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J. Phys. Chem. B 2001, 105, 5852-5855
Salt Effects on the Volume Phase Transition of Ionic Gel Induced by the Hydrophobic Counterion Biding Shigeo Sasaki,* Shogo Koga, Ryota Imabayashi, and Hiroshi Maeda Department of Chemistry and Physics of Condensed Matter, Graduate School of Science, Kyushu UniVersity, 33 Hakozaki, Higashiku, Fukuoka 812, Japan ReceiVed: July 10, 2000; In Final Form: February 2, 2001
The salt effects on the volume phase transition of ionic poly(acrylate) gel with a change in the concentration of hydrophobic counterion, dodecylpyridinium chloride (DPC) in outside solution of the gel is investigated. The transition concentrations increase with the NaCl concentration Cs of the solution and the transition disappears at Cs above 0.5 M. A hysteresis is observed for the transition concentration of DPC; that is, the deswelling transition concentration in the increasing process, Cti is significantly higher than the swelling transition concentration in the decreasing process Ctd. The difference between Cti and Ctd is found to increase with decrease in Cs. The comparison of the gel and the polymer solution reveals the resemblance between the salt effects on their phase transitions.
1. Introduction The phenomena that the gel volumes transitionally change with thermodynamic variables are called as the volume phase transition. The nonionic N-isopropylacrylamide (NIPA) gel is well-known to exhibit the volume phase transition and intensively investigated these decades.1 The coupling effect of the cooperative dehydration of the chain (unbinding of water molecules from the hydrophobic isopropyl group) and the entropy force of chains, both of which are driving forces to shrink the gel, induces the volume phase transition.2 The dehydration is induced by the destabilization of the structured water molecules surrounding the isopropyl group due to their higher chemical potentials than that of the bulk water. The volume phase transition behavior of NIPA gel induced by the addition of salt3,4 or saccharide5 molecules to the solution outside the gel has been well explained by the dehydration mechanism mentioned above.6 The volume phase transition of the gel can be considered to be induced by the coupling of cooperative binding or unbinding of small molecules to the gel chain and the entropy force of chains. That is, the volume phase transition can be regarded as a transition in the degree of binding of small molecules with change in the physicochemical conditions. However, the prevailing explanation for the volume phase transition based on the Flory-Huggins theory,7 in which the χ-parameter being dependent on the volume fraction of gel chain induces the transition, has ignored the binding. It is important to clarify a role of the binding taken in the volume phase transition. For clarifying this, the volume phase transition of the polyelectrolyte gel induced by the hydrophobic counterion binding8 is a suitable object to study, since the electrostatic interaction to induce the binding can be well regulated by the salt concentration. In the volume phase transition of gel, the hysteresis of the transition point is more or less observed between the swelling and deswelling processes.1 Our proposing theory2 has suggested that the hysteresis be caused by the sigmoid nature of the isothermal binding curve and that the magnitude of hysteresis * To whom correspondence should be addressed. E-mail: s-sskscc@ mbox.nc.kyushu-u.ac.jp. Fax: +81-92-642-2607.
