Role of Micellar Size and Hydration on Solvation Dynamics: A Temperature DependentStudy in Triton-X-100 and Brij-35 Micelles

Manoj Kumbhakar, Teena Goel, Tulsi Mukherjee, and Haridas Pal*Radiation Chemistry & Chemical Dynamics DiVision, Bhabha Atomic Research Centre, Trombay,Mumbai 400 085, India

ReceiVed: July 20, 2004; In Final Form: September 15, 2004

The temperature effect on solvation dynamics has been investigated in two neutral micelles, namely, TritonX-100 (TX100) and Brij-35 (BJ-35), using dynamic fluorescence Stokes’ shift method, to explore the role ofmicellar size and hydration on the solvation process. In TX-100, the temperature effect onC(t) is not onlyvery strong but shows an unusual inversion around 298 K. On the contrary, for the BJ-35 micelle, thetemperature effect is not that significant. Present results on solvation dynamics in the two micelles have beenrationalized on the basis of the temperature dependent changes in micellar size and hydration, which arereported to be very large for TX-100 but very marginal for BJ-35. Observed inversion in the solvation ratein TX-100 around 298 K is inferred to be arising due to the interplay of two factors, one is the largelyreduced micellar size at lower temperature, which causes the bulk water to come reasonably closer to theprobe and thus contribute to the solvation process, and the other is the largely increased hydration at highertemperature that makes the micellar structure loose and consequent enhancement in the solvation rates. Withintermediate micellar size and hydration, the solvation dynamics is slowest at around 298 K. For the BJ-35micelle, as the micellar size and hydration does not change to any significant extent, there is almost notemperature effect on the solvation dynamics for this micelle.

1. Introduction

The solvation dynamics in different heterogeneous media,such as micelles,1-5 microemulsions,6-9 lipids,10,11proteins,12-15

DNA,16 cyclodextrin,17,18etc., has been the subject of extensiveresearch in about the last decade. It has been observed that thesolvation process is 2-3 orders of magnitude slower in theconfined media in comparison to that in bulk water.1-21 Overthe years, efforts have been made to understand the details ofthe responses of the water molecules around the probe inheterogeneous media, both experimentally and theoretically.19-21

Understanding the water dynamics in confined media is havinga direct relevance to the biological systems.22 Micelles arepictured as a simplified version of the molecular aggregates,which resemble some of the characteristics of the rather complexand subtle biological systems like proteins, lipids, and DNAenvironments.19-22 Micellar systems also have a wide varietyof scientific, engineering, and technological applications.23

Properties of micelles are largely dependent on their size,shape, composition, etc.3,24-32 In some cases the behaviors ofthe micelles critically depend on the surrounding environments.It is thus important to study the solvation dynamics in micellarmedia under different external conditions to understand howthe changes in the microenvironments affect the dynamics ofthe solvation process in these confined systems. For micelles,it is possible to change their internal as well as externalenvironments by changing the temperature or pressure of thesolution,3,28-31 or by adding a suitable electrolyte.26,27,32 Re-cently, Hara et al.3 have observed substantial changes in thesolvation dynamics in the TX-100 micelle on changing theapplied pressure and explained their results in terms of the

changes in the degree of hydration of the micelle and thechanges in the strength of the intermolecular hydrogen bondingfor the confined water molecules. For the TX-100 micelle, it isreported that the micellar size and hydration increase substan-tially with increasing temperature.30,31 In a recent article, Senet al.5 have reported the temperature effect on solvationdynamics in the TX-100 micelle using 4-aminophthalimide (4-AP) as the fluorescence probe. They have observed a gradualincrease in the solvation rate with an increase in the temperature.These authors have explained this observation simply on thebasis of the enhancement in the rates of the activation-controlledexchange process between the free and bound water moleculesin the micellar phase with temperature. According to theseauthors,5 the temperature dependent changes in the micellar sizeand hydration do not cause any significant effect on the observedsolvation process.5 Because the changes in the micellar size andhydration with temperature are quite large for TX-100,30,31it isdifficult to accept the inference of Sen et al.5 that these changeshave no effect on the observed solvation dynamic. Moreover,if the micellar size and hydration was really not playing anysignificant role in determining the dynamics of the confinedwater molecules, the solvation process in different micelles wasexpected to occur almost in the similar time scale. Literaturereports, however, indicate a completely different picture. Sol-vation dynamics in the cetyltrimethylammonium bromide(CTAB) micelle is seen to be substantially slower than that insodium dodecyl sulfate (SDS) micelle, even though the Sternlayer thicknesses are comparable for both the micelles.1,19,20Itis suggested that the slower solvation process in CTAB than inSDS is due to the comparatively dry Stern layer for the formermicelle than the latter. For a neutral micelle like TX-100, thesolvation rate is much slower than in ionic micelles such asSDS and CTAB and is attributed to the much larger Palisade

* Corresponding author. E-mail: hpal@apsara.barc.ernet.in. Fax: 91-22-25505151 and 25519613.

19246 J. Phys. Chem. B2004,108,19246-19254

10.1021/jp0468004 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 11/17/2004

layer thickness (∼25 Å) for the former micelle than the Sternlayer thickness (∼9 Å) of the latter two micelles.1,19,20Due tothe very thick Palisade layer, most of the fluorescence probesin the TX-100 micelle reside quite deeply into the micelle, wherethe water structure is not as loose as that in the Stern layer ofCTAB and SDS micelles having substantially thinner micellarStern layers. Accordingly, the response of the water moleculesaround the probe in the TX-100 micelle is slower in comparisonto that in CTAB and SDS micelles. From the literature reportsit is thus indicated that the micellar size and hydration play asignificant role in determining the solvation dynamics in micellarmedia. The reports also indicate that the locations of the probein the micelle have a significant role in determining the rate ofthe solvation process.

