Modulation in the Solute Location in Block Copolymer-Surfactant SupramolecularAssembly: A Time-resolved Fluorescence Study

Prabhat K. Singh, Manoj Kumbhakar, Haridas Pal, and Sukhendu Nath*Radiation and Photochemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

ReceiVed: September 12, 2008; ReVised Manuscript ReceiVed: NoVember 12, 2008

Effect of cosurfactant concentration on the location of a dissolved solute in a block copolymer-surfactantsupramolecular system has been investigated using time-resolved fluorescence anisotropy and dynamic Stokes’shift measurements. Pluronic F88 and cosurfactant CTAC have been used to form a supramolecular assembly.The anion of coumarin 343 dye has been used as the solute/probe. It is seen that as the CTAC concentrationis increased in the F88-CTAC supramolecular assembly, the microviscosity around the probe graduallyincreases. The result suggests that the probe undergoes a gradual migration from micellar surface to theinterior of the micelle as the concentration of the CTAC is increased. This is also supported by the dynamicStokes’ shift results. It is seen that as the CTAC concentration is increased in the system, the observed Stokes’shift gradually increases due to the movement of the probe away from the bulk water. By comparing thepresent results with those reported in another pluronic-surfactant system, namely, P123-CTAC, it is indicatedthat the extent of modulation in the position of the probe in such supramolecular systems is largely determinedby the composition of the pluronic, especially on the length of its hydrophilic ethyleneoxide block.

Introduction

Pluronics are an important class of triblock copolymers andhave attracted considerable attention of the researchers for lasttwo decades due to their unique solution behavior1-6 and alsodue to their extensive industrial applications, such as detergents,emulsifiers, lubricants, etc.7,8 Their amphiphilic and nontoxicbehavior have made them very suitable for drug encapsulationand as drug delivery agents.9-15 Because of the formation ofmicroheterogeneous media of varying physical dimensions andcharacteristics, these polymers have also been used extensivelyfor the fabrication of different nanostructures.16-19

Pluronics are, in general, made of two polyethyleneoxide (EO)and one polypropyleneoxide (PO) block with a general formulaof (EO)n-(PO)m-(EO)n. Due to differential solubility behaviorof EO and PO units in water, pluronics are known to formmicelles in aqueous solution. However, this class of polymerforms micelles in water only above a certain concentration,known as the critical micelle concentration (CMC) and alsoabove a certain temperature, known as the critical micellartemperature (CMT). Because of the differences in the solubilityof the EO and PO units, the CMT of a pluronic largely dependson its composition, or more specifically on its EO/PO ratio.Because the water solubility of EO is higher than that of PO,CMT of a pluronic increases with an increase in the EO/POratio. For example, the CMT of 5% w/v solution of twopluronics, F88 (n ) 103 and m ) 39) and P123 (n ) 20 and m) 70), are 30.5 and 12.5 °C, respectively.3 Note that the EO/PO ratio for F88 and P123 are 2.64 and 0.29, respectively. Thisindicates that a pluronic with a lower value of the EO/PO ratiofavors the micellization process of the polymer. The detailedmicellization process of different types of pluronics has beenstudied quite extensively. The micellar structure and theirformation dynamics have been studied using different tech-niques, such as light scattering,5,6,20-23 neutron scattering,4,22-25

fluorescence measurements,3,26-30 and absorption measure-ments.31 All these studies indicate that, structurally, the pluronicmicelles consist of a hydrophobic core formed by PO blocksthat is surrounded by a hydrated layer of EO units, known asthe corona region of the micelle.

The combinations of surfactants with pluronic polymers resultin the formation of complex microheterogeneous systems thathave found several industrial applications.32,33 The propertiesand the solution behavior of these mixed surfactant-polymersystems are important to be understood properly to find any oftheir useful formulations to achieve maximum efficiency in anyapplication. In most of the applications of microheterogeneousmedia, a solute is dissolved in the micellar phase, and theeffectiveness of the dissolved solute for a desired process largelydepends on the physical and chemical properties of the speciesin the micellar media. As pluronics can be obtained with widerange of EO/PO ratios,1,3,5 they can form micelles with widerange of dimensions of their core and the corona regions. Thesesystems can thus provide a wide range of microenvironmentsfor the dissolved solutes. Because of the availability of widerange of microenvironments in these micelles, the solutes mayalso have quite different physical and chemical propertiesdepending on their locations in the micelle. Thus, by changingthe position of a solute in these micelles, it is possible tomodulate the physical as well as the chemical properties of thesolute in these microheterogeneous systems.

