2866
Langmuir 1995,11, 2866-2871
Kinetics in Microemulsion Medium. 4. Alkaline Fading of Crystal Violet in Aqueous (H2O/Aerosol OTDsooctane and H2O/Aerosol OTDecane) and Nonaqueous (Ethylene Glycol/ Aerosol OTDsooctane) Microemulsions Lana Mukherjee, Nita Mitra, P. K. Bhattacharya, and S. P. Moulik" Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta-700 032, India Received March 16, 1994. I n Final Form: October 18, 1994@ The pseudo-first-orderrates of the alkaline fading of crystal violet have been studied in detail in aqueous and ethylene glycol media as well as in water/Aerosol OT (AOTYdecane, water/AOT/isooctane, and ethylene glycoVAOTlisoodane microemulsion media. Both ethylene glycol and its microemulsion have shown reactionacceleratingeffects compared with other environments. The rate of reaction in the microemulsion medium is inversely proportional to the water/AOT or the ethylene glycolIAOT mole ratio; significant increases in rates occur at ratios less than 8 in the former and less than 4 in the latter. The energetics of the reaction have offered distinct features in different environments; the and AS*values have shown isokinetic effects. Normal salt effects have been observed in water-containing media; ethylene glycol alone and in a microemulsion has shown equivalent but abnormal effects. Micelles, microemulsions, liposomes, etc. are compartmentalized liquids which may show special performance toward reaction equilibria and reaction dynamics. Chemical and biochemical reactions have been reported to be f a i r l ~ l -as ~ well as extraordinarily influenced under compartmentalized conditions. Special behaviors of water1 oil microemulsions toward the inversion of sucrose and alkaline phosphatase catalyzed hydrolysis ofp-nitrophenyl phosphate have been r e p ~ r t e d ; ~ significant ,' rate enhancement has been observed. An exceptional rate acceleration of the ferricyanide and iodide reaction in water/Aerosol OT (AOT)/heptane microemulsion medium has been very recently reported.s Similar types of reactions in micellar and microemulsion media have also appeared in the l i t e r a t ~ r e . ~The J ~ alkaline hydrolysis of the dye crystal violet (CV) (a triphenylmethane dye) has been documented on several occasions,11-16but a thorough study, nevertheless, remains pending. While the anionic surfactant sodium dodecyl sulfate decreases and the cationic surfactant cetyltrimethylammonium bromide
* Abstract published in Advance A C S Abstracts, June 15, 1995.
(1) Lopez, P.; Rodriguez, A.; Herrera, C. G.; Sanchez, F. J . Chem. SOC.Faraday Trans. 1992,88,2701. (2) Mishra, B. K.; Valaulikar, B. S.; Kunjappu, J. T.; Manohar, C. J . Colloid. Interface Sci., 1989,127, 373. (3)Fletcher, P. D. I.; Rees, G. D.; Robinson, B. H.; Freedman, R. B. Biochem. Biophys. Acta, 1986,832,204. (4) Ruckenstein, E.; Karpe, P. J . Colloid. Interface. Sci., 1990,139, 408. (5) Fletcher,P. D. I.;Freedman, R. B.; Mead, J.;Oldfield,C.; Robinson, B. H. Colloids Surf. 1984, 10, 193. (6) Das, M. L.; Bhattacharya, P. K.; Moulik, S. P. Langmuir 1990, 6, 1591. (7) Gupta, S.;Mukhopadhyay, L.; Moulik, S. P. Colloids Surf. B 1996,35,533. (8)Mukhejee, K.; Moulik, S. P.; Mukhejee, D. C. Int. J . Chem. Kinet. Submitted. (9) Friberg, S.; Rydhag, L.; Lindblom, G . J . Phys. Chem., 1973,77, 1280. (10)Moya, M. L.; Izquierdo, C.; Casado, J. J . Phys. Chem., 1991,95, 6001. (11)Valiente, M.; Rodenas, E.; J . Colloid, Interface Scz. 1989,127, 522. (12) Valiente, M.; Rodenas, E. J.Phys. Chem., 1991,95, 3368. (13)Leis, J.; Mejuto, J. C.; Pena, M. E. Langmuir, 1993,9,889. (14) Datta, J.; Bhattacharya, A.; Kundu, K. K. 2nd. J . Chem., 1988, 27A, 115. (15) Valiente, M.; Rodenas, E. Langmuir, 1990,6 , 775. (16) Valiente, M.; Rodenas, E. J. Colloid Interface Sci., 1990, 138, 299.
