versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.4 Concepción 2007
J. Chil. Chem. Soc, 52, N° 4 (2007), págs: 1288-1290
EFFECT OF POLY(ACRYLIC ACID) IN THE METAL ION BINDING ABILITY OF POLY(SODIUM 2-(N-ACRYLAMIDO)-2-METHYL-PROPANESULFONATE).
BERNABÉ L. RIVASA)*, EDUARDO PEREIRAA), FERNANDO YÁÑEZA), IGNACIO MORENO-VILLDOSLADAB)
a) Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile.
The binding of Cu2+, Cd2+, Co2+, Pb2+, Zn2+, Ni2+, Cr3+to poly(sodium 2-(N-acrylamido)-2-methyl-propanesulfonate) (PAMPS) in the presence of poly(acrylic acid) (PAA) is studied by diafiltration. It is found an unusual behavior for the polyelectrolyte PAMPS, since it binds irreversibly a fraction of metal ions, while the rest of the ions are eluted form the diafiltration cell without interacting with the polyanion. At pH 3, and in the presence of PAA, the fraction of metal ions bound to the polymer decreases, as the sulfonate molar fraction with respect to the carboxylate groups decreases. A copolymer composed by sodium 2-(N-acrylamido)-2-methyl-propanesulfonate and acrylic acid showed the same binding ability than a mixture of PAMPS and PAA under the same relative sulfonate / carboxylate composition.
Key Words: counterion binding; water-soluble polymers; metal ion interactions.
The ability of polyelectrolytes to bind counterions is generally explained by the counterion condensation theory of G. S. Manning1-3. Manning's theory points out the existence of a layer of counterions bound to the polyion (condensed) that are nonspecifically bound to the polymer chain. They are accumulated around the polyion skeleton within a volume Vp where they are able to move. A large experimental and theoretical literature indicates that this condensed layer of counterions on polyelectrolytes prevents the contourlength charge density of the polyion chain from exceeding a definite critical value. This theory is, then, concerned with the long-range electrostatic interactions between the counterions and the polyion. The central parameter of the theory is the dimensionless charge-density parameter ξ. Experimental and theoretical investigations on several polyions such as poly(4-styrenesulfonic acid) and DNA3 are proved to be explained by the theory.
More complex analyses are required for more complex systems3-10 as: a) systems in which a mixture of univalent and divalent counterions in variable relative concentration are present; b) systems in which specific short range interactions may be found, as hydrogen bond formation, coordination bond formation, aromatic-aromatic interactions, or electrostatic short-range interactions; c) systems in which mixtures of homopolymers or copolymers are present.
Diafiltration has been efficiently used to calculate the binding ability of polyelectrolytes to several counterions5-13. As this is a separation technique, the apparent dissociation constant can be calculated from the rate of filtration of the counterions. To do this, the polymers are previously fractionated and the diafiltration experiments are performed in a batch-like method by contacting the highest molecular weight fractions with a set of metal salts. The initial solution is eluted with water at a definite pH keeping constant the total volume inside the diafiltration cell. Metal ions with high interaction rates with the macromolecules stay retained by the polymer, which is not able to pass through the diafiltration membrane, while other ions are eluted through the membrane. A profile for the retention of the different metal ions by the polymer during filtration can be obtained, and crucial information may be extracted from them. The main magnitudes managed in diafiltration analyses are the filtration factor (F), defined as the ratio between the volume in the filtrate and the constant volume in the diafiltration cell, the concentration in the filtrate of the low molecular-weight species under study (LMWS) (CLMWSfiltrate), the concentration of free LMWS in the cell solution (CLMWSfree), the concentration of LMWS reversibly bound to the water-soluble polymer (WSP) (CLMWSrev-bound), the apparent dissociation constant (KLMWSdiss-WSP), defined as the ratio CLMWSfree/CLMWSrev-bound, the diafiltration parameters ,km j, u, and v, and the polymer concentration in mole per liter of monomeric units (cp). km and y parameters (the absolute value of the slope of the curve In CLMWSfiltrate versus F in the absence and in the presence of the WSP, respectively) are related with the strength of the interaction, while v and u are related with the amounts of LMWS reversibly or irreversibly bound to the polymer, respectively. By irreversibly bound we consider molecules bound in processes that may be reversible with an apparent dissociation constant that tend to zero at the conditions of the experiment. Typical diafiltration profiles are obtained by plotting the logarithm of CLMWSfiltrate versus F. The slope of the profile (parameter/) gives an idea of the rate of filtration of the LMWS, which is related with KLMWSdiss-WSP, following13
where km is the slope obtained in blank experiments (performed in the absence of the WSP), and provided that y ≤ km ≤ 1. From the ordinate at the origin in the profiles, the concentration of LMWS irreversibly bound to the WSP may also be calculated, and thus u and v.
