Journal of Molecular Liquids 115 (2004) 113 – 120 www.elsevier.com/locate/molliq

Photophysics of pyrene-end-capped poly(ethyleneoxide) in aqueous micellar environments Basudeb Haldar, Arabinda Mallick, Nitin Chattopadhyay * Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India Received 21 November 2003; accepted 5 March 2004 Available online 2 July 2004

Abstract The photophysical behavior of pyrene-end-capped poly(ethyleneoxide) (PYPY) has been studied in different aqueous micellar environments like cetyltrimethylammonium bromide (CTAB, cationic), triton X-100 (TX-100, non-ionic) and sodium dodecyl sulfate (SDS, anionic). In pure aqueous medium, the polymer solution shows dual emission corresponding to the monomer and the excimer. The relative intensity of the monomer and the excimer emissions shows interesting variation with the addition of the surfactants leading to the determination of the critical micellar concentrations (CMCs) of the micelles. The determined CMCs for CTAB and TX-100 micelles are in good agreement with the existing literature values. In SDS, however, variation of different fluorometric parameters against surfactant concentration results in the CMC values noticeably different from the reported CMC values for SDS micelles. This has been attributed to the interaction between SDS and the poly(ethyleneoxide) (PEO) chain of the polymer leading to the formation of mixed micelles. The micropolarity around the fluorophore has been determined in the micellar environments from a comparison of the variation of the relative fluorescence intensities of the monomer to the excimer band in water – dioxane mixtures with varying composition. D 2004 Elsevier B.V. All rights reserved. Keywords: Micelle; Critical micellar concentration; Micropolarity; Microenvironment

1. Introduction There has been a long-standing interest in the interaction of hydrophobically modified water-soluble polymers with the amphiphilic molecules [1– 4]. Polymer – micelle interaction may serve as a simplified model for biological binding processes, a mimic to the cell membrane [5]. The steadily growing interest in these systems is driven also by their wide range of technical applications, for example, hydrophobically modified water-soluble polymer – surfactant mixtures are used in various technological formulations such as paints, water-based coating fluids, adhesives, sealants, inks, drug delivery systems, food stuffs and enhanced oil recovery [6,7]. A common reason to choose the hydrophobically modified polymers as thickeners to a technical formulation is that they give different rheological behaviors compared to

* Corresponding author. Tel.: +91-33-2483-4133; fax: +91-33-24146266. E-mail address: [email protected] (N. Chattopadhyay). 0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2004.03.003

the normal hydrophilic thickeners. Furthermore, they can help in increasing the stability of the dispersion [7]. Conformation of polymer chain may change due to polymer – surfactant interaction. The unfolding of globular proteins in the presence of surfactants is considered to be partly due to such interactions [8]. The addition of polymers to micellar solutions of surfactants may modify the micellar properties also, provided there is some interaction between the polymer chain and the micelle. A number of studies have been carried out relating to such polymer – surfactant interactions [9 – 13]. The interaction of poly(ethyleneoxide) (PEO) with the anionic surfactant SDS in water is the most studied system among the various surfactants. The polymer –surfactant interaction mostly occurs at the micellar surface [10]. The interaction of surfactants with nonionic polymers like PEO is affected by different factors. Most important of them is the size and charge of the head group [10]. Cationic detergents tend to experience weaker interaction than the anionic detergents [5,10,14,15]. Hence, no interaction has been observed between cetyltrimethylammonium chloride (CTAC) and PEO or poly(vinylpyrrolidone),

