232
Langmuir 1991, 7, 232-231
Influence of Sodium Dodecyl Sulfate Micelles on the Oxidation of Alcohols by Chromic Acid E. P6rez-Benito and E. R o d e n a s * Departamento de Quzmica Fisica, Universidad de Aka16 de Henares, Alcalh de Henares, Madrid, Spain Received February 22, 1990. I n Final Form: July 2, 1990 Sodium dodecyl sulfate (SDS) micelles produce a large catalytic effect on the chromium(V1) oxidation
of alcohols in the presence of HC104, in the entire of surfactant concentration range used, but the effect
is different depending on the alcohol. The pseudo-first-order rate constants for water-soluble alcohols (benzyl alcohol, 2-propanol, and 1-butanol),increase with SDS concentrations up to a maximum value at high surfactant concentrations, while for water-insoluble alcohols like 1-hexanol and 1-octanol,the pseudofirst-order rate constants increase, reach a maximum value, and decrease with SDS concentrations. These kinetic results have been explained by means of the pseudophase ion exchange kinetic model, by considering that micellar counterions and H+ions compete for the ionic head groups of the micellar surface.
Introduction Many results have been published in the literature showing different catalytic or inhibitory effects of sodium dodecyl sulfate (SDS) in chemical reactions like acidcatalyzed hydrolysi~,l-~ base-catalyzed h y d r o l y s i ~ ,re~?~ placement reactions: halogenation of organic substrate^,^ dimerization of cation radicals,8 oxidation of olefinsg and ferrocenes,1° and electron transfer reaction between transition-metal complexes,ll but only a few of these results have been satisfactorily explained. The oxidation of alcohols is a very important kind of chemical reactions and it has not been studied in these systems. One of the most important oxidation reactants for organic substrates is Cr(VI), and in this paper we study the influence of SDS micelles on the oxidation of alcohols by potassium dichromate in a perchloric acid medium. The alcohols used are benzyl alcohol, 1-butanol,2-propanol, 1-hexanol, l-octanol, and 1-decanol. Alcohol oxidation in aqueous phasel2-l5has been studied extensively. The reactions are first order with regard to the alcohol and Cr(V1) concentrations, but they show a complex order with regards to the H+ concentration. In the experimental conditions the final products correspond to the carbonyl compounds.12 SDS micelles produce a strong catalytic effect on the reactions in the entire surfactant concentration range used,
* Author
to whom correspondence should be addressed. (1)Clark, B. C.; Chamblee, T. S.; Iacobucci, G. A. J. Org. Chem. 1984, 54. 1032. (2) Fadnavis, N. W.; Berg, H. J. van den; Engberts, J. B. F. J. Org. Chem. 1985,50,48. (3)Gonsalves, M.; Probst, S.; Rezende, M. C.; Nome, F.; Zucco, C.; Zanette, D. J. Phys. Chem. 1985,89,1127. (4) Tomida. H.; Yotsunvanaai, - - T.: Ikeda. K. Chem.Pharm. Bull. 1978, 26, 148. (5)Tachiyashiki, S.;Yamatera, H. Inarg. Chem. 1986,25, 3043. (6)Abe, M.; Suzuki, N.; Ogino, K. J . Colloid Interface Sci. 1983,93, 285. (7)Jaeger, D. A.; Robertson, R. E. J. Org. Chem. 1977,42,3298. (8) Quintela, P.A.; Diaz, A.; Kaifer, A. E. Langmuir 1987,4,663. (9)Menger, F.M.;Doll, D. W. J . Am. Chem. SOC.1987,106,1109. (10)Bunton, C. A.; Cerichelli, G. Int. J. Chem. Kinet. 1980,12,519. (11)Bruhn, H.;Holtzwarth, J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82,1006. (12)Sengupta, K. K.; Samanta, T.; Basu, S. N. Tetrahedron 1986,42, 681. Sengupta, K. K.; Samanta, T.; Basu, S. N. Tetrahedron 1985,41, 205.
