•Contact angle (θ) change of an n-hexadecane droplet was tracked in water.•Br− adsorption lowered the Au(1 1 1) electrode/water (S/W) interfacial tension.•Br− adsorption raised θ of the droplets of 1 μL and < ca. 33 pL.•A plot of θ vs. surface charge enabled us to pinpoint Br− desorption potential.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.Download high-res image (127KB)Download full-size image

•n-Hexadecane (HD) on the Au(1 1 1) electrode does not exist as a continuous liquid film but as small droplets.•The size of HD droplet deposited on a Au(1 1 1) surface by a touching method was smaller than 50 μm in diameter.•The height of the HD droplet top rises beyond the double layer thickness on the electrode surface around − 0.55 V.•The HD droplets are not the obstacle to the quasi-reversible redox reaction of solution-phase ferri/ferro-cyanide.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.

•Two-step phase changes of dodecyl sulfate on a Au(1 1 1) electrode showed electroreflectance (ER) signals.•ER spectral feature was of the electroreflectance of Au.•Phase change from interdigitated bilayer to hemi-micellar phase was found to be slower than reverse process.•Second harmonic ER response was recorded.•Kinetics of the transitions was discussed semi-quantitatively based on a simplified model.Electroreflectance (ER) methods (potential-modulated UV-visible reflectance spectroscopy) were used to track the potential dependent phase changes of an n-dodecyl sulfate (DS−) adlayer on a Au(1 1 1) electrode surface. In the aqueous solution containing Na-DS (SDS) at a concentration lower than cmc, two step non-faradaic phase changes, which have been well known so far, were clearly observed as ER signals, although the surfactant is colorless. On the ER spectral structure basis, the origin of the signal was the electroreflectance from Au, which relies on the change of surface free electron density on the Au surface. The second harmonic frequency ER signal helped us gain perspective on the extent of non-linearity and kinetics of the reflectance change in response to the potential modulation. Results of the ER measurements enabled us to find that adsorptive hemi-micelle formation-desorption at -0.18 V (Ag/AgCl/sat-KC1) is faster process than the phase change process between interdigitated bilayer and the hemi-micellar phase at +0.50 V. The phase change from the interdigitated bilayer to the hemi-micellar phase exhibited a nucleation-growth type nature in response to a potential step, and this phase change appeared slower than the reverse change.

An X-ray crystal structure of a viologen type ionic liquid compound, bis(trifluoromethanesulfonyl)imide (TFSI) salt of butyl heptyl viologen [C4VC7·(TFSI)2], with a melting point (mp) of 52 °C was examined. A cis conformer of TFSI (cis-TFSI), known to be less stable than trans-TFSI, was found in the crystal in a 1:1 coexistence with trans-TFSI. In contrast, all TFSI anions found in dimethylviologen TFSI salts [C1VC1·(TFSI)2] were of trans conformation. The lower mp of C4VC7·(TFSI)2 compared to that of C1VC1·(TFSI)2 originates in a smaller electrostatic lattice energy and disordered structure of the side chains of C4VC7. Any charge transfer interaction causing a visible region absorption band was not observed in the solids of either C4VC7·(TFSI)2 (dihedral angle of bipyridinium ϕ = 37.2°) or C1VC1·(TFSI)2 (ϕ = 9°).

A redox-active ionic liquid (IL), 1-butyl-1′-heptyl-4,4′-bipyridinium bis(trifluoromethanesulfonyl)imide, has been synthesized and its transport processes were investigated. The conductivity and viscosity of the IL, as well as the diffusion coefficients of its components were studied over a 50 °C wide temperature range: for the diffusivity studies, both the pulsed-gradient spin–echo (PGSE)–NMR technique and voltammetric measurements have been applied. The measured data are presented in the paper and are compared to each other. It was found that the diffusion coefficients determined by means of NMR and chronoamperometry measurements are, within the range of experimental error, equal—and they are (in accordance with other ionic liquid studies) higher than what the conductivity or viscosity measurements indicate. The results are interpreted in the light of the existing theories. The measured diffusion coefficients and bulk conductivities can be well interrelated based on the “ionicity” concept (that is, by treating the ionic liquid as a weak electrolyte). In agreement with the empirical Walden rule, a direct comparison between the measured conductivities and viscosities is also possible, for which a hole conduction model is utilized. Based on the fact that both the electrochemical and the NMR measurements yield practically the same diffusion coefficients in the system, there is no evidence that interpretations based in other redox-active IL systems on “homogeneous electron transfer” apply to the system studied here.

