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Interaction of polyelectrolyte networks with oppositely charged micelle-forming surfactants
Interaction of polyelectrolyte networks
903
isotropic, narrower than for the corresponding 3d-systems. This correlates with fall in the number of degrees of freedom in the isotropic 2d-system as compared with the 3d. Translated by A . CROZY
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
L. ONSAGER, Ann. N.Y. Acad. Sci. 51: 1949. P. J. FLORY, Proc. Roy. Soc. London A234: 73, 1956. E. DIMARZIO, J. Chem. Phys. 35: 658, 1961. M. WARNER and P. J. FLORY, Ibid. 73: 6327, 1980. W. M A I E R and A. SAUPE, Z. Naturforsch. 14: 882, 1959. P. J. FLORY and G. RONCA, Molec. Cryst. Liquid Cryst. 54: 311, 1979. L. ONSAGER, Phys. Rev. 65: 117, 1944. L . D . LANDAU and Ye. M. LIFSHITS, Statisticheskaya fizika (Statistical Physics) 567 pp., Moscow, 1964. P. DE GENNES, Fizika zhidkikh kristallov (Physics of Liquid Crystals) 400 pp., Moscow, 1977. V. V. RUSAKOV and M. I. SHLIOMIS, Termotropnyi ZhK-perekhod v lineinykh polimerakh. Rol'dliny i zhestkosti makromolekul. (Thermotropic LC Transition in Linear Polymers. Role of Length and Rigidity of the Macromolecules) Pre-print, Sverdlovsk, 1983. 11, L. D. LANDAU, Zh. eksp. teoret, fiz. 7: 627, 1937.
PolymerScienceU.S.S.R. Vol. 32, No. 5, pp. 903--909,1990 Printed in Great Britain.
0032-3950/90 $10.00+ .00 t~ 1991PergamonPressplc
INTERACTION OF POLYELECTROLYTE NETWORKS WITH OPPOSITELY CHARGED MICELLE-FORMING SURFACTANTS* V. R. RYABINA,S. G. STARODUBTSEV and A. R. KHOKHLOV Lomonosov State University, Moscow
(Received 2 February 1989) The conformational properties of polyelectrolyte networks swelling in a solvent containing micelle-forming SAs have been studied. The charge of the SA molecules is opposite to that of the network chains. It is shown that in this case there is heavy sorption of the SA molecules by the gel accompanied by the formation of SA aggregates within the gel and considerable fall in its size. Increase in the ionic strength of the solution leads to fracture of the network-SA complex. These phenomena are due to the fact that firstly on entry of the SA molecules into the gel the low molecular mass counter-ions are released, i.e. heavy gain in translational entropy is reached, and, secondly, aggregation in the gel is more advantageous than micelle formation in the solvent since it leads to immobilization of a smaller number of counter-ions.
RECENTLY there has been growing interest in the study of weakly cross-linked polymer networks synthesized in the presence of a large amount of solvent. Such networks are extremely labile and are capable of significantly changing their size with change in the external factors--composition of the * Vysokomol. soyed. A32: No. 5,969-974, 1990.
V . R . RYABINAet al.
904
solvent, ionic strength, application of mechanical forces--these changes often being realized as sharp abrupt conformational transitions [1-3]. An important new trend in research in polymer sciences is study of the structure and properties of complexes of linear polyelectrolytes with ionogenic SAs [4, 5]. Such complexes are also of considerable practical interest, for example, for producing effective means of purifying industrial effluents [6]. The aim of the present work is to study systems combining the properties of weakly cross-linked polymer gels and linear polyelectrolyte-SA complexes. The work is concerned with various aspects of the interaction of polyelectrolyte networks prepared in the presence of a large amount of solvent with oppositely charged SA molecules capable of forming micelles in aqueous solutions. The test objects were networks based on the copolymers of sodium methacrylate (MAC-) with methacrylic acid (MAC) the charges on which may migrate (Table 1, networks 1-3) and also networks based on TABLE 1.
Network No.