(13)Wiberg,K. B.; Schafer, H. J.Am. Chem. SOC.1969,91,927.Wiberg, K. V.;Schiifer, H. J. Am. Chem. SOC.1969,91,933. (14)Wiberg, K. B.; Mukherjee, S. K. J.Am. Chem. SOC.1971,93,2544. Wiberg, K. B.; Mukherjee, S. K. J . Am. Chem. SOC.1974,96,6647. (15)Hasan, F.;RocBk, J. J. Am. Chem. Soc. 1973,95,5421.
0143-1463/91/2407-0232$02.50/0
but the effect varies depending on the alcohol. Pseudofirst-order rate constants for benzyl alcohol, 1-butanol, and 2-propanol increase with SDS concentrations up to a maximum value a t high surfactant concentrations, but for more insoluble alcohols like 1-hexanol and 1-octanol, pseudo-first-order rate constants increase, reach a maximum value, and then decrease with rising SDS concentrations. The kinetic results can be explained by means of the pseudophase ion exchange kinetic model,16-18by considering that micellar counterions and H+ ions compete for the ionic head groups of the micellar surface, and the effect of the ions and the alcohols on the physical properties of surfactant solutions. Addition of alcohol decreases the critical micelle concentration (cmc) v a l ~ e ,the ~~ fraction ,~~ of micellar head groups neutralized,z1 the aggregation n ~ m b e r , and ~ ~ the - ~ micellar ~ surface effective dielectric c0nstant.2~Addition of H+ ions decreases both cmcZ6and the micellar effective dielectric constantz7 and increases the micellar aggregation n ~ m b e r . ~ ~ - ~ l
Experimental Section Materials. Sodium dodecyl sulfate,SDS (Sigma),potassium dichromate (Sigma),perchloric acid (Merck),acetonitrile (Merck), benzyl alcohol (Merck), 1-butanol (Merck), 2-propanol (Panreac), 1-hexanol (Sigma), 1-octanol (Merck), and 1-decanol (Merck)were of the highestavailablepurity and wereused without further purification. (16)Menger, F. M.; Portnoy, C. E. J. Am. Chem. SOC.1967,89,4689. (17)Bunton, C. A. Catal. Reu.-Sci. Eng. 1979,20,1. (18)Romsted, L. S. Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977;Vol. 2, p 509. (19)Shirahama, K.; Kashiwabara, T. J. Colloid Interface Sci. 1971, 36,65. (20) Singh, H.N.; Swarup, S. Bull. Chem. SOC.J p n . 1978,51,1534. (21)Manabe, M.; Kawamura, H.; Yamashita, A.; Tokunaga, S. J.Colloid Interface Sci. 1987,115,147. (22)Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983,91,256. (23)Malliaris, A.; Lang, J.; Sturm, J.; Zana, R. J.Phys. Chem. 1987, 91,1475. (24)Luo, H.;Boens, N.; Auweraer, M. van der; Schryver, F. C.; Malliaris, A. J. Phys. Chem. 1989,93,3244. (25)Baglioni, P.; Kevan, L. J. Phys. Chem. 1987,91,1516. (26)Singh, H.N.; Swarup, S.; Saleem, S. M. J. Colloid Interface Sci. 1979,68,128. (27) PBrez-Benito,E.; Rodenas, E. J. Colloid Interface Sci. 1990,139, 87. (28) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J. Phys. Chem. 1980,84,1044. (29) Lianos, P.; Zana, R. J. Phys. Chem. 1980,84,3339. (30)Almgren, M.; Lofroth, J. E. J. Colloid Interface Sci. 1981,81,486. (31)Krahtovil, J. J. Colloid Interface Sci. 1980,75, 271.
