J. Phys. Chem. B 2008, 112, 6363–6372
6363
Effects of Block Size of Pluronic Polymers on the Water Structure in the Corona Region and Its Effect on the Electron Transfer Reactions Poonam Verma,† Sukhendu Nath,*,‡ Prabhat K. Singh,‡ Manoj Kumbhakar,‡ and Haridas Pal*,‡ Radioanalytical Chemistry Section and Radiation & Photochemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed: December 11, 2007; ReVised Manuscript ReceiVed: February 18, 2008
Effects of constituent block size of triblock copolymers on the nature of the water molecules in the corona region of their micelles have been investigated using time-resolved fluorescence measurements. The physical nature of the water molecules in the micellar corona region of the block copolymer, Pluronic F88 ([ethylene oxide (EO)]103-[propylene oxide (PO)]39-EO103), has been studied using a solubilized coumarin dye. Solvent reorientation time and rotational correlation time have been measured and compared with another block copolymer, Pluronic P123 (EO20-PO70-EO20), which has a different composition of the constituent PO and EO blocks. It is noted that due to the presence of larger number of EO blocks in F88 as compared with P123, the corona region of the former micelle is more hydrated than that of the latter. The solvent reorientation time and rotational correlation time are found to be relatively shorter for F88 as compared with P123. This indicates that the water molecules in the corona of the F88 micelle are more labile than those of P123, which is also supported from the estimated number of water molecules associated with each EO unit, measured from the size of each type of micelle and its aggregation number. To understand the effect of block size on the chemical reactions in these microheterogeneous media, electron transfer reactions have been carried out between different coumarin acceptors and N,N-dimethylaniline donor. The electron transfer results obtained in F88 micelles have been compared with those obtained in P123, and the results are rationalized on the basis of the relative hydration of the two triblock copolymer micelles. Introduction Block copolymers made up of poly(ethylene oxide (EO)) and poly(propylene oxide (PO)) blocks have attracted considerable attention during last two decades.1–10 This is partly because of their unique solution behavior and partly due to their wide range of applications, for example, as detergents, lubricants, and emulsifiers.11,12 These polymers have also been tested as controlled drug encapsulation and delivery systems, because of their low toxicity and amphiphilic nature.13–19 The general formula of triblock copolymers of these classes is represented by (EO)n-(PO)m-(EO)n, and one commercial name for these copolymers is Pluronic. Although Pluronics do not possess any polar head and nonpolar tail groups like conventional surfactants, they form micelles at higher concentration or at higher temperature due to the different temperature-dependent solubilities of EO and PO units.8,10,20 The solution behavior of Pluronics has been studied extensively using different techniques: neutron scattering,7,8,21,22 X-ray scattering,23 light scattering,3,6,21–23 fluorescence spectroscopy,5,24,25 etc. A general observation of all these studies is that the solution behavior of Pluronic polymers extensively depends on the hydrophilic-lipophilic balance (HLB) of the polymers. In other words, the solution behavior depends on the ratio of the EO and PO units present in the specific copolymer. Thus, a polymer with large number of PO units, which is hydrophobic in nature, enhances the * Authors for correspondence. E-mail addresses:
[email protected];
[email protected]. Tel: 91-22-25593771. Fax: 91-22-25505151. † Radioanalytical Chemistry Section. ‡ Radiation & Photochemistry Division.
