4322
J. Phys. Chem. B 1998, 102, 4322-4326
Electron Transfer in Reverse Micellar Solutions: Influence of the Interfacial Bound Water D. Grand U.R.A.75, C.N.R.S., UniVersite´ de Paris-Sud, 91 405 Orsay Cedex, France ReceiVed: July 29, 1997; In Final Form: December 4, 1997
The thermal decay of the cation radical of tetramethylbenzidine (TMB) with aqueous nucleophiles was examined in positive (benzyldimethylhexadecylammonium chloride, BHDC) and negative (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles. It was shown that the rate constants are slightly affected by the sign of the interfacial charges, contrary to the situation that prevails in direct micelles. A correlation between the dependence of the decay rate constant and the density of the surfactant molecules on the water pool size has been established in the range 5 e ω° e 25 for all the micellar solutions. This result points out the decisive role of the bound water of the interfacial region, when the electron transfer occurs from the aqueous to the lipidic phase.
Introduction In a previous study,1 it has been shown that the fraction of free water in the water pool is the most important factor governing the charge separation efficiency of a hydrophobic chromophore embedded in reverse micelles. Numerous studies have shown that the reaction rates and equilibrium constants are greatly altered in microemulsions compared with those in homogeous solvents and are dependent on the water pool size (ω°). Such an effect was interpreted qualitatively by the peculiar state of solubilized water in the pools2 and changes in microviscosity3 or polarity.4 Different localization and orientation of the hydrophobic substrate were also invoked as well as the dilution of the reactants, which induces an increase of the mean distance between them.5,6 An effect of the sign of the interfacial charges7 was also reported: a great enhancement of the rate constant was observed when the substrate and the surfactant were oppositely charged and only a slight increase was observed when they had the same charge. The purpose of the present study is to examine how the thermal decay of the photocation of tetramethylbenzidine (TMB+) with aqueous nucleophiles is affected by the sign of the interfacial charges (positive or negative) of reverse micelles and by the size of the water pool. The incidence of the water structures will be considered. More precisely, can a quantitative correlation between kTMB+ and ω° be displayed? Experimental Section The surfactants (Surf) and the solvents were used as purchased: sodium bis(2-ethylhexyl) sulfosuccinate (AOT, purity of >99%) and benzyldimethylhexadecylammonium chloride (BHDC, purity of >97%). The solvents (n-heptane, benzene (Bz), and chlorobenzene (ClBz) were of the highest purity available. The surfactant concentration was kept constant: 0.15 M for AOT and 0.27 M for BHDC. The water content of the microemulsion is expressed as the molar concentration ratio ω° ) [H2O]/[Surf]. The preparation of the micellar solutions as well as the photoexcitation of the chromophore TMB (tetramethylbenzidine) has been described previously.1 The decay of the photoproduced
TABLE 1: Variation of the Micellar Concentration [Mi] in mol L-1 and of the Average Number of TMB+int Molecules per Micelle (TMB+int/Mi) on ω° Values in AOT/n-Heptane and BHDC/Benzene Reverse Micellesa AOT/n-heptane ω°
[Mi] × mol/L
5 10 15 20 25 30 35 40
2.080 1.100 0.833 0.652 0.425 0.288 0.234 0.163
45 50
0.140 0.115
103
TMB+int/Mi 0.0015 0.0030 0.0039 0.0050 0.0075 0.0124 0.0173 0.0271 0.0264 0.0340 0.0415
BHDC/benzene [Mi] × 103 mol/L
TMB+int/Mi
1.34 0.925 0.590 0.400 0.290 0.208 0.184 0.143
0.0015 0.0024 0.0037 0.0063 0.0079 0.O12 0.0173 0.0251
0.100
0.04
a
The size parameters of the reverse micelles are taken from refs 14, 19, and 20. TMB+int/Mi is equal to the ratio [TMB+int]/[Mi ], both expressed in mol L-1.
