Solvation Dynamics in Aqueous Polymer Solution and in Polymer

Surajit Chatterjee , Roopali Prajapati , Arpan Bhattacharya , and Tushar K. Mukherjee. Langmuir 2014 ..... Padmaja P. Mishra , Apurba L. Koner , Anind...
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J. Phys. Chem. B 2002, 106, 3763-3769

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ARTICLES Solvation Dynamics in Aqueous Polymer Solution and in Polymer-Surfactant Aggregate Sobhan Sen, Dipankar Sukul, Partha Dutta, and Kankan Bhattacharyya* Physical Chemistry Department, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: May 15, 2001; In Final Form: January 10, 2002

Solvation dynamics of 2,6-p-toluidinonaphthalene sulfonate (TNS) is studied using picosecond time-resolved emission spectroscopy in an aqueous solution of poly(vinylpyrrolidone) (PVP) and in a polymer-surfactant aggregate consisting of PVP and sodium dodecyl sulfate (SDS). On addition of PVP to an aqueous solution of TNS, the emission quantum yield (φf) of TNS increases about 210 times in 0.75 wt % aqueous solution of PVP. The solvation dynamics of TNS in 0.75 wt % aqueous solution of PVP is found to be biexponential with a major component (85%) of 60 ps and a slower one of 800 ps (15%). The retardation of the solvation dynamics in the dilute polymer solution compared to that in the bulk water is attributed to the restricted movement of water molecules in the vicinity of the polymer chains. The solvation dynamics of TNS in the PVP-SDS aggregate is described by two components, 300 ( 20 ps (55%) and 2500 ( 100 ps (45%). The slower solvation dynamics in PVP-SDS aggregate compared to PVP alone or SDS alone indicates serious restrictions on the mobility of the water molecule squeezed between polymer chains and micellar (SDS) surface.

1. Introduction Water molecules confined in a small volume play a key role in controlling the structure, dynamics, and reactivity of many natural systems. In the presence of a macromolecular chain, the three-dimensional hydrogen-bonded network of water is disrupted. Analysis of the Raman spectrum of the O-H stretching bond of water in aqueous polymer solutions indicates that polymer chains introduce hydrogen bond defects.1a In the case of polar polymers, the water molecules in the first hydration shell of the polymer form hydrogen bonds with it. Polymerwater interactions have been studied using dielectric relaxation.1b-d Dielectric relaxation of aqueous solutions of polymer exhibits a component1b-d which is slower by 2-3 orders of magnitude than that of pure water. Kaatze2a reported a dielectric relaxation time of 8 ps for pure water, while the more recent tera-Hertz measurement by Mittelman et al.2b indicate a major subpicosecond component. The dramatically slow relaxation component of polymer-bound water has implications in charge transport in polymer electrolytes.3 Recently, many groups have detected such a slow component of relaxation of water in many confined media using many different experimental techniques,4-18 and this has inspired a number of theoretical investigations.5,19 Among all the experimental techniques, time-resolved fluorescence Stokes’ shift (TDFSS) and the more recent three-photon echo peak shift stand out for their superior time resolution. As a result they have been used to study solvation dynamics of water in many environments such as proteins,4 cyclodextrin,6 DNA,8 reverse micelles,9,10 sol-gel matrix,11 lipid vesicles,12 water surface,14 and so on. It is observed that while solvation dynamics occurs in around a 1-ps time scale in bulk water,16 in * E-mail: [email protected]. Fax: (91)-33-473-2805.

many confined media the solvation dynamics of water exhibits component in the 100-1000 ps time scale.5-19 Though solvation dynamics of water molecules in different confined environments have been extensively studied, there is no such report for aqueous solutions of polymer. Hydrophilic and hydrophobic interactions strongly influence conformation of water-soluble polymers in aqueous solutions. Water molecules present in the hydration layer of polymers are “restricted” and the study of dynamics in aqueous polymer solutions is of fundamental importance to understand the behavior of biological macromolecules. Maeda et al. used Raman spectroscopy1a and Desbrieres et al. used quasi-elastic light scattering20a to study the behavior of aqueous polymer solutions. Argaman and Huppert have studied solvation dynamics in neat liquid polyethers, CH3-(OCH2CH2)n-OCH3 (n ) 2-4).18 Apart from the major ultrafast subpicosecond components, they detected two relatively long components in the 10 and 100 ps time scales, which they attributed to the motion of the polyethers. In the present work, we have studied the solvation dynamics of water molecules in an aqueous solution of poly(vinylpyrrolidone) (PVP, molecular weight ) 29 000 Da) using 2,6-p-toluidinonaphthalene sulfonate (TNS) as a probe. TNS is a well-known probe for many biological systems.21 In aqueous solution, the emission quantum yield of TNS is very small (0.001) and the lifetime is also very short (60 ps). On binding to various organized media, as the probe TNS is transferred from bulk water to the less polar interior of the organized media, the emission intensity increases markedly.21 The fluorescence enhancement of TNS in organized media is attributed to suppression of the main nonradiative pathway, namely, intramolecular charge transfer (ICT) in the relatively nonpolar interior of the organized assemblies.22,23 TNS has recently been used