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while both the polymers form mixed micelles with SDS [16]. Evidence is there to show that PEO does not interact with nonionic detergents such as poly(ethyleneoxide)dodecyl ethers [17]. Hu et. al. [9] studied the nature of interaction of a polymer, bis-1-pyrenyl methyl ether of PEO (M c 8000) with SDS. The extent of polymer surfactant interaction was reported to be dependent on the polymer chain length. To the best of our knowledge, there is no report on the interaction of this polymer with cationic or neutral micellar systems. To obtain more information on the nature of interaction, we have studied the interaction of the polymer, pyrene-end-capped PEO (PYPY, Scheme 1), a relatively mono-dispersed water-soluble polymer, with the three common surfactant systems, viz., cetyltrimethylammonium bromide (CTAB), triton X-100 (TX-100) and sodium dodecyl sulfate (SDS), the representative surfactant systems with cationic, non-ionic and anionic head groups, respectively. The polymer that we have used is qualitatively similar to that used by Hu. et al., but it has a longer chain length (M c 9500). PYPY contains excimer-forming pyrene units at both ends of the chain. They are bonded through ether linkage. Although the first report of bonding of pyrene to poly(ethylene oxide) used an ester linkage [18], it is susceptible to hydrolysis as discussed elsewhere [19]. Hence, we have chosen an attachment of the hydrophobic end caps through the less hydrolysable ether linkage [20]. It is known [9,20,21] that in aqueous solution, the polymer forms an associated pair using the two end pyrene moieties in the ground state (Scheme 2). Photoexcitation leads to a structured pyrene monomer fluorescence in the range 350 – 450 nm and a broad and unstructured excimer emission at around 485 nm [20]. The blue monomer fluorescence shows characteristic vibrational structure with the I3/I1 ratio sensitive to the local polarity of the medium [22]. The ratio is strictly valid for pyrene itself. Normally, this effect is very small in the 1-substituted pyrene derivative, where the substituent itself serves to perturb the electronic symmetry. In PYPY, the PEO chain acts as a h-oxo substituent which has a symmetrizing influence on the S1 ! S0 (Lb) transition. Polar solvents enhance the intensity of the (0,0) band (I1) relative to that of the (0,2) band (I3). Therefore, I3/I1 ratio decreases with increasing solvent polarity and vice versa. Although the magnitudes are different from those in the case of unsub-

Scheme 1. Structure of PYPY.

Scheme 2. Photophysics of PYPY [22].

stituted pyrene, they do give qualitative information about the polarity and hence the environment around the pyrene end group. We have exploited this principle to determine the critical micellar concentrations (CMCs) of different micellar systems. Moreover, an attempt has been made to find out the micropolarity around the fluorophore and the location of the probe in the microenvironment, monitoring the relative intensity of the monomer and the excimer fluorescence from a comparison of this parameter in water – dioxane mixture with varying composition.

2. Experimental Pyrene-end-capped poly(ethyleneoxide) (PYPY) was a generous gift from Professor Mats Almgren (University of Upsala, Sweden). It is relatively mono-dispersed (Mw/ Mn V 1.10) and has a molecular weight of f 9500 determined from dynamic light scattering. Details of the characterization of this polymer has been given elsewhere [23]. The surfactants cetyltrimethylammonium bromide (CTAB), triton X-100 (TX-100) and sodium dodecyl sulfate (SDS) were obtained from Aldrich and were used without further purification. Spectrophotometric grade 1,4-dioxane (Aldrich) was used as received. Triply distilled water was used for the preparation of the experimental solutions. All the experiments were carried out at ambient temperature in air-equilibrated solutions. Absorption spectra were recorded on a Shimadzu MPS2000 spectrophotometer. The fluorescence measurements were carried out on a SPEX Fluorolog 2 spectrofluorimeter equipped with DM3000F data processing software. Since precise determination of I3/I1 for the monomer emission of PYPY involves resolution of the partially overlapping bands, the total emission spectra were resolved using the multiplepeak fit option of Origin 6.1 software. Adopting this technique, the reliability of the estimated I3/I1 values was

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improved very much, and it was within ca. 2– 3%. All the micellar solutions were freshly prepared to avoid any unwanted complications like aging [24]. For the fluorescence experiments, the samples were excited with 330 nm light.