To resolve the paradox whether the temperature effect on thesolvation dynamics in micellar solution is mainly due to theenhancement in the exchange process for the free and boundwater molecules or the changes in the micellar size and hydrationalso play a role, in the present work we have carried outtemperature dependent studies on the solvation dynamics in TX-100 and Brij-35 (BJ-35) micelles, using coumarin-153 (C153)as the fluorescence probe. The chemical structures of thesurfactants, TX-100 and BJ-35, and that of the probe, C153,are shown in Chart 1. Both TX-100 and BJ-35 are reported toform neutral spherical micelles. Due to large differences in thenumber of the hydrophilic oxyethylene units in TX-100 andBJ-35, their micellar characteristics are, however, quite different.Important micellar parameters for TX-100 and BJ-35 micellesare listed in Table 1. As reported in the literature,28-31 the sizeand hydration of the TX-100 micelle increases largely withtemperature, but for BJ-35, there is not much of a temperatureeffect on the micellar size and hydration. Thus, it was expectedthat the temperature effect on the solvation dynamics in TX-100 and BJ-35 micelles will help us answer whether the changesin the micellar size and hydration with temperature do havesome effect or not on the observed changes in the solvationdynamics with temperature.

2. Experimental Section

TX-100 was obtained from Sigma and BJ-35 was obtainedfrom Pierce Chemical Co., and they were used without furtherpurification. Laser grade C153 was obtained from Exiton andused as received. Nanopure water, having a conductivity of∼0.1µS cm-1, was obtained by passing distilled water through a

Barnstead Nanopure Water System and used for the preparationof the micellar solutions.

In the experimental solutions the concentrations of TX-100and BJ-35 surfactants were such that the concentration of themicelles remains in the range of∼1 mM. For all the experi-ments, the concentrations of the probe C153 were kept quitelow, only in the range of∼10 µM. With this experimentalcondition, where the probe concentration is∼100 times lowerthan that of the micelles, it is expected that none of the micellescontains more than one probe molecule in it. Because the dyeC153 is almost insoluble in water,33 and because the dyesolubility increases substantially in micellar solutions, it isexpected that the dye C153 mostly solubilized in the micellarphase.

Steady-state absorption spectra were recorded using a JASCO(Tokyo, Japan) model V530 spectrophotometer. Fluorescencespectra were recorded using a Hitachi (Tokyo, Japan) modelF-4010 spectrofluorometer. Time-resolved fluorescence mea-surements were carried out using a diode laser based spectrof-luorometer from IBH. The instrument works on the principleof time-correlated single-photon counting (TCSPC).34 In thepresent work, a 408 nm diode laser (<100 ps, 1 MHz) wasused as the excitation light source and a TBX4 detection module(IBH) coupled with a special Hamamatsu PMT was used forfluorescence detection. For the present setup, the instrumentresponse function was∼240 ps at fwhm. In the solvationdynamics studies, fluorescence decays were recorded with avertically polarized excitation beam and fluorescence wascollected at magic angle (54.7°). In these measurements, thespectral bandwidth for fluorescence detection was kept at∼8nm. For anisotropy measurements, fluorescence decays werecollected with a larger spectral bandwidth of∼32 nm, to avoidthe effect of the solvent relaxation process on the anisotropyresults. Measurements were repeated for three times, both tocheck the reproducibility and to obtain the average values ofthe relaxation times. In the present work, the temperature ofthe solution was adjusted with the help of a coldfingerarrangement and the temperature was controlled using amicroprocessor based temperature controller (model DS fromIBH), with the temperature accuracy of(1 °C.

For the measurement of the time-resolved emission spectra(TRES), a set of fluorescence decays were collected at differentwavelengths, covering the entire emission band of the probe.These decays were fitted using a triexponential function to obtainthe fitted curvesD(λ,t). The TRES spectra,S(λ,t), were thenconstructed using the fitted decays,D(λ,t), after their normaliza-tion with respect to the steady-state fluorescence spectrum,S0(λ),and applying the following relation.

To obtain smooth TRES, the data points obtained for the spectraS(λ,t) using eq 1 were fitted following the procedure describedby Maroncelli and Fleming,35 considering a long-normal lineshape function for the emission band. The smooth TRES thusobtained were then used to estimate the emission maxima atdifferent times and consequently to construct the dynamicStokes’ shift correlation function,C(t), as is used to measurethe solvation dynamics.1-21,35

Fluorescence anisotropy measurements were carried out bymeasuring the polarized fluorescence decays,I|(t) and I⊥(t),whereI|(t) andI⊥(t) are the fluorescence decays for the paralleland perpendicular polarizations with respect to the vertical

CHART 1

TABLE 1: Micellar Parameters for TX-100 and BJ-35Micelles at Different Temperaturesa

rmicelle(Å) CMC (mM) Nagg

temp, (K) TX-100 BJ-35b TX-100 BJ-35b TX-100 BJ-35b

288 37 0.334 48298 44 44 0.319 0.10 86 40308 54 0.294 150

a The parameters have been obtained from refs 28-31. b For BJ-35there is no significant change in micellar parameters with temperature.

S(λ,t) ) D(λ,t)S0(λ)

∫0

∞D(λ,t) dt

(1)

Role of Micellar Size and Hydration on Solvation Dynamics J. Phys. Chem. B, Vol. 108, No. 50, 200419247

polarization of the excitation beam. From theI|(t) and I⊥(t)decays, the anisotropy decay function was constructed usingthe following relation,34

whereG is a correction factor for the polarization bias of thedetection system.G factor was independently obtained by usingthe horizontally polarized excitation beam and measuring thetwo perpendicularly polarized fluorescence decays.34 Eachmeasurement was repeated at least two or three times, and theaverage values are reported.

