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Effect of Chain Hydrophobicity on Volume Change Behavior of Ionic Gels in Binding of Hydrophobic Counterions Shigeo Sasaki,* Yasuhiro Yamazoe, and Hiroshi Maeda Department of Chemistry and Physics, Graduated School of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan Received November 15, 1999. In Final Form: June 12, 2000 The effect of the hydrophobicity of gel chains on the volume changes of ionic gels in binding of the Dodecylpyridinium (DP) ions was examined. The study was carried out for a series of copolymer gels, acrylate-methylacrylate, acrylate-ethylacrylate, acrylate-n-propylacrylate, and acrylate-n-butylacrylate(BA), in which the molar ratios of acrylate to the alkyl acrylate were 1. The distinctive two regimes; the regime HYS where the volume changes with hysteresis and the regime TRL where it changes in a transitionlike manner without hysteresis, were found in the volume change with the concentration of DP ion (Cout). The hysteresis was more prominent for the more hydrophobic gel and/or the gel in the solution of lower salt concentration. Substantial plateau regions were found between HYS and TRL in the volume change of the more hydrophobic gel. The hysteresis was also exhibited for the change in degree of binding of counterion, β, with Cout. The dissociation rate of DP ion in immersing the gel bound by DP ions into the solution of higher salt concentration is affected by the hydrophobicity of gel chain more than by the osmotic pressure.
1. Introduction Hydrophobic and electrostatic interactions have been well-known to take important roles in assembling various molecules in the aqueous solutions. The combination effect of the hydrophobic and electrostatic interactions, which act on molecules in different manners, assembles or disassembles macromolecules and/or small molecules. The subtle combination of the hydrophobic and electrostatic interactions regulates the cooperative formation of such complexes as a DNA-protein. The quantitative and qualitative relation between the cooperativity and the combination effect has not yet been well clarified. Using a system of the ionic gel and the hydrophobic counterion, we are studying what the are effects of change in the hydrophobicity of the chain on the behavior of counterion binding and chain conformation. Theoretical1-5 and experimental6-9 studies done so far have clarified the highly cooperative nature of the hydrophobic counterion binding to the polyelectrolyte. It has been found that both cooperativity parameter and binding constant increase with increase in the hydrophobicity of the counterion.10 It has been also found that the addition of salt decreases significantly the binding constant and that it induces a subtle change in the cooperativity parameter.10,11 High cooperativity of the binding is induced by the hydrophobic interaction between the bound hydrophobic counterions. Recent studies12,13 (1) Schmitz, K. S.; Schurr, J. M. Biopolymers 1970, 9, 697. (2) Schwarz, G. Eur. J. Biochem. 1970, 12, 442. (3) McGhee, J. D.; von Hippel, P. H. J. Mol. Biol. 1974, 86, 469. (4) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2263. (5) Wei, Y. C.; Hudson, S. M. Macromolecules 1993, 26, 4151. (6) Binana-Limbele, W.; Zana, R. Macromolecules 1987, 20, 1331. (7) Benrraou, M.; Zana, R.; Varoqui, R.; Pefferkorn, E. J. Phys. Chem. 1992, 96, 1468. (8) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180. (9) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (10) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1390. (11) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (12) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502.
have revealed that cross-linked polymer chains hold these features of the hydrophobic counterion binding. Our previous paper14 has reported that the hydrophobicity of chain seriously affects the binding behavior of the dodecylpyridinium (DP) ion to the ionized carboxylate groups of gel chain. The following have been revealed from the binding behavior to the copolymer gels of acrylic acid with methylacrylate (MA), acrylic acid with ethylacrylate (EA), acrylic acid with n-propylacrylate (PA), and acrylic acid with n-butylacrylate (BA). The binding constant increases exponentially with the alkyl carbon number (n) of the alkyl acrylate. The cooperativity parameters decrease with n. It has been concluded that the strong hydrophobic attractive force between the bound counterion and the chain weakens the hydrophobic interaction between the bound counterions and reduces the cooperativity. Our interest here is a detailed relation between the gel volume and the degree of binding to a carboxylate group, β. As reported previously,14 the behavior of volume change with β for the BA gel is somehow different from that for the MA gel at β ) 0.5-0.7. The volume of hydrophilic gel decreases linearly with β as observed for the MA gel14 and poly(acrylate) gel.13 For the more hydrophobic BA gel, however, we have observed a plateau in the volume change between β ) 0.5 and 0.7. No volume change with increases in β is inconsistent with the well-established volume change behavior of ionic gels15 that the counterion binding reduces the gel volume owing to the reduction of osmotic pressure. In the previous study, the detail of volume change with β or concentrations of DP ion outside the gel, Cout has not been well clarified. In the present study, much smaller sized gels than those used in the previous study were used to achieve equilibrium for shorter periods, and more detailed relations between the volume and Cout were (13) Sasaki, S.; Fujimoto, D.; Maeda, H. Polym. Gels Networks 1995, 3, 145. (14) Sasaki, S.; Yamazoe, Y.; Maeda, H. Langmuir 1997, 13, 6135. (15) Schosseler, F.; Illmain, F.; Candau, S. J. Macromolecules 1991, 24, 225.
