J. Phys. Chem. B 2001, 105, 6943-6949
6943
Properties of Microlayers of Ionic Liquids Generated at Microelectrode Surface in Undiluted Redox Liquids. Part II Wojciech Hyk,* Karolina Caban, Mikolaj Donten, and Zbigniew Stojek* Department of Chemistry, Warsaw UniVersity, Pasteura 1, PL-02-093 Warsaw, Poland ReceiVed: February 2, 2001; In Final Form: April 14, 2001
For several simple alcohols such as methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, physical properties of the microlayers of the new-type ionic liquids generated during the electrode processes of these undiluted redox liquids were studied. In all cases, well-reproducible, transport-controlled, anodic voltammograms were obtained at Pt microelectrodes. It has been found that the transport properties of the undiluted alcohols, such as activation energy of diffusion and diffusion coefficients, are strongly affected by the enormously large concentration of ions in the region adjacent to the electrode surface. The results obtained for alcohols were compared to those of diluted 1,1′-ferrocenedimethanol, which compound may be treated as a model redox system. This comparison revealed, in contrast to the Stokes-Einstein relation, that the molecules of each alcohol, while having much smaller size than the molecules of 1,1′-ferrocenedimethanol, diffuse to the microelectrode surface at a similar rate (methanol) or slower (other alcohols). In the sequence from methanol to 1-pentanol the ratio of diffusivities of alcohol and 1,1′-ferrocenedimethanol ranges from 1.16 to 0.62. This may serve as evidence for the formation of a very viscous ionic layer at the electrode surface during the electrolysis of undiluted alcohols. It has also been found that the ionic layer formed during the electrode process of the undiluted alcohol is much more dense than the solvent in the bulk. This difference in densities engenders an extra transport of natural convective (gravitational) character and influences the voltammetric wave heights. For example, at a 25-µm-in-radius electrode, positioned horizontally, natural convection accounts for 10.5, 7, and 5.5% of the total current for methanol, 1-propanol, and 1-pentanol, respectively. The gravitational effect becomes negligible for the electrodes smaller than 5 µm in radius.
Introduction A direct consequence of small size of microelectrodes, and therefore very small currents flowing at such electrodes, is a possibility of doing electrochemical measurements at very high concentrations of substrates, including undiluted redox liquids. The term “undiluted redox liquid” is related to the system where the electroactive species is also the medium of the system studied. A very large concentration of such a substrate together with limited solubility of the supporting electrolyte, which is at least 1 order of magnitude smaller than the concentration of the solvent, results in a large ohmic potential drop, and thus, excludes the use of conventional-size electrodes. It has been found that several commonly used organic solvents and reagents give well-defined voltammograms at microelectrodes. These include the following: nitrobenzene,1,2 benzonitrile,2 aniline and pyrrole,3 methanol and ethanol,4 4-cyanopyridine,5 acetophenone,6 and dimethyl sulfoxide.7 Since a well-defined and transport-controlled plateau of the voltammetric wave is observed for each of these systems, it can be concluded that the limiting conditions are reached. Thus, the concentration of the substrate-medium must drop practically to zero at the electrode surface. This, in turn, implies that a layer consisting of the ionic product and the counterion from supporting electrolyte must be formed at the electrode surface. The formation of such the ionic microlayer and the very large increase in the concentration of ions in the region adjacent to * Corresponding authors. E-mail:
[email protected]. E-mail:
[email protected].
