ARTICLE pubs.acs.org/JPCA
Study of the Alkaline Fading of Phenolphthalein in Microemulsions Shiyan Mao,† Zhiyun Chen,‡ Xueqin An,‡ and Weiguo Shen*,†,‡ † ‡
Department of Chemistry, Lanzhou University, Lanzhou, Gansu, 730000, China Department of Chemistry, East China University of Science and Technology, Shanghai, 200237, China ABSTRACT: The reactions of the alkaline fading of phenolphthalein (PN) have been studied in water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/ isooctane microemulsions by monitoring the absorbance changes of PN in the system with the time and the results compared with those found for the same reactions in aqueous solutions. It was found that the values of the equilibrium constants and the forward reaction rate constants in the microemulsions were significantly larger than that in aqueous solutions and decreased with increasing the molar ratio of water to AOT (ω), except for that with low ω. The temperature dependence of the reaction rate constant was analyzed to obtain the values of free energy, enthalpy, and entropy of activation, which suggests the existence of an isokinetic relationship and a common mechanism for the reactions occurring in the microemulsions with different ω. It was also observed that the competition between the reactions of the alkaline fading of PN and the hydrolyzation of AOT in water/AOT/isooctane microemulsions when the reaction time was sufficiently long.
1. INTRODUCTION Microemulsions are macroscopically homogeneous and thermodynamically stable systems of water nanodroplets dispersed into apolar solvents in the presence of adequate surfactants.1 AOT is one of the few ionic surfactants that forms microemulsions without addition of any cosurfactants due to its double-tailed nature.2 The microemulsions consisting of AOT, water and oil has been widely investigated as a model system, since it is very well characterized, and stable over a wide range of concentration and temperature.35 A microemulsion droplet of water in oil can be considered as a microreactor that provides special medium for chemical reactions and enhances or retards the reaction rates.612 This reactor provides a good approximation to the real conditions under which the biological reactions take place in vivo at the cellular level and a promising future opened to physical chemistry in the many industry fields.3 It has attracted many attentions to compare the behavior of the reactions in conventional aqueous media with those observed in microemulsions. Various spectroscopic methods based on application of indicator dyes are suitable for the investigation of microemulsions, because the spectral properties of these molecular probes are very sensitive to the microenvironment.13 The indicators have been applied to evaluate the proton activity,14 the concentration of hydroxonium ion,15 pH,16,17 and so on, in the water pool of the microemulsions. PN is one of the most common acidbase indicators, and useful not only for acidbase titration but also in simple quantification of ultrasonic intensity18 and monitoring a chemical reaction.19,20 However, the applications of any indicator, particularly in the study of a kinetic process, require a fast rate of the interconversion between the colored and colorless species of the indicator. The kinetic behavior of the alkaline fading of PN in r 2011 American Chemical Society
alkaline aqueous media has been studied by monitoring the absorbance of the reaction system and found that the reaction was reversible and the rate constants of the forward and the reverse reactions and the equilibrium constant were 5.7 103 L mol1 s1, 8.6 105 s1, and 67 L mol1 at 298.2 K, respectively.21 These lower values of the rate constants and the equilibrium constant limit the application of PN as an indicator in its alkaline aqueous solution. The reactions of the alkaline fading of crystal violet (CV), the colored ion of which has the opposite charge of the AOT headgroup, in AOT microemulsions and pure water have been extensively studied,6,8,9 and it was found that the reactivity was significantly lower in microemulsions than that in the aqueous solution. The colored ion of PN has the negative charge as AOT headgroup; therefore, the different kinetic behavior would be expected in AOT microemulsions as comparing with that of CV. However, no attempt was made to the study of the alkaline fading of PN in microemulsions. In this paper, we report the fascinating results found in studying the alkaline fading of PN in water/AOT/isooctane microemulsions. For comparative purposes, the same reaction was also studied in aqueous media.
