0 Copyright 1990 American Chemical Society
The ACS Joumal of
Surfaces and Colloids APRIL 1990 VOLUME 6, NUMBER 4
Articles Electrochemical Investigations in Microemulsion Media. 1. Methylviologen Reduction E. Dayalan and Syed Qutubuddin* Chemical Engineering Department, Case Western Reserve University, Cleveland, Ohio 44106
Abul Hussam Department of Chemistry, George Mason University, Fairfar, Virginia 22030 Received January 6, 1989.In Final Form: June 28, 1989 The electrochemical reduction of methylviologen was investigated in cationic, anionic, and nonionic microemulsions. The reduction takes place in two reversible single-electron-transfer steps. Adsorption of the neutral form of methylviologen on the electrode surface observed in aqueous solutions is eliminated in microemulsions. The half-wave potentials for both reduction steps and the diffusion currents depend on the type and composition of the microemulsion. The observed results are explained on the basis of electrostatic interactions of methylviologen dication and cation radical with the surfactant and the solubilization of the hydrophobic neutral form in the nonaqueous domains of the microemulsion.
Introduction A microemulsion is an isotropic dispersion of two immiscible fluids, generally oil and water (with or without electrolyte), which is thermodynamically stabilized by the presence of surfactant molecules arranged in a microstructure a t the interfaces's2 A cosurfactant such as a short-chain alcohol is often present in microemulsions. Microemulsions are useful in such diverse areas as enhanced oil recovery, biomedical applications, and specialized reactions, including polymerization, mineral extraction, precious metal recovery, coatings, and advanced fuel technology.'-'' Both polar and nonpolar compounds can be solubilized in microemulsions due to the coexistence of aqueous and nonaqueous domains. For electrochem-
* To whom correspondence should be addressed.
(1)Healy, R. N.; Reed, R. L. SOC.Pet. Eng. J. 1977,17, 129. (2)Miller, C.A.; Qutubuddin, S. In Interfacial Phenomena in Apolar Media; Eicke, H. F., Parfitt, G. D., Eds.; Marcel Dekker: New York, 1987;pp 117-185. (3)Improued Oil Recouery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic: New York, 1977. (4)Speiser, P. In Reuerse Micelles: Biological and Technological Releuance of Amphiphilic Structures in Apolar Media; Luisi, P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984.
0743-7463/90/2406-0715$02.50 JO
ical investigations, microemulsions provide the advantages of both aqueous media in terms of high conductivity and nonaqueous media in terms of solubilizing capacity for nonpolar compounds. Electrochemical techniques have been recently used in characterizing micelles and
micro emulsion^.'^-'^
There are a few reports on the stabilization of electrochemically generated reactive intermediates in aqueous micellar so1utions.l6-l8 The observed results were interpreted variously. Some results were explained on the basis ~
~~~~
(5)Mackay, R. A. Adu. Colloid Interface Sci. 1981,15, 131. (6)Jones, C. A,; Weaner, L. E.; Mackay, R. A. J. Phys. Chem. 1980, 84,1495. (7) Moudgil, B. M. ACS Symp. Ser. 1985,272,437. (8) Barber, L. L., Jr. US.Patent 4502926,1985. (9)Goering, C. E.; Schwab, A. W.; Campion, R. M.; Pryde, E. H. Trans ASAE 1983,26,1602. (10)Schwab, A. W.;Nielsen, H. C.; Brooks, D. D.; Pryde, E. H. J. Dispersion Sci. Technol. 1983,4 , 1. (11)Haque, E.; Qutubuddin, S. J . Polym. Sci., Polym. Lett. Ed. 1988,26,429. (12)Wiencek, J. M.; Qutubuddin, S. Colloids Surf. 1988,29,119. (13)Zana, R.; Mackay, R. A. Langmuir 1986,2,109. (14)Mackay, R. A,; Dixit, N.; Agarwal, R.; Seiders, P. J. Dispersion Sci. Technol. 1983,4 , 397. (15)Chokshi, K.R.; Qutubuddin, S.; Hussam, A. J. Colloid Interface Sci. 1989,129, 315.