0 1991 American Chemical Society
SDS Micelles in Oxidation of Alcohols
Langmuir, Vol. 7, No. 2, 1991 233
,
1
U.I
.,
Figure 1. Pseudo-first-orderrate constants for the oxidation of benzyl alcohol in aqueous phase at different alcohol and HC104 concentrations:0 , [H+]= 2.14 M; 0,[H+]= 1.75 M; +, [H+] = 1.36 M; 0,[H+] = 1.00 M; [H+] = 0.20 M. HCr04-
Scheme I + H'
I
I
0 020
0015
"E
Figure 2. Natural logarithm of the pseudo-first-order rate constants for the oxidation of 1-hexanol obtained in CH&N/ HzO mixtures with different dielectric constants, e, at a fixed alcohol concentration,0.26 M, and different HClO, concentra, [H+]= 0.33 M; 0,[H+] tions: 0 , [H+]= 1M; 0,[H+]= 0.5 M; . = 0.2 M.
Results and Discussion H2CT04
KCf
Methods. All the kinetic measurements have been carried out at 25 f 0.1 "C with a Hewlett-Packard 8452A diode array spectrophotometer. The reaction rates have been studied by following the absorbance decrease at 350 nm corresponding to a Cr(V1)absorption maximum. The initial oxidant concentration, potassium dichromate, was 4 X lo4 M, which, because of the high ionic strength involved in all the experiments,took the form of chromic acid in equilibrium with the ionizated f0rm.3~Pseudofirst-order rate constants have been obtained with a standard deviation below 0.5%. The oxidation of water-soluble alcohols (benzylalcohol,2-propanol, and 1-butanol)in aqueous phase has been studied in a H+ concentration range 0.2-2.14 M and alcohol concentration ranges of 0.02-3 M benzyl alcohol and 0.05-1 M for 1-butanoland 1-3.3 for 2-propanol. The rate constants for the oxidation of the water-insoluble alcohols (1-hexanol and 1-octanol)in aqueous phase have been obtained by extrapolation of the pseudo-first-orderrate constants obtained in the mixtures acetonitrile/HzO to an acetonitrile amount of 0. The mixture ratio acetonitrile/HzO was 0.33-0.83 (v/v),with H+ concentration in the range 0.2-1.4 M and a fixed alcohol concentration of 0.265 M for 1-hexanol and 0.05 M for 1-octanol. Kinetic measurements in the micellar phase have been realized with SDSconcentration O.oO3-0.25 M, HClOd concentration 0.2-1 M, and alcohol concentration 0.05-0.3 M for benzyl alcohol,0.05-1 M for I-butanol and 2-propanol, 0.033 M for 1-hexanol, 0.0070.017 M for 1-octanol, and 0.0042 M for 1-decanol. For water-soluble alcohols (benzyl alcohol, 1-butanol, and 2-propanol),mixed micelles SDS-alcohol were prepared "in situ" in the reaction cuvette, while for the water-insoluble alcohols, the alcohol had been previously solubilizedin 0.1M SDS solutions, with a molar concentration ratio of surfactant/alcohol of 1/2, 1/05, and 1/0.1 for 1-hexanol, 1-octanol,and 1-decanol,respectively. (32) Espenson, J. H.;Kinney, R. J. Inorg. Chem. 1971,10, 1971,
Reaction rates for the oxidation of water-soluble alcohols (benzyl alcohol, 2-propanol, and 1-butanol) depend on both alcohol and HC104 concentrations. Pseudo-firstorder rate constants for the oxidation of benzyl alcohol in the aqueous phase are shown in Figure 1. The same kind of results have been obtained for 2-propanol and l-butanol. These kinetic data show the reaction rate is first order with regard to the alcohol. The results have been explained by means of the reaction mechanism given in literature12-15 (Scheme I). This mechanism considers that the chromate ester intermediate is in equilibrium with the protonated alcohol and chromic acid, and the pseudo-first-order rate constant is easily derived as kexp
=
kKKRoHKCr[H+12[RoHl
1 + Kc,[H+l
(1)
an expression that can be reorganized [H+l[ROHl--kexp
l
+
1
k K K ~ItKKR0HKCr ~ ~ [H+]
(2)
