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Potential-dependent morphological change of n-hexadecane small droplets on a Au(1 1 1) electrode in aqueous solution: A voltammetric and electrochemical fluorescence microscopic study
Abstract
A mesoscopic view of active droplets of an alkane on a Au(1 1 1) electrode was described using the results of voltammetric and in situ electrochemical fluorescence imaging measurements. In situ fluorescence imaging studies revealed that the morphology of an adlayer of liquid n-hexadecane (HD) on a Au(1 1 1) electrode surface is reversibly driven by potential, forming micro-droplets with greater height of the droplet's top at more negative potentials. Spreading of HD to be a continuous liquid film never took place even around pzc and even with the amounts of HD ranging from 10 to 10,000 monolayer equivalent. The droplets at negative potentials were smaller than 50 μm in diameter. The total Au electrode area occupied by HD was small even at positive potentials so that quasi-reversible redox reaction of ferri/ferro-cyanide in the solution phase was still observed. The change of the droplet height by the reshaping with the electrode potential was repeatable, and the change took place beyond the double layer thickness region.
When a small immiscible oil droplet is placed on an electrode surface in a surfactant aqueous solution, a drastic decrease of the interfacial differential capacitance of the electrode is observed. To clarify this behavior at a molecular level, interplay of a n-hexadecane (HD) droplet with a dodecyl sulfate anion (DS⁻) adlayer on an Au electrode was described using the results of the measurements of cyclic voltammograms, potential-dependent contact angle, and attenuated total reflection-surface enhanced infrared reflection absorption spectra (ATR-SEIRAS). When DS⁻ adsorbs alone on a Au(1 1 1) electrode, it forms an adlayer of a hemi-micellar structure, which changes to an interdigitated bilayer at positive potentials. When HD stays alone on the electrode surface as droplets, it does not spread as an oil liquid film but an ordered monolayer of HD is formed around droplets as found in the present ATR-SEIRAS study. When coexisting, HD 1 μL droplet showed stronger tendency to spread on a Au(1 1 1) electrode surface than when HD alone there, because DS⁻ adsorption took place at both HD/water and the electrode/water interfaces. When the electrode surface with many HD microdroplets (< 50 μmφ) was immersed in Na-DS (SDS) solution, we found that formation of a mixed adlayer consisting of HD and DS⁻ significantly lowered the interfacial differential capacitance down to 5 μF cm⁻². ATR-SEIRAS revealed that the alkyl phase of this mixed adlayer was more liquid-like than DS⁻ adlayer alone and more solid-like than HD droplet alone. At positive potentials, interdigitated bilayer of DS⁻ was formed regardless of whether HD droplets were present on the electrode surface. At very positive potentials, HD droplet can spread over the DS⁻ adlayer.
Electrowetting of a Au/aqueous solution interface with hexadecane (HD) was largely affected by specific adsorption of Br⁻ on the Au surface. The adsorption distinctly changed the potential dependence of the contact angle (θ) of HD droplets on a Au(1 1 1) electrode surface in line with electrocapillary relationship. Measurements of θ as a macroscopic observable, being sensitive to the atomic level change of the electrode surface such as surface reconstruction and Br⁻ adsorption, allowed us to monitor the state of the Au/aqueous solution interface as demonstrated by pinpointing the Br⁻ desorption potential. The amplitude of potential-controlled reshaping of a HD 1.0 μL droplet was enhanced by specific adsorption of Br⁻. The specific adsorption also affected HD microdroplets (<50 μm diameter ≈ 33 pL volume) in the same way as 1.0 μL droplets as revealed by in situ electrochemical fluorescence imaging measurements. Overall, ionic adsorption provides us with opportunities of fine control of the dynamics of oil droplet in an electrode potential range narrower than 1.5 V.
Extensive computer experiments have been conducted in order to shed light on the macroscopic shear flow behavior of liquid n-hexadecane fluid under isobaric-isothermal conditions through the nonequilibrium molecular dynamic methodology. With respect to shear rates, the accompanying variations in structural properties of the fluid span the microscopic range of understanding from the intrinsic to extrinsic characteristics. As drawn from the average value of bond length and bond angle, the distribution of dihedral angle, and the radius distribution function of intramolecular and intermolecular van der Waals distances, these intrinsic structures change with hardness, except in the situation of extreme shear rates. The shear-induced variation of thermodynamic state curve along with the shear rate studied is shown to consist of both the quasiequilibrium state plateau and the nonequilibrium-thermodynamic state slope. Significantly, the occurrence of nonequilibrium-thermodynamic state behavior is attributed to variations in molecular potential energies, which include bond stretching, bond bending, bond torsion, and intra- and intermolecular van der Waals interactions. To unfold the physical representation of extrinsic structural deformation, under the aggressive influence of a shear flow field, the molecular dimension and appearance can be directly described via the squared radius of gyration and the sphericity angle, R(g)(2) and ϕ, respectively. In addition, a specific orientational order S(x) defines the alignment of the molecules with the flow direction of the x-axis. As a result, at low shear rates, the overall molecules are slightly stretched and shaped in a manner that is increasingly ellipsoidal. Simultaneously, there is an obvious enhancement in the order. In contrast to high shear rates, the molecules spontaneously shrink themselves with a decreased value of R(g)(2), while their shape and order barely vary with an infinite value of ϕ and S(x). It is important to note that under different temperatures and pressures, these three parameters are integrated within a molecular description in response to thermodynamic state variable of density and rheological material function of shear viscosity.