Two phase change processes of diphenyl viologen (dPhV) on a Au(111) electrode in KCl and KBr aqueous solutions were described using the results of voltammetric, electroreflectance (ER), and electrochemical scanning tunneling microscopic (EC-STM) measurements. Both processes exhibited sharp spikelike voltammetric responses. In KCl solution, the phase change at 0.30 V versus Ag/AgCl/saturated KCl was found to be a nonfaradaic order–disorder phase transition, from an ordered adlayer of dPhV dication (dPhV2+) with coadsorbed Cl– at more positive potentials than 0.30 V to a gaslike phase at less positive potentials. The faradaic reaction at −0.09 V was found to be the transition from the gaslike phase to a condensed monolayer of dPhV•+. The EC-STM images of the condensed monolayer showed stripe patterns of rows of π–π stacked dPhV•+. Almost the same set of two processes was observed in KBr solution but not in KF solution. In KF solution, although two voltammetric responses were observed, the peaks were small and broad, indicative of sluggish adsorption state changes of individual dPhV cations. Taken together, specific adsorption of coexistent anions is of critical importance for the occurrence of the sharp nonfaradaic phase transition.

•First order faradaic phase transition of dibenzyl viologen (dBV) at HOPG was studied.•At high Br− concentration, dBV exhibits two-step phase transitions.•As the first reduction step, a dBV+ Br− mesophase emerges at [Br−] > 75 mM.•As the second, phase transition from the mesophase to a 2D condensed phase occurs.•Remarkable effect of Br− upon the transition enabled us to model the mesophase.We found that dibenzyl viologen (dBV) on an HOPG electrode undergoes a two-step first order faradaic phase transition at high concentrations of bromide ion (Br−). Results of voltammetric and electroreflectance measurements were used to describe the mechanism of the two-step transition processes. When [Br−] > 180 mM, the transition step at less negative potential was ascribed to a phase transition between a gas-like adsorption layer of dBV dication (dBV2+) and a mesophase of dBV radical cation (dBV+). Most likely, the mesophase is a two-dimensional (2D) ordered phase composed of co-adsorbed dBV+ and Br− where both are in direct contact with the HOPG surface. The transition step at more negative potential was ascribed to a phase transition between the dBV+ Br− mesophase and a 2D condensed phase of dBV+. In the condensed phase being denser than the mesophase, dBV+ molecules are π-stacked due to face-to-face interaction between bipyridinium radical cations. This transition step involves also a reduction process of dBV2+ to dBV+ followed by its incorporation into the condensed phase. The two-step transition was not observed in KCl solution of any concentration, either in KBr solution of [Br−] < 75 mM. Other viologens examined, including benzyl–heptyl viologen, did not exhibit such a two-step transition but single-step one. The nature of the transition, especially in the [Br−] range from 75 to 180 mM, was closely analyzed.