3. Results and discussion 3.1. Absorption and fluorescence study The absorption spectrum of pyrene shows the lowest energy structured band in the range 300 –370 nm, the band maximum being 334 nm in polar solvents. Pyrene absorption is known to be significantly insensitive to the polarity of the medium. PYPY displays its absorption spectrum qualitatively similar to that of pyrene. The only difference is that the position of the band maximum is a bit right shifted in case of the polymer. The absorption spectrum of polymer is found to be practically insensitive to the solvent polarity, an observation similar to that in the case of pyrene. The band maximum for the polymer system in aqueous solution is observed at 343 nm and the bathochromic shift of this band maximum from that of pyrene itself (334 nm) is nonnegligible. This reflects that pyrene units in PYPY are not equivalent to pyrene itself and perhaps corroborates the earlier proposition [20] that the pyrene units remain as associated pair in PYPY prior to the photoexcitation. The excitation spectra monitoring the monomer and the excimer emissions (at 376 and 485 nm, respectively) of an aqueous solution of PYPY ( f 1.0  10 6 M) are identical and they correspond well to the absorption spectrum indicating again that there is association prior to the excitation. In aqueous medium, PYPY gives its characteristic emission spectrum as described in the report of Duhamel et. al [20]. The spectrum consists of a monomer emission with characteristic structure and a broad and unstructured excimer emission. The emission spectrum of the f 1.0  10 6 M solution of PYPY is modified interestingly in the presence of CTAB. The emissions do not vary appreciably until a certain CTAB concentration is reached beyond which the monomer emission decreases sharply with a concomitant increase in the excimer emission. With further addition of CTAB after a certain point, the variation in the monomer and excimer emissions is reversed and eventually attains saturation. The variation in the emission spectrum of PYPY as a function of CTAB has been presented in Fig. 1. Following the standard practice [9,16], we have plotted the ratio of excimer to monomer emission intensities (IE/IM) against logarithm of CTAB concentration (Fig. 2). The figure also shows the variation of the ratio of emission intensities of peak 3 to peak 1 of the monomer vibronic structure (I3/I1) as a function of logarithm of CTAB concentration. The break points of the individual plots in Fig. 2 correspond with each other very well. These break points have been corresponded to the critical micellar concentrations (CMCs) as usual. The dependence of IE/IM on the surfactant

Fig. 1. Fluorescence spectra of PYPY as a function of CTAB concentration in aqueous solution. In (a) i to viii corresponds to 0.00, 0.072, 0.108, 0.144, 0.360, 0.495, 0.630 and 0.720 mM, and in (b) i to x corresponds to 0.72, 0.81, 0.90, 1.08, 1.35, 1.80, 2.70, 4.14, 4.86 and 6.30 mM of CTAB, respectively (PYPY concentration is f 1.0  10 6 M, kexc = 330 nm).

concentration shows the typical pattern of micellar catalysis operating on the excimer forming process [11]. The variation in the emission spectra and also the variation in IE/IM with varying concentrations of CTAB resemble well with those observed for the PYPY system in aqueous SDS medium by Hu et al. [9]. Thus, it appears that their proposition to explain the variation of monomer and excimer fluorescences in various SDS concentrations is also applicable in the PYPY – CTAB system. The monomer to excimer equilibrium shifts towards the excimer due to the PEO chain cyclization at the premicellar aggregate stage of aqueous CTAB solution. After the formation of full-fledged micelles, i.e., beyond CMC2, the pyrene chain ends might get diluted among newly formed micelles which is reflected in a gradual decrease in the excimer emission intensity and increase in the monomer emission intensity. Further addition of CTAB does not lead to a disturbance in the monomer –excimer equilibrium and hence leads to the attainment of a plateau. Whereas the curves

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Fig. 2. Variation of (a) IE/IM and (b) I3/I1 against logarithm of concentration of CTAB.

IE/IM vs. logC have a similar pattern in CTAB and SDS, this is not the case with the curves I3/I1 in the two environments. The explanation is not obvious. An alternative explanation may be put forward involving the formation of discrete micellelike clusters (at higher CTAB concentrations) around the polymer chain and thereby loosing the random behavior of the polymer chain. This point needs to be established through further experiments.