0743-746319512411-2866$09.00/0
enhances the rate of the reaction below their cmc's, the nonionic surfactant Triton X-100 hardly affects the dynamics of the process below and above its cmc.14 Introduction of 1-hexanol and 1-octanolin cationic micellar medium has a n inhibitory effect whereas cyclohexane has shown a rate accelerating effect.15J6 In microemulsion medium, reversibility of the reaction has been observed; salt (KJ3r)has shown a rate inhibitory effect.l' KC104is especially known for this effect.13 Although several studies on the alkaline fading of CV appear in the literature, a comprehensive work in the area is lacking; in particular, the energetics and salt effects on the process have not been examined in compartmentalized environments. We have, therefore, undertaken a n elaborate investigation on the said reaction in three different microemulsion media, two aqueous (water/AOT/ decane and waterlAOT/isooctane) and one nonaqueous (ethylene glycol/AOTlisooctane). Complementary kinetic measurements in aqueous as well as in ethylene glycol media have been also made. The use of nonaqueous microemulsion in the study of reaction dynamics is only rare. The energetics of the hydrolytic process and the salt effect on it have been also investigated along with the rate influencing effects of the said microemulsions a t a good number of compositions. It is important to note that CV has significant solubility in water or ethylene glycol and poor solubility in both isooctane and decane. Thus the reaction between CV+and OH- essentially takes place in the region least influenced by the interfacial phase between oil and water or oil and ethylene glycol bridged by the surfactant molecules of AOT. This study thus comprises an important part of our program of investigation of reaction dynamics under compartmentalized conditions.
Experimental Section Materials. AOT [bis(2-ethylhexyl)sulfosuccinate(Na salt)] was a product of Sigma having the specification reported earlier.I7 n-Decane,isooctane, and ethylene glycol were characterized AR or Excellar grade products of E. Merck and BDH. The dye crystal
violet and sodium chloride were also Exellar grade BDH products. Doubly distilled water was used throughout the study.
(17)Mukherjee, K.; Mukhejee, D. C.; Moulik, S. P.;Langmuir, 1993, 9, 1727.
0 1995 American Chemical Society
Kinetics i n Microemulsion Medium 1.01
L
It
1
Langmuir, Vol. 11,No.8, 1995 2867
1.6
0.2
1.4 0.
t
1 . 2 0.1
Wavelength ( n m )
mol dm-3) in different Figure 1. Spectra of CV (1.5 x media: 1 , aqueous; 2, isooctane/AOTlwater (w = 12) microemulsion; 3, EG isooctane/AOT/EG microemulsion and isooctandAOT/EGmicroemulsion containing 0.1 M NaCl; 4, decane/ AOTlwater (w = 12).