Few applications of ultramicroelectrodes for the investigation of chemical reactivity are presented. The first example describes how the combination of data obtained at ultramicroelectrodes can be used together with data obtained at classical electrodes to determine the apparent electron consumption of an electrochemical process within the time scale of microelectrolytic techniques. The second shows how submicrosecond electrochemistry can be used to determine rate constants that require usually fast spectroscopic techniques to be determined. The third example is an application of electrochemistry in the absence of supporting electrolyte to the determination of the Collman’s reagent standard potential. The last example describes how electrochemical data obtained in a solvent of extremely low dielectric constant can be used to discuss the mechanism of oxidative addition at zerovalent palladium.
Electrochemical methods were combined with redox-active surfactants to actively control the motions and positions of aqueous
and organic liquids on millimeter and smaller scales. Surfactant species generated at one electrode and consumed at another
were used to manipulate the magnitude and direction of spatial gradients in surface tension and guide droplets of organic
liquids through simple fluidic networks. Solid microparticles could be transported across unconfined surfaces. Electrochemical
control of the position of surface-active species within aqueous films of liquid supported on homogeneous surfaces was used
to direct these films into periodic arrays of droplets with deterministic shapes and sizes.
Four linear terarylene molecules (i) 4-nitro-terphenyl-4''-methanethiol (NTM), (ii) 4-nitro-terphenyl-3'',5''-dimethanethiol (NTD), (iii) ([1,1';4',1''] terphenyl-3,5-diyl)methanethiol (TM) and (iv) ([1,1';4',1''] terphenyl-3,5-diyl)dimethanethiol (TD) have been synthesized and their self-assembled monolayers (SAMs) have been obtained on polycrystalline gold. NTM and NTD SAMs have been characterized by X-Photoelectron Spectroscopy, Kelvin Probe measurements, electrochemistry and contact angle measurements. The terminal nitro group (-NO2) is irreversibly reduced to hydroxylamine (-NHOH), which can be reversibly turned into nitroso group (-NO). The direct comparison between NTM/NTD and TM/TD SAMs unambiguously shows the crucial role of the nitro group on electro-wetting properties of polycrystalline Au. The higher grade of surface tension related to NHOH has been successfully applied for basic operations of digital µ-fluidics, such as droplets motion and merging.
Abstract The use of a single crystal gold bead electrode is demonstrated for characterization of self-assembled monolayers (SAM)s formed on the bead surface expressing a complete set of face centered cubic (fcc) surface structures represented by a stereographic projection. Simultaneous analysis of many crystallographic orientations was accomplished through the use of an in-situ fluorescence microscopic imaging technique coupled with electrochemical measurements. SAMs were prepared from different classes of molecules which were modified with a fluorescent tag enabling characterization of the influence of electrical potential and a direct comparison of the influence of surface structure on SAMs adsorbed onto low index, vicinal and chiral surfaces.. The assembly of alkylthiol, Aib peptide and DNA SAMs are studied as a function of the electrical potential of the interface revealing how the organization of these SAMs depend on the surface crystallographic orientation, all in one measurement. This approach allows for a simultaneous determination of SAMs assembled onto an electrode surface onto which the whole fcc stereographic triangle can be mapped, revealing the influence of intermolecular interactions as well as the atomic arrangement of the substrate. Moreover, this method enables study of the influence of the Au surface atom arrangement on SAMs which were created and analyzed, both under identical conditions, something that can be challenging for the typical studies of this kind using individual gold single crystal electrodes. Also demonstrated is the analysis of a SAM containing two components prepared using thiol exchange. The two component SAM shows remarkable differences in the surface coverage which strongly depend on the surface crystallography enabling estimates of the thiol exchange energetics. In addition, these electrode surfaces enable studies of molecular adsorption onto the symmetry related chiral surfaces since more than one stereographic triangle can be imaged at the same time. The ability to observe a SAM modified surface that contains many complete fcc stereographic triangles will facilitate the study of the single and multicomponent SAMs, identifying interesting surfaces for further analysis.
The fluorescence decays in mono- and multilayer Langmuir–Blodgett (LB) films of 5-(4-N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a) deposited on gold-evaporated glass substrates were measured to explore various channels for excitation energy transfer processes. The decays in the 1-layer LB films of porphyrin 338a separated from gold surface by LB film of arachidic acid were also measured as a function of the thickness of the spacer, i.e. thickness of the LB films of arachidic acid. Consistent with our previous steady-state fluorescence study, the expectation values of the lifetime obtained by fitting the decay curves with two exponential functions for the LB films indicate that at the short separation distance, the energy transfer from the porphyrin molecules to the gold surface overshadows other processes, including intra- and interlayer energy transfers. The deviation of energy transfer rate from Chance et al. (CPS) prediction of inverse distance cubed in our case may be interpreted by assuming the presence of additional excitation energy decay pathways in the LB films of porphyrin 338a on the gold-evaporated glass substrates.