2. EXPERIMENTAL SECTION 2.1. Materials. AOT was purchased from Aldrich Chemical Co. and was dried over P2O5 in a desiccator for 2 weeks. Received: March 9, 2011 Revised: April 25, 2011 Published: May 16, 2011 5560
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Figure 1. Absorption spectra of PN in alkaline aqueous solutions and in water/AOT/isooctane microemulsions at 298.2 K. (a, b) In the aqueous solution at initial time and after the equilibrium of eq 3 achieved, [OH] = 5 103 mol L1; (c) in the aqueous solution after the equilibrium of eq 3 is achieved, [OH] = 2 mol L1; (d, e) in the microemulsions at initial time and 6 h after the reaction began, [OH] = 5 103 mol L1, ω = 12.
Figure 2. Absorbancetime plots for the alkaline fading of PN in the aqueous solution at 298.2 K, [OH] = 1.3 102 mol L1: (a) at 553 nm; (b) at 243 nm.
Isooctane, PN, and sodium hydroxide (NaOH) were supplied by Tianjin Second Chemical Co., Tianjin Jingda Fine Chemical Co., and Tianjin Chemical Reagent Co., respectively, and used as received. All the reagents were of analytic grade. Twice distilled water was used for preparations of all samples. 2.2. Kinetic Measurements. An aqueous solution (1) with the concentration of sodium hydroxide being about 2 mol L1
and a water/PN/ethanol solution (2) with the concentrations of PN and ethanol being 7.9 103 mol L1 and 70% in volume, respectively, were prepared. The accurate concentration of the sodium hydroxide in solution (1) was determined by titration with potassium hydrogen phthalate. For the study of the kinetics of the reaction in the aqueous solution, an appropriate amount of the PN solution (2) was diluted by water to a desired concentration. This solution was sat in a thermostat with required constant temperature at least 20 min, and then an appropriate amount of sodium hydroxide solution (1) was transferred with a microsyringe into it and the kinetic measurement started. The concentration of PN was 2.2 105 mol L1 in the reaction system. Each of the samples for studies of the kinetics of the reactions in the microemulsions were prepared as follows: Appropriate amounts of AOT and water were weighed according to the desired ω and the overall concentration of AOT ([AOT]). The mixture was dissolved and diluted by appropriate amount of isooctane and stirred to form the microemulsion. The PN solution (2) was injected with a microsyringe into the microemulsion to prepare the final reaction medium. This reaction medium was sat in a thermostat at a required temperature at least for 20 min; then an appropriate amount of the sodium hydroxide solution (1) was added into it and the kinetic measurement started. The overall concentrations of AOT and PN were 0.3 mol L1 and 2.2 105 mol L1 in the microemulsion, respectively. Kinetic measurements and spectral studies were carried out in an Agilent 8453E UV spectrophotometer with a thermostatted 5561
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Figure 3. Absorbancetime plots for the alkaline fading of PN in water/AOT/isooctane microemulsions of ω = 12 at 298.2 K and 553 nm: (a) [OH] = 5 103 mol L1; (b) [OH] = 1.3 102 mol L1; and (c) [OH] = 2.6 102 mol L1.
multiple-cell holder, which was capable of keeping the temperature in the sample cell to be constant within (0.1 K.
