1444
Langmuir 2000, 16, 1444-1446
Partition of Salts between N-Isopropylacrylamide Gels and Aqueous Solutions Hideya Kawasaki,* Takayuki Mitou, Shigeo Sasaki, and Hiroshi Maeda Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan Received May 4, 1999. In Final Form: August 9, 1999
Introduction Poly(N-isopropylacrylamide)(NIPA) gel in water has been known to exhibit a discontinuous volume shrinkage (i.e., the volume phase transition) at about 34 °C with increasing temperature.1 Distribution of solutes between the NIPA gel and the external solution is affected by the following two unique characteristics of the NIPA gel. One is that the swelling of the NIPA gel is extremely sensitive to temperature. A size-dependent solute separation controlled by changing the temperature has been proposed for NIPA gels.2-4 The other significant characteristic of the NIPA gel is that the volume phase transition is accompanied by the dehydration of NIPA chains.5 Above the transition temperature, the behavior of the gel is dominated by its hydrophobicity due to the dehydrated NIPA chains. The nonpolar property of the NIPA gel that depends on temperature is expected to affect the interaction of solutes with the gel. The effect of surfactants on the volume phase transition of NIPA gel has been extensively examined.6-8 The increase of the transition temperature with surfactant concentration has been explained by an attractive interaction of the surfactants with the gel-chain. The distribution of the hydrophobic solute (e.g., benzoic acid) between NIPA gel and the external solution was examined, and NIPA gel strongly adsorbed the organic molecule dissolved in water above the transition temperature.9 Recently, however, it has been reported that the collapse of the NIPA gel at the transition has no effect on the distribution behavior of the inorganic electrolytes (zinc, nickel, chromium) between the NIPA gel and the external solution.10 The complex formation between amide groups and transition metals for the NIPA gel10 is dominant in both the swollen gel and the shrunken gel. Almost no effect of the collapse of the NIPA gel on the distribution of the salts will be expected for the NIPA gel. Furthermore, the employed NIPA gels were likely to be heterogeneous, since the gels were synthesized at 60 °C above the transition temperature and were turbid due to the heterogeneity of * Corresponding author. (1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (2) Freitas, R. F. S.; Cussler, E. L. Chem. Eng. Sci. 1987, 42, 97. (3) Gewehy, G.; Nakamura, K.; Ise, N.; Kitano, H. Makromol. Chem. 1992, 193, 249. (4) Jianzhong, W.; Sassi, A. P.; Blanch, H. W.; Prausnitz, J. M. Polymer, 1996, 21, 4803. (5) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (6) Inomata, H.; Goto, S.; Saito, S. Langmuir, 1992, 8, 1030. (7) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir, 1994, 10, 418. (8) Kokufuta, E.; Nakaizumi, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (9) Seida, Y.; Nakano, Y. J. Chem. Eng. Jpn. 1996, 29, 767. (10) Lehto, J.; Vaaramaa, K.; Vesterinen, E.; Tenhu, H. J. Appl. Polym. Sci. 1998, 68, 355.
the network.10 The NIPA gels synthesized below the transition temperature may give a different distribution behavior of salts. The conclusion that there is no effect of the volume phase transition of the NIPA gel on the distribution of salts should be confirmed by further investigation. In this study, we examined the distribution of various kinds of salts (LiCl, KCl, NaCl, CsCl, KNO3, KSCN, K2SO4, CH3COOLi, CH3COONa, CH3COOK, CH3COOCs) between the NIPA gel and the external salt solution at various temperatures. The NIPA gels were synthesized at 5 °C, a temperature that is well below the transition temperature, and were transparent. Similar experiments were also performed for a copolymer gel of NIPA and sodium p-styrensulfonate (NIPA-SSNa) with a very small amount of charge. Here, we first report that the volume phase transition of the NIPA gel has a significant effect on the distribution of the salts: below the transition temperature, the salts are almost equally partitioned between the swollen gel phase and the external solution, while above the transition temperature, they are excluded from the completely collapsed gel, and the exclusion tendency of the salts depends on the kinds of salts. Experimental Section Samples. The NIPA gel and the NIPA-SSNa gel were synthesized at 5 °C in a plate form with a thickness of 1 mm, as reported previously.11 The prepared sample gels were immersed in a large amount of distilled water for a week to wash away residual chemicals. After purification, the gels were dried gently and then under vacuum at room temperature. All salts used in the present study were of reagent grade. NIPA-SSNa gel with a mole fraction of SSNa monomer X in the pre-gelation solution is denoted as NIPA-SSNa(X) gel. Swelling Ratio. The equilibrium swelling ratio, W, was defined as
W ) (Wwet - Wdry)/Wdry
(1)
where Wwet and Wdry are the weights of gels in the equilibrium swollen state and dried state, respectively. The swelling ratio was measured after immersing the gel in the solution for two weeks. Partition of the Salts between the Gel and the External Solution. In the distribution experiments of the shrunken gel above the transition temperature, about 0.2 g of the dry gel was placed in 2 mL of the salt aqueous solution (1 or 0. 3 M) and kept in a water bath at a given temperature ( 0.1 °C for two weeks. In the swollen gel below the transition temperature, about 0.2 g of the dry gel was immersed in 6 mL of the salt aqueous solution. The gel/solution partition coefficient, Kp, is defined as
Kp )
Cgel Cext
(2)
where Cgel and Cext are the solute concentrations inside and outside the gel (mol kg-1). The Cext was evaluated by using a calibration curve showing the relationship between the osmolality of the solution and the concentration of the salt.12,13 Using the following relation, the Cgel was estimated by
CgelWgel + Cext(Wtot - Wgel) ) Nt
(3)
(11) Kawasaki, H.; Sasaki, S.; Maeda, H.; Mihara, S.; Tokita, M.; Komai, T., J. Phys. Chem. 1996, 40, 16282. (12) Weast, C. W. Handbook of Chemistry and Physics, 58th ed.; CRC Press: Boca Raton, FL, 1978; p D219. (13) Dick, D. A. Int. Rev. Cytol. 1959, 8, 387.
