Interactions between a Series of Pyrene End-Labeled Poly (ethylene

Oct 7, 2014 - ABSTRACT: The interactions between a series of poly(ethylene oxide)s covalently labeled at both ends with pyrene pendants (PEO(X)-Py2, ...
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Interactions between a Series of Pyrene End-Labeled Poly(ethylene oxide)s and Sodium Dodecyl Sulfate in Aqueous Solution Probed by Fluorescence Shaohua Chen,†,§ Jean Duhamel,*,† Baoliang Peng,‡ Masuduz Zaman,‡ and Kam C. Tam‡ Institute for Polymer Research, Waterloo Institute for Nanotechnology, †Department of Chemistry and ‡Department of Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada S Supporting Information *

ABSTRACT: The interactions between a series of poly(ethylene oxide)s covalently labeled at both ends with pyrene pendants (PEO(X)-Py2, where X represents the number-average molecular weight of the PEO chains and equals 2K, 5K, 10K, and 16.5K) and an ionic surfactant, namely, sodium dodecyl sulfate (SDS), in water were investigated at a fixed pyrene concentration of 2.5 μM corresponding to polymer concentrations smaller than 21 mg/L and with an SDS concentration range between 5 × 10−6 and 0.02 M, thus encompassing the 8 mM critical micelle concentration (CMC) of SDS in water. The steady-state fluorescence spectra showed that the I1/I3 ratio decreased from 1.73 ± 0.06 for SDS concentration smaller than 2 mM where pyrene was exposed to water to 1.43 ± 0.03 for SDS concentration greater than 6 mM where pyrene was incorporated inside SDS micelles. The ratio of excimer-to-monomer emission intensities (the IE/IM ratio) of all PEO(X)-Py2 samples remained constant at low SDS concentrations, then increased, passed through a maximum at the same SDS concentration of 4 mM before decreasing to a plateau value that is close to zero for PEO(10K)-Py2 and PEO(16.5K)-Py2 but nonzero for PEO(2K)-Py2 and PEO(5K)-Py2. The pyrene end groups of these two latter samples could not bridge two different micelles due to the short PEO chain, and excimer was formed by intramolecular diffusion inside the same SDS micelle. Time-resolved fluorescence decays of the pyrene monomer and excimer of the PEO(X)-Py2 samples were acquired at various SDS concentrations and globally fitted according to the “Model Free” analysis over the entire range of SDS concentration. The molar fractions of various excited pyrene species and the rate constant of pyrene excimer formation retrieved from the analysis of fluorescence decays were obtained as a function of SDS concentration. Interactions between SDS and PEO could not be detected by isothermal titration calorimetry, potentiometry with a surfactant selective electrode, and conductance measurements.

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the HEUR solution.8 The mechanism by which HEURs thicken a solution stems from their ability to form rosette micelles at low polymer concentration. The bridging of the rosette micelles upon increasing the polymer concentration leads to the formation of a polymeric network that increases the solution viscosity most effectively.7−9 Since the viscoelastic behavior of HEURs results from their amphiphilic nature, the rheology of HEUR aqueous solutions is strongly affected by the presence of surfactants.10 Due to the obvious implications for industrial applications, the interactions between PEO-Hyd2 and anionic surfactants such as sodium dodecyl sulfate (SDS) have been the object of a number of studies.10−17 SDS is also known to interact with the hydrophilic backbone of PEO. The binding of SDS to PEO has been investigated by various experimental approaches such as isothermal titration calorimetry (ITC),18−22 surface tension,23,24 viscosity, 25 neutron 11,25,26 and laser light 27 scattering,