=I
1.0 O.! 0.2
i
-
0.8 0.r
0.6-0.:
Preparation of Microemulsions(ReactionMedia). Three types of microemulsions, viz. waterIAOTldecane, waterIAOT1
isooctane, ethylene glycol/AOTIisooctane, were used as the reaction media for the kinetic runs. To the solution of AOT in oil was added the requisite quantity of water and the mixture stirred to prepare the microemulsion. A general method of preparation may be found in earlier reports.' Aqueous solutions of crystal violet ( 1 m mol dm-3) and sodium hydroxide ( 1 mol dm-3) were prepared separately. The crystal violet solution was injected with a microsyringe into the microemulsion solutions to prepare the final reaction medium. The overall concentration ofcrystalviolet in the microemulsionwas 15pm mol dm-3. Sodium hydroxide solutionwas added to the final reaction medium before the kinetic measurements. The overall concentration of OH- in the microemulsion was 10 m mol dm-3. For the study of reaction kinetics in isooctane/AOTlethylene glycol microemulsion medium, both the crystal violet and sodium hydroxide solutionswere prepared in ethylene glycol. The overall concentration of the dye in the microemulsion was 15 pmol dm-3 and of sodium hydroxide was 5 m mol dm-3. In samples containing low percentages of EG, the reaction rates were too fast to measure accurately. In these cases, [OH-] = 2 m mol dm-3 was used. For the study of salt effects, sodium chloride solutions were prepared in water and ethylene glycol for the kinetic studies in decandAOTIwater, isooctandAOTIwater, and isooctane/AOT/ ethylene glycol microemulsions. Kinetic Measurements. Reaction kinetics was studied by spectrophotometric method in a Shimadzu 160 A UV-vis spectrophotometer maintained at a constant temperature (298 K) by circulating water from a constant temperature water bath (accuracy 10.02") surrounding the silica cuvettes (1 cm path length). Twelveternary mixtures of decandAOTIwater, f i h e n ternary mixtures ofisooctandAOTIwater, and eighteen ternary mixtures of isooctaneIA0Tlethylene glycol were used in the study. For the detailed study of temperature and salt effects,decane/ AOTlwater in weight percents 65125110 and 56130114;isooctane/ AOThvater in weight percents 6513015 and 55/30115;and isooctane/ AOTlethylene glycol in weight percents 5140155, 5135160, and 6013515 were used. The spectral measurements were taken at 1 = 590, 595, and 600 nm for aqueous, ethylene glycol, and microemulsion media, respectively. They are the Am= of CV in the respective media. The reaction was started by adding a microamount of sodium hydroxide solution into the microemulsion containing the dye and other components and automatic recording of the reaction course over time with the spectrophotometer.
Results and Discussion The visible spectra of CV in aqueous, decane/AOT/water, isooctane/AOT/water, ethylene glycol (EG), and isooctane/ AOT/ethylene glycol microemulsion media at w = 12 are shown in Figure 1. The spectra consist of a sharp peak at I, = 590 n m for aqueous medium, at 1 = 595 nm for ethylene glycol, and at I, = 600 n m for microemulsion media with a shoulder at I, = 540 n m in water and
0.1 0.4-0.:
0.2- 0. 0-
10
20
30
40
50
E
T i m e / Secs
Figure 2. In &/At) vs time ( t )plot in different environments: 0,0, 0, and W represent EG, aqueous, isooctane/AOT/HzO (w = 3 ) microemulsion isooctane/AOT/EG ( w = 1 ) microemulsion media, respectively.
microemulsion media. For ethylene glycol and nonaqueous microemulsion media, the shoulder appeared at il= 555 nm. The peak and the shoulder correspond to a symmetrical and distorted helical isomer of CV, respectively.12
Rate Dependence on the Medium Composition. Representative diagrams of the kinetic runs in aqueous, ethylene glycol, and microemulsion media are presented in Figure 2. The reaction was made pseudo-first-order with respect to CV by the presence of excess (100-1000fold) NaOH.
CV+ + OH-
+
CV-OH
(1)
--d[CV+l - kl[CV+l dt
where, k l =kp[OH-I is the pseudo-first-order rate constant in excess [OH-] and k p is the second-order rate constant. The integrated form of eq 2 is
[CV+I, A, In -= In - = klt [CV+I, At
(3)
where the subscripts zero and t refer to the time of start and time elapsed during the reaction, respective1y;Ao and A, are the respective absorbance of CV+. Good straight lines are obtained in all the reaction media. The pseudofist-order rate constants (12,) are calculated from the slopes of the straight lines of lnAdAt vs time ( t )according to eq 3. I n microemulsion medium, the reaction rate was found to depend inversely on the concentration of the surfactant, AOT. The OH- ion activitywas reduced by the AOT anion. The effective (kinetic) concentration of the hydroxyl ion ([OH-],r) was, therefore, considered l/[AOT] times lower than its total concentration i.e. [OH-ltot,so kl = kz[OH-l,ff. The knowledge of [OH-] and [AOTI thus helped to estimate Kz from K1. The dependence of k l on [AOT] is depicted in
Mukherjee et al.