COMPOSITION AND CONDITIONS OF SYNTHESIS OF NETWORKS*
[Acrylamide]
[MAA]
[MAC]
tool. % 1 2 3 4 5 6 7 8 9
---95.5 92.5 89.5 69.5 -99.5
[MAC-]
(I)0
Tp, °C
10.0 30.0 50.0 4.0 7.0 10.0 30.0 10.0 --
0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.154 0.077
20 20 20 20 20 20 20 4 20
mol. % -------89.5 --
89.5 69.5 49.5 -------
*The content of BAA in all networks was 0.5 mol.%.
copolymers of M A C - and acrylamide or methacrylamide (MAA) (Table 1, networks 4-8). In the last case the charged network units are fixed. The crosslinking agent was in all cases N,N'methylene-b/s-acrylamide (BAA). The gels were obtained by radical copolymerization of the monomers and the crosslinking agent in aqueous solutions in the presence of fixed amounts of sodium bicarbonate by the standard technique [7]. The temperature of polymerization Tp and the values of the volumetric fraction of the polymer in the gel after synthesis ~0 are given in Table 1. For synthesis we used siliconized glass ampoules of diameter 3 ram. The samples obtained were cut into cylinders 5 mm in length and washed in twice distilled water for three weeks. They were then placed in a vessel containing the SA [cetylpyridinium bromide (CPB) of Chemapol] and water and thermostatted at 25°C for 20 days. The volume of water in the vessel was 10 times the volume of the gel. Extending the experiment to 40 days did not affect the values of the magnitudes obtained. The relative volume of the swollen networks was characterized by the value of the ratio V/Vo where V is the equilibrium volume of the network and V0 the volume of the network after synthesis. The effectiveness of binding of CPB by the gel was characterized by its distribution in the gel-water system: K = cg/cs where cg and cs are the concentration of CPB cations in the gel and solution. The magnitude K was calculated from the formula
g = ( M - DVs e -~) e/VD,
(1)
Interaction of polyelectrolyte networks
905
where M is the total n u m b e r of CPB moles in the solution; e is the extinction coefficient (CPB has a characteristic absorption band in the U V region; Amax -- 259 nm with e = 4100); Vs is the volume of the solution; D is the optical density of the solution at 259 nm. The presence of SA aggregates in the gel was estimated visually and in solution spectrophotometrically from the solubilization of the dyes (Sudan-i, Sudan-3) insoluble in water. The sample was mechanically strained by compressing it 3-5 times between the planes of a folded plate of fine stainless steel. Figure 1 presents the dependences of the relative volume of the gel V/Vo, the concentration of CPB cations in the gel Cg and the degree of filling of the anionic network with CPB cations 3' (Y is the ratio of the n u m b e r of CPB cations in the network to the total number of network units) on the concentration of CPB in the external solution cs for network 8 (Table 1, dependences for other charged networks are similar). It will be seen that the CPB concentration in the gel is 102-104 times higher than in the external solution; for all the values of c~ studied it is well above the critical concentration for micelle formation (CCM). From this it is natural to assume that over a wide interval of cs values micelles of the type depicted in Fig. 2a, do not appear in solution while CPB cations aggregate within the gel.*
g
i
-5
I
'
10 c~ x 104, mole/I
FIG. 1. Dependences of V/Vo(1), cg (2) and y (3) on cs for network 8 (Table 1). Broken line shows CCM in water. ®
® j
®
G
e~
ee
=
'
® ®
~.¢
~
~
&
¢,
®
(a)
¢ e
Flo. 2. Schematic representation of the formation of micelles in (a) SA solution and (b) in the gel.
*The question of the structure of these aggregates needs further study.
906
V . R . RYABINAet al.
It is easy to satisfy oneself of the soundness of this assumption by placing on the surface of the gel particles of a dye insoluble in water (Sudan-I, Sudan-3). On contact with the gel the dye solubilizes staining the sample, staining being uniform over the whole volume of the gel. At the same time solubilization of the dye by the solution does not occur, i.e. SA aggregates (micelles) do not form in solution. We would note that as the SA ions are bound by the gel the structure of the network-SA complex changes manifest, in particular, in the fact that in the region of large y the gels lose their transparency. It may also be seen from Fig. 1 that interaction with the SA sharply reduces the volume of the gels. It is natural to equate this reduction with the phenomenon of collapse of the polymer network [1-3, 8] which in the given case is due to the fact that transfer of the CPB cations to the gel and the formation there of aggregates is accompanied by release of an equimolar quantity of Na + ions (counter-ions in relation to the charges on the network chains) into the external solution. The osmotic pressure of the "gas" of the counter-ions within the network thereby falls (the SA ions in the aggregates are immobilized) and the network collapses through the volumetric interactions of the cetyl groups. The phenomenon of collapse of the networks on interaction with oppositely charged SAs is of a general character (Fig. 3): for relatively small y values the volume diminishes very strongly and only in the region of high y values does it begin to rise weakly because of intense absorption of the voluminous CPB ions by the networks. Figure 4 presents the dependences of the gel-water distribution constant K on 3' for different IogK log V/Vo
