Chemical Physics Letters 385 (2004) 357–361

www.elsevier.com/locate/cplett

Temperature dependence of solvation dynamics in a micelle.4-Aminophthalimide in Triton X-100

Pratik Sen, Saptarshi Mukherjee, Arnab Halder, Kankan Bhattacharyya *

Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

Received 18 November 2003

Published online: 25 January 2004

Abstract

Solvation dynamics of 4-aminophthalimide (4-AP) is studied in a Triton X-100 (TX) micelle at three different temperatures. The

average solvation time hssi has been found to be 800, 400 and 110 ps at 283, 303 and 323 K, respectively. This corresponds to an

activation energy of 9� 1 kcal mol�1. The observed temperature dependence is in qualitative agreement with recent computer

simulations on the solvation dynamics in micelles.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

In bulk water, solvation dynamics occurs in sub-pi-

cosecond time scale [1–3]. The very fast solvation dy-namics in bulk water is due to intermolecular vibrations

and librations and the process involves negligible acti-

vation energy [1–3]. In the case of water molecules con-

fined in complex organized assemblies and in biological

macromolecules, solvation dynamics exhibits a compo-

nent in 100–1000 ps time scale [4–25]. According to a

phenomenological model the slow component of solva-

tion dynamics arises because of an exchange of the watermolecules between the �bound� and the �free� forms [17].

In the limit of very high binding energy (i.e. jDG0bf j), the

slow component of solvation (sslow) is given by [6,17,19]

sslow � 1=kbf ; ð1Þwhere the rate constant for bound-to-free interconver-

sion kbf is,

kbf ¼ ðkBT=hÞ expð�Ea=RT Þ: ð2ÞVery recently, Bagchi and co-workers [18–21] carried

out very detailed computer simulation of solvation dy-

namics of the cesium counter ion in an anionic micelle.

* Corresponding author. Fax: +91-33-2473-2805.

E-mail address: pckb@mahendra.iacs.res.in (K. Bhattacharyya).

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2003.12.115

According to them, the slow component of solvation

dynamics arises because of hydrogen bonding of the

water molecules with the polar head groups (PHG) of

the micelle [20,21]. They introduced an energy criterionthat a water–PHG hydrogen bond exists if the pair en-

ergy between a water molecule and a PHG is less than

�6:25 kcal mol�1 [20]. They found that the average

solvation time increases from 20 to 40 ps as the tem-

perature decreases from 350 to 300 K [18].

In the present work, we report on the temperature

dependence of the solvation dynamics of 4-aminoph-

thalimide (4-AP) in Triton X-100 (TX) micelles. Themain reason for studying the solvation dynamics in TX

micelle instead of a cationic micelle (CTAB) or an an-

ionic micelle (SDS) is the following. According to the

structural studies (small angle X-ray and neutron scat-

tering), a micelle consists of an essentially �dry� corecontaining the hydrocarbon chains which is surrounded

by a hydrated peripheral shell made up of the polar head

groups and considerable amount of water [26,27]. Thishydrophilic shell is called the Stern Layer for an ionic

micelle (e.g., CTAB and SDS) and a palisade layer for

neutral micelle (such as, TX). The thickness of the hy-

drated layer is 6–9 �A for the ionic micelles [27] while for

TX the thickness of this layer is 25 �A [26]. Since the

solvation probe 4-AP is about 8 �A in length, in case of

TX it is totally enclosed inside the hydration layer while

N

O

O

H

H

H

N

Scheme 1. Structure of 4-AP.

450 500 550 600 650

100

200

300

400

500

Em

issi

on I

nten

sity

(a.

u.)

358 P. Sen et al. / Chemical Physics Letters 385 (2004) 357–361

for anionic micelle a considerable part of the probe

sticks out of the hydration layer of the micelle into thebulk water.

Wavelength (nm)

Fig. 1. Emission spectra of 4-AP in 100 mM TX-100.

2. Experimental

4-Aminophthalimide (4-AP, Kodak, Scheme 1) was

purified by repeated recrystallization from methanol-

water mixture. Triton X-100 (Aldrich) was used as re-ceived. The steady-state absorption and emission spectra

were recorded in a Shimadzu UV-2401 spectrophotom-

eter and a Perkin–Elmer 44B spectrofluorimeter, re-

spectively. For lifetime measurements, the samples were

excited at 405 nm using a picosecond diode laser (IBH

Nanoled-07). The emission was collected at a magic

angle polarization using a Hamamatsu MCP photo-

multiplier (2809U). The time correlated single photoncounting (TCSPC) set up consists of an Ortec 935

QUAD CFD and a Tennelec TC 863 TAC. The data are

collected with a PCA3 card (Oxford) as a multi-channel

analyzer. The typical FWHM of the system response is

about 100 ps. The temperature was maintained using a

circulator bath (Neslab, Endocal).