2868 Langmuir, Vol. 11, No. 8, 1995 -
2
.
0
~
-5
-2,*1
I2
L
I d
0
-3 I
-0.35 -0.30
I
-0.25
, , -0.20 -0.15
, -0.10
-0.05
, 0
I
1:
0.05
Log [ A O T 1
Figure 3. log k1 vs log [AOT] plot for isooctane/AOT/water microemulsion at w = 12.35.
waterIAOT1 isooctane
[AOTllmol
12.5125162.5 15130155 17.5135147.5 20/40/40 22.5145/22.5 25150125
0.45 0.54 0.67 0.79 0.93 1.07
[CVI = 1.5 x
dm-3
[OHll,,&mol dm-3 0.143 0.100 0.082 0.065 0.055 0.046
mol dm-3. [OH-l,,,,ii
kl x
kdmol-l
1O31s-l 5.7 3.2 2.8 1.7 1.6 0.83
dm3 s-l
=1 x
I
I
4
8
E
c1
E!
X N
Y
j
-1
Table 1. Rate Constants of Alkaline Fading of CV in Water/AOT/lsooctane at a Constant [H~OY[AOTIRatio (w = 12.36) with Different AOT Concentrations at 298 KO
a
'V
-4
-3.0
-3.21
c
I
I
12
16
I
20
0
0.018 0.017 0.022 0.020 0.027 0.019
mol dm-3.
Figure 3 . The log kl vs log [AOT] plot was found to be fairly linear with least square fitting to the equation log k l = -2.10 log [AOTI - 2.99 with a correlation coefficient of 0.90. It has been observed that the rate constants are lower in decanelAOT1water than in isooctanelAOT1water medium. It is higher in a n isooctane/AOT/ethylene glycol microemulsion environment. Compared to the aqueous medium, the rate constant is higher in ethylene glycol. The rate constants k1 and kp a t constant w in the water1 AOTIisooctane microemulsion medium are presented in Table 1. k 1 has been rightly found to depend on the local [OH-] whereas kz has virtually remained unchanged. On the other hand, in watedAOT1decane and waterlAOT1 isooctane, both kl and k p have been found to decrease (Figure 4a). In the latter medium, while kl decreases with w , kz practically becomes invariant a t w > 8. Leis et al.13 have reported variable kl but invariant kz with w in the same microemulsion medium. The variable kz values support a nonspecific effect (or effects) on the reaction dynamics; k1 is not solely guided by [OH-] as schemed above. The constancy of kz a t w > 8 in water/ AOTIisooctane medium advocates the minor role of the nonspecific effects under the compartmentalized condition. Figure 4b shows the variation of k1 and kp with w (here the mole ratio of EG and AOT) in isooctanelAOT/ethylene glycol microemulsion medium. The nature of variation is the same as the aqueous microemulsion. In both the figures both kl and kp become prominently large at lower w , the rises are earlier in the nonaqueous microemulsion than in the aqueous. The effects are comparatively sharp for kl than k2. Figure 5 depicts the dependence of k1 on the effective concentration of the OH- ion in the aqueous pool of decane/AOT/water microemulsion medium. The slope of the straight line gives the second-order rate constant, K Z = 0.02 mol-' dm3s-l (cf. primed closed triangle point, curve 3, Figure 4a). A similar nature of variation has been observed in CTAB-hexanol reverse micellar media by Valiente et a1.lZ k1 and ka in the different water/
.4
'uo 2
4
6
1 0 1 2
8
EG /AOT
Figure 4. (a) Variation of kl and kz with the mole ratio of [H20]/[AOT](w)in different media: curves 1 and 3, k l and kz for decandAOT1water microemulsion; 2 and 4,kl and KZ for
isooctane/AOT/water microemulsion. (b)Dependence o f & and on w in isooctane/AOT/EG microemulsion. Curves 1and 2, k l and k2.
k2
0.012 c
I
\ x-
0
0.1
0.2
0.3
[OHJ/mol
0.4
0.5
dni3
Figure 5. Dependence of kl on [OH-] in decandAOT1water microemulsion at w = 11.3.