7
L5
<.-.-.\. 2
3
0.5 1
-0,5
3
"I
I 20
I J0 ],,°/o
Fro. 3 FIG. 4 FIG. 3. Dependencesof V/Vo on y for networks 1-3, 6 and 7 (Table 1). FIG. 4. The functions K(y) for networks 1-3, 6 and 7 (Table 1). Arrows denote the points corresponding to the CCM in the ambient solution. types of gels. It will be seen that these dependences pass through a maximum corresponding to the most effective sorption of the SA molecules from solution. The effectiveness of sorption, as a whole~ grows with increase in the density of the charged groups on the network chains (Fig. 4). As Fig. 3 shows the relative change in the volume of the network also increases. At the same time
Interaction of polyelectrolyte networks
907
experiments run on uncharged networks based on acrylamide (Table 1, network 9) showed that they practically do not sorb the SA and do not change their volume in its presence. Figure 5 confirms increase in the effectiveness of sorption with rise in the charging capacity of the network chains. In addition, from these findings it may be concluded that for an identical concentration of the SA in solution, the networks the charges in which may migrate, absorb the SA more effectively than those with fixed charges. However, despite the difference in the structure and degree of chargeability of the gels (including gels containing only 4% charged groups) for all of them the magnitude K remains much larger than unity, this statement also applying to values of cs above the CCM (see Fig. 4, where the CCM values in the external solution are denoted by arrows). Mechanical strain of the gel (its compression 3-5 times) has an insignificant influence on the effectiveness of sorption of the SA by the gel. The gel volume changes by not more than 15% and the K and y magnitudes roughly by the same value. The phenomenon of intense absorption of the SA by the oppositely charged polyelectrolyte gels considered with the formation of SA aggregates in the network and sharp contraction of its volume may occur only for a sufficiently low ionic strength of the solution. Increase in the ionic strength (achieved by adding a low molecular mass salt NaBr) leads to fracture of the network-SA complex (Fig. 6) while the CCM in solution with increase in ionic strength considerably falls. tOO
-7''1°
log VNo ! 0,3
-5
-a
Iogc, [rnolell]
-0,5
Fro. 5
FI~. 6.
~
~
-
Fio. 6
Fro. 5. The functions y(cs) for networks 1-5, 7 and 9 (Table 1). Effect of ionic strength of the solution on the relative volume V I V o of network 7 (Table 1). Degree of filling -/= 0.19 (1), 0.29 (2) and 0.55 (3).
When the polyelectrolyte gel is placed in a large volume of solvent, the low molecular mass ions for entropic reasons (to realize the maximum freedom of translational movement) tend to pass from the gel into the solvent. However, if the solvent does not contain ions such a process is impossible since the gel must remain macroscopically electroneutral. A fundamental different situation arises if, as in the present work, the solvent contains ions (dissociated CPB). In this case the reaction of ion exchange between CPB and the network carrying carboxylate-anions* is possible (2)
I - - C O 0 " N a + + Cj.H ~+N X___~Br- ~
t
Within the gel
Outside the gel
Within the gel
Outside the gel
* For networks with protonated carboxyl groups a side reaction of type (2) is possible with the substitution Na +-* H + . Such a reaction actually occurs for high SA contents in the system although in the conditions of the experiments described here it involves not more than 1-2% of the network units (it is easy to establish this from change in the pH of the medium). Therefore, in the further reasoning the occurrence of this reaction will be disregarded.
908
V . R . RYABINAet al.
If the concentration of CPB in the solvent is far lower than that of the charges in the gel (such a relation is fulfilled for not too high cs) then the equilibrium in reaction (2) will be practically entirely shifted to the right: for entropic reasons it is advantageous for the Na ÷ ions to spread uniformly over the whole volume of the system as a result of which their concentration within the gel falls heavily while the concentration of the SA in the gel increases (Fig. 1). A further reason for the effective absorption of the SA by the gel is that within the gel SA aggregates form. Heavy aggregation of diphilic ions in the gel is due not only to the fact that the SA concentration in the gel increases and may exceed the CCM. It turns out that the actual CCM value in the gel must be considerably lower than in the solvent. In fact, micelle formation in the solvent (Fig. 2a) leads to immobilization of a large number of counter-ions, i.e. to loss by them of translational entropy and, therefore, is less advantageous than aggregation in the gel (Fig. 2b) when the charge of the micelle from the start is neutralized by the immobilized charges of the network chains. Preliminary estimates show that the CCM values in the network containing 10--50% charged units is lower by at least an order than those