the electrode surface lead to some drastic changes in physicochemical properties of this region: first of all viscosity, species diffusivity, and conductivity are affected. The structure and the properties of the depletion layers in nitrobenzene and acetophenone have been investigated by White and co-workers.8,9 They coupled the electrochemical experiments with other techniques, such as interferometric microscopy8 and elevated pressure.9 Some indirect evidence for the formation of the ionic layer in undiluted redox systems has been presented in our previous paper.10 In that paper, we have shown that the magnitude of the steady-state limiting current of some “model” undiluted systems changes with respect to the size of the counterion (for nitrobenzene) and does not change with respect to the size of the co-ion (for methanol) present in the system. A theoretical model developed by us for the voltammetry of undiluted redox systems allowed us to predict these changes. The aim of this work was to learn more about the ionic layer formed at the microelectrode surface by applying totally different procedures. Five simple alcohols: methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, have been chosen for the experimental investigations. The existence of a very viscous region adjacent to the electrode surface should be reflected in relatively large values of the activation energies of the diffusion of molecules of undiluted substrate. The latter may be determined from the temperature dependencies of the alcohol diffusivities. On the other hand, the generation of the microlayer may lead to changes in the density and, as a result, may induce an extra substrate flux of convective nature. By measuring the dependence of the steady-state limiting current on the orientation
10.1021/jp0104233 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001
6944 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Hyk et al. range from 0 to 180°, at every 10°) at least 4 replicates of voltammograms were recorded. Density Measurements. To evaluate the changes in solvent concentration caused by the increase in temperature, density measurements at various temperatures were carried out using a Model MG-2, ECOLAB densitometer. For each temperature considered five replicates of alcohol density measurement were taken. Chemicals. Alcohols: methanol (MeOH), ethanol (EtOH), 1-propanol (PrOH), 1-butanol (BuOH), and 1-pentanol (PnOH), were of p.a. purity and were purchased from POCh Gliwice (Poland) and Fluka. Lithium perchlorate (LiClO4, supporting electrolyte, suprapure) and 1,1′-ferrocenedimethanol (Fe(C5H4)2(CH2OH)2, 98%) were purchased from Aldrich. All chemicals were used as received. Data Treatment
Figure 1. Cross section of the cylindrical cell used for the “gravitational” measurements. The arrow on the left side shows the direction of rotation of the cell. C, W, and R refer to counter, working, and quasi-reference electrodes, respectively. The vertical arrow denoted as g represents the direction of the earth’s gravitational field.
of the electrode with respect to the earth’s gravitational field, we were able to determine the contribution of natural convection (gravitational convection) to the total transport in the undiluted alcohols. To expose the specific properties of the undiluted systems, all the experimental results were compared to those of diluted 1,1′-ferrocenedimethanol, a model system that gives a transportcontrolled, one-electron anodic wave at platinum microelectrodes in all the solvents employed. Experimental Section Voltammetry. Electrochemical measurements were performed using a Model 283, PARC potentiostat which was controlled via software. Two pieces of platinum foil were used as the counter- and the quasi-reference electrodes, to eliminate a possible leak of ions from the bridge. The platinum working microdisks of 5.3, 10, and 25 µm in radius (Project Ltd., Warsaw) were polished with aluminum oxide powder of various size (down to 0.05 µm) on a wet pad. Before each experiment the working electrodes were briefly polished again and were rinsed with a direct stream of ultrapure water (Milli-Q, Millipore). The electrodes were dried using ethyl alcohol. After the measurements, the electrode surface was inspected optically with an Olympus, Model PME 3, inverted metallurgical microscope. No visible changes in the surface and no signs of crystalline or polymer deposits were found. The positive feedback IR-drop compensation was used only for the logarithmic analysis of the voltammetric wave of undiluted systems. The regular cell was water/glycol-jacketed, and temperature was controlled by a refrigerated circulator (Polystat, Cole Parmer). To minimize the electric noise this cell was kept in an aluminum foil Faraday cage. Another, homemade rotative cell was used for “gravitational” measurements. The rotation of the cell body allowed angle θ between the normal to the electrode surface and the vector of the earth’s gravitational field, g, to be varied from 0 to 360° with a very good precision of 1.0°. The rotative cell is illustrated schematically in Figure 1. The angle θ ) 0° corresponds to the typical orientation of the electrode: facing the bottom of the cell. For each temperature (in the range from -14 to 45 °C, at every 3 °C) and angular position of the microelectrode (in the
