13612
J. Phys. Chem. 1994, 98, 13612-13615
Dielectric Study of Water Structure in Polymer Solution Naoki Shinyashiki, Yasuhide Matsumura, Nobuhiro Miura, Shin Yagihara, and Satoru Mashimo* Department of Physics, Tokai University, Hiratsuka-shi, Kanagawa 259-1 2, Japan Received: March 4, 1994; In Final Form: August 3, 1994@
Dielectric measurement using the time domain reflectometry made on poly(vinylpyrro1idone) (PVP)-water mixtures exhibits a relaxation process due to reorientation of water molecules. If the concentration of PVP is lower than 57 wt %, the mixture freezes between 0 and -19 "C. Relaxation strength drops at that point and disappears altogether at -57 "C. On the other hand, mixtures with concentration higher than 57 wt % have no freezing point. These results indicate the existence of three kinds of water in the mixture. The first one, disappearing at the freezing point, is the same as pure water. The second one, which disappears at -57 "C, forms a distorted cluster consisting of five water molecules and a repeat unit of PVP. The third one, which has no freezing point, forms a nonordered structure among polymer chains.
Introduction Polyvinylpyrrolidone (PVP), which is a flexible and randomly coiled polymer, is very soluble in water,' and its aqueous solution is nonelectrolytic. Therefore PVP in aqueous solution is a good sample to investigate the water structure around the coiled polymer, since dielectric measurements can be easily done for such a nonelectrolytic solution. Infrared absorption measurements suggest that water molecules are bonded not only to carbonyl groups but also to C-N sites2 Further, measurement of the sorption isotherm suggests a quite strong interaction between PVP and water m o l e ~ c l e s . ~ Previous dielectric studies on the PVP-water mixtures revealed two kinds of relaxation p r o c e s s e ~ . ~The . ~ highfrequency process is due to the reorientation of water molecules, and the low-frequency process is attributed to the microBrownian motion of the PVP chain.5 The relaxation time for the chain motion is more than 1000 times longer than that for the water molecules. In general, the water-organic compound mixture shows a relaxation peak and a straight line for the plot of the logarithm of the relaxation time against mole fraction of water X, in a region with 0.83 < X, 5 1.6 The universal critical point of X, = 0.83 suggests that one compound molecule associates with five water molecules in the water rich region. In this region such clusters coexist together with pure water clusters and form a high-order ~tructure.~ On the other hand, in the water-poor region water molecules form a nonordered micelle-like cluster around the compound m ~ l e c u l e . ~ Recent dielectric relaxation measurements on PVP-water mixtures have suggested that the observed relaxation process of water is divided into two processes: one is due to the bulk water, and the other, to the hydration water.* In PVP-water mixture^,^ there may be no structurally bound water observed previously in moist collageng and in DNA aqueous Although PVP has no ordered structure, DNA has a highly ordered double helix structure, and tropocollagen has a ternary helical structure. At the freezing point of the PVP mixture, DSC12,13and d i e l e ~ t r i c ' ~measurements ~'~ indicated that water in the mixture is mainly frozen but remains partly in. a liquid state. The permittivity of pure water decreases rapidly at the freezing point.14 It was shown that the freezing points of PVP-water @
Abstract published in Advance ACS Abstracts, December 1, 1994.
0022-365419412098-13612$04.5010
mixtures decrease with increasing PVP concentration.12 Therefore, a highly concentrated solution shows no freezing p ~ i n t . ' ~ - ' ~ Investigation of the dynamical behavior of water molecules in PVP-water mixtures above and below the freezing point is very important to elucidate the structure of water around coiled polymers. In this work, dielectric measurements were performed on PVP aqueous solutions with concentrations of 2070 wt % in a temperature range from +45 to -55 "C, by a time domain reflectometry (TDR) method.16-18 Three kinds of water structures could be assigned from dielectric properties such as relaxation strength and relaxation time. A phase diagram for the water structure can be drawn from the concentration and temperature dependence of the dielectric properties.
