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2007, 111, 1511-1513 Published on Web 02/01/2007
Contrary Hydration Behavior of N-Isopropylacrylamide to its Polymer, P(NIPAm), with a Lower Critical Solution Temperature Yousuke Ono and Toshiyuki Shikata* Department of Macromolecular Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed: December 27, 2006; In Final Form: January 22, 2007
The number of hydrated water molecules per N-isopropylacrylamide in homogeneous aqueous solution was determined to be a constant with a value of 5-6 below and above the lower critical solution temperature, LCST (32 °C), of its polymer, poly(N-isopropylacrylamide), by high-frequency dielectric relaxation techniques.
Poly(N-isopropylacrylamide) (P(NIPAm)) is a well-known water-soluble polymer whose aqueous solutions exhibit lower critical solution temperatures (LCSTs).1-3 P(NIPAm) exists in an expanded conformation in water below the LCST of 32 °C in a homogeneous liquid phase, while its conformation becomes compact, resulting in a separated two-phase above the LCST. From a theoretical viewpoint,3 it has been proposed that the phase transition is governed by cooperative dehydration of P(NIPAm). Recently, the temperature dependence of the hydration number per monomer unit of P(NIPAm) in aqueous solution has been examined accurately. The hydration number was determined to be 11 below the LCST and it sharply diminished above the LCST, irrespective of the molar mass and its distribution of P(NIPAm)s.2 This supports the cooperative dehydration of P(NIPAm)s at the LCST. However, whether the dehydration is originated by characteristics of the monomer, N-isopropylacrylamide (NIPAm), or characteristics resulting from the polymerization of the monomer has not been verified so far. In this study, we found that the temperature dependence of the hydration behavior of the monomer, NIPAm, and its analog, N-isopropylpropionamide (NIPPA), in aqueous solution, is completely contrary to that of P(NIPAm)s using dielectric relaxation (DR) techniques that are powerful enough to detect the dynamics of water molecules hydrated to solutes in aqueous solution. DR measurements in the high-frequency range, up to 20 GHz (1.26 × 1011 s-1 in angular frequency, ω), allowed us to determine relaxation times and strengths for the solutes, NIPAm and NIPPA, and also for the solvent, water. Thus, the experimental results were directly related to the number and dynamics of hydrated water molecules to the solutes.2,4-6 The highfrequency dielectric relaxation behavior, real and imaginary parts (′ and ′′) of the (relative) complex permittivity versus ω, were investigated for aqueous NIPAm and NIPPA solutions7 at concentrations c ) 0.12-1.50 M over a wide ω range, up to 1.26 × 1011 s-1, and at various temperatures, T, from 8 to 39 °C,8 which was higher than the LCST of an aqueous solution of P(NIPAm)s. The NIPAm and NIPPA solutions at c e 1.5 M showed no phase transition in the T range examined as in the * Corresponding author. E-mail:
[email protected].
10.1021/jp068954k CCC: $37.00
Figure 1. Angular frequency, ω, dependence of the real and imaginary parts of the electric permittivity, ′ and ′′, for an aqueous solution of NIPAm at c ) 1.5 M and 25 °C. This figure also contains the contribution of the solute, NIPAm, including hydrated water molecules, ∆′ and ∆′′.
