Complexation of crosslinked polyacrylamide-supported aminoligands with Cu(II): Effect of crosslinking on complexation kinetics

Indian Journal of Chemistry

Vol. 31A, September 1992, pp. 673-677

Complexation of crosslinked polyacrylamide-supported aminoligands with Cu{II): Effect of crosslinking on complexation kinetics

Beena Mathew, P M Madhusudanan &V N Rajasekharan Pillai*

School ofChemical Sciences, Mahatma Gandhi University, Kottayarn, Kerala 6R6 631 Received 1 October 1991; revised and accepted 20 January 1992

Kinetics of CU(I1)complexation of amino ligands supported on polyacrylamides with crosslinking agents which differ in their polarity and flexibility have been studied. The complexation is first order with respect to the metal ion. The rate of Cu(II) intake increases with the hydrophilic and flexible nature of the crosslinks. The Langmuir and Frumkin equations are used to evaluate the adsorption parameters of complexation which suggest that the phenomenon of adsorption depends on the na- ture of the crosslinking agent in the polymer matrix. The interaction between the complexed metal ions is more in the polymer support with rigid crosslinking.

Interaction of metal ions with ligand functions supported on crosslinked polymer matrices is of considerable significance in a number of areas like catalysis^I, metal ion separation/ and in bio- logy'. Generally, this interaction is a heterogene- ous complex formation between an insoluble crosslinked macromolecular ligand and a metal ion in aqueous medium. The structural characteri- stics of the polymer matrix which affect the reac- tivity of the attached functional groups decide the nature and extent of the interaction of the metal ion with the Iigand":". The dependence of the reactivity of the attached functional groups on the nature of the polymer matrix like the polarity of the support, molecular character and extent of crosslinking and the overall topographical nature has been reported earlier from this laboratory ^{7 - 9.}

It was thought of interest to extend this approach to the interaction of metal ions with polymeric li- gands. Thus we report here the results of the kin- etic investigation on the complexation of Cu(II) ions with amino ligands incorporated in polyacryl- amide matrices crosslinked with divinylbenzene (DVB), N,N'-methylene-bis-acrylamide (NNMBA) and tetraethyleneglycol diacrylate (TEGDA). For the adsorption studies, Langmuir and Frumkin equations were used. In Langmuir adsorption, as- sumptions are made that the adsorption sites are equivalent and the ability of the substrate to bind at these sites are independent of whether or not nearby sites are occupied, whereas in Frumkin equation, adsorption depends on the binding as well as the nearby active sites.

Materials and Methods

Polyacrylamides crosslinked with DVB, NNM- BA and TEGDA (8 mole%) were prepared and functionalized with ethylenediamine following the procedure reported earlier ^10. The ligand capacities of these resins were determined by acid-titration.

The resins with 0.5477 (DVB), 0.8417 (NNMBA) and 1.1075 (TEGDA) mmol NH/g were used in these studies. Copper sulphate used was the pu- rest available sample (Merck).

General experimental procedure

For the kinetic studies of DVB-crosslinked ami- no resin, different sets of 8% DVB-crosslinked amino resin (100 mg each) with NH/g (0.5477 mmol) was mixed with CU(Il) salt solution (6.5 x 10 -³ N, 100 mI) at two different tempera- tures in a thermostatically controlled system for different time intervals. The remaining [Cu(II) ions} in each system was estimated iodometrically.

Similar studies were done for the 8% NNMBA- and TEGDA-crosslinked amino resins.

For the adsorption studies, different sets con- taining NHz functionalized resin ( 100 mg) in Cu(II) solution of varying concentrations were used at two different temperatures for the DVB-, NNMBA- and TEGDA-crosslinked systems. The Langmuir and Frumkin equations were used for following the adsorption.

Results and Discussion

Crosslinked. polyacrylamides with DVB-, NNM- BA- and TEGDA-crosslinks are represented in

674 INDIAN J CHEM, SEe. A, SEPTEMBER 1992

Scheme 1. The respective crosslinking agents dif- fer in their relative rigidity and polarity. Cross- links given by DVB are rigid and hydrophobic while those given by TEGDA are hydrophilic and flexible due to the presence of the polar ethylene oxide units between the diacrylate functions.