for CPB in water. In the light of all this the results presented in Fig. 6 on the fracture of the network-SA complex with increase in the ionic strength of the solution become quite understandable. On addition of a low molecular mass salt NaBr the gain in translational entropy through release of Na + counter-ions on entry of CPB cations into the network diminishes as does also the loss in entropy of B r counter-ions on formation of micelles in solution. A consequence of this is the more uniform distribution of CPB ions between network and solution and fracture of the CPB-gel complex. The large decrease in the gel volume on sorption of CPB, as noted, is linked with two factors: decrease in the osmotic pressure of the counter-ions in the gel passing into the external solution and simultaneous accumulation in the gel of CPB cations aggregated as a result of hydrophobic interactions. In refs [8, 9] it is shown that here there may be collapse of the network which, depending on the conditions, may be realized as a discrete (abrupt) or continuous (though occurring in a narrow interval) conformational transition. From Fig. 3 it will be seen that in all the cases considered in this work collapse of the networks on sorption of CPB is of a continuous character. Since for high 3' values the gels become microheterogeneous, it may be assumed that the aggregates formed are not the usual micelles and possibly they are of a lamellar nature. Further structural investigations in this field are of interest in their own right. We would also note that the interaction of the polyelectrolyte networks with oppositely charged SAs permits collapse of the network in a good solvent which may assume practical importance. The presence of a maximum in the curves of Fig. 4 is explained by the factors already mentioned. For low cs (and small 3') there is heavy sorption of CPB and aggregation of the SA cation within the gel leading to reduction in its size with the result that K = CJCg rises. However, as 3' increases the rise in K slows and then gives way to a drop. The reason is that in the region of degrees of filling of the network corresponding to neutralization of the network anions by the SA cations, the entropic gain from the release of Na + counter-ions becomes insignificant and a large fraction of the CPB cations remains in the solvent. The results listed in Fig. 5 show that in study of complexing of a network with SA counter-ions it is necessary to allow for a further factor, namely the presence of elastic stresses which arise in the network on aggregate formation. In fact, such stresses must be more marked when the charges on the network are fixed and cannot migrate. As a result for such systems sorption of the CPB cations must occur relatively less effectively, which was also observed in the experiment. Thus, the experimental results obtained in the present work admit of a simple qualitative explanation. To construct a quantitative theory it is necessary to allow for the fact that the free
Interaction of polyelectrolyte networks
909
energy of a polymer gel is made up of the free energy of the elastic swelling of the gel, the free energy of the volumetric interactions of the units of its components and the gain in free energy on aggregation (decrease in this gain promotes the elastic stresses appearing on aggregation). It is also necessary to allow for the translational entropy of all the SA counter-ions and molecules not brought together into aggregates. As shown above, the last contribution plays a decisive role in the effects considered. As for the free energy of the Coulomb interactions proper of the ions it is easy to show that as in refs [8, 9] this contribution is negligibly small as compared with the others. In conclusion, it should be noted that the range of phenomena studied in the present work is interesting both from fundamental and practical standpoints. Charged gels may be used for clearing water of SAs; the gel complexes with SA micelles are promising as selective adsorbents of organic compounds and, therefore, further study of the above described effects appears highly promising. Translated by A. CROZY
REFERENCES 1. T. TANAKA, D. FILLMORE, S.-T. SUN, I. NISHIO, G. SWISKLOW and A. SHAH, Phys. Rev. Lett. 45:
1636, 1980. 2. S. G. STARODUBTSEV, A. R. KHOKHLOV and V. V. VASILEVSKAYA,Doki. Akad. Nauk SSSR 282: 392, 1985. 3. S. G. STARODUBTSEV, N. R. PAVLOVA, V. V. VASILEVSKAYAand A. R. KHOKHLOV, Vysokomol. soyed. B27: 485, 1985 (not translated in Polymer Sci. U.S.S.R.). 4. Z. Kh. IBRAG1MOVA, V. A. KASAIKIN, A. B. ZEZIN and V. A. KABANOV, Ibid. A28: 1640, 1986 (translated in Polymer Sci. U.S.S.R. A28: 8, 1826, 1986). 5. Z. Kh. IBRAGIMOVA, Dissert. Cand. Chem. Sci. (in Russian) 137 pp., Moscow State Univ., MOSCOW, 1988. 6. A. S. YUSHCHENKO, G. M. KURDYUMOV, T. A. SOLOV'EVA, V. A. KASAIKIN, Z Kh. IBRAGIMOVA, A. B. ZEZIN and V. A. KABANOV, U.S.S.R. Patent 1346586. B.I. No. 39, 6 pp., 1987. 7. M. ILAVSKY, Macromolecules 15: 782, 1982. 8. V. V. VASILEVSKAYAand A. R. KHOKHLOV, Matematicheskiye metody dlya issledovaniya polimerov (Mathematical Methods for Investigating Polymers) p. 45, Pushchino, 1982. 9. Idem, Vyoskomol. soyed. A28: 316, 1986 (translated in Polymer Sci. U.S.S.R. A28: 2,348, 1986).