Fig. 2 reveals that with an increase in the surfactant concentration, the I3/I1 ratio of the monomeric emission of PYPY first decreases, then increases and saturates eventually. The CMC values determined from the variation of IE/ IM and I3/I1 in the CTAB environment are tabulated in Table 1. The CMC values thus determined correspond well with the literature values. An obvious question that arises is whether the PEO chain in PYPY interacts with CTAB or not. There are several reports [10,13 – 15] suggesting that the PEO chain hardly interacts with the cetyltrimethylammonium halide surfactants. An indication of polymer –surfactant interaction may be obtained from the CMC values of the surfactant in the presence of the polymer. The surfactant starts micellization at somewhat lower amphiphilic concentration when a polymer –surfactant interaction occurs [5]. This lower amphiphile concentration is designated as apparent critical micellar concentration of the surfactant. The CMC values estimated here reproduce the normal CMC values of CTAB determined through other independent methods. The result corroborates that the PEO chain in PYPY does not interact perceptibly with CTAB. The variation in the fluorescence spectra of PYPY in the presence of TX-100 is rather different when compared to that in CTAB. With the gradual addition of TX-100, the monomer emission intensity increases monotonically at the cost of the excimer emission intensity (Fig. 3). The variation suggests that unlike CTAB, with increase in the TX-100 monomer formation has been promoted from the existing excimers in aqueous medium. Thus, it appears that the mechanism of interaction of PEO with CTAB at lower concentration range does not apply in the case of the TX100 surfactant system. The cyclization of the PEO chain leading to the formation of excimer is hindered, perhaps due to the bulky head group of TX-100, and the monomer to excimer equilibrium as proposed by Hu et. al. does not shift towards excimer even at the initial stage of micellization. The changes in the IE/IM and also I3/I1 values are plotted against logarithm of concentration of TX-100 and have been presented in Fig. 4(a and b, respectively). The CMC values estimated from the break points of both the plots are included in Table 1.

Table 1 Estimation of CMCs for aqueous CTAB, TX-100 and SDS systems Surfactant

Fluorometric parameter for estimation of CMC

Estimated CMCs (mM) using PYPY as fluorescence probe

Literature CMCs (mM) in absence of any polymer like PEO

Reference

CMC1

CMC2

CMC3

CMC1

CMC2

CTAB

IE/IM

0.31

0.71

1.08

– 0.3

0.8 0.75

– –

[22] [25]

TX-100

I3/I1 IE/IM

0.30 0.23

0.70 0.63

1.07 –

0.24 0.20

– 0.9

– –

[22] [26]

SDS

I3/I1 IE/IM

0.27 0.64

0.60 1.26

– 3.5

2.0 –

6.0 7.5

15.0 46.0

[27] [28]

I3/I1

0.64

1.32

–

CMC3

B. Haldar et al. / Journal of Molecular Liquids 115 (2004) 113–120

Fig. 3. Fluorescence spectra of PYPY in aqueous TX-100 solutions. The concentrations of TX-100 in i to xi are 0, 0.053, 0.155, 0.254, 0.306, 0.367, 0.487, 0.582, 0.954, 1.43 and 2.05 mM, respectively (PYPY concentration is f 1.0  10 6 M, kexc = 330 nm).

The gradual decrease in the micro-polarity around the monomer above the CMC1 has been reflected from the increase in the I3/I1 values. The small variation in the I3/I1 indicates that the monomers do not penetrate much inside the core of the micelles. In connection with the question of interaction between the PEO and TX-100 micelles, reports indicate that the PEO chain does not generally interact with the non-ionic surfactants like n-alkoxypoly(ethyleneoxide) [5,17]. The absence of any apparent CMC values in our experimental results corroborates the idea of non-interaction of the PEO chain with TX-100. It was already mentioned that Hu et. al. [9] have studied the interaction of this polymer system in the SDS environment. They have shown that the association of the SDS molecules leading to the formation of micelle takes place at somewhat lower amphiphile concentration than its normal critical micellar concentration (CMC) and the micelle thus formed has lower aggregation number instead of approximately 60 molecules per micellar unit. Our experimental results with PYPY in SDS medium are just a reproduction of the results of Hu et al. [9]. The variation in the emission spectra with increasing concentration of SDS has been depicted in Fig. 5. Though Hu et al. have not reported the CMC values, the break points we obtained in the plot of IE/ IM against logarithm of SDS concentration are in close proximity with the break points extracted from their plots. The variation in IE/IM and I3/I1 as a function of logarithm of SDS concentration have been given in Fig. 6a and b, respectively. Our estimated CMC values were found to be appreciably lower than the reported normal CMCs for the SDS system as consistent with the observations of Hu et al. and confirm the interaction between SDS and PEO chain. The interaction is ascribed to be specific in nature leading to the incorporation of some parts of the PEO chain into the

