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Chemical Physics Letters 450 (2007) 156–163

Noninvasive probing of aqueous Triton X-100 with steady-stateand frequency-domain fluorometry

Hari Om a,b, Gary A. Baker c, Frank V. Bright d, Krishan K. Verma b, Siddharth Pandey a,*

a Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110 016, Indiab Department of Chemistry, Maharishi Dayanand University, Rohtak 124 001, India

c Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USAd Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY 4260, USA

Received 3 October 2007; in final form 29 October 2007Available online 4 November 2007

Abstract

Most fluorescence spectroscopic methods that investigate the properties of surfactant-based systems are invasive in nature as they relyon information afforded by an externally-introduced probe. External probes may adversely affect the colloidal property of interest. Infor-mation from chromophoric surfactants as endogenous probes of micellization process is presented. Fluorescence from phenyl moiety of acommon nonionic surfactant Triton X-100 in water is demonstrated to be rich in information on surfactant aggregation. While Triton X-100 concentration-dependent fluorescence emission and excitation can be effectively used to monitor the presence of micelles, frequency-domain excited-state fluorescence intensity decay data affords key dynamic parameters characterizing Triton X-100 micelles.� 2007 Elsevier B.V. All rights reserved.

1. Introduction

Although used since the pioneering days in colloidalstudies, fluorescence spectroscopy of organized media hasevolved into one of the most widely used techniques tostudy microheterogeneous media [1–7]. Each of the variousfluorescence methods provides different information aboutthe system, offering high-resolution information about thestructure, local molecular order, interfacial properties andtransport phenomena. Typically, these studies involve treat-ing the system with a fluorescent dye [8–11], either boundpassively to a specific environment or covalently linked toa particular colloidal component. In either case, during suchinvasive probing, the presence of the external probe invari-ably alters the micellar property of interest. Consequently,such methodologies are not able to afford ‘true’ informationon many key properties as a result of the perturbationcaused by the external probe within the system [12,13]. Thisproblem raises serious questions and imposes severe limita-

0009-2614/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2007.10.101

* Corresponding author. Fax: +91 11 26581102.E-mail address: sipandey@chemistry.iitd.ac.in (S. Pandey).

tions on the use of electronic absorbance and fluorescence-based methods for characterization and analysis of surfac-tant solutions. In this Letter, we present a non-perturbingapproach for monitoring organized assemblies which relieson the fluorescence decay characteristics of intrinsic chro-mophores within surfactant monomers [13–16]. For thispurpose, we have selected a common non-ionic surfactantTriton X-100 (TX-100, Scheme 1). Steady-state and fre-quency-domain time-resolved fluorescence from the phenylmoiety of TX-100 are utilized to demonstrate the effective-ness and potential of this approach in monitoring andunderstanding micellar solutions.

2. Materials and methods

2.1. Reagents

The following chemicals were used in this study. Highestpurity 2,5-diphenyl-1,3,4-oxadiazole (PPD), received fromAldrich Chemical Co., was used as fluorescence lifetimestandard. Triton X-100 (TX-100) was purchased in highestpurity possible from Sigma. Doubly-distilled deionized

OOH

9.5

Scheme 1. Structure of non-ionic surfactant Triton X-100 (TX-100).

H. Om et al. / Chemical Physics Letters 450 (2007) 156–163 157

water was obtained from a Millipore, Milli-Q academicwater purification system having P18 MX cm resistivity.

2.2. Spectroscopic measurements

Quartz cuvettes (1 cm2) were used for all spectroscopicexperiments and all measurements were performed at roomtemperature (22 ± 2 �C). All absorbance measurementswere carried out on one of the following two spectropho-tometers: Spectronic model 1201 spectrophotometer (Mil-ton Roy, Rochester, NY) or Perkin-Elmer Lambdabio 20double beam spectrophotometer. Steady-state and time-resolved fluorescence experiments were performed withan SLM-AMINCO 48000 multiharmonic Fourier (MHF)phase-modulation spectrofluorometer (Spectronic Instru-ments, Rochester, NY). Steady-state fluorescence was alsoacquired on model FL 3-11, Fluorolog-3 modular spectro-fluorometer with single Czerny-Turner grating excitationand emission monochromators having 450 W Xe arc lampas the excitation source and PMT as the detector purchasedfrom Horiba-Jobin Yvon, Inc. Spectral response fromappropriate blank was subtracted before any data analysis.All the measurements were performed three separate timesstarting from sample preparation and taken in triplicateevery time before averaging.