The diffusion coefficients and the activation energy of diffusion of the electroactive species were determined from the experimentally measured variables, i.e., limiting current, temperature, and solvent density. Diffusion Coefficients. These parameters for both undiluted (alcohols) and diluted (1,1′-ferrocenedimethanol) redox systems were determined from the expression describing the steady-state limiting current at disk microelectrodes for diffusional transport
IL ) 4nFDcbre
(1)
where n denotes the number of electrons transferred per molecule, F is the Faraday constant, D and cb are the diffusion coefficient and the bulk concentration of the electroactive species, respectively, and re is the radius of microelectrode. The usefulness of the application of eq 1 to the voltammetric responses of undiluted methanol and ethanol was discussed by Ciszkowska and Stojek.4 They found that the height of the anodic voltammetric waves (IL) of undiluted methanol and ethanol is independent of scan rate (V), for relatively small values of V, and then increases slightly with an increase in V. This is exactly what the theory predicts for the processes where the wave height is controlled by the rate of diffusion,11 therefore this behavior may be treated as a proof of the diffusioncontrolled nature of the electrode processes of methanol and ethanol. However, one should note here that in the case of undiluted substrates the parameter D in eq 1 is not just the substrate diffusion coefficient in the solution bulk but rather the transport coefficient of the substrate in the solution layer located next to the electrode surface. This coefficient determines the wave height of an undiluted liquid.10 In this paper the dependence of the limiting current on the scan rate was examined for other alcohols: 1-propanol, 1-butanol, and 1-pentanol. The results obtained are presented in Figure 2. For the scan-rate range from 2 to 100 mV/s the agreement with the theoretical predictions11 is satisfactory (the wave height was practically constant). For higher values of scan rate some deviations are observed. We attribute these deviations to the changes in the viscosity of the ionic layer as the system is departing from the steady state. The theory does not take into account any changes in viscosity at the electrode neighborhood.11 Another uncertainty is related to the number of electrons transferred per molecule in the undiluted alcohols. Such systems resemble solutions of organic compounds in nonaqueous solvents where the number of electrons exchanged per molecule is usually 1.12 To support this assumption we have made the logarithmic analysis of the voltammetric wave (i.e., plot of E
Ionic Liquids Generated at Microelectrode Surface
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6945 2.3, 19.4 and 1.3, and 30 kΩ and 0.23 mol/L for methanol, 1-propanol, 1-butanol, and 1-pentanol, respectively. By rearranging eq 1 and taking into account the changes in solvent concentration (volume) with respect to temperature, one obtains the final formula used for the calculations of D for the undiluted systems:
D)
Msolv IL 4nF red
(2)
where Msolv is the molecular mass of the solvent, and d is density of the solvent at the given temperature. For the diluted systems the respective expression looks as follows:
D)
M ILV(25)d(25) 4nF mred
(3)
where M is the molecular mass of the substrate, m is the mass of the substrate dissolved in volume V(25) of the alcohol at 25 °C, and d(25) is density of the pure alcohol at 25 °C. It is assumed that the changes in the solution volume and mass due to the dissolution of millimolar amounts of either LiClO4 or 1,1′-ferrocenedimethanol are negligible. In other words, the density of the solution bulk at a given temperature is assumed to be equal to the density of the pure solvent. As it can be concluded from eqs 2 and 3, the uncertainty (error) in the determination of diffusivity depends on the uncertainty in the values of the following quantities: limiting current, microdisk radius, density, mass, and volume. Usually, the uncertainty in a measurement is expressed by the standard deviation. Therefore, the calculations of the standard deviation of the diffusion coefficient (s(D)) were done according to the rules of the propagation of random errors. The latter was evaluated using the Taylor series expansion about the mean values of the variables of a given function. If the variables are not correlated the terms containing the second and higher order derivatives can be dropped. This procedure applied to eq 2 leads to the following estimate of s(D) for the undiluted systems: Figure 2. Normalized limiting current plotted versus parameter p defined as (nFVre2/DRT)1/2 for 1-propanol, 1-butanol, and 1-pentanol. Conditions: supporting electrolyte: LiClO4 (c ) 20 mM), scan rate: 2-1000 mV/s, Pt microelectrode radius: 5.3 (fill color: black) and 10 µm (fill color: white), T ) 21 °C.