903
isotropic, narrower than for the corresponding 3d-systems. This correlates with fall in the number of degrees of freedom in the isotropic 2d-system as compared with the 3d. Translated by A . CROZY
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
L. ONSAGER, Ann. N.Y. Acad. Sci. 51: 1949. P. J. FLORY, Proc. Roy. Soc. London A234: 73, 1956. E. DIMARZIO, J. Chem. Phys. 35: 658, 1961. M. WARNER and P. J. FLORY, Ibid. 73: 6327, 1980. W. M A I E R and A. SAUPE, Z. Naturforsch. 14: 882, 1959. P. J. FLORY and G. RONCA, Molec. Cryst. Liquid Cryst. 54: 311, 1979. L. ONSAGER, Phys. Rev. 65: 117, 1944. L . D . LANDAU and Ye. M. LIFSHITS, Statisticheskaya fizika (Statistical Physics) 567 pp., Moscow, 1964. P. DE GENNES, Fizika zhidkikh kristallov (Physics of Liquid Crystals) 400 pp., Moscow, 1977. V. V. RUSAKOV and M. I. SHLIOMIS, Termotropnyi ZhK-perekhod v lineinykh polimerakh. Rol'dliny i zhestkosti makromolekul. (Thermotropic LC Transition in Linear Polymers. Role of Length and Rigidity of the Macromolecules) Pre-print, Sverdlovsk, 1983. 11, L. D. LANDAU, Zh. eksp. teoret, fiz. 7: 627, 1937.
PolymerScienceU.S.S.R. Vol. 32, No. 5, pp. 903--909,1990 Printed in Great Britain.
0032-3950/90 $10.00+ .00 t~ 1991PergamonPressplc
INTERACTION OF POLYELECTROLYTE NETWORKS WITH OPPOSITELY CHARGED MICELLE-FORMING SURFACTANTS* V. R. RYABINA,S. G. STARODUBTSEV and A. R. KHOKHLOV Lomonosov State University, Moscow
(Received 2 February 1989) The conformational properties of polyelectrolyte networks swelling in a solvent containing micelle-forming SAs have been studied. The charge of the SA molecules is opposite to that of the network chains. It is shown that in this case there is heavy sorption of the SA molecules by the gel accompanied by the formation of SA aggregates within the gel and considerable fall in its size. Increase in the ionic strength of the solution leads to fracture of the network-SA complex. These phenomena are due to the fact that firstly on entry of the SA molecules into the gel the low molecular mass counter-ions are released, i.e. heavy gain in translational entropy is reached, and, secondly, aggregation in the gel is more advantageous than micelle formation in the solvent since it leads to immobilization of a smaller number of counter-ions.
RECENTLY there has been growing interest in the study of weakly cross-linked polymer networks synthesized in the presence of a large amount of solvent. Such networks are extremely labile and are capable of significantly changing their size with change in the external factors--composition of the * Vysokomol. soyed. A32: No. 5,969-974, 1990.
V . R . RYABINAet al.
904
solvent, ionic strength, application of mechanical forces--these changes often being realized as sharp abrupt conformational transitions [1-3]. An important new trend in research in polymer sciences is study of the structure and properties of complexes of linear polyelectrolytes with ionogenic SAs [4, 5]. Such complexes are also of considerable practical interest, for example, for producing effective means of purifying industrial effluents [6]. The aim of the present work is to study systems combining the properties of weakly cross-linked polymer gels and linear polyelectrolyte-SA complexes. The work is concerned with various aspects of the interaction of polyelectrolyte networks prepared in the presence of a large amount of solvent with oppositely charged SA molecules capable of forming micelles in aqueous solutions. The test objects were networks based on the copolymers of sodium methacrylate (MAC-) with methacrylic acid (MAC) the charges on which may migrate (Table 1, networks 1-3) and also networks based on TABLE 1.
Network No.
COMPOSITION AND CONDITIONS OF SYNTHESIS OF NETWORKS*
[Acrylamide]
[MAA]
[MAC]
tool. % 1 2 3 4 5 6 7 8 9
---95.5 92.5 89.5 69.5 -99.5
[MAC-]
(I)0
Tp, °C
10.0 30.0 50.0 4.0 7.0 10.0 30.0 10.0 --
0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.154 0.077
20 20 20 20 20 20 20 4 20
mol. % -------89.5 --
89.5 69.5 49.5 -------
*The content of BAA in all networks was 0.5 mol.%.