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micellar units of SDS. In contrast to the SDS micelle, the interactions between PYPY – CTAB and PYPY – TX-100 are believed to be cooperative in nature whereby the interaction is mostly restricted between the micellar units and the pyrene end groups. By now, multiple break points (or CMCs) are established from a number of current scientific reports [26 – 35]. Certain physical properties such as conductivity, surface tension, osmotic pressure when plotted against surfactant concentration show multiple break points assigned to different CMCs [31]. Fishman and Eirich [32] have shown two break points for SDS at 3 and 12 – 13 mM when they plotted reduced viscosity against the surfactant concentration. Basu [33] has shown two break points for TX-100 at 0.25 and 7.3 mM when he plotted nonradiative rate constants against surfactant concentration. Multiple break points for all the surfactants have been shown by Sarkar et al. [34] while monitoring fluorescence quantum yield of 2-(2V-hydroxyphenyl) benzimidazole against surfactant concentrations although they did not report them as CMCs. Similarly, plotting the fluorescence polarization anisotropy of different fluorescent probes against the surfactant concentrations,

Fig. 4. Variation of (a) IE/IM and (b) I3/I1 against logarithm of concentration of TX-100.

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3.2. Binding constant In order to study the binding interaction of PYPY with CTAB and TX-100, the binding constant (K) values have been estimated from the fluorescence intensity data for the micellar systems. The equation suggested by Alamgren et al. [36] has been exploited following the significant difference in the fluorescence intensity of the probe in pure water and aqueous micellar environments. According to the equation ðIl  I0 Þ=ðIc  I0 Þ ¼ 1 þ ðK½M Þ1

ð1Þ

where Il, I0 and Ic are the relative fluorescence intensities under complete micellization, in the absence of micelles and in the presence of intermediate amounts of surfactant, respectively. [M] represents the total concentration of micelle and it is determined by ½M ¼ ð½S  CMCÞ=N

ð2Þ

where [S] represents the concentration of the surfactant and N is the aggregation number of the micelle [37]. It was already mentioned earlier that unlike CTAB and TX-100, SDS forms a mixed micelle instead of the normal

Fig. 5. Fluorescence spectra of PYPY as a function of SDS concentration in aqueous solution. In (a) i to v correspond to 0, 0.36, 0.81, 1.0 and 1.3 mM, and in (b) i to viii correspond to 1.30, 1.99, 2.67, 3.18, 3.65, 5.22, 12.4 and 19.0 mM of SDS, respectively (PYPY concentration is f 1.0  10 6 M, kexc = 330 nm).

Dennison et al. [28] and Chaudhury et al. [35] have observed multiple break points which they have assigned as CMCs for the three surfactants as those used in the present experiment. Table 1 presents the determined CMC values along with the literature values for comparison. Variation of the I3/I1 ratio of the monomer fluorescence of PYPY in different micellar systems needs some discussion. The variation is of similar nature in TX-100 and SDS micelles. The pattern is somehow different in CTAB. However, the I3/I1 ratios at very low and very high concentrations of surfactants are not dramatically different. This suggests that the fluorophore is not included deep inside the micellar units at these concentrations. As will be discussed in a forthcoming section dealing with the determination of the micropolarity around the probe, the probe is located in the micelle – water interfacial region in all the studied micellar environments. Hence, the microplarity is not remarkably different from that of the aqueous phase.

Fig. 6. Variation of (a) IE/IM and (b) I3/I1 of PYPY against logarithm of concentration of SDS.