The excited-state intensity decay kinetics were deter-mined in the frequency domain by using an intracavity-dou-bled argon-ion laser (Model 95 SHG, Lexel Lasers, Inc.)operating at 257.3 nm as the excitation source. Emissionwas monitored in the typical L-format through 295 nmand 345 nm longpass filters. Magic angle polarization con-ditions were used for all excited-state intensity decay kineticexperiments to eliminate bias stemming from fluorophorerotational reorientation. PPD in ethanol was used as theprimary lifetime standard with an assigned lifetime valueof 1.30 ns. The Pockels cell modulator was operated at a5 MHz repetition rate. Typically, data sets were acquiredfor 60–90 s between 5 and 200 MHz (40 total frequencies).At least 10 discrete multifrequency data sets were acquiredfor each sample under a given set of experimental condi-tions. The excited-state intensity decay kinetics were recov-ered from the phase-modulation data by using a standardnon-linear least squares methodology. In all data analyses,we used the true uncertainty in each phase-modulationdatum as the frequency weighting factor.

2.3. Theory

For more than one simultaneously excited, non-interact-ing species, the decay of total fluorescence is described inprinciple [17,18] by:

IðtÞ ¼X

i

ai expð�t=siÞ ð1Þ

where ai are the corresponding pre-exponential factorsassociated with the excited-state lifetimes si. In the simplestcase of excited aggregate formation from free ‘small’ mol-ecules in fluid solution, the decays of uncomplexed chro-mophore are of the form [17–19]:

iMðtÞ ¼ a1 expð�t=sEÞ þ a2 expð�t=sMÞ ð2ÞiEðtÞ ¼ a3½expð�t=sEÞ � expð�t=sMÞ� ð3Þ

Here, sE and sM represent the decay times of the excitedaggregates and the quenched monomer, respectively. Thisscheme of excited aggregate formation was presented byBirks [19] and was used by many researchers over the yearsto explain the excited aggregate formation mechanism andto recover the kinetic parameters of such process for ‘small’molecules.

2.3.1. Analysis of excited aggregate formation reaction by

phase-modulation fluorometry

For an irreversible excited-state reaction where the rateof dissociation of excited aggregate back to monomer, k�1,can be considered insignificant, the phases (u) and modula-tions (m) of the M and E states relative to the excitationwaveform can be expressed [17,18] as

tan uM ¼x

kM þ k1

ð4Þ

tan uE ¼xðkM þ kE þ k1ÞkEðkM þ k1Þ � x2

ð5Þ

mM ¼kM þ k1

½ðkM þ k1Þ2 þ x2�1=2ð6Þ

mE ¼ mMkE

½ðkEÞ2 þ x2�1=2ð7Þ

where x, kM, kE, and k1 represent the frequency, the decayrate of monomer fluorescence (in the absence of any othercompetitive deactivation pathways), the excited aggregatedecay rate, and the rate of the forward reaction (i.e., ex-cited aggregate formation rate), respectively. A carefulexamination of Eq. (5) reveals that if uE0

is the phase angleof the excited aggregate state observed if this state could beexcited directly, we can write [17]

/E ¼ /M þ /E0ð8Þ

The interesting feature of this expression is that the phaseangle /E can exceed 90�. Phase angles of the directly ex-cited species or from a heterogeneous population of fluoro-phores can never have a value of phase angle exceeding 90�.Observation of a phase angle in excess of 90� constitutesproof of an excited state reaction. Similarly, for a reversibleexcited-state reaction scheme the phase angles and modula-tion values corresponding to monomer species and excitedaggregates can be calculated accordingly. The values foruM, uE, mM, and mE are represented, in the case of revers-ible excited-state reaction scheme, by more complicatedmathematical expressions containing the term representing

158 H. Om et al. / Chemical Physics Letters 450 (2007) 156–163

rate of dissociation of excited aggregate back to monomer(k�1). It is important to mention here that the excited statereaction scheme can be extended and applied to the caseswith more than two excited state species undergoing morethan one excited state reactions among themselves. Furtherinformation and extensive reviews of the theory of phase/modulation fluorometry as applied to excited-state reactionscheme can be found elsewhere [17,18,20–26].

