Hydration and Hydrogen Bond Network of Water ... - ACS Publications

Apr 12, 2015 - The structural properties of the water HB network were examined in terms ... otherwise called a volume phase transition, around its clo...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCB

Hydration and Hydrogen Bond Network of Water during the Coil-toGlobule Transition in Poly(N‑isopropylacrylamide) Aqueous Solution at Cloud Point Temperature Keiichiro Shiraga, Hirotaka Naito, Tetsuhito Suzuki, Naoshi Kondo, and Yuichi Ogawa* Graduate School of Agriculture, Kyoto University, Kitashirakawa-oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan ABSTRACT: Aqueous solutions of poly(N-isopropylacrylamide), P-NIPAAm, exhibit a noticeable temperature responsive change in molecular conformation at a cloud point temperature (Tcp). As the temperature rises above Tcp, the extended coil-like P-NIPAAm structure changes into a swollen globule-like conformation as hydration levels decrease and hydrophobic interactions increase. Though water plays an important role in this coil-toglobule transition of P-NIPAAm, the behavior of water molecules and the associated hydrogen-bond (HB) network of the surrounding bulk water are still veiled in uncertainty. In this study, we elucidate changes in the hydration state and the dynamical structure of the water HB network of P-NIPAAm aqueous solutions during the coil-to-globule transition by analyzing the complex dielectric constant in the terahertz region (0.25−12 THz), where bulk water reorientations and intermolecular vibrations of water can be selectively probed. The structural properties of the water HB network were examined in terms of the population of the non-HB water molecules (not directly engaged in the HB network or hydrated to P-NIPAAm) and the tetrahedral coordination of the water molecules engaged in the HB network. We found the hydration number below Tcp (≈10) was decreased to approximately 6.5 as temperature increased, in line with previous studies. The HB network of bulk water becomes more structured as the coil-toglobule phase transition takes place, via decreases in non-HB water and reduction in the orderliness of the tetrahedral HB architecture. Together these results indicate that the coil-to-globule transition is associated with a shift to hydrophobicdominated interactions that drive thermoresponsive structural changes in the surrounding water molecules.

1. INTRODUCTION Poly(N-isopropylacrylamide), P-NIPAAm, as depicted in Figure 1a, is a thermoresponsive polymer which undergoes a

solvated P-NIPAAm is known for its narrow temperature range and sharp transition compared to that of other thermoresponsive polymers.3 Additionally, the phase transition temperature can be tuned by grafting terminals to the P-NIPAAm molecules4,5 or by the addition of soluble molecules (i.e., urea) into the P-NIPAAm aqueous solution.6 Thus, it has the potential to be used as an intelligent substance in a number of practical applications including drug delivery7 and tissue engineering.8 Experimental research into P-NIPAAm aqueous solution has been carried using different techniques such as light scattering,6,9−11 nuclear magnetic resonance,12 infrared spectroscopy,13−20 Raman spectroscopy,21,22 and differential scanning calorimetry,23−27 as well as theoretical studies.28,29 These studies have found that the origin of the volume phase transition of P-NIPAAm at Tcp is as follows: below Tcp, where hydrophilic regions are exposed to the solvent, hydrophilic amide (−NH) and carbonyl (−CO) groups form hydrogen bonds (HBs) with proximal water molecules driven by an enthalpic term (hydrophilic hydration). At the same time, the “cagelike” rigid HB network of water can be found around isopropyl groups, called hydrophobic hydration. For P-

Figure 1. (a) P-NIPAAm chemical formula with the polymerization degree of n. (b) Schematic of the NIPAAm monomer unit; hydrogen (white), carbon (lime), nitrogen (blue), and oxygen (red). (c) Schematic of the NIPAAm trimer unit (n = 3).

reversible coil-to-globule transition, otherwise called a volume phase transition, around its cloud point temperature (Tcp) or lower critical solution temperature (LCST) around 306 K. At Tcp, soluble P-NIPAAm molecules, with a coil-like structure with a larger hydrophilicity on the molecular surface, abruptly become insoluble. This transition is associated with increasing turbidity and a single endothermic behavior.1,2 While such a coil-to-globule transition around Tcp or LCST is manifested in various homopolymers, the coil-to-globule transition of the © XXXX American Chemical Society

Received: January 31, 2015 Revised: April 10, 2015

A

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B NIPAAm solution, once the temperature exceeds Tcp, the entropic term dominates the enthalpic one, leading to an unfavorable Gibbs free energy state, and the hydrophobic groups start to aggregate together (so-called hydrophobic interaction) to compensate for the entropic penalty. According to this hydrophobic interaction, the extended coil-like conformation of P-NIPAAm with the hydrophilic region exposed to the solvent changes into a swollen globule-like structure. During this process, the hydrophilic −NH and −CO groups are mostly invaginated into the inner part of the PNIPAAm molecule forming intramolecular −NH···OC− hydrogen bonds, with the hydrophobic isopropyl residue also being modified because of a cooperative decrease in the number of hydrophobic hydration water.30,31 In addition, recent molecular dynamics (MD) simulation showed that the drastic change in the conformational state of hydrophobic isopropyl group serves as a dynamic trigger for P-NIPAAm coilto-globule transition, including hydrophilic−hydrophobic transition and HB transformation.32 These results indicate that a knowledge of the balance of interactions between water and the hydrophilic and hydrophobic groups at the coil-to-globule transition in the P-NIPAAm aqueous solution is essential for understanding complicated molecular mechanisms in biological systems, such as the stabilization of protein conformation and self-association of lipids. To comprehend these complicated water−P-NIPAAm interactions, previous Fourier transfrom infrared (FTIR) and two-dimensional infrared (2D-IR) spectroscopy studies13−20 have focused on the amide bands, which are sensitive to the HB environment. It has been suggested that in the coil-to-globule transition dehydration of hydrophobic hydration water occurs first, followed by main-chain diffusion and aggregation. Subsequently, some of the −NH and −CO groups are no longer involved in intermolecular HBs with water, instead forming −NH···OC− intramolecular HBs.20 On the basis of UV resonance Raman spectroscopy, Ahmed and co-workers further explored the amide bands in the Raman spectrum, finding that one of the two acceptor HBs between water and −CO group is lost and the donor HBs of the water−amide (−NH) group remain unperturbed even in the globule state above the critical temperature.22 These experimental observations were later supported by molecular dynamics simulation by Deshmukh et al.,33 who insisted donor HBs of the amide group are stronger than acceptor HBs of the carbonyl group. These results indicate hydrophilic hydration is not very sensitive to the coil-to-globule transition of P-NIPAAm molecule. Meanwhile, dielectric spectroscopy studies in the gigahertz region34−36 have shown that the hydration number per P-NIPAAm monomer unit drastically diminished during the coil-to-globule transition, in apparent conflict with Ahmed and co-workers’ result in which only one water molecule interacting with −CO group is affected by the coil-to-globule transition. These discrepancies among experimental approaches obscure understanding of the interaction between water and P-NIPAAm during the volume phase transition. This unsatisfactory situation arises from a lack of knowledge about the specific behavior of water molecules when perturbed by the hydrophilic and/or hydrophobic forces of P-NIPAAm. Because native characteristics of water largely originate from their ability to form intermolecular HBs, knowledge about changes in water HB dynamics during this phase transition would go a long way toward understanding water−P-NIPAAm interactions. From this perspective, an experimental method that could selectively and directly probe

the dynamics of water HBs would facilitate further understanding of water’s role in the coil-to-globule transition of PNIPAAm. Terahertz spectroscopy is one such experimental technique that could directly probe these water HB dynamics, which have relaxational and vibrational behaviors at time scales of picoseconds and subpicoseconds. Because of a strong dipole moment and an intramolecular distribution of the lone pair electrons, liquid water molecules form on average four HBs (average HB lifetime ≈1 ps or subpicoseconds37) with their nearest neighbors in the tetrahedral direction. Continuous assembly of this local tetrahedron, referred to as the HB network of bulk water, undergoes structural reorganization within every 10 ps at room temperature. During the structural reorganization of bulk water, some bulk water molecules are released from the HB network before finding new HB partners and experience individual reorientation at subpicoseconds. Therefore, various dynamics of the water molecules themselves, such as molecular reorientations and HB-related vibrations (i.e., stretching and libration) are at time scales between subpicoseconds and picoseconds. Because the terahertz frequencies correspond to the periodic electric fields from picosecond to subpicosecond time window, the dynamical behaviors of water can be directly probed. On the other hand, dynamics of macromolecules themselves and their hydrated water molecules are so slow compared to the picosecond time window that interferences from solute molecules are negligibly small in the terahertz region. In this respect, terahertz spectroscopy is a unique tool to “selectively” examine the bulk water dynamics related to the HBs, which cannot be intensively investigated by other experimental approaches. In particular, as far as the dielectric responses in the terahertz region are concerned, several relaxational and vibrational processes of water can be observed, such as collective bulk water relaxation, individual bulk water relaxation, and intermolecular HB vibrations in aqueous solutions.38−40 They represent bulk water dynamics; therefore, perturbations imposed on water by solute molecules can be directly evaluated from their complex dielectric constant in the terahertz region. To date though, only a few studies38−42 have used terahertz spectroscopy to probe water−solute interactions. The cause of the present situation comes from the difficulty in the precise measurement of the terahertz spectrum on aqueous solutions because of strong absorption of water in the terahertz region. In this study, we use attenuated total reflection (ATR) spectroscopy to precisely determine the complex dielectric constant of P-NIPAAm aqueous solution between 0.25 and 12 THz in order to investigate the water−P-NIPAAm interactions around Tcp in terms of water dynamics. Changes in the hydration state and modulation of the HB network induced by the coil-to-globule transition of P-NIPAAm from 298 to 312 K are quantified from changes in the complex dielectric constant in the terahertz region.