0 1990 American Chemical Society
716 Langmuir, Vol. 6, No. 4, 1990
of electrostatic interaction of the intermediates with the s u r f a ~ t a n t . ~ Other ~ ~ " results were explained on the basis of the structural aspects of the substrate and surfactant.ls The electrochemical reduction of methylviologen (I,l'-dimethyL4,4'-bipyridiniumdication) in micellar systems has been recently investigated by Kaifer and Bardlg and Quintela and Kaifer." The observed results were explained taking into account both electrostatic and hydrophobic interactions with the surfactant. In the present work, the electrochemical reduction of methylviologen has been investigated in different microemulsions containing cationic (cetyltrimethylammonium bromide, CTAB), anionic (sodium dodecyl sulfate, SDS), and nonionic loctylphenoxypoly(oxyethylene),Triton X-100) surfactants. Methylviologen (MV) was chosen as a model system to investigate the usefulness of microemulsions as media for fundamental electrochemical studies. This reaction involves a water-soluble reactant, a sparingly water soluble intermediate, and a waterinsoluble product. Microemulsions are well suited for carrying out electrochemical investigation of such systems since the three different species can all be solubilized in the same medium. Moreover, the effect of surfactants on the various species will be reflected in the changes in reduction potentials and currents which may provide insight on the nature of the species involved. Methylviologen has been widely investigated in order to understand the electron-transfer mechanism between the various redox forms, namely, the dication MV2+,the cation radical MV'+, and the neutral form MV.ls2l An understanding of the electrochemistry of methylviologen is important because of its frequent use as an electron acceptor in photochemical energy conversion and electrochemical display devices, as a mediator in electron transfer in biological studies, and in reducing electrochemically inactive compounds, etc.lSz3 Experimental Section Materials. Methylviologen dichloride trihydrate, SDS, and CTAB were obtained from Sigma. Triton X-100 was obtained from Rohm and Hass, the alcohols (1-pentanol and 1-butanol) and ferrocene from Aldrich, and the oils (octane and dodecane) from Humphrey. NaBr and NaCl were certified ACS reagents from Fisher. All the solutions were prepared with triple distilled water. Equipment. Electrochemical experiments (cyclic voltammetry and rotating disk electrode voltammetry) were performed by using an electrochemical analyzer (BAS Model 100). A Pine Instrument Co. rotating electrode assembly was used for rotating the electrode. A digital plotter (Bausch and Lomb DMP-40 series) was used for recording voltammograms. Quasielastic light-scattering measurements were performed by using the apparatus described in ref 24. An argon ion laser (Lexel95-4)equipped with an etalon provided the incident radiation. Diffusion coefficient measurements were carried out at 90° scattering angle. Procedure. All the solutions were purged with purified nitrogen to remove dissolved oxygen before experiments. A nitro(16) Meyer, G.; Nadjo, L.; Saveant, J. M. J. Electroanal. Chem. 1981, 229,417. (17) McIntire, C. L.; Blount, H. N. J. Am. Chem. SOC. 1979,101,7720. (18) McIntire, G. L.; Chiappardi, D. M.; Casselberry, R. L.; Blount, H.N. J. Phys. Chem. 1982,86, 2632. (19)Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985,89, 4876. (20) Quintela, P. A.; Kaifer, A. E. Langmuir 1987,3, 769. (21) Bird, C. L.; Kuhn, A. T. Chem. SOC.Reo. 1981,10,49. (22) Gudavicius, A. V.; Razumas, V. J.; Kulys, J. J. J. Electroanal. Chem. 1987,229, 153. (23) Calvert, J. M.; Manuccia. T. J.; Nowak, R. J. J. Electrochem. SOC.1986,133,951. (24) Cheung, H. M.; Qutubuddin, S.; Edwards, R. V.; Mann, J. A., Jr. Langmurr 1987, 3, 744.
