Adsorption of N-Decyl-N,N,N-trimethylammonium triflate (DeTATf), a cationic surfactant, on the Au(111) electrode surface

The adsorption behavior of the cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) on the Au(111) electrode surface was characterized using cyclic voltammetry, differential capacity, and chronocoulometry. The thermodynamics of the ideally polarized electrode have been employed to determine the Gibbs excess and the Gibbs energy of adsorption. The results show that the adsorption of DeTATf has a multistate character. At low bulk DeTATf concentrations, the adsorption state is consistent with the formation of an adsorbed film of nearly flat molecules. At higher concentrations this film may represent a three-dimensional aggregated state. At negative potentials and charge densities close to 0 microC cm-2, the data suggest the formation of a film of tilted molecules oriented with the hydrocarbon tail toward the metal surface and the polar head toward the solution. A surprising result of this study is that DeTATf displays adsorption characteristics of a zwitterionic rather than a cationic surfactant. This behavior indicates that the adsorbed species is an ion pair.     

Electric-field-driven surface aggregation of a model zwitterionic surfactant

Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to describe the electric-field-controlled surface aggregation of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS), a model zwitterionic surfactant, at a Au(111) electrode surface. At concentrations below the critical micelle concentration (CMC), the monomer adsorbs and aggregates at the surface. The charge on the metal (sigmaM) controls the orientation of adsorbed molecules and consequently the film structure. At high negative (sigmaM < -5 microC cm-2) charge densities, a spongy, disordered film is formed in which the polar heads are turned toward the solution. At high positive (sigmaM > +5 microC cm-2) charge densities, a planar film with "blisters" is observed with the polar heads of DDAPS turned to the metal. Hemicylindrical aggregates are observed in the intermediate charge density range (-5 < sigmaM < +5 microC cm-2). At bulk concentrations higher than the CMC, micelles adsorb and the structure of these films is controlled by the fusion of the adsorbed micelles. STM and AFM images provided direct visualization of this field-driven surface aggregation of the zwitterionic surfactant.     

Quaternary ammonium bromide surfactant adsorption on low-index surfaces of gold. 1. Au(111)

The coadsorption of the anionic and cationic components of a model quaternary ammonium bromide surfactant on Au(111) has been measured using the thermodynamics of an ideally polarized electrode. The results indicate that both bromide and trimethyloctylammonium (OTA(+)) ions are coadsorbed over a broad range of the electrical state of the gold surface. At negative polarizations, the Gibbs surface excess of the cationic surfactant is largely unperturbed by the presence of bromide ions in solution. However, when the Au(111) surface is weakly charged the existence of a low-coverage, gaslike phase of adsorbed halide induces an appreciable (~25%) enhancement of the interfacial concentration of the cationic surfactant ion. At more positive polarizations, the coadsorbed OTA(+)/Br(-) layer undergoes at least one phase transition which appears to be concomitant with the lifting of the Au(111) reconstruction and the formation of a densely packed bromide adlayer. In the absence of coadsorbed halide, the OTA(+) ions are completely desorbed from the Au(111) surface at the most positive electrode polarizations studied. However, with NaBr present in the electrolyte, a high surface excess of bromide species leads to the stabilization of adsorbed OTA(+) at such positive potentials (or equivalent charge densities).     

Adsorption of insoluble surfactants at the Au(111)/solution interface

The adsorption of surfactants, which form insoluble monolayers on an aqueous substrate, onto a single crystal gold electrode have been described. Adsorption of this class of surfactants have been characterized using a combination of electrochemistry and Langmuir-Blodgett techniques. We have developed a technique to simultaneously measure the film pressure at the gas-solution (GS) interface and the film pressure of the surfactants that spread to the metal-solution (MS) interface. We have shown that surfactants such as octadecanol and stearic acid, which interact weakly with the metal surface, adsorb at an uncharged MS interface (at the potential of zero charge) and progressively desorb when the electrode surface is charged negatively. The electrode potential (charge density at the metal surface) influences the transfer of the surfactant from the GS interface to the MS interface. The transfer ratio is 1:1 at an uncharged MS interface, and is progressively reduced to zero when the MS interface is charged. We have employed 12-(9-anthroloxy) stearic acid, a surfactant dye molecule, to study the mechanism of potential induced desorption and adsorption of the film of insoluble molecules. With the help of electroreflectance spectroscopy and light scattering measurements, we have shown that if desorbed, the surfactant molecules form micelles (flakes or vesicles) that are trapped under the electrode surface. The micelles spontaneously spread back onto the electrode surface when the charge density at the metal approaches zero. The repeatable desorption and readsorption involve micellisation of the film at negative potentials and spontaneous spreading of the micelles to reform the monolayer at potentials close to pzc.     