NNMBA-crosslinking has intermediate rigidity and polarity. The interaction of the polymer-sup- ported ligand with the metal ions in aqueous me- dium is governed by the extent of availability of the ligands for the metal ions. This depends on the nature of the polymer backbone and molecu- lar character and extent of crosslinking in the po- lymer matrix. (Scheme 1).

Rigid / hydrophobic DVB-Crosslinked IH2

~CH -CH'CH 'CH-CH -CH-

2 2 ,) 2!

~O NH)

':lH

~H) _

NH

NH) CO ^NH)

I , I

-CH)- CH-CH2'CH' CHi CH- Semi rigid Ihydrophilic NNMBA - Crosslinked

~H2

~H2

o

NH2 '('0

! ,

-CH2-CH-CH2' CH'CHfrH-

NH2

Scheme I - Aminopolyacrylamides with DYB-. NNMBA- and TEG DA-crosslinks

Flexible Ihydrophilic TEGDA - Crosslinked

The results of the kinetic studies of the DVB-, NNMBA- and TEGDA-crosslinked systems are given in Fig. 1. In order to evaluate energy of ac- tivation (E) and entropy of activation (~S") of complexation, a parallel run was carried out at higher temperature. The kinetic parameters for complexation was calculated from the Arrhenius equation.

For the DVB-crosslinked system the rate con- stant at 318 K is nearly double as that at 306 K.

The activation energy for complexation is 82.410 kJ/mol, Arrhenius parameter is 1.145 x 10⁹ s -^I and the entropy of activation is -71.628 J. These are comparable with those obtained for solution kinetics.

The nature of the complexation of NNMBA- and TEGDA-crosslinked amino resins was found to be different from that of the DVB-crosslinked system. The complexations of the NNMBA- and TEGDA-crosslinked systems occur faster than the DVB-crosslinked system. This appears to be due to the increased availability of the ligands by the diffusion of the solvent molecules with the metal ion into the polymer matrix. This increases with the increase in flexibility and hydrophilic nature of the crosslinking agent. Therefore the kinetics of the complexation of TEGDA-crosslinked resin could not be followed accurately at higher tem- peratures and they were carried out at 290 K and 303 K only. For the NNMBA-crosslinked system, the kinetic studies were carried out at tempera- tures 303 K and 316 K. The specific rate constant at higher temperature was found to be less in the case of NNMBA- and TEGDA-crosslinked resins.

This can be ascribed to the exotherrnicity of the complexation process and the dissociation of the complex leading to an overall decrease in the rate.

The diffusion of the solvent molecules into the polymer matrix increases with temperature". In

OVB 1.47 TEGOA

1.51 1.46~-'-_-'---'-_....a....---,_....I

o 10 20 30 40 50 0 4 8 12 16 20 0 4 8 12 16 20

, , t

Fig. I - Kinetic curves for the Cu(I1) complexation of DY8-, NNMBA and TEGDAcrosslinked aminopolyacrylamides

1.53 1.57

1.52 1.56

1. 51 -; 1.55

!J ^1.54

,

~

..

~ 1.50

8'

""i 1.'9

1.53

1.48 1.52

x T,3OIIK.K,:6.6.-os..!l"

• T.l18K,",:3.IOS,6'

1.47 1.52 1.51 -;- 1·50,

~J \.49

,

MAlHEW et al: EFFECf OF CROSSUNKING ON COMPLEXATION KINETICS

TEGDA-crosslinked polyacrylamide-support, the hydrophilicity and flexibility of the crosslinking agent enhance the diffusion making the complexa- tion process reversible with increase in tempera- ture.

Diffusion, adsorption and interaction of metal ions with crosslinked polyacrylamide-supported amines

In the complexation of crosslinked polymer- supported ligands with metal ions which is a he- terogeneous reaction, the following fundamental processes have to be considered: (i) diffusion of metal ions into the polymer matrix; (ii) adsorption on the surface and (ill) the reaction which mayor may not be followed by desorption. In the case of adsorption, there are two possible approaches: (i) adsorption without any interaction with neigh- bours - Langmuir-type and (ii) adsorption invok- ing interaction between the neighbours - Frumkin- type^12,I3.

