J. Phys. Chem. B 2009, 113, 1353–1359
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Modulation in the Solute Location in Block Copolymer-Surfactant Supramolecular Assembly: 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-surfactant supramolecular 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 concentration is increased in the F88-CTAC supramolecular assembly, the microviscosity around the probe gradually increases. The result suggests that the probe undergoes a gradual migration from micellar surface to the interior of the micelle as the concentration of the CTAC is increased. This is also supported by the dynamic Stokes’ 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 the present results with those reported in another pluronic-surfactant system, namely, P123-CTAC, it is indicated that the extent of modulation in the position of the probe in such supramolecular systems is largely determined by the composition of the pluronic, especially on the length of its hydrophilic ethyleneoxide block. Introduction Pluronics are an important class of triblock copolymers and have attracted considerable attention of the researchers for last two decades due to their unique solution behavior1-6 and also due to their extensive industrial applications, such as detergents, emulsifiers, lubricants, etc.7,8 Their amphiphilic and nontoxic behavior have made them very suitable for drug encapsulation and as drug delivery agents.9-15 Because of the formation of microheterogeneous media of varying physical dimensions and characteristics, these polymers have also been used extensively for the fabrication of different nanostructures.16-19 Pluronics are, in general, made of two polyethyleneoxide (EO) and one polypropyleneoxide (PO) block with a general formula of (EO)n-(PO)m-(EO)n. Due to differential solubility behavior of EO and PO units in water, pluronics are known to form micelles in aqueous solution. However, this class of polymer forms micelles in water only above a certain concentration, known as the critical micelle concentration (CMC) and also above a certain temperature, known as the critical micellar temperature (CMT). Because of the differences in the solubility of the EO and PO units, the CMT of a pluronic largely depends on 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/PO ratio. For example, the CMT of 5% w/v solution of two pluronics, 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. This indicates that a pluronic with a lower value of the EO/PO ratio favors the micellization process of the polymer. The detailed micellization process of different types of pluronics has been studied quite extensively. The micellar structure and their formation dynamics have been studied using different techniques, such as light scattering,5,6,20-23 neutron scattering,4,22-25 * To whom correspondence should be addressed. E-mail: snath@ barc.gov.in; phone: 91-22-25590306; fax: 91-22-5505151.
fluorescence measurements,3,26-30 and absorption measurements.31 All these studies indicate that, structurally, the pluronic micelles consist of a hydrophobic core formed by PO blocks that is surrounded by a hydrated layer of EO units, known as the corona region of the micelle. The combinations of surfactants with pluronic polymers result in the formation of complex microheterogeneous systems that have found several industrial applications.32,33 The properties and the solution behavior of these mixed surfactant-polymer systems are important to be understood properly to find any of their useful formulations to achieve maximum efficiency in any application. In most of the applications of microheterogeneous media, a solute is dissolved in the micellar phase, and the effectiveness of the dissolved solute for a desired process largely depends on the physical and chemical properties of the species in the micellar media. As pluronics can be obtained with wide range of EO/PO ratios,1,3,5 they can form micelles with wide range of dimensions of their core and the corona regions. These systems can thus provide a wide range of microenvironments for the dissolved solutes. Because of the availability of wide range of microenvironments in these micelles, the solutes may also have quite different physical and chemical properties depending on their locations in the micelle. Thus, by changing the position of a solute in these micelles, it is possible to modulate the physical as well as the chemical properties of the solute in these microheterogeneous systems. It is known from light and neutron scattering studies that some pluronic micelles form unique supramolecular assemblies in the presence of an ionic surfactant, for example, SDS.20-22 In these supramolecular assemblies, it is understood that the hydrophobic chains of the surfactant molecules are dissolved in the core of the pluronic micelles, and the charged head groups of the surfactants reside at the peripheral region of the core, projecting into the hydrated corona region of the micelles.22 Because of this unique structure of these assemblies, a charged layer is formed inside these micelles. Accordingly, any solute having a charge opposite to that of the charged layer inside these