Influence of the electric field on a bio-mimetic film supported on a gold electrode

A model biological membrane was formed by fusion of mixed cholesterol and DMPC (dimyristoylphosphatidylcholine) phospholipid vesicles onto a gold-coated quartz support. The gold surface was charged and the influence of the charge at the solid support on the structure and integrity of the phospholipid bilayer was investigated using the specular reflection of neutrons and electrochemical measurements. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence low water content. When the support's surface is negatively charged the film swells and incorporates water due to the field driven poration of the membrane. When the charge density is more negative then -8 microC cm(-2) the bilayer is detached from the metal surface. However, it remains in close proximity to the metal electrode, suspended on a thin cushion of water. The film thicknesses, calculated from neutron reflectivity, have allowed us to determine the tilt angle of the lipid molecules as a function of the support's charge density. The lipid molecules are tilted 55 degrees from the surface normal at zero charge density but become significantly more perpendicular (30 degrees tilt angle) at charge densities more negative than -8 microC cm(-2). The tilt angle measurements are in very good agreement with previous IR studies. This paper describes the highlights of a more in-depth study which is fully described in [1].     

Electrochemical and PM-IRRAS studies of potential controlled transformations of phospholipid layers on Au(111) electrodes

Chronocoulometry and photon polarisation modulation infrared reflection absorption spectroscopy (PM-IRRAS) have been employed to study the fusion of dimyristoylphosphatidylcholine (DMPC) vesicles onto a Au(111) electrode surface. The results show that fusion of the vesicles is controlled by the electrode potential or charge at the electrode surface (sigmaM). At charge densities of -15 microC cm(-2) < sigmaM < 0 microC cm(-2), DMPC vesicles fuse to form a condensed film. When sigmaM < -15 microC cm(-2), de-wetting of the film from the electrode surface occurs. The film is detached from the electrode surface; however, phospholipid molecules remain in its close proximity in an ad-vesicle state. The state of the film can be conveniently changed by adjustment of the potential applied to the gold electrode. PM-IRRAS experiments demonstrated that the potential-controlled transitions between various DMPC states proceed without conformational changes and changes in the packing of the acyl chains of DMPC molecules. However, a remarkable change in the tilt angle of the acyl chains with respect to the surface normal occurs when ad-vesicles spread to form a film at the gold surface. When the bilayer is formed at the gold surface, the acyl chains of DMPC molecules are significantly tilted. The IR spectra have also demonstrated a pronounced change in the hydration of the polar head region that accompanies the spreading of ad-vesicles into the film. For the film deposited at the electrode surface, the infrared results showed that the temperature-controlled phase transition from the gel state to the liquid crystalline state occurs within the same temperature range as that observed for aqueous solutions of vesicles. The results presented in this work show that PM-FTIR spectroscopy, in combination with electrochemical techniques, is an extremely powerful tool for the study of the structure of model membrane systems at electrode surfaces.     

Redox transformations of adsorbed NO molecules on a Pt(100) electrode

The electrochemical behavior of adsorbed NO molecules on a Pt(100) electrode has been studied in perchloric acid solutions by means of cyclic voltammetry. According to the literature data, a saturated NO adlayer with a coverage of ～0.5 monolayers (MLs) is formed under open circuit conditions in an acidic nitrite solution as a result of a disproportionation reaction. The saturated adlayer is stable in the potential range of 0.4–0.9 V vs. a reversible hydrogen electrode in 0.1 M HClO₄. NO molecules are oxidized at 0.9–1.1 V with the formation of adsorbed nitrite anions, and they can be reduced to ammonia at potentials less than 0.4 V. In this paper it has been shown that the adlayer stability depends on the surface coverage and extent of ordering. An unsaturated NO adlayer demonstrates NO ? NH₃ redox transformations at 0.5–0.8 V.     