INTRODUCTION The particular viscoelastic behavior of aqueous solutions of associative thickeners (ATs) has been taken advantage of in numerous industrial applications where control of the solution viscosity is required such as in cosmetics, paints, and enhanced oil recovery.1−5 Hydrophobically modified ethoxylated urethanes or HEURs are one of three main AT families to be sold commercially, with the two others being hydrophobically modified derivatives of hydroxyethyl cellulose (HMHEC) and alkali swellable emulsion copolymers (HASE).6 HEURs are prepared by the condensation polymerization of a flexible poly(ethylene glycol) spacer and a rigid diisocyanate linker in the presence of a small amount of long chain alcohol.5 The resulting macromolecule behaves as a long poly(ethylene oxide) chain terminated at both ends with a hydrophobe (PEO-Hyd2). The hydrophobe is key to enhancing the viscosity of an aqueous HEUR solution as the same polymer prepared without the hydrophobe shows no thickening behavior.7 In this context, it is hardly surprising that the hydrophilic-to-lipophilic balance (HLB) between the length of the terminal hydrophobe and the hydrophilic PEO chain controls the viscoelastic behavior of © 2014 American Chemical Society

Received: July 25, 2014 Revised: October 7, 2014 Published: October 7, 2014 13164

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NMR,21,24,28−30 fluorescence spectroscopy,31−33 ESR,34 and conductimetry.35,36 Considering their importance in the rheology of aqueous solutions of HEURs, and for that purpose, of any type of AT, it is quite telling that the behavior of the hydrophobes covalently attached to HEURs is hardly ever investigated in the scientific literature at the molecular level as they interact with surfactants. The reason for this state-of-affair is that except for fluorescence, none of the techniques used to characterize AT solutions is sensitive enough to probe the hydrophobic groups of ATs in the dilute regime at polymer concentrations of a few mg/L that are required to prevent intermolecular interactions. In these fluorescence studies, the hydrophobe of an AT is typically replaced by the hydrophobic fluorophore pyrene to yield PyAT.12,37−40 The strong hydrophobicity of pyrene ensures that if it is used in lieu of the hydrophobes of a commercial AT such as HEUR or HASE, the Py-AT exhibits rheological properties that are similar to those of the analogous nonfluorescent AT which justifies the use of Py-ATs as a mimic of a nonfluorescent AT.41,42 Among the photophysical properties of pyrene that are relevant to the study of Py-ATs in water are the ability of pyrene to report on the polarity of its local environment and to form an excimer upon encounter between an excited and a ground-state monomer. For instance, the ratio of the first to the third peak, the I1/I3 ratio, obtained from the fluorescence spectrum of a 1pyrenemethoxy derivative decreases from 1.7 in water to 1.4 in the hydrophobic interior of an SDS micelle. Consequently, this feature can be used effectively to probe whether a hydrophobic pyrene pendant is located inside a surfactant micelle or in aqueous solution.43,44 After absorption of a photon, an excited pyrene can form an excimer instantaneously upon direct excitation of a pyrene aggregate in water or by diffusive encounter with a ground-state pyrene in a micelle, or decay as a monomer if it remains isolated in the solution.37−40 These two photophysical properties of pyrene have been applied for decades to provide qualitative information about the behavior of pyrenyl labels covalently attached onto different water-soluble polymers.37−40 As a result of recent developments introduced by this laboratory, fluorescence decays acquired with Py-ATs can now be analyzed quantitatively with the Fluorescence Blob Model (FBM),45,46 the Sequential Model (SM),47,48 or the Model Free (MF) analysis49 to retrieve the molar fractions fagg, fdiff, and f free of pyrene species that are aggregated (Pyagg*), form excimer by diffusion (Pydiff*), or are isolated (Pyfree*), respectively. By applying the MF analysis to the fluorescence decays acquired with a series of pyrene end-labeled PEOs (PEO(X)-Py2, where X represents the PEO molecular weight ranging from 2000 to 16 500 g/mol) as a function of SDS concentration, the molar fractions fagg, fdiff, and f free were probed as a function of SDS concentration with polymer concentration in the micromolar range, highly dilute conditions that greatly minimize interpolymeric interactions. Considering the importance of surfactant/hydrophobe interactions in understanding the complex rheological behavior of aqueous solutions of surfactants and ATs, the ability to determine the state of the hydrophobes of an AT, namely, the extent by which they are aggregated or isolated in solution, offers a new and important experimental means for the characterization of aqueous solutions of ATs and surfactants. This study represents a first step in this direction by providing the first complete description of the behavior of the hydrophobic end groups of a series of PEO-Py2 constructs used as fluorescent mimics of HEURs.

