Influence of Sphere-Rod Transition in Micelle Shape on Thermal Cis-Trans Isomerization of 4-Dimethylamino-4'-nitroazobenzene SHIGEYOSHI MIYAGISHI, SEIICHI MATSUMURA, TSUYOSHI ASAKAWA, AND MORIE NISHIDA Department of Chemistry and Chemical Engineering, Faculty of Technology, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920, Japan Received May 4, 1987; accepted October 22, 1987 The rate constant of thermal cis to trans isomerization of 4-dimethylamino-4'-nitroazobenzene ( D M N A ) was measured in sodium dodecyl sulfate (SDS) solutions in the presence of NaC1. The rate constant decreased abruptly above a threshold NaC1 concentration which corresponded to a sphere-rod transition in micelle shape o f SDS. Micro-viscosity of the micelle also increased at NaCI concentration above the threshold value, while a discontinuous change in micro-polarity of the micelle was not found at the transition point. The rate constant of cis-DMNA in the spherical micelle was larger than that in the rodlike micelle. © 1988AcademicPress,Inc. INTRODUCTION
Recently attention has focussed on a relation between a reaction in an organized assembly and the latter structure (or size) (14), which could be modified by means of various methods. Our interest is potential application of such modification to control a reaction. A change in structure of the organized assembly brings about variations in its microviscosity and micro-polarity (5-8). In a previous paper (8), we found that micro-viscosity in a micelle can be controlled by a modification of its external environment. A distribution of reactants between bulk and micelle phases is also influenced (8). As the result, a reaction in the micelle must be affected. The factors which determine a micelle structure include chemical structure of the surfactant, its concentration, temperature, organic additives, added salts, etc. Tazuke and his co-workers (9) reported that photoexcitation triggers a conformational change in a surfactant molecule. The phase diagrams of surfactant-water systems generally show that the micelles have different structures depending on the concentration and temperature. Many ionic surfactants associate into rod-like
micelles with increasing micelle concentration, when the salt concentration exceeds a certain threshold value, below which the surfactants form spherical micelles (10-14). Since the sphere-rod transition of miceUe shape due to the addition of NaC1 is followed by an increase in micro-viscosity (5, 15), a reaction in the micelle is expected to be modified at the transition point. Thermal cis-trans isomerization of 4-dimethylamino-4'-nitroazobenzene is one of the most suitable model reactions for the present purpose, although many kinds of reactions have been investigated in the surfactant solutions. Whitten and his co-workers (16-18) reported that the thermal cis-trans isomerization of 4- (dialkylamino)-4"nitroazobenzenes is extremely sensitive to solvent polarity and these azobenzenes are useful to study polarity in the micelles. In addition, the reaction mechanism has been studied extensively in various solvents (18-24). In this paper we report the effect of the spherical-rod transition of a micelle on the thermal cis-trans isomerization of 4-dimethylamino-4'-nitroazobenzene (DMNA) in sodium dodecyl sulfate (SDS) solutions.
JournalofColloidand InterfaceScience,Vol. 125,No. 1, September1988
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of the limited solubility of DMNA. Such effect was not observed in the SDS solutions. The emission intensity of 1.0 × 10 -5 M a u ramine at 500 nm was measured in the SDS solutions at an excitation wavelength of 440 nm with a Hitachi 204S fluorescence spectrophotometer. The vibronic band intensities (Ii and 13) in pyrene m o n o m e r fluorescence were measured in the SDS solutions with a Shimazu RF-540 fluorescence spectrophotometer at an excitation wavelength of 310 nm.