3. Results

3.1. Ground-State Absorption and Fluorescence Studies.Fluorescence spectra of C153 were recorded in TX-100 and BJ-35 micelles and also in different ethanol-water solvent mixturesat ambient temperature to understand the location of the dye inthe micelle and also to know the polarity of the microenviron-ment around the probe. For these purposes, a calibration curvewas first constructed by plotting the fluorescence maxima (νjfl )of the dye in ethanol-water solvent mixtures against the solventpolarity function,∆f, defined as36

whereε is the static dielectric constant andn is the refractiveindex of the solvent. For pure ethanol and water solvents,ε

and n values were taken from the literature.37 For ethanol-water mixed solvents (MS), theε andn values were estimatedfrom the volume fractions (f) of the cosolvents using thefollowing relations,38-43

where the subscripts A and B represent the respective cosolvents.The calibration curve thus obtained is shown in Figure 1, whichis linear within the experimental error. Using this calibrationcurve, and noting theνjfl values for C153 emission spectra inthe TX-100 and BJ-35 micelles, theε values for the micro-environment around the dye in the two micelles were estimatedto be∼22.4 and∼28.8, respectively (assuming then value is

similar to that of water). It should be mentioned here that, whilethe spectral width of the probe emission in the mixed ethanol-water solvent mixtures was compared with that in the twomicelles studied, there was no unusual change observed in themicellar solution, suggesting the suitability of the calibrationcurve shown in Figure 1 for the estimation of the polarity ofthe microenvironment around the probe in the two micelles.Because the micellar core is highly nonpolar (similar tohydrocarbon solvents)44 and the bulk water is highly polar, theestimatedε values in the present systems indicate that the dyepreferentially resides in the micellar Palisade layer of havingan intermediate polarity. The slightly higherε value for the probemicroenvironment in the BJ-35 micelle indicates that thePalisade layer of this micelle is somewhat more polar than thatof the TX-100 micelle.

Fluorescence spectra of C153 dye in TX-100 and BJ-35micelles were also recorded at different temperatures. Noappreciable changes in the shapes and in the peak positions wereobserved in both the micelles for the temperature range 288-308 K, investigated in the present work. These observationsindicate that in a particular micelle the polarity of the microen-vironment around the probe remains more or less unchangedfor the temperature range studied. This result in TX-100 is quiteunexpected considering the fact that the size and hydration ofthis micelle changes substantially with temperature.30,31Presentobservation from the steady-state fluorescence result in TX-100 can, however, be rationalized if we consider that there is arelative migration of the probe toward the micellar core withincreasing temperature of the solution. It should be mentionedthat such a migration of a probe in a micelle is already reportedin the literature based on fluorescence anisotropy studies.31 Inthe BJ-35 micelle, because there is not much of change in themicellar size and hydration with temperature,28,29 the micro-environment around the probe is expected to be similar for allthe temperatures studied.

3.2. Time Dependent Fluorescence Stokes’ Shift Measure-ments. The fluorescence decays of C153 dye measured at itsemission maxima in TX-100 and BJ-35 micelles are found tofit well with a single-exponential function for all the temper-atures studied. The fluorescence lifetime (τf) of C153 measuredin TX-100 changes from 4.82 to 4.53 ns, for changing thetemperature from 288 to 308 K. For BJ-35 micelle, theτf valueremains more or less the similar (∼3.91 ns) for all thetemperatures studied. The nominal change in theτf value inTX-100 and almost no change in the value in BJ-35 thus indicatethat in both the micelles the microenvironment around the proberemains more or less similar for all the temperatures studied,which is also indicated in the steady-state fluorescence resultspresented in section 3.1. The single-exponential nature of thefluorescence decays in the present systems indicates that forboth TX-100 and BJ-35 micelles, the different probe moleculespresent in different micelles in a particular solution experiencealmost similar microenvironments.

Wavelength dependent fluorescence decays, as measured forC153 dye in TX-100 and BJ-35 micelles at different tempera-tures, clearly indicate a time dependent Stokes’ shift for theprobe emission spectra. Typical such decays in TX-100 micelleat 298 K are shown in Figure 2A. The time-resolved emissionspectra (TRES) of C153 in the two micelles at differenttemperatures were constructed using the procedure describedin section 2.35 Typical TRES for C153 dye in TX-100 micelleas obtained at 298 K are shown in Figure 2B.

The time dependent fluorescence Stokes’ shifts, as estimatedfrom TRES, were used to construct the normalized spectral shift

Figure 1. Plot of fluorescence maxima (νjfl in cm-1) against the solventpolarity function ∆f (cf. eq 2) for different ethanol-water solventmixtures. Data points corresponding to the TX-100 and BJ-35 micellesare indicated in the figure.

r(t) )I|(t) - GI⊥(t)

I|(t) + 2GI⊥(t)(2)

∆f ) ε - 12ε + 1

- n2 - 1

2n2 + 1(3)

εMS ) fAεA + fBεB (4)

nMS2 ) fAnA

2 + fBnB2 (5)

19248 J. Phys. Chem. B, Vol. 108, No. 50, 2004 Kumbhakar et al.

correlation functionC(t), defined as

whereν(0), ν(t), andν(∞) are the emission maxima (in cm-1)at time zero,t, and∞, respectively. A detailed procedure forthe spectral reconstruction method and the calculation of theC(t) function has been given by Maroncelli and Fleming.35 TheC(t) function represents the temporal response of the solventrelaxation process, as occurs around the probe following itsphotoexcitation and the associated change in the dipole moment.

The C(t) curves obtained for TX-100 and BJ-35 micelles atdifferent temperatures are shown in Figure 3A and B, respec-tively. It is indicated from this figure that theC(t) curves inboth the micelles follow a bimodal decay characteristics at allthe temperatures studied. The biexponential analysis is seen togive reasonably good fit for theC(t) curves, as are also shownin Figure 3A,B. The two solvation times thus obtained for TX-100 and BJ-35 micelles at different temperatures are listed inTable 2 along with their percentage contributions. The averagesolvation times (⟨τs⟩) in the two micelles were also estimatedby using

whereas1andas2are the percentage contributions correspondingto the fast (τs1) and slow (τs2) solvation times, respectively. The⟨τs⟩ values thus obtained are listed in Table 2.