Porphyrin self-assembly at electrochemical interfaces: role of potential modulated surface mobility

The self-assembly of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) on Au(111) electrodes was investigated. The adlayer structure was found to depend on the electrode potential. At positive potentials (>0.5V(SCE)), a disordered layer of TPyP is formed on the Au(111) electrode. STM images showed that the disordered molecules are immobile. At negative potentials (-0.2V(SCE)), however, the molecules are highly mobile and can no longer be imaged by STM, though they remain on the surface. At intermediate potentials (-0.2 to +0.2V(SCE)), the TPyP formed a highly ordered adlayer. Once the ordered adlayer is formed, it persists even after the potential is stepped to higher values (0.5-0.8 V(SCE)). These results can be explained by the role of potential modulated adsorbate-substrate interaction and surface mobility. This suggests the intriguing prospect of using electrode potential to tune surface interactions and to drive surface processes, e.g., molecular self-assembly, in electrochemical systems.     

Change of Surface Structure upon Foam Film Formation

The charge distribution and coverage with surfactant molecules at foam film surfaces plays an important role in determining foam film structure and stability. This work uses the concentration depth profiling technique neutral impact collision ion scattering spectroscopy to experimentally observe the charge distribution in a foam film for the first time. The charge distribution at the surface of a foam film and the surface of the corresponding bulk liquid were measured for a cationic surfactant solution and the surface excess as well as the electric potential were determined. Describing the internal pressure of foam films by using the electrochemical potential is introduced as a new concept. The foam film can be seen to have a more negative surface charge compared to the bulk liquid surface due to re‐arranging of the surfactant molecules. It is discussed how the change in surface excess and electric potential change the electrochemical potential and the stability of the foam film.     

非水体系苯并咪唑及衍生物吸附的电化学表面增强拉曼光谱研究

应用电化学现场表面增强拉曼光谱(SERS)以及直接电化学合成技术分别研究了非水体系中苯并咪唑及2-巯基苯并咪唑在铜电极表面的吸附行为及其与三苯基膦(pph3)共存的表面过程.在较负电位区间苯并咪唑主要以分子形式吸附在电极表面.在较正电位区间,电极表面生成类高分子(CuBIM)n膜,具有缓蚀作用,对含有pph3的该体系,Cu+首先与pph3配位形成稳定的阳离子,进入溶液之后与BIM配位生成稳定的配合物,导致不能在表面有效地成膜而破坏了苯并咪唑的缓蚀作用.2-巯基苯并咪唑在Cu表面主要通过自组装单层方式在电极表面吸附,且在实验测试的电位区间内,MBI均是以S端与金属表面作用,其吸附取向随电位正移由倾斜逐渐向接近垂直过渡,并在金属表面形成MBI单分子层膜.pph3的加入不影响MBI在Cu电极表面的成膜行为.电化学现场模拟合成及产物结构组成解析为推断表面反应过程提供了直接证据.     

Interaction between a stationary mercury electrode and a hydrogen bubble in an aqueous sodium dodecyl sulphate solution

The stability and rupture of thin liquid films formed from an aqueous solution of Na₂SO₄ (0.05 mol dm⁻³) in the presence of 5 × 10⁻⁵ mol dm⁻³ sodium dodecyl sulphate between a stationary mercury electrode and a hydrogen bubble has been investigated as a function of electrode potential. The electrostatic component of disjoining pressure has been calculated using the results of capacity measurement for the mercury-solution interface. Special attention has been paid to films formed on positively charged mercury surfaces. In this case, despite the positive electrode polarization, the outer Helmholtz plane potential is found to be negative due to the high surface activity of the dodecyl sulphate anion. The van der Waals component of disjoining pressure has been calculated on the basis of a double sheath model of the two interacting surfaces, taking into consideration the orientation of the adsorbed surfactant layers at the interfaces. Calculations of the total disjoining pressure can explain film stability at negative mercury potentials, but do not explain film rupture when the polarization of the mercury is positive. The existence of a hydrophobic attractive interaction is postulated in the latter case.     

Supramolecular nanostructures of 1,3,5-benzene-tricarboxylic acid at electrified Au(111)/0.05 M H2SO4 interfaces: an in situ scanning tunneling microscopy study