10.1021/la991490f CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000
Effect of Chain Hydrophobicity on Volume Changes
obtained. For examining the effect of chain hydrophobicity on the volume change behavior, MA, EA, PA, and BA gels were also used in the present study. For examining the effect of electrostatic interaction, the salt concentration, Cs, dependence of the volume change behavior was investigated again, but in a detailed manner. Distinctive two regimes of Cout were found in the volume change behavior: the regime HYS where the volume change with Cout exhibits a hysteresis and the regime TRL where the change exhibits no hysteresis. For PA and BA gels in the solution at Cs ) 25 mM, substantial plateaus in the volume change were observed between HYS and TRL regimes. The plateau in the volume change has been also observed for the BA gel in the previous study.14 The kinetic study of the gel from the shrinking state to the swelling state was also carried out in the present study, which revealed that the dissociation rate of the bound DP ion from the BA gel was much slower than that from the MA gel. 2. Experiments Copolymer gels of acrylic acid with alkyl acrylate were prepared with radical copolymerization. The polymerization was initiated by R,R′-azobis(isobutyronitrile). The gels were synthesized in the cylindrical glass (inner diameter ) 0.3 mm) and in the space between two glasses separated by a 1 mm thick spacer of poly(tetrafluoroethylene) (Teflon) sheet. The polymerization was carried out in the dimethyl sulfoxide solution of 3 M acrylic acid, 3 M alkyl acrylate, and the cross-linker, N,N′-methylenebis(acrylamide). The concentrations of the cross-linker and the initiator, respectively, were 3 × 10-2 and 4.5 × 10-2 M. The polymerizing solution was degassed before polymerization. The polymerization was carried out at 60 °C for 5 h. After the polymerization, the rod- and platelike gels were immersed into water to shrink the gel and were rinsed thoroughly with methanol. The gels were cut into 10 mm length rods or 10 mm square plates. They were dried gently and thoroughly in a vacuum. Mole ratios of acrylate to the total monomer units in the gels were 0.50 ( 0.04, which were determined by potentiometric titration. The ratios were very close to the compositions of acrylate in the polymerizing solution. This indicates that the acrylate monomers are randomly distributed along the chain.16-18 Dodecylpyridinium chloride (DPC) was recrystallized from acetone three times and was used. All chemicals used were of reagent grade. An apparatus used to determine the diameter of gel, d, has been described elsewhere in detail.19 The apparatus consists of the optical microscope, the camera, a computer system, the cell to set the sample holder, and the water bath to control the temperature to 25.0 ( 0.1 °C. The sample holder made of Teflon tube and silicone rubber tube was set in the cell. One end of the rodlike gel was adhered to the silicon tube by using the glue. The gel fixed to the sample holder was immersed into the solution of given concentrations of salt (NaCl), Cs and DPC. For making the carboxyl groups fully ionized, small amounts of NaOH were added to the solutions to make their pH greater than 10. The size of gel equilibrated with the solution was measured on more than 1 day after immersing the gel into the solution. The DPC concentration outside the gel, Cout, at equilibrium was almost same as that of the solution prepared, since the outside solution was renewed a few times at intervals of 2 h. A diameter of the gel at a various DPC concentrations was obtained by analyzing an image taken by the apparatus mentioned above. The Cout dependence of gel diameter was examined in both the increasing and decreasing Cout processes. The Cout dependence of β for the BA plate gel in the solution at Cs ) 25mM was examined with increasing or decreasing Cout. After the dry gel was suspended in the alkaline salt solution (pH (16) Flory, P. J. Principles of Polymer Chemistry ; Cornell University Press: Ithaca, NY, 1953; Chapter 5. (17) Vollmer, B. Angew. Macromol. Chem. 1968, 3, 1. (18) Niwa, M.; Kobayashi, M.; Matsumoto, T. Koubunshi Roubunshu 1981, 38, 413. (19) Sasaki, S.; Maeda, H. J. Colloid Interface Sci. 1999, 211, 204.