2. EXPERIMENTAL SECTION 2.1. Materials. P-NIPAAm powder (Kohjin Co.) with an average molecular weight of 44 000 (corresponding to the polymerization degree n = 390) was dissolved into pure water to prepare a 3 wt % P-NIPAAm aqueous solution. From the density measurement of the solution, the “stoichiometric” molar concentration of water (Cwater) was determined. Tcp of our P-NIPAAm solution was confirmed to be around 306 K by visual inspection of changes in turbidity. B

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Kramers−Kronig equation proposed by Bertie and Lan,48 the spolarized phase shift spectrum, φs(ω), was calculated. Then, from the Fresnel reflection coefficient, r̃(ω) =(R(ω))1/2 exp[−iφ(ω)], the complex dielectric constant, ε̃( ω), was determined. To avoid absorption of water vapor, the measurement system was evacuated to 100 Pa. Typically, the standard errors, δε̃SE(ω), for the terahertz TDATR spectroscopy setup in the real and imaginary part for the complex dielectric constant of distilled water were 0.2% and 0.3% at 0.5 THz, 0.4% and 0.4% at 1.0 THz, and 1.0% and 0.8% at 2.0 THz, respectively. Also, the measurement errors for FIR FT-ATR reflectance of distilled water were 0.4% at 5 THz and 0.6% at 10 THz. 2.3. Analysis of the Dielectric Spectrum. The dielectric properties of P-NIPAAm aqueous solution in the terahertz region are complex, deriving from different and partially overlapping dielectric responses. As noted previously,38,49 the relaxational and vibrational dynamics of solute molecules themselves are assumed to be zero in aqueous solution because the relaxational dispersion of P-NIPAAm (centered at ∼0.1 GHz)34−36,50,51 is far below the frequency of the terahertz region. Additionally, the inter- and intramolecular vibration of the solute is not apparent in the dispersed aqueous solution. Thus, the complex dielectric constant ε̃(ω) of P-NIPAAm aqueous solutions in the terahertz region is composed of relaxations and vibrations of water molecules: slow relaxation of bulk water, χ̃slow(ω) (collective reorientation of the hydrogenbonded bulk water); fast relaxation of bulk water, χ̃fast(ω) (individual reorientation of the non-hydrogen-bonded bulk water); intermolecular stretching vibration of water, χ̃S(ω) (hindered translation between hydrogen-bonded water molecules); and libration of water, χ̃L(ω) (hindered rotation between hydrogen-bonded water molecules)52−55

2.2. Determination of the Complex Dielectric Constant. The broadband complex dielectric constant between 0.25 and 12 THz of distilled water and 3 wt % P-NIPAAm aqueous solution, at temperatures ranging from 298 to 312 K, was measured by two spectroscopic systems: terahertz timedomain attenuated total reflection (TD-ATR) spectroscopy from 0.25 to 3.0 THz and far-infrared Fourier transform attenuated total reflection (FIR FT-ATR) spectroscopy from 3 to 12 THz. In these systems, when the incident wave satisfies the total reflection geometry, the transmitted wave vector perpendicular to the reflection interface becomes imaginary, which is called an evanescent wave. Because the evanescent wave penetrates into the sample (comparable to approximately one tenth of the incident wavelength), the Fresnel reflection coefficient represents slight changes in the complex dielectric constant of the sample more sensitively than the external reflection geometry.43 For this reason, ATR geometry offers the best accuracy for absorptive samples, such as water, in the terahertz region.44 Terahertz TD-ATR measurements were made using a TAS7500 spectrometer (Advantest Co.) with a silicon ATR prism. Two mode-locked Ti:sapphire lasers (λ = 800 nm; pulse width, ∼40 fs) were linked at a slightly different repetition frequency (Δωdif). One is used as a pump, and the other is used as a probe pulse. This scheme, a so-called asynchronous optical sampling strategy,45 theoretically ensures short sampling intervals (∼1/Δωdif) of a time-domain terahertz pulse with fine time resolution. The p-polarized terahertz pulse emitted from the biased InGaAs photoconductive antenna was separated into two path lines by a beamsplitter. One was focused on the prism−sample interface with an incident angle of 57° to obtain the sample terahertz waveform with an InGaAs photoconductive antenna detector; the other was directly introduced to a different photoconductive antenna detector to monitor the jitter of the laser. This optical setup accurately probed the slight broadening and time delay of the timedomain terahertz pulses E(t). Then, the ratio of the timedomain sample pulse, ESAM(t), to background pulse, EBKG(t), ESAM(t)/EBKG(t) was Fourier transformed into the frequency region: Ẽ SAM(ω)/Ẽ BKG(ω). Here, we experimentally obtained the reflectance R(ω) = |Ẽ SAM(ω)/Ẽ BKG(ω)|2 and the phase shift φ(ω) = Arg[Ẽ SAM(ω)/Ẽ BKG(ω)]. Because the measured Ẽ SAM(ω)/Ẽ BKG(ω) corresponds to the ratio of the complex reflection coefficient r̃SAM(ω)/r̃BKG(ω), the experimental R(ω) and φ(ω) were substituted into Fresnel’s equation. Given the incident angle and complex dielectric constant of the ATR prism, both the real and imaginary part of the complex dielectric constant of the sample, ε̃(ω) = Re[ε(ω)] − iIm[ε(ω)], were determined by solving the equation.46,47 During the measurement, a Peltier element attached to the ATR prism made of monocrystal silicon is used to control the temperature of the liquid sample on top of the ATR prism within a ± 0.1 K fluctuation. Dry air was purged to avoid any signal attenuation due to water vapor in the system. FIR FT-ATR data was measured by a FARIS-1s (Jasco Co.) with a ceramic heater light source and deutrated triglycine sulfate element detector. The incident angle was set at 45°, and the silicon ATR prism was equipped with a Peltier element to maintain prism temperature to within 0.3 K fluctuation. At a 45° incident angle, we measured the reflectance spectrum, R(ω), of unpolarized FIR waves, which were first transformed to the s-polarized reflectance, Rs(ω), via the relationship 2R(ω) ≡ Rs(ω) + RP(ω) = Rs(ω) + Rs2(ω). According to the

ε (̃ ω) = χslow ̃ (ω) + χfast ̃ (ω) + χS̃ (ω) + χL̃ (ω) + ε∞ =

Δεslow Δεfast ΔVSωS2 + + 1 + iωτslow 1 + iωτfast ωS2 − ω 2 + iωγS 2 ΔVLωL + + ε∞ 2 ωL − ω 2 + iωγL (1)

where Δεslow(fast), τslow(fast), ΔVS(L), ωS(L), and γS(L) are the relaxation strength, relaxation time, vibration strength, resonant frequency, and damping constant, respectively. The last term, ε∞, represents the high-frequency limit of the real part, whose dispersion is assumed to be constant in the terahertz region, such as intramolecular vibrations and electron excitations. Equation 1 was applied to both the complex dielectric constant of distilled water and the P-NIPAAm aqueous solution between 0.25 and 12 THz, at temperatures ranging from 298 to 312 K. A nonlinear least-squares fitting calculation based on the Lavenberg−Marquardt algorithm for complex functions56 was performed, with simultaneous fits of the experimentally measured real part Re[ε(ω)] and imaginary part Im[ε(ω)] with the set of free parameters in eq 1 by minimizing the deviation, Π. C

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B ⎡ n 1 ⎢{∑ (Re[ε(ω )] − Re[ε (ω )])}2 Π= j fit j 2(N − M ) ⎢⎣ j = 1

other, indicating that no other relaxational and/or vibrational modes are present, even in the P-NIPAAm aqueous solution. However, the imaginary part of the P-NIPAAm solution was attenuated compared to that of distilled water over the whole frequency regime because the number of water molecules in a unit volume is reduced in the P-NIPAAm aqueous solution. In fact, the difference in the imaginary part between distilled water and the P-NIPAAm solution is at least 5 times larger than the standard error of the experimental system at each temperature. In addition, it was confirmed that the precipitation of PNIPAAm aggregates at the ATR prism was negligible, even above Tcp. The inset in Figure 2 shows the complex dielectric constant of the P-NIPAAm solution with increasing temperature, and the temperature dependence at selected frequencies (0.5 THz and 5 THz) are summarized in Figure 3. As seen in the inset in