Dayalan et al. Table I. Composition of Microemulsions concn
microemulsion cationic
CTAB
component
CTAB 1-butanol
n-octane NaBr water
wt
%
mM
2.3
63
2.3 1.3 89.5
125
4.6
anionic
SDS ME 1
SDS ME 2
SDS 1-pentanol dodecane NaBr water SDS 1-pentanol dodecane NaBr
water
nonionic
Triton X-100
Triton X-100 n-octane NaBr water
5.5 10.3 5.1 1.0
190
2.0
70
100
78.1 5.5
2.0 1.0 89.5
10.0 1.4 1.0 87.6
100
160 100
gen atmosphere was maintained in the cell during experiments. A three-electrode arrangement in a conventional threecompartment cell was used. A glassy carbon working electrode, a platinum foil counter electrode, and a SCE reference electrode were used. All the potentials are referred to SCE. The temperature was maintained at 25 f 1 OC. The working electrode was successively polished with 0.3- and 0.01-pm alumina/water slurries on felt surface and washed thoroughly before use. The area of glassy carbon electrode was estimated by performing cyclic voltammetric experiments with 4 mM K,[Fe(CN),] in 1 M KCl (diffusion coefficient, D = 6.32 X lo4 cm2 9-l) and was found to be 0.0675 cm2. The area of the glassy carbon rotating electrode was estimated to be 0.442 cm'.
Results and Discussion The compositions of the microemulsions used in this study are given in Table I. All the microemulsions used contain sodium bromide as the electrolyte. Therefore, a control experiment in aqueous solution with 100 mM NaBr as base electrolyte was carried out. The cyclic voltammograms for 1 mM MV2+in NaBr solution are shown in Figure 1for three different sweep rates (100,20, and 500 mV/s). The voltammogram a t 100 mV/s (Figure 1A) shows two peaks for the anodic reaction corresponding to the second step of methylviologen reduction. One peak was observed at a potential of -1.0 V and the other at -0.875 V. This means that two forms of the product of the second reduction step (MV) are present, one easily oxidizable compared to the other. The position of the first peak shifts from -1.02 to -0.96 V when the sweep rate is increased from 60 to 500 mV/s. The position of the second peak at -0.875 V is independent of sweep rate in the range 10-200 mV/s. The heights of these two peaks (relative to each other) change with sweep rate. The height of the first peak (at about -1.0 V) decreases (relative to the height of the peak a t -0.875 V) as the sweep rate is decreased below 100 mV/s while that of the second peak (at -0.875 V) increases. Finally, at 20 mV/s only the peak at -0.875 V remains (Figure 1B). Similarly, as the sweep rate is increased above 100 mV/s, the height of the peak at -1.0 V increases (relative to the height of the peak a t -0.875 V) while that of the peak at -0.875 V decreases. Finally, at 500 mV/s the peak at -1.0 V is predominant, and only a very small shoulder is left a t -0.875 V (Figure IC). At still higher sweep rates (e.g.,
Langmuir, Vol. 6, No. 4, 1990 717
Electrochemical Investigations in Microemulsions T LO&
lTo a A
A
A
m T A T
.?ova A
E Volt vs
Figure 1. Cyclic voltammograms of 1 mM methylviologen in aqueous 100 mM NaBr. Sweep rate (mV/s): (A)100; (B) 20;
(C) 500.
EVoItvssc~
1000 mV/s) the peak a t -0.875 V completely disappears. However, the reduction process itself becomes less reversible and the peak potential separations (-), become larger. It is reported that the anodic peak corresponding to the second step of MV2+ reduction is not diffusion contr~lled.'~This is because of the hydrophobic nature of MV, which leads to the adsorption peak. The presence of bromide ions further induces adsorption of the hydrophobic MV and rearrangement to a more stable adsorbed form. Potential- and time-dependent adsorption peaks for hydrophobic cation radicals of higher alkylviologens corresponding to nucleation and transition to more stable forms in the presence of bromide ions have been reported frequently in the literat~re.'~-'' Cyclic voltammograms were also recorded at different sweep rates with MV'+ in 100 mM NaCl aqueous solution (Figure 2). The overall behavior is comparable to that observed in NaBr. The difference between NaBr and NaCl media is that the rearrangement of the adsorbed MV to a more stable form appears to be slow in the presence of NaCl. This is reflected in all the voltammograms in Figure 2. A t 100 mV/s (Figure 2A), only the peak at about -1.0 V is predominant with a very small shoulder at a less negative potential (about -0.925 V). The voltammogram at 20 mV/s still has a peak at -1.0 V apart from the peak at about -0.9 V (Figure 2B). The small shoulder which appears at -0.925 V at 100 mV/s sweep rate disappears as the sweep rate is increased to (25) Bewick, A.; Lowe, A. C.; Wederell, C. W. Electrochim. Acta
1983,28, 1899.
