Kinetics and mechanism of sol-gel transformation on polyelectrolytes of nickel alginate ionotropic membranes

Fur. Polym. J. Vol. 24, No. 3, pp. 281-283, 1988 Printed in Great Britain. All rights reserved

0014-3057/88 $3.00 + 0.00 Copyright © 1988 Pergamon Journals Ltd

KINETICS A N D MECHANISM OF SOL-GEL TRANSFORMATION ON POLYELECTROLYTES OF NICKEL ALGINATE IONOTROPIC MEMBRANES R. M. HASSAN, M. H. WAHDAN and AHMED HASSAN Department of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt

(Received 24 November 1986; /n revised form 17 June 1987) Abstract--The kinetics and mechanism of the gelation reaction on capillary ionotropic membranes of nickel alginate complex have been studied complexometrically. It was found that the exchange between Na ÷ ions of the macromolecular chains of alginate sol and N? + ions of the electrolyte proceed according to a sigmoidal curve with a distinct rapid period initially followed by a decrease in the rate of exchange at longer times. Second-order overall kinetics have been observed. The rate of gelation conforms to d[Gel]/dt = Rj[Alg-][Ni2+] where Rj represents the second order rate of exchange of either the fast or slow exchange in such a gelation reaction. The kinetic parameters were evaluated and a tentative mechanism is discussed.

INTRODUCTION

Alginic acid is a glycuronoglycan of presumably f l ( l - 4 ) linked D-mannuronic acid units [1,2], and some [3] t~-guluronic acid units [4, 5]. Sodium alginate was selected from m a n y polyelectrolytes due to its great importance and wide application such as in medicine, food processing, agriculture and industry. However, no attention has been paid to the kinetics of sol-gel transformation between such an alginate polyelectrolyte and multicharged metal ions. The kinetics of such a gelation process have been considered recently and we have developed a mathematical treatment [8] for a falling drop of sodium alginate sol in a divalent metal ion electrolyte [6]. Therefore, the present investigation was undertaken with a view to obtaining an insight into the detailed reaction mechanism on capillary ionotropic membranes of the nickel alginate complex.

had attained the temperature of the water-bath, fixed known volumes of the alginate sol were syringed out and poured inside the petri dishes up to two-thirds of their heights. The petri dishes were then immersed separately in vessels containing equal volumes of nickel sulphate solutions of known concentrations. The time of contact of alginate sol with the metal electrolyte was recorded for each petri dish. After known time intervals, the formed nickel alginate membranes were carefully removed and washed several times quickly with deionized water until the resultant water became free from Ni 2+ ions. The chelated Ni 2÷ ions in those membranes were exchanged by using dilute acids, collected and determined complexometrically [7]. The variation in the concentration of the chelated Ni 2+ ions as a function of time was recorded. Such conditions were sufficient to allow a kinetic analysis of the results. However, no method could be devised which was specific for alginate sol.

EXPERIMENTAL

All materials were of Analar (BDH) grade. Doubledistilled water was used in all preparations. The temperature was controlled within +0.1°C. Sodium alginate sols of various concentrations were prepared by dissolving the solid material in double-distilled water. This process was performed by stepwise addition of the powder alginate whilst rapidly stirring the water to avoid formation of a lumpy solution which swells with difficulty. Petri dishes of 5 cm diameter and 2 cm in height were smeared with a very thin layer of alginate sol and dried in an electric oven at 120°C for about 20rain. KINETIC MEASUREMENTS

Preliminary experiments indicated that the rate of gelation is altered by changing the metal ion concentration. F r o m this point of view, the kinetics were studied under pseudo first-order conditions where [NF÷ ]0 >> [Alg- ]0. The alginate sol and the nickel ions electrolyte were equilibrated at the desired temperature in a thermostated water-bath. After the reactants

RESULTS

Stoichiometry Ion exchange is inherently a stoichiometric process [8]. Any counter ions which leave the macromolecule chains of the polyelectrolyte are replaced by an equivalent a m o u n t of other metal ions. The stoichiometry of the overall gelation reaction of alginate sol with excess Ni 2+ ions was determined complexometrically. The mixture was kept at room temperature for about 48 hr. The unreacted [Ni 2+] was estimated periodically until it had attained a constant value, i.e. completion of gelation. A stoichiometric ratio of 0.51 + 0.02 ([Ni 2+ ]. . . . . . . d / [ A l g - ] 0 ) was obtained for several different initial concentrations of N ? + ions. The [Ni 2+ ] and [Alg - ] dependencies The order with respect to reactants was determined by working under pseudo first-order conditions 281

R.M. HASSANet al.

