Article pubs.acs.org/Macromolecules
Hydration of Sodium Alginate in Aqueous Solution Kamila Mazur,†,* Richard Buchner,‡ Mischa Bonn,† and Johannes Hunger† †
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Institut für Physikalische und Theoretische Chemie, Universität Regensburg, 93040 Regensburg, Germany
‡
ABSTRACT: Alginates are naturally occurring biocompatible polysaccharides. They have a broad range of applications, mainly in connection to their ability to control the rheology of aqueous solutions. Specifically, addition of a small amount of alginate (10% wt) leads to a ∼100-fold increase in viscosity. Here we explore whether that pronounced retardation of the long-range correlations is accompanied by molecularlevel changes of the water structure. We employ viscometry, dielectric spectroscopy (DS) and femtosecond infrared (fs-IR) pump−probe spectroscopy to study water dynamics in sodium alginate solutions. Remarkably, despite the large rheological effects of alginates in solution, the rotational dynamics of water are remarkably similar to those observed in bulk water. Only a small subensemble of water molecules is slowed down significantly, amounting to 6 ± 2 water molecules per saccharide unit. Furthermore, DS measurements reveal an additional ∼5 water molecules to be slowed down by the counterion (Na+). Our results reveal that the effect of alginate on the dynamics of water is restricted to the first hydration shell. This indicates that the large viscosity increase is determined by the polysaccharide network, with large water pools present between the polysaccharide chains.
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INTRODUCTION Alginate is a water-soluble polysaccharide, which is isolated from brown algal species. It is built up of two uronic acids residues, L-guluronic (G) and D-mannuronic acid (M). Alginates are biodegradable, biocompatible, and nontoxic. Their most important property is related to their viscosifying, stabilizing and gelling properties as well as their ability to retain water. Owing to these properties, they have a broad range of applications, mainly in food industry as thickeners, stabilizing agents and emulsifiers1 and they are also used in the cosmetic and drug industry.2 Thus, it is of technological interest to understand the mechanism of how alginate affects the overall dynamics of soft matter samples. Such insights are also interesting from a fundamental point of view. Already in relatively small amounts, alginate has a dramatic effect on the rheology of aqueous solutions (Figure 1) − the question that presents itself is how the alginate interacts with water to bring about such large changes: can the local interactions of the alginate with the water be related to the global effect of the polysaccharide on the rheological properties? While the hydration of monosaccharides or disaccharides has been studied intensively,3−6 there are only few reports on the water dynamics in the hydrations shell of polysaccharides.7,8 In particular, the effect of polyelectrolyte-based polysaccharides on the dynamics of water are of paramount interest, as in principle Coulomb forces may lead to longer-ranged interactions, compared to neutral polysaccharides. Naturally, the hydration water itself plays an important role in structure, dynamics, and functionality of biomolecules as the hydrating water molecules determine charge screening and the dissociation of the polyelectrolyte. Thus, their hydration has been a subject of intense studies in recent years. © 2014 American Chemical Society
Figure 1. Solution viscosities (red squares, left axis) and densities (green dots, right axis) of NaAlg solutions plotted against the saccharide unit concentration.