Langmuir, Vol. 16, No. 18, 2000 7127 ) 9-10) for a couple of days, a given amount of DPC, mDPC, was added to the solution. The gel was equilibrated for more than 2 weeks. It took about 1 week to reach the equilibrium, which was indicated by an unchangeable gel volume with time. The Cout, which was obtained from the optical absorbency at λ ) 259 nm, gave β by using the relation βWd/Ma + CoutVt ) mDPC, where Vt, Ma, and Wd, respectively, were the total volume of the system (the solution + the gel), the weight of the gel containing 1 mol acrylic acid, and the weight of the dry gel. The β in the Cout increasing process was obtained in the way mentioned above. The β in the Cout decreasing process was obtained in the rather complicated manner as follows. A given amount of DPC was added to the alkaline salt solution, in which the gel was suspended for a couple of days, to make the Cout value of solution more than 2 mM to shrink the gel after suspending it for 2 weeks. The deswelling gel was once taken out of the solution. After the solution attached on the gel surface was carefully removed, the gel was immersed into the alkaline salt solution containing no DPC. The gel was suspended for more than 1 week, the concentration of DPC in the solution (Cout) was measured, and the gel was taken out of the solution and weighed. Then, the gel was immersed into the 1:1 mixture of 100 mM NaCl aqueous solution and methanol to dissociate the bound DP ions. From the DP concentration of the solution (CDP) determined from the optical absorbency, the β was determined by the relation βWd/Ma + Cout WG/FG ) CDPVMix, where WG, FG, and VMix, respectively, are the weight and density of the gel suspended in the second solution and the total volume of the mixture. Here, FG was assumed to be 1. The time profile of the dissociation of bound DP ions from the MA, EA, PA, or BA gel was observed in immersing the DP binding gel into the solution of the Cout at which β was substantially 0 in the equilibrium condition. The DP binding gel was prepared by immersing and suspending the gel in the solutions containing NaCl (Cs ) 25 mM) and DPC (Cout ) 3 mM) for more than 2 weeks. The gel was suspended in the DP-free salt solution (Cs ) 0.2, 0.5, or 1 M) of a given volume (V0 g 50 mL . the gel volume). The concentration of DP ion in the solution, C(tn), was monitored at a time, tn, by taking small amount, VS(tn) of the solution for measuring the optical absorbency. The C(tn) was kept less than the Cout for the gel of β ) 0.05 in any cases for minimizing the effect of concentration fluctuation of DP ion outside the gel on the experiment. In the case that C(tn) was close to the Cout-value, the V0 increased after the measurement by adding the DP free salt solution. The β(tn) at the time of the nth measurement was obtained from the following relation
{
n-1g1
β(tn)Wd/Md ) β(tn)Wd/Md - C(tn) V0 -
}
∑ V (t ) S
k)1
{
C(tn-1) V0 -
k
+
n-2g1
∑ V (t ) S
k)1
k
}
where C(t0) ) 0. The β(t0 ) 0) was regarded as the equilibrated β value of the DP binding gel in the solution of Cout ) 3 mM and Cs ) 25 mM. The experiments of binding and dissociation were carried out at 25.0 ( 0.5 °C.
3. Results Figure 1 shows the normalized diameters, d/do, of MA, EA, PA and BA gels as functions of Cout, where do is the diameter at Cout ) 0. The differences in do among the MA, EA, PA, and BA gels are very small, which indicates much stronger swelling power of the osmotic Donnan pressure than the shrinking power of the hydrophobic attraction among the alkyl chains at β ) 0. This has been also observed in the previous experiment.14 The d/do decreases with increase in Cout for all gels. As reported previously,14 β increased with an increase in Cout. That is, the d/do decreases with β. It is noticeable that the d/do value of the completely shrinking gel of any type at Cs ) 25 mM is smaller than that at Cs ) 100 mM, as shown in Figure 1.
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Figure 1. Shrinking and swelling behavior of MA (a), EA (b), PA (c), and BA (d) gels with change in the DPC concentration outside the gel. Open and close symbols, respectively, represent the processes of increasing and decreasing DPC concentrations. Dotted lines with arrow show the hysteresis. The ordinate is the diameter normalized by the diameter of swollen gel.