(26) Fletcher, S.;Duff, L.; Barradas, R. G . J . Electroanal. Chem. roo,759. (27) Cieslinski, R.;Armstrong, N. R. J. Electroanal. Chem. 1984, 161 59. (28) Vargalyuk, V. F.;Starokozheva, T. I.; Loshkarev, Yu. M. Souiet Electrochem. 1979,15, 1337. 1979,
Figure 2. Cyclic voltammograms of 1 mM methylviologen in aqueous 100 mM NaC1. Sweep rate (mV/s): (A)100; (B) 20; (C) 500.
200 mV/s. The position of the peak which appears at about -1.0 V at 100 mV/s sweep rate becomes less negative with increasing sweep rates and appears a t about -0.925 V at 500 mV/s sweep rate. This type of behavior was not reported in earlier work,lg where 50 mM NaCl was used as the base electrolyte. Therefore, it may be concluded that the concentration and nature of the anion play an important role in the kinetics of MV adsorption and rearrangement to different forms. The cyclic voltammograms recorded at 100 mV/s in the presence of MV'+ in three different types of microemulsions are shown in Figure 3. The electrochemical parameters are listed in Table 11. As is evident from Figure 3 and Table 11, the reduction of MV'+ takes place in two reversible steps in all three types of microemulsions. The cathodic to anodic current ratios are nearly one. The peak potential separation (AE) for both the steps is 6070 mV at sweep rates of 10-280 mV/s as expected for a reversible one-electron-transfer process. The plots of cathodic peak current (I,,,) vs square root of sweep rate (v''') are linear for the first step in all the systems. Some representative plots are shown in Figure 4. The slopes are used in calculating the apparent diffusion coefficients of MV'+ in each system by using the RandlesSevcik equation." The values of the calculated diffusion coefficients are listed in Table 11. The diffusion coefficients of MV'+ in the CTAB and Triton X-100 microemulsions are only slightly different from the values in aqueous solutions. The differences can be ascribed
I
(29) Bard, A. J.; Faulkner, L. R. Electrochemical MethodsFundamentals and Applications; Wiley: New York, 1980.
718 Langmuir, Vol. 6, No. 4, 1990 T
IO&
A
/v3
LO& 7
A
J
EVdtvsSCE
Figure 3. Cyclic voltammograms of 1 mM methylviologen in microemulsions: (A) cationic; (B) nonionic; (C) anionic (SDS ME 1). Sweep rate 100 mV/s.
mainly to the effects of the medium. On the contrary, the diffusion coefficient of MV2+ is very small in SDS microemulsion. This suggests that the MV2+ is associated with (or partitioned into) the microemulsion droplet. The complications observed in aqueous NaBr and NaCl solutions due to the adsorption and rearrangement of MV on the electrode surface are eliminated in the different microemulsions investigated. This is because the microemulsions provide a nonaqueous environment for solubilization of the hydrophobic neutral MV, thereby preventing its adsorption on the electrode surface. A closer examination of the data in Table I1 reveals a number of facts. The peak potentials and also the halfwave potentials ( E l l 2 , determined as the average of cathodic and anodic peak potentials) for the first reduction step of MV2+are almost the same in aqueous NaBr, aqueous NaC1, CTAB microemulsion, and Triton X-100 microemulsion. The peak currents are nearly the same in all the above systems. Therefore, the cationic and nonionic surfactants do not affect the reduction of MV2+ to MV'+. Some differences are observed in the SDS microemulsion. The peak potential is shifted to a less negative value. The peak current is considerably decreased. This shows that the anionic surfactant in the microemulsion interacts with MV2+and MV'+ (reactant and product, respectively, of the first reduction). The decrease in the peak current suggests that there is association of MV2+ with the negatively charged surfactant on the surface of the microemulsion droplet. The shift of E y 2 , , to a less negative value shows that the product MV' is stabilized in the SDS microemulsion. The electrochemical reduction of methylviologen was