case of P(NIPAm) slutions.1 The number of hydrated water molecules per NIPAm and NIPPA molecule were determined as a function of T. Exchange processes of water molecules hydrated to the solutes and rotational relaxations of the solutes will also be discussed. To determine the dielectric contribution of the solutes exactly, including hydrated water molecules, the values of ∆′ and ∆′′, ′ and ′′ of sample solutions were deconvoluted into three Debye-type relaxation modes. The contribution for bulk (free) water, ′w and ′′w, were assigned to the fastest mode found at a relaxation time of τw ) 8.3-11.5 ps at 25 °C slightly dependent on c.10 Then, ∆′ and ∆′′ were obtained as follows: ∆′ ) ′ - 1 - ′w and ∆′′ ) ′′ - ′′w. Figure 1 shows typical dielectric spectra, ′ and ′′, and ∆′ and ∆′′ versus ω, for an aqueous NIPAm solution at c ) 1.5 M and 25 °C. The dependencies of ∆′ and ∆′′ on ω involved two relaxation modes with relaxation times of 33 and 119 ps, and similar spectra were also obtained in all of the samples examined. The ratio Φ ) w/wp represents the fractional dielectric contribution of the bulk water, w, to pure water, wp.10 The value of Φ was evaluated to be 0.60 to 0.635 for the ∆′ and © 2007 American Chemical Society
1512 J. Phys. Chem. B, Vol. 111, No. 7, 2007
Letters
Figure 2. Temperature, T, dependencies of the hydration number for solutes, m: NIPAm (closed) and NIPPA (open symbols), the concentration-normalized dielectric relaxation strength of the exchange process for hydrated water molecules, exc-1, and for that of the rotational relaxation process of the solutes, r c-1. Plural data points for m, exc-1, and r c-1 found at the same temperatures means that those were determined at different c values. A broken line represents m of aqueous P(NIPAm) solutions.2
∆′′ spectra shown in Figure 1.11 The relationship between Φ and c contains important information related to the volume fraction of a solute, φ, and also the number of hydrated water molecules per solute molecule, m. Φ was described accurately by eq 1 using the partial molar volume of the solute, V h s, and that of water, V h w, (in the unit of cm3 mol-1) at measured temperatures12,13
Φ)
hs c 1 - 10-3V 10-3V hs c 1+ 2
- 10-3mV h wc
(1)
where 10-3V h sc ) φ.4,5 The value of m is plotted as a function of T for aqueous NIPAm and NIPPA solutions in Figure 2.14 In this figure, plural data points showing the m values with the coincident symbols are found at the same temperatures, which means that the data were determined at different c values. The obtained m was a constant with a value of ca. 5-6, irrespective of T, even above the LCST of P(NIPAm)s. Nevertheless, the value of m for an aqueous P(NIPAm) solution was kept at ca. 11, and decreased markedly at the LCST of 32 °C 2 as also shown in Figure 2. Consequently, no water molecules dehydrated from both of NIPAm and NIPPA molecules at the LCST of P(NIPAm). These imply that the sharp phase transition of P(NIPAm)s at the LCST is induced by the cooperative dehydration of P(NIPAm), which are characteristics originated by the polymerization of the monomer, NIPAm. Such characteristics, which are necessary for the sharp LCST, possibly require a degree of polymerization higher than a certain value. The precise number of hydrated water molecules in phase-separated P(NIPAm) chains above the LCST is not known due to the difficulty of experiments.2 However, most of 11 water molecules SCHEME 1
hydrated per P(NIPAm) monomer unit possibly dehydrate cooperatively above the LCST because the formation of hydrogen bonds, NH‚‚‚OdC, between isopropylamide groups has been confirmed specifically using spectroscopic techniques such as infrared adsorption measurements15 in phase-separated P(NIPAm) chains. Each isopropylamide group of the monomer, NIPAm (or NIPPA), is mainly hydrated by 5-6 water molecules, and the reason for the hydration has been attributed to the hydrogen bond formation, according to the results obtained by other spectroscopic studies.16 Thus, the value of m ) 11 for the polymer, P(NIPAm), strongly suggested that 5-6 additional water molecules formed hydrogen bonds to the water molecules directly hydrated to isopropylamide groups producing hydrogen bond bridges between many monomers involved in P(NIPAm), as is depicted schematically in Scheme 1. The hydrogen bond bridges formed between monomer units play an important role in sustaining an extended conformation of P(NIPAm) below the LCST. The cooperative loss of the hydrogen bond bridges at the LCST