The interesting points to be noted from Figure 3A,B and Table2 are the following. (i) For all the temperatures studied, the

C(t) curves in the BJ-35 micelle show much faster decays incomparison to those in the TX-100 micelle. (ii) For the TX-100 micelle, theC(t) function shows substantial temperatureeffect but for the BJ-35 micelle, the temperature effect on theC(t) function is not that significant. (iii) For the TX-100 micelle,the changes in theC(t) function with temperature are not agradual one. Thus, theC(t) function shows the slowest decay

Figure 2. (A) Typical fluorescence decay traces obtained at differentwavelengths for C153 dye in TX-100 micellar solution at 298 K. Thedecays at the blue edge of the fluorescence spectra show a distinct fastdecay component and those at the red edge of the spectra show a distinctgrowth component, indicating a time dependent Stokes’ shift in theemission spectra of the dye in the micellar solution. (B) Time-resolvedemission spectra of C153 in TX-100 micellar solution. For spectra 1-6,the time windows are 0.05, 0.55, 1.0, 2.2, 4.0, and 10.0 ns, respectively.The symbols in the spectra correspond to the experimental points andthe continuous curves correspond to the log-normal fits to theexperimental data points.

C(t) )ν(t) - ν(∞)

ν(0) - ν(∞)(6)

⟨τs⟩ ) as1τs1 + as2τs2 (7)

Figure 3. (A) Normalized dynamic Stokes’ shift correlation functionC(t) for the C153 probe in the TX-100 micelle at different tempera-tures: solid line 288 K; dotted line, 298 K; dashed line, 308 K. (B)Normalized dynamic Stokes’ shift correlation functionC(t) for the C153probe in the BJ-35 micelle at different temperatures: solid line, 288K; dotted line, 298 K; dashed line, 308 K. (C) Normalized dynamicStokes’ shift correlation functionC(t) for C151 probe in TX-100 micelleat different temperatures: solid line, 288 K; dotted line, 298 K; dashedline, 308 K.

TABLE 2: Solvation Times Obtained from theBiexponential Fitting of the Normalized Spectral ShiftResponse FunctionC(t) in TX-100 and BJ-35 MicellarSolutionsa

micelle temp (K) as1 τs1 (ps) as2 τs2 (ps) ⟨τs⟩ (ps)

TX-100 288 66.8 416 33.2 3018 1281(probe C153) 298 81.9 981 18.1 4350 1929

308 55.0 281 45.0 1977 1044TX-100 288 61.5 492 38.5 2531 1277(probe C151) 298 56.8 718 43.2 4357 2290

308 77.1 458 22.9 3102 1062BJ-35 288 65.9 124 34.1 766 342(probe C153) 298 50.9 193 49.1 682 433

308 52.8 184 47.2 626 393

a Average solvation times calculated using eq 7 are also listed.

Role of Micellar Size and Hydration on Solvation Dynamics J. Phys. Chem. B, Vol. 108, No. 50, 200419249

at an intermediate temperature of 298 K and the decay becomesfaster on either increasing or decreasing the temperature fromthis value. (iv) For the BJ-35 micelle, as the changes in thesolvation times with temperature are quite small and are almostwithin our experimental error, we cannot give much of emphasison the trend of these changes with temperature.

Faster solvent relaxation process in the BJ-35 micelle thanin the TX-100 micelle indicates a higher degree of hydrationfor the Palisade layer of the former micelle than the latter. Thisis supported by the higherε value for the Palisade layer of theBJ-35 micelle than that of TX-100 micelle, as obtained fromsteady-state fluorescence studies (cf. section 3.1). Consideringthe chemical structures of BJ-35 and TX-100 surfactantmolecules, the number of the hydrophilic oxyethylene units ismore for the former than the latter. However, the aggregationnumber for the BJ-35 micelle is substantially lower than thatof TX-100 micelle, for almost all the temperatures studied. Dueto this smaller aggregation number, the micellar size for BJ-35is also smaller than that of TX-100, even though the number ofoxyethylene units is more in the former case.28-31 From thisinformation it is thus inferred that the Palisade layer for theBJ-35 micelle is relatively loose and allows more waterpenetration than that in the TX-100 micelle.28,31 Due to thishigher water penetration, the mobility of the water moleculesaround the probe is less hindered, causing the solvation processto occur faster in the BJ-35 micelle than in TX-100.

Because the error limit in the solvation dynamics measure-ments is expected to be somewhat higher (results werereproducible within 20%) not much of importance could begiven to the changes in the solvation times in BJ-35 micellesas a function of temperature. For the TX-100 micelle, however,the changes are large enough to carry substantial significance.Moreover, both the solvation times estimated for TX-100 arelonger than the instrument response function of the presentexperimental set up (∼240 ps), and thus the temperaturedependent changes in the solvation times in this micelle aremore evident than those in the BJ-35 micelle, where the fastsolvation component (τs1) is within the instrument responsefunction. When the slow solvation components (τs2) in the twomicelles are compared, it is clearly indicated that unlike TX-100, the changes in the solvation dynamics with temperaturein the BJ-35 micelle are not only nonmonotonic such as TX-100 but also not as large. It is thus inferred that the solvationdynamics in TX-100 has a substantial temperature effect butfor the BJ-35 micelle the temperature effect on the solvationdynamics is not that significant.

Under normal circumstances, the dynamics of a physico-chemical process like solvation was expected to increase withtemperature. Thus, the fact that the changes in the solvationrate with temperature in the TX-100 micelle show an inversionaround∼298 K appears to be a quite unusual observation.Discussion on this unusual behavior will also be presented latterin section 4. In the present context, however, it is to bementioned that to check the genuineness of such an unusualobservation, we also carried out measurements in TX-100micelle using an another coumarin dye, namely, coumarin 151(C151; 7-NH2-4-CF3-1,2-benzopyrone), as the fluorescenceprobe. The results thus obtained also show a clear nonmonotonictemperature dependence on the solvation dynamics in the TX-100 micelle, as are shown in Figure 3C. The solvation timesthus obtained from the TX-100 micelle at different temperaturesusing C151 as the probe are listed in Table 2. Marginaldifferences in the time scales for the solvation components withC151 and C153 as the probes are supposed to be related to the

differences in the hydrophobicities of the two coumarin dyesconcerned (cf. section 4) and also due to the estimation errorsin the solvation dynamics mesurements.