vs ln[(IL - I)/I] with the slope equal to RT/RnF for a totally irreversible process, where R is the transfer coefficent).13 The term RT/RnF contains two unknowns: R and n. They should be determined independently. However, for undiluted systems it is rather impossible. For n ) 1 and R ) 0.5, which numbers are very probable for the systems studied, the theoretical value of RT/RnF equals 0.051 V at 25 °C. The experimental plots of E vs ln[(IL - I)/I] yielded the slopes of 0.046, 0.049, 0.037, and 0.064 V for methanol,14 1-propanol, 1-butanol, and 1-pentanol, respectively. The numbers obtained indicate that if R is approximately 0.5 then n ) 1. It must be stressed here that the I-E response taken to the analysis cannot be distorted by the IR-drop. Since for undiluted liquids the supporting electrolyte concentration is at least several times smaller than the solvent concentration, even if the saturated solutions are prepared, hence, the appropriate compensation of IR-drop was required in order to do such the analysis. We have used the positive feedback method available in the PARC 283 potentiostat. The values of the uncompensated resistance inserted to the ECHEM software and the concentrations of the added supporting electrolyte were as follows: 8.0 and 2.0,14 9.6 and
s(D) =
Msolv 4nF
x(
) (
) (
2 2 1 IL IL s(IL) + 2 s(re) + s(d) red re d red2
)
2
(4)
where s(IL) and s(d) are the standard deviations of limiting current and density of the solvent at the given temperature, respectively, and s(re) equal to 0.06 µm is the standard deviation of the microdisk radius measurement. For example, for methanol s(IL) ) 0.13 µA and s(d) ) 8.6 × 10-4 g/cm3 at 25 °C. For the diluted systems the respective expression looks as follows:
s(D) =
x(
M 4nF
) (
) (
) (
) (
2
2
)
2
V(25)d(25) L ILd(25) (25) ILV(25) (25) s(I ) + s(V ) + s(d ) + mred mred mred
(
2
2
)
2
ILV(25)d(25) ILV(25)d(25) ILV(25)d(25) s(m) + s(re) + s(d) 2 2 m r ed mre d mred2 (5)
where s(V(25)), s(d(25)), and s(m) are the standard deviations of volume of the 25 mL volumetric flask at 25 °C, density of the solvent at 25 °C, and weighting, respectively.
6946 J. Phys. Chem. B, Vol. 105, No. 29, 2001
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Activation Energy of Diffusion. Having the mean values of the diffusion coefficients as well as their standard deviations calculated, the next step in our studies was to determine the activation energy of diffusion of the systems studied. This parameter can be obtained from the slope of the linearized Arrhenius relation which is given by
ln(D) ) ln(A) -
Ea 1 RT
(6)
where D is diffusion coefficient of the substrate, A is the preexponential factor, Ea denotes the activation energy of diffusion for the substrate in a pertinent medium, R is the gas constant, and T is absolute temperature. According to the explanations given above, the uncertainty in the determination of activation energy of diffusion depends on the uncertainty in the value of the slope (a) of eq 6 and is given by the following formula:
s(Ea) ) Rs(a)
(7)
where s(Ea) and s(a) are the standard deviation of the activation energy of diffusion and the slope of eq 6, respectively. The latter was found using the weighted linear regression method. We chose the weighted variant of the linear regression method, since the standard deviations of ln(D) were different over the range of 1/T examined. The weight of ith point (wi) was defined k si-2, where k is the number of points taken to the as ksi-2/∑i)1 linear regression and si is the standard deviation of ith value of ln(D).15 Software. Processing of the experimental data has been performed using the