Our electrochemical system is a hemispherical droplet of nitrobenzene including ferrocene, which is mounted on a glassy carbon electrode in a sodium sulfate aqueous solution. It is composed of a hemispherical oil ∣ water interface, a disk-shaped oil ∣ electrode interface, a water ∣ electrode interface and a ring oil ∣ water ∣ electrode interface. The voltammetric current of the droplet showed two anodic waves for the oxidation of ferrocene. The current at 0.5 V was proportional to the radii of the droplet and concentration of ferrocene, and was independent of the potential sweep rate as well as the concentration of sulfate. These facts indicate that the electrode reaction should occur at the thin ring on the electrode into which the water and the oil phase merge. A model of a microband electrode was suggested and was applied to the limiting current to evaluate the width of the three-phase interface. The evaluated width, 0.23 μm, is much larger than the molecular size.
In this paper, two new thiols, [4-(9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6a) and [4-(2-nitro-9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6b), were synthesized, and self-assembled monolayers (SAMs) of these thiols were formed on gold electrodes. The structure and surface properties of molecular films were investigated by contact angle measurements and attenuated total reflectance infrared spectroscopy (ATR-FTIR). The blocking behavior of Au-6a and Au-6b SAMs was examined with cyclic voltammetry in the presence of redox probes such as K3Fe(CN)6, Ru(NH3)6Cl3 and ferrocene. Electrochemical measurements revealed that the voltammetric behavior of the redox probes was dependent on the nature of the probe molecules, the electrolytic solution composition and the monolayer structure. The optical properties of the SAMs were studied by steady-state and time-resolved fluorescence spectroscopy. It was obtained that the fluorescence emission bands of Au-6a and Au-6b monolayers were red-shifted and broadened compared to those of free thiols in solution as well as significant reduction at their emission intensities. The fluorescence decay profiles of 6a and 6b monolayers were described by a monoexponential function. In order to determine the possible deactivation mechanism between the metal support and photoexcited molecules, spectroelectrochemical steady-state fluorescence and lifetime measurements on the Au-fluorophore electrodes were also performed as a function of the applied potential. These results indicate that the fluorescence quenching occurs via an energy transfer mechanism from the excited molecules to the gold substrate.
The wettability of low-energy liquid surfaces by a series of alkanes has been determined by measuring the relevant surface tensions, interfacial tensions, and spreading pressures. Comparisons are made with solid surfaces for which these data cannot be measured directly. These data are used to test a number of theories currently being advanced in the literature. Questions such as the validity of Young's equation with zero contact angle, the linear relation between cos θ and the surface tensions of the wetting liquids, the relation between critical surface tensions and surface tensions of solids, and the magnitude of spreading pressures of nonwetting liquids on low-energy surfaces are examined critically.
Typical cyclic voltammograms at the three low-index faces of gold are given and discussed comparatively. The influences of various experimental conditions or parameters, e.g. temperature, concentration and nature of the electrolyte, ohmic potential drop, potential limits, etc. are described and related to the atomic surface structures. These findings are recommended as a starting point for further studies on these faces and for obtaining a more satisfactory intercomparison of results from one laboratory to another.
The adsorption of an insoluble monolayer of 12-(9-anthroyloxy) stearic acid (12-AS) onto a single crystal gold electrode has been described. The spreading of the insoluble surfactant onto the metal | solution interface of the gold electrode was initially investigated with the help of cyclic voltammetry and differential capacity. Electrochemical studies indicate that the film of 12-AS molecules spreads onto the metal surface at potentials close to the potential of zero charge (pzc) and desorbs from the electrode surface at potentials that are sufficiently negative. The spreading of the film and its subsequent desorption are repeatable. The 12-AS molecule is a surfactant dye which may be used to perform spectroelectrochemical experiments. Electroreflectance spectroscopy, fluorescence spectroscopy and measurements of the light scattered by the desorbed surfactant molecules were employed to determine the mechanism by which molecules of the insoluble surfactant spread onto and desorb from the metal | solution interface of the gold electrode. With the help of the spectroelectrochemical experiments we have demonstrated that the repeatable potential-induced desorption and adsorption (spreading) of the insoluble molecules involves formation of micelles (or similar molecular organizations) in the subsurface region and spreading of the micelles onto the electrode surface.
In this paper quantitative studies of the adsorption of diethylether on the three low-index faces (111), (100) and (110) of the gold electrode are presented. The equilibrium charge densities at the gold electrode were determined from combined chronocoulometric and admittance measurements. The adsorption parameters such as free energy of adsorption ΔGA, limiting Gibbs excess Γmax, lateral interaction constant a, etc. were calculated and compared with earlier results relative to the mercury electrode surface. These adsorption parameters were next discussed in terms of the properties characteristic for the nature (liquid, solid) and the structure (crystallographic orientation) of the surfaces investigated. Under the assumption that: (1) diethylether adsorption is a water-substitution process, and (2) the ether-metal interactions are weak and change slightly with the nature and the structure of the surface, the free energies of adsorption ΔGA0 were used as a probe of the surface hydrophilicity. The following series of increasing hydrophilicity was proposed:Au(111)≦Hg≦Au(100)<Au(110) and the earlier literature estimations of gold hydrophilicity were critically discussed.
