Langmuir 1997, 13, 6135-6138
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Effect of the Hydrophobicity of Chain on Binding Behavior of Hydrophobic Counterions to Ionic Gels Shigeo Sasaki,* Yasuhiro Yamazoe, and Hiroshi Maeda Department of Chemistry, Faculty of Science, Kyushu University, 33 Hakozaki, Higashiku, Fukuoka 812, Japan Received June 6, 1997X Binding isotherms of the dodecylpyridinum ion to copolymer gels of methyl acrylate/acrylate (MA), ethyl acrylate/acrylate (EA), propyl acrylate/acrylate (PA), and n-butyl acrylate/acrylate (BA) were compared to examine the effect of the hydrophobicity of the gel chain on the binding behavior of hydrophobic counterions. The cooperativity of the binding decreased when lengthening the chain of alkyl acrylate. In the binding to the BA gel, anticooperative binding was observed. The binding constant increased with the chain length of alkyl acrylate. Linear dependencies on the alkyl number of alkyl acrylate were found for both of the logarithm of observed cooperativity parameters and binding constants. The dependency of the gel volume on the degree of the binding for BA gels was found to be significantly different from those for MA, EA, and PA gels.
1. Introduction It is well-known that the binding of hydrophobic counterions to ionic polymer chains is cooperative.1 Recent studies have revealed that the cooperativity increases with increasing hydrophobic nature of counterions2 and that the addition of salt to the solution decreases the binding constant but does not affect the cooperativity.3 The high cooperativity is due to the hydrophobic interaction, and the binding constant is regulated by the electrostatic interaction between ionic chains and counterions. The binding constant increases with the alkyl chain length of the hydrophobic counterion.3 The features of binding of hydrophobic counterion mentioned above have been found to hold even when the polymer chains are cross-linked.4,5 Our interest here is the effect of the hydrophobicity of the polymer chain on the binding behavior. The counterion binding regulated mainly by the electrostatic interaction has anticooperative features as exhibited in the pH titration of poly(carboxylic acids),6 where the acidity of the carboxyl decreases with an increasing degree of ionization and increases on addition of simple electrolytes, which shield the polymer charge. The cooperative binding of hydrophobic counterions is caused by the hydrophobic attractive interaction between the bound and the free counterions. The cooperativity of the counterion binding is enhanced by strengthening the hydrophobicity of the counterion.4 However, our present investigation revealed that the cooperativity was dramatically weakened on lengthening the alkyl chain of side group of the polymer. The strong hydrophobic attraction between the bound counterion and the chain was found to weaken the hydrophobic interaction between the bound and the free counterions. The free energy of the hydrophobic interaction between counterions and polymers is expected to depend on the lengths of alkyl chains. The logarithm of the binding X Abstract published in Advance ACS Abstracts, November 1, 1997.
(1) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2263. (2) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. In Structures/ Performance, Relationships in Surfactants; Rosen, M. J., Ed.; ACS Symp. Ser. 253; American Chemical Society: Washington, DC, 1984; p 225. (3) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930. (4) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502. (5) Sasaki, S.; Fujimoto, D.; Maeda, H. Polymer Gels and Networks 1995, 3, 145. (6) Nagasawa, M.; Murase, T.; Kondo, K. J. Phys. Chem. 1965, 77, 4005.