on-line statistics service (www.chem. uw.edu.pl/stat, Polish version), which is available to the readers of the appropriate book.16 Results and Discussion Temperature Effects. The voltammograms of oxidation of undiluted alcohols and 2 mM 1,1′-ferrocenedimethanol studied in 20 mM LiClO4 solutions obtained at temperature in the range from -14 to +45 °C were well reproducible with the coefficient of variation less than 2.5 %. The voltammetric waves of undiluted alcohols have no symptoms that may indicate the formation of dimers or polymers at the electrode surface. In addition, the long-time chronoamperometric experiments performed for alcohols revealed that after a few seconds the current remains practically constant. This indicates that the electrogenerated ionic microlayer was stable during the voltammetric experiments. The voltammetric results for undiluted alcohols are presented in Figure 3. Figure 4 contains only one exemplary set of the voltammograms of 1,1′-ferrocenedimethanol in 1-propanol, since the voltammetric waves of this redox system are qualitatively similar in all alcohols over the temperature range studied. As it is seen in Figure 3, for higher alcohols (starting from 1-propanolsFigure 3 c-e) a minimum is formed on the wave plateau. The minimum becomes deeper when both temperature and the dielectric permittivity of the solvent decrease. This drop in the steady-state current may indicate a sudden decrease in the real electrode potential which is the difference between the applied potential and the IR-drop. The drop in the potential may be caused by a local drop in the conductivity of the solution at the electrode surface due to the strong short-range interactions between the ionic product and its counterion, what may lead to the formation of either ion pairs or microcrystals of the electrogenerated salt. Such a process would be a consequence of the huge accumulation of the salt
Figure 3. Voltammetric responses of 5.3-µm-radius Pt microdisk at various temperatures in undiluted alcohols: methanol (a), ethanol (b), 1-propanol (c), 1-butanol (d), 1-pentanol (e). Conditions: supporting electrolyte: LiClO4 (c ) 20 mM), scan rate: 50 mV/s.
in the electrode neighborhood. By increasing temperature, the crystallization and association processes may be eliminated and
Ionic Liquids Generated at Microelectrode Surface
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6947
Figure 4. Voltammetric responses of 5.3-µm-radius Pt microdisk at various temperatures in 2 mM 1,1′-ferrocenedimethanol in 1-propanol. Conditions: supporting electrolyte: LiClO4 (c ) 20 mM), scan rate: 10 mV/s.
TABLE 1: Regression Coefficients of Linear Dependencies of ln(D) vs 1/T for Undiluted Methanol, Ethanol, 1-Propanol, 1-Butanol, and 1-Pentanol substrate-solvent
slope a ( s(a)
methanol ethanol 1-propanol 1-butanol 1-pentanol
-1541.2 ( 12.5 -1845.2 ( 13.6 -2435.6 ( 11.4 -2654.0 ( 9.9 -2941.8 ( 8.9
correlation intercept b ( s(b) coefficient r2 -15.39 ( 0.04 -15.31 ( 0.05 -14.03 ( 0.04 -13.61 ( 0.03 -13.00 ( 0.03
0.9993 0.9990 0.9999 0.9999 0.9999
the solubility increased. As a result the minimum vanishes. Interestingly, the same effect might also be achieved by the introduction of a larger amount of supporting electrolyte, thus by increasing the conductivity of the system before the experiment. In such the situation the local drop in the conductivity should not affect sufficiently the overall one, and, as a result, well-shaped voltammograms should be observed. These speculations have been verified positively by us for 1-propanol, 1-butanol, and 1-pentanol containing 2.30, 1.30, and 0.23 M LiClO4, respectively. The