3. RESULTS AND DISCUSSION 3.1. Phenomenon and Mechanism. Alkaline fading of PN was studied both in aqueous solutions and in water/AOT/ isooctane microemulsions with various concentrations of OH in the range of ω from 8 to 20. It was found that the microemulsion reaction systems with ω less than 8 were unstable, some precipitations were observed at the bottom of the sample cells; therefore the investigation for the lower ω is restrained. The reaction of alkaline fading of PN may be described by2123
H2 L h HL þ Hþ
ð1Þ
HL h L2 þ Hþ
ð2Þ
k1
L2 þ OH a LOH3 k1
ð3Þ
where H2L and HL are colorless lactones, L2 is red-pink quinonoid and LOH3 is colorless carbinol. With the advantage of the Agilent 8453E UV spectrophotometer we were capable to record the whole spectrum simultaneously and examine the change of the spectrum in the reaction course. We found that L2 had a maximum of absorption at 553 nm in alkaline aqueous solution and at 557, 555, and 553 nm for ω = 8, 10, and g12 in the microemulsions, while LOH3 had a maximum of absorption
at 243 nm in the alkaline aqueous solution and at 248 nm in the microemulsions. When the reaction took place in an aqueous solution, the absorbance at 553 nm decreased with time and approached to a constant while the absorbance at 243 nm increased with time and approached to a constant as the chemical equilibrium reaches. Increase of the concentration of OH ([OH]) resulted in increasing the equilibrium value of the absorbance at 243 nm and decreasing the equilibrium value of the absorbance to zero at 553 nm. Those behaviors are typically shown in Figure 1ac and more clearly in Figure 2, which confirms the reaction mechanism described by eq 3. However, when the reaction took place in the microemulsions, the spectra showed the different shapes, as demonstrated in Figure 1d,e; even when the concentration of OH is very low, the equilibrium value of the absorbance at 553 nm seemed to approach to zero. The changes of the absorbance with time at 553 and 248 nm are shown more clearly in Figures 3 and 4. The absorbance at 553 nm decreases initially and passes a minimum, then increases until a maximum. After that, it decreases again and very slowly approaches zero after 1000020000 s dependent on [OH], while the absorbance at 248 nm increases first and passes a maximum and then decreases with time and approaches to a constant very slowly as the absorbance at 553 nm decreases slowly to zero. This phenomenon was observed for the microemulsions with different OH concentrations, different molar ratios of water to AOT and different temperatures. This abnormal behavior may be explained by the hydrolyzation of AOT in an alkaline environment,8,24,25 which consumes OH and shifts equilibrium of eq 3 to the left. Because hydrolyzation of AOT is slow reaction, conversion of L2 to LOH3 is dominating until it reaches 5562
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Figure 4. Absorbancetime plots for the alkaline fading of PN in water/AOT/isooctane microemulsions of ω = 12 at 298.2 K and 248 nm: (a) [OH] = 5 103 mol L1; (b) [OH] = 1.3 102 mol L1; (c) [OH] = 2.6 102 mol L1.
equilibrium at the minimum of the absorption of L2 represented by point D in Figure 3 and the maximum of the absorption of LOH3 represented by point F in Figure 4; then the hydrolyzation of AOT turns to be dominating and reduces LOH3 to form L2. As the reaction of AOT hydrolyzation further proceeds, the continuously consumption of OH results in the reductions of both L2 and LOH3 to form HL and H2L through reactions described by eqs 2 and 1 after the particular values of reaction time represented by point E in Figure 3. It is worth mentioning that the values of the reaction time at points D and F in Figures 3 and 4 are correspondingly the same, for example, about 810 and 850 s, indicated in Figures 3b and 4b, respectively. It confirms the mechanism of the complex reactions discussed above and determines the characteristic equilibrium values of the reaction described by eq 3. Table 1 summarized the characteristic equilibrium values of reaction time (te) and the absorbance at 553 nm (Ae). 