1

⎤2 + {∑ (Im[ε(ωj)] − Im[εfit(ωj)])}2 ⎥ ⎥⎦ j=1 n

(2)

In this equation, N is the number of data points in the spectrum and M is the number of free fitting parameters; Re[εfit(ωj)] and Im[εfit(ωj)] are the real and imaginary part in the right-hand side in eq 1, respectively. To successfully fit the experimental ε̃(ω), the fitting calculation was iterated until chi-square becam smaller than 10−9. By substituting ε̃(ω) ± δε̃SE(ω) into the lefthand side of eq 1, the errors in this fitting procedure were determined. To minimize an unreliable fitting result, the relaxation time of the slow relaxation was fixed by the critical slowing formula, τslow = 1.08(T/228−1)−1.73 ps.52 Although this formula originally holds in pure water, we also applied this to the PNIPAAm aqueous solution because it has been found that τslow of bulk water in aqueous solution is independent of the kind of solute and solute concentration and is thus equal to that of pure water.57,58 However, all other parameters were freely fitted by the least-squares calculation. The fitted parameters (Δεslow(fast), τfast, ΔVS(L), ωS(L), γS(L), and ε∞) of distilled water was in good agreement with previously reported values,52,53 confirming the validity of our fitting procedure. Furthermore, the static dielectric constant of the distilled water, εS = Δεslow + Δεfast + ΔVS + ΔVL + ε∞, at each temperature was in very good accordance with the temperature dependence of εS obtained by the regression line in ref 59.

3. RESULTS AND DISCUSSION 3.1. Temperature Dependence of the Complex Dielectric Constant. The complex dielectric constant of the distilled water and the P-NIPAAm aqueous solution, between 0.25 and 12 THz at 312 K, are depicted in Figure 2. For both samples, the frequency dispersions are quite similar to each

Figure 3. Temperature dependence of the complex dielectric constant at 0.5 THz: distilled water (blue circles) and P-NIPAAm solution (red diamonds). Above Tcp (306 K) is represented as the shaded area. The insets are the complex dielectric constant at 5.0 THz. The bars indicate the standard errors of the experimental system.

Figure 2, the real part of the complex dielectric constant tends to increase in the lower frequencies and to decrease in the higher frequencies as temperature increases, with the tipping point around 0.7 THz. Such a temperature dependency of distilled water has been observed in other studies.40,52 In contrast, the imaginary part linearly increased with temperature over the entire measured frequency range. Interestingly, the temperature dependence of P-NIPAAm aqueous solution exhibits a discontinuous change at Tcp (∼306 K) presumably arising from the coil-to-globule transition of P-NIPAAm, while that of distilled water shows a near linear change between 298 and 312 K (Figure 3). To understand the detailed mechanism behind the discontinuous change around Tcp of the P-NIPAAm aqueous solution, all the measured complex dielectric constants, ε̃(ω), were decomposed into their constituent complex susceptibilities: χ̃slow(ω), χ̃fast(ω), χ̃S(ω), and χ̃L(ω), according to eq 1. The original and decomposed spectra are shown in Figure 4. This decomposition revealed that the high-frequency tail of the slow relaxation of bulk water, with a peak around 20 GHz, largely dominates the imaginary part in the low-frequency region of this study. The fast relaxation mode, χ̃fast(ω),

Figure 2. Complex dielectric constant of distilled water and PNIPAAm aqueous solution at 312 K. The insets show the complex dielectric constant of P-NIPAAm solution at various temperatures from 298 to 312 K. D

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 4. Comparison of experimental result (gray circles) with its decomposed components; slow relaxation (blue), fast relaxation (green), intermolecular stretching vibration (orange), libration (purple), and high-frequency limit (red).

originating from the non-HB water shows its dispersion in the sub-terahertz region. This is because single water molecules detached from the HB network can reorient relatively freely because of their own dipole, but water molecules engaged in the HB network that cooperatively reorient with proximal molecules need a long time to reorient because of this restriction. Furthermore, well-damped (broad) vibration modes of intermolecular stretch, χ̃S(ω), and libration, χ̃L(ω), can be found around 5 THz and >12 THz, respectively. Figure 5 shows the temperature dependence of the decomposed imaginary part of χ̃slow(ω), χ̃fast(ω), and χ̃S(ω) for the P-NIPAAm aqueous solution, compared to that of distilled water (inset). First, the slow relaxation, χ̃slow(ω), clearly increased with temperature for both distilled water and the PNIPAAm solution (Figure 5, upper panel) because weakened HBs at high temperatures induce the decrease in τslow, resulting in a blueshift of this whole relaxation mode.60 Accordingly, because the terahertz region locates at the high-frequency tail of the slow relaxation dispersion, the imaginary part of χ̃slow(ω) appears to be increased because of the blueshift. Second, as can be seen in Figure 5 (middle panel), the fast relaxation, χ̃fast(ω), tends to decrease as temperature rises from 298 to 312 K, accompanying a small shift to higher frequencies, for both distilled water and the P-NIPAAm solution. In the case of distilled water, a result in agreement with a previous report,52 one again confirms the validity of our experimental results and analytical procedures. For the P-NIPAAm aqueous solution, interestingly, we found a large jump in the peak position and peak height around Tcp, indicating the coil-to-globule transition affects the molecular dynamics of non-HB water. Finally, a similar discontinuous change in the P-NIPAAm aqueous solution around Tcp was also seen in the intermolecular stretching vibration, χ̃S(ω). While the peak height gradually becomes higher for distilled water (in good accordance with ref 61), the peak height of P-NIPAAm aqueous solution suddenly decreased at Tcp (Figure 5, lower panel). Because this intermolecular stretch sensitively represents the dynamical structure of the water HB network, this result implies the structural modulation of water molecules is driven by the coil-

Figure 5. Imaginary part of the P-NIPAAm susceptibilities of (top panel) slow relaxation, (middle panel) fast relaxation, and (bottom panel) intermolecular stretching vibration at 300, 304, 308, and 312 K. The insets show the imaginary part of distilled water.

to-globule transition of the P-NIPAAm molecule. Further discussions about the slow relaxation (χ̃slow(ω)), fast relaxation (χ̃fast(ω)), and intermolecular stretch (χ̃S(ω)) are found in sections 3.2., 3.3., and 3.4., respectively. 3.2. Hydration State. Although only bulk water is responsible for the measured complex dielectric constant in the terahertz region, we can infer the number of hydrated water molecules per solute (hydration number Nhyd), mainly from the slow relaxation strength, Δεslow. In the presence of hydrated water, Δεslow of aqueous solutions is always smaller than expected in the presence of the solute (dilution effect, i.e., the substitution of water by solute molecules in aqueous solution).38,54,55 The reason for this phenomenon can be explained by the transition of bulk water to hydrated water, because the hydrated water develops distinct relaxation dispersions far below the terahertz region58 and no longer contribute to the slow relaxation mode of bulk water, χ̃slow(ω). In this context, hydrated water is defined as water molecules exhibiting greater retarded relaxation dynamics than HB bulk water. Thus, both enthalpy-driven hydrophilic hydration (water dynamically retarded by forming a direct HB with −NH and −CO groups) and entropy-driven hydrophobic hydration (long-lived water−water HBs formed around the hydrophobic group) schemes are counted as hydrated water in this analysis. Figure 6 shows the best-fitted result of the slow relaxation strength Δεslow of distilled water (blue solid circles, Δεwslow) and 3 wt % P-NIPAAm aqueous solution (red diamonds, Δεsslow), from 298 to 312 K. The blue empty circles (Δεw,Φ slow) represent the slow relaxation strength, under the assumption that no hydrated water is present in 3 wt % P-NIPAAm aqueous solution. Δεw.Φ slow is deduced by E

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 6. Relation between the temperature and Δεslow. (Solid blue circles) Δεslow of the distilled water; (empty blue circles) Δεslow of distilled water multiplied by the water mole fraction in the 3 wt % PNIPAAm solution; (red diamonds) Δεslow of the 3 wt % P-NIPAAm solution. The error bars are the standard errors originating from the fitting procedures. w, Φ w Δεslow (T ) = Δεslow (T ) × Φ

Figure 7. Hydration number of the P-NIPAAm solute from 298 to 312 K. The inset is the hydration number of the monomer unit (NIPAAm). The shaded area represents above Tcp. The error bars are calculated from the standard deviations in the fitting procedures, according to the error-propagation law.

