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Kinetics and mechanism of sol-gel transformation on polyelectrolytes of some transition metal ions, especially cobalt alginate ionotropic membranes
Eur. Po(vm. J. Vol. 25, No. 12, pp. 1209-1212, 1989 Printed in Great Britain
0014-3057/89 $3,00 +0.00 Pergamon Press plc
KINETICS A N D MECHANISM OF SOL-GEL T R A N S F O R M A T I O N ON POLYELECTROLYTES OF SOME TRANSITION METAL IONS, ESPECIALLY COBALT ALGINATE IONOTROPIC MEMBRANES R. M.
HASSAN,A.
M.
SUMMAN,*M.
K. HASSAN and S. A. EL-SHATOURY
Department of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt (Received 10 October 1988; in revised form 6 April 1989)
Abstract--The kinetics of gelation have been studied complexometrically at temperatures between 25 and 40 °. The pseudo-first order plot of exchange between Na + ions of the macromolecular chains of alginate sol and Co 2÷ ions gives a sigmoidal curve with two distinct portions. The initial part was fast and curved significantly at early times followed by a decrease in the rate of exchange at longer times. The rate of gelation conforms to the expression (C~ - C,) = B0 e - n , + P0 e R,~,where R r and Rs are the first order rate constants of gelation for the fast and slow reactions, respectively, There is discussion of the factors which affect the rate of exchange such as the concentrations of metal ions and alginate sol, the diameter of the formed capillaries and the orientation of the macromolecular chains and the solvent molecules toward the metal ions. The kinetic parameters have been evaluated and a consistent gelation mechanism is suggested.
INTRODUCTION
difficulty. Petri dishes (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° for about 20 min.
Alginic acid is a polyelectrolyte [1] which consists o f f l ( l ~ ) linked D - m a n n u r o n i c acid units a n d some L-guluronic acid units [2-5]. W h e n a divalent transition metal ion is allowed to diffuse t h r o u g h a sodium alginate sol, it turns slowly to a gel m e m b r a n e complex with definite structure and o r i e n t a t i o n depending on the m e t h o d of p r e p a r a t i o n a n d the metal ion used [6-10]. These gels are i o n o t r o p h i c in nature, differing from the classical type o f gels; the long chain molecules are held together by a complex chelation between the metal ion a n d the functional groups of the m a c r o m o l e c u l a r chains. In spite o f the great i m p o r t a n c e of ionotropic alginate gels for applications in agriculture, food processing, medicine a n d industry, the kinetics of their gelation processes have not been established. A w a d et al. examined the rate of f o r m a t i o n o f noncapillary metal alginate gels [11, 12]. The proposed m e c h a n i s m could not explain some of the observations. The main object of this report is to extend investigation [9, 13, 14] to capillary metal alginate m e m b r a n e s in order to elucidate suitable m e c h a n i s m s a n d put forward rate laws consistent with the experimental data.
The preliminary experiments indicated that the rate of gelation depended on the metal ion concentration. The kinetic studies of the present work were performed under pseudo-first-order conditions where [Co z+ ]0>>[Alg- ]0. The alginate sol and the cobalt ion electrolyte were equilibrated at the desired temperature. After the reactants had attained the temperature of the water-bath, fixed volumes of the alginate sol were removed by syringe and poured into the Petri dishes up to two-thirds of their heights. The dishes were then immersed separately in vessels containing equal volumes of cobalt sulphate solutions of known concentrations. The time of contact of alginate sol with the metal electrolyte was recorded for each dish. After various intervals, the formed cobalt alginate membranes were carefully removed and washed quickly with deionized water several times till the washings were free of Co 2+ ions. The chelated Co 2÷ ions in those membranes were exchanged by using dilute acids, collected and determined complexometrically [15]. The variation in the concentration of chelated Co s÷ ions as a function of time was recorded. These conditions were sufficient to allow a kinetic analysis of the results. However, no method could be devised which was specific for alginate sol.