Neutron scattering experiments have demonstrated that water determines the confirmation as well as the flexibility of alginate chains.9 The molecular structure and dynamics of water-alginate systems depend on the alginate concentration, temperature, chain length, and nature of the counterion. Monovalent cations form soluble mixtures with alginic acid. Dissolved in water, alginate with monovalent ions dissociate, releasing ions into water. Multivalent ions, except Mg2+, form gels. Received: November 18, 2013 Revised: January 9, 2014 Published: January 14, 2014 771
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Physics, Spitfire ace amplifier) are converted to midinfrared pulses. This is achieved using an optical parametric amplifier (OPA) which through nonlinear optical processes generates IR pulses at the wavelength of the OD-stretch vibration, 4 μm.11 For the fs-IR experiment,12,13 a small fraction of the IR beam is then split off, using a wedged CaF2 window, where front and back reflections are used as a probe and a reference pulse. The portion (∼90%) transmitted through the wedged window is used as the pump pulse. A λ/2 plate sets the polarization of the pump pulse to 45° with respect to the polarization of the probe beam. The timing of the pump pulse with respect to the probe pulse is varied using a delay line. Pump, probe, and reference are focused onto the sample by parabolic mirrors, where the foci of the pump and the probe coincide. After the sample, all beams are recollimated by a second, identical parabolic mirror placed after the sample. A rotating wire-grid polarizer allows selecting the polarization components of the probe beam, parallel or perpendicular to the pump pulse polarization. Using an imaging spectrograph (Horiba Scientific, Triax 180), the probe and the reference beam are spectrally dispersed onto a liquid-nitrogen-cooled mercury−cadmium-telluride array detector (2 × 32 pixels). An optical chopper placed in the pump pathway periodically blocks every second pump pulse which allows for active background subtraction. The reference pulse is used to correct for pulse-to-pulse energy fluctuations in the probe beam. The pump induced change in absorption, Δα can thus be expressed as
In this work we use dielectric spectroscopy (DS) and timeresolved femtosecond infrared spectroscopy (fs-IR) to probe water dynamics in aqueous solutions of sodium alginate (NaAlg) over a wide concentration range at a temperature of 23 °C. Both techniques probe reorientational dynamics of water. However, they probe the rotational motion of water along different axes, i.e. fs-IR probes reorientation of the OH (or equivalent OD) bond of water, while DS is sensitive to the reorientation of the permanent dipole of water molecules. Both techniques principally probe the local dynamics. In the present study, we compare these results to the longer-ranged correlations as reflected in the macroscopic viscosity of the solutions.
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EXPERIMENTAL SECTION
Sodium alginate, from Macrocystispyrifera, was purchased from SigmaAldrich and was used without further purification. Aqueous solutions at saccharide unit concentrations ranging from 0.05 mol/L to 1 mol/L were prepared by dissolving sodium alginate in Millipore Milli-Q water. To all solutions a small amount of D2O (typically 5%) was added leading to the formation of HOD molecules, which are required for fs-IR spectroscopy. Prior to measurements, all solutions were centrifuged for 20 min at 13000 rpm. Dynamic viscosities were measured using an automated rolling ball viscometer, Anton PaarAMVn with stated uncertainty ≤0.005. Solution densities, required for the analysis of the DS results, were measured using a vibrating-tube densimeter, Anton Paar DMA 5000 M to an accuracy of ±0.005 kg m−3. Both viscosity and density measurements were performed at 25 °C (±0.05 °C). Dielectric Spectroscopy. The dielectric measurements were carried out at frequencies ranging from 0.2 to 70 GHz using a vector network analyzer (Anritsu MS4647A) with an open-ended coaxial line sensor. The details on the calibration of the instrument and the extraction of the dielectric spectra can be found elsewhere.10 In the dielectric spectroscopy experiment, the total polarization of a sample is probed as a response to an externally applied electric field, E(ν), where ν is the field frequency. The sample response can be expressed in terms of the complex dielectric spectrum, ε*(ν) = ε′(ν) − iε″(ν), where the real permittivity, ε′(ν)represents the in-phase polarization. The imaginary part, ε″(ν) (the dielectric loss) corresponds to the outof-phase polarization component. For molecular species, polarization originates from the alignment of their permanent dipole moments along the electric field. If the step response function of the polarization follows a single exponential decay, the dielectric spectra can be described with a Debye function with the relaxation time, τ. For multicomponent solutions, several relaxation processes can be observed. These can be modeled by a sum of individual relaxation processes: n