so that, a plot of [H+][ROH]/kexp versus l/[H+] is a straight line from which values of Kcr and the constant product kKKRoH = k' can be obtained. Our kinetic results can be adapted to this rate equation. In the case of 1-hexanol and 1-octanol, which are insoluble in water, the rate constants for the reaction in the aqueous phase have been obtained by extrapolating the rate constants in the mixtures HzO/acetonitrile to an acetonitrile concentration of 0. The experimental results follow a typical rate constant variation with the dielectric constant, and in Figure 2, values of natural logarithm of the pseudo-first-order rate constants for the oxidation of 1-hexanol are plotted versus l / t , showing a linear dependence. The dielectric constant has been obtained from t = xi~itj,3~ where ei and x i are the dielectric constant and the volume fraction of each component i in the solution. The same kind of results have been obtained for l-octanol. The extrapolated values of pseudo-first-order rate constants for these alcohols also fit the proposed mechanism as can be seen in Figure 3, where values of [H+][ROH]/k,pversus 1/[H+]for benzylalcohol and l-oc(33) Decroog, D.Bull. SOC.Chim. Fr. 1964,127.
P6rez-Benito and Rodenas
234 Langmuir, Vol. 7, No. 2, 1991
!
4
J
100
. 1
1
3
2
L
5 l/[H'l (M-11
.
Figure 3. Values of [H+][ROH](k,,, versus 1/[H+] for benzyl and 1-octanol ( 0 )oxidations in aqueous phase. alcohol (0)
I
0.1
Table I
k'/M-2
alcohol benzyl alcohol 1-butanol 2-propanol 1-hexanol 1-octanol a
Kcr/M-'
5-1
1.00 0.049 0.029 0.045n 0.04W
0.23 0.40 0.40 0.40-1.40 0.09-0.40
[SDSI (MI
0.2
Figure 5. Pseudo-first-order rate constants for the oxidation of benzyl alcohol at [alcohol] = 0.08 M and different SDSand HClOI concentrations: [H+] = 0.2 M; B, [H+] = 0.5 M; 0 , [H+] = 1.0
*,
M.
I-
Values of k' obtained by using a value of Kcr = 0.4 M-1.
ky.10 (-3-1)
1.0
o.5 0.5
0.1
0.2
03 [ROHI (M)
Figure 4. Variation of the pseudo-first-order rate constants for the oxidation of benzyl alcohol with the alcohol concentration at a fixed [H+]= 0.2 M and different SDS concentrations: , [SDS] = 0 M; B, [SDS] = 0.05 M; 0 , [SDS] = 0.10 M.
Figure 6. Pseudo-first-order rate constants for the oxidation of benzyl alcohol at a fixed [H+], 0.2 M, and different SDS and alcohol concentrations: +, [ROH] = 0.05 M; B, [ROH] = 0.08 M; 0 , [ROH] = 0.30 M. 15
k,,103 (5-1)
tanol are plotted. The same plots for the other alcohols studied showed straight lines. Kcr and k' values obtained from these plots are given in Table I. It should be pointed out that the values of Kcr are nearly the same for the water-soluble alcohols (benzyl alcohol, 2-propanol, and 1-butanol), while the values for the water-insolublealcohols (1-hexanoland 1-octanol)have a strong indetermination because of the extrapolating method used. The value varies in a range 0.4-1.4 M-' for 1-hexanol and 0.1-0.4 M-' for 1-octanol, but it can be concluded that the same value, nearly 0.4 M-', explains the results for all the alkanols used. This value is smaller than the reported value of the chromic acid reciprocal of the acidity constant,34Kcr = 5.5 M-I; this difference could be related to the strong ionic strength and the alcohol effects in the medium. SDS micelles affect the pseudo-first-order rate constants for these reactions. The variations of the pseudo-firstorder rate constant, kg, for benzyl alcohol oxidation a t different SDS, benzyl alcohol, and H+concentrations are given in Figures 4-6, which shows that the pseudo-firstorder rate constants for this reaction do not vary linearly (34) Handbook of Chemistry and Physics, 61st ed.; Weast, R. C., Ed.; CRC Press, Inc.: Boca Raton, FL, 1980.