EXPERIMENTAL PROCEDURES
RESULTS
All materials were of AR grade (BDH). Twice-distilled water was used in all preparations. The temperature was controlled within 0.1 °. Sodium alginate sols of various concentrations were prepared by dissolving the necessary amounts of solid material in bidistilled water. This process was performed by stepwise addition of the powder alginate while rapidly stirring the water, otherwise it gives lumps which swell with *Chemistry Department, Faculty of Applied Science, UmmA1-Qura University, Mekkah, Saudi Arabia. EPJ
.'5 ~-"
a
Kinetic measurements
Stoichiometry
Ion exchange is inherently a stoichiometric process [6]. A n y c o u n t e r ions which leave the m a c r o m o l e c u l a r chains of the polyelectrolyte are replaced by an equivalent a m o u n t o f o t h e r metal ions. The stoichiometry o f the overall gelation reaction of alginate sol with excess Co 2+ ions was determined complexometrically. The mixture of alginate sol a n d Co 2+ ion electrolyte was m a i n t a i n e d at r o o m temperature for a b o u t 48 hr. The unreacted [Co 2+ ] was
1209
1210
R.M. HASSANet al.
/
estimated periodically until it had attained a constant value, i.e. completion of gelation. A stoichiometric ratio of 0.51 +0.02 ([Co 2+]...... ~d/[Alg-]0) was obtained at several initial concentrations of Co :+ ions. The stoichiometry of such a sol-gel transformation can be expressed by the following equation: Co :+ + 2Na-Alg = Co-Alg: + 2Na + . electrolyte
sol
gel
60
(1)
electrolyte
Also, from the kinetic observations of sol-gel transformation on alginates with polyvalent metal ions [11-14], the stoichiometry of the gelation processes may be written generally as
50
,5 el
M "+ + n N a - A l g = M-AIg, + n Na +
40
where M denotes the metal ion and n its valency. The empirical rate law of gelation can be expressed by d[Gel] dt
1 d[Alg- ] n dt
d[M "+ ] dt
30
= Rj [M "+ ] [Alg- ].
(2)
I
I
I
1
2
3
I 4
Time x lO-2/min
Fig. 1. Typical pseudo-first o r d e r plots for C o 2+ a l g i n a t e
[Co 2+] and [Alg - ] dependencies The order with respect to reactants was determined by working under pseudo-first order conditions with Co z÷ in a large excess over alginate sol. The concentration of Co 2+ ions was varied from 0.35 to 0.78 M, whereas [Alg- ] suitable for the kinetic measurements was 0.05 M. Plots o f l n (Coo - Ct) vs time were found to curve significantly at early times but became linear and relatively slow at longer times. These curves indicate that the gelation reaction obeys the expression (Coo - Ct) = Bo e -Rft + Po e-Rs'; Rf and Rs are the first order rate constants of exchange for the fast and slow gelation steps, while B0 and P0 represent the initial concentration of the sol in the two gelation processes, respectively. The rate constants of exchange shown in Table 1 were obtained by drawing a straight line through the fast linear portion (R0 of the first order plot and extrapolating the line back to zero time (B0). The rate constant of exchange, R~, for the slower gelation reaction was obtained from plots of the form In ( C o - C t ) - ( C - C~) vs time. The quantity (C~ - Ct) represents the experimental point and ( C o - C~) represents the extrapolated point at time t' [16]. The pseudo-first order plots are shown in Fig. 1. Temperature dependency The gelation was studied at temperatures from 25 to 40 °. As shown in Fig. 2, the plots of - I n (h/kT) Rj against 1/T of the Eyring equation [17] gave good straight lines. The enthalpy and entropy of activation were calculated from the slopes and the intercepts Table I. Rates of exchange for Co2+-alginate gelation reaction: [Alg- ] = 0.05 M and temp. = 25 ° [Co2+ ] M 0.35 0.46 0.57 0.68 0.78 Average
102 R I min ~
102 R t M - ] min- =
103 R, min t
102 R2 M I min-
1.75 2.41 2.83 3.49 4.02
5.01 5.23 4.97 5.13 5.16
3.51 5.66 6.21 7.89 8.74
1.00 1.23 1.09 1.16 1.12
5. I0
-
I.12
gelation reaction. [Alg-] = 0.05 M, temp = 25°, [Co2+ ] = (O) 0.68, (O) 0.57, (A) 0.47 M. R2
38
J o
~
e
~
°~
37 R1
r~
_._....o-36
o--"
I
/
/
.....o
Y
35 I
I
3.2
3.3 kKT
Fig. 2. E y r i n g plot o f the gelation reaction between C o 2+
and alginate sol. of the straight lines. The kinetic parameters were calculated using the least-squares method and are summarized in Table 2. DISCUSSION
Mechanism As a divalent transition metal ion electrolyte comes in contact with the alginate sol, a primary membrane will be formed on the surface of the sol on immediate contact with the electrolyte. The membrane will separate the sol from the surrounding electrolyte, whereas the macromolecular chains start to distribute themselves statistically on the lower side of the formed membrane. As equilibrium is approached, the metal ions begin to diffuse through the already formed membrane inside the alginate sol. Simul-
Kinetics and mechanism of sol gel transformation
1211
Table 2. Kinetic parameters of the gelation reaction between alginate polyelectrolyte and transition metals: [Alg ] = 0.05 M, [M 2+ ] = 0.57 M and temp. = 2 5 102R min
AS~'I
AH~'/
AG~8/
kJ'kmol~
kJmol i
kJmol ~
Ref.