increases with the degree of sigmoid nature, which is a monotonically increasing function of the volume change at the transition. Therefore, the magnitude of hysteresis is expected to increase with the volume change. For examining the effect of the volume change on the hysteresis, the volume phase transition of polyelectrolyte gel is suitable to be investigated, since its volume strongly dependents on the salt concentration. It is well-known that the binding of hydrophobic counterion to polyelectrolyte is highly cooperative.9 The binding makes the chain affinity with the water molecules poor and precipitates the polyelectrolyte molecules. The precipitation in the solution is considered to correspond to the volume phase transition.10 However, the relation between the volume phase transition and the precipitation of the polyelectrolyte system has not been well clarified so far. The present experiment revealed that the transitional concentrations of the hydrophobic counterion, dodecylpyridinium chloride (DPC) for inducing the volume phase transition of ionic poly(acrylate) gel and precipitating the poly(acrylate) molecules were very close to each other in the solution at the salt concentration (Cs) below 0.3M but were not observed in the solution at the salt concentration (Cs) above 0.5 M even when the concentrations were beyond the critical micelle concentration (cmc) of DPC. This is essentially same as the experimental results reported by Khokhlov et al.11 and Khandurina et al.12 that the deswelling of polyelectrolyte gel with increase in the hydrophobic counterion concentration is suppressed in the salt solution of higher concentration than a certain value. 2. Experimental Section The poly(acrylate) gels were prepared by the radical copolymerization8,10 in the aqueous solution of 1 M acrylic acid and 0.005 M N,N′-methylenebis(acrylamide) at 60 °C. The gel synthesized in a glass tube (inner diameter ) 0.3 mm) was soaked in the isopropyl alcohol. The cylindrical gel was taken out of the tube, rinsed thoroughly with water, dried, and used for observing the volume transition behavior. The slab gel synthesized into a plate form of about 50 mm square and 1 mm thickness was cut into small pieces (about 5 mm square) and rinsed thoroughly with water, freeze-dried, and used for
10.1021/jp002463x CCC: $20.00 © 2001 American Chemical Society Published on Web 06/05/2001
Volume Phase Transition of Ionic Gel
J. Phys. Chem. B, Vol. 105, No. 25, 2001 5853
the experiment of binding isotherm of DPC. Sodium polyacrylate (Na-PAA) was purchased from Wako Pure Chemical Industries, Ltd., and was used without further purification. According to the supplier, a degree of polymerization of PAA molecule was widely distributed from 22 000 to 70 000. All chemicals used were of analytical grade. Double distilled water was used. An end of the cylindrical gel was inserted into the silicon rubber tube and fixed to it with glue. The gel fixed was immersed into the solution contained in a rectangular cell. The cell was set in a water bath being 25 ( 0.1 °C. The gel was immersed into the solution of given concentrations of DPC (CDPC) and NaCl (Cs), pH of which was adjusted between 9 and 10 by adding a small amount of the 0.1 M NaOH solution. The diameter of the gel was measured using a computer-aided optical microscope system.8 For examining hysteresis, size measurements for one gel were made successively changing CDPC. The measurements were made at an interval of more than 24 h, which were needed to reach the equilibrium. The much greater volume of the solution phase (2.5 mL) than the gel (about 5 µL) ensured that CDPC in the solution phase substantially equal to the setting CDPC even when the DP ions fully bind to the gel chain. A binding amount of DPC to the cylindrical gel was evaluated as follows. The whole solution equilibrating with the gel was taken out of the cell. Then a given volume V t ()2.5 mL) of the 1 M NaCl solution, into which the dodecylpyridinium ion DP+ was released from the gel for several hours, was put into the cell. The releasing amount, which is the product of Vt and the concentration of DPC in the 1 M NaCl solution, is the binding amount, since the binding scarcely occurs in the solution at Cs ) 1 M and the gel volume is negligibly small compared with Vt. A binding isotherm of dodecylpyridinium ion (DP+) to the gel at 25 °C was obtained in the following manner.10 A dried slab gel (weight ) WgD) was suspended in a volume (VT) of NaCl solution of a given concentration (CS). The gel was fully neutralized with NaOH solution, followed by the addition of a given mole amount of DPC (mDPC). The gel was equilibrated with the solution for more than a week since the gel size was lager than that of the cylindrical gel. The gel volume Vgt was determined by the relation Vgt ) Wgs/F, where WgS and F were the weight and the density of the gel, respectively. The F was assumed to be identical to the density of the solution. The liquid attached to the surface of gel was removed before weighing the gel. The degree of DP+ binding β to a ionized site of gel chain is given by
β)
mDPC - VTCDPC WgD
Figure 1. The DPC concentration dependence of the absorbance of the PAA solution. The transition point to generate the precipitate of PAA is indicated by an arrow.