compressibility, like ice, to be hydrated water, whereas we focused on the dynamical aspect of hydrated water, which has much more retarded dynamics than bulk water.64 Commonly, such a “static” hydration number determined by ultrasonic measurement is smaller than “dynamical” hydration; for example, in saccharides, the number of thermodynamically ice-like hydrated water (“static” hydration)65 is always not greater than all the hydrated water that are somehow perturbed by the solute.66 Considering the hydrophilic hydrated water directly hydrogen-bonded to P-NIPAAm solute, amide group (−NH) and carbonyl group (−CO) potentially offer one donor HB and two acceptor HBs, respectively, amounting in total to at most three hydrated water molecules existing per P-NIPAAm monomer unit.22 From this fact we can notice that hydrophobic hydration around the isopropyl group, as well as hydrophilic hydration, plays an important role in an entire hydration phenomenon of P-NIPAAm below Tcp, as indicated by various previous studies.25−29 Accordingly, we presume that the breakdown of Nhyd/n ≈ 10 below Tcp in the present study is as follows: three hydrated water molecules are directly hydrogen-bonded to the hydrophilic part of P-NIPAAm molecule, and the remaining seven are the hydrophobic hydrated water. This observation is consistent with the result from Ono and Shikata, who proposed a “HB bridge” scheme in which additional solid water HBs are formed with the water molecules directly hydrating P-NIPAAm, and also to each other.34 Above Tcp, we found a moderate decrease in Nhyd (Nhyd/n ≈ 6.5 at 312 K, decreased about 35% compared to that below Tcp), more than likely due to the coil-to-globule transition of PNIPAAm molecule. Our result that about 3.5 water molecules per monomer unit are liberated as bulk water during the coil-toglobule transition is in accord with previous literature (≈ 2.5).63 In a globule-like structure with hydrophilic −NH and −CO groups being invaginated, Ahmed et al. estimated two out of three direct hydrophilic hydrated water molecules survive (one of the two acceptor HBs of −CO is lost, and the donor HB of −NH is unperturbed) even above Tcp,22 with a result supported by MD simulation.33 Liu et al. also derived a similar result that the number of intermolecular P-NIPAAm−water HB decreased from about 1.5 to about 1.2 (per monomer) according to the coil-to-globule transition.32 On the basis of this description, the Nhyd/n ≈ 6.5 at 312 K found in our study can be disassembled as two hydrophilic hydration and four or five hydrophobic

(3)

where, Δεwslow(T) is the slow relaxation strength of the distilled water at temperature T and Φ is the water mole fraction of 3 wt % P-NIPAAm aqueous solution to the distilled water. Δεw,Φ slow s was larger than Δεslow at all measured temperatures, corresponding to changes in the amount of hydrated water in P-NIPAAm solution. To quantitatively evaluate the hydration number, Nhyd, we first calculated the molar concentration of bulk water Csbulk s C bulk (T ) =

s s Δεslow (T ) + Δεfast (T ) ρwater w w Δεslow (T ) + Δεfast(T ) M water

(4)

where ρwater is the density of water and Mwater is the molar weight of water.54,58,62 Although both the slow relaxation (Δεwslow and Δεsslow) and the fast relaxation (Δεwfast and Δεsfast) components are taken into account in eq 4, the dominant factor is the slow relaxation because Δεslow ≫ Δεfast (the population of HB bulk water is much larger than that of non-HB bulk water) at all conditions. Then, Nhyd is calculated by Nhyd(T ) =

s s Cwater − C bulk (T ) Csolute

(5)

Cswater

where and Csolute are the molar concentration of water and solute in the 3 wt % P-NIPAAm aqueous solution, respectively. The temperature dependence of the calculated hydration number, Nhyd, can be seen in Figure 7. Note that this Nhyd represents the hydration number per polymer with a polymerization degree n = 390, and the hydration number of the monomer (Nhyd/n) is shown in the inset in Figure 7. These results indicate that the hydration number, Nhyd, is almost constant below 306 K but starts to decrease after the coil-toglobule transition above Tcp. The measured hydration number of the monomer below Tcp, Nhyd/n ≈ 10, is in reasonable agreement with previous dielectric spectroscopic experiments (≈11 per monomer), 34,35 a theoretical study (1.6 g/g, corresponding to ≈10 hydration number per monomer),63 and MD simulation (≈12),33 but slightly larger than the ultrasonic combined with density measurement of Kogure et al. (≈7.5).64 The discrepancy with the absolute value obtained from ultrasonic measurements may be attributed to the difference in the definition of hydration; Kogure et al. considered water with smaller adiabatic F

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B hydration water. From this approximation, we find the number of hydrophobic hydrated water above Tcp is slightly decreased relative to that below Tcp, in line with the recent MD simulation.33 This can be ascribed to the solvent-exposed hydrophobic surface area becoming smaller in the globule-like P-NIPAAm structure as a result of the hydrophobic interactions,28,32 resulting in a reduced hydrophobic hydration number. To summarize, the observed moderate decrease in Nhyd/n of P-NIPAAm above Tcp stems mainly from a hydrophobic interaction accompanying the reduction of hydrophobic hydration water, and to a lesser extent from partial dehydration of hydrophilic hydrated water. 3.3. Population of Non-HB Water Molecules. In the terahertz region, the HB bulk water (slow relaxation) and nonHB bulk water (fast relaxation) can be dynamically distinguished, allowing us to directly look at “isolated” water molecules transiently released from the HB network of the bulk water.52 Identification of such non-HB water is quite difficult for other experimental techniques, such as nuclear magnetic resonance (NMR), infrared spectroscopy, Raman spectroscopy, and X-ray absorption spectroscopy, because their time resolution is either too long or too short to observe the peculiar dynamics of the non-HB water occurring at subpicosecond time scales. Thus, the existence of the non-HB water has not been intensively investigated, leading to the importance of the non-HB water around biomolecules and polymers being underestimated. In this study, we attempt to elucidate the relationship between the coil-to-globule transition of PNIPAAm and the corresponding change in the population of the non-HB water from the fast relaxation χ̃fast(ω). Figure 8 displays the temperature dependence of the relaxation strength, Δεfast, and relaxation time, τfast, for both

was calculated from the fraction of fast relaxation strength to total relaxation strength.54,55 σnon‐HB(T ) =

Δεfast(T ) Δεslow (T ) + Δεfast(T )

(6)

The resulting σnon‑HB(T), displayed in Figure 9, shows that the fraction of non-HB water molecules in distilled water was

Figure 9. Fraction of non-HB water of the distilled water and PNIPAAm aqueous solution at temperatures varying from 298 to 312 K.

almost constant at around 2.7−2.8%, indicating the HB destructuring effect is not strongly dependent on temperature, at least within the range from 298 to 312 K. On the other hand, the percentage of these molecules in P-NIPAAm solution was high and constant between 298 and 306 K (2.9%). The increase in the non-HB bulk water fraction (below Tcp) may originate from the strong destructuring effect of the water HB network imposed by hydrophilic groups in the P-NIPAAm (−NH and −CO groups). On the basis of MD simulation, Tamai et al.67 demonstrated that the probability to produce 0 HB water species around hydrophilic groups of P-NIPAAm is higher than that in bulk and around hydrophobic groups. Godec et al., who also used MD simulation to examine the water HB structure around hydrophilic and hydrophobic solutes, concluded the highly disordered HB structure found around the hydrophilic region is probably due to the steric constraint (such as the HB angle and distance) imposed by the hydrophilic groups.68 Accordingly, we assume the reason for the larger fraction of non-HB water below Tcp in this study is a result of the more highly exposed hydrophilic −NH and −CO groups, which generate more non-HB water molecules. It is noteworthy that σnon‑HB clearly fell off in P-NIPAAm aqueous solution above Tcp, whereas the σnon‑HB of distilled water exhibited a slight temperature dependency. From a thermodynamic point of view, the enthalpic penalty arising from the loss of water HBs around hydrophobic groups would lead to an increase in entropy in order to minimize the Gibbs free energy in the system,69 in turn enhancing the water HB structure in the vicinity of the hydrophobic region69−71 and reducing the population of non-HB water molecules.68 Given that MD simulation suggests that hydrophilic groups are more likely to produce a highly distorted HB structure than hydrophobic groups,63 the temperature dependence of σnon‑HB of P-NIPAAm aqueous solution in this study could be associated with the transition from a hydrophilic structure (below Tcp) to a hydrophobic one (above Tcp). In other words, the number of hydrophilic groups exposed to solvent decreased when the temperature exceeds Tcp thereby increasing the ratio of water-exposed hydrophobic groups. Accordingly, this result suggests that the coil-to-globule structure of P-NIPAAm

Figure 8. Temperature-dependent fast relaxation strength of the distilled water and 3 wt % P-NIPAAm aqueous solution. The inset is the fast relaxation time in picoseconds.