The potential-induced adsorption and self-assembly of 1,3,5-benzene-tricarboxylic acid (TMA) was investigated at the electrified Au(111)/0.05 M H2SO4 interface by in-situ scanning tunneling microscopy (STM) and surface enhanced infrared reflection absorption spectroscopy (SEIRAS) in combination with electrochemical techniques. Depending on the applied electric field, TMA forms five distinctly different, highly ordered supramolecular adlayers on Au(111) surfaces. We have elucidated their real-space structures at the molecular scale. In the potential range -0.25 V < E < 0.20 V, planar-oriented TMA molecules form a hexagonal open-ring honeycomb structure, Ia, a hydrogen-bonded ribbon-type phase, Ib, and a herringbone-type phase, Ic, stabilized by directional hydrogen bonding and weak substrate-adsorbate interactions. Interfacial water molecules are being replaced. In 0.20 V < or = E < 0.40 V, e.g., around the potential of zero charge, and at slightly higher coverages, a close-packed physisorbed adlayer of hydrogen-bonded TMA dimers, II, was observed. Further increase of the electrode potential to positive charge densities causes an orientation change from planar to upright. An initially disordered phase, IIIa, transforms into an ordered, stripelike chemisorbed adlayer, IIIb, of perpendicularly oriented TMA molecules. One carboxylate group per molecule is bound to the electrode surface, while the two other protonated carboxyl groups are directed toward the electrolyte and act as structure-determining components of a hydrogen-bonded two-dimensional ladder-type network. Structural transitions between the various types of ordered molecular adlayers are attributed to (hole) nucleation and growth processes.     

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in mixed micelles of zwitterionic and positively charged surfactants

The rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, NBOC, was determined in micelles of N-hexadecyl-N,N,N-trimethylammonium bromide or chloride (CTAB or CTAC), N-hexadecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (HPS), N-dodecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (DPS), N-dodecyl-N,N,N-trimethylammonium bromide (DTAB), hexadecylphosphocholine (HPC), and their mixtures. Quantitative analysis of the effect on micelles on the velocity of NBOC decarboxylation allowed the estimation of the rate constants in the micellar pseudophase, k(m), for the pure surfactants and their mixtures. The extent of micellar catalysis for NBOC decarboxylation, expressed as the ratio k(m)/k(w), where k(w) is the rate constant in water, varied from 240 for HPS to 62 for HPC. With HPS or DPS, k(m) decreased linearly with CTAB(C) mole fraction, suggesting ideal mixing. With HPC, k(m) increased to a maximum at a CTAB(C) mole fraction of ca. 0.5 and then decreased at higher CTAB(C). Addition of CTAB(C) to HPC, where the negative charge of the surfactant is close to the hydrophobic core, produces tight ion pairs at the interface and, consequently, decreases interfacial water contents. Interfacial dehydration at the surface in equimolar HPC/CTAB(C) mixtures, and interfacial solubilization site of the substrate, can explain the observed catalytic synergy, since the rate of NBOC decarboxylation increases markedly with the decrease in hydrogen bonding to the carboxylate group.     

Co-adsorption of CO onto a Ag-modified Pt(111)--restructuring of a Ag UPD layer monitored by EC-STM

The effect of co-adsorption of CO on an underpotentially deposited (UPD) silver monolayer on a Pt(111) single crystal electrode in 0.05 M sulfuric acid is investigated for the first time by means of electrochemical scanning tunneling microscopy (EC-STM). Pure electrochemical experiments suggest that the co-adsorption of CO onto Pt single crystal electrodes previously modified by a monolayer of Ag, forces Ag atoms of the first UPD monolayer into a second adlayer. The present EC-STM studies reveal the formation of a large-area Ag network after the co-adsorption of CO. The resulting Ag nanostructures formed on wide Pt(111) terraces are approximately 0.5 nm high and 10 nm wide. The desorption of the newly formed second Ag adlayer, the oxidation of CO and the desorption of Ag atoms from the first adlayer are monitored by EC-STM and simultaneously detected in the corresponding CVs in three different oxidation peaks. EC-STM images recorded afterwards show the unchanged Pt surface. The presence of Ag on the surface leads to a downward shift of the onset of oxygen adsorption on the Pt(111) surface.     

TPPS和Gemini表面活性剂的复合膜及其手性的研究

研究了一种新的gemini表面活性剂(C12H24-α,ω-(C12H25N＋(CH3)2Br－)2, （简写为C12-C12-C12）和TPPS在气液界面上形成的复合膜及其手性.实验发现,单独C12-C12-C12不能在纯水表面形成稳定的单分子膜，但当亚相中存在TPPS时，可形成稳定的单分子膜.通过水平提拉法将复合膜转移到固体基板上，发现在适当的pH值条件下，TPPS可在复合膜中形成J-聚集体，并且发现，尽管Gemini表面活性剂和TPPS 都 是非手性的，TPPS的J-聚集体表现出强烈的Cotton效应.另外，gemini表面活性剂的两个正电荷中心对TPPS的J-聚集体的手性并不能表现出协同效应.     

