30 September 1994

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CHEMICAL PHYSICS LETTERS

Chemical Physics Letters 228 (1994) 79-82

Effect of urea on micellization of CTAB: probed by ESPT of carbazole Santi Kundu, Nitin Chattopadhyay * Departmentof Chemistry,Jadavpur University,Calcutta700 032, India Received 17 January 1994; in final form 30 June 1994

Investigation of the micellar characteristics of aqueous cetyltrimethylammonium bromide (CTAB), through the excited state proton transfer reaction of carbazole reports two critical micellar concentrations for the surfactant. Steady state as well as timeresolved studies establish that both the CMCs are affected in the presence of urea. Observation further revealed that urea disfavours the ESPT process in micellar solution. This has been interpreted in the light that urea expels the fluorescer molecule from the micellar surface to the bulk water.

1. Introduction Owing to the importance of micelles as model systems mimicking biomembranes in biological processes, attention has been drawn to the effect of micelles on the nature and kinetics of reactions [ l-41. Excited state proton transfer (ESPT) has been taken as a realistic reaction for this purpose [4-61. In a previous work, we have explained the effect of micellization on the ESPT reaction of carbazole (CAZL) [ 41. It was shown that due to the cationic nature of the surfactant cetyltrimethylammoniumbromide (CTAB), ESPT is favoured and the fluorescer prefers to reside at the periphery of the micellar aggregates. There is interest in understanding that effect of urea on micellar solutions [ 7- 111. The interest has grown from the observation that urea causes denaturation of proteins and inhibits formation of micellar aggregates. The effect has been observed from two angles *Corresponding author.

[ 9- 13 1. According to the first school, urea is a water structure breaker and the second school says that urea displaces water around the hydrophobic group [ 3 1. In this Letter we have studied the ESPT and CAZL in aqueous CTAB solution. We have monitored two CMCs for the system and determined them by steadystate as well as time-resolved methods. This work also reveals that urea expels the probe fluorescer from the micellar surface to bulk water.

2. Experimental CAZL was purified as before [ 141. CTAB obtained from Fluka was used as received. Analytical grade NaOH and urea (both from BDH) were used as such. Water used for the preparation of solutions was triply distilled. The micellar solutions were freshly prepared just before the experiments to avoid ageing [ 15 1. All the experiments were performed at 26 ’ C. A Shimadzu MPS 2000 spectrophotometer and Spex fluorolog spectrofluorimeter were used for re-

0009-2614/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZOOO9-2614(94)00913-9

S. Kundu, N. Chattopadhyay /Chemical Physics Letters 228 (1994) 79-82

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cording the absorption and emission spectra respectively. Time-resolved studies were performed using a time-correlated single-photon counting technique [ 16 1. The experimental details have been discussed elsewhere [ 141, only the exciting laser source was replaced by a nanosecond nitrogen flash lamp (Edinburgh Instruments, 199 fluorescence spectrometer). The fwhm of our exciting pulse was 1.2 ns.

3.7

t

2 z

P

e

Fig. 1 presents the effect of urea on the steady-state relative luminescence intensity (neutral/anion) of carbazole bound to CTAB in the presence of 0.0 15 M aqueous NaOH. Fig. 2 depicts a similar effect through decay analyses of the neutral emission of the fluorescer. The figures clearly indicate the existence of two critical micellar concentrations (CMC) for the CTAB system. The CMCi and CMCz values come out to be 3.0 x 1Oe4 and 7.5 x 1Oe4 M, respectively, from both steady-state and time-resolved studies (Table 1) . The values agree, extremely well, to the CMC values reported individually [ 4,18,19], although both were assigned, wrongly, to CMC,. Figs. 1 and 2 show that as long as the surfactant concentration is below the CMC, urea affects the relative fluorescence yield (neutral/anion) of the excited state prototropic species of carbazole negligibly. This indicates that urea hardly affects the ESPT

I

2.7

r"

3. Results and discussion

I

I

I 16

I

Fig. 1. Plot of relative fluorescence yield (neutral/anion) of carbazole as a function of CTAB concentration in the presence of (a) 0 M, (b) 1 M, (c) 2.5 M and (d) 4 M urea (the solution is 0.015 M in NaOH).