From the reported binding constant of 4-AP with TX

[28] in 100 mM TX, nearly 85% of the probe (4-AP)molecules remain bound to the micelle. In this work, the

concentration of TX was kept at 100 mM for all the

three temperatures.

Fig. 2. Fluorescence decays of 4-AP in 100 mM TX-100 at 283 K at (i)

450, (ii) 500 and (iii) 600 nm.

3. Results

3.1. Steady-state spectra

The absorption spectrum of 4-AP in 100 mM TX

micelles at all the three temperature remains unchanged

and exhibits an absorption maximum at 373 nm. The

emission intensity of 4-AP in water is very low with

a quantum yield of 0.01 [28]. However, in presence of

100 mM TX, the emission intensity increases about three

times and exhibits a blue shift to 530 nm [14]. This in-dicates that the probe molecules experience a less polar

micellar environment as compared to that in bulk water

(Fig. 1). The emission maximum of 4-AP remains un-

altered with the variation of temperature from 283 to

323 K.

3.2. Time-resolved studies

The fluorescence decays of 4-AP in the 100 mM TX

micelles are found to be markedly dependent on the

emission wavelength at all the three temperatures. For

example, at 283 K, at the blue end (450 nm) the fluo-

rescence decay of 4-AP is biexponential with two decay

components of 530 ps (65%) and 5300 ps (35%), while at

the red end (600 nm) the decay of time constant 5040 ps

is preceded by a distinct rise with a time constant of 160ps. Fluorescence decays of 4-AP in 100 mM TX micelles

at 283 K are shown in Fig. 2. At 323 K, at the blue end

(450 nm) the fluorescence decay of 4-AP in TX micelle is

biexponential with two decay components of 260 ps

(60%) and 3350 ps (40%), while at the red end (600 nm)

4-AP exhibits a decay component of 2080 ps and a

distinct growth component of 100 ps.

Following the procedure prescribed by Maroncelliand Fleming [29], the time-resolved emission spectra

(TRES) were constructed using the parameters of best fit

to the fluorescence decays and the steady-state emission

spectrum. The TRES of 4-AP in 100 mM TX micelles at

283 K are shown in Fig. 3. The solvation dynamics is

described by the decay of the solvent correlation func-

tion CðtÞ, defined as,

15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0t

Nor

mal

ized

Int

ensi

ty

Wavenumber (cm )-1

Fig. 3. Time resolved emission spectra of 4-AP in 100 mM TX-100 at

283 K at 0 ps (j), 150 ps (s), 600 ps (N) and 4000 ps (r).

P. Sen et al. / Chemical Physics Letters 385 (2004) 357–361 359

CðtÞ ¼ mðtÞ � mð1Þmð0Þ � mð1Þ ; ð3Þ

where mð0Þ, mðtÞ and mð1Þ are the peak frequencies attime 0, t, and 1, respectively. The decay of CðtÞ of 4-AP

in 100 mM TX at 283 K, is found to be biexponential

with one component of 150 ps (30%) and another

1080 ps (70%), with an average solvation time,

hssi ¼ 800� 100 ps (Fig. 4, Table 1). The solvation dy-

0 1000 2000 3000 40000.0

0.2

0.4

0.6

0.8

1.0

C(t

)

Time (ps)

0 100 200 300 4000.0

0.2

0.4

0.6

0.8

1.0

Fig. 4. Decay of response function, CðtÞ of 4-AP in 100 mM TX-100 at

283 K (j), at 303 K (s), and at 323 K (N). The points denote the

actual values of CðtÞ and the solid line denotes the best fit to an ex-

ponential and/or biexponential decay.