ε*(ν) = ε∞ +
∑ j=1
Sj 1 − αj βj
[1 + (2πνiτj)
]
+
σ0 2πνiε0
⎡ I ⎤ probe/ Ireference ⎥ Δα = − ln⎢ ⎢⎣ Iprobe,0/Ireference,0 ⎥⎦
(2)
with the frequency dependent intensities of the probe, Iprobe, and reference pulses, Ireference. The subscript “0” denotes the IR intensities in the absence of the pump pulse. The isotropic absorption changes recorded at the magic angle (or alternatively constructed from the parallel and perpendicular signals Δαiso = (ΔαII + 2Δα⊥)/3) are independent of any rotational dynamics and solely represents the population dynamics. The pump pulse excites the OD-stretch mode of HOD, which leads to the change in absorption, Δαiso due to three contributions. First, the pump pulse creates the population in the first excited state of HOD molecules (0 → 1 transition). This results in a decrease in absorbance due to a depopulation of OD groups in the ground state. Stimulated emission (1 → 0) further decreases the transient absorption spectra. Because of the anharmonicity of the OD potential, the 1 → 2 transition has lower energy than the 0 → 1 and thus results in a red-shifted induced absorption. The vibrational excitation changes in the absorption induced by the pump will decay as a function of time, t. This characteristic decay time, i.e., the vibrational lifetime of the OD-stretch vibration of HOD in H2O is 1.8 ± 0.2 ps.14As the vibrational excitation decays, the absorbed energy is dissipated in the sample and will eventually result in a small rise of the temperature. This is manifested by a persistent absorbance change at the long delay times (t > 10 ps). This thermal contribution was subtracted following the procedures described previously.14 Δα is not only time dependent, but it also depends on the relative polarization of the pump and probe beams. The pump beam excites most efficiently those HOD molecules which have their transition dipole moment aligned parallel to the pump polarization, thus creating an anisotropic distribution of excited molecules. Because of molecular rotation the excited molecules lose their orientational memory and eventually their distribution becomes isotropic. Measuring Δα in the polarization perpendicular (Δα⊥) and parallel (ΔαII) with respect to the pump polarization enables to construct the anisotropy parameter, R(t), which reflects the orientational memory of the molecules:
(1)
where 0 ≤ αj < 1 and 0 < βj ≤ 1 account for symmetric and asymmetric broadening of the relaxation modes, respectively. For α = 0 and β = 1, the relaxation corresponds to a Debye function. For the formal description of the experimental spectra of aqueous solutions of NaAlg we tested several models based on different band-shapes and number of relaxation processes, n. Here, Sj is the relaxation amplitude and τj is the relaxation time of each individual process. ε∞ is the permittivity at infinite frequencies, which accounts for polarization effects like vibrations, librations, etc., which are resonant at frequencies outside the present experimental frequency range. For electrically conducting samples, the direct current (dc) conductivity (σ0) associated with charge transport creates an additional Ohmic loss term, which is accounted for by the last term in eq 1, with ε0 being the permittivity of free space. Femtosecond Infrared Spectroscopy. In the fs-IR experiments, laser pulses (800 nm, 50 fs) from a regenerative amplifier (Spectra-
R(t ) =
ΔαII − Δα⊥ ΔαII + 2Δα⊥
(3)
Thus, obtained R(t) decays are averaged over frequencies ranging from 2420 to 2550 cm−1 as they are independent of absorption frequency.14 For neat water R(t) exhibits a single exponential decay with a characteristic decay time of 2.5 ps,14 which directly corresponds 772
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to the second order rotational correlation time of the excited HOD molecules in the sample.15 In contrast to that, the dielectric relaxation times can be related to the first-order rotation time. As a result, the time constants measured with the two techniques differ by a factor of ∼3.16,17
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RESULTS Dielectric Spectroscopy. The dielectric spectra of aqueous solutions of NaAlg as a function of concentration are shown in Figure 2. All spectra are dominated by a pronounced peak in ε″
Figure 3. Dielectric permittivity (ε′) and dielectric loss spectra (ε″) for a solution of 0.29 mol/L NaAlg. Symbols are experimental spectra and solid lines show the fit using eq 1. The magenta symbols represent the total loss spectra, while green symbols show loss spectra with the conductivity contribution subtracted (last term of eq 1). The shaded areas indicate the contribution of the counterion fluctuation and the water reorientational relaxation, respectively.