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EXPERIMENTAL SECTION

Materials. The detailed synthesis of the PEO(X)-Py2 samples has been described in a recent publication.50 The general chemical structure of the polymers is shown in Figure 1. The pyrene content of each sample

Figure 1. Chemical structure of the PEO(X)-Py2 samples. n equals 45, 113, 227, and 375 for PEO(X)-Py2 with X = 2K, 5K, 10K, and 16.5K, respectively. was determined by UV−vis absorption and indicated that all PEO samples were fully end-capped with pyrene groups.50 SDS was purchased from EM Science and used as received. Milli-Q water was used to prepare all aqueous solutions. Steady-State Fluorescence Measurements. Fluorescence emission spectra were acquired via a Photon Technology International LS100 steady-state fluorometer with a continuous Ushio UXL-75Xe xenon arc lamp as the light source. A fluorescence microcell (3 mm × 3 mm) purchased from Hellma was used with the usual right angle configuration. Emission spectra were acquired by exciting the samples at 344 nm. The fluorescence intensities of the monomer (IM) and the excimer (IE) were calculated by taking the integrals under the fluorescence spectra from 372 to 378 nm for the pyrene monomer and from 500 to 530 nm for the pyrene excimer. Taking the ratio of IE over IM yields the IE/IM ratio. The I1/I3 ratios were determined from the intensity of the first, I1, and third, I3, peaks in the fluorescence spectrum of the pyrene monomer taken at 374 and 385 nm, respectively. Time-Resolved Fluorescence Measurements. The fluorescence decay profiles were acquired with the time-correlated single photon counting (TCSPC) technique using an IBH time-resolved fluorometer and an IBH 340 nm LED with a 500 kHz repetition rate as the excitation source. For all PEO(X)-Py2 solutions, the excitation wavelength was set at 344 nm. The decay curves were obtained by setting the emission wavelength at 374 nm for the monomer and 510 nm for the excimer. To block potential light scattering leaking through the detection system, filters were used with a cutoff at 370 and 495 nm during acquisition of the fluorescence decays for the monomer and excimer, respectively. All fluorescence decays were acquired over 1024 channels ensuring a minimum of 20 000 counts at their maximum. A time per channel of 2.04 ns/ch was used for the acquisition of the monomer and excimer decays. For all the decay profiles, reference decays of degassed solutions of PPO [2,5-diphenyloxazole] in cyclohexane (τ = 1.42 ns) for the pyrene monomer and BBOT [2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene] in ethanol (τ = 1.47 ns) for the pyrene excimer were used to obtain the instrument response function (IRF) via the MIMIC method.51 The fluorescence decays were analyzed according to the Model Free (MF) analysis which is described in detail in the Supporting Information (SI). In particular, the SI includes the process applied to obtain the average rate constant of excimer formation ⟨k⟩ and the molar fractions of the pyrene species that form excimer by diffusion ( fdiff), do not form excimer ( f free), or form pyrene aggregates ( fagg). ITC Measurements. The enthalpies for the binding of SDS to the PEO(X)-Py2 constructs were determined using a Microcal isothermal titration microcalorimeter with a reference cell and a sample cell having a volume of 1.35 mL. The titration was carried out by injecting 30 times 250 μL of concentrated 0.2 M SDS titrant solution into the sample cell filled with water or PEO(X)-Py2 titrate solution. The tip of the syringe behaved as a stirrer that ensured a continuous mixing efficiency at 307 rpm. The time interval between each injection was set at 2.5 min, and each injection was completed within 4 s. All ITC experiments were conducted at a constant temperature of 25.0 ± 0.1 °C. Electromotive Force (EMF) and Conductivity Measurements. A Metrohm surfactant membrane electrode selective to SDS monomers 13165

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and a Metrohm electrode were used for the EMF and conductivity measurements, respectively. The surfactant selective electrode was used to monitor the SDS monomer concentration during the binding of SDS to PEO(X)-Py2 by measuring the EMF values relative to a Metrohm bromide ion reference electrode. Conductivity was measured with a conductometer supplied by Metrohm. The titration was conducted by injecting a concentrated 0.8 M SDS titrant solution placed in a 200 mL reservoir into the sample container filled with 50 mL of water or PEO(X)-Py2 titrate solution. Each titration consumed 0.03 mL of SDS solution. Solutions of SDS having concentrations of 0.001, 0.01, 0.1, and 1 mM were used to check the stability of the instruments and to obtain the reference EMF values. All experiments were conducted at a constant temperature of 25.0 ± 0.1 °C, which was controlled via a VWR water bath.