Dayalan et al. also studied in the micellar system containing 70 mM SDS in 100 mM NaBr aqueous solution. The cyclic voltammogram recorded a t 100 mV/s is shown in Figure 5A. The electrochemicalparameters are included in Table 11. The peak potentials and the diffusion coefficient (determined from I ,c vs u112 plot) for MV2+ for 70 mM SDS in 100 mM d B r nearly agree with the values reported for 70 mM SDS in 50 mM NaCl (Table I1 and ref 19). The concentration of SDS is 190 mM in the SDS microemulsion (SDS ME 1). The shift in peak potential (to less negative values) in this microemulsion is less than the shift observed in 70 mM SDS micellar solution. The peak current in the microemulsion is also slightly higher than in micellar solution. This reveals that the interaction of MV2+ and the stability of MV" are less in the SDS microemulsion compared to the SDS micelles. This is due to the reduced charge caused by the presence of cosurfactant in the microemulsion droplet compared to the micelle. The diffusion coefficients of the microemulsion droplets and micelles measured by a quasielastic light-scattering technique are presented in Table 111. The apparent diameter of the droplets and micelles as calculated by using the Stokes-Einstein equation is also listed in Table 111. The addition of MV2+ does not alter the diffusion coefficients. Compared to aqueous solutions, the peak potential and E l l z values for the reaction MV'+ + e MV (Table 11) are less negative for all the systems investigated with the exception of SDS micellar solution. As indicated earlier, the reduction product (MV) is nonpolar and is soluble in the organic domains of the microemulsions. This decreases the free energy of formation of the product, and the reduction takes place at less negative potentials. But there are certain differences in E l l z values observed in various microemulsions. SDS micelles are known to stabilize MV'+ making its reduction difficult, thereby shifting the potential of reduction to more negative values. SDS microemulsions also stabilize MV'+ (shift of Ellz,l to less negative potentials). However, it is the increased solubility of MV in the microemulsion compared to micellar solution (due to the increased volume fraction of organic components) which is responsible for the shift of E1/2,2 to less negative potentials compared to micellar and aqueous solutions. The differences in E1,2,2values between different microemulsions are mainly due to the variation in composition and concentrations of organic components (see Table I) apart from the contribution due to electrostatic interaction in the case of SDS microemulsions. A few experiments were carried out to observe the effects of adding an organic component (alcohol) and changing the composition of the organic components. 1-Pentanol (the cosurfactant used in SDS microemulsion) was added to aqueous NaBr and aqueous micellar solutions. A SDS microemulsion of a different composition (SDS ME 2) was also prepared. This microemulsion contained 70 mM SDS (same as in SDS micelles), and the total volume of organic components is less than in SDS ME 1. MV2+ reduction was carried out in all the above solutions. The cyclic voltammogram recorded a t 100 mV/s in SDS ME 2 is shown in Figure 5B. The electrochemical parameters for MV2+reduction in these systems are also included in Table 11. There is no change in the behavior of MV2+ in the alcohol-saturated NaBr aqueous solution compared to pure NaBr solution. This is because very little pentanol is dissolved in this solution. There is a considerable difference between the values obtained in the SDS
Langmuir, Vol. 6, No. 4, 1990 719
Electrochemical Investigations in Microemulsions
CTAB ME Triton X-100 ME SDS ME 1 70 mM SDS + 100 mM NaBr 100 mM NaBr (saturated with 1-pentanol) 70 mM SDS + 100 mM NaBr + 3% 1-pentanol , SDS ME 2
0.71 0.71 0.69 0.67 0.71 0.69 0.69
0.65 0.65
0.68
0.68 0.66
0.63 0.61 0.65 0.63 0.64
0.64 0.68 0.66 0.67
17.7 20.9 18.0 20.4 16.2 15.8 15.1 17.9 6.9 11.8 5.7
6.2
14.3 21.7
8.2 14.4 7.6 12.7
1.05 1.05 1.01 0.98 1.01 1.16
1.05 1.05
1.03
1.00
0.99 0.95 0.92
0.94 1.05 1.00
1.00
0.97
1.02 1.02 0.98
0.95 0.98
1.10 1.02 1.02 1.00
14.3 16.8 15.3 35
13.9 20.9 14.8 17.3 6.7
5.1
8.5 7.5
4.5 5.8
4.7 7.4 12.9 18.5
5.9 6.1 7.4 5.3 0.82 0.94
4.1 1.9 1.4
Sweep rate = 100 mV/s. E values in V vs SCE; I values in MA.