AOTIisooctane microemulsion media are presented in Table 1. It has been reportedl8 that the alkaline fading of CV in water is accelerated by the presence of cosolvents (18)Mandal, U.; Sen, S.; Das, K.; Kundu, K. K. Can.J.Chem., 1986, 64, 1638.
Langmuir, Vol. 11, No. 8, 1995 2869
Kinetics in Microemulsion Medium Table 2. Energetics ParameterP for the Alkaline Fading of CV in Different Media at 298 K AG*kJ AH%J AS*/JK-l medium aqueous ethylene glycol
mol-’
mol-1
- 4.0
mol-’
- 3.8
87 (75) 32 (32) -184 (-145) 86 (73) 43 (43) -143 (-99)
waterlAOTldecane 87 (83) 24 (24) -226 (-195)
-3.6
84 (81) 9 (9) -249 (-242) 87 (82) 11 (11) -254 (-239)
-3.4
ethylene glycoVAOTIisooctane w = 12.28 87 (76) 39 (39) -163 (-124) w = 1.5 79 (73) 29 (29) -169 (-149)
- 3.2
w = 8.2
waterlAOT1isooctane w = 4.12 w = 12.35
a
Values in parentheses correspond to the second-orderreaction.
such as dioxane and ethylene glycol having low dielectric constants. The lowering of the dielectric constant of the reaction medium enhances the reaction between the CV+ and OH- ions.ll The results presented in Figure 4a,b reveal that the rate constants for the nonaqueous microemulsion are greater than the aqueous microemulsion. This is justified because the dielectric constant of the reaction medium in the ethylene glycol pool is lower than that the aqueous pool. It is oRen reported that the reaction rate influencing effect in microemulsion medium depends on w but is independent of the concentration of the amphiphile. The results presented in Table 1 are contradictory to this expectation. At w = 12.35, there has been an &fold rate decrease for a 2.4-fold increase in [AOTI. The results presented in Figure 4a for isooctane/AOT/ water (curves 2 and 4) are with references to measurements taken a t variable [AOT]. Activation Parameters for the Reaction. The energetics of the activation process of the fading kinetics in aqueous ethylene glycol and microemulsion media are presented in Table 2. Processingofthe temperature effect on the reaction rate constant to evaluate E 2 (energy of activation) according to the Arrhenius equation, K = Ae-EJRT, is presented in Figure 6. The enthalpy of activation then followed from the relation AI+ = E,* RT. The estimation of the free energy of activation from the relation kl = (kT/h)e-AG*’RT (where k and h are the Boltzmann constant and Planck’s constant, respectively) then resulted in the evaluation of AS* from the Gibb’s Helmholtz equation AG* = - TAS*. The free energy of activation in isooctane/AOT/water microemulsion medium a t w = 4.12 is to some extent lower than that in the aqueous medium. At w = 12.35, the free energy of activation is more or less the same as in the aqueous medium. The enthalpy of activation in the microemulsion medium increases with an increase in w . Accordingly, in aqueous medium, the enthalpy of activation is a maximum. The lowering of the enthalpy of activation in microemulsion medium supports the catalysis of the reaction through preferential incorporation of CV in the microdroplets of either water or ethylene glycol to favorably react with OH-. For the nonaqueous microemulsion, the enthalpy of activation is lower at [EG]/[AOTl = 1.5 than at 12.28. Both the enthalpy and entropy of activation increase in ethylene glycol medium. The free energies of activation are more or less the same in ethylene glycol and isooctane/ AOT/ethylene glycol microemulsion a t [EG]/[AOT]= 12.28. The ASt of activation does not follow the AS*= - 4 1 . 8 2 ~ 2 ~ correlation approximately observed for ionic reactions.l9 The AS* values are strikingly negative and greater in aqueous microemulsion media. Stabilization of the activated complex seems to be a primary requirement for (19)Laidler, K.J.Chemical Kinetics, 3rd ed.; Harper and Row: New York, 1987;p 195.