reference. The maximum in the SDS dilution profile indicates the critical micelle concentration (CMC) of SDS in water is found to equal 8.4 mM. The titration curves obtained with the 6.1 × 10−5 M PEO(16.5K) and PEO(16.5K)-Py2 solutions show differences compared to the SDS dilution curve. For the 6.1 × 10−5 M PEO(16.5K) solution, the titration curve exhibits a critical aggregation concentration (CAC) of 4.2 mM. The CAC of the 6.1 × 10−5 M PEO(16.5K)-Py2 solution is much smaller ( 3 mM) did a rise time appear in the excimer fluorescence decays and the global analysis of the decays could be conducted. As mentioned in the Experimental Section, the natural lifetime of pyrene, τM, set to equal 155 ns in the analysis of the fluorescence decays with eqs S1 and S2 in the SI was determined by fitting the monomer decays of PEO(2K)-Py1 in the presence and absence of SDS. While the τM value of 155 ns proved to yield 13169

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Table 3. Parameters Retrieved from the Global MF Analysis of the Pyrene Monomer and Excimer Fluorescence Decays Acquired with a 1.25 × 10−6 M PEO(10K)-Py2 Solution as a Function of SDS Concentration monomer

excimer

[SDS] mM

τ1, ns

a1

τ2, ns

a2

τM, ns

f Mfree

f Ediff

τE0, ns

f EE0

χ2

15.0 10.0 8.0 7.5 7.0 6.5 5.8 5.0 4.0 3.5 3.3 2.4 1.0 0.5 0.2

38.7 40.0 39.4 36.2 30.4 25.0 23.9 18.9 48.9 21.7 28.0 16.3 28.5 15.0 22.2

0.11 0.12 0.11 0.14 0.25 0.23 0.32 0.29 0.44 0.53 0.54 0.14 0.12 0.02 0.04

117 122 117 119 82 81 66 56 21 57 95 124 128 118 110

0.12 0.12 0.12 0.10 0.27 0.38 0.48 0.58 0.41 0.28 0.20 0.65 0.74 0.87 0.82

165 165 165 165 165 165 165 155 155 155 155 155 155 155 155

0.77 0.76 0.75 0.76 0.48 0.39 0.20 0.13 0.16 0.20 0.26 0.21 0.15 0.11 0.14

0.77 0.77 0.76 0.74 0.88 0.89 0.91 0.90 0.87 0.86 0.80 0.84 0.81 0.82 0.81

71 70 73 78 67 60 49 50 47 45 45 42 45 46 42

0.23 0.23 0.24 0.26 0.12 0.11 0.10 0.10 0.13 0.14 0.20 0.16 0.19 0.18 0.19

1.03 1.03 0.99 1.10 1.25 1.16 1.09 1.14 1.23 1.26 1.11 1.26 1.02 1.08 1.17

Table 4. Parameters Retrieved from the Global MF Analysis of the Pyrene Monomer and Excimer Fluorescence Decays Acquired with a 1.25 × 10−6 M PEO(16.5K)-Py2 Solution as a Function of SDS Concentration monomer

