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Langmuir 2004, 20, 7064-7069
Specific Swelling Behaviors of Poly(4-vinyl phenol) Gels in Tetraalkylammonium Chloride Solutions Ling Xu,† Eisuke Yokoyama,† Hiroko Watando,† Rei Okuda-Fukui,‡ Susumu Kawauchi,§ and Mitsuru Satoh*,† Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-0033, Japan, Hewlett-Packard Japan, Limited, 2-2-24 Higashishinagawa, Shinagawa-ku, Tokyo, 140-8641 Japan, and Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan Received March 30, 2004. In Final Form: May 31, 2004 Swelling behaviors of poly(4-vinyl phenol) (P4VPh) gel in aqueous tetraalkylammonium chloride (TAACl) solutions were investigated to find a very specific swelling behavior. Especially for the tetrabutylammonium chloride (TBACl) system, P4VPh gel remarkably deswelled with increasing salt concentration (e2.0 M) and then sharply reswelled in a higher concentration region (g ca. 2.1 M). A similar swelling profile was also observed in the swelling time-course; upon immersion of a water-swollen P4VPh gel into 2.5 M TBACl solution, the gel first deswelled in an early stage (∼0.1 h) and then remarkably reswelled with time. Relative amounts (mol/mol) of TBA cation and water per monomer residue were estimated as ca. 1 and 0 for the deswollen state and 5 and 50 for the reswollen state, respectively. This result, together with those of attenuated total reflection Fourier transform infrared measurements performed for the gels swollen in various kinds of TAACl and inorganic salt solutions, suggested that in the highly deswollen and almost dehydrated state, phenol rings aggregated with intervening TBA cations, while the aggregation reswelled upon further binding with TBA cations.
Introduction The Hofmeister series1,2
is classically known as a series of ions’ ability to salt-out polymers from their aqueous solutions. For example, strongly hydrated anions, such as F- and SO42-, are most effective salting-out agents, while weakly hydrated ones such as I- and SCN- are least effective and sometimes even show a salting-in effect. On the other hand, cations’ effects are much less significant than those by anions, and the relative effectiveness among cations can be inverted depending on polymer systems. In preceding studies,3-6 we proposed a possible mechanism that explains the ion effects on the polymer solubility and gel swelling in water by taking into account changes in water’s abilities for electron-pair donation (EPD) and acceptance (EPA) upon ionic hydration. According to the mechanism, the salting-out effect by strongly hydrated anions is caused by destabilization of hydrogen-bonding hydration to polar groups on the polymer, because EPA of water molecules is significantly reduced upon hydration to the relevant anions. Irregular effects for the swelling behavior by cations were successfully interpreted as resulting from a subtle balance of a salting-in effect by * To whom correspondence should be addressed. Fax: 81-3-57342888. E-mail:
[email protected]. † Department of Chemistry and Materials Science, Tokyo Institute of Technology. ‡ Hewlett-Packard Japan, Ltd. § Department of Organic and Polymeric Materials, Tokyo Institute of Technology. (1) Eagland, D. In Water; Franks, F., Ed.; Plenum Press: New York, 1969; Vol. 4, p 424. (2) Ninham, B. W.; Yaminsky, V. Langmuir 1997, 13, 2097-2108. (3) Takano, M.; Ogata, K.; Kawauchi, S.; Satoh, M.; Komiyama, J. Polym. Gels Networks 1998, 6, 217-232. (4) Muta, H.; Miwa, M.; Satoh, M. Polymer 2001, 42, 6341-6344. (5) Muta, H.; Ishida, K.; Tamaki, E.; Satoh, M. Polymer 2002, 43, 103-110. (6) Muta, H.; Kawauchi, S.; Satoh, M. Colloid Polym. Sci. 2003, 282, 149-155.
cations, which can stabilize hydrogen-bonding hydration of polymer polar groups through enhancing water EPA, and an ordinary salting-out effect due to decrement of water activity in a concentrated salt solution. The supposed changes in water EPD and EPA upon ionic hydration have been in fact confirmed by ab initio molecular orbital calculations7-9 on several typical model systems consisting of hydrated ion clusters and small molecules with polar groups. In such an ab initio study,8 we have found that hydrogen-bonding hydration of phenol OH is stabilized by anions rather than cations, being contrary to cases of usual alcoholic OH groups. This result triggered our research on poly(4-vinyl phenol) (P4VPh) gel10 and allowed us to find two extraordinary swelling behaviors of the gel. One is “super saltresistivity”, showing no appreciable deswelling even in saturated solutions of many kinds of inorganic salts including typical salting-out agents such as fluorides and sulfates. The other is its high water content (>90%) irrespective of the polymer’s insolubility in water. As a matter of fact, the P4VPh gel, once dried, loses its swelling ability in water. This strongly suggests that the highly swollen state in water is a metastable state and that direct interaction among polymer substrates such as tight stacking of the aromatic rings without intervening water molecules would retard the gel swelling or water absorption. As for the mechanism of the salt-resistivity, we suggested10 stabilization of hydrogen-bonding hydrations by anions and cations in the system; anions are able to (7) Muta, H.; Kojima, R.; Kawauchi, S.; Tachibana, A.; Satoh, M. J. Mol. Struct. (THEOCHEM) 2001, 536, 219-226. (8) Muta, H.; Sin, T.; Yamanaka, A.; Kawauchi, S.; Satoh, M. J. Mol. Struct. (THEOCHEM) 2001, 574, 193-209. (9) Muta, H.; Kawauchi, S.; Satoh, M. J. Mol. Struct. (THEOCHEM) 2002, 620, 65-76. (10) Muta, H.; Taniguchi, T.; Watando, H.; Yamanaka, A.; Takeda, S.; Ishida, K.; Kawauchi, S.; Satoh, M. Langmuir 2002, 18, 9629-9631.