copolymers of M A C - and acrylamide or methacrylamide (MAA) (Table 1, networks 4-8). In the last case the charged network units are fixed. The crosslinking agent was in all cases N,N'methylene-b/s-acrylamide (BAA). The gels were obtained by radical copolymerization of the monomers and the crosslinking agent in aqueous solutions in the presence of fixed amounts of sodium bicarbonate by the standard technique [7]. The temperature of polymerization Tp and the values of the volumetric fraction of the polymer in the gel after synthesis ~0 are given in Table 1. For synthesis we used siliconized glass ampoules of diameter 3 ram. The samples obtained were cut into cylinders 5 mm in length and washed in twice distilled water for three weeks. They were then placed in a vessel containing the SA [cetylpyridinium bromide (CPB) of Chemapol] and water and thermostatted at 25°C for 20 days. The volume of water in the vessel was 10 times the volume of the gel. Extending the experiment to 40 days did not affect the values of the magnitudes obtained. The relative volume of the swollen networks was characterized by the value of the ratio V/Vo where V is the equilibrium volume of the network and V0 the volume of the network after synthesis. The effectiveness of binding of CPB by the gel was characterized by its distribution in the gel-water system: K = cg/cs where cg and cs are the concentration of CPB cations in the gel and solution. The magnitude K was calculated from the formula
g = ( M - DVs e -~) e/VD,
(1)
Interaction of polyelectrolyte networks
905
where M is the total n u m b e r of CPB moles in the solution; e is the extinction coefficient (CPB has a characteristic absorption band in the U V region; Amax -- 259 nm with e = 4100); Vs is the volume of the solution; D is the optical density of the solution at 259 nm. The presence of SA aggregates in the gel was estimated visually and in solution spectrophotometrically from the solubilization of the dyes (Sudan-i, Sudan-3) insoluble in water. The sample was mechanically strained by compressing it 3-5 times between the planes of a folded plate of fine stainless steel. Figure 1 presents the dependences of the relative volume of the gel V/Vo, the concentration of CPB cations in the gel Cg and the degree of filling of the anionic network with CPB cations 3' (Y is the ratio of the n u m b e r of CPB cations in the network to the total number of network units) on the concentration of CPB in the external solution cs for network 8 (Table 1, dependences for other charged networks are similar). It will be seen that the CPB concentration in the gel is 102-104 times higher than in the external solution; for all the values of c~ studied it is well above the critical concentration for micelle formation (CCM). From this it is natural to assume that over a wide interval of cs values micelles of the type depicted in Fig. 2a, do not appear in solution while CPB cations aggregate within the gel.*
g
i
-5
I
'
10 c~ x 104, mole/I
FIG. 1. Dependences of V/Vo(1), cg (2) and y (3) on cs for network 8 (Table 1). Broken line shows CCM in water. ®
® j
®
G
e~
ee
=
'
® ®
~.¢
~
~
&
¢,
®
(a)
¢ e
Flo. 2. Schematic representation of the formation of micelles in (a) SA solution and (b) in the gel.
*The question of the structure of these aggregates needs further study.
906
V . R . RYABINAet al.
It is easy to satisfy oneself of the soundness of this assumption by placing on the surface of the gel particles of a dye insoluble in water (Sudan-I, Sudan-3). On contact with the gel the dye solubilizes staining the sample, staining being uniform over the whole volume of the gel. At the same time solubilization of the dye by the solution does not occur, i.e. SA aggregates (micelles) do not form in solution. We would note that as the SA ions are bound by the gel the structure of the network-SA complex changes manifest, in particular, in the fact that in the region of large y the gels lose their transparency. It may also be seen from Fig. 1 that interaction with the SA sharply reduces the volume of the gels. It is natural to equate this reduction with the phenomenon of collapse of the polymer network [1-3, 8] which in the given case is due to the fact that transfer of the CPB cations to the gel and the formation there of aggregates is accompanied by release of an equimolar quantity of Na + ions (counter-ions in relation to the charges on the network chains) into the external solution. The osmotic pressure of the "gas" of the counter-ions within the network thereby falls (the SA ions in the aggregates are immobilized) and the network collapses through the volumetric interactions of the cetyl groups. The phenomenon of collapse of the networks on interaction with oppositely charged SAs is of a general character (Fig. 3): for relatively small y values the volume diminishes very strongly and only in the region of high y values does it begin to rise weakly because of intense absorption of the voluminous CPB ions by the networks. Figure 4 presents the dependences of the gel-water distribution constant K on 3' for different IogK log V/Vo
7
L5
<.-.-.\. 2
3
0.5 1
-0,5
3
"I