The lifetime of 3B3u pyrazine near Ag surfaces has been measured down to a separation of ∼35 Å, where it has decreased by a factor of ∼600 from the isolated molecule value. There is no evidence for saturation of the energy transfer rate. The technique and apparatus are described. Numerical solutions to the classical double interface theory of energy transfer show energy transfer rates should be sensitive to adsorbed gas on top of the excited molecule.
Through the application of a potential between a water droplet on a hydrophobic coating and the underlying conductive substrate, we have shown control over the contact angle of the droplet. The same phenomenon can be used to control capillary rise of a liquid in a coated capillary over several centimeters in a short time. Interesting applications such as a temporary improvement of adhesion (drying of paint), electrical control of liquid lenses, and patternwise and dynamic filling of a capillary array are in sight.
Molecular structure of monolayers formed at the interface between a Au(111) surface and solutions containing n-alkanes has been studied by in situ scanning tunneling microscopy at room temperature. For increasing CnH2n+2 length from n = 10 up to 50 with even n values, we found that rectangular and tilted arrangements of alkanes within the self-organized layers alternate. This alternation is related to the periodic decrease of the sliding force for molecules displaying a length close to mT (m-integer), where T is the period of commensurability between the CH2-CH2-CH2 period along alkyl chains and the interatomic distance along Au 110. The observation of rectangular lamellae for all the alkane monolayers grown at higher temperature supports this model.
The reduction of Fe(CN)(6)(3-) at the Au(111) electrode coated with a thin film of 4-pentadecylpyridine (C15-4-Py) was investigated. The C15-4-Py molecules form a multilayer at the Au(111) surface which may be repeatedly adsorbed and desorbed by varying the electrode potential. At E > -0.2 V (SCE) the film has a very compact structure. Its capacity is lower than 1 mu F cm(-2) and it exhibits strong blocking behavior with respect to the reduction of Fe(CN)(6)(3-). At potentials -0.6 V (SCE) < E < -0.3 V (SCE) the film is less rigid. Its capacity increases to about 2 mu F cm(-2) and it is more porous. The rate of Fe(CN)(6)(3-) reduction at the film covered electrode is characteristic of nonlinear diffusion to active pores separated by inactive areas. The average pore size changes between 10 mu m and about 30 mu m and the pore separation varies between 200 and 500 mu m for -0.6 V (SCE) < E < -0.3 V (SCE). In this range the pore density changes from 100 to 6 x 10(3) pores/cm(2).
Electrochemical desorption of a patterned, self-assembled monolayer (SAM) of an organothiol on a gold surface can be used to control the shape of drops of liquid hexadecanethiol (HDT) on the surface. The contact angle of HPT drops on a thin, transparent film of gold (100 Angstrom thick) under an aqueous electrolyte solution could be varied from 128 +/- 2 degrees when the gold was held at -1.7 V (vs Ag pseudoreference) to 37 +/- 2 degrees when the gold was held at 0.0 V. Switching between these two limiting contact angles could be performed rapidly, reversibly, and reproducibly using this system. These drops could act as lenses and could be used as an optical switch which focuses/defocuses light. An array of 1-nL drops could be formed by differential wetting of a gold surface patterned with methyl- and carboxylic acid-terminated organothiol, and the contact angles of drops in this array could be switched in ca. 50 ms. Alternatively, drops could be confined to an array of gold pads on a conducting glass and switched in a similar fashion.
In situ scanning tunneling microscopy revealed the formation of the two-dimensional (2D) crystals of n-alkanes (CnH2n+2, n = 12−17) on a Au(111) surface in neat liquid at room temperature. The molecules were adsorbed on the gold surface with their molecular axis parallel to the surface plane. The molecular rows of even- and odd-numbered alkanes ran in the nearest-neighbor (NN) atomic direction and the next-nearest-neighbor atomic direction of the gold surface, respectively. The molecular axis was oriented close to the NN direction of the gold surface in both odd- and even-numbered alkanes. Although there are two NN directions with respect to the direction of the bridging row of the herringbone structure due to the reconstruction of the Au(111) surface with crossing angles of 30° and 90°, the molecular axis was preferentially oriented in the NN direction with a crossing angle of 30°. The 2D crystal of alkanes was not formed on the iodine-modified Au(111) surface, confirming that the molecule−substrate interaction played an important role in forming the 2D crystal of alkanes.
Electrovariable optical components based on liquid/liquid interfaces encompass microfluidic lenses and mirrors, fiber-optic switches, and display elements. These devices function on the basis of the electrowetting effect, where a liquid interface is deformed by an applied electric field. Existing electrovariable lenses use electrowetting on dielectrics; however, because of voltage losses across the dielectric layer typically a voltage variation of 60 V is required to achieve a full focal range. To date the lowest recorded voltage for shape deformation in display elements is 25 V, which is not ideal for portable devices. Herein we investigate the properties of an alternative system, based on the interface between two immiscible electrolytic solutions (ITIES). This system shows promise, reducing the voltage requirement for a substantial electrowetting response by 2 orders of magnitude. Presented are the first experimental demonstration of ITIES-based ultra-low-voltage electrowetting and a preliminary study of its dynamics.