It is known from light and neutron scattering studies that somepluronic micelles form unique supramolecular assemblies in thepresence of an ionic surfactant, for example, SDS.20-22 In thesesupramolecular assemblies, it is understood that the hydrophobicchains of the surfactant molecules are dissolved in the core ofthe pluronic micelles, and the charged head groups of thesurfactants reside at the peripheral region of the core, projectinginto the hydrated corona region of the micelles.22 Because ofthis unique structure of these assemblies, a charged layer isformed inside these micelles. Accordingly, any solute having acharge opposite to that of the charged layer inside these

* To whom correspondence should be addressed. E-mail: snath@barc.gov.in; phone: 91-22-25590306; fax: 91-22-5505151.

J. Phys. Chem. B 2009, 113, 1353–1359 1353

10.1021/jp808123m CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/09/2009

supramolecular assemblies can experience an electrostaticattraction toward the core-corona interface of the micelles. Theextent of the electrostatic attraction will naturally depend onthe concentration of the ionic-surfactant in these systems.34 Dueto the presence of such an electrostatic attraction, the solutecan move from the surface to the interior of the micelle. Theaim of the present study is to see if the position of a dissolvedsolute in a mixed pluronic-surfactant system can be changedby changing the composition of the mixed micellar system. Forthis purpose, we have used F88 as the block-copolymer micelleand CTAC as the cationic surfactant. The anionic form of thecoumarine-343 (C343, cf. Scheme 1 for molecular structure)dye has been used as molecular probe in this study. As thecharacteristics of the pluronic micelles are largely dependenton their EO/PO ratios, the results obtained in the present studyfor the F88-CTAC system have also been compared with thoseof the P123-CTAC system,35,36 because P123 has a largelydifferent EO/PO ratio than F88.

Methods and Materials

Ground state absorption and steady-state fluorescence mea-surements were carried out using a Shimadzu (Japan, modelUV-160A) spectrophotometer and a Hitachi (Japan, model4010F) spectrofluorimeter, respectively. Fluorescence spectrathus recorded were corrected for the wavelength-dependentinstrument responses by measuring the spectrum of quininesulfate and comparing it with the reported standard spectrum.37

The measured spectra, I(λ), were in wavelength domain andwere converted to the frequency domain, I(νj), using thefollowing equation.37

Time-resolved fluorescence measurements were carried outusing a time-correlated single-photon counting (TCSPC) instru-ment from IBH (UK). Sample was excited with 408 nm diodelaser. The emission was collected at a right angle to theexcitation beam using a monochromator and a Peltier cooledPMT (IBH, UK, model TBX-04). The instrument response ofthe TCSPC setup was measured by collecting the scattered lightfrom a TiO2 suspension in water. The instrument responsefunction thus measured had a fwhm of ∼230 ps.

Time-dependent anisotropy was measured using the followingequation.

where I|(t) and I⊥ (t) are the decay of fluorescence intensity forthe parallel and perpendicular polarizations, respectively, withrespect to the polarization (vertical) of the excitation beam.Polarization sensitivity of the detector has been incorporated

through the correction factor G, which was measured indepen-dently. All these measurements were carried out 2-3 times tocheck the reproducibility and to obtain the average values forthe rotational relaxation times.

F88 was a gift from the BASF corporation, Edison, NJ, USA.CTAC (Aldrich) and C343 (Exciton) were used as received.Solution of F88 was prepared by taking 0.05 g of F88 per 1 mlof nanopure water from a Millipore Milli Q system. The solutionwas stirred with a magnetic stirrer at room temperature for 24 h.To form the supramolecular assembly, an appropriate amountof CTAC was added to the F88 solution. C343 was addeddirectly to the polymer-surfactant solution. Concentration ofthe coumarin dye in the polymer solution was kept very low(∼1 µM) so that the possibility of having more than one dye ina micelle is negligible. It was shown earlier that the CMT for5% w/v F88 is ∼34 °C and, accordingly, the micellizationprocess gets effectively completed at ∼40 °C.28,38,39 Becauseof this reason, all the present measurements were carried out at40 °C. Molecular structures of all the chemicals used in thepresent study are shown in Scheme 1.