excimer

[SDS] mM

τ1, ns

a1

τ2, ns

a2

τM, ns

f Mfree

f Ediff

τE0, ns

f EE0

χ2

10.0 8.0 7.5 6.5 5.8 5.0 4.5 4.0 3.5 3.2 3.0

33.5 34.6 26.8 30.5 31.0 22.8 25.7 25.6 22.6 24.7 26.2

0.08 0.08 0.14 0.19 0.17 0.22 0.30 0.47 0.34 0.31 0.19

157 158 94 83 60 60 50 68 52 98 113

0.10 0.10 0.37 0.42 0.61 0.66 0.55 0.31 0.37 0.23 0.29

165 165 165 165 155 155 155 155 155 155 155

0.82 0.82 0.49 0.39 0.22 0.12 0.15 0.22 0.29 0.46 0.52

0.78 0.77 0.80 0.91 0.91 0.91 0.90 0.81 0.87 0.81 0.77

88 88 44 59 50 51 49 45 49 45 45

0.22 0.23 0.20 0.09 0.09 0.09 0.10 0.19 0.13 0.19 0.23

1.05 1.06 1.04 1.12 1.01 0.96 1.19 1.21 1.18 1.14 1.21

ground-state pyrene dimers.50,58 However, in this study, the spike is unlikely due to ground-state dimers because no spike was observed (see Figure S1 in the SI) with all PEO(X)-Py2 samples in aqueous solution where pyrenes are known to form groundstate aggregates.48 On the contrary, the spike was only observed when the pyrene monomers were located in SDS micelles where excimer should occur via diffusion. Therefore, the spike in the excimer decay is presumably due to the presence of a small amount of pyrene degradation products or pyrene impurities which emit with a short lifetime. Analysis of the fluorescence decays was started 1−5 channels after the instrument response function (IRF) maximum so that the spike was not included in the analysis of the fluorescence decays. As noted in an earlier publication,50 the spike is observed only under conditions where little excimer fluorescence is detected in the steady-state fluorescence spectra. The fits of the decays were good with all χ2 smaller than 1.30, and residuals and autocorrelation functions of the residuals randomly distributed around zero. Examples of the fit for PEO(5K)-Py2 at low and high SDS concentrations are shown in Figures S1 and S2, respectively. The decay times, pre-exponential factors ai, and fractions f Mdiff, f Mfree, f Ediff, and f EE0 retrieved from the analysis are listed in Tables 1−4. The fractions were used to determine the molar fractions of aggregated pyrenes ( f E0), pyrenes forming excimer by diffusional encounter (fdiff), isolated

pyrenes that do not form excimer ( f free) according to eqs S7−S9, while the average rate constant of excimer formation (⟨k⟩) was determined using eq S10. The fractions fdiff, f free, and f E0 and the rate constant ⟨k⟩ are listed in Table S1 in the SI and plotted as a function of SDS concentration in Figure 6. At small SDS concentrations, Figure 6 shows that in water most of the pyrene pendants are associated for PEO(2K)-Py2 and the molar fraction f E0 decreases from 0.69 ± 0.05 to 0.44 ± 0.01, and further to 0.16 ± 0.01 for PEO(X)-Py2 samples having an Mn of 2K, 5K, and 10K, respectively. For each sample, addition of a sufficient amount of SDS to the solution results in a drop of f E0 suggesting that SDS is disrupting the pyrene aggregates. At high SDS concentrations, f E0 decreases to around 0.05 for all PEO(X)-Py2 samples confirming the disappearance of the pyrene aggregates for large SDS concentration. However, f E0 remains larger than zero at high SDS concentration, suggesting that some residual pyrene aggregation is still present in the SDS micelles as has been found previously with Py-HASE.42,44 Figure 6 also indicates that, at low SDS concentrations, not all pyrene excimer is formed by direct excitation of the pyrene aggregates, and that some pyrene excimer is generated by diffusive encounters between an excited and a ground-state pyrene.48 The fraction of pyrenes forming excimer via diffusion, fdiff, at low SDS concentrations is found to equal 0.25 ± 0.04, 0.50 ± 0.01 and 0.71 ± 0.02 for PEO molecular weights of 2K, 5K, and 13170

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Figure 6. Fractions fdiff (◇), f free (□), and f E0 (△) and ⟨k⟩ (◆) as a function of SDS concentration for (A) PEO(2K)-Py2, (B) PEO(5K)-Py2, (C) PEO(10K)-Py2, and (D) PEO(16.5K)-Py2. Vertical line represents the CMC of SDS in water.