B. Haldar et al. / Journal of Molecular Liquids 115 (2004) 113–120

micelle when it interacts with the PEO chain of PYPY. As the characteristics of a mixed micelle widely differ from a normal micelle and the incorporation of the PEO chain decreases the aggregation number of the SDS micelle, we have restricted ourselves from estimating the binding constant value for the SDS system. Since PEO does not intervene in the formation of normal micelles in the case of CTAB and TX-100, it is reasonable to assume that the N values should not differ much from the N values of the normal micelles. Following the report of Saroja et al. [37], the N values used for the calculation of [M] are 60 for CTAB, and 143 for TX-100. Both the monomer and the excimer fluorescences were monitored for the study. A representative plot based on Eq. (1) for PYPY (monitoring the excimer emission) in aqueous TX100 is given in Fig. 7. The binding constants determined monitoring the two emissions are the same within experimental limits and the mean values are 3  105 and 8.5  105 M 1 for CTAB and TX-100, respectively. The free energy changes associated with the probe –micelle binding process for the two micellar systems have been calculated at ambient temperature, and they come to be  31.5 and  34.1 kJ mol 1 for the CTAB and TX-100 systems, respectively. 3.3. Polarity of the micellar microenvironment around the fluorophore Micelles are characterized by three distinct regions: a nonpolar core formed by the hydrocarbon tails of the surfactant, a compact Stern layer having the head groups and a relatively wider and diffused Gouy – Chapman layer that encompasses the majority of the counter ions [38]. Depending on the nature of the probe and the micelle, a probe molecule can be bound either to the nonpolar core of the micelles or to the micelle – water interface. Fluorometric study has been made with PYPY in different water – dioxane mixtures, for which the ET(30) values are known [39] to

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Fig. 8. Variation of IM/IE of PYPY against ET(30) of water – dioxane mixture with varying composition.

monitor the variation in IE/IM with gradual increase in dioxane composition. The IE/IM ratio varies as a function of ET(30) in a parabolic fashion. So, IM/IE (the reciprocal of IE/IM) has been plotted against ET(30) for different water – dioxane mixtures to obtain a linear variation. Fig. 8 depicts such a plot. Since IM/IE values of the PYPY emission in fully micellized condition in all the three surfactant systems are higher than that in pure aqueous phase, it is expected that the polarities around the pyrene groups are to some extent lower than those in pure water. Superposition of the IM/IE values of PYPY in the micellar environments at fully micellized condition on the plot for dioxane –water mixtures gives the estimated ET(30) values which are 57.5, 58.8 and 59.8 for CTAB, TX-100 and SDS, respectively. The values suggest that the microenvironments around the end groups in all the micelles do not differ very much. The values are in agreement with the ET(30) values reported for interfacial polarity of the studied micellar systems [40]. The observation suggests that even at fully micellized condition, the fluorophore does not penetrate into the micellar core and the pyrene groups bind only at the micelle –water interfacial region.

4. Conclusion The experiment reveals the following points:

Fig. 7. Plot of (Il  I0)/(Ic  I0) against [M]  1 for TX-100 (monitoring the excimer emission of PYPY).

The relative intensity of the excimer to the monomer emission as well as the ratio of the prominent vibronic bands of the pyrene monomer fluorescence (I3/I1) of the PYPY fluorophore can serve as sensitive parameters for the determination of CMCs of different micellar systems. CMC values suggest that PYPY interacts with SDS forming mixed micelles but there is hardly any interaction of this sort with CTAB and TX-100.

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The estimated PYPY – micelle binding constants for CTAB and TX-100 systems reveal that the probe has very similar binding efficiency with the two micelles. The estimated micropolarity around the probe in the micellar environments suggests that the fluorophore does not go into the micellar core but binds at the micelle – water interfacial region.

[13] [14] [15] [16] [17] [18] [19] [20]

Acknowledgements

[21]

Financial supports both from the Council of Scientific and Industrial Research, and the Department of Science and Technology, Government of India, are gratefully acknowledged. BH thanks the CSIR for awarding the research fellowship.

[22]

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