10.1021/jp808123m CCC: $40.75 2009 American Chemical Society Published on Web 01/09/2009
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SCHEME 1: Molecular Structure of the Chemicals used in the Present Study
supramolecular assemblies can experience an electrostatic attraction toward the core-corona interface of the micelles. The extent of the electrostatic attraction will naturally depend on the concentration of the ionic-surfactant in these systems.34 Due to the presence of such an electrostatic attraction, the solute can move from the surface to the interior of the micelle. The aim of the present study is to see if the position of a dissolved solute in a mixed pluronic-surfactant system can be changed by changing the composition of the mixed micellar system. For this purpose, we have used F88 as the block-copolymer micelle and CTAC as the cationic surfactant. The anionic form of the coumarine-343 (C343, cf. Scheme 1 for molecular structure) dye has been used as molecular probe in this study. As the characteristics of the pluronic micelles are largely dependent on their EO/PO ratios, the results obtained in the present study for the F88-CTAC system have also been compared with those of the P123-CTAC system,35,36 because P123 has a largely different EO/PO ratio than F88. Methods and Materials Ground state absorption and steady-state fluorescence measurements were carried out using a Shimadzu (Japan, model UV-160A) spectrophotometer and a Hitachi (Japan, model 4010F) spectrofluorimeter, respectively. Fluorescence spectra thus recorded were corrected for the wavelength-dependent instrument responses by measuring the spectrum of quinine sulfate and comparing it with the reported standard spectrum.37 The measured spectra, I(λ), were in wavelength domain and were converted to the frequency domain, I(νj), using the following equation.37
I(ν¯ ) ) λ2I(λ)
(1)
Time-resolved fluorescence measurements were carried out using a time-correlated single-photon counting (TCSPC) instrument from IBH (UK). Sample was excited with 408 nm diode laser. The emission was collected at a right angle to the excitation beam using a monochromator and a Peltier cooled PMT (IBH, UK, model TBX-04). The instrument response of the TCSPC setup was measured by collecting the scattered light from a TiO2 suspension in water. The instrument response function thus measured had a fwhm of ∼230 ps. Time-dependent anisotropy was measured using the following equation.
r(t) )
I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)
(2)
where I|(t) and I⊥(t) are the decay of fluorescence intensity for the parallel and perpendicular polarizations, respectively, with respect to the polarization (vertical) of the excitation beam. Polarization sensitivity of the detector has been incorporated
Figure 1. (A) Absorption and (B) emission spectra of C343 at different pH in water, pH ) 3.5 ( · · · ), pH)10.5 (---), and in 5% w/v F88 solution (s).
through the correction factor G, which was measured independently. All these measurements were carried out 2-3 times to check the reproducibility and to obtain the average values for the 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 ml of nanopure water from a Millipore Milli Q system. The solution was stirred with a magnetic stirrer at room temperature for 24 h. To form the supramolecular assembly, an appropriate amount of CTAC was added to the F88 solution. C343 was added directly to the polymer-surfactant solution. Concentration of the coumarin dye in the polymer solution was kept very low (∼1 µM) so that the possibility of having more than one dye in a micelle is negligible. It was shown earlier that the CMT for 5% w/v F88 is ∼34 °C and, accordingly, the micellization process gets effectively completed at ∼40 °C.28,38,39 Because of this reason, all the present measurements were carried out at 40 °C. Molecular structures of all the chemicals used in the present study are shown in Scheme 1. Results and Discussion Ground-state Absorption and Steady-state Fluorescence Studies. Ground-state absorption spectra of the probe, C343, were recorded in aqueous solution at different pH and are shown in Figure 1A. The absorption spectrum of C343 in F88 solution is also shown in Figure 1A for comparison. It is evident from Figure 1A that the absorption characteristic of C343 in F88 solution is very similar to that in water at alkaline pH. Present results indicate that the dye C343 mainly exists in its anionic form in F88 micellar solution.
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Figure 2. Steady-state emission spectra of C343 in 5% w/v F88 solution at different CTAC/F88 molar ratio: 0.0 (s), 0.1 ( · · · ), 0.2 (---) and 0.5 ( · - · - · - · ).