In this paper, we analyze the interaction of Cu2+, Cd2+, Co2+, Pb2+, Zn2+, Ni2+, Cr3+ with mixtures of PAMPS and PAA in different compositions, as well as with a copolymer composed of both comonomers.
Reagents. Commercially 2-(#-acrylamido)-2-methyl-propanesulfonic acid (Aldrich) acrylic acid (Aldrich), and ammonium peroxydisulfate (Fluka) were used to synthesize the homo- and copolymers. Nitrates of Cu2+, Cd2+, Co2+, Pb2+, Zn2+, Ni2+, Cr3* (Aldrich) were used to prepare the solutions. The pH was adjusted with minimum amounts of NaOH (Merck) and HN03 (Merck). D20 (Aldrich, 98 %) was used to prepare the solutions for 1H-NMR analyses.
Polymer synthesis. Synthesis of poly(2-(N-acrylamido)-2-methyl-propanesulfonic acid): To an aqueous solution (100 mL) of 2-(N-acrylamido)-2-methyl-propanesulfonic acid (10.1 g, 0.0487 mol) was added ammonium peroxydisulfate (0.115 g, 5-10-4 mol). The solution was deoxygenated under a N2 stream, and then kept at 60°C for 24 h. The polymer was purified by diafiltration over a diafiltration polyethersulfone membrane with a molecular weight cut off of 100,000 g mol-1 (Amicon). The WSP was isolated by lyophilization of the diafiltration cell solution.
Synthesis of poly(acrylic acid): The same procedure was followed with freshly distilled acrylic acid (10.5 g, 0.15 mol) adding 0.340 g (1.5-10-3 mol) of ammonium peroxydisulfate as initiator (AP).
Synthesis of poly(2-(N-acrylamido)-2-methyl-propanesulfonic acid-co-acrylic acid) 2/1: The same procedure was followed mixing 2-(N-acrylamido)-2-methyl-propanesulfonic acid (3.05 g, 0.015 mol), freshly distilled acrylic acid (1.051 g, 0.015 mol), and ammonium peroxydisulfate (0.0679 g, 3-10-4 mol). The comonomer composition was calculated to be 2/1 by 1H-NMR, comparing the relative intensity of the well resolved signal at 3.2 ppm corresponding to the methylene bound to the sulfonate group.
The structures of the three polymers are shown in Figure 1.
Equipment. The unit used for diafiltration studies consisted of a filtration cell with a magnetic stirrer, a membrane with an exclusion rating of 10,000 or 100,000 g / mol-1 (Amicon), a reservoir, a selector, and a pressure source. Metal ion concentrations were measured by atomic absorption on a Unicam Solaar 5M. The pH was controlled on a Digital InoLab WTW pH meter. 1H-NMR spectra were obtained in a Bruker AC 250 P spectrometer. The samples were lyophilized in a Thermo Savant ModulyoD equipment.