3.3. Anisotropy Measurements.Fluorescence anisotropy forC153 dye was measured in TX-100 and BJ-35 micelles atdifferent temperatures to understand more about the character-istics of the microenvironments around the probes in the twomicelles. For all the temperatures studied, the measured ani-sotropy decay,r(t), shows biexponential behavior. Fitted ani-sotropy decay functions at different temperatures for TX-100and BJ-35 micelles are shown in Figure 4A,B, respectively. Theexperimental data points for all the anisotropy decay curves atdifferent temperatures are not shown in this figure, except justone set for each of TX-100 and BJ-35 micelles, just to avoidthe jumble up of the data points for different decay curves inthe figure. The two rotational time constants (τr1 and τr2)estimated for both TX-100 and BJ-35 micelles at differenttemperatures are listed in Table 3 along with their relativeamplitudes (ar1 and ar2). It is seen from Table 3 that therotational relaxation times are unusually slower in both themicelles in comparison to the picosecond rotational timesreported in bulk water.45 The slower anisotropy decays in bothTX-100 and BJ-35 micelles indicate that the probe C153experiences much higher microviscosity in these micellarsolutions in comparison to that in bulk water and consequentlysuggest that the probe resides in the micellar phase (Palisadelayer).

It is seen that both fast and slow rotational time constants(τr1 and τr2, respectively) are quite strongly temperature de-pendent for both TX-100 and BJ-35 micelles. Present resultsthus suggest that there is an increase in the fluidity inside thePalisade layer of both the micelles on increasing the temperature.

Figure 4. (A) Fluorescence anisotropy decay (fitted curves) for theC153 probe in the TX-100 micelle at different temperatures: solid line,288 K; dotted line, 298 K; dashed line, 308 K. (B) Fluorescenceanisotropy decay (fitted curves) for C153 probe in the BJ-35 micelleat different temperatures: solid line, 288 K; dotted line, 298 K; dashedline, 308 K. Experimental data points are shown only for 288 K, bothfor TX-100 and BJ-35 micelles in the respective insets.

19250 J. Phys. Chem. B, Vol. 108, No. 50, 2004 Kumbhakar et al.

This is quite expected because, even if there is no change inthe micellar parameters, like size and hydration, the microvis-cosity inside the micelle is expected to change following aninverse relation with temperature. Comparing the relativechanges in the rotational relaxation times with temperature inTX-100 and BJ-35 micelles, it is, however, indicated that thetemperature effect is more prominent in the former micelle thanin the latter. This is probably due to the fact that in the case ofTX-100 there is also a large change in the micellar size andhydration with temperature, which is not the case for BJ-35.28-31

In a micelle, three different kinds of motions, namely (i) thewobbling motion of the probe in the micelle, (ii) the lateraldiffusion of the probe along a spherical surface, and (iii) therotational motion of the whole micelle, can contribute to theoverall anisotropy decay. Compared to motions ii and iii, whichare relatively slow and isotropic, wobbling motion i is quitefast and anisotropic.28,31,46-48 As discussed in a two-step modelfor anisotropy decay in micellar solutions, the experimentallymeasuredτr1 andτr2 values are determined by the correlationtimes for the wobbling motion, lateral diffusion, and the wholemicelle rotation (τW, τL, andτM, respectively) as

As an approximation,τM values can be estimated using Stokes-Einstein-Debye equation as28,31,46,47

whereη is the viscosity of water,kB is Boltzmann’s constant,T is the absolute temperature, andrM is the radius of the micelle.The τM values thus estimated for TX-100 and BJ-35 micellesare all in the few tens of nanoseconds (cf. Table 3). Thus, fromeq 9 it is expected thatτr2 is effectively the measure ofτL.Similarly, as theτr2 values are about 5-6 times higher than theτr1 values, it is expected from eq 8 thatτr1 is effectively themeasure ofτW.

From the time constantτr2, it is possible to have anapproximate estimate for the lateral diffusion coefficientDL forthe probe using the following relation,28,31,46,47

whererd is the radius of the sphere, on the surface of whichthe probe undergoes its lateral diffusion. With the probe mainlyresiding in the micellar Palisade layer, on average, it can be

assumed that the probe is always at a distancerd from the centerof the micelle and its lateral diffusion occurs on a sphericalsurface of radiusrd. For the BJ-35 micelle, because there is notmuch of a change in the micellar size with temperature,rd

remains almost invariant for all the temperatures. Thus in thismicelle, the relative increase inDL values (cf. Table 3) shouldbe almost similar to the relative decrease in theτr2 values withtemperature. Accordingly, for a increase in the temperature from288 to 308K,DL is expected to increase only by about 1.5 times,which we expect is due to the increased fluidity inside themicellar Palisade layer due to an increase in temperature. ForTX-100, as the micellar size increases with temperature,28-31

relative increase in theDL values cannot be simply equalizewith the relative decrease in theτr2 values with temperature. Ifwe assume that with temperature therd increases at the sameproportion as that ofrM, eq 11 suggests an unusually large, about5 times, increase in theDL values on increasing the temperaturefrom 288 to 308 K. Because the probe molecules are supposedto be entangled with the surfactant chains, their lateral diffusionis really not expected to increase that sharply with temperature,even if the fluidity inside the micellar Palisade layer increasesto a reasonable extent. Thus, we feel that in TX-100 micelle,the increase inrd with temperature is much less than that ofrM.This effectively implies that the probe C153 undergoes a relativemigration toward the micellar core as the micellar size increaseswith temperature. Under this situation, the microenvironmentaround the probe does not change that extensively withtemperature, even though there is an increase in the micellarsize and hydration. Accordingly, the steady-state fluorescencemeasurements indicate more or less similar polarity for themicroenvironment around the probe even for TX-100 for allthe temperatures studied (cf. section 3.1). It should be mentionedthat the migration of the probe toward the micellar core onincreasing the micellar hydration has also been reported in theliterature from similar fluorescence anisotropy studies.31