Experimental Section A sample of PVP with a weight-average molecular weight of 10000 was purchased from Sigma Chemical Co. Ltd. Deionized and distilled water was obtained from Whittaker Bioproducts, Inc. The solutions used were prepared by dissolving the PVP in water for 24 h before the measurements. Dielectric measurements were performed by employing the TDR method over the frequency range 300 kHz to 15 GHz. A detailed procedure for measuring complex permittivity was reported already.5*'8 The concentrations of PVP employed were 20, 40, 60, and 70 wt %, respectively. If the measuring temperature was lower than room temperature, the solution was cooled slowly from room temperature. When the temperature reached the measuring point, it was kept for 30 min before measurement. After another 30 min, the same measurement was carried out again. We confirmed that the two reflected pulses were completely of the same form. If the measuring temperature was higher than room temperature, the solution was heated slowly until the temperature reached the measuring temperature. The temperature was varied from +45 to -55 "C.
Results PVP-water mixtures lose fluidity and turn white when the temperature reaches the freezing point T,. The freezing points thus determined for mixtures with 20 and 40 wt % PVP are -5 and -7 "C, respectively. However mixtures with concentrations higher than 57 wt % do not freeze in the temperature range 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 51, 1994 13613
Water Structure in Polymer Solution
1
20 WATER
5
1
-40t
A+B
WATER
~
I WATER
B
C
I
20
0 10’ E
A L L W .4T E R
FREEZES
-80 0
100
60 C(Wt%)
40
20
I’
80
100
lo-’
6
7
Figure 1. Phase diagram for the three kinds of water in PVP aqueous solutions.
8 log f(Hz)
9
10
Figure 2. Dielectric dispersion and absorption curves for PVP aqueous
measured. Figure 1 shows plots of freezing temperature Tc against PVP concentration which are shown as closed circles. Dielectric dispersion and absorption curves for the 20 wt % mixture at various temperatures are shown in Figure 2. Two distinct relaxation processes are observed for these solutions above the freezing point. Both peaks shift to low frequencies as polymer concentration is increased or temperature is decreased. The high-frequency process due to reorientation of water molecules is described well by the Cole-Cole equation. The low-frequency process due to the micro-Brownian motion of the main chain of PVP was indicated to have a response function of the K o h l r a u s ~ h - t y p e .Two ~ ~ ~ relaxation ~ processes are also observed for the 60 and 70 wt % solutions. Figure 3 shows dielectric dispersion and absorption curves for the 70 wt % solution. The dielectric relaxation observed for the aqueous solution of PVP above Tc is thus expressed as a sum of the two relaxation processes as
€*(a) = E;(W)
+
6i(W)
+ E,
solution with 20 wt % PVP at various temperatures.
e’
6
7
8 log (Hz)
9
10
Figure 3. Dielectric dispersion and absorption curves for PVP aqueous (1)
solution with 70 wt % PVP at various temperatures.
where ~ t ( w and ) E;(OJ) are complex permittivities corresponding to the high-frequency and low-frequency processes, respectively, and are given as E;(@)
=
“h
1
+
(jWrh)Bh
1
nn
and -60
Here h € h and A61 are the relaxation strengths of the high- and low-frequency processes, respectively, r h and tl are the corresponding relaxation times, P h is the Cole-Cole parameter describing the distribution of relaxation times, P k is the coupling parameter of the Kohlrausch function, E , is the limiting highfrequency dielectric constant, and w is the angular frequency. For the solution with 20 or 40 wt % PVP, the relaxation peak due to the micro-Brownian motion of the PVP chain disappears below Tc, as is seen in Figure 2. However the tail of another relaxation process, whose relaxation peak must be located at a frequency lower than 300 kHz, was observed in the lowfrequency region measured. This comes from the ice phase. The high-frequency process below Tc has a small relaxation
-40 -20
0
20
40
Figure 4. Temperature dependence of ALE^ for four concentrated PVP aqueous solutions with various concentrations of PVP.
strength, and its peak shifts discontinuously to low frequency at Tc. The dielectric relaxation below Tc is described experimentally by a sum of the two Cole-Cole relaxation processes as
where h and IC denote the high- and low-frequency processes, respectively.
Shinyashiki et al.
13614 J. Phys. Chem., Vol. 98, No. 51, 1994
0.91
-71
-4
I
/
1
I8
3.0
.,,'
,
/
/i
0.5
3.5 4.0 4.5 1 0 3 / T (K-l)
'.I
-60 -40
I
-20 0 T ("C)
20
40
Figure 5. Plot of log t h against l/T(K-') for four concentrated PVP aqueous solutions.