Co:+
R~ = 4.97 R 2 = 1.09
- 176,64 - 188.53
37.8 38.03
90.44 94.21
This work
Ni2+
R I = 2.34 R2 = 0.64
-224.49 -235.70
25,29 25.34
92.19 95,58
[14]
Cu2~
R I =2.13 R2 = 0.39
-260.89 -275.23
14.77 14.68
92.5 96.7
[27]
M
taneously, the counter ions resulting from the dissociation of the sol, Na ÷, start to diffuse through the membrane into the outside metal ion electrolyte. The net process of the exchange reaction leads to the formation of the gel membrane complex with definite structure and orientation. When the metal ions are allowed to diffuse upward, non-capillary gel membranes are formed. This method of gel preparation is known as the ascending technique. When the metal ions diffused in the downward direction, there are capillaries that are straight, parallel and nearly identical to each other. The process of diffusion takes place stepwise to give capillary ionotropic polymembranes. Under microscopic investigation, these capillaries appear as fine pores of the same radius in a transverse section [9]. This method of gel preparation is termed the descending technique. In these ionotropic gels, the metal ion chelates the functional groups of the macromolecular chains. A sort of bridge between two carboxylates and one or two pairs of hydroxyl groups is formed, depending on the coordination number of the metal ion. These chelates may be either intermolecular (I) [18] when the carboxylate and hydroxyl groups belong to different chains.
%~O m H
o~
~o 0,~
0
II
/0
H 0
HI IH H H
0 (1)
O/ H H
or intramolecular (II) [9] in which the functional groups belong to the same chain thus %C~0-J"--'Q
/o,, /h
H
//
/
,~"
M--O~C ~*0 "\ HAI--'--O H 0
"-,I/~,.
H
H
(Ii)
\1
g
In exchange reactions, two possible rate determining steps have been considered. These are the counter ion exchange across the interface between the sol and the electrolyte [19] and the actual exchange reaction at the fixed ionic groups [20-22]. Rate control by ion exchange across the interface is very unlikely [23]. The fast initial part of the sigmoidal curve, Rr, can be explained by the formation of primary membrane, whereas the slow part of the curve shown at longer times, R s, is attributed to the steady exchange of the interdiffused counter ions, Na + and Co 2+, through the already formed membrane. The slow process includes some other factors such as the penetration power of Co -'+ ions and the orientation of the macromolecular chains and solvent molecules toward the chelated Co 2+ ion. It is worthwhile to consider the exchange of the interdiffused ions in either the fast or the slow gelation steps as rate-controlling. It is noticed that the rate constants of gelation were in the order Co > Ni > Cu alginates as shown in Table 2. The magnitude depends on the ionic radii of the transition metal ions and on the diameter of the formed capillaries. Cobalt (II) ion is the largest [24] and has the greatest rate as experimentally observed. The width of the formed capillaries will enhance the exchange process between the interdiffused metal ions. The diameters of capillaries lie in the order Co > Ni > Cu alginates [8] in good accord with the experimental observation of rate of exchange. Furthermore, the large negative entropy of activation can be explained by the necessity for the small metal ions to penetrate the large carboxylate groups in order to bring them together and form the ionotropic gel complexes. It was found that the orientations of the macromolecular chains and solvent molecules towards the metal ions strongly influence the kinetic parameters of sol-gel transformation and tend to affect the entropy of activation. Since the entropy of activation increases with decreasing orientation factor, the magnitude of orientation should be in the order Co < Ni < Cu alginates [7, 25]. This is consistent with the values of entropy of activation obtained for these gel complexes. The kinetic data indicate that the stability of the gel complexes lies in the order Cu > Ni > Co alginates in good agreement with that reported elsewhere [10, 26]. In spite of the variety of the transition metal ions used, it seems that the free energy of activation remained unaltered. This fact may suggest that the mechanisms of the sol-gel transformation for these transition metal ion alginates are nearly similar.