1.7 I 0

I

I 6

4 STAB]

x 10%

I 12

I 16

-

Fig. 2. Plot of lifetime values of neutral carbazole as a function of CTAB concentration in the presence of (a) 0 M and (b) 4 M urea (the solution is 0.015 M in NaOH).

process below the CMC, thus it has only an insignificant effect on the water structure [ 3,11,20]. As the surfactant concentration approaches the CMC or goes above it, urea affects the neutral/anion ratio. A lowering of the relative fluorescence yield is observed at a much higher CTAB concentration, in the presence of urea (Fig. 1). The neutral lifetime starts decreasing much later in the presence of urea. These imply that in the presence of urea, the CMCs of CTAB increase. The CMC, and CM& values shift from 3.0~10~~ and 7.5~10~~ M to 8~10~~ and 13.5 x 10e4 M, respectively, in 4 M urea solution (Table 1). A similar increase in the CMC values for triton X-100 and SDS micelles, in the presence of urea, were also found by others [ 3,7]. The relative fluorescence yield (neutral/anion) of aqueous carbazole solution in the presence of 0.0 15 M NaOH is 1.0 and the ratio finally drops to 0.14 after CMCz is achieved. It is interesting to note that under similar conditions of experiment other than the addition of urea, the final ratio gradually goes up, and for 4 M urea solution it is as high as 0.4 (Fig. 1). The discrepancy in the final values is too large to be explained by urea-induced change in aggregation number of the micelle [ 3,2 1] or by change in the microviscosity or micropolarity. Thus, in the presence of urea, even after CMC2 is achieved, ESPT has been disfavoured as compared to its absence. The same is

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S. Kundu, N. Chattopadhyay /Chemical Physics Letters 228 (1994) 79-82 Table 1 CMC values of CTAB (in presence of 0.0 15 M NaOH) Cont. of urea (M)

Method of estimation

CMC values ( 1O-“ M ) estimated CMC,

0.0

1.0 2.5 4.0

Ref.

2.5 7.5 2.2 8.2

[171 141

CMC2

steady state

3.0

1.5

time resolved

3.0

7.5

steady state steady state steady state time resolved

4.4 6.6 8.0 7.8

9.0 11.0 13.5 13.5

also revealed from the increase in the neutral lifetime of carbazole in the micellar solution in the presence of urea. Both observations can be explained by the mode that the fluorophore is expelled from the micellar periphery either to the core of the micelle or to the bulk water. As pointed out earlier, with increase in urea concentration the final ratio gradually increases. This is understandable as more and more fluorophore molecules are expelled by the greater number of urea molecules present in the solution. The possibility of urea-induced penetration of the fluorescer into the micellar core is ruled out on the following ground. Since the core is a hydrophobic region, OH- cannot reach the excited carbazole sitting there and ESPT is restricted. This would lead to a biexponential decay of carbazole, the longer component corresponding to a value of about 8 ns ( r_carbazole depends slightly on the solvent polarity and is around 8 ns in alcoholic or even cyclohexane solvent). However, the neutral decays are found to be single exponential with r values much less than 8 ns (Fig. 2). Thus carbazole molecules are pushed, not to the micellar core, but to the bulk aqueous phase. The existing idea that urea removes some water molecules from the micellar interface so that desolvation of the probe molecule at the interface compels it to go to the bulk is befitting here. Since carbazole locates itself at the CTAB micellar surface and due to the cationic nature of the interface the ESPT reaction is favoured [ 41; expulsion of the fluorophore molecules from the micellar surface to the bulk reduces the ESPT process

literature

(0.01 M NaOH) (0.015 M NaOH) (0.01 M NH&l) (0.001 M KBr)

and, thus, increases the neutral/anion ratio.

1181 I181

fluorescence

4. Conclusion The above discussion leads to the following points. (i) CTAB has two CMCs instead of a single as reported earlier. (ii) Both the CMCs are increased in the presence of urea. (iii) Urea expels the fluorophore molecules from the micellar periphery to the bulk aqueous phase.

Acknowledgement

Financial support from the Council of Scientific and Industrial Research, Government of India (Project No. 01(1283)/93/EMR-II) is gratefully acknowledged.

References [ 1] K. Kalyansundaram, Chem. Sot. Rev. 7 (1978) 453. [2] K.L. Mittal and B. Lindman, eds., Surfactants in solution, Vols. l-3 (Plenum Press, New York, 1984). [ 31 N. Sarkar and K. Bhattacharyya, Chem. Phys. Letters 180 (1991) 283; K. Das, N. Sarkar and K Bhattacharyya, J. Chem. Sot. FaradayTrans. II 89 (1993) 1959.

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S. Kundu, N. Chattopadhyay /Chemical Physics Letters 228 (1994) 79-82

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