Redox behavior of diphenyl viologen (dPhV) on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode was described using the results of voltammetric and electroreflectance measurements. Its characteristics were compared to those of dibenzyl viologen (dBV), which undergoes the first-order faradaic phase transition. Unlike dBV, dPhV-dication (dPhV2+) was found to take a strongly adsorbed state on an HOPG surface. This is due to much stronger π–π interaction between phenyl rings of dPhV2+ and HOPG surface than between benzyl groups of dBV2+ and the surface. The participation of this strongly adsorbed dPhV2+ in the redox process can be avoided by (1) a shorter than ∼3 min time period elapsing from touching a freshly cleaved HOPG surface to dPhV solution until the start of potential scan, (2) complete equilibration at the electrode potentials at which superficial dPhV molecules are fully reduced, or (3) multiple cyclic potential scanning to repeat oxidation–reduction of adsorbed species. Even in such conditions, although voltammograms of thin-layer electrochemistry for the surface-confined dPhV•+/dPhV2+ couple are obtained with peak widths being as narrow as those of dBV, it is not the first-order phase transition. The participation of strongly adsorbed dPhV2+ molecules results in another new voltammetric feature with a broader peak. The film formed by strongly adsorbed dPhV2+ was hydrophilic, whereas dBV2+ does not form such a film but only a gas-like layer. Measurements using X-ray photoelectron spectroscopy confirmed that the film consists of dPhV2+ with coexistent water. These results reveal a typical case that delicate interaction balance among V2+, V•+, and electrode surface determines whether the two-dimensional first-order transition takes place or not.

Phase transitions of an adsorption layer of dibenzyl viologen (dBV) as a typical diaryl viologen on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode are described using voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and electroreflectance (ER) spectroscopy. A monolayer redox process at less negative potential than the bulk redox process was found to be the first-order faradaic phase transition between a gaslike adsorption layer of dication (dBV2+) and a 2D condensed monolayer of radical cation (dBV•+). Comparison of the results of cyclic voltammetry and potential step chronoamperometry was made with those of heptyl viologen (HV), which also undergoes a faradaic phase transition of the first order. It suggested that the contribution of intermolecular π–π interaction between benzyl groups of dBV to the phase transition is minor and apparently equivalent to interchain interaction between the heptyl chains of HV. In situ EC-STM images of the 2D condensed monolayer demonstrated stripe patterns of the rows of dBV•+ molecules forming 3-fold rotationally symmetric domains. The results of the ER measurements also revealed that the orientation of the longitudinal molecular axis of the bipyridinium moiety of dBV•+ molecules lying flat on the HOPG electrode surface, most likely with a side-on configuration.

Tetraoctylammonium bromide stabilized gold nanoparticles (TOAB-AuNPs) attached to 1,6-hexanedithiol (HDT) modified Au electrode was used for the simultaneous determination of paracetamol (PA) and ascorbic acid (AA) at physiological pH. The attachment of TOAB-AuNPs on HDT modified Au surface was confirmed by attenuated total reflectance (ATR)-FT-IR spectroscopy and atomic force microscope (AFM). The ATR-FT-IR spectrum of TOAB-AuNPs attached to the HDT monolayer showed a characteristic stretching modes corresponding to –CH2 and –CH3 of TOAB, confirming the immobilization of AuNPs with surface-protecting TOAB ions on the surface of the AuNPs after being attached to HDT modified Au electrode. AFM image showed that the immobilized AuNPs were spherical in shape and densely packed to a film of ca. 7 nm thickness. Interestingly, TOAB-AuNPs modified electrode shifted the oxidation potential of PA towards less positive potential by 70 mV and enhanced its oxidation current twice when compared to bare Au electrode. In addition, the AuNPs modified electrode separated the oxidation potentials of AA and PA by 210 mV, whereas bare Au electrode failed to resolve them. The amperometry current of PA was increased linearly from 1.50 × 10−7 to 1.34 × 10−5 M with a correlation coefficient of 0.9981 and the lowest detection limit was found to be 2.6 nM (S/N = 3). The present method was successfully used to determine the concentration of PA in human blood plasma and commercial drugs.