I 20
I J0 ],,°/o
Fro. 3 FIG. 4 FIG. 3. Dependencesof V/Vo on y for networks 1-3, 6 and 7 (Table 1). FIG. 4. The functions K(y) for networks 1-3, 6 and 7 (Table 1). Arrows denote the points corresponding to the CCM in the ambient solution. types of gels. It will be seen that these dependences pass through a maximum corresponding to the most effective sorption of the SA molecules from solution. The effectiveness of sorption, as a whole~ grows with increase in the density of the charged groups on the network chains (Fig. 4). As Fig. 3 shows the relative change in the volume of the network also increases. At the same time
Interaction of polyelectrolyte networks
907
experiments run on uncharged networks based on acrylamide (Table 1, network 9) showed that they practically do not sorb the SA and do not change their volume in its presence. Figure 5 confirms increase in the effectiveness of sorption with rise in the charging capacity of the network chains. In addition, from these findings it may be concluded that for an identical concentration of the SA in solution, the networks the charges in which may migrate, absorb the SA more effectively than those with fixed charges. However, despite the difference in the structure and degree of chargeability of the gels (including gels containing only 4% charged groups) for all of them the magnitude K remains much larger than unity, this statement also applying to values of cs above the CCM (see Fig. 4, where the CCM values in the external solution are denoted by arrows). Mechanical strain of the gel (its compression 3-5 times) has an insignificant influence on the effectiveness of sorption of the SA by the gel. The gel volume changes by not more than 15% and the K and y magnitudes roughly by the same value. The phenomenon of intense absorption of the SA by the oppositely charged polyelectrolyte gels considered with the formation of SA aggregates in the network and sharp contraction of its volume may occur only for a sufficiently low ionic strength of the solution. Increase in the ionic strength (achieved by adding a low molecular mass salt NaBr) leads to fracture of the network-SA complex (Fig. 6) while the CCM in solution with increase in ionic strength considerably falls. tOO
-7''1°
log VNo ! 0,3
-5
-a
Iogc, [rnolell]
-0,5
Fro. 5
FI~. 6.
~
~
-
Fio. 6
Fro. 5. The functions y(cs) for networks 1-5, 7 and 9 (Table 1). Effect of ionic strength of the solution on the relative volume V I V o of network 7 (Table 1). Degree of filling -/= 0.19 (1), 0.29 (2) and 0.55 (3).
When the polyelectrolyte gel is placed in a large volume of solvent, the low molecular mass ions for entropic reasons (to realize the maximum freedom of translational movement) tend to pass from the gel into the solvent. However, if the solvent does not contain ions such a process is impossible since the gel must remain macroscopically electroneutral. A fundamental different situation arises if, as in the present work, the solvent contains ions (dissociated CPB). In this case the reaction of ion exchange between CPB and the network carrying carboxylate-anions* is possible (2)
I - - C O 0 " N a + + Cj.H ~+N X___~Br- ~
t
Within the gel
Outside the gel
Within the gel
Outside the gel
* For networks with protonated carboxyl groups a side reaction of type (2) is possible with the substitution Na +-* H + . Such a reaction actually occurs for high SA contents in the system although in the conditions of the experiments described here it involves not more than 1-2% of the network units (it is easy to establish this from change in the pH of the medium). Therefore, in the further reasoning the occurrence of this reaction will be disregarded.
908
V . R . RYABINAet al.
If the concentration of CPB in the solvent is far lower than that of the charges in the gel (such a relation is fulfilled for not too high cs) then the equilibrium in reaction (2) will be practically entirely shifted to the right: for entropic reasons it is advantageous for the Na ÷ ions to spread uniformly over the whole volume of the system as a result of which their concentration within the gel falls heavily while the concentration of the SA in the gel increases (Fig. 1). A further reason for the effective absorption of the SA by the gel is that within the gel SA aggregates form. Heavy aggregation of diphilic ions in the gel is due not only to the fact that the SA concentration in the gel increases and may exceed the CCM. It turns out that the actual CCM value in the gel must be considerably lower than in the solvent. In fact, micelle formation in the solvent (Fig. 2a) leads to immobilization of a large number of counter-ions, i.e. to loss by them of translational entropy and, therefore, is less advantageous than aggregation in the gel (Fig. 2b) when the charge of the micelle from the start is neutralized by the immobilized charges of the network chains. Preliminary estimates show that the CCM values in the network containing 10--50% charged units is lower by at least an order than those for CPB in water. In the light of all this the results presented in Fig. 6 on the fracture of the network-SA complex with