This has been also observed but obscure in the previous experiment.14 The lower limit of Cout at which d/do starts decreasing corresponds to a critical aggregation concentration (cac). The cac decreases with the alkyl carbon number of the side chain, n, as shown in Figure 1. This has been also observed in the previous study.14 The decreasing tendency of cac with n is due to the strengthened hydrophobic attractive force between the DP ion and the gel chain. Figure 1 shows that the gel’s shrinking range of Cout widens with increase in n. This is consistent with the fact that the cooperativity of DP ion binding decreases with n, which has been also observed in the previous experiment.14 Figure 1 shows distinct two regimes: the regime HYS where the d/do changes with hysteresis and the regime TRL where it changes in a transition-like manner without hysteresis. For PA and BA gels in the solution at Cs ) 25 mM, substantial plateaus in the d/do change are observed between HYS and TRL regimes as indicated by a notation of PL in Figure 1. In the previous study,14 the plateau has been distinctive for only BA gel but obscure for PA gel. The present examination of the volume change behavior in a detail manner with respect Cout elucidates this more clearly. The hysteresis in the HYS region is more clearly observed for the gels in the solution Cs ) 25 mM than that in the solution Cs ) 100 mM, as shown in Figure 1. The hysteresis for the gel with larger n tends to be more prominent. Figure 2 shows Cout dependence of β value in HYS regions of BA gel in the solution Cs)25mM. It is obvious that the β value in the increasing Cout process is lower
Figure 2. Relation between the degree of DP binding and the size of the gel in the HYS regime of BA gel in the solution of Cs ) 25mM.
than that in the decreasing Cout process at the same Cout. This corresponds to the larger d/do in the increasing Cout process than that in the decreasing Cout process. This indicates that the hysteresis of d/do is originated from the hysteresis of β. Figure 3 shows the time profiles for the dissociation of bound DP ions from the MA, EA, PA, and BA gels into 1 M NaCl solution. The DP binding gels used in the experiment were prepared in the 25 mM NaCl solution.
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Figure 3. Time profile of the dissociation of DP from the MA, EA, PA, and BA gels in immersing DP bound gels into 1 M salt solution. The β0 is β(t0 ) 0).
It is obvious that the dissociation rate decreases with an increase in n, although the dissociation rates from the BA and PA gels are very close to each other. The hydrophobic attractive force seems to reduce the dissociation rate. Figure 4 shows that the dissociation rates of the bound DP ions from the EA, PA, and BA, gels are insensitive to the Cs of the solution in which the gels are suspended for the dissociation, and that the dissociation from the MA gel is sensitive to the Cs. It should be mentioned that the reproducibility of the dissociation rate was qualitatively good but was not quantitatively. That is, the trend of dissociation rates mentioned above always holds, although the subtle changes in the size and shape of gel used in the experiment slightly affects the time profiles of dissociation. 4. Discussion The binding of counterion reduces the swelling osmotic pressure and makes the gel shrink. In the present experiment it was found that the shrinkage stops in the regime PL of PA or BA gel despite the counterion binding. In the previous experiment,14 this has been also observed for BA gel at β between 0.5 and 0.7. This can be explained by the reduced flexibility of chain bound by the counterion. The stiff chain expands more than the flexible chain, since a distance between the cross-links due to the entropy is approximately proportional to Lfb1-f, where f, L, and b, respectively, are the Flory exponent being less than 1, the contour length of chain, and the statistical segment length. The force to stiffen the chain or to lengthen b gives the expanding force of chain as a result and can compensate for the shrinking effect due to the reduction of osmotic pressure. We speculate the mechanism to stiffen the chain with the binding as follows. The alkyl chain of the bound DP ion can twist around the gel chain owing to the hydrophobic attraction between them. The twist, which suppresses the freedom of rotation around the C-C bonds of the main chain, makes the chain stiffer and lengthens b. The degree of the stiffness increases with the degree of the entanglement that increases with increase in the alkyl chain length as well as β. This can explain that the stoppage of shrinkage in the regime PL of the relatively hydrophobic BA and PA gels is prominent. The shrinking with Cout is transition-like in the regime TRL, as shown in Figure 1. In the regime TRL, the highly hydrophobic block of successive chain segments bound by DP ion might contact with each other and the gel greatly shrinks or collapses. From the previous experiment,14 we
Figure 4. Salt concentration dependence of the time profile of the dissociation of DP from the MA (a), EA (b), PA (c), and BA (d) gels. The β0 is β(t0 ) 0).