Short time immobilization of densely packed tetraoctylammonium bromide (TOAB) stabilized gold nanoparticles (AuNPs) were established on a Au electrode modified with a self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDMT). The quartz crystal microbalance experiment showed densely packed TOAB–AuNPs single layer formation on both SAMs was achieved within 20 min. AFM images demonstrated that the immobilized TOAB–AuNPs on the SAMs were densely packed and the AuNPs film thickness was 6–7 nm. The electronic communication between the immobilized AuNPs and the underlying bulk electrode was confirmed by cyclic voltammetry and electroreflectance spectroscopy. A reversible electron transfer reaction was observed for both [Fe(CN)6]4−/3− and [Ru(NH3)6]2+/3+ at TOAB–AuNPs immobilized on HDT (Au/HDT/AuNPs) and BDMT (Au/BDMT/AuNPs) modified electrodes. The electroreflectance spectra show a red-shifted strong positive-going plasmon resonance bands at 551 nm and 584 nm, respectively, for the Au/BDMT/AuNPs and Au/HDT/AuNPs electrodes. The observed reversible redox response for the solution redox species and red-shifted plasmon resonance bands for the immobilized AuNPs again indicated that the AuNPs were immobilized on the SAMs in a densely packed manner. An advantage of TOAB–AuNPs modified electrode prepared by short time immersion over citrate-stabilized AuNPs modified electrode was demonstrated by the enhanced oxidation of ascorbic acid (AA) at these electrodes. The oxidation of AA was shifted to 90 mV less positive potential with higher oxidation current at TOAB–AuNPs modified electrode when compared to citrate-stabilized AuNPs modified electrode.

Potential dependent behaviors and structures of binary-component bilayers and multiple layers of two different water-insoluble 4-pyridyl-terminated surfactants (C15–O–Py and C15–(CO)–Py, see Fig. 1) on a Au(1 1 1) electrode were investigated by voltammetric techniques. Film preparation was based on repetitive horizontal touching of the electrode to the spread film of the surfactants at a gas/water interface. When bilayer formation was made by sequential touching to the neat spread films of the two different surfactants, the resultant bilayer structure was always of Au(1 1 1)/C15–O–Py/C15–(CO)–Py, being independent of whether the first touching was made with C15–O–Py or C15–(CO)–Py. This reflects the stronger affinity of C15–O–Py for the electrode surface, which works dominantly over mixing. When multiple touching was made to the spread film of two-component mixture at a given mixture ratio, the resultant first layer appeared a C15–O–Py monolayer as long as its monolayer amount was supplied. Further increase in the number of the multiple touchings to the mixed spread film resulted in the formation of C15–O–Py bilayer in the bilayer thickness region of the film. Implications of these results were discussed in regards to the structures and properties of the films. When binary-component bilayers and multi-layers are prepared by horizontal touching methods such as a Langmuir–Schaefer (LS) technique, care must be taken to the fact that the resultant film structure does not always turn out as the procedural deposition order.

The major factors determining the potential-driven phase changes of the films of 4-pyridyl terminated water-insoluble neutral surfactants on a Au(1 1 1) 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–(CO)–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.

Phase transition behavior of two-component viologen adsorption layers at a HOPG electrode was described using the results of voltammetric measurements. In the coexistence of heptyl viologen (HV) and its bis-carboxylated derivative in the solution phase, a well-mixed condensed monolayer of the radical cations was formed at any molar fraction. In sharp contrast, the binary system of HV and butyl viologen (BV) exhibited phase separation in the molar fraction range where BV is saturated in the predominantly formed condensed phase of HV. It was, however, found that this separation, being opposed to the prediction based on the adsorption free energy, occurs only when the time period enough for full condensation of HV is not given. The significant features of phase transition of two-component viologen adsorption layers on a HOPG electrode surface were highlighted in comparison with the formation and reductive desorption of the self-assembled monolayers of alkanethiols.

The state of the heme of myoglobin molecules incorporated in a didodecyldimethylammonium bromide (DDAB) film on a pyrolytic graphite electrode was described using the results of electroreflectance measurements. It was found that the hemes are released from the myoglobin molecules. The ER spectrum of PG electrode|Mb–DDAB film was indistinguishable from the spectrum of PG electrode|hemin–DDAB film, even in the presence of NaBr, but clearly different from PG electrode|imidazole-coordinated hemin–DDAB. These results support the claim of de Groot and coworker [M.T. de Groot, M. Merksx, M.T.M. Koper, J. Am. Chem. Soc. 127 (2005) 16224; M.T. de Groot, M. Merkx, M.T.M. Koper, Electrochem. Commun. 8 (2006) 999]. It is likely that DDAB is not a strong inhibitor of imidazole coordination but acts on the protein, resulting in conformational change and the heme release.