-3.0
- 2.8
~
3.2
3.3
3.4
3.5
i : 10’ Figure 6. In k vs T-l plot in different environments: 1, isooctane/AOT/water microemulsion (w = 4.1); 2, isooctane/ AOT/EG microemulsion (w = 1.5); 3, decane/AOT/water microemulsion (w = 8.2);4,EG;5, isooctane/AOT/water microemulsion (o= 12.3);6,aqueous; 7,isooctane/AOT/EG microemulsion (w = 12.28).
the propagation of the reaction. This is reversed in EG medium as well as in isooctane/AOT/EG microemulsion at w = 12.28. The Al? and AS* values obtained in different media have shown isokinetic behaviorz0(Figure 7). The kl- and &related best fit (least squares) lines are presented in the figure. The compensation temperatures derived from the slopes are 276 and 236 Kfor the first-order and secondorder kinetics, respectively. The experimental temperature was 298 K. Effect of Ionic Strength. The dependence of the rate constant k1 on ionic strength 01) for aqueous, ethylene glycol, and microemulsion media is presented in Figure 8 and Table 3. Linear variations are obtained in all cases; the Bronsted-Bjerrum equation (4) is obeyed:19
log K , = log &),
+ l.OlSZ,Z,~
(4)
where (k& is the hypothetical rate constant a t zero ionic strength and ZAand ZBare the charges on the reacting species CV+ and OH-, respectively. Since ZAZB= - 1, the slopes of the lines are expected to be -1.018 a t 298 K. The reaction rates decrease in the presence of added NaCl in the aqueous and nonaqueous microemulsions. A reduction (20)Pommier, C.F.C.; Guiochon, G.;J.Phys. Chem.,1971,75,2632; Pimental, G.C.; McClellan, A. L.; Annu. Rev. Phys. Chem., 1971,22, 347;Moulik, S.P.;Ray, S.; Das, A. R. J.Phys. Chem., 1978,80, 157; Lumry, R.; Rajender, S. Biopolymers, 1970,9,1125.
Mukherjee et al.
2870 Langmuir, Vol. 11, No. 8, 1995
Table 3. Effect of Ionic Strengthaon the Pseudo-First-OrderRate Constant of Alkaline Fading of CV in Different Media at 298 K p/mol dm-3 kl x 10-3/s-1 film01 dm-3 kl x 103/s-l ethylene glycol medium aqueous medium 0 5.89 0 4.57 0.005 4.80 0.01 3.08 0.025 4.75 0.11 1.85 0.055 4.27 0.21 1.74 0.105 4.03 0.31 1.50 0.205 3.28 0.61 1.12 0.305 2.75 1.21 0.36
- 200
-300
-100
AS#/ J K - ‘ ~ ~ I - ’ Figure 7. Enthalpy-entropy compensation plot in different reaction media: lines 1 and 2,kl and kz. 0,A, 0,0 , O represent aqueous, EG, decane/AOT/water (w = 8.2),isooctane/AOT/EG and isooctane/AOT/water ( w = 4.121,respectively. Primed and unmimed svmbols rewesent the data at w = 12.28 and 1.5, respectively, for the isboctane/AOT/EG system. For line 1,i point 0’is excluded from the least square calculation. ’I correlation coefficient for line 1 is 0.98,line 2, 0.99.
ethylene glycol/AOT/isooctane
water/AOT/isooctane w = 4.14 0 34.00 0.24 10.00 0.34 9.70 0.44 7.60 0.54 6.20 0.84 5.24 1.44 3.31
w = 9.86 0 0.009 0.170 0.243 0.330 0.420 0.560
3.90 3.10 2.90 2.80 2.72 2.20 2.34
Zero ionic strength value obtained by extrapolation,
ethylene glycol and isooctane/AOT/ethylene glycol ( w = 9.86)microemulsion medium, the slopes obtained are -0.6 and -0.61, respectively. These values are 40%lower than expected. Ethylene glycol, having a lower dielectric constant than water, may cause the NaCl to be partly dissociated. The actual ionic strength of NaCl in EG and in microemulsions containing EG may be lower than that in water. However, the Debye-Huckel coefficient (Q)in the slope of eq 4 (1.018) is 0.51 for aqueous medium.lg Making use of the dielectric constant of EG, the true coefficient Q in ethylene glycol is 2.14. Equation 4 then takes the form
or
log 12, = log (kJ1
0.2
0.4
0.6
0.8
1.0
1.2
p‘/2
Figure 8. Dependence of log K on 4 in different environments: 1, isooctane/AOT/water microemulsion (w = 4.1); 2, EG; 3,isooctane/AOT/EG microemulsion (w = 9.8);4,aqueous.