10.1021/jp0118672 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/23/2002

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SCHEME 1: Structure of Polymer-Surfactant Aggregate

Figure 1. Distribution of the particle size of 0.75 wt % aqueous PVP solution by dynamic light scattering experiment.

TABLE 1: Dynamic Light Scattering Data

system

to study solvation dynamics in many biological systems.24 Zewail et al. detected subpicosecond components in the solvation dynamics of TNS in histone.24a Pierce and Boxer 24b and Bashkin et al.24c on the other hand, reported solvation dynamics of TNS on a 10 ns time scale in other proteins. In this work, we have employed solvation dynamics of TNS to understand the interaction of PVP with a surfactant, sodium dodecyl sulfate (SDS). Interaction of water-soluble polymers with surfactants in aqueous solutions has attracted a lot of recent attention because of its various biological and technological implications.25-31 The structure of polymer-surfactant aggregates is very different from that of the polymer or the surfactant assemblies. According to the “necklace model”, the polymer-surfactant aggregates consist of spherical micellar beads, which are surrounded by polymer chains and connected by polymer strands (Scheme 1).26-31 Such a structure is quite different from that of a spherical micelle. In a micelle, the surface of the micelle remains largely exposed to bulk water. But in a polymer-surfactant aggregate, the surface of the micelle remains shielded from bulk water by polymer chains. Among the various polymer-surfactant aggregates, the one involving the polymer, poly(vinylpyrrolidone) (PVP), and the surfactant sodium dodecyl sulfate (SDS) is perhaps most wellstudied.25-31 We have earlier demonstrated that shielding of the micellar surface by polymer chains results in dramatic retardation of the excited-state proton-transfer process,30a fluorescent anisotropy decay, and photoisomerization.30b In the present work, we focus our attention on the solvation dynamics of water molecules squeezed between the polymer chain and SDS micelles, in a PVP-SDS aggregate. 2. Experimental Section 2,6-p-Toluidinonaphthalene sulfonate, sodium salt of TNS (Sigma) is purified by repeated recrystallization from a methanolwater mixture. Poly(vinylpyrrolidone) (PVP, molecular weight, MW ) 29 000 Da, Aldrich) and sodium dodecyl sulfate (SDS, Aldrich) were used as received. The steady-state absorption and emission spectra were recorded in a JASCO 7850 spectrophotometer and a Perkin-Elmer 44B spectrofluorimeter, respectively. The extinction coefficient of TNS at 300 nm is 17 300 M-1 cm-1. The intrinsic viscosity of the polymer solution is determined using a Schruz-Immergut type variable shear viscometer.