Materials. SDS (specially prepared reagent for investigation of protein, Nakarai Chemicals) was purified by the procedure described before (8). DMNA (guaranteed reagent, Tokyo Kasei Kogyo Co. Ltd.) was recrystallized from toluene. Its purity was checked by chromatography on silica gel. Auramine was the same sample as that in the previous paper (8). Pyrene (guaranteed reagent, Tokyo Kasei Kogyo Co. Ltd.) was used without further purification. RESULTS AND DISCUSSION Procedure. All kinetic runs were performed on a flash photolysis apparatus (Union Gikken Kinetics of cis-DMNA in SDS solution. As RA-412) equipped with a spectrophotometer a micelle of SDS was reported to change in (RA-401) and a data-processing apparatus shape from spherical to rod-like when added (RA-451). The thermal cis-trans isomeriza- NaC1 concentration exceeds 0.5 M (10, 11), tion was monitored at 490 n m after irradiation we investigated influence of the sphere to rod on a sample solution (3 X 10 -5 M D M N A ) transition resulting from addition of NaC1 on in a 20-mm path length uv cuvette with a Xe- a thermal cis-trans isomerization rate constant non lamp (50 J, 10 #sec) and a Toshiba L-39 (kobsd) of cis-DMNA. The rate constant defilter. The measurement was repeated ten creased with increasing NaC1 concentration times on the identical sample solution. At least when the salt concentration exceeded a certain three independent experiments were done for threshold value, below which the isomerizaeach sample and the mean value for the isom- tion rate was nearly constant. Figure 1 comerization rate constant was determined. Errors pares the effects of NaC1 concentration on kob~d in the rate constants were within 6%. The er- at 25 °, 30 °, and 35°C. At higher temperature, rors were larger in lower SDS concentration kobsa decreased more steeply and the threshold or at higher temperature, but became smaller NaC1 concentration became higher. The dein higher concentration of SDS or at lower temperature. The sample solution was prepared as follows: after an aliquot of acetone solution of DMNA was placed in a test tube, 30 o O the acetone was then evaporated in vacuo and an appropriate SDS solution or solvent ( 5 ml) ~25 was added into the test tube, which was sonicated 15 min on a Shimazu SUS-300. The OI SDS solutions were prepared in 10 m M N a O H O2.0 solution. Absorption spectra of 1.5 × 10 -3 M x DMNA were recorded on a Hitachi 124S I spectrophotometer. The sample solutions were ~ 1.5 prepared in the same manner as that in the 25"C measurement of the rate constant. The results I I I of the rate constants and the absorption spectra 0.0 0.2 0.4 0.6 0.8 were not influenced by addition (less than 1 NaCl /M vol%) of acetone. Some sample solutions were left overnight at 25°C and then the measureFIG. 1. Dependenceof ko~aon NaCI concentration at ments were repeated in order to study effect various temperatures. SDS = 0.1 M.
Journal of Colloid and Interface Science, Vol. 125, No. 1, September 1988
MICELLE STRUCTURE AND REACTIVITY
crease in kob~ suggests a variation in the microenvironment surrounding the reagent molecule. The threshold NaCI concentration also depended on SDS concentration (C) as seen in Fig. 2. There was some uncertainty in the determination of the break point. However, the experimental errors (6%) were not so large as to preclude determination of the region of the threshold NaC1 concentration. The lines in Figs. 1 and 2 were drawn to demonstrate the trends but were not computer fits. The region of the threshold concentration is shown as a function of the SDS concentration in Fig. 3. The threshold NaC1 concentration was about 0.5 M below 0.07 M SDS, above which the threshold NaCI concentration decreased. The micro-environment must be different in two regions which are divided by the threshold region in Fig. 3. As shown in Figs. 2 and 4, the values of kob,d are strongly dependent on SDS concentration. The plots of kob~ vs 1/ C had a break point in 0.3 and 0.5 M NaC1 solutions, respectively. Both SDS and NaC1 concentrations at the break point were nearly equal to the threshold values in Fig. 3. Ikeda et aL found that below 0.5 M NaCI, SDS forms spherical micelles and above 0.5
o 0.2 Z
SDS/M FIO. 3. NaC1 and SDS concentrations at break points.
M, rod-like micelles are formed in high SDS concentration (11). The threshold region in Fig. 3 appears to correspond to the sphericalrod transition point of the SDS micelle. The values at the intercepts in Fig. 4 are the rate constants in the corresponding micelles (kin). In the systems exhibiting the break point, two values of kin, km (sphere) and km (rod), are obtained and listed in Table II. They are the values of km in spherical and rod-like mi-
? - -
NaCI /M FIG. 2. Dependence of ko~ on NaCI concentration in various SDS concentrations at 25.0°C.