a dramatic increase of f free as more pyrene groups are isolated in different SDS micelles. After the CMC, f free retains a high value because almost all pyrene pendants are separated in different SDS micelles. In the study of the interactions taking place between Py-HASE and SDS,42,44 f free was found to peak at intermediate SDS concentrations below [SDS]IpE/IM due to the release of free pyrene pendants from the hydrophobic pyrene junctions upon addition of SDS. This observation was confirmed by surface tension measurements.44 However, this effect was not observed in this study because for the short-chain samples PEO(2K)-Py2 and PEO(5K)-Py2, the pyrene end groups are subjected to strong intramolecular hydrophobic interaction48 and they cannot be released into the aqueous phase as a free pyrene. On the other hand, f E0 is small for the long-chain samples PEO(10K)-Py2 and PEO(16.5K)-Py2 indicating that most pyrene end groups are not associated. Under this condition, the addition of SDS brings the pyrene pendants together instead of releasing more free pyrene groups into the solution as observed with Py-HASE. The average rate constants of pyrene excimer formation ⟨k⟩ were calculated for the PEO(X)-Py2 samples according to eq S10 and plotted as a function of SDS concentrations in Figure 6. At low SDS concentrations where SDS has little effect on the fluorescence behavior of PEO(X)-Py2, ⟨k⟩ decreases significantly with increasing polymer chain length as the pyrene end groups are held at a greater distance from each other.48,50 Further addition of SDS results in an increase of ⟨k⟩ due to two effects. First, SDS melts the pyrene aggregates of the short PEO(X)-Py2 constructs to allow more pyrene excimer to be formed by

10K, respectively. With an increase in SDS concentration, fdiff increases for PEO(2K)-Py2 and PEO(5K)-Py2 until it reaches a maximum value at the CMC of SDS in water above which it remains constant. However, for PEO(10K)-Py2 and PEO(16.5K)-Py2, fdiff decreases after passing through a maximum at an SDS concentration of 5 mM ([SDS]fpdiff = 5 mM). At this concentration, most pyrene groups are incorporated into mixed micelles as indicated by the I1/I3 ratio shown in Figure 5 and pyrene excimer is formed by diffusion. The molar fraction fdiff at [SDS]fpdiff of both PEO(10K)-Py2 and PEO(16.5K)-Py2 was found to equal ∼0.80. Increasing the SDS concentration past [SDS]fpdiff results in a drop in fdiff as the pyrene pendants distribute themselves into different micelles in a process that decreases the IE/IM ratio. At SDS concentrations higher than the CMC of SDS in water, PEO(10K)-Py2 and PEO(16.5K)-Py2 form little excimer. At low SDS concentrations, the fraction of pyrenes that do not form excimer, f free, is found to equal 0.05 ± 0.01, 0.05 ± 0.00, and 0.13 ± 0.02 for PEO molecular weights of 2K, 5K, and 10K, respectively. These nonzero f free values result from the presence of pyrene monolabeled PEO that act as fluorescent impurities for the shorter chains48,50 and pyrene groups which are too far to interact with each other for the longer chains,50 or a combination of both effects. For PEO(2K)-Py2 and PEO(5K)-Py2, f free remains small over the entire range of SDS concentrations within experimental error. PEO(10K)-Py2 shows a trend that is similar to those obtained with the PEO(2K)-Py2 and PEO(5K)I /I Py2 samples before [SDS]pE M. Increasing the SDS concentration fdiff past [SDS]p for PEO(10K)-Py2 and PEO(16.5K)-Py2 results in 13171

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Figure 7. Schematic overview of the interactions between SDS and PEO(X)-Py2 as a function of SDS concentration.