of 50% of the reaction rate by the addition of 1.5 mol dm-3 NaCl to the isooctane/AOT/water microemulsion has been reported by Leis et al.13 Slopes of the lines for the aqueous, decane/AOT/water (w = 11.341, and water/AOT/isooctane (w = 4.14) microemulsion media obtained are - 1.0, -1.1, and -0.96, respectively, in the present study. The data of Leis et al.13correspond to a slope of -0.9. The values are very close to the theoretical value of -1.018, For
+ 4.28ZAZ,&
(5)
The true slope should be 7 times greater than what has been experimentally realized. To adjust 0.60 to 4.28, the ionic strength of NaCl in EG medium has to be insignificant, which rules out the possibility of rationalization on the basis of the much altered dissociation of NaCl in EG. Conductance measurements of KC1 and NaCl in EG medium have evidenced no ion association but complete ionization of the electrolytes in the solvent.21s22It is the changed (lower) dielectric constant of EG that has increased the coefficient of eq 4 from 1.018 t o 4.28 (eq 5). A reduction of it to 0.60 (the experimental coefficient) requires the polarity of EG to be about 4 times greater than that of water, which is impossible. The salt effect in EG medium thus remains little understood. Valiente and Rodenas13have reported special salt effect of NaC104 on CV+ and OH- reaction. A rate inhibition of 400 times by 1.2 mol dm-3 NaC104 has been observed in aqueous medium. The clod- ion specifically interacts with CV+. It may be likely that CV+ specially interacts with C1- ion in EG medium, leading to a strong inhibitory effect. But spectral evidence (Figure 1)supports no such interaction that Valiente and Rodenas13observed for CVf and C104-. The rate reducing effect of NaCl in EG medium thus remains unsolved. The dielectric constant ofwater in the microaqueous pool of the watedoil microemulsion is only (21)Accascina, F.;DAprano, A.; Goffredi, M. Ric. Sci. Rend. Sez. A, 1964,6, 151. (22) Santos, M. C.; Spiro, M.
J.Phys. Chem., 1972,76, 712.
Langmuir, Vol. 11, No. 8, 1995 2871
Kinetics i n Microemulsion Medium
Table 4. The Parameters of Equations 6 and 6 for the Alkaline Fading of CV in Different Media at 298 K Bronsted-Bjerrum medium
log (kdl
aqueous water/AOT/isooctane (w = 4.12) ethylene glycol ethylene glycoVAOT/isooctane (w = 9.8)
-2.3 -1.4 -2.2 -2.4
slope 1.0 0.96(3.3) 0.6(4.3) 0.6 (4.3P
Bronsted-Bjerrum-Pitzer In (k0h
slope
A22
-5.5 -4.5 -5.1 -5.6
0.80 1.24 0.60 0.60
2.10 1.90 0.30 0.30
a The evaluation of Q of eq 5 for an ethylene glycol/AOT/isooctanemicroemulsion is based on a dielectric constant of 32. Theoretical slopes are shown in the parentheses.