/ SOS(M) ]FIG. 4. Plots ofko~ vs reciprocal concentration of SDS. Journal of Colloid and Interface Science, Vol. 125, No. 1, September i 988
MIYAGISHI ET AL.
celles, respectively. Obviously km (sphere) is larger than km (rod). As the isomerization rate constant of DMNA is larger in the more polar solvent (see below), the spherical micelles may provide more polar environment than the rodlike micelles. However, we should also pay attention to the fact that generally a reaction rate varies with viscosity of the medium, although there has been no report about viscosity dependence of the isomerization rate constant of cis-DMNA. Absorption spectra of trans-DMNA. The behaviour of trans-DMNA differed considerably in the micelle solutions from that of cisDMNA, as described below. As shown in Fig. 5a, the maximum position of a visible absorption spectrum of DMNA (Xmax) in the SDS solution was independent of NaC1 concentra500
E 490 E
0.3M /x &
SDS/M FIG. 5. Absorbance and Xmaxof t r a n s - D M N A . In (a): e , 0.1 M S D S ; ©, 0.15 M S D S . Journal of Colloid and Interface Science, Vol. 125,No. 1, September1988
tion above a certain concentration, which was very low compared with the threshold NaC1 concentration in Fig. 3. The Xmax shifted slightly to the longer-wavelength side as the SDS concentration increased, but above 0.1 M SDS the value was independent of the SDS concentration. Therefore the values in Fig. 5a are nearly equal to those in the SDS micelles. As the Xm,xis sensitive to polarity of the medium as described later, the change in X~ax observed at the low NaC1 concentration suggests some change in polarity of the micelle. The Xmaxwas not altered in the region of the sphere-rod transition (the threshold region in Fig. 3) by addition of NaC1 (Fig. 5a). Such a trend was also observed in the systems of 1.0 × 10 -5 M DMNA. That is, the influence of the sphere-rod transition on polarity of the micelle was not clearly detected. This fact was considered to conflict with the result of the isomerization rate constant. In order to resolve this conflict, the following two experiments were done. One was estimation of viscosity in a micelle by using auramine as a probe, and the other was to detect polarity change in the threshold region by using the pyrene fluorescence spectrum. Micro-viscosity and micro-polarity. In order to examine a variation in micro-viscosity below and above the threshold value, fluorescence intensity ratios of auramine solubilized in the SDS micelles were measured at various NaCI concentrations (Fig. 6 ). The micelle solutions exhibited a sharp increase in the fluorescence intensity ratio above the threshold NaC1 concentration given in Fig. 3. The increase in the fluorescence intensity ratio reflects an increase in the micro-viscosity (8, 25). In the previous paper (8), we reported that the micro-viscosity in the SDS micelle increases when the micelle form changes from a sphere to a rod. These results suggest that the threshold value in kob~ocorresponds to the sphere-rod transition point of the SDS miceUe and the decrease in kob~dis correlated with a change in the micro-viscosity. The fluorescence spectrum of pyrene is often used to estimate polarity in a micelle (26).
MICELLE STRUCTURE AND REACTIVITY 20
Schanze and Whitten (17) reported the following equations for 4-diethylamino-4'-nitroazobenzene (DENA):
Auramine=10/zM 15 0
AG* = -6.331r* + 20.77 kcal,
r = 0.935
Ema x =
r = 0.986.
-6.881r* + 63.00 kcal,
Whitten and his co-workers (16) applied kct and ~kmax to study polarity in micelles, microemulsions, and vesicles, and concluded that its cis form prefers a more polar environment 0 I I I I 0.0 O.2 0.4 0.6 O.8 than the trans form. In other words, they suggested that the solubilization site of the cis NaCI / M form differs from that of the trans form. As FIG. 6. Dependence of fluorescence intensity of aura- DMNA is very analogous to DENA, the above mine on NaCI concentration. conclusions for DENA must also be correct for DMNA. We can estimate zr* in the micelle solutions The intensity ratio between the first and third by using their data and Eq.  or . The vibration bands of its fluorescence (I1/I3) in estimated values from kct were 1.35, 1.32, 1.02, the SDS solution is plotted against NaC1 con- and 1.23 for SDS CTAB, Brij 35, and centration in Fig. 7. The ratio monotonically DODAB, respectively. On the other hand, the decreased with an increase in NaC1 concen- estimated values from ~kmax w e r e 1.08, 1.08, tration and did not exhibit discontinuity near 0.99, and 1.08. The former values were always the threshold value in Fig. 2. As the ratio can larger than the latter values. However, we be a measure of polarity, this finding suggests should pay attention to the following facts. that the sphere-rod transition of the SDS micelle does not bring about a discontinuous change in polarity at the solubilization site of pyrene and agrees with the trend in ~'maxof
trans-DMNA. Polarity parameter, ~r*. Table I lists the rate
constant (k), the Gibbs function of activation for the thermal cis-trans isomerization (AG*), and the energy of charge transfer absorption of the trans form (Emax) in various solvents along with Taft Kamlet ~r* parameters (27). The value of Emax is equal to hc/~max. Equation [ 1] is the least-squares fit for AG* vs ~r* and Eq. [ 2] is that for E m a x VS 71"*:
1.05 ~ S D S = 0 . 0 7 M
AG* = --6.2777r* + 20.46 kcal,
r = 0.967
 Emax = -7.3251r* + 64.79 kcal,
r = 0.991.