10
0.5
Figure 7. Pseudo-first-order rate constants for the oxidation of isopropyl alcohol at [H+] = 1 M and different SDS and alcohol concentrations: 0,[ROH] = 0.05 M; 0 , [ROH] = 1 M.
with the alcohol concentration as the reaction in aqueous phase. The same variation with alcohol concentration is found for the other alcohols. Pseudo-first-order rate constants for the oxidation of 1-butanol,2-propanol, 1-hexanol, 1-octanol, and 1-decanol at different SDS,H+,and alcohol concentrations are given in Figures 7-11. As one can see, SDS micelles produce a catalytic effect in the entire concentration range used,
SDS Micelles in Oxidation of Alcohols
Langmuir, Vol. 7, No. 2, 1991 235 I
' I
o.2 LSDSl (M)
01
.,
Figure 11. Pseudo-first-order rate constants for the oxidation of 1-octanoland 1-decanol at a fixed [H+],1 M, and different Figure 8. Pseudo-first order-rate constants for the oxidation of SDS and alcohol concentrations: 0 , [1-octanol] = 0.0167 M; 0, 1-butanol at a fixed alcohol concentration, [ROH] = 0.1 M, and , [1-octanol] = 0.0067 M; [1-decanol] = 0.0042 M. different SDS and HClO4 concentrations: +, [H+]= 0.2 M; . [H+]= 0.5 M; 0 , [H+]= 1.0 M. used, most of the Cr(V1) is in the chromic acid form in 4 equilibrium with the ionized form, HCr04-. HCr04- ions are considered nondistributed in the micellar phase ky.lO2 because of the repulsion with the negatively charged mi(5-1) cellar surface, and the following exchange equilibrium 3 between H+ and Na+ counterions in the micellar surface18 has been considered O.'
0.1
[=SI (MI
H+,
2
+ Na',
+ H+,
+ Na',
to be defined with the following ion-exchange equilibrium constant
L
I
I
0.1
I
I
1
o.2
1
[SDSl (MI
Figure 9. Pseudo-first-order rate constants for the oxidation of 1-butanol at a fixed [H+],1 M, and different SDS and alcohol concentrations: e, [ROH]= 0.05M;m, [ROH]= 0.1 M; 0 ,[ROH] = 1 M.
where [H+,], [Na+w], [H+,], and [Na+,] denote the reactive ions and micellar counterions in the aqueous and micellar phases, respectively. The value of KHNa= 0.82,35 which is near 1, indicates there is no difference between specific absorption of these ions onto the micellar surface, and the ions can be considered to be statistically distributed between aqueous and micellar phases. The H+ concentration in the micellar phase, H+,, is given by the following quadratic equation
1 ,
,
0.1
1
02 [SDSI (MI
I
J
Figure 10. Pseudo-first-order rate constants for the oxidation of 1-hexanolat a fixed alcohol concentration, [ROH] = 0.033 M, and different SDS and HC104ooncentrations: 0,[H+]= 0.2 M; M, [H+]= 0.33 M; 0,[H+]= 0.5 M; 0 , [H+]= 1 M. but the effect is different depending on the alcohol. In the case of 1-decanol oxidation, SDS does not produce any effect, according to Figure 11. The kinetic results can be explained by means of the micellar pseudophase kinetic mode1.16J7 This model considers the micelle to be separate from the aqueous phase and that the reaction occurs in both phases. At the potassium dichromate and H+ concentrations
that depends on K H ~and , the fraction of micellar head groupsneutralized,P= ([H+m]+ [Na+m])/[D~],orfor the expression in the case KHNa = 1
(5) This variation in KHNa, between 0.82 and 1, does not produce any significant variation in the fitting of the kinetic results. P values in the presence of benzyl and isopropyl alcohols have been obtained by conductivity measurements as ratios of the slopes above and below the cmc. These results show benzyl alcohol incorporation produces greater ionization than 2-propanol. Values of 0for the other alcohols have been extrapolated from the conductivity (35) Romsted, L. S.;Zanette, D.J. Phys. Chem. 1988, 92, 4690.