3.2. Kinetic Data Analysis. The mechanism of the reversible alkaline fading reaction of PN may be represented by eq 3, in which k1 is the second-order rate constant for the forward reaction and k1 is the first-order rate constant for the reverse reaction. Since the concentration of OH was much larger than that of PN in the experimental design, thus it was practically constant in the reaction process, we could combine it with k1 to form a pseudo-first-order rate constant k10 , that is, k10 = k1[OH]. A reversible first-order reaction obeys the kinetic law: d½L2 0 ¼ ðk1 þ k1 Þð½L2 e ½L2 Þ dt
Table 1. Characteristic Equilibrium Values of Reaction Time (te) and the Absorbance at 553 nm (Ae) for the Reaction Described by eq 3 in the Text for the AOT Microemulsion Medium with Various ω and [OH] at 298.2 K ω
102[OH]/mol L1
te/s
Ae
8
1.3
580
0.054
10
1.3
690
0.037
12
0.5
1480
0.134
12
1.3
890
0.036
14 14
0.5 1.3
1590 910
0.142 0.039
16
1.3
1010
0.045
18
1.3
1160
0.050
20
1.3
1300
0.052
2 2 where [L ] is the concentration of colored form L at any time 2 2 t and [L ]e is the equilibrium concentration of L . Combining with the LambertBeer law and integration of eq 4 yields 0
A ¼ Ae þ ðA0 Ae Þexpððk1 þ k1 ÞtÞ
ð5Þ
where A0 is the absorbance when all PN is in the colored form, Ae and A are the absorbance at equilibrium and at any time t, respectively. A0 and Ae are constants. The equilibrium constant K is calculated from K ¼
ð4Þ 5563
½LOH3 e A0 Ae ¼ 2 ½L e ½OH Ae ½OH
ð6Þ
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Table 2. Values of Ae, k1, k1, and K for the Alkaline Fading of PN in Microemulsions and Aqueous Solutions with Various ω and [OH] at 298.2 Ka ω
102[OH]/mol L1
Ae
102k1/L mol1 s1
104k1/s1
K/L mol1
8b
1.3
0.053
4.15 ( 0.46
3.40 ( 0.28
122 ( 10
10b 12b
1.3 0.5
0.036 0.132
4.85 ( 0.57 4.11 ( 0.18
1.84 ( 0.16 6.93 ( 0.25
264 ( 21 59 ( 2
12b
1.3
0.035
4.93 ( 0.27
1.49 ( 0.06
331 ( 12
14b
0.5
0.140
3.96 ( 0.15
5.14 ( 0.16
77 ( 2
14b
1.3
0.038
4.68 ( 0.40
2.48 ( 0.16
189 ( 11
16b
1.3
0.043
4.04 ( 0.48
2.56 ( 0.23
157 ( 13
18b
1.3
0.048
3.66 ( 0.86
2.62 ( 0.45
140 ( 23
20b
1.3
0.050
3.35 ( 0.16
3.01 ( 0.11
111 ( 4
aqueousb aqueousb
0.5 1.3
0.579 0.413
0.52 ( 0.04 0.64 ( 0.03
0.78 ( 0.07 0.97 ( 0.05
67 ( 1 66 ( 1
aqueousc
0.5
(0.580)
0.53 ( 0.03
0.77 ( 0.06
69 ( 1
aqueousc
1.3
(0.412)
0.66 ( 0.03
0.96 ( 0.04
69 ( 1
aqueousd
1.0
0.57
0.86
67
The values of Ae in the bracket are from measurements. b Data from fitting eq 5 with Ae being an optimal parameter. c Data from fitting eq 5 with Ae being measured values. d Data from ref 21. a
Table 3. Values of k1 for the Alkaline Fading of PN in Aqueous Solutions and Microemulsions with Various ω under Four Temperatures and [OH] = 5 103 mol L1 293.2 K ω
a
298.2 K
1 1
2
10 k1/L mol
s
2
10 k1/L mol
301.2 K
1 1
s
2
10 k1/L mol
303.2 K
1 1
s
10 k1/L mol 1 s1 2
8
1.87 ( 0.10
3.25 ( 0.17
3.38 ( 0.17
3.46 ( 0.18
10
2.44 ( 0.13
4.00 ( 0.20
4.46 ( 0.23
4.72 ( 0.24
12
2.99 ( 0.15
4.02 ( 0.21
4.57 ( 0.23
5.22 ( 0.27
14
2.72 ( 0.14
3.58 ( 0.18
4.36 ( 0.22
4.85 ( 0.25
16
2.54 ( 0.13
3.47 ( 0.18
4.01 ( 0.20
4.67 ( 0.24
18
2.39 ( 0.12
3.27 ( 0.17
3.84 ( 0.20
4.45 ( 0.23
20 aqueous
2.16 ( 0.11 0.45 ( 0.03
3.11 ( 0.16 0.57 ( 0.03
3.65 ( 0.19 0.66 ( 0.04
4.10 ( 0.21 0.73 ( 0.04
aqueousa
0.45
0.57
Data from ref 21.