Figure 2. Salt concentration (Cs) dependent behavior of diameter of poly(acrylate) gel with an increase (open symbols) or a decrease (closed symbols) in CDPC. The arrows indicate the transitions. No diameter changes of the gels in the solutions at Cs ) 0.5 and 0.6 M are indicated by the horizontal broken and dotted lines.
outside the optical path. Figure 1 shows the typical relation between the maximum absorbency observed after the addition and the CDPC. The critical CDPC for the precipitation was determined from a crossing point of two extrapolated lines at higher and lower CDPC as shown in Figure 1. 3. Results and Discussions
Ma
(1)
where Ma ()72) is a molecular weight of acrylic acid. The concentration of DPC in the solution was obtained from the optical absorbency at λ ) 259 nm. The phase diagram of PAA-DPC system was obtained monitoring the generation of precipitation in the solution of 0.1 mM PAA and a given CS with increase in CDPC. The generation was indicated by an increase in the optical absorbency at λ ) 450 nm. The change in the absorbency of the thoroughly stirring solution was monitored after adding a small amount of 100 mM DPC solution in which the salt concentration was same as CS of the PAA solution. When the precipitation was generated, the absorbency increased within 1 min after the addition and then gradually decreased because the precipitates gathered to be
Figure 2 shows the Cs dependence of volume change behavior with increase or decrease in CDPC. The transitional volume change, which has been rather obscure in the experiments performed before,10-12 is clearly observed at Cs less than 0.4 M. The fact that the CDPC at transition increases with Cs indicates that the principal driving force to binding is the electrostatic interaction between the DP+ ion and the ionized gel chain. A hysteresis is clearly observed in the transition behavior. The DPC concentration at the transition in the increasing process Cti is higher than the concentration at the transition in the decreasing process Ctd. It is remarkable that the magnitude of hysteresis, |Cit - Cdt | increases with decrease in Cs as shown in Figure 2. As shown in Table 1, the amount of binding DP+ ion to the shrunken cylindrical gel in the solution at a midpoint of the hysteresis, CDPC ) 1.1 mM and Cs ) 0.1 M is 25 times the
5854 J. Phys. Chem. B, Vol. 105, No. 25, 2001
Sasaki et al.
Figure 3. Patterns observed on the surfaces of volume phase transition gels in the solution at Cs ) 0.1 M. The CDPC value is denoted under the each pattern.
TABLE 1: Binding Amount of DP+ Ion to the Hysteretic Cylindrical Gel in the 0.1 M NaCl Solution gel state
CDPC/M dm-3
total binding amount/M
swollen shrunken shrunken
1.1 × 10-3 1.1 × 10-3 1.5 × 10-3
1.1 ( 0.1 × 10-7 27.3 (1.0 × 10-7 26.6 (1.0 × 10-7
Figure 5. Phase maps of the salt concentration (Cs) and the DPC concentration (CDPC) for the transitions of the PAA gel and the PAA solution. The transition points, Cti from the swollen to the shrunken state and Ctd from the shrunken to the swollen state, respectively, are denoted by the symbols of O and b. The transition points of the slab gel and the critical generation points of the precipitation of PAA, respectively, are denoted by the symbols of × and 0. The micelles of DPC are formed in the solution at Cs and CDPC being above a broken line.14
Figure 4. Correlation between the transitions of the gel volume (V) and the degree of DP+ binding (β). The value V is a gel volume occupied by one ionized group of the chain. Vertical dot lines represent the transitions.