Results and Discussion

Ground-state Absorption and Steady-state FluorescenceStudies. Ground-state absorption spectra of the probe, C343,were recorded in aqueous solution at different pH and are shownin Figure 1A. The absorption spectrum of C343 in F88 solutionis also shown in Figure 1A for comparison. It is evident fromFigure 1A that the absorption characteristic of C343 in F88solution is very similar to that in water at alkaline pH. Presentresults indicate that the dye C343 mainly exists in its anionicform in F88 micellar solution.

SCHEME 1: Molecular Structure of the Chemicals usedin the Present Study

I(ν) ) λ2I(λ) (1)

r(t) )I|(t) - GI⊥ (t)

I|(t) + 2GI⊥ (t)(2)

Figure 1. (A) Absorption and (B) emission spectra of C343 at differentpH in water, pH ) 3.5 ( · · · ), pH)10.5 (---), and in 5% w/v F88 solution(s).

1354 J. Phys. Chem. B, Vol. 113, No. 5, 2009 Singh et al.

The steady-state emission spectra of C343 in aqueous solutionat different pH along with that in F88 solution are shown inFigure 1B. Unlike absorption spectra, the dye does not showmuch change in the emission spectra with pH of the solution.A close look at Figure 1B indicates that the emission spectrumof C343 in F88 is very similar to that of C343 in alkalinesolution. This result is in agreement with that obtained in theabsorption measurement. Thus, from the present results we inferthat the dye C343 exists in its anionic form in the F88 micellarsolution. This is in good agreement with the earlier reportedresults where it has been shown that in different micellarsolutions, including some pluronic micelles, C343 exists in itsanionic form.35,36,38-43

To see whether there is any change in the microenvironmentfor the probe, C343, in the F88 micellar solution due to theaddition of CTAC, we have recorded the steady-state emissionspectra of C343 in F88-CTAC systems at different CTACconcentrations. These results are shown in Figure 2, where it isindicated that with an increase in the CTAC concentration theemission spectra of C343 undergoes a small (∼7 nm) but definiteblue shift. This hypsochromic shift of the emission spectra ofC343 suggests that, with an increase in the CTAC concentrationin F88 solution, the microenvironment for the dye graduallybecomes more nonpolar in nature.

Due to the ionic nature of the probe, its excitation spectrumis also expected to be quite sensitive to its microenvironment.With this expectation, we have also recorded the excitationspectra of the probe in an F88 micelle at different concentrationsof CTAC, and the results are presented in the Figure 3A. It isevident from Figure 3A that the excitation spectrum shows agradual blue shift as we increase the concentration of CTAC inthe F88 micellar solution. Figure 3B shows the variation of themaxima of the excitation spectra with the CTAC concentration.It is evident from Figure 3B that the changes in the maxima ofthe excitation spectra are more prominent at the lower CTACconcentration region and saturates to a plateau value at ∼0.4of CTAC/F88 molar ratio. This result clearly indicates that thereis a change in the microenvironment of the probe with thechanges in the CTAC concentration.

Time-resolved Fluorescence Studies. To understand thedetails of the effect of the added CTAC on the photophysicalbehavior of the probe, C343, in the F88 micellar solution, wehave investigated the rotational relaxation and dynamic Stokes’shift studies as discussed below.

a. Time-resolWed Rotational Relaxation Studies. Time-resolved fluorescence anisotropy measurements have beencarried out to understand how the microenvironment of the

probe, C343, in F88 micelle changes with the addition of CTAC.The changes in the fluorescence anisotropy decay for C343 inF88 micellar solution at different CTAC concentrations areshown in Figure 4A. The anisotropy decay for C343 in bulkwater is also shown in Figure 4A for comparison. It should bementioned, however, that the exact reorientation time for C343in bulk water could not be estimated correctly because it is muchshorter than the time-resolution of our TCSPC instrument.