0.4 0.6 NaCI / M
FIG. 7. Dependence of 11/13 of pyrene on NaC1 concentration. Journal of Colloid and Interface Science, Vol. 125, No. 1, September 1988
MIYAGISHI ET AL.
TABLE I Rate Constants and Free Energy of Activation for cis-trans Isomerization, and Emaxfor DMNA in Various Solvents at 25.0°C and Their 7r* Values w*
Hexane Cyclohexane Diethyl ether Methylethyl ketone Acetone
-0.081 0.000 0.273 0.674 0.683
Acetonitrile Cyclohexanone DMF
0.713 0.755 0.875
Formamide SDS micelle in 0.12 M NaCI
k (see -~)
0.0028a 0.040 11.1 2.5b 9.57c 13.5 27.2 8.35c 50b 98 120b 339 2200 1510
AG* (kcal. mol -l)
E~ (kcal. rnol -~)
66.0 64.30 62.86 59.93 59.72
16.0 16.9b 16.V 15.9 15.5 16.2c 15.1b 14.7 14.6b 14.0 12.9 13.1
59.59 58.66 58.36 57.30 57.20 58.13
"Ref. (11). b Ref. (12). CRef. (13).
The value of rr* determined from Eq. [ 3 ] or  in profic solvents tends to be larger than the true value, and its deviation from the true value is particularly larger in the value from kot (18). For example, the 7r* values of methanol and tert-butanol are 0.586 and 0.534 (27), but the estimated values are 0.943 and 0.675 from AG*, and 0.669 and 0.610 from Emax, respectively. In an aqueous micelle solution, water molecules can penetrate into a micelle and have a chance to contact a molecule solubilized near the micelle surface. The micro-environment of the solubilized molecule ( D M N A or D E N A ) is similar to a protic solvent rather than an aprotic solvent. These facts permit a different explanation about the result that the polarity parameter from AG* is different from the parameter based on Emax in the micelle solution. The explanation is as follows: both cis and trans forms are solubilized in an identical site and do not migrate even if the isomerization occurs. However, the probe does not always act correctly as expected from Eqs. [3 ] and Journal of Colloid andlnterface Science, Vol. 125, No. 1, September 1988
[4 ], but can give only qualitative information of polarity in the micelle solutions. We applied Eqs.  and [2 ] to the present results to estimate qualitatively the polarity parameters. In 0.1 M SDS solution, the ~r* varied from 0.96 to 0.88 when ~'m~x varied from 495 to 490 n m (Fig. 5a), while the values estimated from kob~ varied from 1.18 to 1.17 when the kob~ decreased from 1.75 × 10 3 to 1.52 × 10 3 sec -1 (Fig. 2). The values of 7r* were 1.14 and 1.10-1.12 in the spherical and rod-like micelles, respectively, in 0.5 M NaC1 (Table II). The parameters from the rate constants were always larger than those from the absorption spectra. Also, the results suggest that the micro-polarity in the spherical micelles is apparently higher than that in the rodlike micelles. This is similar in tendency with the result of Fig. 7, indicating that the micropolarity becomes less polar with an increase of NaC1 concentration. Association constants of cis- and transDMNA (Kcis and gtrans) with micelle. In order to examine the interaction of D M N A with the
MICELLE STRUCTURE AND REACTIVITY
TABLE II Properties of DMNA in SDS Micelles at 25.0°C cis-DMNA
Sphere NaCl (M)
k~ (105 sec-')
0.12 0.30 0.50
1.51 + 0.005 1.35 + 0.03 1.13 _____0.03
1.17 1.16 1.14
K~ (104 M-')
2.6-4.0 1.8-6.8 1.5-7.6
k~ (103 sec-')
K~ (104 M -~)
492 490 490
0.91 0.88 0.88
spherical and rod-like micelles in more detail, we attempted to evaluate the association constants of c is- and t r a n s - D M N A with the micelles. As the time scale of the thermal isomerization was comparable with that of the temperature j u m p in the micelle systems (28), the situation was complex. Therefore discussion is limited to a qualitative comparison of the association constants, which were roughly estimated as follows. According to Eq. (4.7) in Ref. (29), the rate constant kobsa can be expressed by ko~