Electrowetting (wetting under the influence of an applied electric field) of three fluoropolymer surfaces (amorphous Teflon, DuPont) by electrolyte solutions was studied with the sessile drop method. The electrowetting curve (contact angle/potential) is analogous to the electrocapillary curve (surface tension/potential) and may be described by a combination of the Young and Lippmann equations. The influence of the electrical double layer at the polymer/solution interface has been neglected in the past because the overall interfacial capacitance is mainly determined by the capacitance of the insulating polymer layer. We demonstrate that for some surfaces a systematic deviation occurs at positive potentials. This departure from the constant capacitance regime is attributed to double layer effects, namely, the adsorption of hydroxide and halide anions. The pH, ionic strength, and polymer composition can all influence electrowetting behavior.
Insoluble alkanes have been observed by electrochemical scanning tunneling microscopy (STM) to form ordered monolayers at the Au(111)/0.1 M HClO4 solution interface. Hexadecane molecules self-assemble into ordered layers that are stable over the potential range from 0.15 VSCE to 0.55 VSCE, on both reconstructed and unreconstructed Au(111) surfaces under electrochemical control. The hexadecane molecules appear as 2.2 nm long and 0.45 nm wide rods, suggesting an extended conformation. STM images show that a reversible order−disorder transition can be induced by moving the electrode potential positive or negative of the stable potential region (0.15 VSCE to 0.55 VSCE). The hexadecane molecules aggregate immediately after lifting of the surface reconstruction by a positive potential step to 0.65 VSCE. Reversible disappearance of the ordered alkane structure is also observed when the potential is stepped below 0.15 VSCE. The molecules resume an ordered lamellar structure upon return of the substrate to the stable potential region. A critical role for the aqueous solvent in inducing the phase transition is proposed.
Self-assemblies of n-hexadecane and n-hexatricontane on the Au(111) surfaces in their neat liquid or solution have been studied by using scanning tunneling microscopy (STM). Ordered monolayers are observed on the reconstructed Au(111) surfaces, while no overlayer could be observed on the unreconstructed surface at room temperature. The reconstructed surface is found to be perfect for the n-alkanes to self-assemble on it, because the alkane monolayer geometrically matches well to the reconstructed surface. The reconstruction stabilizes the self-assembled monolayer on the Au(111) surface, which facilitates the observation of STM.
Extensive nonequilibrium molecular dynamics simulations have been carried out for liquid decane, hexadecane, and tetracosane at densities corresponding to atmospheric pressure and near ambient temperatures. The strain-rate-dependent viscosity has been obtained for strain rates ranging over several orders of magnitude. At high strain rate, the viscosities for all alkanes studied here have similar values and exhibit similar power-law shear-thinning behavior with a slope between about -0.40 and -0.33. Accompanying this shear thinning is the onset of orientational order and the alignment of the alkane molecules with the flow direction. The alignment angle tends to 45 degrees at very low strain rate and is significantly smaller at high strain rate, This suggests that the chains substantially align in the flow direction and that the dominant motion at high strain rate is the sliding of the chains parallel to the flow. At low strain rate, the shear viscosity shows a transition to Newtonian behavior. The Newtonian viscosity can be obtained from the plateau value of the shear viscosity at the lowest strain rates calculated from the nonequilibrium molecular dynamics simulation (NEMD). This is demonstrated by comparing the viscosity of decane obtained by extrapolating the NEMD simulation with an independent calculation using the standard Green-Kubo method. The transition from the non-Newtonian regime to the Newtonian regime is also correlated with the disappearance of orientational order and with the longest relaxation time of the liquid alkanes simulated. (C) 1996 American Institute of Physics.
The structure of self-assembled monolayers of long n-alkane molecules deposited on an Au(111) surface has been studied by in situ scanning tunnelling microscopy at 300 K. These monolayers are formed on Au(111) either at the interface between gold and a solution of hexatriacontane (C36H74) in tetradecane, or by ultrahigh vacuum (UHV) deposition of octatriacontane (C38H78).At the liquid/solid interface, the monolayer consists of a close-packed arrangement of lamellae separated by troughs. Perfect side-to-side packing of C36H74 molecules, the long axis of which stays parallel to Au 〈110〉 directions, forms each lamella. Lamella present two different widths (4.8 ± 0.1 and 4.5 ± 0.1 nm) determined by the molecular orientation with respect to troughs. In contrast, the monolayers obtained after UHV deposition at 300 K show poor organization with meandering lamellae and troughs that present strong variations in width. Nevertheless, all the C38H78 molecules stay parallel to the 〈110〉 directions of Au(111). Annealing such monolayers at 350 ± 5 K yields a structure that looks like the one observed at the liquid/solid interface.These results show a directional epitaxy relationship perpendicular to the 〈110〉 directions of Au(111) in a monolayer of C36H74 and C38H78 molecules. We discuss the origin of the differences observed between the molecule packing at the liquid/solid interface and the one in monolayers built in UHV at 300 K. Copyright © 2000 John Wiley & Sons, Ltd.