The fluorescence anisotropy decays for C343 dye in F88micellar solutions were seen to fit well with a biexponentialfunction of the following form.

where ari and τri are the amplitude and time constant, respec-tively, for the ith decay component. The average rotationalrelaxation time, <τr>, was calculated using the followingequation.44

All the fitting parameters, including <τr> values, for C343dye in F88 micellar solution at different CTAC concentrationsare presented in Table 1. The variation in the <τr> values withCTAC concentration is presented in Figure 4B. It is obviousfrom Figure 4A and Table 1 that the reorientation time of theprobe is much slower in F88 micellar solution compared to thatin bulk water. This result clearly indicates that the probe, C343,gets preferentially bound to the micellar phase rather thanresiding in the bulk water phase. Since the present probe is

Figure 2. Steady-state emission spectra of C343 in 5% w/v F88solution at different CTAC/F88 molar ratio: 0.0 (s), 0.1 ( · · · ), 0.2 (---)and 0.5 ( · - · - · - · ).

Figure 3. (A) Excitation spectra of C343 in 5% w/v F88 solution atdifferent CTAC/F88 molar ratio: 0.0 (s), 0.1 ( · · · ), 0.2 (---) and 0.5( · · - · · - · · -). (B) Variation in the excitation spectra maxima of C343with the CTAC/F88 molar ratio.

r(t) ) ∑i)1

2

ari exp(-t/τri) (3)

<τr> )∑i)1

2

ariτri2

∑i)1

2

ariτri

(4)

Solute Location in a Supramolecular Assembly J. Phys. Chem. B, Vol. 113, No. 5, 2009 1355

anionic in nature, it will, however, prefer to stay at the micellarsurface rather than entering into the interior of the micelle.Because of the ionic nature of the probe there is also aprobability that a small fraction of the dye might also remainpartitioned in the bulk water. In fact, it is reported in theliterature that in some other micellar media, including pluronicmicelles, C343 anions get distributed between the micelle andthe bulk water, although the distribution is preferentially shiftedtoward the micellar phase.35,36,38-42 Due to the presence of largehydrophobic group in C343, the percentage of C343 in the bulkwater phase is expected to be much less as compared to that inthe micellar phase. In the present anisotropy results, thecontribution of the dye in bulk water phase is, however, expectedto be very small as the <τr> values in these systems are muchhigher than that in aqueous solution. From Figure 4B it is also

evident that the reorientation time of C343 increases with CTACconcentration and reaches a plateau when the CTAC/F88 molarratio reaches a value of ∼0.4.

The above results on the reorientation dynamics of the solutein the F88-CTAC system can be explained on the basis of theformation of the supramolecular assembly between F88 micelleand CTAC surfactant. It is proposed that as we add CTAC toF88 micellar solution; they form a supramolecular assembly suchthat the hydrophobic long chain of the CTAC molecule getsdissolved in the core of the F88 micelle and its cationicheadgroup resides at the interfacial region of the core and coronaof the micelle. Formation of such a supramolecular assemblyis supported from the fact that the detailed light- and neutron-scattering studies have shown that ionic surfactants such as SDSand CTAC can form the above kind of supramolecular as-semblies with other pluronic micelles.20-22 The formation ofsuch a supramolecular structure is also supported by severalphotophysical studies reported in the literature.29,30,35,36 Bydrawing an analogy we can also predict the formation of sucha supramolecular assembly between F88 micelle and CTACsurfactant. Formation of such a supramolecular assembly, wherethe charged head groups of CTAC reside at the interface of thecore and corona region, results in the development of a positivelycharged layer inside the F88 micelle on the addition of CTAC.The charge density of this layer naturally increases graduallyas we increase the concentration of CTAC in the solution. Asthe probe used in the present study is anionic in nature, itexperiences an electrostatic attraction by the positively chargedlayer developed inside the F88 micelle due to the presence ofCTAC. This electrostatic interaction leads to the movement ofthe anionic probe from the surface of the micelle to its interiorregion as we increase the CTAC concentration. Thus, theincrease in the reorientation time of the probe with an increasein the CTAC concentration in F88 micelle is due to the followingtwo reasons. First, the microviscosity experienced by the probein the interior of the micelle is much higher than that at themicellar surface. Second, the increase in the electrostaticinteraction between probe and the charged layer inside themicelle acts against the rotational motion of the probe. Thus,there is a gradual increase in the <τr> values of the dye in F88micelle with an increase in the CTAC concentration.