12 mM SDS 0.75 wt % PVP 0.75 wt % PVP + 12 mM SDS

hydrodynamic diam, nm

diffusion coefficient × 106, cm2/s

polydispersity index

0.9 3.6 6.4

5.69 1.54 0.79

0.31 0.41 0.60

For lifetime measurements, the sample was excited at 300 nm by the second harmonic of a rhodamine 6G dual jet dye laser with DODCI as saturable absorber (Coherent 702-1), synchronously pumped by a CW mode-locked Nd:YAG laser (Coherent Antares 76s). The emission was collected at magic angle polarization using a Hamamatsu MCP photomultiplier (2809U). Our time correlated single photon counting (TCSPC) set up consists of Ortec 935 QUAD CFD and Tennelec TC 863 TAC. The data are collected with a PCA3 card (Oxford) as a multichannel analyzer. The typical fwhm of the system response is about 50 ps. Dynamic light scattering (DLS) data were recorded in a DLS700 instrument (Otsuka Electronics Co. Ltd., Japan) fitted with a 5 mW He-Ne laser, operating at 632.8 nm, by placing the sample tube in the thermostated chamber of the goniometer. All measurements were taken at 90°. The DLS intensity data were processed by using the instrumental software to obtain the hydrodynamic diameter, the polydispersity index, and the diffusion coefficient of the samples. 3. Results 3.1. Dynamic Light Scattering and Viscosity Data. The dynamic light scattering data (DLS) of 0.75 wt % PVP (7.5 mg/mL) in water is shown in Figure 1. The average hydrodynamic diameter of the sample is found to be 3.6 nm. The polydispersity index and the diffusion coefficient are listed in Table 1. Since Figure 1 clearly shows a single Gaussian distribution of the particle size, there is no aggregation of the polymer at this concentration (0.75 wt %). The macroscopic viscosity of 0.75 wt % aqueous PVP solution is found to be 1.25 cP at 25 °C and intrinsic viscosity is 5.17 M-1 (segmentwise). The DLS data of 12 mM SDS indicates an average hydrodynamic diameter of 0.9 nm. The hydrodynamic diameter in a 0.75 wt % PVP solution containing 12 mM SDS is found to be 6.4 nm. Thus the hydrodynamic diameter of the PVPSDS aggregate is greater than that of PVP alone and SDS alone. The very high hydrodynamic diameter of the PVP-SDS aggregate indicates formation of a “necklace” consisting of PVP and SDS. The increase in hydrodynamic diameter on addition of SDS to PVP is consistent with fluorescence correlation spectroscopic studies.26 The diffusion coefficients in SDS alone and PVP-SDS aggregates are listed in Table 1.

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Figure 2. Absorption spectrum of 1 × 10-5 M TNS in 0.75 wt % aqueous PVP solution.

Figure 4. (a) Steady-state emission spectra of 1 × 10-5 M TNS in an aqueous solution containing (i) 80 mM SDS, (ii) 0.75 wt % PVP, (iii) 0.75 wt % PVP, and 12 mM SDS. (b) Variation of quantum yield (φf) of 1 × 10-5 M TNS in an aqueous solution containing 0.75 wt % PVP with increasing concentrations of SDS.

Figure 3. (a) Steady-state emission spectra (λex ) 315 nm) of aqueous solution of 1 × 10-5 M TNS in the presence of (i-xiv) 0.006, 0.012, 0.025, 0.05, 0.075, 0.10, 0.20, 0.30, 0.40, 0.60, 0.75, 1.0, 1.5, and 2.0 wt % PVP. (b) Variation of quantum yield (φf) of 1 × 10-5 M TNS with increasing concentrations of PVP.

3.2. Steady-State Spectra. 3.2.1. TNS in PVP. Figure 2 shows absorption spectrum of a 1 × 10-5 M aqueous solution of TNS containing 0.75 wt % PVP. The emission quantum yield (φf) of TNS is very small (0.001) in pure water with the emission maximum at ∼465 nm. On addition of PVP to an aqueous solution containing 1 × 10-5 M TNS, φf of TNS increases about 210 times from 0.001 in water to 0.21 in 0.75 wt % PVP in water (Figure 3). Along with this, the emission maximum exhibits a blue shift from 465 nm in water to 455 nm in 0.75 wt % PVP (Figure 3). The 210-fold increase in φf of TNS in the presence of PVP relative to water may be ascribed to the shielding of polymer-bound TNS from bulk water by polymer

chains and consequent suppression of the nonradiative ICT process of TNS.22,23 The absorption and excitation spectra of TNS in 0.75 wt % PVP in water are identical to those in pure water. The plot of φf against PVP concentration (Figure 3b) indicates that the φf saturates above a PVP concentration of 0.6 wt %. This suggests that above this concentration of PVP one can assume almost all the TNS molecules present in the solution remain bound to the polymer and experience a microenvironment very different from that of bulk water. Thus the time-resolved studies are carried out in 0.75 wt % PVP solution. 3.2.2. TNS in PVP-SDS Aggregate. On addition of SDS to an aqueous solution of TNS containing 0.75 wt % PVP, φf increases sharply above a SDS concentration of ≈1 mM up to 0.31 at a SDS concentration of g10 mM SDS (Figure 4). The emission maximum of TNS in a solution containing 12 mM SDS and 0.75 wt % PVP is found to be at 445 nm, i.e., blueshifted by 10 nm from that in PVP alone. This indicates that the microenvironment of TNS in PVP-SDS aggregates is very different from that in PVP alone. In the absence of PVP, in water the emission intensity of TNS increases marginally on addition of 12 mM SDS, while the emission maximum is very similar to that in water.22 The microenvironment of TNS in

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Figure 5. Fluorescence decays of 1 × 10-5 M TNS in the aqueous solution of 0.75 wt % PVP at (i) 405, (ii) 455, and (iii) 550 nm.