S0743-7463(97)00598-2 CCC: $14.00
constant of alkylpyridinium ion to dextran sulfate was found to be linear in the alkyl number of alkylpyridinium.3 However, no systematic investigation concerning with the dependence on the hydrophobic strength of the side group of polymer chain has been reported so far. The present investigation revealed that the logarithm of the binding constant of dodecylpyridinium (DP) ion to copolymer gels of alkyl acrylate and acrylate increased linearly with the alkyl number(na) of the alkyl acrylate. It was also found that the logarithm of the cooperativity parameters observed in the binding behavior of the system mentioned above decreased linearly with the alkyl number. These findings strongly suggest that the hydrophobic interaction between the bound hydrophobic counterions and the chain increases linearly with alkyl chain length of the side group. 2. Experiments Gels were prepared by radical copolymerization in dimethyl sulfoxide (DMSO) solution of acrylate (1 M) and alkyl acrylate (1 M), and N,N′-methylenebis(acrylamide) (5 mM). Copolymer gels with methyl acrylate, ethyl acrylate, propyl acrylate, and n-butyl acrylate as the alkyl acrylate component are denoted by MA, EA, PA, and BA, respectively. The polymerization was initiated by R,R′-azobis(isobutyronitrile) and was carried out at 60 °C. Gels, synthesized in a plate form of 1 mm thickness, were rinsed thoroughly with methanol, cut into small pieces, and dried under vacuum. Mole ratios of acrylate to the total monomeric units of BA, PA, EA, and MA were 0.48, 0.52, 0.49, and 0.49, respectively, determined by the potentiometric titration. The ratios were very close to the composition of acrylate (0.5) in the polymerizing solution, and the comonomers were expected to be randomly distributed along the chain.7-9 In the titration curve corrected for the blank titration, a pH jump from 7 to higher than 10 at the end point was clearly observed. Carboxylate groups of the acrylate are fully dissociated at pH higher than 8. Dodecylpyridinium chloride (DPC) was recrystallized from acetone several times before use. All chemicals used were reagent grade. Details of the experiments of the DPC binding to gels are described elsewhere.5 Experiments were carried out at 25.0 ( 0.5 °C. Small pieces of dry gel were suspended in a NaCl solution of a given concentration Cs. Neutralization of the gel was carried out by adding NaOH by the amount of 1.1 times as large as that of the carboxylate, which raised the pH to higher than 8 and ionized the carboxylate fully. After an addition of a given amount (7) Vollmert, B. Angew. Makromol. Chem. 1968, 3, 1. (8) Niwa, M.; Kobayashi, M.; Matsumoto, T. Kobunshi Ronbunshu 1981, 38, 413. (9) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; Chapter 5.
© 1997 American Chemical Society
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Table 1. K and u Values for the Cooperative Binding of DPC to MA, EA, PA, and BA Gels K, dm3 M-1
u
Cs/M
MA
EA
PA
BA
0.025 0.05 0.075 0.100 0.025 0.05 0.075 0.100
(20 ( 1) × 102 (12 ( 1) × 102 (11 ( 1) × 102 (8.8 ( 1) × 102 18 ( 5 17 ( 5 27 ( 5 16 ( 5
(36 ( 2) × 102 (22 ( 1) × 102 (18 ( 1) × 102 (15 ( 1) × 102 5(1 4(1 6(1 7(1
(58 ( 2) × 102 (38 ( 2) × 102
(120 ( 20) × 102 (68 ( 2) × 102
(23 ( 1) × 102 1.6 ( 0.2 1.9 ( 0.2
(42 ( 2) × 102 0.66 ( 0.1 0.61 ( 0.2
1.7 ( 0.2
0.62 ( 0.11
Figure 2. Cooperativity parameter u as functions of the alkyl number of side group of the chain na. A relation of ln(u) ) 3.9 - 1.1na is shown by a solid line.
Figure 1. Binding isotherm of DPC. The solid lines are the calculated β values for the K and u values tabulated in Table 1. of DPC, the gel was equilibrated with the solution phase for more than 2 weeks. The gel volume per 1 mol of the acrylate, V was determined by the relation V ) (Wg/Wd)(Ma/F), where Wg, Wd, Ma, and F, respectively, denote the weight of gel, the weight of the dry gel, the weight of the gel containing 1 mol acrylate group, and the density of the gel. It was assumed that F is identical to the density of solution outside the gel. The apparent degree of binding of DPC ion, β, to the acrylate group of the gel was obtained from the relation, (βWd/Ma) + CoutV0 ) mDPC, where mDPC, V0, and Cout, respectively, are the total mole amount of DPC, the total volume of the system (gel + the outer solution), and the concentration of DPC in the solution outside the gel. Here the volume of gel chain is assumed to be negligibly small compared with the gel volume. Cout was obtained from the optical absorbance at 259 nm.