(i) Langmuir type: The adsorption (complexation)

of 8% DVB-, NNMBA- and TEGDA-crosslinked polyacrylamide-supported amines with Cu(II) ions was carried out at different temperatures with definite amount of the resin and varying concentr-

675

ations of Cu(II) salt solution till equilibrium is reached. The Langmuir equation can be taken in the form

... (1) where C^f and C^m are the concentrations of un- complexed and complexed Cu(II) ions, K is the stability constant and A,; is the surface area covered by the metal ions. The plot of Cfl C^m against C^fis linear (Fig. 2). The stability constant (K), surface area covered (A,;) and the free energy of activation (~G") for the various systems at different temperatures are given in Table 1. For the DVB-crosslinked resin, the specific rate con- stant decreased with temperature whereas the sur- face area covered and the free energy of activa- tion at the two different temperatures are almost same. This can arise from the rigid nature of the polymer-support. The activation energy for com- plexation, Arrhenius parameter and the entropy of activation are given in Table 1.

Parallel studies were carried out for the 8%

NNMBA- and TEGDA-crosslinked poly(N-2- aminoethylacrylamide )s. The curves obtained by

DVB NNMBA TEGDA

5 ⁶ 5

X T : J06K X T: 303K

•

^{T: 318 K • T:290K}

4 5

.9.. 3 ~ ⁴ ~ ³

em

:j/:

^{T:T : 316 K303K em 3 em}

1L-~ __ ~ __ ~~~

o .2 .4 .6 .8 0 .2 .4 .6 .8 0 .2 .4 .6 .8

c,(mmol) e,(m ",01) e, (m mol)

Fig. 2- Langmuir plots for the Cu(II) complexation of DYB-, NNMBA- and TEGDA-crosslinked aminopolyacrylamides

Crosslinking (8%)

Table 1-Adsorption and kinetic parameters for the Cu(II) complexation of 8%DYB-, NNMBA- and TEGDA-crosslinked polyac- rylamide-supported amines

Kinetic Parameters Temp.

K

Adsorption Parameters

E A IlS" K A, IlC'"

kl/rnol ^(s-I) J mol-^I k-^I klima! kllmo!

DYB 31.9 8.9 x lO~ +8.3 306 32.1 0.1301 - 26.4

318 20.1 0.1286 -26.2

NNMBA 18.6 1.1xlO5 +8.3 303 66.5 0.1871 - 92.3

316 1.6 0.3032 - 61~1

TEGDA 88.8 2.2 x lO20 - 225.5 290 22.5 0.1740 - 36.2

303 lO9.3 0.1960 -96.5

676

^{INDIAN JCHEM, SEe. A.SEYfEMBER 1992}

plotting

GI c.n

versus C^mfor the different temper- atures are given in Fig. 2. The corresponding sta- bility constants, surface area covered and the free energy of activation at different temperatures are given in Table 1. The specific rate constant dec- reases with increase in temperature but the free energy of activation becomes more positive. Larg-

er the surface area covered, higher will be the re-

pulsive force. This leads to an increase in the number of collisions for complexation to occur. A more positive value for the free energy of activa- tion suggests a decrease in the spontaneity of the complexation. The kinetic parameters for the complexation are given in Table 1. The activation energy for complexation in the case of lEGDA- crosslinked system is less than that of the DVB- crosslinked system because of the former's low ri- gidity and hydrophobicity. The surface area covered by the metal ions at room temperature is 16% and at higher temperature it is 25%. This may be due to the increased availability of the reactive sites by the increased solvation at higher temperature.

In the case of the 8% lEGDA-crosslinked polyacrylamide-supported amino resin, the com- plexation was very fast at higher temperature.