where [LOH3]e is the equilibrium concentration of LOH3. Thus, the measured absorbance values A at any time and Ae at the infinite time may be used to determine the values of (k10 þ k1), and A0 through eq 5, and K through eq 6, and further to obtain the values of k1 and k1. The measured absorbance A at 553 nm and various reaction time for the PN-alkaline fading in the aqueous solution with the [OH] being 5 103 mol L1 and 1.3 102 mol L1 at 298.2 K were used to determine the values of k1, k1, and K, and are listed in the last third line and the last second line of Table 2, respectively. In the initial reaction stage, the reverse reaction is negligible, thus ln A ¼ k1 0 t
ð7Þ
The absorbance data measured in the initial reaction stage for the PN-alkaline fading in aqueous solution with [OH] being 5 103 mol L1 at 293.2, 298.2, 301.2, and 303.2 K were used to fit eq 7 to obtain the vales of k1 at the above four temperatures, which are listed in the last second line of Table 3. The goodness of the fitting is shown in the plots of ln A against t in Figure 5. The results from the above two methods are in well agreement with
each other and with that reported in the literature,21,23 which are listed in the last lines of Tables 2 and 3. When the reactions took place in the microemulsion media, hydrolysis of AOT consumed OH and resulted in a competition with the reaction of the alkaline fading of PN in AOT microemulsions, however the second-order rate constant for the hydrolysis of AOT was estimated25 to be 4.4 104 L mol1 s1, and thus it was assumed to be negligible in the time range before the equilibrium point D in Figure 3 in the following discussions. We monitored the absorbance changes at the wavelength 553 nm with the reaction time for the microemulsions of different molar ratios ω of water to AOT with an overall concentration of OH being 1.3 102 mol L1 at 298.2 K. The experimental results were fitted to eq 5 to obtain the optimal values of Ae, (A0 Ae) and (k10 þ k1), combination of eq 6 and replacing [OH] by [OH]w (the concentration of OH in the water pool), gave the values of k1, k1, and K. The above fitting procedure was also used for analysis of the kinetic data of the reactions in aqueous solutions with the [OH] being 5 103 mol L1 and 1.3 102 mol L1 at 298.2 K. The results are listed in Table 2. The comparisons of the experimental values of the absorbance and 5564
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Figure 5. Plots of ln A vs t for the alkaline fading of PN in water/AOT/ isooctane microemulsions with various ω and the aqueous solution at 298.2 K and [OH] = 5 103 mol L1. The points represent experimental results; lines represent the results calculated by eq 7: (9) ω = 8, (O) ω = 10, (b) ω = 12, (0) ω = 14, (2) ω = 16, (4) ω = 18, (1) ω = 20, (3) the aqueous solution.