3. Results Figure 1 shows the binding isotherms of DPC to MA, EA, PA, and BA at Cs ) 25 and 100 mM. The gel type
dependent and independent portions of binding isotherms are separately observed at β less than 1 and at β more than 1, respectively. The former and the latter are called regimes 1 and 2, respectively. The binding behavior in regime 1 depends on Cs while that in regime 2 is almost independent of Cs. This indicates that the binding in regime 1 is regulated by the electrostatic interaction and that the binding in regime 2 is not. The binding in regime 1 starts at Cout much lower than Cs. This indicates that the binding is of the site-binding type rather than the territorial binding which can be described by the counterion condensation theory.10 The cooperativity of the binding in regime 1 decreases with increasing na irrespective of Cs as shown in Figure 1. The cooperative binding behavior was analyzed by the Zimm-Bragg theory11
β)
(
1 1+ 2
s-1
)
x(s - 1)2 + 4s/u
; s ) KCout
(1)
where K and u, respectively, are the binding constant and a cooperativity parameter. In the present analysis the K and u values were obtained so as to minimize the sum of difference between the prediction of eq 1 and the data points in the range of β less than 0.7. The obtained values are tabulated in Table 1. It is noticeable that the u value decreases with the increase in na. For BA gel, the u values were found to be less than 1, which indicated an anticooperative binding. The cooperativity of binding of hydrophobic counterion decreases with an increasing hydrophobicity of the gel chain. A linear relation between the logarithm of u and na as shown in Figure 2 was obtained, irrespective of Cs. A relation, u ) 50 exp(-1.1na), is shown by a solid line in (10) Manning, G. S. Q. Rev. Biophys. 1978, 11, 179. (11) Zimm, B. H.; Bragg, J. K. J. Chem. Phys. 1959, 31, 526.
Counterion Binding to Ionic Gels
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Figure 3. Cooperativity parameter u as functions of the salt concentration Cs. Figure 5. Binding constant K as functions of the salt concentration Cs. The relations of ln K ) 5.4 - 0.58 ln Cs, ln K ) 5.8 - 0.64 ln Cs, ln K ) 6.2 - 0.67 ln Cs, and ln K ) 6.5 - 0.78 ln Cs are shown by solid lines from the bottom to the top.
of MA, EA, PA, and BA at β ) 0 and Cs ) 25 mM were similar, between 10 and 12 dm3 per 1 mol of the acrylate group. This indicates that the degree of the cross-linkage is almost same among the gels. The volumes decrease with binding. This is mostly due to the reduction of the osmotic pressure, which decreases the concentration of free counterion. The poor affinity of the polymer segments in the bound state to water molecules makes the chain conformation compact and also reduces the volume. 4. Discussions
Figure 4. Binding constant K as functions of the alkyl number of side group of the chain na. The relations of ln K ) 7.0 + 0.59na (upper), ln K ) 6.5 + 0.58na (middle), and ln K ) 6.3 + 0.51na (lower) are shown by solid lines.