amount binding to the swollen gel in the same solution. The former amount is almost same as the amount binding to the shrunken gel in the solution at CDPC ) 1.5 mM and Cs ) 0.1 M. In this experiment the used gels were all the same and a dry weight of the gel was not measured. The experimental result indicates that the hysteresis of the volume is also a hysteresis of β. In shrinking or swelling, there appear regular patterns on the surface of the gel. Figure 3 shows the shrinking patterns on the surface of the gel in the solution at Cs ) 0.1 M. The smooth surface at CDPC less than 1.21 mM changed into being barklike at CDPC ) 1.22 mM and more bumpy with increase in CDPC. The surface of the completely shrinking gel at CDPC ) 1.30 mM turned to smooth again. The patterns have been also observed on the surface of the gels that exhibit the other types of volume phase transition, the copolymer gel of N-isopro-
pylacrylamide and acrylate13 in raising temperature and the copolymer gel of acrylamide and acrylate in the acetone-water mixture.14 The pattern formation seems always accompanied by the volume phase transition of gel.8 Figure 4 demonstrates that the change in β synchronizes with the change in volume of the slab gel. This indicates that the volume phase transition is also the transition of β. The coexistence of the shrunken and swelling states in the slab gel makes the β and gel volume be between their values at the shrunken and swelling states as shown in Figure 4. In such a large gel as the slab gel, the shrunken and swelling states can coexist.10 The transition of slab gel corresponds to that of the increasing process of CDPC (the process from swelling to shrinking of the cylindrical gel), since the dried gel immersed in the solution first swells before binding because of higher diffusivity of water molecules than DP+ in the gel. As a matter of fact, the CDPC at the transition is very close to Cti as shown in Figure 5. Figure 5 shows a phase diagram of the gel and PAA solution. It should be mentioned that the transition DPC concentrations CDPCtr of the PAA solution are lower than those of the gel. This might be related to the large difference in the degree of polymerization between the cross-links: about 22 000 for PAA molecule being much larger than 100 for the gel. The expanding conformation of the longer chain is generally the more destabilized in a poor solvent.15 The binding makes the chain affinity with water molecules poor. The binding might be promoted more by the destabilization of the expanding confor-
Volume Phase Transition of Ionic Gel
J. Phys. Chem. B, Vol. 105, No. 25, 2001 5855
mation due to the coupling effect of the binding and the conformation on the free energy.2 No transition is observed for the gel in the solution at Cs above 0.5 M, as shown in Figures 2 and 4. The monomer concentration of DPC cannot exceed the cmc, which decreases with Cs as shown by a broken line in Figure 5.16 The volume change behavior related to the cmc has been also reported by Khandurina et al.12 The Cti and Ctd shown in Figure 5 are less than cmc and thus are the monomer concentrations in the solution. The extrapolated values of Cti and Ctd to Cs above 0.5 M are evaluated to be more than cmc, and the monomer concentrations of Cti and Ctd cannot be achieved in the experiment. This is why no transition occurs in the solution at Cs above 0.5 M. It is natural to expect that a multivalent counterion of micelle binds to ionized gel chain even in such the high salt concentration as 0.5 M. However, the binding was scarcely observed as shown in Figure 4. It should be mentioned that no transitional change in the intensity of light scattered from the PAA solution of Cs ) 0.6 M was observed at CDPC ) cmc in increasing CDPC and that no generation of precipitation of PAA was observed in the solution of 100 mM DPC and Cs above 0.6 M. These facts indicate no binding of micelle particles to ionized polymer chains. It is important to note that the observed smallest diameter ds of the gel in the solution at Cs ) 0.4 M is significantly larger than the ds values at Cs less than 0.4 M. This is related to the fact that the monomer concentration of DPC in the solution cannot exceed a critical micellar concentration (cmc) even when CDPC is larger than cmc. Figure 2 shows that the diameter of the gel in the 0.4 M NaCl solution reaches a constant value at CDPC ) 2.5 mM, which is very