That the changes in the reorientation times of C343 are notdue to any structural change of the micelle but due to itschanging location in the micelle with the added CTACmolecules can be supported from the following considerations.From detailed studies in other pluronic-CTAC systems, it isreported by Jansson et al.20,21 and Mali et al.45 that the changesin the micellar structural characteristics are only marginal evenup to the CTAC concentration, much higher than what is usedin the present study. To substantiate their inference and also tosee if the possible changes in the structure of F88 micelle dueto the addition of CTAC can affect the reorientation dynamicsof a probe that reside on the micellar surface, we have carriedout the fluorescence anisotropy measurement of a cationic probe,Rhodamin-110 (R110) in F88 micellar solution at differentCTAC concentration. The average reorientation time measuredfor R110 in F88 micelles at CTAC/F88 molar ratio of 0.0 and0.4 are effectively the same, ∼0.33 ns. The cationic probe, R110,is reported to reside on the surface of the micelle,45 and it showsreorientation time very similar to C343 in F88 micelle in theabsence of CTAC surfactant. These results with R110 dye thusclearly indicate that, even if there could be a marginal changein the micellar structure due to the addition of CTAC, it doesnot change the reorientation time of a probe that resides on the

Figure 4. (A) Temporal profile of the fluorescence anisotropy for C343dye in water (O) and in F88 micelles at different concentration ofCTAC: (∆) 0.0, (×) 0.1, and (0) 0.3 CTAC/F88 molar ratio. Solidlines are biexponential fitting to the data points. (B) Variations of theaverage reorientation time, <τr>, for C343 dye in F88 micelle with theCTAC/F88 molar ratio.

TABLE 1: Time-resolved Anisotropy Parameters for C343Dye in F88 Micelle with Varying CTAC/F88 Molar Ratio

[CTAC]/[F88] τr1 (ns) a1 τr2 (ns) a2 <τr> (ns)

0.00 0.15 0.38 0.48 0.62 0.350.01 0.18 0.44 0.69 0.56 0.470.02 0.20 0.41 0.85 0.59 0.590.04 0.20 0.30 1.21 0.70 0.900.05 0.17 0.21 1.21 0.79 0.990.06 0.23 0.30 1.39 0.70 1.040.10 0.28 0.30 1.53 0.70 1.150.20 0.31 0.27 1.66 0.73 1.290.25 0.28 0.17 1.62 0.83 1.390.30 0.36 0.27 1.83. 0.73 1.430.40 0.31 0.21 1.75 0.79 1.460.50 0.36 0.25 1.81 0.75 1.44

*ai ) ariτri/ ∑ i)1

2ariτri.

1356 J. Phys. Chem. B, Vol. 113, No. 5, 2009 Singh et al.

surface of the micelle. Thus, the observed changes in thereorientation times of C343 with the addition of CTAC mustnot be due to the changes in the micellar structure but ratherare due to the changes in the microenvironment of the probe asit arises due to its movement form the surface to the interior ofthe micelle.

b. Dynamic Stokes’ Shift Measurements. As mentionedabove, the position of the probe, C343, in F88 micellar phasechanges on changing the concentration of CTAC in the solution.It is thus quite expected that the microenvironment around theprobe, especially the microstructure of the water moleculesaround the probe, will also change accordingly. The structureof water molecules around a solute can affect its physical aswell as chemical behavior in the micelle. Thus, understandingthe nature of the water molecules around a solute is essentialin order to understand the physical and chemical properties ofthe dissolved solute. For this purpose, we have carried out adetailed dynamic Stokes’ shift measurement in F88-CTACsupramolecular assembly for the probe C343.

To study the dynamic Stokes’ shift, emission transients werefirst recorded at 10 nm intervals over the entire emissionspectrum of the probe. The wavelength-dependent emissiondecays were then converted to the time-dependent emissionspectra by the standard method.35,46-48 The time-resolvedemission spectra (TRES) thus constructed for the probe C343in F88 micellar solution are shown in Figure 5A. It is to benoted that, along with the decrease in the emission intensitywith time, there is a concomitant small but definite shift in theemission peak toward the lower frequency. To have a detailedidea about the changes in the emission spectra with time, theareas of each of these transient emission spectra were normalizedand are plotted in Figure 5B. It is clearly evident from the time-resolved area-normalized emission spectra (TRANES)49,50 thatan isoemissive point exists in the present system. The appearanceof such an isoemissive point suggests the presence of twoemissive species in this system.49,50 The existence of twoemissive species can be explained on the basis of the distributionof the probe between micelle and bulk water phase. As

mentioned earlier that the probe C343 mainly exists as an anionin the present system, thus there is always a possibility that asmall fraction of the dye will be present in the bulk water. Suchdistribution of C343 in different microheterogeneous media havealready been reported in the literature.35,38-42 Presence of a smallbut definite amount of C343 in bulk water is thus inferred tobe responsible for the appearance of the isoemissive point inFigure 5B, as TRANES is suggested to be quite sensitive tothe number of emissive species in the solution.49,50