Table 1

Decay parameters of CðtÞ of 4-AP in the presence of 100 mM TX-100 at diff

Temperature (K) Dma (cm�1) a1 s1 (ps)

283 600 0.30 150

303 600 0.40 80

323 600 1.00 110

a�50 cm�1.b hssi ¼ a1s1 þ a2s2.

namics of 4-AP in 100 mM TX at 303 and 323 K are

found to be very different than that at 283 K. In a so-

lution of 100 mM TX at 303 K, the decay of CðtÞ for

4-AP, is found to be biexponential with one component

of 80 ps (40%) and another 620 ps (60%), with an av-erage solvation time, hssi ¼ 400� 50 ps. At 323 K, the

decay of CðtÞ is found to be single exponential with a

time constant 110� 50 ps (Fig. 4, Table 1). The total

Stokes shift is observed to be 600� 50 cm�1 for all the

three temperatures.

4. Discussion

The most important observation of the present work

is the marked decrease in the average solvation time by

almost eight times from 800� 100 ps at 283 K to

110� 50 ps at 323 K. Assuming an Arrhenius depen-

dence (Eq. (2)) of the rate constant (¼ hssi�1), the acti-

vation energy (Ea) for the solvation may be evaluated

from the slope of the plot of logarithm of hssi�1against

(1/T) (Fig. 5). The magnitude of Ea is obtained as 9� 1

kcal mol�1.

It may be noted that the temperature dependence of

solvation dynamics detected in this work is in qualitative

agreement with that observed in the molecular dynamics

simulation carried out by Pal et al. [18]. However, the

magnitude of temperature dependence detected in this

work is higher than that reported in the simulation. It

0.0030 0.0033 0.0036

20

21

22

23

24

ln (1

/<τ s

>)

1/T (K )-1

Fig. 5. Plot of ln(1=hssi) against 1=T .

erent temperatures

a2 s2 (ps) hssib (ps)

0.70 1080 800� 100

0.60 620 400� 50

– – 110� 50

360 P. Sen et al. / Chemical Physics Letters 385 (2004) 357–361

should be emphasized that the simulation deals with

solvation of an ion (cesium) which is much smaller in

size (diameter¼ 3.6 �A) than that of the solvation probe,

4-AP (8 �A). Also the structure of the anionic micelle

studied by simulation is different from that of the TXmicelle.

The activation energy (9� 1 kcal mol�1) determined

in this work is higher than the hydrogen bond energy

between two water molecules in bulk water (�5.5

kcal mol�1) [20]. However, it should be noted that ac-

cording to computer simulations [20] hydrogen bond

energy of interfacial water molecules forming one or two

hydrogen bonds with PHG of a micelle is �13–14kcal mol�1, i.e., 7–8 kcal mol�1 stronger than a water–

water hydrogen bond. Though, as noted earlier, the

simulations [20] were carried out on an anionic micelle

having a different structure, the activation energy (9� 1

kcal mol�1) determined in the present work is very close

to the difference in between the water–micelle and wa-

ter–water (�7–8 kcal mol�1) hydrogen bond energies

[20]. Thus, it appears that the rate determining step inmicellar solvation is removal of water molecules from

the interfacial region to bulk water. This is consistent

with the earlier phenomenological model [17].

Several authors reported that the hydration number

(i.e., the number of water molecules per molecule of

surfactant) changes with rise in temperature [30–32].

Streletzky and Phillies [32] studied the temperature de-

pendence of structure and hydration of TX micelle usingquasi-elastic light scattering spectroscopy. According to

them, the hydration number of TX micelle increases

from 6 to 16 with increase in temperature from 283 to

323 K [32]. If the eightfold decrease in average solvation

time from 283 to 323 K in TX micelle were due to solely

changes in hydration number and structure of TX mi-

celles, the solvation time should not involve any acti-

vation barrier and should not display a linear Arrheniusplot. The linearity of the Arrhenius plot (Fig. 5) suggests

the changes in structure and hydration number have a

minor role in the observed temperature dependence of

solvation dynamics.

In a recent work, it has been proposed that slow

solvation dynamics of a probe in the water pool of a

microemulsion (reverse micelle) involves self-diffusion of

the probe [13]. The self-diffusion of the probe is mani-fested in the time-dependent change in spectral width.

No such time dependent change in the emission spectral

width is observed in the present work.

5. Conclusion

This work shows that solvation dynamics of 4-AP inTX micelles exhibit marked temperature dependence.

The activation energy of the slow component of solva-

tion dynamics is found to be 9� 1 kcal mol�1. This is

very close to the difference between the water–micelle

and water–water hydrogen bond energy as reported in a

recent simulation [20]. The role of temperature depen-

dent change in the structure and hydration number of

TX micelles appears to be minor.