As already mentioned, theτr1 values are effectively themeasure of theτW values.28,31,46,47For both the TX-100 andBJ-35 micelles, theτr1 values are seen to decrease graduallywith temperature, suggesting a reduced viscosity in the micellarPalisade layer on increasing the temperature. When theτr1 valuesand their relative contributions in the two micelles at differenttemperatures are compared, it is indicated that the microviscosityexperienced by the probe in the BJ-35 micelle is probablysomewhat lower than that in TX-100 micelle, which we supposeis due to a higher hydration of the Palisade layer of BJ-35 thanthat of TX-100, as is also indicated from the estimated somewhathigher polarity for the Palisade layer of the former micelle thanthat of latter from steady-state measurements (cf. section 3.1).When the temperature effect on theτr1 values is compared, itis seen that the effect is somewhat higher for TX-100 than forBJ-35. Because for BJ-35 there is no significant change inmicellar hydration with temperature,28-31 the reduction in theτr1 values in this micelle simply reflects the thermal effect onthe microviscosity and consequently to the wobbling rate ofthe probe. For TX-100 micelle, as the hydration also increaseswith temperature, a relatively large reduction in theτr1 valuesin this micelle with temperature is supposed to have someconsequences with the changes in the hydration of this micellewith temperature.

4. Discussions

Inside the micellar Palisade layer, the movement of the watermolecules is supposed to be very much hindered. Accordingly,the solvation process is expected to be substantially slower in

TABLE 3: Time Constants of the Anisotropy Decays (τr1and τr1) Obtained from the Biexponential Fit along withTheir Relative Amplitudes (ar1 and ar2)a

micelle temp (K) ar1

τr1

(ps) ar2

τr2

(ps)τM

(ns)1010DL

(m2 s-1)

TX-100 288 33.6 774 66.4 4190 38 5.11298 40.8 395 59.2 2440 77 12.81308 56.1 367 43.9 1880 112 25.42

BJ-35 288 61.13 553 38.87 2483 102 12.68298 81.75 401 18.25 2193 77 14.29308 91.57 347 8.43 1615 54 19.39

a The correlation times for the rotation of the whole micelle (τM)and lateral diffusion coefficient (DL), calculated using eqs 10 and 11,respectively, are also listed.

τr1-1 ) τW

-1 + τr2-1 (8)

τr2-1 ) τL

-1 + τM-1 (9)

τM )4πrM

3η3kBT

(10)

τr2 )rd

2

6DL(11)

Role of Micellar Size and Hydration on Solvation Dynamics J. Phys. Chem. B, Vol. 108, No. 50, 200419251

micellar media than in bulk water. Moreover, it is likely thatthe response from the few water molecules that are adjacent tothe probe will be much slower than the collective response ofthe water molecules that are somewhat away from the probe.Thus, in micellar media, the faster solvation component (τs1) issupposed to be mainly determined by the collective responseof the water molecules that are somewhat away from the probeand the slower solvation component (τs2) is supposed to bemainly determined by the response of the few water moleculesthat are adjacent to the probe.

Our results on solvation dynamics in BJ-35 and TX-100micelles clearly indicate that the solvation process in BJ-35 isfaster than in TX-100 (cf. Table 2). Recently, Dutt hasextensively studied the rotational relaxation dynamics of ahydrophobic probe DMDPP (2,5-dimethyl-1, 4-dioxo-3, 6-di-phenylpyrrolo[3,4-c]pyrrole) in BJ-35 and TX-100 micelles andobserved the relaxation process to be much faster in the formerthan in the latter micelle.28,31 It has thus been inferred that themicroviscosity around the probe in BJ-35 is lower than in TX-100, which we suppose to be due to the higher hydration of theformer micelle than the latter. Accordingly, the solvation processin BJ-35 is expected to occur faster than in TX-100, which isactually indicated from the present results.

Considering the qualitative picture of the micellar structure,as is depicted in Figure 5, it is understood that the Palisadelayer is composed of the micellar headgroups (oxyethylene units)and large number of water molecules. These water moleculesare either just mechanically trapped within the micellar structureor thermodynamically bound to the oxoethylene groups viaintermolecular hydrogen bonding.28,31It is possible that amongthe mechanically trapped water molecules, some are inter-molecularly hydrogen bonded with themselves and some arefree.28,31 Considering these features for the water moleculesinside the micelle, it is expected that the thermodynamicallybound water molecules will be relatively less labile than thoseof the mechanically trapped water molecules. It is likely thatthere is a dynamic exchange between the thermodynamicallybound and the mechanically trapped water molecules in themicellar Palisade layer, which is proposed to play a significantrole in determining the solvation dynamics.3,19-21 Due to thepresence of long surfactant chains and their participation inhydrogen bonding with water molecules around, the water

structure in the Palisade layer is much more rigid in comparisonto that in bulk water. With this qualitative picture, and with theobservation that the polarity of the Palisade layer of BJ-35micelle is somewhat higher than that of TX-100, it is expectedthat more water molecules (higher hydration) are present in thePalisade layer of the BJ-35 micelle than in that of the TX-100micelle, which is also supported by the fluorescence anisotropyresults. Though the reduced size of the BJ-35 micelle (cf. Table1)28,29 suggests that the oxyethylene chains in these micellesexist with much more folded configurations than in TX-100micelles, the largely reducedNagg value for the former micelleprovides more space for the water molecules in the Palisadelayer of this micelle. Due to higher hydration, the proportionof the mechanically trapped water is more in the Palisade layerof BJ-35 micelles in comparison to that in TX-100 micelles,resulting in the relatively lower microviscosity and the relativelyfaster solvation dynamics in the former micelle than in the latter.