10.1021/la990542y CCC: $19.00 © 2000 American Chemical Society Published on Web 11/13/1999
Notes
Langmuir, Vol. 16, No. 3, 2000 1445 Table 1. Kp Values of Various Types of Salts for NIPA Gel in Salt Solutions of 1 M for a Swollen Gel and a Collapsed Gel 20/°C W/W0
Kp
W/W0
Kp
LiCl NaCl KCl CsCl KNO3 KSCN K2SO4 CH3COOLi CH3COONa CH3COOK CH3COOCs
22.0 ( 1 9.6 ( 0.1 32.6 ( 0.1 28.5 ( 0.1 23.2 ( 0.1a 17.3 ( 0.1a -
0.97 ( 0.05 0.91 ( 0.05 0.99 ( 0.05 0.98 ( 0.05 0.96 ( 0.05a 0.94 ( 0.05a -
0.4 ( 0.2 0.4 ( 0.2 0.4 ( 0.2 0.4 ( 0.2 0.4 ( 0.2 0.7 ( 0.2 0.4 ( 0.2 0.4 ( 0.2 0.4 ( 0.2 0.4 ( 0.2 0.4 ( 0.2
0.29 ( 0.06 0.07 ( 0.07 0.04 ( 0.04 0.04 ( 0.04 0.44 ( 0.05 1.05 ( 0.05 0.04 ( 0.04 0.09 ( 0.05 0.06 ( 0.05 0.11 ( 0.04 0.05 ( 0.04
a
Figure 1. Temperature dependence of the Kp value of NaCl (a) and that of the swelling ratio (b) for NIPA gel and NIPASSNa(X ) 0.01) gel in a NaCl solution of 0.3 M. The transition temperatures of these gels are indicated by dashed lines in Figure 1(a). where Wgel, Wtot, and Nt, respectively, are the weight of the water (kg) in the measured gel, that of the whole system, and the total mole amount of the salt.
Results and Discussion In a previous paper, we reported that the NIPA gel and the NIPA-SSNa(X ) 0.01) gel in 0.3 M NaCl solution exhibited a discontinuous volume shrinkage with increasing temperature.14,15 Figure 1(a) shows the temperature dependence of the Kp value for the NIPA gel and the NIPASSNa(X ) 0.01) gel in NaCl solution at a concentration of 0.3 M. In Figure 1(b), the temperature dependence of the swelling ratio (W) for these gels is also shown. The transition temperatures of the NIPA gel and the NIPASSNa gel are indicated by dashed lines in Figure 1(a). The transition temperature in a NaCl solution of 0.3 M was slightly lower than that of NIPA gel in water, which was due to the reported salt effect on the NIPA gel.14 Below the transition temperature (swollen NIPA gel), the Kp values are almost equal to unity (Kp ∼ 1). This indicates that NaCl is almost equally partitioned between the swollen gel phase and the external solution. Above the transition temperature (shrunken gel), however, the Kp values are less than unity (Kp < 1). Above the transition point, the decrease of the Kp value continues with shrinkage of the gel when increasing the temperature. (14) Kawasaki, H.; Sasaki, S.; Maeda, H. J. Phys. Chem. B 1997, 101, 4184. (15) Maeda, H.; Kawasaki, H.; Sasaki, S. Hydrocolloids 1: Physical Chemistry and Industrial Application of Gels, Polysaccharides, and Proteins; Elsevier: in press.
40/°C
sample
Measured at 12 °C.