kb + km" gcis" M =
1 + K¢is" M
where kb and km are the rate constants in a bulk phase and miceUe phase, Kcis is the association constant for the formation of a cisD M N A - m i c e l l e complex (29), and M is the micelle concentration. As M is equal to ( C - C M C ) / N, and N i s the aggregation n u m b e r of a micelle, rearranging Eq. [ 5 ] yields 1
kob~ - M
kb - k ~
1 gci s + - -- (Ckb-kmN
Kn~ (10s M -j)
1.6-2.4 1.6-4.0 2.4-3.0
vs ( C - C M C ) are shown in Figs. 8 and 9. In the region of the spherical micelle, the values at the intercepts were 8.6 X 10-5-1.54 X 10 -4, 3.8 X 10-5-1.47 × 10 -4, and 3.4 X 10-5-1.53 X 10 -4 in 0.12, 0.3, and 0.5 M NaC1, respectively. Therefore, the value of kb was in the range of 0.8 X 104-1.3 X 104, 0.8 X 104-2.8 X 10 4, and 0.8 X 104-3.0 X 10 4 sec -1, respectively. The direct determination ofk~ was very difficult due to its large value and the low solubility of D M N A in water, but kb was about 1 X 10 4 sec-1. Similarly the values of Kcis were estimated and listed in Table II with other parameters. The value ofgcis in the region of the rod-like micelle varied over a wide range because of the large experimental errors. Association constants of t r a n s - D M N A were roughly estimated from the data ofabsorbance (A) in Fig. 5b by using
3.0 C M C ) . 
The values of ksphere and kcod were used as km in the regions of spherical and rod-like micelles, respectively. Various least-squares procedures, which have different methods of weighting, were tried to fit the experimental data to Eq. [ 6 ]. The plots of 1/(kobed -- kin)
-1 2° 1.0
FiG. 8. Plots of 1 / ( k o b s a M NaCI.
kin) vs ( C - CMC) in 0.12
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MIYAGISHI ET AL.
This result agreed with a tendency for the micro-viscosity to be lower in the f o r m e r micelle than in the latter micelle.
3. T h e Xmax of t r a n s - D M N A was less sensitive to the s p h e r e - r o d transition of the SDS miceUe c o m p a r e d with the t h e r m a l isomerization of c i s - D M N A .
C m cmc FIG. 9. Plots of 1/(kob~ - kin) vs (C - CMC) in 0.3 and 0.5 M NaC1.
A - Ab
Am - Ab 1
4. T h e polarity p a r a m e t e r s 7r* estimated f r o m the t h e r m a l isomerization o f c i s - D M N A were always larger in the SDS solution t h a n those f r o m the charge transfer absorption o f t r a n s - D M N A . This divergence m a y result f r o m p o o r applicability o f Eqs. [ I ] - [ 4 ] to polar solvents a n d micelles rather than a difference in a solubilization site between the cis and trans isomers.
where Ab and Am are absorbance in the bulk and micelle phases, and g t r a n s is the association constant for the f o r m a t i o n of a transD M N A - m i c e l l e complex. T h e values o f Ktr~ns were determined by least-squares methods and are shown in Table II. T h e results in Table II indicate that Kcjs of the spherical micelle is greater than Ktra~s. If a m o r e polar environm e n t is favorable for the cis form, the relation should be opposite. Therefore it is not always correct that the cis f o r m transfers to a m o r e polar micro-environment when the trans form is irradiated in the micelles to b e c o m e the cis form. However, large experimental errors m a d e it impossible to discuss this point in m o r e detail. CONCLUSION 1. T h e rate constant of t h e r m a l cis-trans isomerization of D M N A decreased with an increasing concentration o f NaC1 when the spherical micelles of SDS began to grow into the rod-like micelles. 2. T h e value o f km in the spherical micelle was larger than that in the rod-like micelle. Journal of Colloid and Interface Science, Vol. 125, No. 1, September 1988
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