distilled water and 3 wt % P-NIPAAm aqueous solution. The result observed for distilled water from 298 to 312 K closely approximates that obtained by Yada et al.,52 confirming the validity of our measurement and analysis procedure. The relaxation strength, Δεfast, of distilled water and P-NIPAAm gradually decreases with increasing temperature at almost the same rate below 306 K, but above Tcp, that of P-NIPAAm drops off more rapidly than that of distilled water. Likewise, the relaxation time, τfast, of P-NIPAAm aqueous solution follows that of distilled water below Tcp but was significantly different above it. To quantitatively estimate the population of non-HB water molecules, the fraction of isolated water molecules (σnon‑HB) G

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B changes at Tcp with the invagination of hydrophilic −NH and −CO groups into the inner part of a molecule and that a large part of the outer surface area of P-NIPAAm is occupied with hydrophobic isopropyl groups above Tcp. This is in agreement with previous FTIR measurements,13−18 which concluded large number of hydrophilic group abandon to form amide−water or carbonyl−water HBs above Tcp and start to form additional intramolecular HBs (−NH··· OC−), suggesting that less hydrophilic region is exposed to water above Tcp. Furthermore, MD simulations33,72,73 and laser light scattering experiments9−11 that have calculated the gyration radius of PNIPAAm below and above Tcp have found that the gyration radius becomes smaller above Tcp. This is interpreted to be the result of hydrophobic interactions between separated hydrophobic isopropyl groups inducing changes to the whole molecular structure of P-NIPAAm into a globule-like configuration, and this configuration is stabilized by additional intramolecular −NH···OC− HBs. This reflects the fact that hydrophobic forces are more dominant than hydrophilic ones above Tcp. In view of these previous results, the decrease in σnon‑HB around Tcp (shown in Figure 9) is interpreted as a hydrophilic-to-hydrophobic phase change that accompanies the coil-to-globule transition of P-NIPAAm. Additional intramolecular HBs formed between −NH and −CO groups in the globule-like P-NIPAAm molecule may influence σnon‑HB, but the details of this are still unclear. This is because although PNIPAAm molecules at Tcp simultaneously undergo a hydrophilic-to-hydrophobic phase change and form additional intramolecular −NH···OC− HBs, these two events cannot be separated in our analysis. Quantitative changes in the population of non-HB water at the coil-to-globule transition were calculated from the non-HB number, Nnon‑HB (number of P-NIPAAm-induced non-HB water per solute). First, the molar concentration of the non-HB water in P-NIPAAm aqueous solution (Cnon‑HB) was derived from eq 7. Cnon‐HB(T ) = σnon‐HB(T )C bulk(T )

Figure 10. Number of the non-HB water per P-NIPAAm solute as a function of temperature. The inset shows the number of the non-HB water molecules per monomer unit.

Tcp. As discussed above, this temperature dependence around Tcp originates from the hydrophilic (water destructuring agent) to hydrophobic (water structuring agent) phase change that accompanies the coil-to-globule transition of P-NIPAAm. We found a significant reduction in the non-HB number; for example, Nnon‑HB at 312 K was 75% smaller than that below Tcp. This reduction (75% in Nnon‑HB) is much larger than that for Nhyd (∼32%), providing evidence that at the coil-to-globule transition Nnon‑HB undergoes a far larger change than Nhyd. 3.4. Tetrahedral Coordination of Water Molecules Interacting with P-NIPAAm. While vibrations at frequencies above the infrared region reflect transient dipole dynamics within a molecule, those in the terahertz region originate from large-amplitude and collective motions. Particularly, the intermolecular stretching vibration mode, χ̃S(ω), located around 5 THz, which is assigned to the hindered translational motion among hydrogen-bonded partners of water molecules aligned in the tetrahedral direction.53,74−78 The translational vibration mode is originally Raman-active and generates a strong absorption band in the Raman spectrum,79−81 but in the case of χ̃S(ω), it becomes infrared-active because of an intermolecular charge flux accompanying the dipole transfer.76−78 Yada et al. found the oscillator strength of the intermolecular stretch (f S) of water isotopes (H2O, D2O, and H218O) to have a systematic isotope effect in f S (H2O > D2O > H218O),53 which is correlated with the local asymmetry of the HB network.82 In other words, the oscillator strength, f S, is enhanced as a dynamical fluctuation of a local tetrahedral structure.53,54 As such, oscillator strength of the intermolecular stretching can be used as an indicator of the “dynamical” property of the tetrahedral HB network, that is, the degree of displacement of water molecules located at each vertex of water tetrahedrons. To understand the HB dynamical structure of distilled water and 3 wt % P-NIPAAm aqueous solution, the oscillator strength ( f S) was derived by53

(7)

Note that this Cnon‑HB contains two kinds of non-HB water molecules; one is originally inherent non-HB water in the bulkphase (adequately far from the solute), and the other is soluteinduced non-HB water in the vicinity of the P-NIPAAm molecule. Here, the former can be approximated to be equal to that in distilled water and our interest is the latter because we would like to know how many water molecules are isolated from the water HB network in the presence of P-NIPAAm. To achieve this, the non-HB number (Nnon‑HB) was calculated by54,55 Nnon‐HB(T ) =

w Cnon‐HB(T ) (T ) C bulk(T ) Δεfast − w w Csolute Δεslow (T ) + Δεfast(T ) Csolute (8)

where the first term represents the total number of the non-HB water in 3 wt % P-NIPAAm aqueous solution and the second term represents that in the bulk-phase unperturbed by the solute molecule. Thus, Nnon‑HB determined by eq 8 reflects the number of non-HB water induced by the presence of a PNIPAAm solute molecule. The resulting temperature dependence of Nnon‑HB is shown in Figure 10, and the inset shows the non-HB number per monomer unit (Nnon‑HB/n). It can be seen that Nnon‑HB is large below Tcp where the water−hydrophilic group interactions are more dominant than those of the water− hydrophobic groups, whereas Nnon‑HB abruptly decreases above

fS ∝

∫ ωIm[χS̃ (ω)] dω

(9)

The f S(T) for the distilled water and P-NIPAAm solution is shown in Figure 11a. The f S of distilled water increases linearly with temperature, suggesting the HB network becomes more dynamically disordered at higher temperature environments because of the larger thermal fluctuations. This temperature dependence of water is opposite to that seen in the lowfrequency Raman spectrum.80 This apparent discrepancy can be H

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

resonant frequency, ωS, and damping constant, γS, describe rather “static” aspect of the tetrahedral structure of the HB network. As shown in Figure 12, ωS of the distilled water is

Figure 11. (Upper panel) Oscillator strength, f S, of the distilled water (solid blue circles) and 3 wt % P-NIPAAm aqueous solution (red diamonds). f S of the distilled water multiplied by the water mole fraction of the P-NIPAAm solution (empty blue circles) is shown for comparison. (Lower panel) f S normalized by the distilled water at each temperature.

Figure 12. Temperature dependence of resonant frequency, ωS, and damping constant, γS, of the distilled water and 3 wt % P-NIPAAm solution.

explained by the fact that the Raman-active mode measures symmetric (ordered) translational motions of water, while infrared-active terahertz spectroscopy probes the asymmetric (disordered) motions. For P-NIPAAm aqueous solution, f S also continues to increase below Tcp, even though it is slightly smaller than that of distilled water. To further probe this phenomenon, we calculate the theoretical oscillator strength, f w,Φ S , which represents the oscillator strength of water molecules in 3 wt % P-NIPAAm solution, using eq 10. f Sw, Φ (T ) = f Sw (T ) × Φ

almost constant but γS increases linearly with elevating temperature between 298 and 312 K, a result in good accordance with a previous study.80 Because the resonance frequency of this stretching vibration is proportional to Young’s longitudinal modulus,81 which is closely associated with the intermolecular distance in this case, the nearly unchanged ωS of distilled water seems to come from the fact that water−water molecular distance (HB distance) in the liquid state is not strongly dependent on the temperature, as revealed by the radial distribution function.83 On the other hand, the linear increase in γS of distilled water stems from diverse distribution of the HB distances and HB angles as temperature increases because damping constants become large when the vibrational energy level reaches widespread distribution. Therefore, we can assume that temperature dependence in γS represents the disordered tetrahedral HB structure from a “static” point of view. For 3 wt % P-NIPAAm solution, the resonant frequency, ωS, was also constant and overlaps with that of distilled water within the error bars. Therefore, on average, water−water distance in P-NIPAAm aqueous solution is expected to be almost equal to that in distilled water. However, the damping constant, γS, shows a slightly different feature around Tcp: although increasing tendency similar to the distilled water is observed below Tcp, γS reaches a plateau above 306 K. As a result, γS of P-NIPAAm aqueous solution above Tcp is smaller than that expected from the temperature dependency of the distilled water, though the error bars were still partly overlapped. This experimental result implies the HB distances and angles exhibit small variance in water in P-NIPAAm solution, compared to that in the distilled water. In this section, on the basis of the intermolecular stretching vibration mode of water locating around 5 THz, we find that both dynamically and statically the tetrahedral HB structure of water becomes more ordered in P-NIPAAm aqueous solution above Tcp, as evidenced by the small oscillator strength, f S, and