accumulation of ions in the depletion layer, due to the solvent electrolysis, affects strongly the transport properties of the electroactive species even if the concentration of added supporting electrolyte is several orders of magnitude smaller than that of the substrate (solvent). Therefore, it should be pointed out that the diffusion coefficients of undiluted systems determined voltammetrically differ significantly from their selfdiffusion coefficients.10 The regression coefficients of the linearized temperature dependencies of alcohols’ diffusivities are presented in Table 1. The characteristic parameters of these dependencies, i.e., the activation energies of diffusion (Ea) and the diffusion coefficients at 25 °C (D(25)), are given in Table 2. The results obtained for
alcohols were compared to those obtained for 1,1′-ferrocenedimethanol. Table 3 contains the regression coefficients of the linearized temperature dependencies of the diffusion coefficients of 1,1′-ferrocenedimethanol in all alcohols examined. The parameters Ea and D(25) for 1,1′-ferrocenedimethanol are listed in Table 4. As expected, the data in Tables 2 and 4 demonstrate that the increase in the solvent viscosity (η), which goes along with the molecular mass of the alcohols in the sequence methanolpentanol, leads to the larger activation barrier in the transport process for both undiluted and diluted systems. As a result, the values of diffusion coefficients decrease. However, a comparison of the data obtained for the undiluted systems with those for the diluted model system (Tables 2 and 4) reveals some essential differences. It can be seen, that in the sequence from ethanol to 1-pentanol the differences between the D(25)-values for the undiluted alcohols and their equivalents for the diluted system become bigger. For methanol the D(25) numbers are similar and this is consistent with the observations presented by Hyk and Ciszkowska who used this system to expose the differences in the transport properties in gel and liquid media.14 The fact that the diffusion coefficients observed for undiluted alcohols are smaller and smaller than those for 1,1′-ferrocenedimethanol in the corresponding alcohol leads to the conclusion that smaller molecules of alcohols diffuse slower than relatively larger molecules of 1,1′-ferrocenedimethanol (the latter is roughly three times larger than, for example, 1-propanol). In the sequence from ethanol to 1-pentanol the ratio of diffusivities of alcohol and 1,1′-ferrocenedimethanol, λ, ranges from 82 to 62% (see Table 2). This is in contrast to the StokesEinstein relation which predicts inverse proportionality between the diffusion coefficient and the radius of the diffusing species in the given medium. However, the discrepancies observed will not be surprising if one realizes what is the medium at the electrode surface these species diffuse through. 1,1′-ferrocenedimethanol diffuses through the diluted (20 mM) solution of supporting electrolyte in a neutral solvent, and the viscosity of this solution does not differ from the solvent viscosity. Such the system satisfies the Stokes-Einstein relation. The alcohols, in turn, at the potentials of the voltammetric wave plateau, are transported through the layers of very large concentration of electrogenerated electrolyte (up to several moles per liter). These totally different fluid properties of the undiluted and diluted systems explain the reported above abnormalities. Gravitational Effects. The angular dependencies of the normalized limiting currents for methanol, 1-propanol, and 1-pentanol containing 20 mM LiClO4, drawn as plots of IθL/I0L vs θ, where IθL is the limiting current measured at angle θ, and I0L is the limiting current measured at θ ) 0°, are shown in Figure 5. The coefficient of variation of the ratio IθL/I0L was less than 0.5% for the alcohols examined. A 25-µm-radius microdisk was chosen for the measurements, since for the smaller electrodes, the enhanced diffusion rate (which is