Acknowledgements

Thanks are due to Department of Science and

Technology of India (DST), the �Femtosecond Laser

Facility� and to Council of Scientific and Industrial

Research (CSIR) for generous research grants. P.S.thanks DST and A.H. thanks CSIR for awarding fel-

lowships. K.B. thanks Professor B. Bagchi for many il-

luminating discussions.

References

[1] B. Bagchi, Annu. Rep. Prog. Chem. Sect. C 99 (2003) 127.

[2] R. Jimenez, G.R. Fleming, P.V. Kumar, M. Maroncelli, Nature

369 (1994) 471.

[3] N. Nandi, S. Roy, B. Bagchi, J. Chem. Phys. 102 (1995) 1390.

[4] N. Nandi, K. Bhattacharyya, B. Bagchi, Chem. Rev. 100 (2000)

2013.

[5] K. Bhattacharyya, Acc. Chem. Res. 36 (2003) 95.

[6] S.K. Pal, J. Peon, B. Bagchi, A.H. Zewail, J. Phys. Chem. B 106

(2002) 12376.

[7] N.E. Levinger, Curr. Opin. Coll. Interface Sci. 5 (2000) 118.

[8] X.J. Jordanides, M.J. Lang, X. Song, G.R. Fleming, J. Phys.

Chem. B 103 (1999) 7995.

[9] P. Dutta, P. Sen, A. Halder, S. Mukherjee, S. Sen, K.

Bhattacharyya, Chem. Phys. Lett. 377 (2003) 229.

[10] E.B. Brauns, M.L. Madaras, R.S. Coleman, C.J. Murphy, M.A.

Berg, Phys. Rev. Lett. 88 (2002), 158101-1.

[11] L. Frauchiger, H. Shirota, K.E. Uhrich, E.W. Castner Jr., J. Phys.

Chem. B 106 (2002) 7463.

[12] S. Sen, D. Sukul, P. Dutta, K. Bhattacharyya, J. Phys. Chem. B

106 (2002) 3763.

[13] P. Dutta, P. Sen, S. Mukherjee, A. Halder, K. Bhattacharyya,

J. Phys. Chem. B 107 (2003) 10815.

[14] A. Dutta, D. Mandal, S.K. Pal, S. Das, K. Bhattacharyya, J. Mol.

Liq. 77 (1998) 121.

[15] S.K. Pal, D. Sukul, D. Mandal, S. Sen, K. Bhattacharyya, Chem.

Phys. Lett. 327 (2000) 91.

[16] K. Hara, H. Kuwabara, O. Kajimoto, J. Phys. Chem. A 105

(2001) 7174.

[17] N. Nandi, B. Bagchi, J. Phys. Chem. B 101 (1997) 10954.

[18] S. Pal, S. Balasubramanian, B. Bagchi, J. Chem. Phys. 117 (2002)

2852.

[19] S. Balasubramanian, S. Pal, B. Bagchi, Phys. Rev. Lett. 89 (2002),

115505-1.

[20] S. Pal, S. Balasubramanian, B. Bagchi, J. Phys. Chem. B 107

(2003) 5194.

[21] S. Pal, S. Balasubramanian, B. Bagchi, Phys. Rev. E 67 (2003)

061502.

[22] J. Faeder, M.V. Albert, B.M. Ladanyi, Langmuir 19 (2003) 2514.

[23] S. Senapati, M.L. Berkowitz, J. Chem. Phys. 118 (2003) 1937.

[24] C.D. Bruce, S. Senapati, M.L. Berkowitz, L. Perera, M.D.E.

Forbes, J. Phys. Chem. B 106 (2002) 10902.

[25] D.A. Panatano, D. Laria, J. Phys. Chem. B 107 (2003) 2971.

[26] H.H. Paradies, J. Phys. Chem. 84 (1980) 599.

P. Sen et al. / Chemical Physics Letters 385 (2004) 357–361 361

[27] S.S. Berr, J. Phys. Chem. 91 (1987) 4760.

[28] G. Saroja, A. Samanta, Chem. Phys. Lett. 246 (1995) 506.

[29] M. Maroncelli, G.R. Fleming, J. Chem. Phys. 86 (1987) 6221.

[30] L.S. Romsted, J. Yao, Langmuir 12 (1996) 2425.

[31] L.S. Romsted, J. Yao, Langmuir 15 (1999) 326.

[32] K. Streletzky, G.D.J. Phillies, Langmuir 11 (1995) 42.