and an increase in fdiff in Figure 6. The rate constant ⟨k⟩ is more difficult to analyze, as it depends on a combination of many factors that include hydrophobic interactions between pyrene pendants in the aqueous phase and whether excimer formation occurs intra- or intermolecularly in the water phase or in the hydrophobic domains created by SDS. Increasing SDS concentration past [SDS]IpE/IM results in an increase in the number of pyrene pendants being incorporated into the mixed micelles as indicated by the continuous decrease in the I1/I3 ratio in Figure 5. With all pyrene groups being solubilized inside the mixed micelles, I1/I3 plateaus for SDS concentrations larger than the CMC and takes the constant value of 1.43 ± 0.02 for all PEO(X)-Py2 constructs. At all SDS concentrations, IE/IM took larger values for shorter PEO(X)-Py2 constructs. This effect is certainly due to the higher local pyrene concentration obtained with the shorter PEO(X)-Py2 samples. When the SDS concentration is larger than [SDS]IpE/IM, increasing amounts of SDS resulted in a decrease in both ⟨k⟩ and IE/IM due to a decrease in the local pyrene concentration. The pyrene groups distribute themselves into different hydrophobic junctions formed by the SDS molecules, isolating more pyrene end groups and preventing them to form an excimer. Addition of excess amounts of SDS results in the formation of free SDS micelles whose oily interior solvates the pyrene groups. Because the distance between pyrene end groups is constrained by the PEO chain length, they cannot be isolated in different SDS micelles for short PEO(X)-Py2 constructs and pyrene excimer is formed intramolecularly. In this concentration regime, fdiff is large and f E0 is small for PEO(2K)-Py 2 and PEO(5K)-Py 2 . Interestingly, the same fluorescence behavior was found for the PEO(2K)-Py2 and PEO(5K)-Py2 samples for SDS concentration larger than the CMC suggesting that intramolecular pyrene excimer formation is independent of polymer chain length. In the case of the longer PEO(10K)-Py 2 and PEO(16.5K)-Py 2 constructs, the pyrene end groups can be located in different SDS micelles which results in a large f free value in Figure 6. Above

diffusion. This process is accompanied by a decrease in fagg and an increase in fdiff as shown in Figure 6A and B. Second, adding SDS is expected to bring pyrene groups of different chains into a same micelle so that intra- and intermolecular pyrene excimer formation takes place by diffusional encounter between many pyrene units located in a same micelle which results in a larger ⟨k⟩. This effect is more predominant with the long-chain PEO(X)-Py2 constructs which do not form a large amount of pyrene aggregates at low polymer concentration. After passing through a maximum at an SDS concentration ([SDS]⟨k⟩ p ), ⟨k⟩ decreases and plateaus at the CMC of SDS in water. The decrease in ⟨k⟩ is due to the separation of the pyrene pendants into different SDS micelles. Interestingly, for SDS concentrations larger than the CMC, ⟨k⟩ takes a similar value of 6.4 (±1.4) × 10−6 s−1 for all PEO(X)-Py2 samples, suggesting that it represents the rate constant for pyrene excimer formation that occurs intramolecularly inside an SDS micelle and that it is independent of the polymer chain length. Although IE/IM of the long-chain samples takes a value that is close to zero and a large fraction f free is obtained in Figure 6C and D, there still remains a small fraction fdiff of pyrene pendants that form excimers intramolecularly inside SDS micelles for PEO(10K)-Py2 and PEO(16.5K)-Py2. These constructs form excimers with the same rate constant ⟨k⟩ as the PEO(2K)-Py2 and PEO(5K)-Py2 samples. Interactions between SDS and PEO(X)-Py2. It was established earlier48 that, in pure water without SDS, the shorter PEO(X)-Py2 constructs form pyrene aggregates intramolecularly while most pyrene pendants attached onto the longer PEO chains are isolated. Upon the addition of SDS, SDS molecules target the pyrene groups and form mixed micelles, which results in an increase in IE/IM. At this transition point, the I1/I3 ratio starts to decrease reflecting a change in the environment surrounding the pyrenes. For the short-chain samples, addition of SDS dissociates the pyrene aggregates and creates an environment that solubilizes the pyrene groups allowing them to form excimer by diffusion. This effect leads to a decrease in f E0 13172