TABLE 2: Transport Properties of Undiluted Methanol, Ethanol, 1-Propanol, 1-Butanol, and 1-Pentanol
a
substrate-solvent (cb [M] at 25 °C)
ηa [mPa × s]
Ea ( s(Ea) [kJ/mol]
(D(25) ( s(D(25))) × 1010 [m2/s]
λ ( s(λ)b
methanol (24.56) ethanol (17.06) 1-propanol (13.31) 1-butanol (10.87) 1-pentanol (9.20)
0.544 1.074 1.945 2.544 3.619
12.81 ( 0.10 15.33 ( 0.11 20.24 ( 0.09 22.06 ( 0.08 24.45 ( 0.07
12.00 ( 0.13 4.61 ( 0.05 2.32 ( 0.02 1.69 ( 0.02 1.15 ( 0.01
1.16 ( 0.02 0.82 ( 0.02 0.71 ( 0.02 0.71 ( 0.01 0.62 ( 0.01
Kinematic viscosity of the pure solvent at 25 °C.19
b
(25) (25) λ ) Dalcohol /DFc(CH . 2OH)2
6948 J. Phys. Chem. B, Vol. 105, No. 29, 2001
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TABLE 3: Regression Coefficients of Linear Dependencies of ln(D) vs 1/T for 1,1′-Ferrocenedimethanol in Methanol, Ethanol, 1-Propanol, 1-Butanol, and 1-Pentanol solvent
slope a ( s(a)
intercept b ( s(b)
correlation coefficient r2
methanol ethanol 1-propanol 1-butanol 1-pentanol
-1661.9 ( 18.6 -2005.5 ( 16.4 -2466.9 ( 18.0 -2660.7 ( 26.9 -2880.9 ( 15.8
-15.10 ( 0.07 -14.56 ( 0.06 -13.57 ( 0.06 -13.22 ( 0.09 -12.75 ( 0.05
0.9950 0.9989 0.9990 0.9974 0.9999
TABLE 4: Transport Properties of 1,1′-Ferrocenedimethanol in Methanol, Ethanol, 1-Propanol, 1-Butanol, and 1-Pentanol solvent
Ea ( s(Ea) [kJ/mol]
(D(25) ( s(D(25))) × 1010 [m2/s]
methanol ethanol 1-propanol 1-butanol 1-pentanol
13.81 ( 0.15 16.67 ( 0.14 20.50 ( 0.15 22.11 ( 0.22 23.94 ( 0.13
10.34 ( 0.14 5.63 ( 0.11 3.29 ( 0.10 2.39 ( 0.03 1.85 ( 0.03
adjacent to the electrode surface. According to the theoretical predictions given in our previous paper,10 it can be assumed that for the limiting conditions and for the supporting electrolyte used (LiClO4) the concentrations of both the product and the counterion (the only two components of the layer), cP-C, are equal to the half of the substrate bulk concentration, cb. Knowing both: the concentration of the electrogenerated electrolyte of the general formula R-(OH)+ ClO4-, where R is the alkyl group, and its molecular mass MP-C, one can easily calculate the electrolyte density as the product: cP-CMP-C. The following results were obtained for 25 °C: 1.62, 1.06, and 0.86 g/cm3 for methanol, 1-propanol, and 1-pentanol, respectively. The corresponding densities of pure alcohols at 25 °C are as follows: 0.787, 0.800, and 0.811 g/cm3. Extrapolating these numbers to the appropriately longer alkyl alcohols, one should obtain similar values of the densities of the depletion layer and the solution bulk, and, as a consequence, one may expect to obtain the value of IL at θ ≈ 90° very close to that of IL at θ ) 0°. For the diluted redox system of 2 mM concentration, the steady-state limiting current was practically constant within 1.5% in the entire range of θ. These gravitational ferrocenedimethanol results are consistent with those presented by Gao et al. for the oxidation of Fe(CN)64- of 10 mM concentration at 25 µm electrodes.17 Conclusions
Figure 5. Angular dependencies of steady-state limiting current of undiluted methanol (a), 1-propanol (c), and 1-pentanol (e). The current was normalized with respect to the limiting current at 0°. Conditions: supporting electrolyte: LiClO4 (c ) 20 mM), scan rate: 50 mV/s, Pt microelectrode radius: 25 µm, T ) 21 °C.