Eventually, a significant decrease of the Kp value (Kp < 0.1) takes place at a high temperature (i.e., completely collapsed state at 50 ˚C). This indicates that NaCl is excluded significantly from the completely collapsed gel at 50 °C. A similar trend is also observed for the NIPASSNa(X ) 0.01) gel. In the case of NaCl, the significant reduction of the Kp value takes place in a very narrow range of the swelling ratio (1.4 < W < 0.4) for both the NIPA gel and the NIPA-SSNa gel(X ) 0.01). This swelling ratio corresponds to the water number per NIPA residue (Nw), 2 < Nw < 9. Higher water contents of the gel lead to larger Kp values of NaCl for the shrunken gel above the transition temperature. The remarkable reduction of the Kp value in the collapsed gel may relate to the low water content of the gel. The distribution of various types of salts was examined for the NIPA gel. The Kp values for NIPA gels in 1 M salt solutions are given in Table 1 for a swollen gel and a collapsed gel. For the swollen gels at 12 °C or 20 °C (below the transition temperature), the Kp value was nearly equal to unity, irrespective of the kinds of salts. For the collapsed gel at 40 °C (above the transition temperature), on the other hand, the Kp values are remarkably small (Kp < 1), except for KSCN, and strongly depend on the kinds of salts. Anion types have a more pronounced effect on the Kp value than do cation types. The Kp value for a series of potassium salts is in the order (SCN- > NO3- > Cl- ∼ CH3COO- ∼ SO42-). The cation type seems to be less effective on the Kp value. The Kp value for a series of chloride salts is in the order (Li+ > Na+ ∼ K+ ∼ Cs+). The Kp value of LiCl in the collapsed gel is higher than that of the other salts examined. The Kp value of CH3COOLi in the collapsed gel is, on the other hand, not as high as that of LiCl. The Kp value of a salt is determined by the summation of exclusion tendency of the cation and that of the anion. In CH3COOLi solution, the Kp value may be smaller than that of LiCl due to a strong exclusion tendency of the acetate ion from the collapsed gel. In a previous paper, we reported the distribution of a saccharide (glucose, sucrose) in NIPA gel.11 The Kp values of the saccharides at 25 °C were in the range of 0.85 < Kp < 1 in the collapsed gel. This is because the Kp values were determined near the transition temperature, and the gel was not in the completely collapsed state. In the present study, we confirmed that the Kp value was 0.08 ( 0.05 in the completely collapsed gel in a glucose solution of 1 M at 40 °C. Here, we mention an extra precaution for the distribution experiment in the shrunken NIPA gel. It takes a long time for gels of large sizes to reach the equilibrium shrunken state, due partly to the collective diffusive
1446
Langmuir, Vol. 16, No. 3, 2000
process of the gel-network in solvent 16 and partly to the formation of a skin layer on the surface of the gel.17 It is also well-known that the gel becomes opaque after the temperature jump above the transition temperature, indicating the heterogeneous nature of the network extending over the area that is longer than the wavelength of light.18,19 The gel becomes transparent when reaching the equilibrium shrunken state.19 Thus, the NIPA gel in the equilibrium shrunken state should be transparent. In the present distribution experiments, we prepared the shrunken gel by immersing a dry gel in the solution above the transition temperature. In this procedure, the amount of the solvent and the volume change to reach the equilibrium shrunken state are both small, and therefore it is expected that the equilibrium shrunken state will be attained for a short period. The swelling ratio of the shrunken gel thus prepared was consistent with that of the shrunken gel prepared by warming the swollen gel. All gels measured in the present distribution experiments were transparent. It was confirmed that the Kp values of NaCl in the collapsed gel determined by immersing the gel in the solution for two weeks and for a month were the same within experimental error. (16) Tanaka, T.; Fillmore, J. D. J. Chem. Phys. 1979, 70, 1214. (17) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Nature 1995, 240, 374. (18) Li, Y.; Wang. G.; Hu, Z. Macromolecules 1995, 28, 4194. (19) Shibayama, M.; Shiritani, Y.; Hirose, H.; Nomura, S. Macromolecules 1997, 30, 7307.
Notes
Our results on the partition of salts for the NIPA gel are different from the results for the turbid NIPA gel prepared above the transition temperature,10 in which the collapse of the NIPA gel at the transition had almost no effect on the distribution behavior of the salts.10 A swollen NIPA gel at 20 °C in 1 M NaCl solution became opaque after the temperature jump above the transition temperature (i.e., 40 °C) and the opaque state of the gel continued for 3 days at least. It was noted that the turbid gel was not in the equilibrium state. We found a higher Kp value in this turbid gel (i.e., Kp ∼ 0.7). For the heterogeneous gel, it is considered that the salts exist in both the dense and the dilute regions of the network. The Kp value of a salt is determined by the summation of the salt concentration of these two regions. In the turbid NIPA gel, the concentration of the salt in the dilute regions may be higher than that in the dense regions due to the higher water content, resulting in the high Kp value even above the transition temperature. Acknowledgment. This work is partly supported by research fellowships of the Japan Society for the Promotion of Science for Young Scientists and partly by a Grantin-Aids for Scientific Research (No. 10640494) from the Ministry of Education, Science, Sports, and Culture of Japan. LA990542Y