IntroductionFluorescence Microscopy and Fluorescence ProbesFluorescence near Metal SurfacesDescription of a Fluorescence Microscope for Electrochemical StudiesElectrochemical Systems Studied with Fluorescence MicroscopyConclusions and Future Considerations
The spectro-electrochemical behavior of carbonate and bicarbonate ions at the Au(111) electrode surface was studied using the infrared reflection absorption spectroscopy (IRAS). An absorption band caused by the adsorbed carbonate ions was observed in the wavenumber region of 1425–1511 cm−1 both in Na2CO3 and NaHCO3 solutions. It was concluded that the adsorbed carbonate ions co-ordinate with the electrode surface in the unidentate state with their symmetry axis normal to the substrate. This orientation is retained in the whole potential region where carbonate ions adsorb on the electrode surface in contrast to the behavior of the carbonate ions adsorbed on the Pt(111) electrode surface.
By describing studies of three prototypical surfactants with similar hydrophobic tails but differently charged headgroups, this review provides a summary of the rich phase behavior of soluble surfactant molecules at electrified interfaces. With the use of electrochemical, scanning probe microscopy, and neutron scattering techniques we have been able to fully explore the adsorption and surface aggregation of these molecular systems. Furthermore, we have been able to provide compelling evidence of electric field-driven phase transitions in these surfactant films and their aggregated structures. Cumulatively, our results demonstrate that the electrical state of a surface (namely surface charge or applied potential) plays an integral role in determining the morphology of surfactants at solid interfaces. Unlike other aggregate shape determining factors such as the surfactant packing parameter, the electrical parameter can readily be adjusted in situ, providing a tunable means to control films of soft condensed matter.
The sections in this article are
Introduction
Thermodynamic Aspects
Phase Transitions and Order Parameter
Adsorption Isotherms and Lateral Interactions
Kinetics of 2 D Phase Formation and Dissolution
Diffusion, Adsorption, Autocatalysis
Nucleation and Growth Mechanisms
Nucleation
Growth
Coupling of Nucleation and Growth
Dissolution of 2 D Condensed Monolayers
Examples
Phase Transitions in Anionic Adlayers
Introduction
Phase Formation in Halide Adlayers
Phase Transitions in Adlayers of Oxoanions
Phase Transitions in UPD —Adlayers
Introduction
Underpotential Deposition of Copper Ions on Au ( hkl )
Underpotential Deposition of Copper Ions on Pt ( hkl )
Underpotential Deposition of Lead on Ag ( hkl )
Phase Transitions in Organic Monolayers
General Aspects
Thermodynamic Aspects
Kinetic Aspects
Case Study I—Coumarin at the Mercury–Aqueous Electrolyte Interface
Case Study III —2,2 ′ ‐Bipyridine (2,2 ′ ‐ BP ) on Au ( hkl )
Conclusion and Outlook
Acknowledgment
Epi-fluorescence imaging of the adsorption and desorption of a 3 mol% 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate/octadecanol layer from/to a Au(111) electrode \ electrolyte interface is demonstrated. Since fluorescence near a metal surface is quenched, only molecules away from the metal surface could be observed. At adsorption potentials, no fluorescence was observed. An increase in the fluorescence and the number of desorbed particles was observed when scanning the potential to the negative desorption values. Scanning the potential back to the adsorption value resulted in a decrease in the fluorescence and the number of desorbed particles. A significant hysteresis in the adsorption/desorption processes was observed and the existence of an intermediate pre-adsorbed form of the surfactant was demonstrated and characterized. The morphology of the desorbed layer was strongly influenced by the potential program used to effect desorption. Desorption by potential step created aggregates that were larger than those created by potential sweep. For this system, the potential controls the separation or distance between the aggregates or desorbed species and the metal surface in a repeatable way. (C) 2002 Published by Elsevier Science B.V.
Switching of wettability is achieved in situ, which is a challenge of materials science. Generally, changing liquid droplet is required to ex situ study the wettability response before and after the surface given a treatment, in the sense that the liquid impregnation in the surface structures is irreversible. Herein, an in situ wettability switch is achieved by utilizing the same liquid droplet to characterize the dynamic wettability when the conducting polymer is being stimulated. The oil droplet is facilitated to escape from the nanoscale traps through electrochemically tuning surface composition and surface micro/nanostructures, permitting a reversible and rapid transition between partly wetting and superantiwetting state. This in situ switch is promising for integration into a microfluidic system for the control of the liquid droplet's motion.