3. Results and discussion

3.1. Electronic absorbance data

Electronic absorbance spectra of different concentra-tions of aqueous TX-100 solutions are collected (Fig. S1:Supplementary data). The [TX-100] was varied from8 · 10�5 M to 8 · 10�4 M. The lowest energy absorbancemaxima appeared at 276 (±2) nm for all [TX-100]. Theabsorbance spectral behavior of TX-100 is similar to thatof p-methoxytoluene chromophoric moiety. Any shift inthe absorbance peak due to the change in the [TX-100] iswithin the experimental error (i.e., ±2 nm) and any bandbroadening due to the change in the [TX-100] is insignifi-cant. The linear regression analysis of the maximum absor-bance at 276 (±2) versus the concentration resulted in anr2 = 0.9989 with a molar extinction coefficient (e) = 1.58(±0.05) · 103 M�1 cm�1. It is clear from the spectra pre-sented in Fig. S1 that electronic absorbance is not sensitiveenough to detect or probe micellization process withinaqueous TX-100.

3.2. Steady-state fluorescence emission and excitation data

Fluorescence emission and excitation spectra of [TX-100] from 8 · 10�5 M to 8 · 10�2 M were acquired. Forhigh absorbing samples where appropriate, fluorescencesignals were corrected for self-absorbance using primaryand secondary inner-filtering correction methodologies[17]. All the emission features are in the range 280–440 nm (Fig. 1). For excitation at 257 nm, the fluorescenceintensity increases in the [TX-100] range 8 · 10�5 to8 · 10�4 M with the maximum of the intensity centered at307 ± 1 nm (Fig. 1A). However, at further higher concen-trations the intensity stops to increase and starts todecrease. This would be expected due to the high self-absorbance at 257 nm of the TX-100 solutions at higherconcentrations. More importantly, a very interesting fea-ture develops in the normalized fluorescence emission spec-tra as [TX-100] increases (Fig. 1B). Clearly, a shouldergrows with [TX-100] in the vicinity of 350 nm. This couldbe attributed to the presence of surfactant aggregates (liter-ature reports suggest cmc of TX-100 to be 0.22–0.25 mM[27]). Therefore, the shoulder around 350 nm in thesteady-state fluorescence emission spectra of TX-100 indi-cates the presence of micelles in the solution. This is furtherevidenced by linear regression analysis of the fluorescenceemission intensity at kmax versus [TX-100]. Although an

r2 of 0.9870 is recovered, a clear downward trend can beseen upon close inspection (inset Fig. 1A). Out of curiosity,fluorescence emission spectrum of 8 · 10�2 M was acquiredand compared with previous spectra (one would expect sig-nificant self-absorbance at such high concentration, itcould be used to explain the data qualitatively nonetheless).The spectrum is very interesting (inset Fig. 1B). Apart froma peak at 307 ± 1 nm, a huge red-shifted band centered at344 ± 2 nm is observed. This band has almost double thefluorescence intensity as compared to the band at307 nm. One would expect the micellar concentration tobe very high at such high [TX-100] giving rise to this highlyintense red-shifted fluorescence emission.

Red-edge excitation (REE) at 310 nm is used to furtherexplore the micelle formation as the [TX-100] is increased.Fluorescence emission is collected in the [TX-100] range of8 · 10�4 M to 8 · 10�3 M (Fig. 2). As expected, a broadband centered ca. 345 ± 2 nm is obtained. Indeed anincrease in fluorescence emission intensity with increasing[TX-100] is observed up to 8 · 10�3 M [TX-100] (insetFig. 2). Interestingly, a linear regression analysis resultedin an r2 = 0.9999 indicating a linear increase in micelle con-centration with [TX-100]. It is clear that steady-state fluo-rescence emission signal from surfactant itself providesimportant information regarding micellization process.

