Polymer International 46 (1998) 353È356

Competitive Binding of Divalent Cationsto Poly(a-hydroxyacrylic acid)

Toru Tamura, Noriaki Kawabata, Susumu Kawauchi, Mitsuru Satoh*& Jiro Komiyama

Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan

(Received 12 September 1997 ; accepted 30 March 1998)

Abstract : The competitive binding of divalent cations (Mg2`, Ca2`, Mn2`,Co2`, Ni2`, Cu2` and Zn2`) on poly(a-hydroxyacrylic acid) (PHA) andpoly(acrylic acid) (PAA) was investigated by equilibrium dialysis. In the Mg/Camixed system, binding selectivity for Ca2` over Mg2` was signiÐcantly higher inPHA than in PAA; this was attributed to coordination of a-OH groups on PHAto Ca2`. The binding ability and selectivity for the transition metal cations werealmost the same for PHA and PAA at neutral pH, while PHA maintained appre-ciably higher degrees of binding than PAA in acidic solutions (pH about 3). Thiscation binding ability of PHA was ascribed to the lower value in the rele-pKavant pH region. 1998 SCI.(

Polym. Int. 46, 353È356 (1998)

Key words : poly(a-hydroxyacrylic acid) ; competitive binding ; cation-selectivity ;transition metal cations

INTRODUCTION

Poly(a-hydroxyacrylic acid) (PHA) is a unique poly-electrolyte which forms a lactone ring between a car-boxyl group and a neighbouring OH group. We haverecently started systematic studies on PHA to elucidatethe physicochemical properties in aqueous solution andgel systems.1h3 In one of the studies,1 we found thatPHA has signiÐcantly lower values in an acidicpKasolution than poly(acrylic acid) (PAA). The lower pKavalue was ascribed to the induction e†ect by a-OHgroups (indirect e†ect). In another study,2 divalentcounter-ion activity was measured for PHA and PAAsolutions in the presence of Mg2` or Ca2` and it wasfound that the degrees of binding are in the order,PHA/Mg2`\ PAA/Mg2`\ PAA\ Ca2`\ PHA/Ca2`.This was interpreted as being caused by e†ective coordi-nation of a-OH to Ca2` (direct e†ect).

Although there have been many studies on counter-ion binding of polycarboxylates in single divalent

* To whom all correspondence should be addressed.

counter-ion systems, namely, on competitive bindingbetween one species of monovalent counter-ion and onespecies of divalent counter-ion,4,5 few studies are avail-able for competitive binding among several species ofdivalent counter-ions.

In the present study, we have measured the competi-tive binding degrees of divalent metal ions (Mg2`,Ca2`, Mn2`, Co2`, Ni2`, Cu2` and Zn2`) on PHAand PAA in mixed ion systems at di†erent pH values.The direct and indirect e†ects of a-OH groups on thebinding ability of PHA are investigated.

EXPERIMENTAL

PHA (mol. wt. 59 000) was kindly supplied as anaqueous solution of the sodium salt by Nippon Peroxi-de Co. Ltd. PAA (mol. wt. 60 000) as an aqueous solu-tion was purchased from Chemisciences Inc. Thesesolutions were puriÐed by removing low molecularweight substances by ultraÐltration (UP-20, Toyo RoshiCo.), and collected by freeze-drying. All of the reagents

3531998 SCI. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

354 T . T amura et al.

(HCl, NaCl, MgCl2 , CaCl2 , MnCl2 , CoCl2 , NiCl2 ,and were of analytical grade and wereCuCl2 ZnCl2)

used without further puriÐcation.Equilibrium dialysis was performed using a glass dial-

ysis cell and a regenerated cellulose membrane (UnionCarbide Co. Ltd.). It was conÐrmed that the membraneshows no leak for the polymers used here. PHA or PAAsolution (50 ml) was placed in one compartment of thedialysis cell (compartment i) and the concentrations ofpolymer solutions were adjusted to 1É0 ] 10~3 basecpmol l~1. NaCl and the divalent metal salts of more thantwo species were added to both compartments(compartment i and compartment o). The concentra-tions of NaCl and total divalent metal chloridesc1, c2 ,were adjusted to 1É0 ] 10~2mol l~1 and3É0 ] 10~4mol l~1, respectively. Equilibrium experi-ments under acidic conditions were carried out byadding appropriate amounts of HCl and NaCl to keepthe total monovalent salt concentration toc11É0 ] 10~2mol l~1. The cell was immersed in a bathmaintained at 25^ 0É2¡C for 7 days to establish equi-librium. The solution pH in each compartment wasmeasured before and after the dialysis experiment withan F-8 pH meter (Horiba Seisakusho Co. Ltd.). Theconcentrations of the divalent metal ions in each com-partment were determined with an atomic absorptionspectrophotometer (Shimadzu AA-640-12). The Cl~activity in each compartment was estimated with anOrion Ionanalyzer 901 and a Cl~ ion-sensitive electrode(Orion 9417B).