can infer that this regime exists at β above 0.8. The hydrophobic stiff chains might be stuck together in a sideby-side manner to collapse in the TRL regime. In the regime HYS, the hysteresis in the Cout increasing and Cout decreasing processes is observed. As shown in Figure 2, the hysteresis of the gel volume reflects the hysteresis of β. Larger gel volumes in the increasing process than those in the decreasing process correspond to smaller β-values in the former than those in the latter at the same Cout. Figure 2 also shows that the large change in the volume of the increasing process is accompanied by the large β-value change and vice versa. However, this
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does not necessarily suggest the 1-1 correspondence between the β-values and the volumes in the HYS regime. The very small increase in β with decrease in the Cout could be explained by the stabilization of the bound DP ions with twisting around the gel chain. There might be the activation barrier for the bound DP ions to untwist. Most of the bound DP ions in the increasing process in the HYS regime are not considered to twist around the gel chain. The twist of bound DP ion might take an important role in the hysteresis. For the twist of the hydrophobic part (alkyl chain) of the counterion, the ionic part of the hydrophobic counterion should tightly bind to the ionized group of the gel chain. In increasing Cs, the binding is loosened by weakening the electrostatic attraction between the counterion and the ionized group because of the shielding effect of salt. The fact that the diameters of the collapsed gels in the solution at Cs ) 100 mM are larger than those in the solution at Cs ) 25 mM suggests that the binding in the former is more loose than the binding in the latter. Therefore the hysteresis in the case of Cs ) 100 mM is smaller than that in the case of Cs ) 25 mM, as shown in Figure 1. The reduction of electrostatic attraction between the DP ion and ionized groups on the chain with increase in the salt concentration at the binding site induces the dissociation of DP ion and decreases β. The dissociated DP ions should diffuse in the gel for other bound DP ions to dissociate furthermore, and they are released from the gel to be detected in the experiment of dissociation rate. For the diffusion, a certain amount of water molecules should penetrate into the shrunken gel. The swelling force of osmotic pressure drives the penetration. On the other hand, the shrinking force due to the hydrophobic bound DP ion and gel chain suppresses the penetration of water. It should be noted here that the gel appeared to homogeneously swell with the dissociation and that the increase in the gel volume in the 1 M NaCl solution stopped at an early stage: t ∼ 3 × 102 min for the MA gel, t ∼ 5 × 102 min for the EA gel, t ∼ 2 × 103 min for the PA gel, and t ∼ 2 × 103 min for BA gel (not shown). The increase in swelling force due to the dissociation is compensated with the reduction of osmotic swelling pressure due to an increase in the Cs from 25 mM to 1 M at the early stage.
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From the fact that the times at which the increase in the volume stopped are much shorter than the times for the dissociation to stop, we can say that the penetration of water molecules takes on a less important role in regulating the rate of the dissociation. In the diffusion, the dissociated DP ions bind again to the ionized site near by which the DP ion concentration fluctuates highly. The binding during the diffusion process decreases the diffusion coefficient. Since the binding to the gel chain with the more hydrophobic side chain occurs at the lower concentration of DP ion,14 the diffusion coefficient in the gel decreases with increase in n. The lower diffusion coefficient results in the slower dissociation in the experiment. The mechanism mentioned above can well explain the result shown in Figure 3. This can also explain the Cs dependence of dissociation rate for the MA gel as shown in Figure 4, since the more binding occurs in the lower Cs. The rather Cs independent dissociation rate for the BA, PA, and EA gels as shown in Figure 4 indicates that the weakening effect of the salt on the electrostatic interaction does not affect the hydrophobic attraction so much. 5. Conclusions The present investigation on the effect of the hydrophobicity of gel chains on the volume changes of ionic gels in binding of the DP ions revealed the following. (1) The volume change behavior shows distinct two regimes: the regime HYS where the volume changes with hysteresis and the regime TRL where it changes in a transition-like manner without hysteresis. (2) The more prominent hysteresis is observed for the more hydrophobic gel and/ or the gel in the solution of lower salt concentration. (3) Between HYS and TRL, substantial plateau regions exist in the volume change of PA and BA gels. (4) The hysteresis of volume change accompanies the hysteresis of β. (5) The dissociation rate of DP ion in immersing the gel bound by DP ions into the solution of higher salt concentration is affected by the hydrophobicity of gel chain more than by the osmotic pressure. LA991490F