Figure 7. Decay of the response function, C(t) of 1 × 10-5 M TNS in an aqueous solution of 0.75 wt % PVP (9) and in an aqueous solution containing 0.75 wt % PVP and 12 mM SDS (0). The points denote the actual values of C(t) and the solid line denotes the best fit to a biexponential decay. The inset shows initial portion of the two decays.

TABLE 2: Decay Parameters of C(t) of TNS Bound to 0.75 wt % Aqueous PVP Solution and PVP-SDS Aggregate Containing 0.75 wt % PVP and 12 mM SDS system

∆ν, cm-1

a1

τ1,a ps

a2

τ2,a ps

,a,b ps

PVP PVP + SDS

900 625

0.85 0.55

60 300

0.15 0.45

800 2500

170 1290

a

Figure 6. Time-resolved emission spectra of 1 × 10-5 M TNS in aqueous solution of 0.75 wt % PVP at 25 (9), 50 (∆), 200 (b), 3000 ps (0).

PVP-SDS aggregate is very different from that in SDS micelles, because even at a SDS concentration of 80 mM (i.e., 10 times cmc) the φf of TNS remains about 20 times less than that in 0.75 wt % PVP and 12 mM SDS. 3.3. Time-Resolved Studies. 3.3.1. TNS in PVP. In pure water, the lifetime of TNS is very short (60 ps). In 0.75 wt % aqueous PVP solution, the fluorescence decays of TNS exhibit a very long component of about 8 ns and the fluorescence decays are found to be markedly dependent on the emission wavelength (Figure 5). For example, at 405 nm (blue end) the fluorescence decay is biexponential with two decay components of 670 ps (25%) and 7.4 ns (75%), while at 550 nm (red end) the decay component is 9.2 ns, which is preceded by a distinct growth component with a rise time of 60 ps. Such a wavelength dependence clearly indicates that in aqueous PVP solution TNS molecules undergo solvation dynamics. Following the procedure given by Fleming and Maroncelli,15c the time-resolved emission spectra (TRES) have been constructed by using the parameters of best fit to the fluorescence decays and the steady-state emission spectrum. The TRES clearly show a time-dependent Stokes shift of the emission of TNS in 0.75 wt % PVP solution (Figure 6). The solvation dynamics is described by the decay of the solvent correlation function C(t), defined as

C(t) )

ν(t) - ν(∞) ν(0) - ν(∞)

where ν(0), ν(t), and ν(∞) are the peak frequencies at time 0, t, and ∞, respectively. The decay of C(t) is shown in Figure 7 and the decay parameters are summarized in the Table 2. It shows that decay of C(t) is biexponential, with a very fast

( 5%. b ) a1τ1 + a2τ2.

component of 60 ps (85%) and a slow component of 800 ps (15%). The total Stokes shift of the TNS emission is found to be 900 cm-1. Following Fee and Maroncelli,15d one may calculate the amount of solvation missed in a picosecond setup. According to this method, the difference between the emission frequency at time zero (νpem(0)) and the absorption frequency in a polar solvent (νpabs), is approximately equal to the difference between steady-state frequencies of emission (νnpem) and absorption (νnpabs) in a nonpolar solvent. So that