close to cmc of DPC16 in the 0.4 M NaCl solution. The constant diameter indicates no change of β with increase in CDPC. The fact that the ds value at Cs ) 0.4 M is greater than those at Cs less than 0.4 M suggests that the β value for the former ds value is less than the β value for the latter ds value. Figure 5 shows that the magnitude of hysteresis increases with a decrease in Cs. There seems the tendency that the magnitude of hysteresis increases with increase in the difference between the volumes of gels in the swelling and shrinking states, which increase with decrease in Cs. The same tendency has been observed for the magnitude of hysteresis in the volume phase transition of copolymer gel of NIPA and acrylate.13 This tendency of the magnitude of hysteresis can be qualitatively explained by the previously reported theory2 as follows. The binding equilibrium can be described in terms of the chemical potentials as
µ0DP + T ln CDPC + µCOO ) µCOODP
(2)
where µ0DP , T, µCOO, and µCOODP, respectively, are the standard chemical potential of DP ion, the Boltzmann temperature, the chemical potentials of the ionized carboxylate, and the DP bound carboxylate. According to the theory, khS, khµ0W, µd, and µh of eq 21 in ref 2, respectively, can correspond to CDPC, µ0DP, µCOO, and µCOODP in the present case. The theoretical calculations have demonstrated that a big change in the gel volumes with a transition between the swelling and the shrinking states results in a big change in khS ()CDPC) values at the swelling and shrinking transitions as shown by Figures 3-5 in ref 2. The magnitude of hysteresis is parallel to the difference in ln CDPC values at the swelling and shrinking transitions. This relation between the magnitude of hysteresis and the volume change
Figure 6. Schematic descriptions of the binding isotherm of DP ion to AA gel and the volume phase transition.2 Curve 1 describes the binding isotherm for the case of no volume change with β. The free energy reduction of chain due to shrinking of the gel with β diminishes µCOODP - µCOO and makes the binding isotherm the S-shape as shown by the curve 2. When the volume change with β is less, the binding isotherm is described by the curve 2′. The arrows directing upper and lower, respectively, indicate the volume and the binding phase transitions at CtI and Ctd.2 The volume curves 2V, 2′V, and 1V as the functions of T ln(CDPC), respectively, correspond to the cases of the binding curves 2, 2′, and 1.
with the transition can be explained semiquantitatively as follows. Figure 6 schematically depicts the theoretical relations among β, V, and µCOODP - µCOO ()µ0DP + T ln CDPC). If the volume does not change with β, µCOODP - µCOO is a monotonically increasing function of β as shown by the curve 1 in Figure 6. The big volume change inducing the big free energy change due to the big entropy change of the chain makes the binding curve sigmoid as shown by the curve 2 or 2′ in Figure 6. The bigger volume change with β makes the curve the more sigmoid, and the difference between ln(CtI) and ln(Ctd) increases as shown by the curve 2 in Figure 6. The observed tendency of (Cti/Ctd) to increase with a decrease in Cs is a reflection of the large free energy reduction of the chain in a shrinking gel. References and Notes (1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (2) Sasaki, S.; Maeda, H. Phys. ReV. E 1996, 54, 2761. (3) Park, T. G.; Hoffman, A. S. Macromolecules 1993, 26, 50454. (4) Suzuki, A. AdV. Polym. Sci. 1993, 110, 199. (5) Kawasaki, H.; Sasaki, S.; Maeda, H.; Mihara, S.; Tokita, M.; Komai, T. J. Phys. Chem. 1996, 100, 16282. (6) Sasaki, S.; Kawasaki, H.; Maeda, H. Macromolecules 1997, 30, 1847. (7) Erman, B.; Flory, P. J. Macromolecules 1986, 19, 2342. (8) Sasaki, S.; Maeda, H. J. Colloid Interface Sci. 1999, 211, 204. (9) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (10) Sasaki, S.; Fujimoto, D.; Maeda, H. Polym. Gels Networks 1995, 3, 145. (11) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E.; Starodubtzev, S. G. Macromolecules 1992, 25, 4779. (12) Khandurina, Yu. V.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Polym. Sci. 1994, 36, 195. (13) Matsuo, E. S.; Tanaka, T. J. Chem. Phys. 1988, 89, 169. (14) Matsuo, E. S.; Tanaka, T. Nature 1992, 358, 482. (15) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: 1953; Chapter 13. (16) Fujio, K.; Ikeda, S. Bull. Chem. Soc. Jpn. 1992, 65, 1406.