Figure 2. No strong tendency was observed in the Cs dependence of u, as shown in Figure 3. Table 1 shows that the K value increases with an increase in na or with a decrease in Cs. The K value increases with an increase in the strength of the attractive forces between the counterion and the binding site due to the hydrophobic interaction and the electrostatic interaction. When the salt concentration is low or the chain is more hydrophobic, the attractive interactions are stronger and, hence, the binding constant is greater. A linear relation was found for the logarithm of the K value and na as shown in Figure 4. The power relation K ∝ Cs-v was observed as shown in Figure 5. The binding constant is described by11 K ) exp(∆Gb/kT), where ∆Gb is a free energy gap between the bound and unbound states of the site next to a bound site. Figures 4 and 5 give the approximate relation
∆Gb/kT ) (0.55 ( 0.05)na - (0.65 ( 0.10) ln Cs + (4.5 ( 05) Figure 6 shows the volume change with binding for the MA, EA, PA, and BA at Cs ) 25 and 100 mM. The volumes
The hydrophobic interaction energy is considered to be the free energy needed for forming a water sheath covering a hydrophobic domain. Therefore, the hydrophobic interaction energy is a monotonously increasing function of the surface area of the domain, which increases with the dimension of the nonpolar residues. In the present experiment, the hydrophobic interaction energy is proportional to the length of alkyl side chain as indicated by the observed linear relation between log K and na. This has been also found in the linear relation between log K and the length of the alkyl residues of the hydrophobic counterion.3 In this respect, it is interesting that the linear relation is observed between logarithm of u and na as shown in Figure 2. The cooperativity parameter u is described by11 u ) exp(E/kT), where E is the difference in the binding free energy between a site adjacent to the free sites and a site adjacent to the bound site. The former and the latter, respectively, will be denoted by I-binding and A-binding. The E value increases with the size difference of hydrophobic domains formed by the I-binding and the A-binding.5 The E value has been reported to increase with the length of the alkyl residue of the hydrophobic counterion4 in the binding to hydrophobic polyions. However, Figure 2 indicates that E values linearly decrease with the alkyl number of side group of the chain as described by E ) [(3.9 ( 0.2) - (1.1 ( 0.1)na]kT. This can be explained as follows. A part of the alkyl chain of bound hydrophobic counterion is buried in the hydrophobic domain of the chain and only the unburied part of the alkyl chain is newly exposed to the water phase on binding. The unburied part in the case of the I-binding is lager
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difference can be explained as follows. When the binding is cooperative, the nonpolar side chains in the bound state result in segregating from those segments in the unbound state along the chain. The bound portions might produce domain structures. The dimension of the domain increases with β. The gel volume may decrease almost linearly with β in this case as demonstrated in Figure 6. This is observed for the case of MA, EA, and PA gels and poly(acrylic acid) gel.5 When the binding is not cooperative as in the case of BA, the bound states will be randomly distributed along the chain. A bound site of one chain may be close to another bound site of a different chain. The strong hydrophobic attraction between them might produce a physical cross-link in the gel, so that the gel shrinks. The shrinkage of BA gel at β less than 0.2 might be due to the physical cross-links. The flexibility of the bound portion of the polymer chain decreases with increasing β since the alkyl chain of the hydrophobic counterion sticks to the hydrophobic side chain. This tendency is strongest in the case of BA. The less flexible the polymer chains are, the more they expand.12 The reduced Donnan osmotic pressure due to the counterion binding might be compensated by the effect of the reduced flexibility of the bound chain. The compensation results in the plateau of the BA gel volume at β from about 0.2 to 0.7 as shown in Figure 6. In the case of MA, EA, and PA gels, the flexibility is not much reduced by binding of hydrophobic counterions and the gel volumes decrease with increase in β as shown in Figure 6. The hydrophobic attractive interaction among the bound portions shrinks the gel completely at β of about 0.8. The collapsed gel can bind more hydrophobic counterions than expected from the charge neutralization because of its high hydrophobicity. This explains the fact that β exceeds 1 at high Cout as shown in Figure 1. It is interesting that Cout at β ) 1 is much smaller than the critical micelle concentration13 of DPC. This suggests that the hydrophobicity of the gel in this state is stronger than that of the micelle. The fact that the gel volume is almost constant in the range of β over 1 suggests that chloride ions, the counterions of the bound dodecylpyridinium ions, are bound considerably the charges (now positive) of the surfactant-gel chain complex domain. It is inferred that in this hydrophobic domain ions are present as salts NaCl and DPC. Figure 6. Gel volume V normalized by V at β ) 0 as functions of the degree of ion binding β.
than that in the case of A-binding. Thus E value decreases with na. The gel volume of BA normalized by V at β ) 0 shows different β dependency from those of MA, EA, and PA as shown in Figure 6. The mechanism leading to this
Acknowledgment. This work was partially supported by Grant-in Aids for Scientific Research (B) (No. 08454184) and (C) (No. 08640742) from the Ministry of Education, Science, Sports and Culture of Japan. LA970598X (12) Sasaki, S.; Ojima, H.; Yataki, K.; Maeda, H. J. Chem. Phys. 1995, 102, 9694. (13) Fujio, K.; Ikeda, S. Bull. Chem. Soc. Jpn. 1992, 65, 1406.