282

39

Table I. Rates of exchange for Ni2÷-alginate gelation reaction. [Alg- ] = 0.05 M and Temp = 25°C [Ni2+] (M)

102 R/ (min -I)

103 R, (min -I)

102 R t (M l min-I)

103 R2 (M -I rain -L)

0.37 0.47 0.58 0.69 0.79

0.94 1.13 1.36 1.87 1.98

2.34 3.06 3.72 4.39 4.96

2.55 2.40 2.34 2.71 2.50

6.34 6.51 6.41 6.36 6.28

2.50

6.38

Average

where Ni 2+ is present in large excess over alginate sol. The concentration of Ni 2÷ ions was varied from 0.37 to 0.79 M, whereas [Alg-]0 suitable for the kinetic measurements was 0.05 M. Plots of -in(Ct - Co) vs time were found to curve significantly initially but became linear at longer times. These curves indicate that the behaviour of the gelation reaction obeys the expression (C t - Coo)= Boe-R:' + Poe-n~'. Here, R/ and R, are the first order rates of exchange for the fast and slow gelation steps, while B0 and P0 represent the concentration change for the fast and slow reacting species, respectively. The rates of the exchange shown in Table 1 were obtained by drawing a straight line through the fast time linear portion (R/) of the first order plot and extrapolating to zero time (B0). The rate of the exchange, Rs, for the slower gelation reaction was obtained from plots of the form -ln[(Ct-Co~)-(C~-Co~)] vs time. The quantity ( C t - C ~ ) represents the experimental point and (C~ - Coo) represents the extrapolated point at time t [9]. The pseudo first order plots are shown in Fig. 1.

//

/

R2

.~ t~e

38

.37 36

I 3.2

I 3.3

I

K/T

Fig. 2. Eyring plot of the gelation reaction between Ni2+ and alginate sol.

Temperature dependency The gelation reaction was studied at temperatures from 25 to 40°C. As shown in Fig. 2, the plot of - I n h/kTRj against lIT of the Eyring equation [10] gave good straight lines. The enthalpy and entropy of activation were calculated from the slopes and the intercepts of the lines. The kinetic parameters were calculated using the least-squares method and are summarized in Ta]91e 2. DISCUSSION

The experimental results indicate that the gelation process can be expressed stoichiometrically by the following exchange reaction 2 Na-Alg + NiSO4 = Ni-Alg2 + Na:SO4

./

sol

electrolyte

gel

electrolyte

The empirical rate law of such gelation is d[Gel] dt = Rj[Ni2+][Alg-] Here, Rj is the second order rate of exchange and represents either R 1 and RE, obtained by dividing the appropriate first order rates of exchange (R: or Rs, respectively) by [Ni2+]o.

Mechanism of gelation ~Y

When a divalent metal ion such as Ni 2+ ion is allowed to diffuse through a sodium alginate sol placed in a petri dish, a primary membrane will be formed on the surface of the alginate sol on immediate contact with the electrolyte. The formed membrane will separate the sol from the surrounding electrolyte, whereas the macromolecular chains of the polyelectrolyte start to distribute themselves statistically downside the formed membrane. As equilibrium is approached, the Ni 2+ ions of the electrolyte begin to diffuse through the already formed membrane inside the alginate sol. Simultaneously, the counter

I

I

2

3

4

5

6

7

Table 2. Activation parameters of the Ni 2+-alginate gelation reaction

Time x 102/min

Fig. 1. Typical pseudo first-order plots for Hi2+ alginate gelation reaction. [AIg-]=0.05M, Temp=25°C and [Ni2÷] (O) 0.69, ((D) 0,58, (A) 0.47 M.

Rj

AS* (J/mol deg)

AH* (kJ/mol)

AG* (kJ/mol)

Rj R2

-224.94 _+ 14.23 -235,70+_ ll.41

25.29 + 1.25 25.34+_ 1 . 1 1

92,19 +- 4,61 95.58+_3.73

Sol-gel transformation ions resulting from the dissociation of the sol, Na ÷, start to diffuse through the membrane outside into the electrolyte. Due to the different mobilities of the exchanged ions, a diffusion potential will be established [11]. The electric field resulting from the diffusion potential flattens the negatively charged chains of the alginate sol. The exchanged Na ÷ ions form a sort of bridge between the carboxylate groups of the macromolecules [12]. In reality, these bonds are not just simple - C O O - N i - O O C - , but a kind of chelate complex between two carboxylate, one or two pairs of hydroxyl groups and the Ni 2÷ ion as shown below,

.~z.o/.Y"

C__0__ Ni__O__C ~0

/o

"

% ' "- ,.' .4 /A"- o .I

-,0

0//

HI In H

H

H

H

This process takes place gradually to give a capillary type structure [13] in an ionotropic polymembrane. [14] Generally, in exchange reactions the ratedetermining step is found to be an actual chemical exchange [15-17] at the fixed ionic groups rather than counter ion exchange at the interfaces [18]. Hence, the initial part of the sigrnoidal curve shown in Fig. 1 is fast and can be explained by the formation of a primary membrane which separates the alginate sol from the surrounding electrolyte. On the other hand, the slow part at longer times is attributed to a steady chemical exchange resulting from the inter-diffusion of ions. It is worth considering whether the rate of the chemical exchange in either the fast or the slow

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gelation processes is rate-controlling in such a sol-gel transformation. The large negative value of AS* can be explained in part by a necessity for the Ni 2÷ ions to penetrate the spaces between the carboxylate group in order to crosslink them and form the nickei-alginate ionotropic membrane complex. REFERENCES

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