RESULTS AND DISCUSSION

Method of estimation of the degree of binding

According to Nordmeier and Dauwe,6 the degrees ofbinding of the divalent metal ions h, were estimated bythe following equation which was derived on the basisof the Donnan equilibrium.

h \ ic2b/cp\ ic2/cpM1 [ (oa~/ia~)2(oc2/ic2)N (1)

where and are the concentrationsic2b , ic2 , oc2 , ia~ oa~of bound divalent metal ion in compartment i, the con-centration of free divalent metal ion in compartmenti and compartment o, and the activity of Cl~ ion incompartment i and compartment o, respectively. Itshould be noted here that eqn (1) has been derivedunder an assumption that the activity of free divalentcounterions in compartment i is free from electrostaticinteraction with the polymer. This assumption is justi-Ðed if the ionic strength of compartment i is muchhigher than the concentration of the polymer. In fact,this condition seems to be satisÐed in the present studybecause c1 ? cp [ c2 .

Binding selectivity for Ca2½ and Mg2½

Degrees of binding estimated by eqn (1) for each ion inthe mixed systems are shown in Table 1. The degrees ofbinding for Mg2` and Ca2` were obtained as PHA/Mg \ PAA/Mg\ PAA/Ca\ PHA/Ca. This order isthe same as that obtained by the counterion activitymeasurement in the single divalent counter-ion system.2Thus, the present study also conÐrmed the higher Ca2`selectivity for PHA in the mixed counter-ion systems. Ina previous study,2 the selectivity of Ca/Mg wasexplained by considering the binding process as thefollowing three steps : (1) scission of the hydrogenbond between COO~ and OH, which was suggested byab initio calculation studies ; (2) binding of Mg2` orCa2` to COO~ ; (3) coordination of OH to the centralcation. The second step is common to both polymers ;

TABLE 1. Degrees of binding of divalent metal

cations to PHA and PAA

Polymer Mixed system Ion h

PHA Mg/Ca Mg 0·080

Ca 0·190

Co/Ni/Cu Co 0·095

Ni 0·101

Cu 0·190

Mn/Cu/Zn Mn 0·081

Cu 0·213

Zn 0·127

Mn/Ca Mn 0.130

Ca 0·202

Co/Ca Co 0·197

Ca 0·182

Ni/Ca Ni 0·204

Ca 0·174

Cu/Ca Cu 0·300

Ca 0·097

Zn/Ca Zn 0·202

Ca 0·171

Co/Ni/Cu Co 0·005

(pH 3·33) Ni 0·024

Cu 0·055

Co/Ni/Cu Co 0·006

(pH 2·05) Ni 0·006

Cu 0·015

PAA Mg/Ca Mg 0·130

Ca 0·180

Co/Ni/Cu Co 0·099

Ni 0·109

Cu 0·215

Co/Ni/Cu Co 0·003

(pH 3·37) Ni 0·004

Cu 0·014

Co/Ni/Cu Co 0·002

(pH 2·30) Ni 0.002

Cu 0·002

POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998

Competitive binding of divalent cations to PHA 355

the Ðrst and third steps add a negative and a positivecontribution to the total binding strength for PHA.Thus, the strongest binding of Ca2` ion to PHA maybe ascribed to a larger contribution by the third stepthan that by the Ðrst, and the opposite should be thecase for Mg/PHA system. The reason why the OHgroups do not contribute so much to the stabilization ofCOO~/Mg as to that of COO~/Ca, may be the smallsize and the higher dehydration energy of the formercation. Namely, an optimum form of the carboxylanions and OH groups coordinated to Mg2` ion wouldonly be realized at a greater cost of dehydration energyand restriction of polymer chain compared with thecase of Ca2` ion (Fig. 1). In fact, it is known that, unlikeCa2` ion, Mg2` ion forms a 1 : 1 complex with aCOO~ group rather than a 1 : 2 one in aqueous solu-tions of PAA.7 Although this interpretation should bevalid for the results of both the present and a previousstudy, the coexistence of Ca2` ion may a†ect the Mg2`ion binding. This is because the entropy of the chainmobility may be reduced through 1 : 2 binding of Ca2`ion with carboxyl anions which are far apart on thepolymer chain, and the entropic disadvantage for Mg2`ion binding would thus be largely lightened. In order tosee if this is the case, we estimated the binding strengthusing a counterion condensation theory (IntermediateModel, IMM)7,8 for the present Mg/Ca mixed systems(Table 2). For the calculation in the present mixed ionsystems, the same extra interaction energy for*EexCa2` ion was used as estimated in the single ion

Fig. 1. Schematic illustration of counter-ion binding forPHA/Ca2`, Mg2` (broken line indicates hydrogen bonding).

TABLE 2. Extra interaction energies (kcal molÉ1)

and degrees of binding h for PHA and PAA

Polymer Parameter Mg Ca

PHA h 0.08 0·19

DEex

0·84a–1·9b 3·1a

PAA h 0·13 0·18

DEex

1·5a–2·0b 2·8a

a Estimated from activity data with IMM in the single diva-

lent counterion systems.

b Estimated from dialysis data with IMM in the present

mixed counterion systems.

system2 ; a value of for Mg2` ion was obtained*Eexthat reproduced the experimental h values of bothcations. As expected, the extra interaction energies ofMg2` ion thus estimated are appreciably higher thanthose in the single ion systems. The values esti-*Eexmated suggest that in the mixed system, the ion selec-tivity remains unchanged while the relative selectivity isreduced. In the following, the e†ect of a-OH on thebinding selectivity for transition metal cations wasinvestigated.