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the CMC, ⟨k⟩ is constant for all PEO(X)-Py2 samples suggesting that the intramolecular formation of pyrene excimer inside SDS micelles is independent of PEO chain length. While the sequence of events is similar, with excimer formation being first promoted by intermolecular associations upon addition of SDS and then disfavored as the pyrene pendants are isolated in different micelles, the length of the PEO chain appear to have a major effect on the overall behavior of the PEO(X)-Py2 constructs. On the one hand, short PEO chains do not allow one pyrene end group to escape from the hydrophobic environment generated by either the other pyrene end group in water or an SDS micelle. On the other hand, longer chains act as efficient spacers that hold the two pyrene units away from each other whether in water or in different SDS micelles. The results obtained thus far have been summarized schematically in Figure 7 to describe the interactions taking place between SDS and the PEO(X)-Py2 samples. Figure 7 consists of two sequences of binding stages that apply for the short-chain samples, PEO(2K)-Py2 and PEO(5K)-Py2, on the one hand, and the long-chain samples, PEO(10K)-Py2 and PEO(16.5K)-Py2, on the other hand. At low SDS concentrations, SDS molecules target the pyrene pendants and bring them together to form mixed micelles, which results in an increase of the IE/IM ratio. Due to the stronger hydrophobic interactions between the pyrenes attached onto the short-chain samples and the shorter distance separating the pyrene end groups, it is more difficult to separate them inside the different hydrophobic domains that are created in solution upon addition of SDS, which leads to a larger IE/IM ratio observed than that of the longer chain samples. With excess amounts of SDS, free SDS micelles are formed and the pyrene end groups of the shorter chain samples cannot be isolated into different SDS micelles. This effect results in intramolecular excimer formation inside a same SDS micelle. On the other hand, most pyrene end groups attached onto the longer chain PEOs can be successfully separated and the PEO backbone can interact with the surface of the SDS micelles to reduce the electrostatic repulsion between two micelles and further insulate the hydrophobic domains of the micelles from the aqueous phase.

low SDS concentration and were found to increase with sample chain length. Unfortunately, f free could not be obtained for the PEO(16.5K)-Py2 constructs at low SDS concentration. Increasing SDS concentration past the CMC resulted in a significant increase of f free for PEO(10K)-Py2 and PEO(16.5K)-Py2 as most pyrene groups became isolated in different SDS micelles. However, the PEO(2K)-Py2 and PEO(5K)-Py2 samples were too short to allow SDS micelles to isolate the pyrene ends and pyrene excimer was still formed by intramolecular diffusion of the pyrene end groups inside the SDS micelles. This observation demonstrates that too short a PEO chain prevents PEO-Hyd2 to bridge two surfactant micelles which affects the ability of this polymer to form a network with important consequences for the rheological behavior of the solution of this polymer. A scheme describing the interactions between SDS and the short and long PEO(X)-Py2 constructs was proposed to rationalize the fluorescence results. This study demonstrated the important role of PEO chain length on the interaction of PEO-Hyd2 constructs with SDS in water.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the Model Free (MF) analysis and tables of the molar fractions and average rate constant retrieved from the MF analysis of the fluorescence decays. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Present Address §

S.C.: EOR Research Department, Petroleum Exploration & Production Research Institute, Sinopec, Beijing 100083, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to thank NSERC and a Petroleum Research Fund grant for financial support. S.C. and J.D. thank Alice Yang for her help in measuring the IE/IM ratios of the polymer samples.

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CONCLUSIONS The interactions between SDS and a series of PEO(X)-Py2 constructs were investigated in aqueous solution by ITC, potentiometry, conductimetry, and fluorescence measurements. ITC, EMF, and solution conductivity results showed that the binding between SDS and the PEO backbone takes place at higher polymer concentration. No such interactions were detected at the low polymer concentrations used for the fluorescence experiments. The pyrene monomer and excimer decays of PEO(X)-Py2 were globally fitted according to the MF analysis in the whole range of SDS concentrations. The fractions of the different excited pyrene species were calculated at different SDS concentrations. The molar fraction of aggregated pyrenes (f E0) was found to decrease with increasing polymer chain length and SDS concentration, indicating that SDS molecules decompose the pyrene aggregates. For PEO(2K)-Py2 and PEO(5K)-Py2, the molar fraction of pyrenes forming excimer by diffusional encounter (fdiff) increased with SDS concentration and remained constant above the CMC of SDS in water, while fdiff of PEO(10K)-Py2 and PEO(16.5K)-Py2 peaked at 5 mM SDS before decreasing with increasing SDS amounts. The molar fractions of isolated pyrenes that do not form excimer ( f free) for PEO(2K)-Py2, PEO(5K)-Py2, and PEO(10K)-Py2 are small at

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