Procedure for diafiltration. Polymeric molecular weight fractions over 100,000 Dalton were dissolved in 10 mL of twice distilled water together with the metal ion salts in order to achieve the final concentrations shown in Table 1. The solutions were placed into the diafiltration cell. The pH of the aqueous solution contained in the reservoir was adjusted to the same value as in the cell solution. In order no macromolecule is filtered, the filtration runs were carried out over a membrane with a molecular weight cut-off of 10,000 Dalton under a total pressure of 3 bar, keeping constant the solution volume in the cell by creating a continuous flux of liquid through the cell solution from the reservoir (around 0.02 mLs-1). Vigorous stirring is held in order to minimize concentration polarization and fouling. Filtration fractions (10 mL) were collected and the metal ion concentrations analyzed by atomic absorption spectroscopy.
RESULTS AND DISCUSSION
Interaction with mixtures PAMPS-PAA. Both PAMPS and PAA have different characteristics. PAMPS is a strong polyelectrolyte and a strong acid. Its sulfonate group does not form coordination bonds with metal ions. On the contrary, PAA is a weak acid which is able to form coordination bonds in its basic form with several metal ions as Cu2+. Both homopolymers in excess (1.0 10-2 M) quantitatively retained all the seven metal ions (whose individual concentration was 2.5 10-4 M) at pH over 4. This situation did not change for mixtures of both homopolymers in different compositions so that the total amount of charges was kept in 1.0 10-2 M. At pH 3, PAA protonates and looses its ability to bind the metal ions. The dialfiltration profiles for all the metal ions are shown in Figure 2, and it can be seen that the j parameter tend to 1 in all the cases, so that Kdiss tend to infinite, following equation (1). On the contrary PAMPS still keeps its binding ability (see Table 1). Upon the analyses of the diafiltration profiles, it can be concluded that a fraction of the metal ions is irreversibly bound to PAMPS, while the rest of the metal ions are eluted showing no significant interaction with the polyelectrolyte. This can be inferred, respectively, from the values of u and y, which take values higher than 0 for the former, and close to 1 for the latter. Typical polyelectrolyte behavior produce equilibrium binding of the counterions, a fact that is reflected in u values close to 0, and j values smaller than 1, from which the apparent dissociation constant can be calculated. So, our results show that the mechanism of metal ion retention may be different from pure long-range electrostatic interactions. As PAMPS is a hydrophobic polymer, the possibility of formation of ion pairs between the sulfonate groups and the metal ions will produce more hydrophobic domains and thus induce polymer folding and trapping of the metal ions.
The tendency found for Cu2+ and shown in Table 1 and Figure 3 is general for all the divalent metal ions. As can be seen in Figure 3, there is a linear dependence on the binding ability with the molar concentration (Table 1) and molar fraction of PAMPS (in monomeric units) (figure 3).
Interaction with a copolymer P(AMPS-AA) 2/1. As stated in the introduction, the binding of counterions to copolymers represent a higher degree of complexity since specific interactions between the comonomer functional groups as well as the physical properties of the macromolecules may influence the copolymer behavior. In principle, the linear charge density in the copolymers may be modulated by the comonomer composition, for example if one of the comonomers is not charged, as in acrylamide-acrylic copolymers. In previous works, we have studied the metal ion binding ability of copolymers such as poly [acrylamide-co-(acrylic acid)] (P(AAm-AA)) and found a decrease on this ability by the presence of AAm moieties7. Although this fact may be attributable to a decrease on the linear charge density in the copolymers, it was found that the ability of the poly(acrylic acid) (PAA) homopolymer also decreased in the presence of poly(acrylamide) (PAAm). Thus, even if the PAA chains had the same linear charge density, mixtures of both homopolymers behave as the corresponding copolymers. Such a behavior could be reasonably explained invoking specific interactions between the amide and the carboxylic groups, as hydrogen bond formation.