I
."I
Table 111. Diffusion Coefficients and Sizes from Light-Scattering (LS)and Electrochemical (EC)
Measurements D X lo6, cmz s-l LS EC 0.46 0.72
system SDS ME 1 70 mM SDS 100 mM NaBr 70 mM SDS + 100 mM NaBr + 3% 1-pentanol SDS ME 2
+
I
01 0.00
0.10
0.10
SCAN RATE
OAO
0.10
"*
(VOLT1SEC)
010
"*
Figure 4. Plots of cathodic peak current vs root of scan rate for 1 mM methylviologen in (1) 100mM NaBr; (2) cationic microemulsion; (3) anionic microemulsion (SDS ME 1); (4) SDS micelles.
9
E von vs SCE
Figure 5. Cyclic voltammograms of 1 mM methylviologen in (A) SDS micelles and (B) anionic microemulsion (SDS ME 2). Sweep rate 100 mV/s .
micellar solution when 3% alcohol was added to it. The shift in E1/2,1to a less negative value is smaller compared to pure micellar solution. The addition of alcohol increases the size of the micelles (Table 111). This may result in a decrease in the apparent surface charge density of the mixed micelle and diminished electrostatic interaction with MV2+ as well as MV'+. The increased organic component results in increased solubility of MV in the mixed micelle, and E1,2,2is shifted to less negative values.
d, A
LS
EC
1.04
0.96 0.78
106 35 47
68 51 63
0.41
0.2
120
247
1.41
The results from SDS ME 2, which has a lower amount of organic components compared to SDS ME 1 (Table 11),also reveal the same behavior. The apparent diameter of the droplet as obtained from light scattering (Table 111) is 120 A for SDS ME 2, slightly larger than the size of SDS ME 1 droplets. The ratio of alcohol to surfactant is higher in SDS ME 2 compared to SDS ME 1 (Table I). Therefore, the apparent charge density on the microemulsion droplet in SDS ME 2 is less compared to that in SDS ME 1. The electrostatic interaction of SDS ME 2 droplets with MV2+and MV'+ is less than that of SDS ME 1. This results in a slight change in Ellz,l and Z values. The decreased organic volume fraction in E%& ME 2 is reflected in the smaller shift of E1f2,2to a less negative value than observed in SDS ME 1. Additional electrochemical experiments were carried out to determine the diffusion coefficients of micelles and microemulsion droplets and to estimate the partitioning of MV2+between the aqueous phase and micelles or microemulsion droplets. The information obtained from these experiments provides additional justification for the explanation given above for the observed differences in El12 and peak current values in different systems. The diffusion coefficients of micelles and microemulsion droplets were determined by using ferrocene as a hydrophobic electroactive probe. A known concentration (- 1 mM) of the probe was solubilized in the micelles or microemulsion droplets, and the diffusion current for its anodic oxidation was measured. The diffusion coefficients calculated correspond to the diffusion coefficients of micelles or microemulsion droplets. The applicability of this method using cyclic voltammetry and rotating disk voltammetry techniques has been demonstrated for CTAB micro emulsion^.'^ In the present investigation, the cyclic voltammetry data could not be utilized for the calculation of diffusion coefficients because the plots of Z vs ul/' were not linear, and the slope increased with u'f? This nonlinearity is characteristic of a weak reactant adsorption on the electrode surface. The reason for this behavior of ferrocene in SDS systems is being investigated and will be reported later. The limiting currents (IL) from RDV experiments varied linearly with the square root of the angular velocity of electrode rotation, wl/'. Diffusion coefficients calculated from the slopes of I , vs w 1 f 2plots by using the Levich equation" and
Dayalan et al.