Hence, the adsorption studies were carried out at

290 and 303 K. The plots of the

Glc;.

against

G

at 290 and 303 K are given in Fig. 2. The specific rate constant, surface area covered and the free energy of activation at the two temperatures are given in Table 1. The high energy of activation, specific rate constant and the more negative value of free energy of activation support the view that

increased complexation can occur at higher tem-

perature. With increase in temperature, the latent reactive sites in the possibly coiled crosslinks be- come more exposed thereby enhancing the com- plexation. The entropy of activation at higher temperature delineates the disordering of the.or- dered structure of the polymer' matrix by the in- creased diffusion of the solvents into the cross- links, making them uncoiled and resulting in in- creased complexation. The surface area covered by Cu(II) ions at 290 K is 17.4% and at room temperature it is 19.6%.

(ii) Frumkin type: In order to investigate the in- teraction effect in complexation, Frumkin equa- tion (Eq. 2) was chosen. Here

G

and C^mare the concentrations of uncomplexed and complexed Cu(II) ions and K,and ^K'lare constants.

. .. (2)

ovs ^{2 NNMBA I TEGOA}

J4

I

~is z

<,is

~o~~~--~~~~~~

c".

tl -c-2

...,_{•••....I} J_,

.••••.0J t-~---::"~~--:...-~

J .1

<,

J.-1

~ 4

I

'-'J

~ ~r-~~~~~--~~~~

.!.a

oS

-4

~~---~

Fig. 3 - Frumkin plots for the Cu(lI) complexation of DVB-, NNMBA- and TEGDA-crosslinked aminopolyacrylamides

Crosslinking (8%)

Table 2 - Adsorption/interaction parameters and kinetic parameters of the Cu(II) complexation of 8% DVB-, NNMBA- and TEG- DA-crosslinked polyacrylamide-supported amines

Kinetic Parameters Temp. Adsorptionllnteraction Parameters K

E A I1S" K, K2 I1C'"

kJ/mol (s-II Imol-I k" mol"' kJ/mol kJ/mol

DVB 6.25 1.3x 102 -204.7 306 19.05 11.08 - 25.07/- 23.69

318 9.00 12.16 - 24.90/ - 24.10

NNMBA 13.1 1.7x10·' - 183.1 303 2.94 9.62 - 20.10/ - 23.10

316 3.11 11.92 - 21.10/ - 24.70

TEGDA 2.3 2.4x 101 -218.63 290 2.30 9.70 - 18.60/ - 22.10

303 3.20 10.10 - 20.30/ - 23.20

MATHEW et al.:EFFECT OF CROSSUNKING ON COMPLEXATION KINETICS

This can be transformed into Eq. (3)

In[ _Cr(C

_c

^m_{r- Cm)}

1

^{= InKJ +K2 C}

_c

^{m ... (3)}

\

For the complexations of DVB-, NNMBA- and TEG DA-crosslinked polyacrylamide-supported amines with Cu(U) ions, the experimental data ob- tained for the Langmuir type was used. The plots of In[Cm/QCr- Cm)] versus Cm/Cr are given in Fig. 3. The interaction constant (K2) and adsorp- tion constant (K1)' free energy of activation at dif- ferent temperatures, the energy of activation for interactions, Arrhenius parameter and the entropy of interaction are given in Table 2.

The free energy of activation for interaction is of the same order of magnitude for all systems.

This is exemplified by the very low value of acti- vation energy for interaction. Again it was found that the free energy of interaction is higher than the free energy of adsorption in the DVB-cross- linked system. This may be due to the possibility of increased interaction by the active sites in the DVB-crosslinked polymer matrix. The rigidity and hydrophobicity of the crosslinks make the ac- tive sites mainly on the surface resulting in in- creased interaction. For NNMBA- and TEGDA- crosslinked systems, the free energy of interaction is lower than the free energy of adsorption. This may arise from the flexibility of the crosslinking agents which results in extensive swelling in aque-

677

ous medium. But in the case of NNMBA-cross- linked system, the semi-rigidity of the polymer matrix needs higher activation energy for interac- tion. The entropies of interactions are negative and are of the same order of magnitude. Thus, the free energy of interaction depends on the na- ture of the polymer-support.

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