TMB+ was monitored by recording the optical density at 474 nm ( ) 40 000 M-1 cm-1)8 versus time (sweep time from 2 µs to 2 s). Results and Discussion I. Photocation-Nucleophile Reaction. Kinetic studies in micellar media require the knowledge of the reactant distribution and concentration between the different phases. Under our experimental conditions,1 the hydrophobic photocation, TMB+, is only generated at the micellar interface. With respect to its charge, TMB+ is attracted toward water in negative micelles (AOT) or slightly repelled to the hydrocarbon phase by the positive interface of BHDC micelles. The average number of TMB+ molecules per micelle (TMB+int) is estimated from the ratio of the micelle and photocation concentrations, before TMB+ decay takes place. TMB+int value is thus found to be much smaller than 1 in AOT as well as in BHDC micelles (Table 1). It appears that the interfacial capacity to solvate the photocation is of the same order in AOT/n-heptane and BHDC/Bz systems. In addition, the TMB+int value increases
S1089-5647(97)02484-X CCC: $15.00 © 1998 American Chemical Society Published on Web 05/12/1998
Electron Transfer in Reverse Micellar Solutions
J. Phys. Chem. B, Vol. 102, No. 22, 1998 4323 TABLE 2: Relative Values of the Interfacial Potential (Ψ in mV) and kTMB+ (s-1) Variations in AOT/n-Heptane Micelles at Various ω° Values
Figure 1. Variation of the rate constant of decay of TMB+ (kTMB+) vs ω°: AOT (curve a, O); BHDC/Bz (curve b, b); BHDC/ClBz (curve c, ×).
with the interfacial area (rw)2, with rw the radius of the water pool, and this finding is consistent with an interfacial location site for the photocation. TMB+ neutralization occurs with the various nucleophiles (Nu) present in the medium. The nucleophile species may be the SO3-, from surfactant AOT molecule, or Cl- in the case of BHDC molecule. Negative species may also be formed from attachment of the aqueous photoelectron, as OH- 9 in every micellar solution and AOT- 10 in anionic micelles. In BHDC micelles, the formation of a stable addition product, TMBCl, characterized by the absorption increase at 310 nm, has been observed, showing that TMB+ is effectively quenched by the aqueous Cl- counterion. In AOT micelles, no addition products could be detected by absorption spectroscopy. By analogy with BHDC micelles, we assume that the main Nu might be the sulfate group or OH- (see section II.2.a). Since TMB+ exhibits an amphiphilic character and a long lifetime compared with homogeneous solutions,11 the reaction should take place in the Stern layer. We have also checked that the thermal decay of TMB+ is not due to impurities, since cycles of irradiation and decay always give rise to the same rate constant values. II. Kinetics of TMB+ Decay. At a given ω°, the kinetics of the TMB+ decay exhibits an initial rapid exponential decay (millisecond time scale) followed by a slower one ( seconds time scale). Figure 1 reports the rate constant values (kTMB+) of the slowest decay vs ω°, measured in the three micellar solutions. (1) Effect of the Variations of the Electrical Interfacial Potential (Ψ). First of all, it should be noted that kTMB+ values lie in the same time range whatever the micelles, showing that TMB+ decay is slightly affected by the sign of the interfacial charges. Along these lines, it appears that TMB+ reactivity with the same Nu is much faster in reverse micelles than in direct micelles, where the TMB+ lifetime τ (τ ) 1/kTMB+) drops from 8 h (in anionic micelles) to 3 s in cationic micelles.11 These data suggest that common properties of the micellar core rather than the variations of the interfacial electrical potential (Ψ) would mainly govern the photocation reactivity. Another remark that emphasizes the above observation is the comparison of the behavior of kTMB+ on one hand and Ψ on the other with ω° variation in AOT micelles. In a previous study,1 it was shown that determinations of the first pKa1 of TMB allow evaluation of the relative Ψ values. Measurements of pKa1 of TMB are now extended to the range
ω°
Ψ (mV)
kTMB+ (s-1)
10 20 30 40
-39.5 -52 -88.5 -92
1.19 0.64 0.30 0.14
10 e ω° e 40. Table 2 gathers the corresponding Ψ values based on Ψ ≈ -52 mV at ω° ) 18.1 The absolute value of Ψ is found to increase slightly with the water pool size, owing to increasing hydration of the counterion and in agreement with theoretical studies.12,13 It is obvious that the energy barrier for transinterface electron transfer (ET) increases as the value of the interfacial potential Ψ becomes more negative. Consequently, the Ψ variation would easily explain the observed decrease of kTMB+ in AOT micelles. But Ψ decreases in the whole ω° range, whereas kTMB+ reaches a limit value for ω° g 30 (see section II.2), and moreover, no significant relationship between kTMB+ and Ψ is found as in the case of direct anionic micelles.11 Furthermore, in positive reverse micelles, the increase of Ψ with ω° will conversely lower the energy barrier for the ET reaction; hence, if the variation of the interfacial potential was a main parameter in this ET reaction, kTMB+ would increase with the size of BHDC micelles, but the experimental data exhibit an opposite trend of variation of kTMB+ with ω°. All these observations lead us