The binding ability of the copolymer P(AMPS-AA) 2/1 was evaluated in conditions that the total apparent concentration of sulfonate groups was 1.2-10 -2 M. It was found that the u value found was equivalent to that found for a mixture of PAMPS and PAA showing the same molar fraction of functional groups (sulfonate and carboxylic), as can be seen in Figure 3. Note that for the copolymer, the apparent concentration of sulfonate groups is nearly twice their apparent concentration in the mixture of the homopolymers, although the systems show the same binding ability. Moreover, although the total concentration of sulfonate groups in the respective mixtures of homopolymers decreases, they are still in excess. So, the respective sulfonate / carboxylic ratio seems to be the important variable that determine the binding ability, rather than the absolute sulfonate concentration provided in excess. This may indicate a possible interaction between both homopolymers at pH 3, probably due to hydrophobic forces.
The binding of Cu2+, Cd2+, Co2+, Pb2+, Zn2+, Ni2+, Cr3+ to poly(sodium 2-(Ar-acrylamido)-2-methyl-propanesulfonate) (PAMPS) was studied by diafiltration. At the conditions of the experiments, this polymer showed a binding mode far from equilibrium. Some metal ions were irreversibly bound to the polymer as if they were trapped inside the polymer domain. At pH 3 in the presence of PAA the fraction of metal ions bound to PAMPS decreased, following linearly the decrease on the total concentration of sulfonate groups and their molar fraction with respect to the carboxylate groups. A copolymer composed by sodium 2-(N-acrylamido)-2-methyl-propanesulfonate and acrylic acid showed the same binding ability than a mixture of PAMPS and PAA under the same relative sulfonate / carboxylate composition, although the total concentration of sulfonate groups was nearly twice the concentration in the homopolymer. Thus, the molar fraction of sulfonate groups seems to be the important parameter, rather than the absolute concentration providing that they are in excess, and this may indicate a possible interaction between both homopolymers at pH 3, probably due to hydrophobic forces.
The authors thank FONDECYT (Grants No 1070542, No 1060191 and No 1061018) for financial support.
1. Manning, G.S. Quart. Rev. Biophys. 11, 179, (1978) [ Links ]
2. Manning, G.S. J. Phys. Chem. 88, 6654, (1984) [ Links ]
3. Nordmeier, E. Macromol. Chem. Phys. 196, 1321, (1995) [ Links ]
4. Hao, M.H.; Harvey, S.C. Macromolecules 25, 2200, (1992) [ Links ]
5. Rivas B.L.; Moreno-Villoslada, I. J. Phys. Chem. B 102, 6994, (1998) [ Links ]
6. Rivas B.L.; Moreno-Villoslada, I. J. Phys. Chem. B 102,11024, (1998) [ Links ]
7. Rivas B.L.; Moreno-Villoslada, I. Macromol. Chem. Phys. 199, 1153, (1998) [ Links ]
8. Rivas B.L.; Moreno-Villoslada, I. J Membrane Sci. 187, 271, (2001) [ Links ]
9. Rivas B.L.; Moreno-Villoslada, I. J. Phys. Chem. B 106, 9708, (2002) [ Links ]
10. Moreno-Villoslada, I.; Jofré, M.; Miranda, V.; Chandia, P.; González, R.; Hess, S.; Rivas, B.L.; Elvira, C; San Román, J.; Shibuhe, T.; Nishide, H. Polymer 47, 6496, (2006) [ Links ]
11. Moreno-Villoslada, I.; Miranda, V.; Oyarzun, F.; Hess, S.; Luna, M.; Rivas, B.L. J. Chil. Chem. Soc. 49, 121, (2004) [ Links ]
12. Moreno-Villoslada, I.; Miranda, V.; Gutiérrez, R.; Hess, S.; Muñoz, C; Rivas, B.L. J. Membrane Sci. 244, 205, (2004) [ Links ]
13. Moreno-Villoslada, I.; Miranda, V.; Chandia, P.; Villatoro, J. M.; Bulnes, J. L.; Cortés, M; Hess, S.; Rivas, B. L. J. Membrane Sci. 272, 137, (2006) [ Links ]