The major factors determining the potential-driven phase changes of the films of 4-pyridyl terminated water-insoluble neutral surfactants on a Au(111) electrode were described using the results of dc and ac voltammetric measurements. In total six surfactants were used to elucidate the effects of electronic property of the 4-pyridyl head group, hydrogen bonding ability between amide groups, direction of the amide bond, and bulkiness around the head group upon the potential dependent reorientation and adsorption-desorption processes.Comparison of the behavior of pentadecyl 4-pyridyl ether (C15-O-Py) and 1-pyridin-4-yl-hexadecan-1-one (C15-(C=O)-Py), possessing respectively, the highest and lowest excess electronic charge on the pyridyl nitrogen among the six surfactants, revealed that the latter exhibits more distinct reorientation of pyridyl group corresponding to phase transition between condensed and loose-packed states and narrower potential region of the compact film formation. Introduction of amide group in the alkyl chains of the surfactants reduced the packing of the adsorbed film in the potential region of the compact film formation,as evidenced by an increase of the differential capacitance. It was found that the existence of amide group facilitates the potential-driven phase transition of closely packed film from a condensed state to a loosely packed state, whereas it suppresses the transition of loosely packed film. The concomitant steric effect was also discussed
In this paper, we prepared "dual-parallel-channel" shape-gradient surfaces, on which water droplets can reversibly and orientedly move between two adjacent pools under the guidance of an external voltage. Furthermore, it is found that the motion speed is governed by several parameters, including bath condition, gradient angle, and the working voltage. In this self-transportation process of water droplets, the external voltage works like a traffic light, which can give "moving", "stopping", "turning" and "straight-going" signals to the water droplets.
The problem of the decay rates for molecules at rough metallic surfaces is considered, where the classical electromagnetic energy-transfer theory of Chance, Prock, and Silbey for a flat surface is generalized to the case of a rough boundary. A dynamical theory is constructed through the combination of the Sommerfeld antenna theory and the integral equation formalism of Maxwell's equations at rough boundaries established mainly by Maradudin, Mills, and Agarwal. Perturbative solutions are obtained and numerical results are given with reference to a shallow sinusoidal grating surface. The results, when compared with those obtained previously from the application of the image field theory, show that this latter theory can be very inaccurate for cases involving highly conducting substrates or large molecule-surface distances, consistent with previous observations for the case of flat surfaces.
We report the observation by scanning tunneling microscopy of the reentrant self-organization of n-alkanes (C(n)H(2n+2)) in monolayers adsorbed on Au(111) induced by a variation of the chain length. In the investigated range of lengths ( 10</=n</=38 with even n values), the presence of self-assembled monolayers which consist of a close-packed arrangement of molecules lying flat on the surface is evidenced except for alkanes with 18</=n</=26. The unexpected transition between ordered and disordered phases is related to the mismatch between Au(111) lattice and the CH2-group period along the chain.
The self-assembled monolayers (SAMs) of normal alkanes (n-C(n)H(2n+2)) with different carbon chain lengths (n=14-38) in the interfaces between alkane solutions (or liquids), and the reconstructed Au (111) surfaces have been systematically studied by means of scanning tunneling microscopy (STM). In contrast to previous studies, which concluded that some n-alkanes (n=18-26) can not form well-ordered structures on Au (111) surfaces, we observed SAM formations for all these n-alkanes without any exceptions. We find that gold reconstruction plays a critical role in the SAM formation. The alkane monolayers adopt a lamellar structure in which the alkane molecules are packed side-by-side, to form commensurate structures with respect to the reconstructed Au (111) surfaces. The carbon skeletons are found to lie flat on the surfaces, which is consistent with the infrared spectroscopic studies. Interestingly, we find that two-dimensional chiral lamellar structures form for alkanes with an even carbon number due to the specific packing of alkane molecules in a tilted lamella. Furthermore, we find that the orientation of alkane molecules deviates from the exact [011] direction, because of the intermolecular interactions among the terminal methyl groups of neighboring lamellae; this results in differences of molecular orientation between mirror structures of adjacent zigzag alkane lamellae. Structural models have been proposed, that shed new light on monolayer formation.
The reversible transportation of droplets was realized by spatiotemporal control of the wetting gradient. The surface wetting was reversibly regulated by using electrochemical reactions of the ferrocenyl (Fc) alkanethiol monolayer, and the wetting gradient was generated by the application of the in-plane bias voltage to the substrate. The back-and-forth motion of the wetting boundary, where the surface changed from wetting to repulsive, sequentially caused a droplet unidirectional spreading and shrinking on the surface. These unidirectional deformations resulted in the net transport of the droplet in an inchwormlike manner. The droplet moved backward when the direction of the in-plane bias voltage was reversed.
Substrate-supported planar lipid bilayer membranes are attractive model cellular membranes for biotechnological applications such as biochips and sensors. However, reliable fabrication of the lipid membranes on solid surfaces still poses significant technological challenges. In this study, simultaneous surface plasmon resonance (SPR) and surface plasmon fluorescence spectroscopy (SPFS) measurements were applied to the monitoring of adsorption and subsequent reorganization of phospholipid vesicles on solid substrates. The fluorescence intensity of SPFS depends very sensitively on the distance between the gold substrate and the fluorophore because of the excitation energy transfer to gold. By utilizing this distance dependency, we could obtain information about the topography of the adsorbed membranes: Adsorbed vesicles could be clearly distinguished from planar bilayers due to the high fluorescence intensity. SPSF can also incorporate various analytical techniques to evaluate the physicochemical properties of the adsorbed membranes. As an example, we demonstrated that the lateral mobility of lipid molecules could be estimated by observing the recovery of fluorescence after photobleaching. Combined with the film thickness information obtained by SPR, SPR-SPFS proved to be a highly informative technique to monitor the lipid membrane assembly processes on solid substrates.