(10)

f wS (T)

In eq 10, is the oscillator strength of distilled water at temperature T and Φ is the water mole fraction of 3 wt % PNIPAAm aqueous solution. As a result, f w,Φ (blue empty circles S in Figure 11) comes very close to the oscillator strength of PNIPAAm aqueous solution (red diamonds). The reason for this slight reduction in the oscillator strength of P-NIPAAm aqueous solution was identified to be the decrease in water molecules that are replaced by P-NIPAAm solute molecules (water dilution effect). No other distinct features were found between distilled water and the P-NIPAAm aqueous solution below Tcp, implying that the dynamical fluctuation in the tetrahedral HB structure is not strongly modulated by hydrophilic −NH and −CO groups. Interestingly though, the difference in the oscillator strength between distilled water and 3 wt % P-NIPAAm solution widens above Tcp. To examine this effect in detail, the oscillator strength normalized by f wS at each temperature is shown in Figure 11 (lower panel). Normalized f w,Φ is constant because Φ (relative water molar fraction) in eq S 10 is unchanged, but the normalized oscillator strength of PNIPAAm solution drastically drops off above Tcp. Because f S reflects the dynamical aspect of the structural disorder of water tetrahedron (not P-NIPAAm itself), it is suggested that the globule-like P-NIPAAm molecule above Tcp reduces the dynamical fluctuation of a tetrahedral unit of five water molecules embedded in the HB network. While the oscillator strength, f S, represents the “dynamical” fluctuation of the tetrahedral structure of water HBs, the I

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B small damping constant, γS. In fact, it should be noted that the temperature dependence in f S and γS reflects the spatially averaged behavior of water molecules (including such as bulk and hydrated water) in P-NIPAAm aqueous solution, but discontinuous temperature dependences at Tcp is dominantly responsible for the hydrated water near the P-NIPAAm molecule because bulk water does not exhibit such discontinuous trend in the examined temperature region. For this reason, we can assume that the hydrated water above Tcp can be characterized to form more ordered tetrahedral HB structure, with smaller dynamical fluctuation and relatively homogeneous HB environment. In recent MD simulation, Galamba calculated the orientational order parameter, q, of bulk and hydrophobic hydrated water, finding the degree of tetrahedrality is enhanced around the hydrophobic region.70,71 Coupled with the simulation result, the discontinuous temperature dependence in f S and γS at Tcp in the present study can be ascribed to the dominant influence of water−hydrophobic group (isopropyl group) interaction above Tcp.

likely to be aligned in a perfect tetrahedral structure,70,71 are the dominant force for P-NIPAAm in water. It is interesting to note that the reduction in the number of non-HB water (max. 75%) was significantly greater than that in the hydration number (max. 35%) during the coil-to-globule transition. This result implies that the conformational change of P-NIPAAm strongly affects the structure of the water HB network, rather than the hydration state. Although to identify the reason for this should be an important task to solve in the future, it may be speculated that not only water−P-NIPAAm interaction (i.e., hydration state) but also water−water dynamics around the P-NIPAAm molecule plays an essential role in P-NIPAAm conformation in the solvent.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 75 753 6169. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

4. CONCLUDING REMARKS The complex dielectric constant, ε̃(ω), of water and 3 wt % PNIPAAm aqueous solution in the terahertz region (0.25 and 12 THz) with temperature ranging from 298 to 312 K, was determined by terahertz-ATR spectroscopy. The frequency dispersion in ε̃(ω) of P-NIPAAm aqueous solution was found to be similar to that of the distilled water, which confirms the complex dielectric constant in the terahertz region is selectively sensitive to water relaxations and vibrations at picosecond or subpicosecond time scales. Therefore, ε̃(ω) of both distilled water and 3 wt % P-NIPAAm solution was successfully disentangled into four constituent elements (the slow relaxation of bulk water, fast relaxation of bulk water, intermolecular stretching, and libration of water molecule), allowing us to individually discuss the hydration state and the HB network of water in detail. As a result, the number of hydrated water molecules per monomer unit was found to be 10 below Tcp, in quite good agreement with previous experiments34,35 and simulation.33 This number included not only hydrophilic hydrated water via water−amide (−NH) or water−carbonyl (−CO) HBs but also hydrophobic hydrated water around the isopropyl group. However, a gradual dehydration started at Tcp because of coil-to-globule transition, and the reduction in the hydration number amounted to 35% (from 10 below Tcp to 6.5 above Tcp). Predominantly, the dehydration above Tcp was identified as the loss of hydrophobic hydrated water, according to the minimization of solvent-exposed hydrophobic surface area driven by the hydrophobic interactions. The modulation in the water HBs was discussed in two ways: one is the population of the non-HB water molecules isolated from the HB network, and the other is the tetrahedral structure of water engaged in the HB network. First, it was shown that the number of solute-induced non-HB water undergoes a clear drop off at Tcp (75% decrease at maximum), indicating the water HB structure is significantly strengthened above Tcp. Second, from the oscillator strength and damping constant of the intermolecular stretching vibration mode, the tetrahedral coordination of the HB network was found to be both dynamically and statically ordered above Tcp, relative to that below Tcp. This is because in the globule-like structure, water− hydrophobic interactions, where water molecules can be more

ACKNOWLEDGMENTS We are grateful to Mr. Motoki Imamura and Mr. Akiyoshi Irisawa (ADVANTEST Corporation, Japan) for their technical support. We also acknowledge Professor Garry John Piller (Graduate School of Agriculture, Kyoto University, Japan) for his help and useful discussions. Financial support was provided by Industry-Academia Collaborative R&D from Japan Science and Technology Agency and JSPS KAKENHI Grant 26295.

■

REFERENCES

(1) Schild, H. G. Poly(N-isopropylacrylamide): Experiment, Theory and Application. Prog. Polym. Sci. 1992, 17, 163−249. (2) Heskins, M.; Guillet, J. E. Solution Properties of Poly(Nisopropylacrylamide). Macromol. Sci. Chem. 1968, A2, 1441−1455. (3) Kubota, K.; Fujishige, S.; Ando, I. Single-Chain Transition of Poly(N-isopropylacrylamide) in Water. J. Phys. Chem. 1990, 94, 5154− 5158. (4) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Temperature-Responsive Bioconjugates. 1. Synthesis of TemperatureResponsive Oligomers with Reactive End Groups and Their Coupling to Biomolecules. Bioconjugate Chem. 1993, 4, 341−346. (5) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Dynamic Contact Angle Measurement of Temperature-Responsive Surface Properties for Poly(N-isopropylacrylamide) Grafted Surfaces. Macromolecules 1994, 27, 6163−6166. (6) Gao, Y.; Yang, J.; Ding, Y.; Ye, X. Effect of Urea on Phase Transition of Poly(N-isopropylacrylamide) Investigated by Differential Scanning Calorimetry. J. Phys. Chem. B 2014, 118, 9460−9466. (7) Chung, J. E.; Yokoyama, M.; Yamato, M.; Aoyagi, T.; Sakurai, Y.; Okano, T. Thermo-Responsive Drug Delivery from Polymeric Micelles Constructed Using Block Copolymers of Poly(N-isopropylacrylamide) and Poly(butylmethacrylate). J. Controlled Release 1999, 62, 115−127. (8) Kwon, O. H.; Kikuchi, A.; Yamato, M.; Sakurai, Y.; Okano, T. Rapid Cell Sheet Detachment from Poly(N-isopropylacrylamide)Grafted Porous Cell Culture Membranes. J. Biomed. Mater. Res. 2000, 50, 82−89. (9) Wu, C.; Zhou, S. Laser Light Scattering Study of the Phase Transition of Poly(N-isopropylacrylamide) in Water. 1. Single Chain. Macromolecules 1995, 28, 8381−8387. (10) Wang, X.; Qiu, X.; Wu, C. Comparison of the Coil-to-Globule and the Globule-to-Coil Transitions of a Single Poly(N-isopropylacrylamide) Homopolymer Chain in Water. Macromolecules 1998, 31, 2972−2976.