increase in the ionic strength of the solution become quite understandable. On addition of a low molecular mass salt NaBr the gain in translational entropy through release of Na + counter-ions on entry of CPB cations into the network diminishes as does also the loss in entropy of B r counter-ions on formation of micelles in solution. A consequence of this is the more uniform distribution of CPB ions between network and solution and fracture of the CPB-gel complex. The large decrease in the gel volume on sorption of CPB, as noted, is linked with two factors: decrease in the osmotic pressure of the counter-ions in the gel passing into the external solution and simultaneous accumulation in the gel of CPB cations aggregated as a result of hydrophobic interactions. In refs [8, 9] it is shown that here there may be collapse of the network which, depending on the conditions, may be realized as a discrete (abrupt) or continuous (though occurring in a narrow interval) conformational transition. From Fig. 3 it will be seen that in all the cases considered in this work collapse of the networks on sorption of CPB is of a continuous character. Since for high 3' values the gels become microheterogeneous, it may be assumed that the aggregates formed are not the usual micelles and possibly they are of a lamellar nature. Further structural investigations in this field are of interest in their own right. We would also note that the interaction of the polyelectrolyte networks with oppositely charged SAs permits collapse of the network in a good solvent which may assume practical importance. The presence of a maximum in the curves of Fig. 4 is explained by the factors already mentioned. For low cs (and small 3') there is heavy sorption of CPB and aggregation of the SA cation within the gel leading to reduction in its size with the result that K = CJCg rises. However, as 3' increases the rise in K slows and then gives way to a drop. The reason is that in the region of degrees of filling of the network corresponding to neutralization of the network anions by the SA cations, the entropic gain from the release of Na + counter-ions becomes insignificant and a large fraction of the CPB cations remains in the solvent. The results listed in Fig. 5 show that in study of complexing of a network with SA counter-ions it is necessary to allow for a further factor, namely the presence of elastic stresses which arise in the network on aggregate formation. In fact, such stresses must be more marked when the charges on the network are fixed and cannot migrate. As a result for such systems sorption of the CPB cations must occur relatively less effectively, which was also observed in the experiment. Thus, the experimental results obtained in the present work admit of a simple qualitative explanation. To construct a quantitative theory it is necessary to allow for the fact that the free
Interaction of polyelectrolyte networks
909
energy of a polymer gel is made up of the free energy of the elastic swelling of the gel, the free energy of the volumetric interactions of the units of its components and the gain in free energy on aggregation (decrease in this gain promotes the elastic stresses appearing on aggregation). It is also necessary to allow for the translational entropy of all the SA counter-ions and molecules not brought together into aggregates. As shown above, the last contribution plays a decisive role in the effects considered. As for the free energy of the Coulomb interactions proper of the ions it is easy to show that as in refs [8, 9] this contribution is negligibly small as compared with the others. In conclusion, it should be noted that the range of phenomena studied in the present work is interesting both from fundamental and practical standpoints. Charged gels may be used for clearing water of SAs; the gel complexes with SA micelles are promising as selective adsorbents of organic compounds and, therefore, further study of the above described effects appears highly promising. Translated by A. CROZY
REFERENCES 1. T. TANAKA, D. FILLMORE, S.-T. SUN, I. NISHIO, G. SWISKLOW and A. SHAH, Phys. Rev. Lett. 45:
1636, 1980. 2. S. G. STARODUBTSEV, A. R. KHOKHLOV and V. V. VASILEVSKAYA,Doki. Akad. Nauk SSSR 282: 392, 1985. 3. S. G. STARODUBTSEV, N. R. PAVLOVA, V. V. VASILEVSKAYAand A. R. KHOKHLOV, Vysokomol. soyed. B27: 485, 1985 (not translated in Polymer Sci. U.S.S.R.). 4. Z. Kh. IBRAG1MOVA, V. A. KASAIKIN, A. B. ZEZIN and V. A. KABANOV, Ibid. A28: 1640, 1986 (translated in Polymer Sci. U.S.S.R. A28: 8, 1826, 1986). 5. Z. Kh. IBRAGIMOVA, Dissert. Cand. Chem. Sci. (in Russian) 137 pp., Moscow State Univ., MOSCOW, 1988. 6. A. S. YUSHCHENKO, G. M. KURDYUMOV, T. A. SOLOV'EVA, V. A. KASAIKIN, Z Kh. IBRAGIMOVA, A. B. ZEZIN and V. A. KABANOV, U.S.S.R. Patent 1346586. B.I. No. 39, 6 pp., 1987. 7. M. ILAVSKY, Macromolecules 15: 782, 1982. 8. V. V. VASILEVSKAYAand A. R. KHOKHLOV, Matematicheskiye metody dlya issledovaniya polimerov (Mathematical Methods for Investigating Polymers) p. 45, Pushchino, 1982. 9. Idem, Vyoskomol. soyed. A28: 316, 1986 (translated in Polymer Sci. U.S.S.R. A28: 2,348, 1986).