9992
J. Phys. Chem. C 2007, 111, 9992-10002
Electrochemical Investigation of Hemispherical Microdroplets of N,N-Didodecyl-N′,N′-diethylphenylenediamine Immobilized as Regular Arrays on Partially-Blocked Electrodes: A New Approach to Liquid|Liquid Voltammetry Denise Rayner,† Nicole Fietkau,† Ian Streeter,† Frank Marken,‡ Benjamin R. Buckley,§ Philip C. Bulman Page,§ Javier del Campo,| Roser Mas,| Francesc Xavier Mun˜ oz,| and Richard G. Compton*,† Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom, Department of Chemistry, UniVersity of Bath, Bath BA2 7AY, United Kingdom, Department of Chemistry, Loughborough UniVersity, Leicestershire, LE11 3TU, United Kingdom, and Centro Nacional de Microelectro´ nica, IMB-CNM. CSIC, Campus de la UniVersidad Auto´ noma de Barcelona, Bellaterra 08193, Spain ReceiVed: March 16, 2007; In Final Form: April 20, 2007
A regular array of identically sized microdroplets of 2.5 µm radius of the water-insoluble liquid N,N-didodecylN′,N′-diethylphenylenediamine (DDPD) is immobilized on the nonconducting hydrophobic polymer blocks of a gold partially blocked electrode. Cyclic voltammetric and chronoamperometric measurements for the oxidation of the DDPD microdroplets immersed in an aqueous solution are then recorded for different electrolytes (NaClO4, NaCl, NaBr, NaNO3, Na2SO4, and NaF). Specifically, the cyclic-voltammetric measurements allow us for the first time ever to observe a pre-peak, which can be interpreted as the movement of charge across the surface of the hemispherical droplets before the bulk material of the droplets gets oxidized. Conversion of the whole bulk material in all droplets is obtained by chronoamperometry. The resulting currenttime responses show Cottrellian diffusion at sufficiently short times and are modeled by simulating diffusion through the droplet revealing complex behavior, which is likely to be related to anion dehydration and/or tight and weak ion-pair formation.
1. Introduction Electrochemical processes at microdroplet-modified electrodes immersed in an aqueous solution present an interesting modeling challenge, which yields important insights into a plethora of chemical and biochemical phenomena.1-12 The majority of studies of oil microdroplets in the past have focused on the oxidation of N,N,N′,N′-tetraalkyl-para-phenylenediamines (TAPDs), partially due to their readily discernible redox moiety and their hydrophobic character given by long n-alkyl chains attached to the main redox body.13-19 To maintain electroneutrality within the microdroplets, upon oxidation of the TAPD to the radical cation TAPD+•, anion uptake from, or cation expulsion into the aqueous solution takes place. In particular, Schro¨der et al. showed that the potential of insertion of these ions is controlled by the Gibbs energy of transfer of the electrolyte anion.19 For anions for which the free energy of transfer is low, such as hexafluorophosphate, perchlorate, thiocyanate, and nitrate, anion uptake upon electroxidation takes place, whereas for anions with high free energies of transfer, such as fluoride and sulfate, the radical cation is expelled into the aqueous solution to maintain electroneutrality. Also, much attention has been given to immobilized organic-oil droplets, in which a redox-active solid was dissolved.20-25 * Corresponding author. E-mail:
[email protected]. Tel: +44(0) 1865 275 413. Fax: +44(0) 1865 275 410. † Oxford University. ‡ University of Bath. § Loughborough University. | Campus de la Universidad Auto ´ noma de Barcelona.
Three different types of mechanistic models for the redox conversion of microdroplets at the electrode surface have been considered in previous computational studies and are depicted in Figure 1.9,10,26 In the first case, the redox conversion is assumed to commence at the internal electrode|oil interface. This model requires that electrolyte ions partition into the oil droplets and assumes that no polarization across the oil droplet or across the oil|electrolyte interface occurs (Figure 1a). In the second model, the electrochemical process is assumed to begin at the oil|electrolyte interface accompanied by fast electron conduction across this interface with diffusion of cation-anion pairs as the dominant transport mechanism (Figure 1b). In the third model, the electrochemical reaction is thought to commence at the oil|electrolyte|electrode, at the so-called three-phase boundary. This model assumes that the electron conduction across the oil|electrolyte interface is slow and that anion insertion only occurs in the region of the three-phase boundary (Figure 1c). Various experiments have been designed to illustrate that the electron transfer occurs at the oil|electrolyte or electrode|electrolyte|oil interface. Shi et al. reported evidence for the electron transfer at the oil|electrolyte interface for the case of continuous thin-film deposits of organic solvents.23,27 To localize the site of electron transfer at the three-phase boundary, experiments at droplet-modifed electrodes are undertaken in unsupported organic phases. Marken et al. showed that the oxidation of yellow THPD microdroplets immobilized on a siliconised ITO and immersed in aqueous 0.1 M NaClO4 solution took place around the edges of the microdroplets by simply “viewing” the electrochemically induced color change through an optical
10.1021/jp0721301 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007
A New Approach to Liquid|Liquid Voltammetry
J. Phys. Chem. C, Vol. 111, No. 27, 2007 9993
Figure 2. Structure of N,N-didocecyl-N′,N′-diethylphenylenediamine (DDPD).