micelle formation. For example, the EO/PO ratio for Pluronic F88 (EO103-PO39-EO103) and P123 (EO20-PO70-EO20) is 2.64 and 0.29, respectively. The critical micellar temperature (CMT) for 5% w/v solutions of F88 and P123 are 30.5 and 12.5 °C, respectively.5 This indicates that the micellization process is more favorable for P123 than F88 because of the large number of PO units in the former as compared with latter. Pluronics form micelles above critical micellar concentration (CMC) or above their CMT. In a triblock copolymer micelle, because PO units have more hydrophobic character than EO units, the PO units form the core and the EO units form the hydrated exterior, known as the corona region. These micellar systems provide a complex medium with different local environments. Due to the presence of substantially different local environments in a single micelle, they can provide wide range of interactions, from hydrophobic to hydrophilic interactions. The nature of these interactions can be easily tuned by tuning the composition of the triblock copolymer. The presence of wide range of interactions in these microheterogeneous media results in the dissolution of wide range of substrates in a single-phase formulation. This property has been utilized extensively for many technical applications.11,12 Although considerable efforts have been devoted in understanding the structure of micelles formed by this class of surfactants, efforts to understand the dynamical aspects within these microheterogeneous systems are scant. These dynamical aspects, for example, the motion of the water molecules and the motion of a solute in the micellar phase, reveal information regarding the nature of the microenvironments in the vicinity of the site of solubilization of the reactants. These dynamical
10.1021/jp711642x CCC: $40.75 2008 American Chemical Society Published on Web 04/26/2008
6364 J. Phys. Chem. B, Vol. 112, No. 20, 2008 processes play an important role in the potential application of the micelles as microreactors for carrying out specific chemical reactions and as vehicles for drug delivery systems. Very recently there have been some studies that explore these dynamical aspects in the Pluronic block copolymer micelles. Most of these studies concentrate on the effect of temperature and concentration of polymer on these dynamical processes. For example, Dutt et al. have investigated using fluorescence anisotropy measurements how the friction experienced by a solute changes by changing the temperature of the solution and by changing the polymer concentration.26–28 They have observed that the friction experienced by a solute in the micellar phase and in the gel phase of the block copolymer is very similar though the macroscopic viscosities in these two phases are very different. They have also shown how the photoisomerization reaction is affected due to the phase transition from sol to gel phase of the copolymer solution.29 Grant et al. have used three coumarin dyes of different hydrophobicity to probe different locations in the F88 micellar systems.30,31 From temperaturedependent anisotropy and changes in the emission maxima, they have concluded that these dyes act as local reporters for three different regions of the micelles. Very recently Bhattacharyya et al.32,33 and Kumbhakar et al.34 have studied the motion of the water molecules around a dye molecule in P123 and F127 (EO100-PO70-EO100) micelles. They have shown that the water molecules in the corona region of these micelles are more restricted compared with bulk water. Bhattacharyya et al.32,33 have used different excitation wavelengths in their solvation dynamic studies in P123 micelles to disentangle the contributions of the probes localized at different microenvironments of the micelles. In their study, excitation with a blue light is suggested to monitor the probes that are solubilized in the more nonpolar region, and red wavelength excitation is suggested to monitor probes that are solubilized in more polar regions of the micelles. Kumbhakar et al. have shown how the microviscosity and the solvent motion changes in F127 micellar system as a result of addition of electrolytes.34 As indicated from the literature reports, all dynamical processes in polymer micelles are largely dependent on the nature of the copolymer, which again depends largely on the composition of the EO-PO units in the copolymer. As mentioned earlier, the physical properties of the Pluronic micelles largely depend on the relative number of the EO and PO units. Thus tuning the EO/PO ratio of the polymer allows one to optimize the properties of the micelles to meet the specific requirements in different applications. The block size of the triblock copolymers has a large influence on the physical properties of these micelles. The aim of the present study is to understand the effect of the block size on dynamical aspects in the copolymer micellar systems. Thus, we have studied the dynamics of the water molecules and the motion of the solute molecules in the micelles formed by F88 copolymer. Timedependent fluorescence measurements have been used to understand these dynamical processes. The results obtained in F88 micelles have been compared with the results obtained with P123 micelles,35 which has a very different EO/PO ratio than that of F88. We have also explored the effect of the block size on the bimolecular electron transfer (ET) reaction in Pluronic micelles. Thus, we have studied the ET reactions between several coumarin dyes, which act as electron acceptors, and N,Ndimethylaniline (DMAN), which acts as electron donor, in F88 micelles. The ET dynamics in F88 micelles have been compared with the result obtained in P123 micelles,36 and the observed differences have been explained on the basis of the differences