roughly known. According to a recent e ~ t i m a t ea, ~ tw = 4.12 in water/AOT/isooctane, the dielectric constant is nearly 55. Thus, the Q coefficient of eq 5 and consequently its slope becomes 3.3; the observed slope is much lower (0.96). The salt effect in microemulsion medium is, therefore, also not straightforward. In Table 4, the log (k& and the Bronsted-Bjerrum slopes in different media are compared. Very recently, TajedaZ3 has combined Pitzer’s equation with the Bronsted-Bjenum equation to rationalize the salt effect on the Fe(CN)s3- and Ireaction in aquo-organic solvents. The combined equation has the following form:
In K, = In (KO), + U 2 A #
(6)
where X =
1
+1.24
+ 2ln(l+ 1 . 2 4 )
1 A, = 41.3287 3
1.2
x 105~)/(~T)3’2
Q being the density of the medium. The other terms have their usual significance. The slopes obtained by plotting In k l vs x are also given in Table 4 along with In (k& and AZ2 values. In aqueous and in water/oil microemulsion media the slope values obtained are 0.8 and 1.24, respectively, and the corresponding AZz values are 2.1 and 1.9. The expected AZ2 = 22 - (12 12)= 2. The salt effect as represented in the Bronsted-Bjenum-Pitzer (B-B-P) theory has been reasonably coroborated by the aqueous and aqueous microemulsion media. There has been wide departure in the case of EG and EG-containing microemulsions. The theory with the dielectric constant correction in the microwater pool appears to be sufficient for the quantification of the kinetic process in the aqueous salt environment. Other salt-induced effects on the kinetics are, therefore, considered minor. The above rationale fails in the EG environment. But the identical lower slopes and AZ2 values obtained for the B-B-P relation in presence of NaCl rules out the possibility of additional effects in the microenvironment of the micro EG pool t h a t can influence the reaction kinetics. This exceptional effect of EG needs critical attention. In ethylene glycol and a n EGIisooctane microemulsion the B-B-P slopes are both 0.6 and the corresponding AZ2 is 0.3. It is much lower than the expected value of 2.0. The abnormality of the reaction in EG and EG-containing microemulsion is again evident. The B-B-P theory is thus a better proposition to rationalize the salt effect.
+
General Remarks One may debate about the location of CV+ in compartmentalized liquids. Leis et al.13have proposed that CV+ (23)Tajeda, P.P.J. Chem. SOC.Faraday Trans. 1993,89,2011.
preferentially resides in the location where water has its bulk property; in a watedoil microemulsion this may not be necessarily the interior of the aqueous pool but a t a depth below the watedoil interface. From a partitioning experiment, we have observed that CVf exclusivelyresides in the aqueous medium rather than either the isooctane or decane medium; this preference is also maintained when water is replaced by ethylene glycol. It may be argued that CV+ remains very close to the interface of the compartments in the vicinity of the AOT anion; the reaction with OH- ion is thus inhibited. But a t lower w , where chances of intimacy between CV+ and AOT- are greater, the rate constant increases instead. CVf species thus reside well below the interface and the increased rate constant under the compartmentalized condition is due to other effects. This is further supported by the nearly equal Bronsted-Bjenum slope observed both in aqueous medium and in the aqueous micropool (Table 4) at a small w value of 4.12. The significant lowering of the BronstedBjermm slope for the salt (NaC1)effect in ethylene glycol medium speaks in favor of nonspecific solvent effects. The fact that14 CTAB accelerates and SDS decelerates the reaction rate below their cmc values and the inertness of the nonionic surfactant T X l O O toward the process suggest the positive role of ionic environment on the kinetic process. At the concentrations above cmc, however, a rate decelerating effect has been observed. The activation energies (i.e. the activation enthalpies) are much lower in an aqueous microemulsion but higher in EG and a n EG/ AOT/isooctane microemulsion. Catalytic pathways (both positive and negative) are involved for the apparently straightforward reaction. The stabilization of the activated complex before product formation (reflected on the entropy of activation) is a key factor to the catalytic process. On the whole, the compartmentalized systems are considerably complex in nature; for a physicochemical understanding, further kinetic studies are wanted.
Conclusions The results presented led to the following conclusions: (1)Pure ethylene glycol and ethylene glycol in microemulsion medium accelerate the alkaline fading of CV+ compared to the aqueous environment. (2) The rate enhancement is significantly affected a t lower mole ratios of water/AOT and ethylene glycol/AOT. (3)The energetics of the process exhibit stabilization of the activated complex in a microemulsion and a n isokinetic effect. (4) The salt effect is normal in water-containing media but quite different in media containing ethylene glycol.
Acknowledgment. This work was funded by the Department of Science and Technology, Government of India, with fellowships to L.M. and N.M. LA940236E