J

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

isopropylacrylamide) Gels in Water. Macromolecules 2010, 43, 5103− 5113. (30) Shibayama, M.; Morimoto, M.; Nomura, S. Phase Separation Induced Mechanical Transition of Poly(N-isopropylacrylamide)/ Water Isochore Gels. Macromolecules 1994, 27, 5060−5066. (31) Shibayama, M.; Mizutani, S.; Nomura, S. Thermal Properties of Copolymer Gels Containing N-isopropylacrylamide. Macromolecules 1996, 29, 2019−2024. (32) Liu, S. M.; Taylor, C.; Chong, B.; Liu, L.; Bilic, A.; Terefe, N. S.; Stockmann, R.; Thang, S. H.; De Silva, K. Conformational Transitions and Dynamics of Thermal Responsive Poly(N-isopropylacrylamide) Polymers as Revealed by Molecular Simulation. Eur. Polym. J. 2014, 55, 153−159. (33) Deshmukh, S. A.; Sankaranarayanan, S. K. R. S.; Suthar, K.; Mancini, D. C. Role of Solvation Dynamics and Local Ordering of Water in Inducing Conformational Transition in Poly(N-isopropylacrylamide) Oligomers through the LCST. J. Phys. Chem. B 2012, 116, 2651−2663. (34) Ono, Y.; Shikata, T. Hydration and Dynamic Behavior of Poly(N-isopropylacrylamide)s in Aqueous Solutions: A Sharp Phase Transition at the Lower Critical Solution Temperature. J. Am. Chem. Soc. 2006, 128, 10030−10031. (35) Ono, Y.; Shikata, T. Contrary Hydration Behavior of Nisopropylacrylamide to its Polymer, P(NIPAm), with a Lower Critical Solution Temperature. J. Phys. Chem. B 2007, 111, 1511−1513. (36) Satokawa, Y.; Shikata, T.; Tanaka, F.; Qiu, X.-P.; Winnik, F. M. Hydration and Dynamics Behavior of a Cyclic Poly(N-isopropylacrylamide) in Aqueous Solution: Effects of the Polymer Chain Topology. Macromolecules 2009, 42, 1400−1403. (37) Bagchi, B. Water Dynamics in the Hydration Layer around Proteins and Micelles. Chem. Rev. (Washington, DC, U.S.) 2005, 105, 3197−3219. (38) Arikawa, T.; Nagai, M.; Tanaka, K. Characterizing Hydration State in Solution Using Terahertz Time-Domain Attenuated Total Reflection Spectroscopy. Chem. Phys. Lett. 2008, 457, 12−17. (39) Hishida, M.; Tanaka, K. Long-Range Hydration Effect of Lipid Membrane Studies by Terahertz Time-Domain Spectroscopy. Phys. Rev. Lett. 2011, 106, 158102. (40) Hishida, M.; Tanaka, K. Transition of the Hydration State of a Surfactant Accompanying Structural Transitions of Self-Assembled Aggregates. J. Phys.: Condens. Matter 2012, 24, 284113. (41) Ebbinghaus, S.; Kim, S. J.; Heyden, M.; Yu, X.; Heugen, U.; Gruebele, M.; Leitner, D. M.; Havenith, M. An extended Dynamical Hydration Shell around Proteins. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 20749−20752. (42) Heyden, M.; Bründermann, E.; Heugen, U.; Niehues, G.; Leitner, D. M.; Havenith, M. Long-Range Influence of Carbohydrates on the Solvation Dynamics of Water − Answers from Terahertz Absorption Measurements and Molecular Modeling Simulations. J. Am. Chem. Soc. 2008, 130, 5773−5779. (43) Mirabella, F. M. Internal Reflection Spectroscopy; Marcel Dekker, Inc.: New York, 1993. (44) Jepsen, P. U.; Cooke, D. G.; Koch, M. Terahertz Spectroscopy and Imaging − Modern Techniques and Applications. Laser Photonics Rev. 2011, 5, 124−166. (45) Yasui, T.; Saneyoshi, E.; Araki, T. Asynchronous Optical Sampling Terahertz Time-Domain Spectroscopy for Ultrahigh Spectral Resolution and Rapid Data Acquisition. Appl. Phys. Lett. 2005, 87, 061101. (46) Hirori, H.; Yamashita, K.; Nagai, M.; Tanaka, K. Attenuated Total Reflection Spectroscopy in Time Domain Using Terahertz Coherent Pulses. Jap. J. Appl. Phys. 2004, 43, L1287−1289. (47) Nagai, M.; Yada, H.; Arikawa, T.; Tanaka, K. Terahertz TimeDomain Attenuated Total Reflection Spectroscopy in Water and Biological Solution. Int. J. Infrared Millimeter Waves 2006, 25, 505− 515. (48) Bertie, J. E.; Lan, Z. An Accurate Modified Kramers-Kronig Transformation from Reflectance to Phase Shift on Attenuated Total Reflection. J. Chem. Phys. 1996, 105, 8502−8514.

(11) Wang, X.; Wu, C. Light-Scattering Study of Coil-to-Globule Transition of a Poly(N-isopropylacrylamide) Chain in Deuterated Water. Macromolecules 1999, 32, 4299−4301. (12) Spevácek, J. Phase Separation in Aqueous Polymer Solution as Studied by NMR Methods. Macromol. Symp. 2005, 222, 1−13. (13) Lin, S.-Y.; Chen, K.-S.; Run-Chu, L. Drying Methods Affecting the Particle Sizes, Phase Transition, Deswelling/Reswelling Processes and Morphology of Poly(N-isopropylacrylamide) Microgel Beads. Polymer 1999, 40, 2619−2624. (14) Maeda, Y.; Higuchi, T.; Ikeda, I. Change in Hydration State during the Coil-to-Globule Transition of Aqueous Solutions of Poly(N-isopropylacrylamide) as Evidenced by FTIR Spectroscopy. Langmuir 2000, 16, 7503−7509. (15) Maeda, Y.; Higuchi, T.; Ikeda, I. FTIR Spectroscopic and Calorimetric Studies of the Phase Transitions of N-isopropylacrylamide Copolymers in Water. Langmuir 2001, 17, 7535−7539. (16) Ramon, O.; Kesselman, E.; Berkovici, R.; Cohen, Y.; Paz, Y. Attenuated Total Reflectance/Fourier Transform Infrared Studies on the Phase-Separation Process of Aqueous Solutions of Poly(Nisopropylacrylamide). J. Polym. Sci. 2001, 39, 1665−1677. (17) Kesselman, E.; Ramon, O.; Berkovici, R.; Paz, Y. ATR-FTIR Studies on the Effect of Strong Salting-out Salts on the Phase Separation Scenario in Aqueous Solutions of Poly(N-isopropylacrylamide) [PNIPA]. Polym. Adv. Technol. 2002, 13, 982−991. (18) Meersman, F.; Wang, J.; Wu, Y.; Heremans, K. Pressure Effect on the Hydration Properties of Poly(N-isopropylacrylamide) in Aqueous Solution Studied by FTIR Spectroscopy. Macromolecules 2005, 38, 8923−8928. (19) Lai, H.; Wu, P. A Infrared Spectroscopic Study on the Mechanism of Temperature-Induced Phase Transition of Concentrated Aqueous Solutions of Poly(N-isopropylacrylamide) and Nisopropylacrylamide. Polymer 2010, 51, 1404−1412. (20) Sun, B.; Lin, Y.; Wu, P.; Siesler, H. W. A FTIR and 2D-IR Spectroscopic Study on the Microdynamics Phase Separation Mechanism of the Poly(N-isopropylacrylamide) Aqueous Solution. Macromolecules 2008, 41, 1512−1520. (21) Tsuboi, Y.; Nishino, M.; Sasaki, T.; Kitamura, N. Poly(Nisopropylacrylamide) Microparticles Produced by Radiation Pressure of a Focused Lase Beam: A Structural Analysis by Confocal Raman Microspectroscopy Combined with a Laser-Trapping Technique. J. Phys. Chem. B 2005, 109, 7033−7039. (22) Ahmed, Z.; Gooding, E. A.; Pimenov, K. V.; Wang, L.; Asher, S. A. UV Resonance Raman Determination of Molecular Mechanism of Poly(N-isopropylacrylamide) Volume Phase Transition. J. Chem. Phys. B 2009, 113, 4248−4256. (23) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Thermal Analysis of the Volume Phase Transition with N-isopropylacrylamide Gels. Macromolecules 1990, 23, 283−289. (24) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Effect of Comonomer Hydrophilicity and Ionization on the Lower Critical Solution Temperature of N-isopropylacrylamide Copolymers. Macromolecules 1993, 26, 2496−2500. (25) Grinberg, V. Y.; Dubovik, A. S.; Kuznetsov, D. V.; Grinberg, N. V.; Grosberg, A. Y.; Tanaka, T. Studies of the Thermal Volume Transition of Poly(N-isopropylacrylamide) Hydrogels by HighSensitivity Differential Scanning Microcalorimetry. 2. Thermodynamics Functions. Macromolecules 2000, 33, 8685−8692. (26) Cho, E. C.; Lee, J.; Cho, K. Role of Bound Water and Hydrophobic Interaction in Phase Transition of Poly(N-isopropylacrylamide) Aqueous Solution. Macromolecules 2003, 36, 9929−9934. (27) Ding, Y.; Ye, X.; Zhang, G. Microcalorimetric Investigation on Aggregation and Dissolution of Poly(N-isopropylacrylamide) Chains in Water. Macromolecules 2005, 38, 904−908. (28) Okada, Y.; Tanaka, F. Cooperative Hydration, Chain Collapse, and Flat LCST Behavior in Aqueous Poly(N-isopropylacrylamide) Solution. Macromolecules 2005, 38, 4465−4471. (29) Kojima, H.; Tanaka, F. Cooperative Hydration Induces Discontinuous Volume Phase Transition of Cross-Linked Poly(NK