Figure 6. Temperature dependence of /3h for four concentrated PVP aqueous solutions.
Figure 4 shows the temperature dependence of relaxation strength A E h for the high-frequency process, and Figure 5 shows the plot of log z h against 1/T, where T is the absolute temperature. For solutions with both 60 and 70 wt % PVP, the relaxation strength increases monotonically with decreasing temperature and the relaxation time depends largely on l/T. There was found no transition in h € h and log 'rh in the whole temperature range measured. The freezing point can be determined by the break point of A E h , as shown in Figure 4,as well as by the break point of log t h in Figure 5. The temperature thus determined agrees completely with that determined by the fluidity. Undoubtedly, part of the water freezes at T,, but some water remains unfrozen. Further, as the temperature decreases from T,, the relaxation strength A E h decreases gradually and reaches zero at -57 "C both for 20 and 40 wt % solutions.
of 57 wt % corresponds to a value of 4.7 for n g . This suggests that there are about five water B molecules around one PVP repeat unit. The value of 4.7 for n g is very close to the value of six estimated from eq 3. Water A molecules must be located apart from the PVP chain and form essentially the same structure as that of bulk water. On the other hand, water B does not freeze easily like water A. However in a temperature range between Tc and -57 "C, water B molecules are gradually incorporated into the ice structure of water A and finally all of them disappear at -57 "C. At concentrations higher than 57 wt %, the solution has no water A and B but only water C. The density of water C molecules is too small to form the ice lattice in this case. PVP chains may also prevent the formation of an ordered structure. The relaxation process for water C has a broad distribution of relaxation times in contrast to that for pure water, since P h for the 60 or 70 wt % solution does not change and takes a small value, as seen in Figure 6. This result also suggests that water C has a disordered structure. As mentioned above, some of the water B molecules are bonded directly to the carbonyl group or C-N site of PVP. Therefore their motion is restricted and the structure of water is affected by the PVP chain. At concentrations lower than 57 wt % above T,, about five water molecules form a water B cluster together with a PVP repeat unit, and water B molecules move cooperatively with water A molecules. The number of water molecules in the cluster is kept constant independent of PVP concentration. In contrast, water C exists only in a concentration region higher than 57 wt %, and it has a nonordered structure. Water C molecules are surrounded by PVP chain segments, and their mobility is strongly restricted by the chain. For PVP-water mixtures with a concentration lower than 57 wt % at 25 "C, the plot of log t h against mole fraction of water, X,, gives a straight line. This result is certainly explained by assuming the existence of two kinds of water, such as waters A and B, and a cooperative motion between them. The relaxation time of water in this case is described by6
Discussion Water molecules which form ice crystals at Tc do not contribute to the high-frequency relaxation process of water observed below T,. The sudden decrease of A e h at the freezing point results from a phase transition from liquid to ice crystal. On the other hand, a small but well defined value of Aq, below T, is due to another kind of water. This indicates that the mixture with PVP concentration lower than 57 wt % has two kinds of water: one freezes at T,, and the other remains in the liquid state even below T,. In this work, the former is called water A and the latter is called water B. Further, in the case of a solution with a concentration higher than 57 wt %, none of the water freezes in the temperature range concerned; this kind of water is called water C. Figure 1 shows the phase diagram for these three kinds of water. The number of water B molecules per one PVP repeat unit, n g , is estimated roughly from the change of A E h at , 'Z as