the calculated results through eq 5 are shown in Figure 6. The optimal values of Ae are consistent with the equilibrium values determined at infinite reaction time in aqueous solutions and at the equilibrium point D listed in Table 1 in the microemulsion medium within the experimental uncertainties ((0.003). The measured absorbance in the initial reaction stage for various ω in the microemulsions at 293.2, 298.2, 301.2, and 303.2 K were used to fit eq 7 to obtain the values of k1, which are also listed in Table 3. The goodness of the fitting is also shown in the plots of ln A against t in Figure 5. The values of k1 obtained from the above two methods for ω being 12 and 14 and the overall concentration of OH being 5 103 mol L1 are reasonably consistent within the experimental uncertainties. From Tables 2 and 3 we can see that when the alkaline fading of PN occurs in water/AOT/isooctane microemulsions, the rate constants k1 and k1 are larger than that taking place in the conventional aqueous solution. It may be attributed to the anionic reactants that are repelled by the negatively charged heads of the AOT molecules, which would increase the local activities of the reactants and enhance the reaction rate. According to the transition-state theory,3,7,26 the reactants contact to form the activated complex and some solvent water molecules pass from the reactant solvation spheres to the bulk water in the droplet during the reaction process. The overall activation may be written as3,7 3 L2 solv þ OHsolv f LOHsolv # þ nH2 O
ð8Þ
The forward rate constant k1, reverse rate constant k1, and the equilibrium constant K take the forms3,7,26 k1 ¼ k0
γL2 γOH 1 γLOH3 # RnH2 O
ð9Þ
½LOH3 γ 2 γ ¼ K 0 L OHn ½L2 ½OH γLOH3 RH2 O
ð10Þ
k1 ¼ k1 =K ¼ k01 γLOH3 =γLOH3 #
ð11Þ
K ¼
Figure 6. Plots of A vs t for the alkaline fading of PN in water/AOT/ isooctane microemulsions with various ω and the aqueous solution at 298.2 K and [OH] = 1.3 102 mol L1. The points represent experimental results; lines represent the results calculated by eq 5: (0) ω = 8, (9) ω = 10, (O) ω = 12, (b) ω = 14, (2) ω = 16, (4) ω = 18, (1) ω = 20, (3) the aqueous solution.
where γL2, γOH, γLOH3, and γLOH3# are the activity coefficients of L2, OH, LOH3, and the activated complex, respectively; RH20 is the activity of solvent water. The activity coefficient of a solute is a measure of the deviation of the solute in a real aqueous solution from the ideal behavior at the infinite dilution limit, which is dependent on the temperature, the pressure, the composition, and the characteristic of the water pools in the microemulsion reaction systems we studied. In eqs 911, K0, k01, and k01 are the equilibrium constant and the rate constants of the forward and reverse reactions in the infinite dilution solution, respectively, and are independent of the composition of the system. The value of RH20 less than 1 is an alternative interpretation why the rate constant is larger than that in the conversional aqueous solution. Higuchi et al.27 found that water activity in water/AOT/oil microemulsions decreases with ω. Moreover, the activity-coefficient ratio in eq 9 possibly also increases with a decrease of ω due to changes in the repulsion effect of the negatively charged heads of AOT and the strength of the interactions between the ions in the droplets; which also could explain way the forward rate constant k1 decreases with an increase in the amount of water for ω g 12. However, the reverse rate constant k1 seems to be increasing with ω when ω g 12, indicating that the ratio of γLOH3/γLOH3# in eq 11 increases with ω for ω g 12. It was also found that the values of the equilibrium constants K are significantly larger than that in aqueous solutions and increase with a decrease of ω for ω g 12, which implies that the activity-coefficient ratio in eq 10 for the microemulsions is significantly larger than that for the aqueous solutions and increases with a decrease of ω when ω g 12. However, for ω < 12, unexpected dependences of the above three constants on ω were found, which was also observed for the alkaline fading of CV in the same medium.8 A possible explanation is that the microdroplets with the small water pool radius are not able to accommodate PN in an “easy way”. Loading of this large molecule in the small water pools probably results in a polydispersity in sizes, so that microdroplets containing PN become larger than those that are vacant. This phenomenon 5565
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Figure 7. Plots of ln(k1/T) vs 1/T for the alkaline fading of PN in the aqueous solution and water/AOT/isooctane microemulsions with various ω and [OH] = 5 103 mol L1: (9) ω = 12, line 1; (0) ω = 14, line 2; (b) ω = 16, line 3; (O) ω = 18, line 4; (2) ω = 20, line 5; (4) the aqueous solution, line 6.