Verma et al. in the microenvironments of the two micelles due to the difference in their EO/PO ratio. Experimental Section Steady-state fluorescence measurements were carried out using a Hitachi spectrofluorimeter, model 4010F. The emission spectra were corrected for the wavelength-dependent instrument responses. All these measurement were carried out under magic angle condition because all the time-resolved measurements for solvation and quenching studies have also been made with that polarization condition. Temperature of the solution was controlled within (1 °C, using a thermoelectric controller from Eurotherm. For every measurement, the solution was allowed to equilibrate for at least 20 min at a preset temperature. The time-resolved fluorescence measurements were carried out using a diode laser based time-correlated single-photon counting (TCSPC) spectrometer from IBH, UK. A 408 nm diode laser with 1 MHz repetition rate was used for the sample excitation. A photomultiplier tube (PMT)-based detector (TBX4, IBH) was used for the detection of the emitted photons through a monochromator. For solvation dynamics studies, emission transients were collected at 10 nm intervals to cover the entire range of the fluorescence spectrum. For fluorescence quenching studies, the fluorescence was collected at the wavelength where the contribution from the fast blue-edge decay and fast rededge growth due to slow solvation process get compensated, and the fluorescence decay can be fitted by a single-exponential function reasonably in the absence of the quenchers. Except for anisotropy measurements, all the fluorescence transients were collected at magic angle (54.7°) with respect to the vertically polarized excitation beam to avoid the effect of rotational relaxation of fluorescence probes on their transient decays. The instrument response function (IRF) was measured by collecting the scattered excitation light from suspended TiO2 particles in water. The IRF thus measured was ∼230 ps. All the measurements were carried out at 38 °C to ensure that the micelization process is complete (see later). The temperature was controlled within (1 °C using a thermoelectric controller, model DS from IBH, UK. For solvation dynamics study, all fluorescence transient decays were fitted with a multiexponential equation using the iterative convolution method. Time-resolved emission spectra were reconstructed using the best fitting parameters of the fluorescence decays measured at different wavelengths following the method proposed by Maroncelli and Fleming.37 The solvation times were obtained from the time-dependent changes in the emission maxima. Time-dependent anisotropy was measured using the following equation:
r(t) )
I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)
(1)
where I|(t) and I⊥(t) are the time-dependent emission decays for parallel and perpendicular polarizations with respect to the vertically polarized excitation beam. The polarization sensitivity of the detection system has been incorporated in the factor G. All these measurements were carried out 2-3 times to check the reproducibility and to obtain the average values for the relaxation times. Redox potentials of the coumarin dyes and the DMAN in 5% w/v F88 solution at 38 °C were measured using a cyclic voltammetry (CV) method with an Eco-Chemie potentiostat/ Galvanostat-20 coupled with GPES software. CV measurements were then carried out using a hanging mercury electrode as
Effects of Block Size of Pluronic Polymers
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CHART 1: Chemical Structures of Coumarins and Amine Used in the Present Study
working electrode, a carbon black rod as a counter electrode, and a saturated calomel electrode as the reference electrode. Dynamic light scattering (DLS) studies were carried out using a Malvern 4800 autosizer coupled with a 7132 digital correlator. An argon ion laser, emitting 514.5 nm light, was used as the light source. The light scattered from the sample was collected at a right angle using an avalanche photodiode (APD). Diffusion coefficients of the scattering particles were calculated from the autocorrelation function, C(t), according to following equation: -2Dq2t
C(t) ) 1 + e
(2)
where D is the diffusion coefficient of the scattering particles and q is the scattering vector defined by the following equation:
q)
4πn ( ) sin θ/2 λ
(3)
where n is the refractive index of the medium, λ is the wavelength of the laser light, and θ is the scattering angle, 90° in the present case. The hydrodynamic radius, Rh of the scattering particles was calculated using the Stokes-Einstein equation.