νpem (0) ) νpabs - [νnpabs - νnpem] Using 1,4-dioxane as the nonpolar solvent for TNS, νpem(0) for TNS bound to PVP is calculated to be 23 365 cm-1. From our time-resolved data νpem(0) is found to be 23 030 cm-1 and νpem(∞) is 22 130 cm-1. Thus, in our picosecond set up we have missed 27% of the total dynamic spectral shift. It may be recalled that even in a femtosecond setup a substantial part of the solvation is often missed. For instance even in a femtosecond setup Fleming et al.16b missed about 25% of total solvation of coumarin 343 in water. 3.3.2. TNS in PVP-SDS. In 80 mM SDS, the lifetime of TNS is 700 ( 20 ps and the emission decay exhibits no dependence on emission wavelength (Figure 8). This suggests that TNS bound to SDS micelles does not exhibit solvation dynamics, presumably because solvation dynamics of TNS in SDS is much faster than the response time (50 ps) of our setup. In an aqueous solution containing 0.75 wt % PVP and 12 mM SDS, the emission decays of TNS are found to be markedly wavelength dependent (Figure 9). A decay is observed at the blue end of the emission spectrum, while at the red end, the decay is preceded by a distinct growth. For instance, at the blue end (390 nm) the decay is fitted to a biexponential with two components of 600 ps (55%) and 7.3 ns (45%). At the red end, e.g., at 530 nm, the temporal decay of TNS in PVP-SDS aggregates exhibits a rise time of 290 ps and decay of 9.5 ns.

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Figure 8. Fluorescence decay of 1 × 10-5 M TNS in an aqueous solution containing 80 mM SDS at 460 nm. Figure 10. Time-resolved emission spectra of TNS in an aqueous solution containing 0.75 wt % PVP and 12 mM SDS at 0 (9), 500 (∆), and 10 000 ps (b).

C*, different polymer chains entangle with each other and make a pseudo-network.20b The value of C* is given by1a, 20b

C* )

Figure 9. Fluorescence decays of 1 × 10-5 M TNS in an aqueous solution containing 0.75 wt % PVP and 12 mM SDS at (i-iv) 390, 420, 450, and 530 nm.

Such a decay at the blue end and rise at the red end indicate that TNS undergoes solvation dynamics in PVP-SDS aggregates at a time scale >50 ps. From the steady-state emission spectra and fluorescence decays, the time-resolved emission spectra (TRES, Figure 10) of TNS are constructed following the procedure described by Maroncelli and Fleming.15c The decay of C(t) in the case of the PVP-SDS aggregate is shown along with that for PVP alone in Figure 6. The decay of C(t) for TNS bound to PVP-SDS aggregate is found to be biexponential with one component of 300 ( 20 ps (55%) and another of 2500 ( 100 ps (45%) (Table 1). Evidently, as shown in Figure 6, decay of C(t) is much faster in the case of PVP alone compared to PVP-SDS aggregate. Following the FeeMaroncelli procedure we found that we missed about 28% of total solvation dynamics. 4. Discussions The most important finding of this work is obviously the significantly slow solvation dynamics in the polymer environment or inside the polymer-surfactant aggregate. In a 0.75 wt% aqueous solution of PVP, the solvation dynamics is found to be biexponential with a major component (85%) of 60 ps and a slower one of 800 ps (15%). However, in PVP-SDS aggregate the solvation dynamics is found to be much slower with components of 300 ps (55%) and 2500 ps (45%). Before discussing this, it is necessary to recapitulate the behavior of polymers in aqueous solutions. An important feature of aqueous solution of a polymer is the interaction between polymer chains.1a,20 In a sufficiently dilute solution, when the concentration of the polymer is below the crossover value (C*), each polymer chain remains isolated from others. At a concentration below C*, the polymer chains undergo self-entanglement and the probe TNS along with water molecules remain trapped inside the pseudo-network formed by selfentangled polymer chains. At a polymer concentration above

3Ns 4π3/2

where, NS is degree of polymerization (i.e., the number of polymer segments) and is the unperturbed mean square radius of gyration of the polymer. In the present case of PVP, MW is 29 000 and Ns is 261. The unperturbed radius of gyration (Rg) is R0/x6. From polymer handbook20c for PVP in water at 25 °C, (R0/MW1/2) x 104 (nm) is 720 ( 40. Thus for a MW ) 29 000, the radius of gyration (Rg) is 5 nm. Using the value of radius of gyration (Rg), the segmentwise C* is calculated to be 0.83 M for PVP. In the present case, a 0.75 wt % aqueous solution of PVP (MW ) 29 000) corresponds to 0.07 M (segmentwise). Obviously, this concentration of PVP (0.07 M) is much below the crossover region (0.83 M). As a result, we are dealing with self-entangled polymer chains. We will now discuss the observed slow solvation dynamics. In 0.75 wt % PVP in water, TNS exhibits bimodal solvation dynamics with a major (85%) fast component of 60 ps and a slow component of 800 ps (15%). In our setup with an instrument response function of 50 ps, we have obviously missed the components of solvation with a time scale faster than 50 ps. Dielectric relaxation studies on aqueous PVP solutions indicate that polymer-bound water molecules displays relaxation in nanosecond time scale.1b-d According to the continuum theory,15 the solvation time, τs, is (∞/0) τD, where ∞ and 0 are the dielectric constants at infinite and zero frequency, respectively, and τD is the Debye relaxation time. The observed emission maximum (455 nm) of TNS in PVP corresponds to a methanol-like environment (with ET(30) ≈ 55, 0 ) 32.5).23b The high-frequency dielectric constant (∞) for aqueous PVP environment may be assumed to be same (≈5) as that of water.15e Using a τD ≈ 1 ns, τS for the PVP environment is calculated to be ∼150 ps. This crude estimate is between the 60 and 800 ps components detected by us. Evidently, the long τD in aqueous PVP solution1b is responsible at least partially for the slow components of solvation dynamics. There could be several reasons for the observed slow component of solvation dynamics and dielectric relaxation. According to Nandi and Bagchi,5 aqueous solutions of macromolecules involve a dynamic equilibrium between the “free” bulk water molecules and the “bound” water molecules. The “bound” water molecules refer to those which are hydrogen