Table 4. Values of Free Energy Δ‡G°, Enthalpy Δ‡H°, and Entropy Δ‡S° of Activation in Aqueous Solutions and the Microemulsions with Various ω and [OH] = 5 103 mol L1 Δ‡G° (T = 298.2 K)/ ω
a
kJ mol1
Δ‡H°/kJ mol1
Δ‡S°/J K1 mol1
12
80.0 ( 5.3
37.9 ( 1.8
144.3 ( 16.3
14
81.2 ( 3.2
40.6 ( 1.3
136.0 ( 9.8
16
81.3 ( 4.6
41.5 ( 2.0
133.5 ( 13.7
18
81.5 ( 3.2
42.7 ( 1.5
130.2 ( 9.4
20
81.6 ( 4.4
44.8 ( 2.3
123.5 ( 12.4
aqueous
85.9 ( 4.9
33.2 ( 0.6
176.6 ( 16.2
aqueousa
85.3 ( 3.0
33.9 ( 0.4
174.0 ( 8.6
Data from ref 21.
could be accompanied by the “exclusion” of OH from these “filled” microdroplets, thus, reducing the rate constant of the forward reaction. The experimental results also showed that an increase of [OH] yielded the increase of k1 and decrease of k1 and, thus, significantly increased K. These phenomena may also be explained by the influences of the activity coefficients in eqs 911, which are possibly resulted from the increase in the ionic strength in the system due to an increase of [OH]. 3.3. Activation Parameters for the Reaction. As it may be seen in Table 3, k1 increases with temperature. According to the transition state theory, the temperature dependence of k1 may be used to obtain the values of enthalpy Δ‡H°, entropy Δ‡S°, and free energy Δ‡G° of activation for the bimolecular reaction in the microemulsions through the following expressions:28 k1 Δ ‡ H ° Δ ‡ S° kB þ þ ln ln ¼ ð12Þ T RT R h where T, kB, h, and R are the absolute temperature, the Boltzmann constant, the Planck constant, and the molar gas constant. The good linear plots of ln(k1/T) against 1/T are shown in Figure 7 with the linear correlation coefficients (r2) being 0.993 for line 1, 0.997 for line 2, 0.993 for line 3, 0.997 for
Figure 8. Plot of Δ‡H° vs Δ‡S° for the alkaline fading of PN in the water/AOT/isooctane microemulsions with various ω and [OH] = 5 103 mol L1: (b) microemulsions, (O) aqueous solutions. The line represents the result of the fit for the microemulsions with five ω.
line 4, 0.993 for line 5, and 0.999 for line 6. Least-squares fits gave the values of Δ‡H° and Δ‡S°, which together with their standard deviations are listed in Table 4. The values of Δ‡G° for various ω and temperatures were calculated from Δ‡G° = Δ‡H° TΔ‡S°, and the values of Δ‡G° at 298.2 K are also listed in Table 4. It was found that Δ‡H° and Δ‡S° in the microemulsions are larger than that in the aqueous solution. The value of Δ‡H° increases with ω, and the same trend seems to hold for Δ‡S°, although the standard deviations of Δ‡S° are as large as 9% of the values in average, while Δ‡G° is almost the same. It implies that there possibly exists a so-called compensation effect or isokinetic relationship that enthalpy variations through the series of reactions in the microemulsion droplets with various ω are compensated by entropy changes. This compensation effect may be examined by a plot of Δ‡H° versus Δ‡S° and more reliably by the plots of ln(k1/T) versus 1/T for a series of the reactions.29 If the isokinetic relationship is subsistent, the plot of Δ‡H° versus Δ‡S° should yield a straight line with a slope being the compensation temperature (Tiso) at which all reactions in the series proceed at the same rate, and the plots of ln(k1/T) versus 1/T should yield a series straight lines intersecting at a single point namely isokinetic temperature Tiso.29 Such plots are shown in Figures 8 and 7, respectively. Figure 8 displays a good linear relation between Δ‡H° and Δ‡S° and gives r2 being 0.999 and Tiso being (334 ( 0.1) K, while Figure 7 demonstrates an approximate common intersection for five straight lines of ln(k1/T) versus 1/T for five microemulsion reaction media with different ω and gives Tiso being (331 ( 3) K. However, it is clear shown in above two figures that the reactions in aqueous solutions are out of the isokinetic relationship. This behavior suggests that there is a common mechanism30 for the reactions occurring in the microemulsions with different ω but not for that in the aqueous solutions, which may be attributed to the different reaction environments between microemulsions and the aqueous solutions.31