Figure 1. Schematic representation of the three mechanistic models for the electro-insertion of anions into microdroplets. (a) Reaction commencing at the internal electrode|oil interface, (b) reaction starting at the oil|electrolyte interface accompanied by fast electron conduction across this interface, and (c) slow conduction of electrons across the oil|electrolyte interface and a reaction commencing from the threephase boundary oil|electrolyte|electrode.
microscope.13
Schro¨der et al. confirmed this by using color imprints on paper after various electrolysis times.17 Wadhawan et al. then used the deposition of metallic silver to “fingerprint” the site of electrolysis and used scanning-electron microscopy to view the results.28 Also, the concentration profile of the formation of ferrocinium ions within immobilized ferrocene/ nitrobenzene droplets was monitored by using a 20 µm platinum microelectrode.22 There is also evidence from surface coverage experiments and methods based on a geometric analysis of the voltammograms.10,15,24,25,28-32 Electrode-transfer processes at the liquid|liquid interface, i.e., across the droplet surface, were only reported to take place if the nonaqueous droplets are supported in some way. For these cases, it was shown that electrolyte ions partition into the organic phase from the aqueous phase. The most common method was based on a liquid|liquid protonation step.12,17,29,33 Amatore et al. observed that the variation of peak currents for electrodes modified with dendrimer molecules with redox sites at their surface was linear with respect to the square root of the scan rate, even though the dendrimers were shown to be immobilized on the electrode surface and hence expected to behave like simple adsorbed species.34 It was therefore postulated that there was a charge “hopping” mechanism between the redox sites on the dendrimer surface, and cyclic voltammetry was employed to probe this effect. Thompson et al. then built on this work by modeling electron hopping as a diffusional process constrained to the surface of anthraquinone-derivatized graphite powder and glassy-carbon spheres.35,36 The effect of having arbitrary electrode kinetics was then investigated, and the phenomena observed in chronoamperometric and voltammetric responses were rationalized in terms of the facility of divergent or convergent diffusion over a spherical surface. This was an important effect since the diffusion was constrained to the surface of the sphere and explained why the nonsymmetry in voltammetry was much more pronounced for the sphere and
Figure 3. Schematic illustrating the modification of a partially blocked electrode (PBE) with hemispherical microdroplets. (a) Unmodified PBE and (b) DDPD-modified PBE. Note that the DDPD droplets have contact with the electrode which is the source of electrons.
inverted hemisphere geometries than for the hemispherical particle, as well as rationalizing the point at which the nonsymmetry appears with increasing sweep rate. Here, we present a new method for liquid|liquid voltammetry: the hydrophobic nature of a partially blocked electrode is used to immoblise a regular array of identically sized microdroplets of the water-insoluble liquid N,N-didodecyl-N′,N′diethylphenylenediamine (Figure 2) on the electrode surface (Figure 3). Cyclic-voltammetric and chronoamperometric measurements are then recorded in different background electrolytes: sodium perchlorate, sodium chloride, sodium bromide, sodium nitrate, sodium sulfate, and sodium fluoride. The cyclicvoltammetric measurements allow us for the first time ever to observe a pre-peak, which can be interpreted as the movement of charge across the surface of the hemispherical droplets, before the bulk material of the droplets gets oxidized. Additionally, we interpret the chronoamperometric currents as the oxidation of the bulk of the droplets. This oxidative current is then modeled by simulating diffusion through the droplet. 2. Numerical Simulation The current response of a droplet of DDPD in a potentialstep experiment may be modeled by considering the species in eq 1.
A h B + e-
(1)
kb
B {\ }C k f
Species A, the DDPD, undergoes a single-electron oxidation at the surface of the droplet. The product of this oxidation, species
9994 J. Phys. Chem. C, Vol. 111, No. 27, 2007
Rayner et al.
(∆T)k ) γk(∆T)0
TABLE 1: Dimensionless Parameters parameter
expression
concentration concentration radial coordinate time forward rate constant backward rate constant
b ) [B]/[A]bulk c ) [C]/[A]bulk R ) r/rd T ) Dt/rd2 Kf ) kfrd2/DB Kb ) kbrd2/DB
TABLE 2: Boundary Conditions boundary initial conditions (T ) 0) subsequent conditions (T > 0)
R