is likely an essential trigger for the phase transition of the aqueous P(NIPAm) solution. Consequently, both water molecules with a value of 5-6 bridging hydrated isopropylamide groups owing to hydrogen bonding and most of water molecules directly hydrated to each isoprolylamide group in P(NIPAm) chains with the value similar to that above, depicted schematically in Scheme 1, dehydrate at a time cooperatively at the LCST. A fast relaxation mode in ∆′ and ∆′′ at c ) 1.50 M and 25 °C was described by a set of Debye-type relaxation functions9 with a relaxation time of τex ) 33 ps, as seen in Figure 1. This mode, of which τex decreased with increasing T with an activation energy, E*ex, ) 27 kJ mol-1,17 was assigned to the exchange process of water molecules hydrated to NIPAm and NIPPA. The exchange process for hydrated water molecules to P(NIPAm) was also observed at a relaxation time similar to τex.2 The concentration-normalized relaxation strength of the exchange process, exc-1, seemed to be a constant of 9.0 ( 0.5 M-1 irrespective of c as seen in Figure 2. Plural data points for exc-1 found at the same temperatures also means that the data were determined at different c values. This significantly sustained our assignment of the relaxation mode. Although the source dipoles belonged to water molecules in both the exchange and rotational relaxation processes of pure water, the magnitude of the relaxation strength per unit concentration of the hydrated water molecules in the exchange process, ex(mc)-1 ) 1.5 ∼ 1.6 M-1 assuming m ) 6, was greater than that of pure water, 10-3wpV h w ()1.3 M-1 at 25 °C), by a factor of 15 to 20%. Such a discrepancy has also been observed in the exchange process in the aqueous P(NIPAm) system showing ex(mc)-1 ∼ 1.5 M-1 below the LCST.2 The other slow relaxation mode was described by a set of Debye-type functions with a relaxation time of τr ) 119 ps at
Letters c ) 1.50 M and 25 °C as shown in Figure 1. The value of τr decreased with increasing T with an activation energy identical to that of the relaxation time for pure water molecules, E*w.17 Moreover, the concentration-normalized relaxation strength of the slow relaxation mode, r c-1, seemed to be a constant with a value of 7.0 ( 0.5 M-1 independent of c and T as seen in Figure 2. From these values, the slow relaxation was assigned to the overall rotational relaxation of NIPAm and NIPPA molecules, carrying dipoles of isopropylamide groups, dissolved in water with a viscosity governed by E*w. In the aqueous P(NIPAm) system, a rotational process of monomer units involving isopropylamide groups was observed at 104 ps as broad relaxations with the concentration-normalized total relaxation strength similar to the r c-1 value of 7.0 ( 0.5 M-1.2 This marked discrepancy between relaxation times for the rotational processes of isopropylamide groups in each system implies that the presence of polymer backbones in P(NIPAm) significantly reduced the rates of rotations of monomer units rather than those of free rotations of NIPAm or NIPPA in water. The longitudinal relaxation times, T1, of 13C NMR for a D2O solution of NIPAm were measured at c ) 1.3 M and 25 °C to determine the time scale for rotations of NIPAm molecules in aqueous (D2O) solution.18 The obtained T1 data yielded a rotational relaxation time, τrNMR, of ca. 90 ps for NIPAm.19 The reasonable agreement between τr and τrNMR values supported our assignment of the slow relaxation mode to the rotational relaxation of NIPAm molecules in water. Acknowledgment. We thank Dr. C. Hashimoto and Professor Y. Ozaki, Kwanseigakuin University, for her kind supply of a NIPPA sample. Y.O. expresses special thanks to the Center of Excellence (21COE) program “Creation of Integrated EcoChemistry of Osaka University”. References and Notes (1) (a) Shibayama, M.; Morimoto, M.; Nomura, S. Macromolecules 1994, 27, 5060-5066. (b) Shibayama, M.; Mizutani, S.; Nomura, S. Macromolecules 1996, 29, 2019-2024. (c) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311-3313. (d) Kogure, H.; Nanami, S.; Masuda, Y.; Toyama, Y.; Kubota, K. Colloid Polym. Sci. 2005, 283, 11631171. (e) Winnik, F. M. Macromolecules 1990, 23, 233-242. (2) Ono, Y.; Shikata, T. J. Am. Chem. Soc. 2006, 128, 10030-10031. (3) Okada, Y.; Tanaka, F. Macromolecules 2005, 38, 4465-4471. (4) Ono, Y.; Shikata, T. J. Phys. Chem. B 2006, 110, 9426-9433. (5) Pottel, R. Water; Franks, F., Ed.; Plenum: New York, 1973; Vol. 3, Chapter 8. (6) Kaatze, U. J. Chem. Eng. Data 1989, 34, 371-374.