The solvation dynamics results in BJ-35 and TX-100 micellesat different temperatures clearly show that the temperature effectis very significant in TX-100 micelle and the effect is not thatprominent in the case of BJ-35 micelle. For BJ-35 micellethough, the fast solvation componet (τs1) apparently shows asomewhat higher temperature effect, we could not give muchimportance to these changes, as theτs1 values are within theinstrument response function (240 ps). Further, as the error limitin the solvation dynamics measurements is expected to besomewhat higher,35 the variations in the solvation times in BJ-35 with temperature could not be given enough significance.For TX-100, however, as the two solvation times are relativelyhigher and because the temperature dependent changes in thesolvation times are also relatively large, the trend in thetemperature effect on the solvation dynamics is very evident.The most striking observation in the present study is that in theTX-100 micelle the changes in the solvation rate with temper-ature show an inversion around 298 K, instead of showing agradual increase with temperature. As mentioned earlier, in thesolvation dynamics studies in the TX-100 micelle using 4-APas the probe, Sen et al.5 observed an Arrhenius type ofrelationship between solvation rate and temperature and at-tributed this to the activation barrier for the exchange processbetween the thermodynamically bound (they defined as bound)and mechanically trapped (they defined as free) water moleculesinside the micelle. In the present work no such Arrheniuscorrelation is observed in the same TX-100 micelle using C153as the probe. Further, for BJ-35 micelle, the present resultsindicate that the temperature effect on the solvation dynamicsis not that significant. These results thus suggest that theobserved temperature effect on the solvation dynamics in amicellar media cannot be simply assigned to the activationbarrier for the exchange process between the bound and freewater molecules. If this was the case, both TX-100 and BJ-35micelles should have shown similar temperature effects on thesolvation dynamics, as the activation barrier for the aboveexchange process is expected to be more or less similar for boththe micelles. From the present results, thus, we infer that it isthe micellar characteristics rather than the activation barrier forthe exchange process of the bound and free water that play themajor role in determining the temperature effect on the solvationrates.

For the BJ-35 micelle, the observation that the polarity forthe microenvironment around the probe remains more or lesssimilar for all the temperatures studied (cf. section 3.1) is quiteeasy to understand, as there is not much change in the micellarsize and hydration with temperature.28,29 But for the TX-100

Figure 5. Qualitative picture of the Palisade layer of TX-100 and BJ-35 micelles. Some of the water molecules are directly hydrogen bondedto oxyethylene groups of the surfactant molecules (thermodynamicallybound water), and some are either free or intermolecularly hydrogenbonded among themselves (mechanically trapped water). The dye (largesolid circle) resides in the Palisade layer of the micelles.

19252 J. Phys. Chem. B, Vol. 108, No. 50, 2004 Kumbhakar et al.

micelle, for which the micellar size and hydration changessubstantially with temperature, to rationalize the fact that forthis micelle also there is no observable change in the polarityof the microenvironment with temperatures, we had to considerthat the probe undergoes a gradual migration toward the micellarcore as the micellar size and hydration increases with temper-ature. This is in fact supported by the fluorescence anisotropyresults, as obtained in the present work, and is also reported inthe literature.30,31 As the hydration of the TX-100 micelleincreases with temperature, under normal circumstances wewould have expected that the solvation dynamics in this micelleshould have become gradually faster with increasing tempera-ture. The unusual inversion in the solvation rate with temper-ature, however, indicates that the temperature effect on thesolvation dynamics in TX-100 cannot be explained simply onthe basis of the changes in hydration of the micelle withtemperature but there must be some other factors governing theprocess, as discussed in the following paragraphs.

For the TX-100 micelle, as the temperature is increased from298 to 308 K, the average solvation time⟨τs⟩ reduces by a factorof ∼2. Contrary to this result, the⟨τs⟩ for BJ-35 micells doesnot change to any significant extent for the same temperaturechange. Because the temperature changes the hydration of theTX-100 micelle but not that of the BJ-35 micelle,30,31the aboveresults suggest that the changes in the hydration of the micellecertainly play a role in determining the micellar solvationdynamics. The controversy however arises as the solvationdynamics in the TX-100 micelle also become faster when thetemperature is reduced from 298 to 288 K, even though thehydration of the micelle is supposed to be reduced for the abovetemperature change.30,31 As the temperature is decreased from298 to 288 K, along with the reduction in the micellar hydration,there is also a large reduction in the micellar size.30,31We thusfeel that the micellar size also plays a role in determining thesolvation dynamics at lower temperatures.

As mentioned earlier, the fast component of the solvationprocess is expected to arise from the collective response of thewater molecules that are somewhat away from the probe. At298 and 308 K, because the size of the TX-100 micelle issubstantially large, it is possible that the water molecules thatare contributing collectively to the fast component of thesolvation process are still within the micellar Palisade layer.At these temperatures, the bulk water molecules that are outsidethe micelle (we say it is bulk water, which includes the watermolecules that are on the micellar surface or close to it, but notthe neat water that is quite away from the micellar surface), aresubstantially away from the probe and cannot participate in thecollective solvent response. At 288 K, as the size of the TX-100 micelle is largely reduced,30,31 the bulk water moleculesjust outside the micellar surface are now not that far from theprobe and are able to contribute to the collective response ofthe solvation process. As the response of these bulk watermolecules is supposed to be faster than the responses of thewater molecules inside the micelle, it can subdue the effect ofreduced hydration of the TX-100 micelle, causing the solvationprocess to become faster even on a decrease in temperature from298 to 288 K. Thus, we feel that the large reduction in themicellar size for TX-100 at 288 K introduces the possiblecontribution of the bulk water to the solvation process, resultingin the temperature effect on the solvation dynamics in TX-100showing an inversion, as observed in the present work.