10.1021/la0491798 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/22/2004
Swelling Behavior of Poly(4-vinyl phenol) Gels
stabilize a hydrogen-bonding hydration between the phenol OH proton and the water oxygen, and cations are involved with stabilization of π hydrogen-bonding hydration to the phenol ring as well as one between the phenol OH oxygen and water proton. In the above interpretation on the super salt-resistivity of P4VPh gel, ionic effects on the hydrogen-bonding hydrations were only considered as a main cause. However, hydrophobic hydration should exist around the polymer, and ion effects on the hydration must also contribute to the overall swelling behavior of the gel. In fact, it has been found that as an exception to the super saltresistivity, P4VPh gel showed a slight but appreciable deswelling in tetramethylammonium chloride solution. This finding suggested that more hydrophobic ions may cause some more significant deswelling. Thus, in the present study, we examined swelling behaviors of P4VPh gel in aqueous solutions of three kinds (methyl, ethyl, and tert-butyl) of tetraalkylammonium chlorides (TAACl) to see how the hydrophobicity of ions affects the gel swelling. Further, interactions of the polymer with hydrophobic cations as well as inorganic salts, especially those at the OH group and π system, were probed by Fourier transform infrared spectroscopy. Experimental Section Materials. Poly(4-vinylphenol) (Mw ) 22 000) and ethylene glycol diglycidyl ether (EGDGE, 50% solution) as a cross-linking agent were purchased from Polyscience Inc. and Aldrich Chemical Co. Ltd., respectively. Tetramethylammonium chloride (TMACl, Kanto Chemical), tetraethylammonium chloride (TEACl, Kanto Chemical), tetrabutylammonium chloride (TBACl, ACROS Organics), and other inorganic salts (MgCl2, CaCl2, LiCl, NaCl, KF, KCl KBr, KI, and Na2SO4 from Kanto Chemical) were of analytical grade and were used as received. Deionized water was used for all the experiments. Gel Preparation. An adequate amount of the polymer was dissolved in 1 M NaOH aqueous solution to prepare 22 wt % P4VPh solution. Then, EGDGE was added into the P4VPh solution. The amount of EGDGE was set to 5, 10, or 15 mol % of phenol hydroxyl groups of P4VPh used. Hereafter, gel samples will be described as, for example, “5% gel”, to show differences in the cross-linking density or water content. The cross-linking reaction was performed in a glass capillary (Drummond Scientific Co. Ltd., 25 µL, Φ ) 0.690 mm) at 25 ( 0.1 °C for 24 h. For attenuated total reflection Fourier transform infrared (ATR/FTIR) measurements, film-type gel samples (5% gel) were also prepared. The scheme of gelation was reported in our previous work.10 Measurement of Swelling Degree. Rod-type P4VPh gel samples prepared as above were immersed in water for 10 days to remove low molecular weight substances and non-cross-linked polymer, and finally equilibrium swelling in water was established prior to use. Water contents (%) of the gel samples after this treatment were estimated as 100(Wsg - Wdg)/Wsg, where Wsg is the weight of water-swollen gel, and Wdg is the weight of dried gel (in a vacuum oven at 120 °C for 48 h). The water contents thus calculated were 97.3, 90.3, and 89.8% for 5, 10, and 15% gels, respectively. Water-swollen gel samples were immersed in aqueous salt solutions, the concentration of which was increased in sequence. The gel diameter was periodically observed using a microscope (Diaphot 200, Nikon Co., Ltd.) until each equilibrium swelling was reached typically after several days. In measurements for temperature dependence, swelling degrees were obtained after 24 h. The swelling degree was defined as d/d0, where d and d0 are gel diameters swollen in aqueous salt solutions and deionized water, respectively. All the measurements were performed at room temperature (25 °C) unless otherwise described. ATR/FT-IR Measurement. ATR/FT-IR measurement was carried out using a Shimadzu FT-IR-8200PC with an ATR-8200H attachment (resolution, 4 cm-1; scan, 40 times). For film-type P4VPh gel samples immersed in aqueous solutions of the three
Langmuir, Vol. 20, No. 17, 2004 7065
Figure 1. Dependence of the swelling ratio (d/d0) on TAACl concentration for P4VPh gel (5% gel). kinds of TAA salts as well as many kinds of inorganic salts (KF, MgCl2, CaCl2, LiCl, KCl, KBr, KI, and Na2SO4), the CdC stretching band of the phenol ring and the C-O stretching band of the phenol OH group were monitored around 1512 and 1240 cm-1, respectively. All the gel spectra were measured more than two times, and peak positions were obtained from differential spectra with the corresponding immersing solutions. Ab Initio Calculations. To obtain reference data for the aromatic CdC stretching band upon interaction with a water or a TAA cation, ab initio molecular orbital calculations were performed for small model systems; we employed benzene for an aromatic ring instead of phenol and TMA+ as a typical TAA+, due to a limitation of calculation scale. Several plausible configurations for π-hydrogen and cation-π complexes were full optimized with the second-order Møller-Plesset perturbation theory with the 6-31++G(d,p) basis set (denoted as MP2(full)/ 6-31++G(d,p)). Then, vibrational frequency analyses at the same level of theory were carried out to confirm that the normal modes for the complexes have all positive frequencies as true equilibrium structures. All the calculations were done by using the Gaussian03 program.11
Results and Discussion Swelling Behavior in TAACl Solutions. The swelling behavior of 5% gel in the three kinds of TAACl solutions is shown in Figure 1. In the case of TMACl and TEACl, the swelling degree monotonically decreased with increasing TAACl concentration. However, in TBACl solution, P4VPh gel showed a quite specific swelling behavior. In a lower TBACl concentration region (