to conclude that, in reverse micelles, the ET, from the aqueous to the lipidic phase, does not seem to be mediated via variations of the interfacial potential. As a final remark, substitution of chlorobenzene by benzene, which increases significantly the water pool radius of BHDC micelles,14 does not give rise to a marked decrease of kTMB+ value in favor of the same reaction site, i.e., a confinement of the two reactants at the interface. Contribution of an additional polarization due to the more polar character of ClBz when opposed to Bz might explain the higher stabilization of TMB+ in BHDC/ClBz than in BHDC/Bz micelles. (2) Influence of ω°. A further analysis of Figure 1 shows that the rate constant kTMB+ decreases monotonically as a function of ω° in the three micellar solutions. The photocation decay appears significant especially at low water content, in agreement with literature data.4,15 At higher ω° values, the kTMB+ value tends to level off in all the systems. The experimental data are found to fit an exponential decay upon ω° variation. The magnitude of this dependence is shown in Figure 2 on a logarithmic scale. The rate constant kTMB+ decay upon ω° variation obeys the following expression
kTMB+ ∝ exp(1/ω°)
(1)
below a critical value of ω°(ω°crit) that depends on the nature of the surfactant and of the solvent, thus delineating two areas of ω° influence. For ω° < ω°crit, the curve slopes are of the same order for the three micellar solutions and close to -1 in the limit of experimental accuracy. The ω° increase seems to prevent the interaction of the photocation with the Nu and hence the complex formation. For ω° > ω°crit, despite a slight scatter in the data due to the experimental accuracy, ln kTMB+ reaches a limit value, suggesting a similar microenvironment for the two reactants or an invariance of their concentration in this ω° range.
4324 J. Phys. Chem. B, Vol. 102, No. 22, 1998
Grand TABLE 3: Aggregation Number (Na) and Radius of the Water Pool (rw) in Reverse Micelles at 20 °C for BHDC (0.27 M) in Benzene19 and in Chlorobenzene14 and for AOT (0.15 M) in n-Heptane20 BHDC/benzenea ω°
Figure 2. Variation of ln(kTMB+) vs ω°: AOT (curve a, O); BHDC/ Bz (curve b, b); BHDC/ClBz (curve c, ×).
a
(a) Effect of the Reactant Concentration. One plausible explanation of the observed variation of kTMB+ would be the effect of the reactant concentrations. When ω° is increased, [Nu] is found to decrease whereas [TMB+int] increases. In addition, with respect to the time scale of the studied decay, the intermicellar exchange processes cannot be ruled out, and furthermore, these processes become faster with increasing micellar size.16,17 Such an assumption does not account for the experimental data. First, the variation of the reactant concentrations takes place in the whole ω° range (5 < ω° < 50), whereas the above relationship (1) between kTMB+ and ω° is observed only in a limited ∆ω° range. Second, we have to keep in mind that the number of filled micelles (with a photocation) is kept small with respect to the unfilled micelles, which always contain an excess of nucleophile, since the average dissociation constant of the surfactant molecule turns around 28%.18 This excess of nucleophile must ensure a pseudo-first-order decay, as observed experimentally. Such remarks lead us to consider that in AOT micelles, the Nu species would be SO3- rather than AOT- or OH-. In short, we believe that the photocation reactivity responds predominantly to changes in surface activity that can be due to the organization of the solubilized species being present at the interface, i.e., surfactant, water, and oil molecules. (b) Effect of the Charge Density. The role of the surfactant is to separate the oil-rich and water-rich parts of the microstructure in a stable way. From a pragmatic and simplified point of view, the interfacial region may be described by the density of the surfactant molecules (Na/(rw)2) at the micelle-solvent interface. The parameter Na/(rw)2 may give a rough idea of the excess pressure resulting from the presence of the curved interface, which must influence the solubility of the solvents (water and oil). In reverse micelles, the geometrical relationships between the water pool radius (rw), the aggregation number Na and ω° are well-known.6 Assuming that all the water is inside the spherical and monodisperse micelles, Na/(rw)2 is effectively found to be proportional to 1/ω° as in eq 1. The surface density of each micellar solution is calculated from the known values of Na and rw (Table 3). The plots of kTMB+ versus Na/(rw)2 are displayed in Figure 3. The most striking feature is their convergence at a limit value of about 2.2. × 1015 (AOT) to 2.35 × 1015 (BHDC/ClBz) and 2.8 × 1015 (BHDC/Bz) surfactant molecules per cm2, in the limits of the experimental accuracy and of the micellar characteristics. This result clearly points out that structural changes due to
Na
8.5 10 15 20 25 26.3 30 30.3 35 40 45 49.4 50
rw (Å)
BHDC/chlorobenzenea Na
rw (Å)
292
27.5
70
17.8
677
45.9
330
36.9
1297
65.3
645
52.4
1463 1890
71.6 81.5
1050
67.6
1530
82.4
AOT/n-heptanea Na
rw (Å)
115
18.6
180 230
26.8 31.8
409
42.4
520
48.3
920
64
1300
76.4
If necessary, the values were interpolated from the above references.