(3) where AEBis the relaxation strength of water B at T,, A E A +is~ that of waters A and B at T,, and n, is the total number of water molecules per one repeat unit of PVP. The value of n g estimated from eq 3 is about 6 for the solutions with both 40 and 20 wt % PVP. It should be noted that this estimation is carried out by assuming that the effective dipole moment of water A is equivalent to that of water B. Further, the freezing point contains an unavoidable error. Therefore, the values of AEBand AEA+Bhave uncertainties. On the other hand, if the PVP concentration is higher than 57 wt %, the solution does not freeze. There is no water A in it. The critical concentration
+
log th= m[X, - (m - l)/m] log tA m( 1 - X,) log zg (4) where t~ and t g are the relaxation times of water A and water B, respectively, and m is the number of water molecules involved in the water A cluster, which was indicated previously to have a value of 6.6 The water B cluster consists of m - 1 water molecules and a repeat unit of PVP. It is noted that m -
J. Phys. Chem., Vol. 98, No. 51, 1994 13615
Water Structure in Polymer Solution 1 = 5 is in good conformity with the present result which gives m - 1 = 4.7. The dielectric relaxation in supercooled liquid shows generally a non-Arrhenius type of temperature dependence for the relaxation time. The plot of log z against 1/T in such a case is described by the Vogel-Tammann-Fulcher equation asZo logt,=A+-
B T - To
(5)
where A, B, and TOare constant. The values of A, B, and TO can be determined from the experimental results by a least square fitting procedure. Estimation of the glass transition temperature Tgof water using water mixtures with concentrated organic compounds has been carried out by differential thermal analysis (DTA). The values of Tgthus estimated are 143-136 K.21-23 The plot of log z h against 1/T for solutions with 60 and 70 wt % PVP in all the temperature ranges measured and those for solutions with 20 and 40 wt % PVP above Tc can be described well by eq 5. If we take TOas the glass transition temperature Tg,water in solutions with 20, 40, 60 , and 70 wt % PVP has a Tgof 144, 148, 158, and 165 K, respectively. The obtained TOvalue depends linearly on the mole fraction of the PVP repeat unit, like that of the water-organic compound mixture. The extrapolated value for pure water is 140 K. This value agrees well with that obtained by DTA.21-23
References and Notes (1) Franks, F. Water, A Comprehensive Treatise; Plenum Press: New York, 1982; Vol. 7, Chapter 3.
(2) Rothschild, W. G. J. Am. Chem. SOC. 1972,94, 8676. (3) MacKenzie, A. P. Philos. Trans. R. SOC.London 1977,8278,167. (4) Shinyashiki, N.; Asaka, N.; Mashimo, S.; Yagihara, S. J. Chem. Phys. 1990,93, 760. (5) Miura, N.; Shinyashiki, N.; Mashimo, S.J . Chem. Phys. 1992,97, 8722. (6) Mashimo, S.;Umehara, T.; Redlin, H. J . Chem. Phys. 1991,95, 6257. (7) Mashimo, S.; Miura, N. J . Chem. Phys. 1993,99, 9874. (8) Kaatze, U. Adv. Mol. Relax. Processes 1975,7,71. (9) Shinyashiki, N.; Asaka, N.; Mashimo, S.; Yagihara, S.; Sasaki, N. Biopolymers 1990,29, 1185. (10) Mashimo, S.; Umehara, T.; Kuwabara, S.; Yagihara, S. J . Phys. Chem. 1989,93, 4963. (11) Umehara, T.; Kuwabara, S.; Mashimo, S.; Yagihara, S. Biopolymers 1990,30, 649. (12) Jellinek, H. H. G.; Fok, S. Y. KolloidZ. Z. Polym. 1967,220,122. (13) Ling, G. N.; Zhang, Z. L. Physiol. Chem. Phys. 1983,15, 391. (14) Amrhein, E. M.; Luyet, B. Biodynamica 1966,I O , 61. (15) Jain, S. K.; Johaxi, G. P. J. Phys. Chem. 1988,92, 5851. (16) Cole, R. H.; Mashimo, S.; Winsor, P. J., N.J . Phys. Chem. 1980, 84, 786. (17) Cole, R. H.; Berberian, J. G.; Mashimo, S.; Chryssikos, G.; Bums, A.; Tombari, E. J. Appl. Phys. 1989,66, 793. (18) Miura, N.; Asaka, N.; Shinyashiki, N.; Mashimo, S. Biopolymers 1994,34, 357. (19) Rendell, R. W.; Ngai, K. L.; Mashimo, S. J. Chem. Phys. 1987, 87,2359. (20) Williams,M. L.; Landel, R. F.; Ferry, J. D. J . Am. Chem. SOC. 1955,77,3701. (21) Rasmussen, D. H.; MacKenzie, A. P. J . Phys. Chem. 1971,75, 967. (22) Luyet, B.; Rasmussen, D. Biodynumicu 1968,10, 167. (23) Luyet, B.; Rasmussen, D. Biodynumica 1968,10, 137.
JF940561A