The steady-state emission spectra of C343 in aqueous solution at different pH along with that in F88 solution are shown in Figure 1B. Unlike absorption spectra, the dye does not show much change in the emission spectra with pH of the solution. A close look at Figure 1B indicates that the emission spectrum of C343 in F88 is very similar to that of C343 in alkaline solution. This result is in agreement with that obtained in the absorption measurement. Thus, from the present results we infer that the dye C343 exists in its anionic form in the F88 micellar solution. This is in good agreement with the earlier reported results where it has been shown that in different micellar solutions, including some pluronic micelles, C343 exists in its anionic form.35,36,38-43 To see whether there is any change in the microenvironment for the probe, C343, in the F88 micellar solution due to the addition of CTAC, we have recorded the steady-state emission spectra of C343 in F88-CTAC systems at different CTAC concentrations. These results are shown in Figure 2, where it is indicated that with an increase in the CTAC concentration the emission spectra of C343 undergoes a small (∼7 nm) but definite blue shift. This hypsochromic shift of the emission spectra of C343 suggests that, with an increase in the CTAC concentration in F88 solution, the microenvironment for the dye gradually becomes more nonpolar in nature. Due to the ionic nature of the probe, its excitation spectrum is also expected to be quite sensitive to its microenvironment. With this expectation, we have also recorded the excitation spectra of the probe in an F88 micelle at different concentrations of CTAC, and the results are presented in the Figure 3A. It is evident from Figure 3A that the excitation spectrum shows a gradual blue shift as we increase the concentration of CTAC in the F88 micellar solution. Figure 3B shows the variation of the maxima of the excitation spectra with the CTAC concentration. It is evident from Figure 3B that the changes in the maxima of the excitation spectra are more prominent at the lower CTAC concentration region and saturates to a plateau value at ∼0.4 of CTAC/F88 molar ratio. This result clearly indicates that there is a change in the microenvironment of the probe with the changes in the CTAC concentration. Time-resolved Fluorescence Studies. To understand the details of the effect of the added CTAC on the photophysical behavior of the probe, C343, in the F88 micellar solution, we have investigated the rotational relaxation and dynamic Stokes’ shift studies as discussed below. a. Time-resolWed Rotational Relaxation Studies. Timeresolved fluorescence anisotropy measurements have been carried out to understand how the microenvironment of the
Figure 3. (A) Excitation spectra of C343 in 5% w/v F88 solution at different CTAC/F88 molar ratio: 0.0 (s), 0.1 ( · · · ), 0.2 (---) and 0.5 ( · · - · · - · · -). (B) Variation in the excitation spectra maxima of C343 with the CTAC/F88 molar ratio.
probe, C343, in F88 micelle changes with the addition of CTAC. The changes in the fluorescence anisotropy decay for C343 in F88 micellar solution at different CTAC concentrations are shown in Figure 4A. The anisotropy decay for C343 in bulk water is also shown in Figure 4A for comparison. It should be mentioned, however, that the exact reorientation time for C343 in bulk water could not be estimated correctly because it is much shorter than the time-resolution of our TCSPC instrument. The fluorescence anisotropy decays for C343 dye in F88 micellar solutions were seen to fit well with a biexponential function of the following form. 2
r(t) )
∑ ari exp(-t/τri)
(3)
i)1
where ari and τri are the amplitude and time constant, respectively, for the ith decay component. The average rotational relaxation time, , was calculated using the following equation.44 2
)
∑ ariτri2 i)1 2
∑ ariτri
(4)
i)1
All the fitting parameters, including values, for C343 dye in F88 micellar solution at different CTAC concentrations are presented in Table 1. The variation in the values with CTAC concentration is presented in Figure 4B. It is obvious from Figure 4A and Table 1 that the reorientation time of the probe is much slower in F88 micellar solution compared to that in bulk water. This result clearly indicates that the probe, C343, gets preferentially bound to the micellar phase rather than residing in the bulk water phase. Since the present probe is
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Figure 4. (A) Temporal profile of the fluorescence anisotropy for C343 dye in water (O) and in F88 micelles at different concentration of CTAC: (∆) 0.0, (×) 0.1, and (0) 0.3 CTAC/F88 molar ratio. Solid lines are biexponential fitting to the data points. (B) Variations of the average reorientation time, , for C343 dye in F88 micelle with the CTAC/F88 molar ratio.
TABLE 1: Time-resolved Anisotropy Parameters for C343 Dye in F88 Micelle with Varying CTAC/F88 Molar Ratio τr1 (ns)
[CTAC]/[F88] 0.00 0.01 0.02 0.04 0.05 0.06 0.10 0.20 0.25 0.30 0.40 0.50 *ai ) ariτri /
∑
2 i)1
0.15 0.18 0.20 0.20 0.17 0.23 0.28 0.31 0.28 0.36 0.31 0.36 ariτri.