Figure 3. Variation of kTMB+ vs the surfactant density Na/(rw)2 AOT (curve a, O); BHDC/Bz (curve b, b); BHDC/ClBz (curve c, ×).
surfactant packing in reverse micelles of different sizes modulate the photocation reactivity. Such an effect of the headgroup packing is in agreement with an earlier study21 where an oppositive influence of the surfactant density was observed in the case of an opposite reaction, i.e., penetration of an exciplex from the lipidic phase to the aqueous core of AOT or BHDC micelles. At last, the slopes in Figure 3 are very steep and differ by 1 order of magnitude between AOT and the two BHDC micelles. The difference in the slopes might partly reflect the differences in the distances between the two reactants, where a greater distance would correspond a steeper slope,22 or in the character of the nucleophile species in AOT and BHDC micelles. In short, the above data give evidence that the thermal decay of the photocation, in anionic as well as in cationic reverse micelles, seems mainly governed by the composition of the interfacial layer. In addition, any change in the interfacial structure may affect the penetration and the solvation of the ionic reactants in a significant way. Two questions arise now. First, what are the the different states of water that compose the micellar core? Second, how does the Nu react; would the dehydration of the surfactant headgroups be the most important factor governing the formation of the complex TMB+-Nu, i.e., the determining step in the photocation neutralization?
Electron Transfer in Reverse Micellar Solutions
J. Phys. Chem. B, Vol. 102, No. 22, 1998 4325
TABLE 4: Values of the Interfacial Viscosity (ηint in cP) in AOT/Isooctane Micelles26 and of kTMB+ (s-1) in AOT/ n-Heptane Micelles vs ω° ω° 5 5.5 7.4 9.2 10 11.1 15 18.5 20 25 29.6 30 40 50 51.8
ηint (cP)
kTMB+ (s-1) 2.41
85 43 28 1.19 21 0.76 12 0.64 0.38 8 0.30 0.14 0.10 7
(c) Influence of the Bound Water of the Pool. The heterogeneous state of water in AOT micelles had been investigated by numerous techniques18,23,24 over a large ω° range (2 < ω° < 50). At low ω°, water is found to be highly immobilized in the micellar interior (trapped and bound water), while for ω° > 25, the presence of free water increases its mobility. It follows a high variation of the interfacial viscosity25 (ηint), which drops on going from ω° ) 5 to ω° ) 30, before ηint reaches a constant value (Table 4). It is interesting to note that for the AOT/ isooctane system, the invariance of ηint, observed at ω° ≈ 30, corresponds to a Na/(rw)2 value of about 2.1 × 1015 surfactants/ cm2. If the rate-determining step was the activation (energy diffusion regime), then an increase in the interfacial viscosity must increase the reaction rate. This is not found to be experimentally in agreement with Bakale’s data26 who clearly showed that rate became diffusion-controlled when the AOT micellar radius is greater than 70 Å or ω° ≈ 30. The present data, which display a parallel behavior of kTMB+ and ηint, might suggest a reduced mobility of the reactants and (or) a change in their respective solvation shells on ω° variation. With respect to BHDC micelles, the water core structure as the microviscosity are less known. But, according to Zana,14 the film rigidity might be described by the rate constant of exchange, ke, which depends strongly on the nature of the oil in the surfactant layer. It was thus found that for chlorobenzenze, ke is slightly dependent on water content, while, when benzene is used as solvent, a 5-fold increase in ke is observed on going from ω° ) 10 to ω° ) 40. At a given ω°, the substitution of benzene by chlorobenzenze lowers ke by 1 order of magnitude. A comparison of the present data with ke data clearly shows that it is precisely for the more rigid interface (lower ke value) that kTMB+ values, as well their variations on ω°, are found to be lower. (d) Influence of the Hydration Degree of the Ions. Within the hypothesis of a rigid cage around the complex and assuming that the overall structure of the cage remains essentially the same, despite the different orientation of water molecules in positive and negative micelles, the second point to be considered is the influence of the solvation of the reactants on the complex formation. At this stage of our investigations and partly due to the complexity of the system at the molecular level (lack of the precise chemical composition), the detailed mechanism involved is still poorly understood and only plausible statements can be made.