inversely proportional to the microelectrode radius) made the natural convection difficult to detect. Figure 5 shows clearly that the maximum value of IL occurs when the electrode is oriented perpendicularly to the gravitational field. At 180° the minimum value of IL is obtained. As it has been stated in the previous section, the oxidation of the undiluted alcohols produces a highly concentrated electrolyte near the electrode. Thus, the density of that region is increased significantly with respect to the solution bulk. When the electrode is positioned at θ ) 0°, then, due to the gravitational force, a dense depletion layer tends to move downward the cell. This results in an extra supply of the substrate along the electrode insulating glass sheath. As it could be easily anticipated, the extra transport contribution is the largest for θ ) 90°, thus the maximum of the IL value is observed there. Conversely, when the electrode is positioned at θ ) 180°, then the convective flow of the denser layer is mechanically blocked by the electrode surface, and this results in a decrease in the mass transport rate of the substrate and thus in a decrease in the electrode process intensity. Figure 5 also reveals that the variations in the limiting current become smaller for the solvents of larger density in the bulk. This is seen particularly for the characteristic point of these dependencies, i.e., the maximum value of IL at θ ≈ 90°. The trend observed is due to the decrease in the difference between the density of the electrogenerated layer and that of the solvent bulk. This explanation can be supported by the following approximate calculations of the density of the ionic layer
The results reported in this paper prove indirectly the hypothesis, previously drawn by White and co-workers and the present group, that an ionic microlayer is generated during the electrode process of undiluted alcohols at microelectrodes. The formation of such the layer drastically alters the transport properties of the undiluted alcohol redox systems. This was demonstrated by comparing the values of the activation energy of diffusion (Ea) and of the diffusion coefficients at 25 °C (D(25)) obtained for the undiluted alcohols with those obtained for the model diluted systems1,1′-ferrocenedimethanol. It has been found that the molecules of the alcohols, which are much smaller than the molecules of 1,1′-ferrocenedimethanol, diffuse to the electrode surface (at the potential corresponding to the plateau of the solvent wave) at the rate similar to or even smaller (to 62%) than that of 1,1′-ferrocenedimethanol molecules. The claim that a highly concentrated ionic layer is formed during the electrolysis of a solvent can also be strengthened by the fact that for the alcohols studied the Ea values are significantly larger than those predicted on the basis of the simple empirical relation expressed as Ea ) 3.74RTm.18 This relation correlates the activation energy of diffusion (Ea) of undiluted systems (from molten salts through room-temperature liquids to gases), determined nonelectrochemically, with their melting points (Tm). Interestingly, according to this equation, the Ea values of the undiluted alcohols determined voltammetrically are located in the region of molten salts. Finally, the electrogeneration of the ionic microstructure was also confirmed by the angular dependencies of the steady-state limiting current. The occurrence of the maximum and the minimum on these dependencies at θ ≈ 90 and 180°, respectively, demonstrates the significant contribution of natural convection to the total transport in the undiluted systems. Thus, it provides evidence that, at the limiting conditions, the region adjacent to electrode is much more dense than the solution bulk. The gravitational effect diminished strongly with decreasing the electrode radius. For 5.3-µm-in-radius electrodes it was negligible, therefore the temperature measurements of diffusivity were done with a 5.3 µm electrode.
Ionic Liquids Generated at Microelectrode Surface Acknowledgment. This work was supported by Grant 3 T09A 146 19 from KBN, the Polish State Committee for Scientific Research, and by a Warsaw University Grant No. BW1483/7/2000. References and Notes (1) Malmsten, R. A.; White, H. S. J. Electrochem. Soc. 1986, 133, 1067. (2) Malmsten, R. A.; Smith, C. P.; White, H. S. J. Electroanal. Chem. 1986, 215, 223. (3) McCarley, R. L.; Morita, M.; Wilbourn, K. O.; Murray, R. W. J. Electroanal. Chem. 1988, 245, 321. (4) Ciszkowska, M.; Stojek, Z. J. Electroanal. Chem. 1993, 344, 135. (5) Morris, R. B.; Fischer, K. F.; White, H. S. J. Phys. Chem. 1988, 92, 5306. (6) Ragsdale, S. R.; Lee, J.; Gao, X. P.; White, H. S. J. Phys. Chem. 1996, 100, 5913. (7) Koncka, M.; Stojek, Z. Electroanalysis 1995, 7, 1010.
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