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B (49) Naito, H.; Ogawa, Y.; Hoshina, H.; Sultana, S.; Kondo, N. Analysis of Intermolecular Interaction of Poly(N-isopropylacrylamide) Solution with Attenuated Total Reflectance Terahertz Spectroscopy. Appl. Phys. Lett. 2012, 100, 191102. (50) Nakano, S.; Sato, Y.; Kita, R.; Shinyashiki, N.; Yagihara, S.; Sudo, S.; Yoneyama, M. Molecular Dynamics of Poly(N-isopropylacrylamide) in Protic and Aprotic Solvents Studies by dielectric Relaxation Spectroscopy. J. Phys. Chem. B 2012, 116, 775−781. (51) Füllbrandt, M.; Ermilova, E.; Asadujjaman, A.; Hölzel, R.; Bier, F. F.; von Klitzing, R.; Schönhals, S. Dynamics of Linear Poly(Nisopropylacrylamide) in Water around the Phase Transition Investigated by Dielectric Relaxation Spectroscopy. J. Phys. Chem. B 2014, 118, 3750−3759. (52) Yada, H.; Nagai, M.; Tanaka, K. Origin of the Fast Relaxation Component of Water and Heavy Water Revealed by Terahertz TimeDomain Attenuated Total Reflection Spectroscopy. Chem. Phys. Lett. 2008, 464, 166−170. (53) Yada, H.; Nagai, M.; Tanaka, K. The Intermolecular Stretching Vibration Mode in Water Isotopes Investigated with Broadband Terahertz Time-Domain Spectroscopy. Chem. Phys. Lett. 2009, 473, 279−283. (54) Shiraga, K.; Suzuki, T.; Kondo, N.; Ogawa, Y. Hydration and Hydrogen Bond Network of Water around Hydrophobic Surface Investigated by Terahertz Spectroscopy. J. Chem. Phys. 2014, 141, 235103. (55) Shiraga, K.; Suzuki, T.; Kondo, N.; De Baerdemaeker, J.; Ogawa, Y. Quantitative Characterization of Hydration State and Destructuring Effect of Monosaccharides and Disaccharides on Water Hydrogen Bond Network. Carbohyd. Res. 2015, 406, 46−54. (56) Grant, E.; Sheppard, R.; South, G. Dielectric Behavior of Biological Molecules in Solution; Clarendon Press: Oxford, U.K., 1978. (57) Sato, T.; Buchner, R.; Fernandez, S.; Chiba, A.; Lunz, W. Dielectric Relaxation Spectroscopy of Aqueous Amino Acid Solutions: Dynamics and Interactions in Aqueous Glycine. J. Mol. Liq. 2005, 117, 93−98. (58) Cametti, C.; Marchetti, S.; Gambi, C. M. C.; Onori, G. Dielectric Relaxation Spectroscopy of Lysozyme Aqueous Solutions: Analysis of the δ-Dispersion and the Contribution of the Hydration Water. J. Phys. Chem. B 2011, 115, 7144−7153. (59) Vidulich, G. A.; Evans, D. F.; Kay, R. L. The Dielectric Constant of Water and Heavy Water between 0 and 40°. J. Phys. Chem. 1967, 71, 656−662. (60) Ronne, C.; Thrane, L.; Åstrand, P.-O.; Wallqvist, A.; Mikkelsen, K. V.; Keiding, A. R. Investigation of the Temperature Dependence of Dielectric Relaxation in Liquid Water by THz Reflection Spectroscopy and Molecular Dynamics Simulation. J. Chem. Phys. 1997, 107, 5319− 5331. (61) Zelsmann, H. R. Temperature Dependence of the Optical Constants for Liquid H2O and D2O in the Far IR Region. J. Mol. Struct. 1995, 350, 95−114. (62) Cametti, C.; Marchetti, S.; Onori, G. Lysozyme Hydration in Concentrated Aqueous Solutions. Effect of an Equilibrium Cluster Phase. J. Phys. Chem. B 2013, 117, 104−110. (63) Lele, A. K.; Hirve, M. M.; Badiger, M. V.; Mashelkar, R. A. Predictions of Bound Water Content in Poly(N-isopropylacrylamide) Gel. Macromolecules 1997, 30, 157−159. (64) Kogure, H.; Nanami, S.; Masuda, Y.; Toyama, Y.; Kubota, K. Hydration and Dehydration Behavior of N-isopropylacrylamide Gel Particles. Colloid Polym. Sci. 2005, 283, 1163−1171. (65) Furuki, T. Effect of Molecular Structure on Thermodynamics Properties of Carbohydrates. A Calorimetric Study of Aqueous Di- and Oligosaccharides at Subzero Temperatures. Carbohyd. Res. 2002, 337, 441−450. (66) Shiraga, K.; Ogawa, Y.; Kondo, N.; Irisawa, A.; Imamura, M. Evaluation of the Hydration State of Saccharides Using Terahertz Time-Domain Attenuated Total Reflection Spectroscopy. Food Chem. 2013, 140, 315−320.

(67) Tamai, Y.; Tanaka, H.; Nakanishi, K. Molecular Dynamics Study of Polymer-Water Interaction in Hydrogels. 1. Hydrogen-Bond Structure. Macromolecules 1996, 29, 6750−6760. (68) Godec, A.; Smith, J. C.; Merzel, F. Increase of Both Order and Disorder in the First Hydration Shell with Increasing Solute Polarity. Phys. Rev. Lett. 2011, 107, 267801. (69) Frank, H.; Evans, M. W. Free Volume and Entropy in Condensed Systems III. Entropy in Binary Liquid Mixtures; Partial Molal Entropy in Dilute Solutions; Structure and Thermodynamics in Aqueous Electrolytes. J. Chem. Phys. 1945, 13, 507−532. (70) Galamba, N. Water’s Structure around Hydrophobic Solutes and the Iceberg Model. J. Phys. Chem. B 2013, 117, 2153−2159. (71) Galamba, N. Water Tetrahedrons, Hydrogen-Bond Dynamics, and the Orientational Mobility of Water around Hydrophobic Solutes. J. Phys. Chem. B 2014, 118, 4169−4176. (72) Longhi, G.; Lebon, F.; Abbate, S.; Fornili, S. L. Molecular Dynamics Simulation of a Model Oligomer for Poly(N-isopropylacrylamide) in Water. Chem. Phys. Lett. 2004, 386, 123−127. (73) Tucker, A. K.; Stevens, M. J. Study of the Polymer Length Dependence of the Single Chain Transition Temperature in Syndiotactic Poly(N-isopropylacrylamide) Oligomers in Water. Macromolecules 2012, 45, 6697−6703. (74) Agmon, N. Tetrahedral Displacement: The Molecular Mechanism behind the Debye Relaxation in Water. J. Phys. Chem. 1996, 100, 1072−1080. (75) Martí, J.; Padro, J. A.; Guàrdia, E. Molecular Dynamics Simulation of Liquid Water along the Coexistence Curve: Hydrogen Bonds and Vibrational Spectra. J. Chem. Phys. 1996, 105, 639−649. (76) Sharma, M.; Resta, R.; Car, R. Intermolecular Dynamical Change Fluctuation in Water: A Signature of the H-Bond Network. Phys. Rev. Lett. 2005, 95, 187401. (77) Torii, H. Intermolecular Electron Density Modulations in Water and Their Effects on the Far-Infrared Spectral Profiles at 6 THz. J. Phys. Chem. B 2011, 115, 6636−6643. (78) Torii, H. Cooperative Contributions of the Intermolecular Charge Fluxes and Intramolecular Polarizations in the Far-Infrared Spectral Intensities of Liquid Water. J. Chem. Theory Comput. 2014, 10, 1219−1227. (79) Walrafen, G. E. Raman Spectral Studies of Water Structures. J. Chem. Phys. 1964, 40, 3249−3256. (80) Amo, Y.; Tominaga, Y. Low-Frequency Raman Study of Water Isotopes. Phys. A (Amsterdam, Neth.) 2000, 276, 401−412. (81) Perticaroli, S.; Sassi, P.; Moressi, A.; Paolantoni, M. LowWavenumber Raman Scattering from Aqueous Solutions of Carbohydrates. J. Raman Spectrosc. 2008, 39, 227−232. (82) Bergmann, U.; Nordlund, D.; Wernet, Ph.; Odelius, M.; Pettersson, L. G. M.; Nilsson, A. Isotope Effects in Liquid Water Probed by X-Ray Raman Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 024202. (83) Narten, A. H.; Levy, H. A. Liquid Water: Molecular Correlation Functions from X-Ray Diffraction. J. Chem. Phys. 1971, 55, 2263− 2269.

L

DOI: 10.1021/acs.jpcb.5b01021 J. Phys. Chem. B XXXX, XXX, XXX−XXX