Pgrez-Benito and Rodenas
236 Langmuir, Vol. 7, No. 2, 1991
Scheme 11. + Dn === ROH,
Table I1 alcohol
lalcoholl / M
8
benzyl alcohol benzyl alcohol benzyl alcohol 2-propanol 2-propan o1 1-butanol 1-butanol 1-butanol 1-hexanol 1-octanol 1-octanol
0.05
0.56
0.08 0.30 0.05 1.00 0.05 0.10 1.00 0.033
0.017 0.007
ROH,
K.
0.48 0.29 0.65 0.38 0.65
0.60 0.48
0.58
xw
-P
km
kw
0.73 0.73
measurements in the literature,21 which show that the shorter the alcohol chain, the greater the ionization produced in the micellar aggregate. The fraction of micellar head group neutralized, 6, can be considered as a constant with surfactant concentration, and the corresponding values at different alcohol concentrations are given in Table 11. Alcohol distribution between aqueous and SDS micelles has been studied by using fluorescence22 and NMR23 techniques and the corresponding - values of the partition coefficient P, = [ROHm]/[ROH,] are given. [ROH,] and [ROH,] denote the effective concentrations in the micellar and in the aqueous phase, respectively, that are related to the analytical concentrations by
x,
a X, and X, correspond to the chromate ester intermediate in the aqueous and micellar phases, whose concentrations along with protonated alcohol in both phases can be neglected.
Table I11 k’mo,
alcohol benzyl alcohol 1-butanol 2-propanol 1-hexanol 1-octanol
P, 8 3 2 190 1900
M-’ s-l 19.87 0.63
0.25
a, M-I
0.48 1.00
k’,([H+] = 1 M), M-1 5-1 32
0.60
1.71
0.80 0.46 0.41
k’,o/ k‘w 9.6 10.4 8.1 3.2 3.4
The alcohol concentrations in aqueous and micellar phase are given by the following expressions [ROHI
[RoHwl = 1 + K,[Dn]
[ROHml [ROH,] = [Dn]8
and
and [ROH,] =
WHWl 1- [Dnlo
(7)
The protonation of alcohol in the micellar phase is given by
where is the volume of micellar phase per mole of micellized surfactant. This volume has been considered as given by
o=o,+-
[ROHml[Dn] URoH
where b, and OROH are molar volumes of surfactant and alcohol, respectively. Dm = 0.25 L / m 0 1 ~ ~ a nalcohol d molar volumes from densities are reported values. The partition coefficient is related to the substrate binding constant used in the pseudophase micellar model17by the expression K, =
(
k, = k’,[H+,][ROH,]
+
PsO 1- [Dn]8
(9)
In order to explain all the kinetic results, we have used different treatments, but the mechanism given in Scheme 11, which agrees with the proposed reaction mechanism for the reaction in aqueous phase,12is the best fit to the kinetic data. This mechanism considers that the reaction in the micellar phase occurs between the micellized protonated alcohol and the chromic acid in the aqueous phase across the micellar boundary, according to other authors for other results in the l i t e r a t ~ r e . ~ ’It, ~should ~ be pointed out that consideration of any kind of HZCr04 distribution between aqueous and micellar phases does not fit the results. (36) Stilbs, P. J. Colloid Interface Sci. 1982,87, 385. (37) Bunton, C. A.; Romsted, L. S.; Savelli, G. J.Am. Chem. SOC. 1979, 101, 1253. (38)Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, I. G. J. Phys. Chem. 1982,86, 1881.