J. Phys. Chem. B, Vol. 111, No. 7, 2007 1513 (7) Highly purified NIPAm and NIPPA samples were kindly supplied by Kohjin Co. Ltd. (Tokyo) and a group from Kwanseigakuin University, respectively. (8) An RF LCR meter (Agilent Technologies, 4287A) equipped with a homemade electrode cell was used to determine dielectric relaxation spectra, ′ and ′′, for sample solutions over the frequency range from 1 MHz to 3 GHz at various temperatures from 8 to 39°C. In the frequency range from 50 MHz to 20 GHz, ′ and ′′ were determined using a dielectric material probe system (Hewlett-Packard, 85070B) consisting of a network analyzer (Hewlett-Packard, 8720ES); details were described in ref 2 and elsewhere (Imai, S.; Shiokawa, M.; Shikata, T. J. Phys. Chem. B 2001, 105, 4495-4502.). (9) Fro¨hlich, H. Theory of Dielectrics; Clarendon Press: Oxford, U.K., 1949. (10) According to the dielectric theory,9 real and imaginary parts of the relative complex permittivity are described by the summation of Debyetype relaxation functions of a mode i; ′i ) i/(1 + ω2τi2) + i∞ (real) and ′′i ) iωτi/(1 + ω2τi2) (imaginary), where τi, i, and i∞ represent, respectively, the relaxation time, strength, and permittivity in the high ω limit for the mode i. For pure water at 25 °C, the values: τwp ) 8.3 ps, wp ) 73.3, and w∞p ) 5.1 are well known.6 (11) Assuming w∞ ) Φ(w∞p - 1), ′w ) Φ(′wp - 1) holds as used in ref 2. (12) V h s was evaluated to be between 116.16 and 116.93 cm3 mol-1 for NIPAm via density measurements of aqueous P(NIPAm) solutions at 20.040.0 °C using a DMA5000 density meter (Anton Paar, Graz, Austria). The temperature dependence of V h s was described by the relationship V h s )115.47 + 0.0369T (in °C). V h w varied very slightly from 18.01 cm3 mol-1 at 10 °C to 18.13 cm3 mol-1 at 40 °C. (13) Because τw was slightly longer than τwp, depending on c, even at the same temperature, whether the wp value simply determined at each T can be accepted for the evaluation of Φ is not clear. If wp is considered as a function of τwp, then the relationship wp ) 58.763 + 2.05τwp - 0.040(τwp)2 (in ps) would provide wp by substituting τw instead of τwp.4,6 The Φ value evaluated via the simple method was slightly greater than that of the other (τw control) method. It is likely that the former simple method gives the upper limiting value and the latter gives the lower limiting value for Φ. (14) The average value of m obtained from the two (upper and lower limiting) Φ values for each temperature was plotted in Figure 2. (15) Maeda, H.; Higuchi, T.; Ikeda, I Langmuir 2000, 16, 7503-7509. (16) Maeda, Y.; Kitano, H. Spectrochim. Acta, Part A 1995, 51, 24332466. (17) The magnitude of the activation energy, E*ex ) 27 kJ mol-1, which was greater than that of the rotational relaxation time for pure water, i.e., E*w ) 19 kJ mol-1,6 was related to the energy necessary to break hydrogen bonds of water molecules hydrated to NIPAm and was essentially the same as that in the aqueous P(NIPAm) solutions.2 (18) T1 13C-NMR measurements were performed using a JEOL EX270 spectrometer at resonance frequency for 13C: ωr ) 67.80 MHz and 25 °C via a conventional inversion recovery pulse sequence under the deuterium lock mode. The obtained sharp 13C NMR spectra satisfied the extremely narrowing condition at the resonance frequency. (19) The obtained T1 values, 1.7 and 0.88 s for R and β vinyl carbons of NIPAm, provided a correlation time of τc ∼ 29 ps (Lyerla, J. R., Jr.; Levy, G. C. Topics in Carbon-13 NMR Spectroscopy; Levy, G. C., Ed.; Wiley: New York, 1974; Vol. 1, pp 79-148.). This τc value was subsequently converted to the rotational relaxation time with the same physical meaning as the dielectric relaxation times, using the expression τrNMR ) 3τc.