720 Langmuir, Vol. 6, No. 4, 1990 Table IV. Percentage of MV2+ Partitioned into Nonaaueous Domains in Various Systems
~-
~
~~
system
% MV2+ partitioned
70 m M SDS + 100 mM NaBr 70 mM SDS + 100 mM NaBr + 3% 1-pentanol SDS ME 1 SDS ME 2
100 68
96 63
the corresponding apparent sizes are given in Table I11 for different systems. The diffusion coefficient values obtained by electrochemical and light-scattering methods do not agree with each other. Light-scattering experiments involve very low time scales on the order of microseconds and yield mutual diffusion or self-diffusion coefficients depending on the concentration. Electrochemical measurements are carried out in the millisecond or longer time scale and provide self-diffusion coefficients. However, good agreement between the diffusion coefficients from these two different methods is observed in systems with a low volume fraction of surfactants and organic components and a t low concentrations of electrolyte. In concentrated systems, thermodynamic and hydrodynamic interactions between the particles contribute to the differences in the values obtained by the two different methods. These differences are useful in providing insight about the nature of the interaction^.^' The electrochemicaldiffusion coefficient values obtained by using ferrocene were used alon with the diffusion coefficients of methylviologen (MVR) obtained in aqueous solution and in the micellar solutions or microemulsions to calculate the amount of MVz+ partitioned into the nonaqueous domain. The determination of partitioning of electroactive substances between aqueous and nonaqueous domains in micelles or microemulsions is based on the contributions to the current by diffusion of MV2+ from the aqueous phase and the diffusion of micelles or microemulsion droplets containing MV2+. For cyclic voltammetry experiments, the relationship is (la) or equivalently C, = (D1'I2- D'/2)C/(D11/2 aPP - D2'I2)
Ob)
where D , is the diffusion coefficient of the electroactive substance (MV") in aqueous solution, D , the diffusion coefficient of the micelle or microemulsion droplet, D, the apparent diffusion coefficient of the electroactive sugstance in the micellar solution or microemulsion as determined by cyclic voltammetry, C the total concentration of electroactive substance in the micellar solution or microemulsion, C, the concentration of electroactive substance in the aqueous domain of the micellar solution or microemulsion, C, the concentration of electroactive substance in the nonaqueous domain of the micellar solution or microemulsion, and C z equal to C - C,. The equation takes the following slightly different form if Dappis estimated by using rotating disk electrode voltammetry:
The values of C, were calculated by using eq lb, and the percentage of MV2+ partitioned into the nonaque(30)Dayalan, E.; Qutubuddin, S. Manuscript in preparation.
ous phase in the various systems studied was calculated. The results are summarized in Table IV. MV2+is highly polar and insoluble in the hydrophobic core. The partitioning of MV2+into nonaqueous domains is possible due to electrostatic attraction by the negatively charged SDS surfactant. The partitioning values in Table IV are directly related to the effective charge density on the micelle or microemulsion droplet. Therefore, the strength of the electrostatic interaction of MV2+ with the surfactant in the different systems follows the order SDS micelles > SDS ME 1> (SDS + 1-pentanol) = S D S M E 2 (3)
,,,
The E , values will be affected by the relative stability of the reactant (MV2*) and the product (MV'+) of the first electron transfer in the given medium. Stabilization of MV2+ is expected to shift the El/,,, to more negative values compared to an aqueous system. On the contrary, stabilization of MY'+ will lead to less negative values of Ellz,,. In all the systems containing SDS, Ellz,, is shifted to less negative values. This shows that E,,,,, is influenced by the stabilization of MV'+ rather than MV2+. MV'+ stabilization can take place in two ways, by the electrostatic attraction toward the negatively charged surfactant and by solubilization in the nonaqueous phase due to its partial hydrophobic character. The stabilization of MV'+ by electrostatic attraction is expected to change to less negative values in the various systems in the same order as eq3. The stabilization of MV" by hydrophobic interaction is expected to shift E1/2,1to less negative values in the following order based on the amount of nonaqueous components: SDS ME 1 > SDS ME 2 > (SDS + 1-pentanol) > SDS micelles (4) The experimental values (Table 11) show that the shift of E , 2,1 to less negative values follows the order dictated 6y electrostatic interaction. This clearly shows that the electrostatic attraction of MV" toward the negatively charged surfactant plays the main role in its stabilization. This is probably the reason why there is no change in the El/,,, values in cationic and nonionic microemulsions where electrostatic attraction is not possible. The E l I 2,.v+alues will be affected by the relative stability of MV and MV. As discussed above, the stabilization of MV" is mainly by electrostatic attraction toward the negatively charged surfactant. This is expected to shift to more negative values compared to aqueous solutions. The stability of neutral hydrophobic MV due to solubilization in the hydrophobic core is expected to shift E , ,, to less negative values. In all the systems except SDk micelles, the experimental values are less negative compared to aqueous solutions (Table 11). Solubilization of MV in the nonaqueous domains is expected to shift to less negative values in the following order:
+
SDS ME 1 > SDS ME 2 > SDS 1-pentanol (5) This is the order experimentally observed; therefore, the stabilization of MV by solubilization in the hydrophobic core determines the values. The value is shifted to less negative values in the CTAB and Triton X-100 microemulsions also due to solubilization of MV in the nonaqueous domains. In SDS micelles, the electrostatic attraction toward MV'+ is very strong due to the higher negative charge density on the micelles, and this results in the shift of E,/,,, to more negative values.