from 0.05 to 0.49 mol/L. Thus, our results are consistent with the solute relaxation being due to the fluctuations of the Na+ counterions. However, due to the finite conductance of the samples an additional Ohmic loss contribution arises (Figure 3, magenta symbols) that rapidly increases with increasing concentration and hampers the accurate separation of lowfrequency modes like the present counterion fluctuations. Moreover, orientational relaxation of water molecules hydrating the hydrophobic polysaccharide fragments may also contribute at low frequencies.28,42 For these reasons, a more detailed analysis of this lower frequency mode and in particular a meaningful discussion of its amplitude is not feasible at present and has to be postponed until more accurate data can be obtained in the 0.01−0.5 GHz region. The relaxation mode centered at 18 GHz (SH2O, τH2O) originates from the alignment of the permanent electric water dipoles, against thermal motion, to the externally applied field.24 Using eq 1 to model the experimental spectra, we extract a water reorientation time of τH2O = 8.9 ps, which is virtually constant over the investigated concentration range. This indicates that the dynamics of bulk-like water molecules are not affected by the polyelectrolyte. The amplitude of the relaxation mode SH2O associated with bulk water decreases with increasing NaAlg concentration. The relaxation amplitude is directly related to the number of water molecules (per volume) that contribute to the observed relaxation.25 For electrolyte solutions, kinetic depolarization leads to a reduction of the observed water relaxation strength, SH2O, by an amount Skd. Ions drifting through the solution in the external electric field set up a nonuniform flow which counteracts the rotational motion of the water molecules in the ion vicinity, thus decreasing SH2O. The reduction of the relaxation amplitude, Skd, originates from the coupling of the ions translation to the water rotation and can be quantified using the continuum theory by Hubbard and Onsager:26,27
Figure 2. Real (ε′) and imaginary (ε″) part of the permittivity spectra obtained for NaAlg solutions. The arrow indicates increasing concentration. Symbols correspond to experimental data (others omitted for visual clarity). Lines represent the fits according to eq 1. Note that the conductivity contribution (last term of eq 1) has been subtracted from the dielectric loss for visual clarity.
and a corresponding dispersion in ε′ centered at ∼18 GHz. As the concentration of NaAlg increases, a second relaxation peak centered at 0.27 to 0.79 GHz (depending on concentration) appears and its contribution increases with increasing solute concentration. Accordingly, we fitted the experimental spectra to a superposition of two Debye modes (n = 2 in eq 1; α1 = α2 = 0, β1 = β2 = 1). As we will show below, the two modes can be assigned to the solute (j = 1; S1 = Ssolute, τ1 = τsolute) and to water (j = 2; S2 = SH2O, τ2 = τH2O). The contribution of these two relaxation modes to the experimental spectra are indicated in Figure 3. The amplitude of the low-frequency mode (Ssolute), centered between 0.27 and 0.79 GHz, increases linearly with concentration up to 0.2 mol/L and then moderately levels off reaching a value of 6.3 at the highest concentration of the present study. We note that the amplitude of this low-frequency mode is too high to be consistent with the lower frequency mode being solely due to weakly bound water.18 However, such low frequency relaxations are commonly observed for aqueous solutions of polyions19−23 and are assigned to fluctuations of counterions. The dissociated counterions can fluctuate within the correlation length, ξ, which characterizes the average distance between polyion chains. These fluctuations result in a macroscopic polarization of the sample. The relaxation time of the loosely bound ions, perpendicular to the polyion backbone, is related the correlation length by:22 τ ∝ ξ2. As the distance between chains decreases with increasing concentration, the relaxation time due to the counterion fluctuation is predicted to decrease with increasing NaAlg concentration. This indeed is consistent with our experimental τsolute values, which decrease from 580 to 202 ps as the concentration of NaAlg increases
S kd = σ0 773
2τH2O εs ,H2O − ε∞ ,H2O 3
εs ,H2Oε0
(4)
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where εs,H2O, ε∞,H2O, and τH2O are the static permittivity, the infinite frequency permittivity and the relaxation time of pure water, respectively. The thus obtained overall water relaxation strength, SH2O + Skd can be related to the apparent water concentration, cH2O,app, as detected with DS, using the Cavell equation:25 NAc H2O,app 2εs + 1 2 (SH2O + S kd) = μ kBε0T eff ,H2O εs
(5)
where εs is the static permittivity (=ε∞ + ∑Sj), T is the thermodynamic temperature, NA and kB are the Avogadro and Boltzmann constants, respectively. Assuming the dipole moment of water, μef f,H2O, to be equal to the effective dipole moment of neat water (μeff,H2O = 3.78D),28 the apparent concentration of ‘bulk-like’ water, cH2O,app, is obtained. Thus, obtained values were compared to the analytical concentration of water as calculated from the sample composition: cH2O = (ρw)/MW, where ρ is solution density (Figure 1), w is the weight fraction of water, and MW is the molecular mass of water. As can be seen from Figure 4, we observe a difference between cH2O and cH2O,app which increases with increasing solute concentration.