TRES have also been constructed in F88 micelle at differentCTAC concentration. Figure 6 shows the TRES and TRANESfor C343 dye in the F88-CTAC system at a CTAC/F88 molarratio of 0.5. It is evident from Figure 6 that, like in the F88system, in the F88-CTAC system there is also a gradual shiftwith time in the emission maxima toward the lower frequencyside. A comparison of Figure 5A and 6A clearly indicates thatthe frequency shift in the emission spectra is relatively largein the presence of CTAC. Another important difference betweenthe F88 and F88-CTAC systems is the absence of an isoemis-sive point in the TRANES plot for the latter system (cf. Figures5B and 6B). Unlike in the F88 system, the absence of anisoemissive point in the F88-CTAC system clearly indicatesthe presence of only one emissive species in the latter system.The disappearance of the isoemissive point in the F88-CTACsystem clearly suggests that the presence of CTAC in the F88micelle results in the association of all the dyes in the solutionwith the micellar phase. This result is in good agreement withthe result obtained in the time-resolved anisotropy measurements.

As mentioned, the extent of the frequency shift in the TRESis relatively larger in the case of the F88-CTAC system ascompared to that in the F88 system. To estimate the relativeamount of the dynamic Stokes’ shift observed in the presentsystems, we first calculated the expected total dynamic Stokes’shift (∆total) using the following equation.51

Figure 5. (A) Time-resolved emission spectra (TRES) and (B) time-resolved area normalized emission spectra (TRANES) of C343 in F88micellar solution at different time (0.1-2 ns).

Figure 6. (A) Time resolved emission spectra (TRES) and (B) time-resolved area-normalized emission spectra (TRANES) of C343 in F88micellar solution at CTAC/F88 molar ratio of 0.5 at different time(0.1-2 ns).

Solute Location in a Supramolecular Assembly J. Phys. Chem. B, Vol. 113, No. 5, 2009 1357

where ωabsm and ωabs

CH are the absorption frequencies and ωemm and

ωemCH are the emission frequencies for C343 dye in micelle and

cyclohexane (nonpolar) solution, respectively. Total expectedStokes’ shift (∆total) and the observed Stokes’ shift (∆obs) valuesfor the present systems are tabulated in Table 2. Percentage ofthe total Stokes’ shift observed (∆%

obs) are also shown in Table2. It is evident from Table 2 that the ∆total value decreasesgradually, whereas the ∆obs and, accordingly, ∆%

obs valuesincrease gradually as we increase the CTAC concentration inthe F88 solution. The variation of the ∆%

obs with the added CTACconcentration are shown in Figure 7.

The observed changes in the ∆%obs values with the added

CTAC concentration can be explained on the basis of theformation of the supramolecular structure between F88 micelleand CTAC as mentioned earlier. In the absence of CTAC, thedye C343 resides at the micellar surface and is thus partiallyexposed to the bulk water. As the solvent relaxation rate forthe bulk water is much faster (<1 ps)52,53 than the time resolutionof our instrument, we miss most of the expected total Stokes’shift in this case. However, due to the association of the probein the micellar phase, it is also partly covered by some watermolecules entrapped in the micellar phase. The relaxation ratefor these entrapped water molecules is reported to be muchslower than the bulk water54,55 and could be observed in thepresent study. Thus, the observed 6% Stokes’s shift in F88micellar solution is due to these entrapped water moleculesaround the probe. As we increase the concentration of CTACin the F88 micellar solution, the probe moves from the micellarsurface to the interior of the micelle due to the electrostaticinteraction between the probe and the head groups of CTAC.As the probe moves inside the micelle, it is moving away fromthe bulk water and, subsequently, the probe feels less responsefrom the bulk water but feels more response from the waterentrapped in the micellar media. Because of this, the observedStokes’ shift increases as we increase the CTAC concentration.Thus, the present dynamic Stokes’ shift results also support theinferences made earlier that due to the addition of CTAC theprobe molecule moves from the micellar surface to micellarinterior. This is also supported from the fact that as we increasethe concentration of CTAC the total expected Stokes’ shift(∆total) also decreases. This decrease in ∆total values clearlyindicates that with the addition of CTAC the probe moleculegradually moves toward the relatively more nonpolar region,that is, toward the core of the micelle.