In the BJ-35 micelle, the temperature effect on solvationdynamics is not that significant, a result that correlates withthe fact that the size and hydration of this micelle remains more

or less similar for all the temperatures studied.28,29 It should bementioned here, however, that unlike solvation dynamics, thefluorescence anisotropy results in the BJ-35 micelle show asubstantial temperature effect, indicating a reasonable decreasein the microviscosity with temperature. This is probably due tothe reduction in the orderliness in the micellar Palisade layerstructure with temperature. The reduced orderliness in thePalisade layer structure will in effect cause an enhancement inthe movement of the water molecules present inside the micelleand accordingly affect the solvation dynamics also to someextent. This is in fact reflected by a small but gradual decreasein theτs2 values in BJ-35 micelle on increasing the temperature.In this context, however, we restrain ourselves to comment muchon the temperature dependent changes in theτs1 values in BJ-35 micelle, as these changes are within the time-resolution ofthe present measurements.

The observation that the temperature effect on the solvationrates in BJ-35 micelle is only nominal and that in TX-100micelle the effect is very large and also show unusual inversionclearly indicates that the changes in the micellar size andhydration together play the role in determining the changes inthe solvation dynamics with temperature. Thus, the suggestionmade by Sen et al.,5 that the temperature effect on the solvationdynamics in TX-100 micelle is simply due to the enhancementin the barrier crossing rates for the exchange of the mechanicallyconfined (free) and the thermodynamically bound (bound) waterin the micelle seems to be grossly incorrect, because, if thiswas the case both BJ-35 and TX-100 should have shown similartemperature effects. Further, the inversion in the solvation ratewith temperature, as observed in the present study in the TX-100 micelle, is also not possible to explain on the basis of thesimple barrier crossing mechanism. From the present results,we thus strongly feel that the changes in both micellar size andhydration should be considered to understand the temperatureeffect on the solvation dynamics in the TX-100 micelle. It isinteresting to mention here, that, similar to our results, aninversion in the solvation rate in the TX-100 micelle was alsoobserved by Hara et al.3 on changing the pressure of the solution.We feel that the reason for the observation made by Hara etal.3 is also very similar to what we infer in the present work.Both the present results and those of Hara et al.3 clearly suggestthat the barrier crossing mechanism, as suggested by Sen etal.,5 is not sufficient to explain the observed temperature effectin the solvation dynamics in the TX-100 micelle, and thechanges in the micellar size and hydration are supposed to playa dominating role in determining the observed temperature (orpressure) effect on solvation dynamics.

The last point, which is also very important to address, iswhy, unlike the present results, Sen et al.5 observed a Arrheniustype of correlation for the changes in the solvation dynamis inTX-100 with temperature. The basic difference between thepresent work and that of Sen et al.5 is in the selection of theprobe. In the work of Sen et al.,5 the probe 4-AP used in thesolvation dynamics measurements is a strongly polar moleculeand is also of hydrophilic in nature. Thus, this probe is expectedto reside closer to the micellar surface and will not be able tosense the changes in the micellar size with temperature, thoughthe increased hydration of the micelle will cause a change inthe solvent response experienced by this probe at the micellarsurface. As there is a gradual increase in the micellar hydrationwith temperature, the solvation rate measured using 4-AP asthe probe also shows a gradual enhancement, as observed bySen et al.5 In the present work, the probe used is the C153 dye,which is a molecule of intramolecular charge transfer character

Role of Micellar Size and Hydration on Solvation Dynamics J. Phys. Chem. B, Vol. 108, No. 50, 200419253

but at the same time reasonably hydrophobic in nature, withalmost no solubility in water.33,49 Thus, the C153 dye prefersto reside somewhat deep in the micellar Palisade layer, to avoidexcessive water around it. For the same reason, as the hydration(also size) of TX-100 increases with temperature, the probeC153 undergoes a relative migration toward the micellar core.Thus, the solvation dynamics measured using C153 as the probecan reflect the effect of both micellar size and hydration changeswith temperature (or pressure) quite satisfactorily. Accordingly,the present results can show the unusual inversion in thesolvation rate with temperature, as it arises due to the mutualeffect of the micellar size and hydration changes at lowertemperatures. When we change the C153 dye with a relativelyless hydrophobic dye C151 (due to the presence of 7-NH2

group), we find the temperature dependent inversion in thesolvation rates; though in the present case, the effect on thefast time constant is observed to be somewhat reduced,suggesting that the dye C151 resides not that deep inside themicellar Palisade layer as that of C153. Still, the results withC151 indicate that it is not as exposed to the bulk water as isthe case expected with 4-AP as the probe. Thus, unlike theresults with 4-AP, the results with C151 dye also display theinversion behavior on the temperature dependent changes in thesolvation dynamics, as also observed with C153 dye. We thusfeel that the difference between the present results and those ofSen et al.5 arise due to the differences in the selection of theprobes and the consequent differences in the probe locationinside the micelle.

5. Conclusions

From the temperature dependent solvation dynamic studiesin two neutral micelles, TX-100 and BJ-35, using C153 andC151 as the probes, the following conclusions have been drawn.In both the micelles, the solvation process is much slowercompared to that in bulk water. In comparison to the TX-100micelle, the solvation process in the BJ-35 micelle is a few timesfaster. This has been rationalized in terms of the higher degreeof hydration for the Palisade layer of the BJ-35 micellecompared to that of the TX-100 micelle. An unusual inversionin the temperature effect on the solvation dynamics has beenobserved in the TX-100 micelle, which has been attributed tothe mutual effect of the temperature dependent changes on themicellar size and hydration on the micellar solvation rates. ForBJ-35 micelle, as the micellar size and hydration do not changemuch with temperature, the solvation dynamics show onlynominal temperature effect on the solvation dynamics. Thepresent results on the solvation dynamics measurements aresupported by the fluorescence anisotropy results obtained in thepresent work. The difference between the present results andthat reported earlier on the temperature dependent changes onthe solvation dynamics in the TX-100 micelle has beenrationalized on the basis of the nature of the probes and theirlocation in the micelle. It is understood from the present studythat the micellar size and hydration play an important role indetermining the solvation dynamics in a micellar media.

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