1212
R.M. HASSANet al.
REFERENCES I. A. Katahaisky and I. Michaell. J. Polym. Sci, 69, 15 (1955). 2. S. K. Chanda, E. L. Hirst, E. G. V. Percival and A. G. Ross. J. chem. Soc. 1833 (1952). 3. F. G. Fischer and H. Dfrfel. Z. Physiol. Chem. 302, 186 (1955). 4. R. L. Whistler and R. G. Schweiger. J. Am. chem. Soc. 80, 5701 (1958). 5. R. L. Whistler and K. W. Kirby. Z. Physiol. Chem. 31,t, 46 (1955). 6. A. Haugh and O. Smidsr~d. Acta chim. stand. 19, 341 (1965). 7. H. Thiele and C. Anderson. Koll. Z. 76, 140 (1955). 8. H. Thiele and Hallich. Koll Z. 151, 1 (1957). 9. R. M. Hassan, A. Awad and A. Hassan. J. Polym. Sei. In press. 10. S. Olay. Aeta chim. stand. 19, 329 (1965). 11. A. Awad, F. E1-Cheikh and A. Shaker. CoU Polym. Sci. 258, 1244 (1980). 12. A. Awad, F. E1-Cheikh and A, Shaker. J. Ind. chem. Soc. 57, 728 (1980). 13. A. Awad, F. E1-Cheikh and R. M. Hassan. Rev. Roum. Chem. 24, 563 (1979).
14. R. M. Hassan, M. H. Wahdan and A. Hassan. Fur. Polym. J. 24, 281 (1988). 15. A. I. Vogel. A Text Book of Quantitative Inorganic Analysis, 3rd edn (1961). 16. A. A. Frost and R. G. Pearson. Kinetics and Mechanism, 2nd edn, p. 162. Wiley, New York (1965). 17. S. Glasstone, K. J. Laidler and H. Eyring. The Theory of Rate Processes. McGraw-Hill, New York (1941). 18. R. G. Schweiger, J. org. Chem. 27, 1789 (1962). 19. G. Dickel and A. Mayer. Z. Electrochem. 57, 901 (1953). 20. D. Reichenberg. J. Am. chem. Soc. 75, 589 (1953). 21. M. Tetenboum and H. P. Gregor. J. phys. Chem. 58, 1158 (1954). 22. D. Richman and H. C. Thowas. J. phys. Chem. 60~ 237 (1956). 23. G. Dickel, A. Mayer and L. Nieeieki. Z. Electrochem. 59, 228 (1955). 24. F. A. Cotton and G. Wilkinson. Advanced Inorganic Chemistry, 3rd edn (1972). 25. K. Plohnke. Diff. Arbeit. Kiel (1960). 26. R. Schweiger. Koll. Z. 47, 196 (1964). 27. R. M. Hassan, S. A. EI-Shatoury, M. A. Mousa and A. Hassan. Eur. Polym. J. 24, 1173 (1988).
0014-3057/89 $3,00 +0.00 Pergamon Press plc
KINETICS A N D MECHANISM OF SOL-GEL T R A N S F O R M A T I O N ON POLYELECTROLYTES OF SOME TRANSITION METAL IONS, ESPECIALLY COBALT ALGINATE IONOTROPIC MEMBRANES R. M.