3768 J. Phys. Chem. B, Vol. 106, No. 15, 2002 bonded to the macromolecule; as a result their motion becomes coupled with that of the slow macromolecules. In one of the early MD simulations of two Lennard-Jones solute particles and 214 water molecules, Stillinger et al.33 found that in the first solvation shell of the hydrophobic solute, the translational and rotational motion of the water molecules are at least 20% slower than those in bulk water. This, however, does not explain quantitatively the 60-800 times slower solvation dynamics of water in the vicinity of the polymer chains, observed in this work. Olander and Nitzan carried out a molecular dynamics simulation of polyethers.32 They detected a long component of the order of 100 ps and attributed this to segmental motion of the polymer chains. The structures of polyethers and PVP are very different, and, hence, it is not reasonable to compare the simulations on polyethers with our work on PVP. The major component (85%) of the 60 ps detected in our work for PVP is somewhat close to the 100-ps component detected by Olander and Nitzan. However, the 800-ps component detected by us is much slower. Also, the 300 and 2500 ps components observed in the PVP-SDS aggregate are much slower compared to the 100-ps component detected by Olander and Nitzan. In a very recent simulation carried out by Michael and Benjamin,19a it is found that the solvation dynamics at liquid-liquid interfaces exhibits a slow component which is absent in bulk water or at the water-vapor interface.14 The polymer-surfactant aggregate may be considered as a liquidliquid interface. Thus, the present study is qualitatively consistent with the simulation carried out by Michael and Benjamin.19a The subnanosecond and a few nanosecond component in polymer and polymer-surfactant aggregates, respectively, cannot be due to the chain dynamics of surfactants which occurs in the 100-ns time scale.34 Still one cannot rule out completely the role of local motion of the polar headgroups of polymer or surfactant. But it should be noted, if the slow component is due to the local motion of polar headgroups of the polymer or of the surfactant, they are unlikely to be affected so dramatically in PVP-SDS compared to PVP alone and SDS alone. We therefore feel the role of local motion of polar headgroups is relatively unimportant in explaining the slow component of solvation dynamics. 5. Conclusions In aqueous solutions of the polymer PVP, TNS molecules bind strongly with the polymer. The polymer-bound TNS molecule exhibits nearly 210-fold increase in fluorescence intensity. This is attributed to the suppression of the nonradiative pathway in the relatively less polar microenvironment where the polymer chain wraps around the TNS molecule and shields it from bulk water. The polymer-bound TNS molecule displays substantially slower solvation dynamics compared to bulk water. Compared to the about 1-ps solvation time in bulk water, polymer-bound TNS exhibits a major component of 60 ps and a minor component of 800 ps. The slow component of solvation dynamics in aqueous polymer solutions is consistent with the dielectric relaxation studies,1 the early MD simulations of relaxation dynamics around a hydrophobic solute in water,26 and an analytical model.5 The slow relaxation dynamics is attributed to the binding of the water molecules to the slow macromolecules. Perhaps the most interesting observation is that solvation dynamics in PVP-SDS aggregate is very different and slower than that in PVP alone or SDS alone. Solvation dynamics in the PVP-SDS aggregate (of components 300 and 2500 ps) is found to be dramatically slower than that in PVP alone or in SDS alone. This indicates that the contribution of

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