Figure 5. (a) Anisotropy decay of the OD stretching vibration in aqueous solutions of NaAlg. Symbols are experimental data and lines represent the fit to eq 6. (b) Fraction of water molecules, which exhibit slowed down rotational dynamics. Green squares are water molecules slowed down by Na+ and alginate as obtained with DS. Red squares represent the fraction of water molecules affected by alginate inferred from fs-IR (open symbols) and DS (solid symbols). Error bars in slow water fractions obtained with DS corresponds to the reproducibility within two measurements. Error bars for fs-IR measurements correspond to a 2% increase in the reduced error function, χ2, when fitting eq 6 to the experimental R(t) decays. Figure 4. Concentration of bulk-like water molecules obtained using DS (red) together with the total water concentration as determined from the density measurements (green). The blue shaded area represents the fraction of water molecules bound by the solute. Error bars were estimated from two independent measurements.
double exponential model, where a small fraction, fs, of water molecules has distinctively slower dynamics than bulk-likewater:
This difference corresponds to 11 (±2) water molecules per NaAlg unit. These water molecules are assigned to water molecules in the hydration layers of both sodium and alginate. Because of their interaction with the solute their rotational dynamics are significantly slowed down and thus these water molecules do not contribute to the bulk water relaxation centered at ∼18 GHz. Femtosecond Infrared Spectroscopy. Complementary to our DS results, we study the rotational dynamics of water using fs-IR spectroscopy. In Figure 5a we show the decay of the excitation anisotropy at different concentrations of NaAlg. As can be seen from Figure 5a, the measured R(t) decays slow down, as the solute concentration increases. We note that the observed R(t) decays are not consistent with a monoexponential decay, which would corresponds to a slow-down of the rotational dynamics of all water molecules in solution. However, the observed R(t) can be well described with a
In eq 6 R0 is the anisotropy at t = 0 delay and τb,IR is the reorientational time of bulk water 2.3 (±0.2) ps. As can be seen in Figure 5, such fits provide an excellent description of the experimental R(t) decays. From these fits we find the reorientational time of slowed down water molecules of τs,IR > 10 ps, which is outside our accessible time-window. Thus, the exact rotation time of the slow fraction cannot be unambiguously resolved. In Figure 5b, we show the fraction of water molecules slowed down by the solute as obtained from the fs-IR experiments (open red symbols). The values of fs increase linearly with increasing concentration.