Self-assembled monolayers (SAMs) of alkanols (1-C(N)H(2N+1)OH) with varying carbon-chain lengths (N = 10-30) have been systematically studied by means of scanning tunneling microscopy (STM) at the interfaces between alkanol solutions (or liquids) and Au(111) surfaces. The carbon skeletons were found to lie flat on the surfaces. This orientation is consistent with SAMs of alkanols on highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces, and also with alkanes on reconstructed Au(111) surfaces. This result differs from a prior report, which claimed that 1-decanol molecules (N = 10) stood on their ends with the OH polar groups facing the gold substrate. Compared to alkanes, the replacement of one terminal CH3 group with an OH group introduces new bonding features for alkanols owing to the feasibility of forming hydrogen bonds. While SAMs of long-chain alkanols (N > 18) resemble those of alkanes, in which the aliphatic chains make a greater contribution, hydrogen bonding plays a more important role in the formation of SAMs of short-chain alkanols. Thus, in addition to the titled lamellar structure, a herringbone-like structure, seldom seen in SAMs of alkanes, is dominant in alkanol SAMs for values of N < 18. The odd-even effect present in alkane SAMs is also present in alkanol SAMs. Thus, the odd N alkanols (alkanols with an odd number of carbon atoms) adopt perpendicular lamellar structures owing to the favorable interactions of the CH3 terminal groups, similar to the result observed for odd alkanes. In contrast to alkanes on Au(111) surfaces, for which no SAMs on an unreconstructed gold substrate were observed, alkanols are capable of forming SAMs on either the reconstructed or the unreconstructed gold surfaces. Structural models for the packing of alkanol molecules on Au(111) surfaces have been proposed, which successfully explain these experimental observations.
A single-crystal Au(111) electrode modified with an adsorbed layer of 1-octadecanol (C18OH) or oleyl alcohol (OLA) in pure or mixed composition was characterized using electrochemical and in situ fluorescence microscopy. Cyclic voltammetry and differential capacitance measurements revealed a repeatable, potential-induced adsorption/desorption process of the surfactant to/from the electrode surface while charge density and film pressure measurements indicated quasi-ideal mixing of the two adsorbed alcohols. A layer less defective than pure C18OH was created with incorporated OLA. Optical characterization was accomplished using epi-fluorescence microscopy combined with electrochemistry (electro-fluorescence microscopy) through the incorporation of two fluorescent probes into the adsorbed surfactant layer. Since molecular luminescence is quenched by a nearby metal, fluorescence was only observed when the fluorescent dye/alcohol layers were desorbed and therefore separated from the metal surface. When desorbed, the structure of the alcohol layers were similar in character, revealing aggregated features which did not change in morphology over numerous desorption/re-adsorption cycles. We have also used the electro-fluorescence technique to estimate the distance separating the metal and desorbed surfactant and believe that the molecules are displaced from the electrode surface by a distance not more than 40 nm.
(Figure Presented) One-off analysis: Quantitative, kinetic, and spatial monitoring of single biological events involving vesicular secretion such as exocytosis may be achieved by combining electrochemistry with fluorescence microscopy at indium tin oxide (ITO) transparent microelectrodes. This method paves the way for real-time tracking of the intracellular fate of vesicles while monitoring precisely their dynamics of fusion.
(Graph Presented) Wetting properties: Contact angle measurements of an oil droplet put on H2SO4|Pt(111) and H2SO 4|Au(111) electrochemical interfaces have revealed that particular atomic arrangements of adsorbed water molecules determine the macroscopic wetting properties.
The reductive and oxidative desorption of a BODIPY labeled alkylthiol self-assembled monolayer (SAM) on Au was studied using electrochemical methods coupled with fluorescence microscopy and image analysis procedures to monitor the removal of the adsorbed layer. Two SAMs were formed using two lengths of the alkyl chain (C10 and C16). The BODIPY fluorescent moiety used is known to form dimers which through donor-acceptor energy transfer results in red-shifted fluorescence. Fluorescence from the monomer and dimer were used to study the nature of the desorbed molecules during cyclic step changes in potential. The reductive desorption was observed to occur over a small potential window (0.15 V) signified by an increase in capacitance and in fluorescence. Oxidative readsorption was also observed through a decrease in capacitance and a lack of total removal of the fluorescent layer. Removal by oxidative desorption occurred at positive potentials over a broad potential range near the oxidation of the bare Au. The resulting fluorescence showed that the desorbed molecules remained near the electrode surface and were not dispersed over the 20 s waiting time. The rate of change of the fluorescence for oxidative desorption was much slower than the reductive desorption. Comparing monomer and dimer fluorescence intensities indicated that the dimer was formed on the Au surface and desorbed as a dimer, rather than forming from desorbed monomers near the electrode surface. The dimer fluorescence can only be observed through energy transfer from the excited monomer suggesting that the monomers and dimers must be in close proximity in aggregates near the electrode. The fluorescence yield for longer alkyl chain was always lower presumably due to its decreased solubility in the interfacial region resulting in a more efficient fluorescence quenching. The oxidative desorption process results in a significantly etched or roughened electrode surface suggesting the coupling of thiol oxidative removal and Au oxide formation which results in the removal of Au from the electrode.
Dynamic behaviors of molecular assemblies and nano-substances at electrified interfaces
- Sagara