In instances where fluorescence emission is highlightedwith features depicting aggregation process, emission wave-length-dependent fluorescence excitation may provideadditional information. We acquired fluorescence excita-tion spectra with kem = 307 nm (i.e., at the monomer fluo-rescence maxima) in the [TX-100] range 8 · 10�5 M to8 · 10�3 M (Fig. 3A). At [TX-100] < cmc, as expected, fluo-rescence excitation spectra show one band between 250 and300 nm with maxima centered at 276 ± 1 nm. This is simi-lar to the kmax of absorbance for aqueous TX-100 (vide

supra). However, for [TX-100] > cmc, excitation spectraare drastically changed. As [TX-100] is increased, the singleband for the monomer starts to broaden significantly andsplits into two separate bands, one red shifted while theother blue shifted with respect to the original band. Theblue as well as the red shift with respect to the monomerband increases as [TX-100] is increased. The origin of thered-shifted band could be attributed to the presence ofthe monomers within the micellar aggregates. The blueshift, however, is not easily assignable at this point andcould be due to the monomers encountering, on the aver-age, more hydrophobic region due to the presence of themicelles during their exit and entry into the aggregates.Considering the dynamic nature of the micelles a surfactantconcentration-dependent red and blue shift in the fluores-cence excitation spectra is conceivable as in such steady-state measurements (under continuous illumination) infor-mation on the average property is obtained.

Next, fluorescence excitation spectra were collected withkem = 350 nm (i.e., at the fluorescence maxima of micellaraggregates) in the [TX-100] range 1.6 · 10�4 M to8 · 10�3 M. Similar to kex = 307 nm data as expected,

Rel

ativ

e F

luo

resc

ence

Inte

nsi

ty

0

1

2

3

4

[TX-100] M0 2e-4 4e-4 6e-4 8e-4

Flu

ore

scen

ce In

ten

sity

0

1

2

3

4

Emission Wavelength (nm)

280 320 360 400 440

No

rmal

ized

Flu

ore

scen

ce In

ten

sity

0

1

Emission Wavelength (nm)290 340 390

Rel

ativ

e F

luo

resc

ence

Inte

nsi

ty

0

1

2

3

4[TX-100] = 0.08 M

Increasing [TX-100]

Increasing [TX-100]A

B

r 2 = 0.9870

λex = 257 nm

Fig. 1. Steady-state fluorescence emission spectra of aqueous TX-100 at ambient conditions (kex = 257 nm, and excitation and emission slits are 8 and2 nm, respectively). See text for more details.

H. Om et al. / Chemical Physics Letters 450 (2007) 156–163 159

below cmc the bands are again centered at 276 ± 1 nm, andas the [TX-100] is increased, the band broadens signifi-cantly affording two bands – one red and one blue shifted(Fig. 3B). However, one key additional feature accompa-nies these fluorescence excitation spectra. A broad feature-less band appears at 317 ± 2 nm whose intensity increaseslinearly with [TX-100] (r2 = 0.9999). This band could beeasily attributed to be arising from the aggregated surfac-tant molecules that form micelles in the solution. Extremelylow intensity of this band at 0.16 mM [TX-100] suggests theonset of micellization and/or presence of pre-micellaraggregates. Emission at kex = 275 nm and excitation atkem = 310 nm of aqueous [TX-100] in the range 0.1–0.8 mM provide further support to these observations(Fig. S2: Supplementary data). Simple steady-state fluores-

cence measurements where the fluorophoric moiety inher-ent to the surfactant itself is utilized to probemicellization provide important insightful information onsurfactant aggregation in a noninvasive fashion.

3.3. Frequency-domain excited-state fluorescence intensity

decay data

Table 1 presents the recovered decay times from the sin-gle exponential fit to the excited-state intensity decay for[TX-100] in the range 8.0 · 10�3 M to 8.9 · 10�6 M. Acareful examination of the data reveals several key features.The recovered decay times decrease as the [TX-100] isdecreased. Minor changes in the optical densities of theTX-100 solutions at the excitation wavelength (i.e.,

Emission Wavelength (nm)320 340 360 380 400

Rel

ativ

e F

luo

resc

ence

Inte

nsi

ty

0

1

2

3

[TX-100] M0.000 0.002 0.004 0.006 0.008 0.010

Flu

ores

cenc

e In

tens

ity

0

1

2

3

Increasing [TX-100]

r 2 = 0.9999λex = 310 nm

Fig. 2. Red-edge excited fluorescence spectra at higher aqueous TX-100 concentrations at ambient conditions (kex = 310 nm, and excitation and emissionslits are 8 and 4 nm, respectively). See text for more details.