R(t ) = R 0((1 − fs ) exp(−t /τb,IR ) + (fs exp(−t /τs,IR ))) (6)
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DISCUSSION Both, fs-IR and DS show that the rotational dynamics of a subensemble of water molecules is significantly slowed down compared to bulk-like water. While the DS results indicate that the number of slow water molecules corresponds to 11 ± 2 774
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sodium formate approximately 3 water molecules are slowed down by the carboxylate group of the formate ion. Similarly, Rahman et al.38 studied hydration of formate ions by means of dielectric spectroscopy and found that there are 1−2 strongly and ∼5 weakly bound water molecules in the hydration shell of the formate anion. It was shown that the dynamics of this weakly bound water is only slightly slowed-down with respect to bulk water (2−3 times slower). In our present study we do not find evidence for the presence of such slow water molecules in aqueous solutions of sodium alginate. On the one hand, the relaxation strength of such a mode is estimated to be S < 2.2, which means that it cannot be resolved due to its location between the intense water relaxation and the lower frequency solute mode. Moreover, these ∼5 slightly slowed down water molecules are located within the cage around the formate ion. In the case of alginate the steric exclusion due to the polymer backbone makes the formation of such extended hydration cages highly improbable. Thus, our results suggest that out of the 6 ± 2 immobilized water molecules observed in our study, 1 to 3 water molecules are immobilized by the carboxyl group of the alginate unit. The remainder likely originates from water molecules hydrating the hydrophobic fragments of the saccharide rings. Such water molecules in the vicinity of hydrophobic alkyl moieties undergo retardation of their reorientational dynamics, as has been demonstrated with both fs-IR experiments39−41 and dielectric spectroscopy.42 The DR and fs-IR spectroscopy results show that only a small fraction of water molecules is affected with respect to their rotation dynamics, while the majority exhibits rotation times similar to bulk-like water. On the other hand, the longrange correlations, as determined by the solutions viscosity increase significantly already at the very low concentration, with η values reaching 225 mPa·s at 0.19 mol/L (Figure 1). At this concentration, as little as ∼1−2 mol/L out of a total of 54.3 mol/L of water is affected, while the viscosity increases by more than 2 orders of magnitude. Such a strong increase in the viscosity in case of polymer solutions is mainly associated with intramolecular repulsion between equal charges, which increases the chain size while attractive forces between same charge species contract the chain. Intermolecular repulsion leads to intermolecular ordering of charged coils and the associated obstruction effect is reflected in macroscopic solution viscosity.43 However, the local viscosity affecting the rotational dynamics of the bulk-like water molecules is obviously not affected as DS and fs-IR relaxation times do not scale with η. This suggests the existence of large water pools confined between polysaccharide chains. Water in those pools exhibit dynamics characteristics of bulk like water.
water molecules per saccharide repeating unit, the slow fraction of water as detected with fs-IR corresponds to 6 ± 2 per saccharide. The fundamental difference between both techniques is the rotational degrees of freedom both techniques are sensitive to. While fs-IR measures the rotation of the OD group of HOD molecules, dielectric spectroscopy is sensitive to the rotational dynamics of the permanent dipole moment of water. It has been shown experimentally29,30 and theoretically31 that the rotation of water molecules in the first hydration shell of the sodium cation is anisotropic due to the geometry of the hydration shell, with water’s oxygen coordinating to the cation. This geometry results in the dipole moments of the water molecules next to Na+ being essentially immobile,18,32 while the OH bond is free to rotate, with rotation times similar to bulklike water.13 Thus, the formation of a fairly rigid hydration shell with radially oriented water dipoles is observed around Na+ with dielectric spectroscopy, whereas fs-IR spectroscopy is blind to that. Our present results indicate that the difference in the number of slowed-down water molecules as detected with DR and fs-IR spectroscopy corresponds to ∼5 H2O per repeating unit. This suggests that the observed discrepancy can be fully accounted by the hydration of the sodium ion, which strongly binds ∼5.2 H2O molecules per ion in aqueous solution.32,33 This is also broadly consistent with other experimental techniques yielding values for hydration numbers of 3 (magnetic resonance),34 5 (X-ray scattering)35 and 6 (molecular