a1
τr2 (ns)
a2
(ns)
0.38 0.44 0.41 0.30 0.21 0.30 0.30 0.27 0.17 0.27 0.21 0.25
0.48 0.69 0.85 1.21 1.21 1.39 1.53 1.66 1.62 1.83. 1.75 1.81
0.62 0.56 0.59 0.70 0.79 0.70 0.70 0.73 0.83 0.73 0.79 0.75
0.35 0.47 0.59 0.90 0.99 1.04 1.15 1.29 1.39 1.43 1.46 1.44
anionic in nature, it will, however, prefer to stay at the micellar surface rather than entering into the interior of the micelle. Because of the ionic nature of the probe there is also a probability that a small fraction of the dye might also remain partitioned in the bulk water. In fact, it is reported in the literature that in some other micellar media, including pluronic micelles, C343 anions get distributed between the micelle and the bulk water, although the distribution is preferentially shifted toward the micellar phase.35,36,38-42 Due to the presence of large hydrophobic group in C343, the percentage of C343 in the bulk water phase is expected to be much less as compared to that in the micellar phase. In the present anisotropy results, the contribution of the dye in bulk water phase is, however, expected to be very small as the values in these systems are much higher than that in aqueous solution. From Figure 4B it is also
evident that the reorientation time of C343 increases with CTAC concentration and reaches a plateau when the CTAC/F88 molar ratio reaches a value of ∼0.4. The above results on the reorientation dynamics of the solute in the F88-CTAC system can be explained on the basis of the formation of the supramolecular assembly between F88 micelle and CTAC surfactant. It is proposed that as we add CTAC to F88 micellar solution; they form a supramolecular assembly such that the hydrophobic long chain of the CTAC molecule gets dissolved in the core of the F88 micelle and its cationic headgroup resides at the interfacial region of the core and corona of the micelle. Formation of such a supramolecular assembly is supported from the fact that the detailed light- and neutronscattering studies have shown that ionic surfactants such as SDS and CTAC can form the above kind of supramolecular assemblies with other pluronic micelles.20-22 The formation of such a supramolecular structure is also supported by several photophysical studies reported in the literature.29,30,35,36 By drawing an analogy we can also predict the formation of such a supramolecular assembly between F88 micelle and CTAC surfactant. Formation of such a supramolecular assembly, where the charged head groups of CTAC reside at the interface of the core and corona region, results in the development of a positively charged layer inside the F88 micelle on the addition of CTAC. The charge density of this layer naturally increases gradually as we increase the concentration of CTAC in the solution. As the probe used in the present study is anionic in nature, it experiences an electrostatic attraction by the positively charged layer developed inside the F88 micelle due to the presence of CTAC. This electrostatic interaction leads to the movement of the anionic probe from the surface of the micelle to its interior region as we increase the CTAC concentration. Thus, the increase in the reorientation time of the probe with an increase in the CTAC concentration in F88 micelle is due to the following two reasons. First, the microviscosity experienced by the probe in the interior of the micelle is much higher than that at the micellar surface. Second, the increase in the electrostatic interaction between probe and the charged layer inside the micelle acts against the rotational motion of the probe. Thus, there is a gradual increase in the values of the dye in F88 micelle with an increase in the CTAC concentration. That the changes in the reorientation times of C343 are not due to any structural change of the micelle but due to its changing location in the micelle with the added CTAC molecules can be supported from the following considerations. From detailed studies in other pluronic-CTAC systems, it is reported by Jansson et al.20,21 and Mali et al.45 that the changes in the micellar structural characteristics are only marginal even up to the CTAC concentration, much higher than what is used in the present study. To substantiate their inference and also to see if the possible changes in the structure of F88 micelle due to the addition of CTAC can affect the reorientation dynamics of a probe that reside on the micellar surface, we have carried out the fluorescence anisotropy measurement of a cationic probe, Rhodamin-110 (R110) in F88 micellar solution at different CTAC concentration. The average reorientation time measured for R110 in F88 micelles at CTAC/F88 molar ratio of 0.0 and 0.4 are effectively the same, ∼0.33 ns. The cationic probe, R110, is reported to reside on the surface of the micelle,45 and it shows reorientation time very similar to C343 in F88 micelle in the absence of CTAC surfactant. These results with R110 dye thus clearly indicate that, even if there could be a marginal change in the micellar structure due to the addition of CTAC, it does not change the reorientation time of a probe that resides on the
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Figure 5. (A) Time-resolved emission spectra (TRES) and (B) timeresolved area normalized emission spectra (TRANES) of C343 in F88 micellar solution at different time (0.1-2 ns).