Rh )
kT 6πηD
(4)
where k is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the medium. F88 was a gift from BASF corporation, Edison, NJ. All coumarin dyes were purchased from Exciton and used as received. The DMAN is from Spectrochem, India, and freshly vacuum distilled just before use. The molecular structures of all coumarin dyes and DMAN are given in Chart 1. Solution of F88 was prepared by taking 0.05 g of solid F88 polymer per 1 mL of nanopure water from Milli Q system. The solution was stirred at room temperature for about 24 h with a magnetic stirrer. Coumarin and amine were added directly to the polymer solution and stirred for 4-6 h keeping the solution temperature at 38 °C (see later). Concentration of the coumarin dyes in the polymer solution was kept very low so that the possibility of having more than one dye in a micelle is negligible. Results and Discussion Steady-State Fluorescence and Dynamic Light Scattering Studies. The solution behavior of F88 polymer has been studied extensively to understand its micellization process. Though the CMT for 5% w/v solution of F88 is reported in the literature,30,31 we preferred to measure this parameter independently in the present study because of the following reasons. Unlike, conventional low molecular weight surfactants, SDS, TX100, etc., composition polydispersity of Pluronic surfactants is quite appreciable. Because of this polydispersity in their composition,
Figure 1. Variation of (A) fluorescence emission maximum of C153 dye and (B) average radius of the scattering particle measured by DLS technique in aqueous solution of 5% w/v F88. The vertical solid line indicates the position of 38 °C, which is the experimental temperature for other studies (see text).
there is a lack of sharp CMC and CMT for this class of compounds, and hence a wide range of CMC and CMT values are reported in the literature for the same Pluronic polymer.3 This polydispersity also varies from batch to batch.38 Because of these reasons, we independently determined the CMT value for 5% w/v F88 solution using both fluorescence and DLS measurements. Fluorescence techniques have been extensively used to determine the onset of micelle formation using a fluorescence probe that is sparingly soluble in water but preferentially soluble in the micellar phase.5,39 Due to this preferential solubilization of the probe in the micellar phase, there will be a distinct and large change in the fluorescence properties of the solubilized dye at the onset of micelle formation. From the changes in the fluorescence properties, e.g. emission intensity, peak position, etc., we can determine the CMC, as well as CMT value of these micelles. Different fluorescence probes like pyrene and DPH (diphenyl hexatriene) have been used in the literature to characterize the micellar systems, especially to understand the micellization process.5,24,40 Since the hydrophobic probes will mostly reside in the hydrophobic core of the micelles, they are not suitable to probe the microenvironments in the micellar corona region, which is relatively polar due to the presence of a large number of diffused water molecules. Because in the present study we are mostly interested to understand the nature of water molecules in the corona region of the micelles, the probe should be somewhat polar or dipolar in character. In this respect, coumarins are one of the most suitable probes, because these dyes show a considerable polarity-dependent Stokes’ shift and thus can be utilized suitably to probe the extent of micellar hydration via solvation dynamics studies. In the present study, the coumarin 153 (C153) dye, which is sparingly soluble in water, has been used as the fluorescence probe. Figure 1A shows the temperature-dependent variation in the emission maximum of the dye in 5% w/v F88 solution. It is noted from Figure 1A that there is a sudden blue shift in the emission maximum of the dye at around 34 °C. The blue
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Figure 2. Steady-state fluorescence spectra of C153 in water ( · · · ), isopropyl alcohol (---), liquid PPO (- · · ) and 5% w/v of F88 solution (s) at 38 °C.
shift in the emission maximum indicates the onset of micelle formation because the dye in the micelle experiences a microenvironment that is significantly less polar in nature as compared with bulk water. Since C153 is a hydrophobic probe, as the polymer forms micelles, the dye prefers to reside in the micellar phase rather than in the polar bulk water. We infer from these results that the CMT for a 5% w/v solution of F88 is around 34 °C. This result is in good agreement with the result reported earlier by Grant et al.30,31 To confirm the onset of micellization further and to determine the size of the micelles, we carried out the DLS measurements with 5% w/v of F88 solution at different temperatures. Figure 1B shows the variation in the particle size determined from the light scattering experiments at different temperatures. It is evident from Figure 1B that at low temperature (