Binding selectivity of transition metal ions for PHAand PAA

For the PHA//Co/Ni/Cu system, the h value of Cu2`ion was twice as high as those of Co2` and Ni2` ions inthe neutral pH region (pH B 6É0). For the PHA//Mn/Cu/Zn system, the h value of Cu2` ion was three timesand twice as high as those of Mn2` and Zn2`ion, respectively. With these results, the selectivity byPHA follows the order Cu2`? Zn2` [ Ni2`BCo2`[ Mn2`. The competitive binding of the tran-sition metal ions and Ca2` ion was also investigated.Table 1 shows that, except for Mn2`, the degrees ofbinding of the transition metal ions are higher thanthose of Ca2`. For Cu/Ca, almost all the Cu2` ionswere bound to polymer chain ; h (Cu2`) B 0É30 and theselectivity was about 3É0. The overall selectivityobtained by using Ca2` as a reference is almost consis-tent with the above selectivity : Cu2`? Zn2`BNi2`B Co2`[ Mn2`. These selectivities are, in fact,almost the same as those for citric acid and lacticacid, which may be considered as monomer ana-logues of PHA, i.e. Cu2`? Ni2`B Zn2`[ Co2`[Mn2` and Cu2`[ Ni2`[ Co2`[ Zn2`? Mn2`, re-spectively.9h13 Because PAA also shows a similar select-ivity, the above sequences for PHA seem to be commonto the COO~ group acting as a ligand to those di-valent cations. Thus a-OH does not seem to appreciablycoordinate any of the above cations. These resultssuggest that the hardness of the OH group on PHA isnot sufficient for it to be an e†ective ligand to thetransition metal cations, although it plays an essential

POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998

356 T . T amura et al.

Fig. 2. Degrees of binding of Cu2`, Ni2` and Co2` to PHAand PAA at various pH values.

role in discriminating between Mg2` and Ca2`.

Effect of the solution pH

Selective binding was also investigated under lower pHconditions. Figure 2 shows the results for the Co/Ni/Cusystem on PHA and PAA. Appreciably higher h valueswere observed for PHA at pHB 3 compared with PAA,the total degree of binding being four times larger thanthat for PAA (8% and 2%, respectively). This bindingability maintained in a low pH region may be ascribedto the signiÐcantly low of PHA in acidic solutions.pKaAccording to our previous study,1 the of PHA atpKaa \ 0É1 is about 2É9 while that of PAA is about 5É0.This suggests that COO~ groups are stabilized bya-OH groups through induction e†ects, as is known tooccur with small hydroxycarboxylic acids. For example,the monomer analogue of PHA lactic acid has a inpKawater at 25¡C which is 1É0 lower14 than that of propi-

onic acid. Thus, because of the indirect e†ect of the OHgroup, PHA seems to maintain the binding ability andselectivity for transition metal cations in weakly acidicsolutions.

In summary, the present paper reports that the directand indirect e†ects of a-OH explain the divalentcounterion binding of PHA. These Ðndings will be uti-lized in subsequent studies of the swelling behaviour ofPHA gels that respond to various cations.

ACKNOWLEDGEMENT

We thank the Nippon Peroxide Co. Ltd. for the gener-ous supply of PHANa samples which made this workpossible.

REFERENCES

1 Tamura, T., Kawauchi, S., Satoh, M. & Komiyama, J., Polymer, 38(1997) 2093.

2 Ogawara, K., Kawazoe, S., Tamura, T., Kawauchi, S., Satoh, M. &Komiyama, J., Polymer, 39 (1998) 437.

3 Tamura, T. T., Kawauchi, S., Satoh, M. & Komiyama, J., Polymer,38 (1997) 2093.

4 Towlson, S. M. & Wright, P. V., Polymer, 27 (1986) 409.5 Qian, C., Asdjodi, M. R., Spencer, H. G. & Savitsky, G. B., Macro-

molecules, 22 (1989) 995.6 Nordmeier, E. & Dauwe, W., Polym. J., 23 (1991) 1297.7 Satoh, M., Kawashima, T. & Komiyama, J., Polymer, 32 (1991)

892.8 Satoh, M., Kawashima, T. & Komiyama, J., Biophys. Chem., 31

(1988) 209.9 Li, N. C., Lindenbaum, A. & White, J. M., J. Inorg. Nucl. Chem.,

12 (1959) 122.10 Lefebvre, J., J. Chim. Phys., 54 (1957) 601.11 Matsushima, Y., Chem. Pharm. Bull., 11 (1963) 566.12 Evans, W. P. & Monk, C. B., J. Chem. Soc., (1954) 550.13 Davies, P. B. & Monk, C. B., T rans. Faraday Soc., 50 (1954) 132.14 Martell, A. E. & Smith, R. M., Critical Stability Constants, Vol. 1,

Plenum, New York, 1974.

POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998