Pt电极上CO的同位素取代吸附机理研究

本文依据偶极耦合理论和相干势近似方法,合理选择粗糙电极上吸附分子的频率分布函数、一氧化碳(CO)吸附层的结构参数以及偶极耦合作用常数,对13CO/12CO同位素取代过程记录的红外光谱进行了拟合.研究发现,只有在拟合过程中引入低频CO分子优先取代,就可成功地模拟整个同位素取代过程的红外光谱随表面吸附的13CO/12CO组分的变化,并由此提出了吸附驱动的脱附机理,COad的脱附不是热激发脱附,而是吸附到表面的CO分子为其邻近位置COad的脱附提供能量.伸缩振动频率较低的COad处于台阶或缺陷位等较开阔的位置(尽管其吸附能较高),周围有较大的空间,利于来自溶液的CO分子的吸附,因此在台阶或缺陷位优先发生同位素的取代.     

Adsorption of<Emphasis Type="Italic"> N</Emphasis>-dodecyl-<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethyl-3-ammonio-1-propanesulfonate (DDAPS), a model zwitterionic surfactant,on the Au(111) electrode surface

Electrochemical measurements including cyclic voltammetry, differential capacity, and chronocoulometry have been used to characterize the adsorption behaviors of the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) on the Au(111) electrode surface. The thermodynamics of the ideally polarized electrode have been employed to determine the Gibbs excess and the Gibbs energy of adsorption. The results show that the adsorption of DDAPS has a multistate character. The first two states are observed at potentials close to zero charge. At low bulk DDAPS concentrations, it corresponds to the formation of a film of nearly flat adsorbed molecules. At higher concentrations it is converted into a hemimicellar state. The second state is formed at negative potentials and charge densities close to 0 C cm^–2. It corresponds to a film of tilted molecules oriented with the hydrocarbon tail towards the metal and the polar head toward the solution.Dedicated to Professor Zbigniew Galus on the occasion of his 70^th birthday and in recognition of his many contributions to electrochemistry.     

Electrochemically‐controlled DNA Release under Physiological Conditions from a Monolayer‐modified Electrode

An indium tin oxide (ITO) electrode prepared on a flexible polymeric support was modified with an amino‐silane and then functionalized with trigonelline and 4‐carboxyphenylboronic acid covalently bound to the amino groups. The trigonelline species containing quarterized ammonium group produced positive charge on the electrode surface regardless of the pH value, while the phenylboronic acid species were neutral below pH 8 and negatively charged above pH 9 (note that their pK_a=8.4). The total charge on the monolayer‐modified electrode was positive at the neutral pH and negative at pH>9 (note that 4‐carboxyphenylboronic acid was attached to the electrode surface in excess to trigonelline, thus allowing the negative charge to dominate on the electrode surface at basic pH). Single‐stranded DNA molecules were loaded on the modified electrode at pH 7.0 due to their electrostatic attraction to the positively charged surface. By applying electrolysis at −1.0 V (vs. Ag/AgCl reference) electrochemical oxygen reduction resulted in the consumption of hydrogen ions and local pH increase in the vicinity of the electrode surface. The process resulted in the transition to the total negative charge due to the negative charges formed on the phenylboronic acid species. This resulted in the electrostatic repulsion and release of the loaded DNA. The developed approach allowed the electrochemically‐triggered DNA release not only in the aqueous solutions, but also in human serum solution, thus giving promise for future biomedical applications.     

Adsorption of anionic surfactant on porous and non-porous polyethylene terephthalate films

We study the adsorption of anionic surfactant, sodium dodecyl diphenyloxide disulfonate (SDDD) on three types of polyethylene terephthalate substrates from aqueous solutions of SDDD of different concentrations. Neutral electrolyte (KCl) was added to the solutions to vary the ionic strength. The three types of substrates were: (1) original polymer film, (2) etched non-porous film, which was obtained from pristine film by chemical etching and bears negative charge on the surface, (3) etched porous membranes, which were fabricated from pristine film by ion irradiation and subsequent chemical etching. The membranes have negative charge on the flat surface and on the inner pore walls. The comparison of original and etched nonporous films shows that the negative charge on the flat surface has weak effect on adsorption of the anionic surfactant. The comparison of etched non-porous and porous films shows that the SDDD adsorption on the inner walls of pores is much weaker than on flat surface—even in case the pore radius is significantly larger than the Debye length. This “exclusion” effect strongly depends on ionic strength of solution. For the porous films, the effect of the pore size and shape on the anionic surfactant adsorption is presented and discussed.