160 H. Om et al. / Chemical Physics Letters 450 (2007) 156–163

257.25 nm) can not explain the large changes in excited-state fluorescence decay times. Importantly, the decreaseis fairly drastic as the [TX-100] is decreased till it reachesthe cmc (7.4 ns to 4.8 ns for [TX-100] = 8.0 · 10�3 M to2.7 · 10�4 M). The decay times do not change as dramati-cally for [TX-100] below cmc. It is interesting to note thatalthough the goodness-of-the-fit for the single-exponentialdecay is acceptable (lower v2) for [TX-100] < cmc, thehigher v2 for [TX-100] > cmc clearly indicate the inappro-priateness of this rather simple model for solutions con-taining micelles. Decay times as well as v2 provide anindication of the presence of micelles. It is important tomention that an attempt to fit the excited-state intensitydecay data to a double-exponential decay model resultedin higher v2 at all [TX-100]. Although the fits of theexcited-state intensity decay data to Gaussian and Lorentz-ian distribution did provide lower v2 than those for doubleexponential model, the goodness-of-the-fit were not betterthan those for the single exponential model. It is easy tocomprehend that for [TX-100] < cmc, the single-exponen-tial decay best describes the excited-state intensity decay;one would assume the presence of only the monomer sur-factant molecules of TX-100 in the solution. However,the data for [TX-100] > cmc appear to be more complex.It is well documented in the literature that during themicelle formation the appropriate number (i.e., the aggre-gation number) of surfactant monomers associate to formmicelles and then dissociate [3–7]. The excited phenyl moi-ety within a surfactant monomer during its excited-statelifetime may get involved into forming the micellar aggre-gate. On the other hand, surfactants possessing phenylmoieties in the ground state within the micellar aggregatesmay get excited.

One of the most exciting features of our frequency-domain excited-state fluorescence intensity decay data isthe indication of the presence of excited state reaction.Observation of a phase angle in excess of 90� constitutesproof of an excited state reaction (vide supra). A carefulexamination of the data shows the phase angles consis-tently and significantly in the excess of 90� for [TX-100] > cmc (Fig. 4). Thus, excited-state reaction process isclearly evident; more pronounced when a 345 nm long passfilter is used as compared to a 295 nm long pass filter asexpected. For [TX-100] < cmc, even with 345 nm long passemission filter phase angles never exceed 90�. The decreasein the excited-state fluorescence lifetimes with decreasing[TX-100] can be explained, in part, based on the presenceof the excited-state aggregates. If the excited aggregateshave longer excited-state fluorescence lifetimes than theexcited monomers, increase in [TX-100] will give rise toincreased excited aggregates-to-excited monomers ratioincreasing the overall recovered excited-state fluorescencelifetimes. A second contribution toward increase inexcited-state fluorescence lifetime with increasing [TX-100] may be from the fact that a change in the [TX-100]introduces altered microenvironment (i.e., the cybotacticregion) surrounding the phenyl moiety giving rise to alteredlifetimes. It is important to mention here that in TX-100the phenyl fluorophore is located more towards the tailand away from the head group region (Scheme 1). It is hardto imagine that for [TX-100] > cmc the fluorophoreencounters altered surrounding as the [TX-100] is changedbecause the environment inside the micelles are similar andindependent of the surfactant amount in wide range ofconcentrations. On the other hand, [micelles] increases([micelles] = {[TX-100] � cmc}/Nagg, where Nagg is the

λem = 350 nm

Excitation Wavelength (nm)

240 260 280 300 320 340

No

rmal

ized

Flu

ore

scen

ce In

ten

sity

0

1

240 260 280 300

No

rmal

ized

Flu

ore

scen

ce In

ten

sity

0

1

λem = 307 nm

A

B

Fig. 3. Emission wavelength-dependent fluorescence excitation spectra of aqueous TX-100 (excitation and emission slits are 8 and 4 nm, respectively). (A)kem = 307 nm, and [TX-100] = 0.08 mM (filled circles, dashed line), 0.16 mM (open hexagons), 0.27 mM (filled triangles), 0.40 mM (open diamonds),0.80 mM (filled squares), 1.6 mM (open triangles), and 8.0 mM (filled circles, solid line). (B) kem = 350 nm, and [TX-100] = 0.16 mM (open diamonds),0.80 mM (filled squares), 1.6 mM (open triangles), and 8.0 mM (filled circles).