surface of the micelle. Thus, the observed changes in the reorientation times of C343 with the addition of CTAC must not be due to the changes in the micellar structure but rather are due to the changes in the microenvironment of the probe as it arises due to its movement form the surface to the interior of the micelle. b. Dynamic Stokes’ Shift Measurements. As mentioned above, the position of the probe, C343, in F88 micellar phase changes on changing the concentration of CTAC in the solution. It is thus quite expected that the microenvironment around the probe, especially the microstructure of the water molecules around the probe, will also change accordingly. The structure of water molecules around a solute can affect its physical as well as chemical behavior in the micelle. Thus, understanding the nature of the water molecules around a solute is essential in order to understand the physical and chemical properties of the dissolved solute. For this purpose, we have carried out a detailed dynamic Stokes’ shift measurement in F88-CTAC supramolecular assembly for the probe C343. To study the dynamic Stokes’ shift, emission transients were first recorded at 10 nm intervals over the entire emission spectrum of the probe. The wavelength-dependent emission decays were then converted to the time-dependent emission spectra by the standard method.35,46-48 The time-resolved emission spectra (TRES) thus constructed for the probe C343 in F88 micellar solution are shown in Figure 5A. It is to be noted that, along with the decrease in the emission intensity with time, there is a concomitant small but definite shift in the emission peak toward the lower frequency. To have a detailed idea about the changes in the emission spectra with time, the areas of each of these transient emission spectra were normalized and are plotted in Figure 5B. It is clearly evident from the timeresolved area-normalized emission spectra (TRANES)49,50 that an isoemissive point exists in the present system. The appearance of such an isoemissive point suggests the presence of two emissive species in this system.49,50 The existence of two emissive species can be explained on the basis of the distribution of the probe between micelle and bulk water phase. As
Figure 6. (A) Time resolved emission spectra (TRES) and (B) timeresolved area-normalized emission spectra (TRANES) of C343 in F88 micellar solution at CTAC/F88 molar ratio of 0.5 at different time (0.1-2 ns).
mentioned earlier that the probe C343 mainly exists as an anion in the present system, thus there is always a possibility that a small fraction of the dye will be present in the bulk water. Such distribution of C343 in different microheterogeneous media have already been reported in the literature.35,38-42 Presence of a small but definite amount of C343 in bulk water is thus inferred to be responsible for the appearance of the isoemissive point in Figure 5B, as TRANES is suggested to be quite sensitive to the number of emissive species in the solution.49,50 TRES have also been constructed in F88 micelle at different CTAC concentration. Figure 6 shows the TRES and TRANES for C343 dye in the F88-CTAC system at a CTAC/F88 molar ratio of 0.5. It is evident from Figure 6 that, like in the F88 system, in the F88-CTAC system there is also a gradual shift with time in the emission maxima toward the lower frequency side. A comparison of Figure 5A and 6A clearly indicates that the frequency shift in the emission spectra is relatively large in the presence of CTAC. Another important difference between the F88 and F88-CTAC systems is the absence of an isoemissive point in the TRANES plot for the latter system (cf. Figures 5B and 6B). Unlike in the F88 system, the absence of an isoemissive point in the F88-CTAC system clearly indicates the presence of only one emissive species in the latter system. The disappearance of the isoemissive point in the F88-CTAC system clearly suggests that the presence of CTAC in the F88 micelle results in the association of all the dyes in the solution with the micellar phase. This result is in good agreement with the result obtained in the time-resolved anisotropy measurements. As mentioned, the extent of the frequency shift in the TRES is relatively larger in the case of the F88-CTAC system as compared to that in the F88 system. To estimate the relative amount of the dynamic Stokes’ shift observed in the present systems, we first calculated the expected total dynamic Stokes’ shift (∆total) using the following equation.51
1358 J. Phys. Chem. B, Vol. 113, No. 5, 2009 m CH CH ∆total ) [ωm abs - ωem] - [ωabs - ωem ]
Singh et al.
(5)
m CH where ωabs and ωabs are the absorption frequencies and ωm em and CH ωem are the emission frequencies for C343 dye in micelle and cyclohexane (nonpolar) solution, respectively. Total expected Stokes’ shift (∆total) and the observed Stokes’ shift (∆obs) values for the present systems are tabulated in Table 2. Percentage of the total Stokes’ shift observed (∆obs % ) are also shown in Table 2. It is evident from Table 2 that the ∆total value decreases gradually, whereas the ∆obs and, accordingly, ∆obs % values increase gradually as we increase the CTAC concentration in the F88 solution. The variation of the ∆obs % with the added CTAC concentration are shown in Figure 7. The observed changes in the ∆obs % values with the added CTAC concentration can be explained on the basis of the formation of the supramolecular structure between F88 micelle and CTAC as mentioned earlier. In the absence of CTAC, the dye C343 resides at the micellar surface and is thus partially exposed to the bulk water. As the solvent relaxation rate for the bulk water is much faster (