From the present study, it is evident that the position of asuitable anionic probe in the F88 micellar phase can be changedby adding a positively charged cosurfactant CTAC. Thepositively charged layer thus formed in the F88 micelle helpsto pull the anionic probe from the micellar surface to its interior.The extent of movement of the probe inside the micelle is largelydependent on the magnitude of the electrostatic attraction andhence on the added CTAC concentration. Thus, the presentresults indicate that the position of the probe in the micelle canbe modulated by tuning the concentration of the added surfac-tant. As mentioned earlier, the physical as well as chemicalproperties of the solute largely depends on its location in themicelle. Thus, the important aspect of the present result is thatwe can tune the physical and chemical properties of a soluteby changing the concentration of the added surfactant. Thismight have a large implication in several applied areas, such asuse of these supramolecular assemblies as reaction media,56-58

as drug carriers,59-61 etc. The changes in the position of the

probe due to the addition of CTAC in F88 micellar solutionare schematically presented by Scheme 2.

It is to be noted from Figures 4B and 7 that the concentrationof the CTAC required to reach a limiting value in thereorientation time and in the observed dynamic Stokes’ shiftcorresponds to a CTAC/F88 molar ratio of ∼0.4. Such a limitingvalue for the P123-CTAC micellar system was found earlierto be less than 0.2.35,36 The large difference in the CTACconcentration required to achieve the limiting value of thereorientation time and the observed Stokes’ shift in these twoblock copolymers can be explained on the basis of the differencein the EO block sizes of these two polymers. As mentionedearlier, the corona region is mainly composed of the relativelyhydrophilic EO blocks, and the numbers of EO units are 20and 103 for P123 and F88, respectively. The presence of a largenumber of EO units in the F88 block copolymer results in amuch thicker corona region for the F88 micelle as compared tothat of the P123 micelle. This is also supported by the reportedvalues for the thickness of the corona region for these two typesof micelles. The reported thickness for the corona region are46 and 6 Å for F88 and P123 micelle, respectively.1,25,28 Becauseof the larger thickness of the corona region for the F88 micelleas compared to that for the P123 micelle, the electrostaticinteraction between the probe, residing at the micellar surface,and the head groups of the CTAC, residing at the core-corona

∆total ) [ωabsm - ωem

m ] - [ωabsCH - ωem

CH] (5)

Figure 7. Plot of the percentage of the Stokes’ shift observed (∆%obs)

for the C343 dye in the micellar solution as a function of CTAC/F88molar ratio.

TABLE 2: Solvation Parameters with Varying CTAC/F88Molar Ratio

Stokes’ Shift

[CTAC]/[F88] ∆total/cm-1 ∆obs/cm-1 ∆%obs

0.00 2340 147 6.30.06 2297 201 8.80.10 2238 288 12.80.20 2196 379 17.30.30 2153 445 20.70.50 2127 470 22.1

SCHEME 2: Schematic Representation of the Changesin the Location of an Anionic Solute with the Addition ofa Cationic Surfactant in a Pluronic Micelle

1358 J. Phys. Chem. B, Vol. 113, No. 5, 2009 Singh et al.

interface, is much weaker in the former micelle. Thus, to pullthe negatively charged probe molecule from the micellar surfaceto the interior of the micelle, a much higher concentration ofCTAC is required for the F88 micellar system as compared tothat for the P123 micellar system. As pluronics are availablewith a wide range of EO block sizes, it is possible to pro-vide different microenvironments to the dissolved solute in asingle pluronic micelle. Present results indicate that by properchoice of the pluronics and with the proper concentration ofthe ionic surfactant, the microenvironment of a dissolve solutecan be modulated suitably over a wide range, which in turncan be used to modulate their physical and chemical propertiesover a wide range for different applications.

Acknowledgment. We are thankful to Dr. G. B. Dutt for hisuseful suggestions during this work. We also thank Dr. T.Mukherjee, Director, Chemistry Group and Dr. S. K. Sarkar,Head, Radiation and Photochemistry Division for their constantencouragement and support during this work.

References and Notes

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