Langmuir 1990,6, 721-727
Conclusions Microemulsions provide suitable media for investigating electrochemical behavior of organic compounds. There are advantages of using microemulsions, especially for studies involving redox forms with varying degree of polarity and solubility. While interpreting the observed behavior, it is necessary to take into account the interactions of charged species with the surfactant, the effect of relative charge densities of the micelles or microemulsion droplets, and the concentration of nonaqueous components. In the absence of interactions (electrostatic and hydrophobic) between the various reactive species and surfactants, the electrochemical behavior and the halfwave potentials are not significantly affected.
721
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This research was also supported by grants from the National Science Foundation (PYI Award CBT 85-52882), Eastman Kodak Co., and PPG. The contributions by Alex Ho, Kalpesh Chokshi, Anjum Razaq, and Dominic Gervasio in the early stages of this work are gratefully acknowledged. Registry No. MV, 1910-42-5;Triton X-100,9002-93-1;SDS, 151-21-3;CTAB, 57-09-0;NaBr, 7647-15-6;C, 7440-44-0; 1-butanol, 71-36-3; n-octane, 111-65-9; 1-pentanol, 71-41-0; dodecane, 112-40-3.
Surface-EnhancedRaman Scattering from Model Acrylic Adhesive Systems F. J. Boerio* and P. P. Hong Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221
P. J. Clark and Y. Okamoto Loctite Corp., 705 North Mountain Rd., Newington, Connecticut 06111 Received April 12,1989. In Final Form: November 2, 1989 Sdface-enhanced Raman scattering (SERS) has been observed from a model acrylic adhesive deposited onto silver island films. SERS spectra of the adhesive system were very similar to SERS spectra of o-benzoicsulfimide (saccharin), a component of the curing system of the adhesive, and to normal Raman spectra of the sodium salt of saccharin. When saccharin was replaced in the curing system by benzoic acid, SERS spectra of the adhesive were similar to SERS spectra of benzoic acid and to normal Raman spectra of benzoic acid salts. The intensity of the SERS spectra was independent of the thickness of the adhesive films, indicating that the observed Raman signal was characteristic of the interface and not of the bulk of the films. These results demonstrate that saccharin and benzoic acid were preferentially adsorbed at the silver surface to form metal salts and that surface-enhancedRaman scattering can be used for nondestructive characterization of interphases between polymer films and SERS-active metal substrates as long as the films are not so thick that normal Raman scattering from the bulk is comparable in intensity to the SERS from the interphase.
I. Introduction Surface-enhanced Raman scattering (SERS) is a phenomenon in which the Raman scattering cross section of molecules adsorbed onto the roughened surfaces of certain metals is enhanced by as much as 10' compared to the cross section for normal Raman scattering. Although many theories of S E W have been reported, it now appears that two mechanisms are responsible for most of the enhancement. One is associated with the large electric fields that can exist at the surfaces of metal particles with small radii of curvature and is only obtained for metals for which the complex part of the dielectric constant is small.' The other mechanism is related to distortions of the polarizability of the adsorbed molecules (1) Metiu, H. In Surjace-Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eda.; Plenum Press: New York, 1982; p 1.
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by formation of charge-transfer complexes with the metal surface.' Enhancement due to the charge-transfer mechanism is restricted to the molecules immediately adjacent to the substrate, but enhancement due to electromagnetic mechanisms may extend a few tens of angstroms from the metal surface. Since the enhancement in SERS is very large but restricted to the first few molecular layers adjacent to the substrate, we considered that SERS would be an extremely effective technique for nondestructive determination of the molecular structure of interphases between polymer systems and metals. We investigated SERS from thin films of poly(a-methylstyrene) spin-coated onto silver island films from dilute solutions and found that the intensity of the spectra remained approximately constant when the thickness of the films was varied by 1 order of magnitude, from approximately 100 to lo00 0 1990 American Chemical Society