On one hand, the rate of complex formation is limited by the distance of closest approach of the reactants, i.e., by the tightly bound hydration shell of charged species. The rate of complex formation would be faster the lower the number of water molecules surrounding the ions. At low ω° values, the number of water molecules might not be enough to hydrate all the headgroups of surfactant molecules and to solvate completely the photocation. When these two factors are taken into account, the experimental data might be explained from a qualitative point of view. At low ω° values, the Nu capture by TMB+ might be easier, its binding to the cation stronger, leading to an increase of the rate constant of the cation decay. In contrast, the electrostatic cation-nucleophile interactions would be screened and hence strongly reduced on increasing the number of free water molecules (ω° > 30), and kTMB+ is found to decrease. On the other hand, owing to the fact that the aqueous phase is such a good solvent for ions compared with lipid media of lower dielectric constant, the energy required to move a small ion across a membrane is very high and consequently the interfaces cannot be easily crossed without the aid of ion channels. The presence of ions in the bound layer produces a large pertubation on the H-bonded network with a nonuniform penetration of water molecules into the oil phase. This might ensure the formation of preexisting cavity (“ion pore”), which might affect the surface roughness and the rates of ion permeation. Since the ionic motion from the core to the interior requires substantial rearrangement of the solvation water, such an assumption might suggest that differences in solvation energy of the ions might also be an important factor governing the rate of ion migration with micellar size. Conclusion The present experiments, performed in reverse micelles, are another illustration of the fact that ∆ψ variation is not the main parameter that governs the electron transfer, contrary to the situation prevailing in direct micelles. The quantitative approach of the biphasic decay, from aqueous to lipidic phase, points out the influence of the density of surfactants at the interface, which affects the interfacial structure. Despite the complexity of the interplay of the different factors involved in the ion permeation of an interface, the present work emphasizes the part played by the bound and trapped water of the water pool. The lack of solvating water molecule would be the determining factor in the kinetics of complex formation, and ions appear as “internal indicators” of the electrochemical properties of the solvent. References and Notes (1) Grand, D.; Dokutchaev, A. J. Phys. Chem. B. 1997, 101, 3181. (2) Barbaric, S.; Luisi, P. L. J. Am. Chem. Soc. 1981, 103, 4239. (3) Fendler, J. H. Membrane Mimetic Chemistry; Wiley Interscience: New York, 1982. (4) Wong, M.; Thomas, J. K.; Gratzel, M. J. Am. Chem. Soc. 1976, 98, 2391. (5) El Seoud, O. A.; Pivetta, F.; El Seoud, M. I.; Farah, J. P. S.; Martins, A. J. Org. Chem. 1979, 44, 4832. (6) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 209. (7) Martinek, K.; Levashov, A. V.; Klyachko, N. L.; Pantin, V. I.; Berezin, I. V. Biochim. Biophys. Acta 1981, 657, 277. (8) Alkaitis, S. A.; Gratzel, M. J. Am. Chem. Soc. 1976, 98, 3549. (9) Kang, Y. S.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1992, 96, 8647. (10) Pileni, M. P.; Hickel, B.; Ferradini, C.; Pucheault, J. Chem. Phys. Lett. 1982, 92, 308. (11) Bernas, A.; Grand, D.; Hautecloque, S.; Giannotti, C. J. Phys. Chem. 1986, 90, 6189. (12) Karpe, P.; Ruckenstein, E. J. Colloid Interface Sci. 1990, 137, 408.
4326 J. Phys. Chem. B, Vol. 102, No. 22, 1998 (13) Tomic, M.; Kallay, N. J. Phys. Chem. 1992, 96, 3874. (14) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. (15) Petit, C.; Brochette, P.; Pileni, M. P. J. Phys. Chem. 1986, 90, 6517. (16) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 7403. (17) Verbeeck, A.; De Schryver, F. C. Langmuir 1987, 3, 494. (18) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730. (19) Jada, A. Thesis, University of Strasbourg, France, 1984.
Grand (20) (21) (22) 3, 157. (23) (24) 7409. (25) (26)
Cabos, C.; Marignan, J. J. Phys. Lett. 1987, 46, L-267. Kikuchi, K.; Thomas, J. K. Chem. Phys. Lett. 1988, 148, 245. Hautecloque, S.; Grand, D.; Bernas, A. Trends Phys. Chem. 1992, Maitra, A. J. Phys. Chem. 1984, 88, 5122. Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. Bakale, G.; Beck, G.; Thomas, J. K. J. Phys. Chem. 1981, 85, 1062.