which accounts for the fact that the reaction only occurs in the micellar surface. According to this mechanism, the deduced pseudo-firstorder rate constant is
where k’, = kwKwKROH,and Kc, are the constant values obtained for the reaction in the aqueous phase. The kinetic results have been adapted to this mechanism by simulation techniques using the known values of parameters k’w,Kcr, K H ~ *and , p, and by considering P, and the constants product k’, = kmKmKROH, as adjustable parameters. The cmc has been neglected as both alcohols and protons decrease the value in SDS aqueous solutions.1’+21 Values of P, that fit the kinetic results for the different alcohols are given in Table 111. In the case of waterinsoluble alcohols (1-hexanol and 1-octanol), P, values given in the literature36are the values used to explain the kinetic results, while for the water-soluble alcohols, the P, values that explain the kinetic data are smaller than the values given in l i t e r a t ~ r e , ~according 6 to Table 111. The different P, value for water-soluble and water-insoluble alcohols explains the SDS effect. While for water-soluble
SDS Micelles in Oxidation of Alcohols alcohol an increase in surfactant concentration increases the micellar alcohol solubilization and the experimental rate constant, for water-insoluble alcohol, once all the substrate is solubilized into the micelle, an increase in surfactant concentration increases the micellar counterions that displace H+ions out of the micellar surface, with the decrease in the pseudo-first-order rate constant. A constant value of k’, is not able to explain kinetic results in the whole acid concentrations range used and the following empirical expression has been used
h’, = k’,o e ~ p ( a [ H + ~ ] ) taking into account our experimental evidence showing the strong dielectric constant effect on this reaction, and the decrease that H+ions produce in the micellar surface effective dielectric constant, according to our fluorescence results.27 In Figures 4-11, the k~ calculated values with this treatment are shown by solid lines. The same kind of fitting has been obtained for all the kinetic results with all the alcohols in the concentration ranges used. Values of k’,~ and a, which fit the kinetic results for the different alcohols, are given in Table 111. In order to compare the kinetic rate constants for the reaction in aqueous and micellar phases, it is necessary to take into account the volume element where the reaction occurs. Table I11 gives k’, and the ratio k’mD/k‘w a t [H+]
Langmuir, Vol. 7, No. 2, 1991 237 = 1 M and a t a micellized alcohol to surfactant ratio,
[ROH,]/[Dn], of 0.5. These results indicate that the micellar phase produces a catalytic effect on these oxidation reactions for all the surfactant, alcohol, and H+ concentrations studied, a catalytic effect that is mainly concerned with the increase in the micellar phase rate constant, because of the micellar surface effective dielectric constant decrease with H+ and alcohol concentrations in solution. It should be pointed out that in order to explain some other kinetic data in cetyltrimethylammonium bromide micelles, we are using a treatment that considers ions distribution around micelles according to the PoissonBoltzmann nonlinearized equation39 with specific interactions for some of the ions in solution with the micellar surface and a substrate distribution between aqueous and micellar phases depending on the micellar surface potentiaL4O In the case of these reactions with noncharged substrates and with ions in solution that are nonspecifically adsorbed onto the micellar surface, this treatment reduces to the pseudophase ion exchange kinetic model that explains these results. Registry No. SDS,151-21-3; benzyl alcohol, 100-51-6; 1-butanol, 71-36-3; 2-propanol, 67-63-0; 1-hexanol, 111-27-3; l-octanol, 111-87-5; 1-decanol, 112-30-1. (39) Ortega, F.; Rodenas, E. J. Phys. Chem. 1987, 91, 537. (40) Rodenas, E.; Dolcet, C.; Valiente, M. J. Phys. Chem. 1990, 94, 1472.