dynamics simulation).36 As can be seen from Figure 5b, the fraction of immobilized water molecules as determined by fs-IR spectroscopy agree very well with those determined using DS spectroscopy after subtracting the contribution of Na+ hydration from the latter. This observation suggests that the remainder of ∼6 water molecules per repeat unit bound by the solute can be assigned to H2O molecules in the hydration shell of the polysaccharide itself. This hydration number is remarkably similar to that of another saccharide-based polyelectrolyte, hyaluronan (∼7 H2O/monosaccharide), and that of a neutral polysaccharide, dextran (∼6 H2O/monosaccharide), both determined using fsIR and terahertz spectroscopy.7 Interestingly, Paolantoni et al.5 determined the hydration number for glucose in water to be somewhat higher (∼10 H2O) using depolarized Rayleigh scattering, while the hydration number of trehalose was found to be ∼17.6 These numbers are higher than those inferred for alginate in the present study, which indicates that the surface of the saccharide accessible to the solvent is reduced upon polymerization of the saccharides. Hence, the number of water molecules in the hydration shell is reduced. Interestingly, the nature of the polysaccharide, i.e., being a polyelectrolyte or a neutral polymer, appears to have little effect on the hydration of the biopolymers. Our results rather indicate that a hydration shell consisting of ∼6 water molecules per saccharide unit, having significantly slower dynamics than bulk, is characteristic for polysaccharides. This finding also implies that the presence of the electric field perpendicular to the polymer backbone due to the dissociation of the counterions has a negligible effect on the dynamics of water molecules in the hydration shell of the polysaccharides. A question that remains to be addressed is which groups of the alginate monomer are responsible for the observed slow rotational dynamics of water. Pastorczak et al.37 studied aqueous solutions of carboxylate groups with alkali cations using femtosecond infrared spectroscopy. They found that for
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CONCLUDING REMARKS We studied orientational dynamics of water molecules in aqueous solutions of sodium alginate using femtosecond timeresolved infrared and dielectric spectroscopy. Both experiments evidence the existence of a subensemble of water molecules with slowed-down reorientational dynamics. These water molecules are essentially immobilized within our experimental time window. Here, 6 ± 2 water molecules are slowed down by alginate as inferred from fs-IR and DS studies. In part these molecules are expected to be bound to the carboxylic group of alginate. Furthermore, the exposure of the hydrophobic fragments of the saccharide to water likely leads to their retardation due to hydrophobic hydration. Additionally, DS spectroscopy reveals ∼5 water molecules to be slowed down in 775
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(29) Tielrooij, K. J.; Garcia-Araez, N.; Bonn, M.; Bakker, H. J. Science 2010, 328, 1006−1009. (30) Tielrooij, K. J.; van der Post, S. T.; Hunger, J.; Bonn, M.; Bakker, H. J. J. Phys. Chem. B 2011, 115, 12638−12647. (31) Stirnemann, G.; Wernersson, E.; Jungwirth, P.; Laage, D. J. Am. Chem. Soc. 2013, 135, 11824−11831. (32) Buchner, R.; Hefter, G. T.; May, P. M. J. Phys. Chem. A 1999, 103, 1−9. (33) Eiberweiser, A.; Buchner, R. J. Mol. Liq. 2012, 176, 52−59. (34) Vangeet, A. L. J. Am. Chem. Soc. 1972, 94, 5583−5587. (35) Mahler, J.; Persson, I. Inorg. Chem. 2012, 51, 425−438. (36) Megyes, T.; Balint, S.; Grosz, T.; Radnai, T.; Bako, I.; Sipos, P. J. Chem. Phys. 2008, 128, 044501. (37) Pastorczak, M.; Van der Post, S. T.; Bakker, H. J. Phys. Chem. Chem. Phys. 2013, 15, 17767−17770. (38) Rahman, H. M. A.; Hefter, G.; Buchner, R. J. Phys. Chem. B 2012, 116, 314−323. (39) Rezus, Y. L. A.; Bakker, H. J. J. Phys. Chem. A 2008, 112, 2355− 2361. (40) Rezus, Y. L. A.; Bakker, H. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18417−18420. (41) Bakulin, A. A.; Pshenichnikov, M. S.; Bakker, H. J.; Petersen, C. J. Phys. Chem. A 2011, 115, 1821−1829. (42) Tielrooij, K. J.; Hunger, J.; Buchner, R.; Bonn, M.; Bakker, H. J. J. Am. Chem. Soc. 2010, 132, 15671−15678. (43) Utracki, L. A.; Jamieson, A. M. Polymer Physics: From Suspensions to Nanocomposites and Beyond, 1st. ed.; Wiley: Hoboken, New Jersey, 2010.
the hydration shell of the sodium ion. Comparison of these findings with viscosity measurements indicates that the effect of solute on the water dynamics is restricted to the first hydration shell of the solute, while the major fraction of water molecules is located in larger water pools with bulk-like reorientational dynamics.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (K.M.)
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/ma4023873 | Macromolecules 2014, 47, 771−776