H. Om et al. / Chemical Physics Letters 450 (2007) 156–163 161

aggregation number) as the [TX-100] is increased; moreand more surfactant molecules are involved in forming

Table 1Concentration dependence of the excited-state fluorescence lifetimes (s) ofaqueous Triton X-100 at ambient conditions

[TX-100]/M s (ns) v2

8.0 · 10�3 7.4 751.6 · 10�3 6.5 178.0 · 10�4 6.0 3.34.0 · 10�4 5.3 3.12.7 · 10�4 4.8 2.41.6 · 10�4 4.4 1.28.0 · 10�5 4.4 1.52.7 · 10�5 4.3 1.68.9 · 10�6 4.0 0.9

The lifetimes are recovered from single-exponential fit to the decay data,and v2 represent the goodness-of-the-fit. The imprecision in the s is 65%RSD.

micellar aggregates. This, in turn, will increase theexcited-state fluorescence lifetimes as the [TX-100] isincreased.

It is noteworthy that above cmc the fit of the fluores-cence intensity decay data to single- and multi-exponentialdecay schemes is not satisfactory. As mentioned earlier afitting procedure incorporating excited-state reaction pro-cess was undertaken. The frequency-domain phase datafor [TX-100] < cmc strongly suggested absence of the for-mation of any micellar aggregates starting from excitedmonomers. As suggested by some key publications, thedynamics of micelle formation has been a topic of curiosityfor a long time [3–7,28–34]. Based on experimental evi-dence micellar dynamics is characterized by two relaxationtimes – the fast one indicating association-dissociationinvolving the exchange of surfactant monomers betweenbulk aqueous and micellar phases and the slower one

Frequency (MHz)

0 40 80 120 160

Ph

ase

An

gle

(o )

0

30

60

90

120

log[TX-100]/M-4.0 -3.5 -3.0 -2.5 -2.0 -1.5

χ2

0.0

0.4

0.8

1.2For the fit toexcited-statereaction scheme

λex = 257.25 nm (Lexel Laser)

Fig. 4. Concentration-dependent frequency-domain excited-state fluorescence intensity decay data for aqueous TX-100 at ambient conditions showing thephase angles at various frequencies (kex = 257.25 nm from Lexel laser), and [TX-100] = 0.16 mM (i.e., below cmc, filled circles), 0.80 mM (i.e., above cmc,open diamonds), 1.6 mM (filled triangles), and 8.0 mM (open squares). Inset shows recovered v2 for the fit of the frequency-domain phase data to theexcited-state reaction scheme as described in the text.

162 H. Om et al. / Chemical Physics Letters 450 (2007) 156–163

implying the complete formation and dissolution of themicellar assemblies [28–34]. Further, exit and re-entry rateconstants (k� and k+, respectively) for the surfactantmonomers were introduced. For TX-100 at 25 �C, k� andk+ were calculated to be 1.1 · 106 s�1 and 3.7 · 109

M�1 s�1, respectively [28–30]. Conveniently, a global anal-ysis-based data fitting procedure for the frequency-domainexcited-state intensity decay involving the excited-statereaction process directly recovers the two rate constantsk� and k+ in our case. It is clear that recovered v2 havedrastically improved (inset Fig. 4). We recoveredk� = 3.05(±0.42) · 106 s�1 and k+ = 3.67(±0.51) · 109

M�1 s�1 using the globally-linked (across all [TX-100] >cmc) data fitting protocol. While k+ is similar to thatreported earlier, k� is slightly different. At this juncture,we attribute this difference to the possible differences inthe sample quality and/or the differences in the techniquesused to obtain k�. Inclusion of the [TX-100] < cmc datadegrades the overall v2 recovered from the global analysisprotocol dramatically (global v2 = 0.65 for [TX-100] >cmc versus 4.05 for entire range of [TX-100]). Further,the fit of the frequency-domain intensity decay data for[TX-100] < cmc to an excited-state reaction scheme wasfound to be unacceptable (v2 P 10).

4. Conclusions

Using TX-100 as its own probe, steady-state and fre-quency-domain time-resolved fluorescence measurementsfrom the phenyl fluorophore moiety of the surfactant pro-vides key information on the surfactant aggregation pro-cess noninvasively in a simple and effective fashion.Surfactant concentration-dependent emission and excita-tion spectra are rich in information on micellization. Whilethe phase angles clearly indicate aggregate formation, aglobal analysis of the data provides information on aggre-gate dynamics.

Acknowledgements

S.P. would like to thank DST, India and CSIR, Indiafor the partial support. H.O. would like to thank CSIR, In-dia for the award of research fellowship to him.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.cplett.2007.10.101.

H. Om et al. / Chemical Physics Letters 450 (2007) 156–163 163

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