer.     

The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface (C^o_M) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution (C_M), the exact value depending on the deposition time t_d. Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C_L/C_M, is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C^o_M/C_M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C_L/C_M. The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.     

Effect of Cs+ ions on the electrochemical nanogravimetric response of platinum electrode in acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the presence and absence of Cs⁺ ions by electrochemical quartz crystal nanobalance (EQCN). An unusual potential dependence of the quartz crystal frequency response has been observed in the presence of Cs⁺ ions. The frequency decrease is more pronounced in the region of the underpotential deposition of hydrogen, and the frequency decrease in the double layer region diminishes as the concentration ratio of Cs⁺ and H⁺ ions increases. After immersion in Cs₂SO₄ solutions the frequency change was higher than that expected taking into account the density and viscosity. The effects observed can be explained by the specific adsorption of Cs⁺ ions on the Pt surface, which competes with the hydrogen adsorption. At more positive potentials than the potential of zero charge (pzc) a desorption of the Cs⁺ ions starts. In this potential region both Cs⁺ and HSO₄^? ions are adsorbed at the platinum surface. In the double layer region the mass change caused by the desorption of Cs⁺ ions and the starting adsorption of sulfate ions compensates each other.     

Domain mobility and rotational tunnelling of hydrogen in graphite intercalates

The rotational tunnelling spectrum of hydrogen molecules adsorbed in the ternary intercalate C₂₄Cs(H_s)_x reveals the presence of at least four sorption sites whose occupancy changes with the degree of filling. The striking changes in the tunnelling spectrum at high filling are explained by the mobility of the caesium ions at 77 K and a reconstruction of the layer lattice.     

长链烷基季铵盐插层氧化石墨的结构变化

基于改进的Hummers法制备氧化石墨(GO),并以长链烷基季铵盐(C_nTAB)对其进行插层处理;通过改变C_nTAB的链长、浓度,得到系列C_nTAB/GO插层复合物。采用XRD和元素分析对产物的最大底面间距及C_nTAB插入量进行表征。结果表明,随着C_nTAB链长的增长、C_nTAB浓度的增大,C_nTAB/GO插层复合物的最大底面间距逐渐增大。C_nTAB通过离子键作用和疏水键作用插入到GO层间,在GO片层上的吸附规律符合修正型(Modified)Langmuir模型,即C_nTAB以单分子层吸附在GO片层上。根据C_nTAB/GO插层复合物最大底面间距及C_nTAB插入量的变化规律分析,得出C_nTAB在GO层间的排布模式有单层平躺模式、类双层平躺模式、单层倾斜模式和单层直立模式。     

长链烷基季铵盐插层氧化石墨的结构变化

基于改进的Hummers法制备氧化石墨(GO),并以长链烷基季铵盐(CnTAB)对其进行插层处理;通过改变CnTAB的链长、浓度,得到系列CnTAB/GO插层复合物。采用XRD和元素分析对产物的最大底面间距及CnTAB插入量进行表征。结果表明,随着Cn TAB链长的增长、CnTAB浓度的增大,CnTAB/GO插层复合物的最大底面间距逐渐增大。CnTAB通过离子键作用和疏水键作用插入到GO层间,在GO片层上的吸附规律符合修正型(Modified)Langmuir模型,即CnTAB以单分子层吸附在GO片层上。根据CnTAB/GO插层复合物最大底面间距及CnTAB插入量的变化规律分析,得出CnTAB在GO层间的排布模式有单层平躺模式、类双层平躺模式、单层倾斜模式和单层直立模式。     

Simulation of Coadsorption of Inorganic Ions and Organic Molecules on Electrodes

Electrode coadsorption of two components, one of which reduces the dense-layer capacitance and the other leaves it unchanged, is examined within the framework of Alekseev–Popov–Kolotyrkin model supplemented by Frumkin isotherm. The adsorption behavior of model systems 0.1 M NaF + n-C₄H₉OH and 0.1 M NaCl + n-C₄H₉OH is analyzed, and the results of calculations are compared with experimental data. It is shown that, in choosing the electrical variable for the adsorption of components that reduce the dense-layer capacitance, the preference should be given to the potential drop in the dense layer. It is concluded that the height of anodic maximum in C vs. E curves can serve as a criterion of the lateral interaction between coadsorbed particles at the known parameters of individual adsorption.     

Effect of Alkyl Chain Length on the Interfacial Dilational Properties of Hydroxy-Substituted Alkyl Benzenesulfonates

Dilational rheological behaviors of adsorption layers of three surfactants, sodium 2-hydroxy-3,5-dioctyl benzene sulfonate (C₈C₈), sodium 2-hydroxy-3-octyl-5-decyl benzene sulfonate (C₈C₁₀), and sodium 2-hydroxy-3-octyl-5-dodecylbenzenesulfonate (C₈C₁₂) formed at air–water and decane–water interfaces, have been investigated as a function of concentration and frequency (0.002–0.1 Hz) by the oscillating bubble/drop method. The experimental results show that the dilational moduli of hydroxy-substituted alkyl benzenesulfonates are obviously higher than those of the common surfactants, because the interfacial interactions between alkyl chains are improved drastically by the unique arrangement of C₈C₈ molecules at the interface. However, the moduli at the decane–water interface are much lower than those at the surfaces because decane molecules will insert into the surfactant molecules adsorbed at the interface and destroy the interactions between alkyl chains. With an increase in the number of carbon atom of 5-alkyl, the surface dilational modulus decreases because the orientation of the surfactant molecules at the surface varies from parallel to tilt. On the other hand, the diffusion-exchange process dominates the interfacial behavior and the interfacial modulus improves with the increase in the length of the alkyl chain.     

Analysis of the differential capacity of the mercury | simple alcohol solution interface

A method of analysing the differential capacity developed by Amokrane and Badiali is applied to investigations of the structure of the interface between mercury and simple aliphatic alcohols. Capacitance data for methanol, ethanol, n-propanol and n-butanol are discussed. The solvent contribution C_s is obtained by subtracting the contribution C_M due to the metal from the inner layer capacitance C_i according to the procedure proposed by Amokrane and Badiali. The C_S values are found to be very similar for all aliphatic alcohols studied. The C_S-σ_M curves have a similar shape with a single maximum at a positive charge density and a characteristic inflection at a negative charge density. The effect of ions in the various supporting electrolytes on the C_S-σ_M profiles was also studied.