4. CONCLUSIONS We have studied the kinetics of the alkaline fading of PN both in water/AOT/isooctane microemulsions and in aqueous 5566
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The Journal of Physical Chemistry A solutions by monitoring the absorbance changes of PN in the system with the time. It was found that the values of the equilibrium constants and the forward reaction rate constants in the microemulsions were significantly larger than that in aqueous solutions and decreased with increasing ω, except for that with low ω. We examined the temperature dependence of the reaction rate constants and obtained the values of free energy, enthalpy, and entropy of activation, which suggests the existence of an isokinetic relationship and a common mechanism for the reactions occurring in the microemulsions with different ω. We also observed the competition between the reactions of the alkaline fading of PN and the hydrolyzation of AOT in the water/AOT/isooctane microemulsions as the reaction time was sufficiently long, which provides a possible way to study the kinetics of the hydrolyzation of AOT in the water/AOT/isooctane microemulsions by a spectroscopic method using PN as an indicator.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ86-21-64250047. Fax: þ86-21-64252510. E-mail:
[email protected].
ARTICLE
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’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Projects 20973061 and 21073063), the Chinese Ministry of Education (Key Project 105074), and Committee of Science and Technology of Shanghai (Projects 0652 nm010 and 08jc14081). ’ REFERENCES (1) Luisi, P. L.; Straub, B. E. Reverse Micelles; Plenum Press: New York, 1984. (2) Sager, W. F. C. Langmuir 1998, 14, 6385–6395. (3) Moya, M. L.; Izquierdo, C.; Casado, J. J. Phys. Chem. 1991, 95, 6001–6004. (4) García-Río, L.; Leis, J. R.; Moreira, J. A. J. Am. Chem. Soc. 2000, 122, 10325–10334. (5) García-Río, L.; Herves, P.; Mejuto, J. C.; Perez-Juste, J.; Rodríguez-Dafonte, P. Ind. Eng. Chem. Res. 2003, 42, 5450–5456. (6) Chen, Z. Y.; Zhao, J. H.; He, W.; An, X. Q.; Shen, W. G. Int. J. Chem. Kinet. 2008, 40, 294–300. (7) Izquierdo, C.; Casado, J.; Rodríguez, A.; Moya, M. L. Int. J. Chem. Kinet. 1992, 24, 19–30. (8) Leis, J. R.; Mejuto, J. C.; Pena, M. E. Langmuir 1993, 9, 889–893. (9) Mukherjee, L.; Mitra, N.; Bhattacharya, P. K.; Moulik, S. P. Langmuir 1995, 11, 2866–2871. (10) Holmberg, K. Eur. J. Org. Chem. 2007, 5, 731–742. (11) Majumdar, T.; Mahapatra, A. Colloids Surf., A 2007, 302, 360–365. (12) Levinger, N. E. Curr. Opin. Colloid Interface Sci. 2000, 5, 118–124. (13) Vodolazkaya, N. A.; Mchedlov-Petrossyan, N. O.; Salamanova, N. V.; Surov, Y. N.; Doroshenko, A. O. J. Mol. Liq. 2010, 157, 105–112. (14) Pe~nacoba, I. A.; García, B.; Navarro, A. M.; Hoyuelos, F. J.; Leal, J. M. J. Colloid Interface Sci. 2010, 352, 465–469. (15) Amire, O. A. J. Colloid Interface Sci. 1988, 126, 508–516. (16) Fujii, H.; Kawai, T.; Nishikawa, H. Bull. Chem. Soc. Jpn. 1979, 52, 2051–2055. (17) Fujii, H.; Kawai, T.; Nishikawa, H.; Ebert, G. Colloid Polym. Sci. 1982, 260, 697–701. 5567
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