HASSAN,A.
M.
SUMMAN,*M.
K. HASSAN and S. A. EL-SHATOURY
Department of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt (Received 10 October 1988; in revised form 6 April 1989)
Abstract--The kinetics of gelation have been studied complexometrically at temperatures between 25 and 40 °. The pseudo-first order plot of exchange between Na + ions of the macromolecular chains of alginate sol and Co 2÷ ions gives a sigmoidal curve with two distinct portions. The initial part was fast and curved significantly at early times followed by a decrease in the rate of exchange at longer times. The rate of gelation conforms to the expression (C~ - C,) = B0 e - n , + P0 e R,~,where R r and Rs are the first order rate constants of gelation for the fast and slow reactions, respectively, There is discussion of the factors which affect the rate of exchange such as the concentrations of metal ions and alginate sol, the diameter of the formed capillaries and the orientation of the macromolecular chains and the solvent molecules toward the metal ions. The kinetic parameters have been evaluated and a consistent gelation mechanism is suggested.
INTRODUCTION
difficulty. Petri dishes (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° for about 20 min.
Alginic acid is a polyelectrolyte [1] which consists o f f l ( l ~ ) linked D - m a n n u r o n i c acid units a n d some L-guluronic acid units [2-5]. W h e n a divalent transition metal ion is allowed to diffuse t h r o u g h a sodium alginate sol, it turns slowly to a gel m e m b r a n e complex with definite structure and o r i e n t a t i o n depending on the m e t h o d of p r e p a r a t i o n a n d the metal ion used [6-10]. These gels are i o n o t r o p h i c in nature, differing from the classical type o f gels; the long chain molecules are held together by a complex chelation between the metal ion a n d the functional groups of the m a c r o m o l e c u l a r chains. In spite o f the great i m p o r t a n c e of ionotropic alginate gels for applications in agriculture, food processing, medicine a n d industry, the kinetics of their gelation processes have not been established. A w a d et al. examined the rate of f o r m a t i o n o f noncapillary metal alginate gels [11, 12]. The proposed m e c h a n i s m could not explain some of the observations. The main object of this report is to extend investigation [9, 13, 14] to capillary metal alginate m e m b r a n e s in order to elucidate suitable m e c h a n i s m s a n d put forward rate laws consistent with the experimental data.
The preliminary experiments indicated that the rate of gelation depended on the metal ion concentration. The kinetic studies of the present work were performed under pseudo-first-order conditions where [Co z+ ]0>>[Alg- ]0. The alginate sol and the cobalt ion electrolyte were equilibrated at the desired temperature. After the reactants had attained the temperature of the water-bath, fixed volumes of the alginate sol were removed by syringe and poured into the Petri dishes up to two-thirds of their heights. The dishes were then immersed separately in vessels containing equal volumes of cobalt sulphate solutions of known concentrations. The time of contact of alginate sol with the metal electrolyte was recorded for each dish. After various intervals, the formed cobalt alginate membranes were carefully removed and washed quickly with deionized water several times till the washings were free of Co 2+ ions. The chelated Co 2÷ ions in those membranes were exchanged by using dilute acids, collected and determined complexometrically [15]. The variation in the concentration of chelated Co s÷ ions as a function of time was recorded. These conditions were sufficient to allow a kinetic analysis of the results. However, no method could be devised which was specific for alginate sol.
EXPERIMENTAL PROCEDURES
RESULTS
All materials were of AR grade (BDH). Twice-distilled water was used in all preparations. The temperature was controlled within 0.1 °. Sodium alginate sols of various concentrations were prepared by dissolving the necessary amounts of solid material in bidistilled water. This process was performed by stepwise addition of the powder alginate while rapidly stirring the water, otherwise it gives lumps which swell with *Chemistry Department, Faculty of Applied Science, UmmA1-Qura University, Mekkah, Saudi Arabia. EPJ
.'5 ~-"
a
Kinetic measurements
Stoichiometry
Ion exchange is inherently a stoichiometric process [6]. A n y c o u n t e r ions which leave the m a c r o m o l e c u l a r chains of the polyelectrolyte are replaced by an equivalent a m o u n t o f o t h e r metal ions. The stoichiometry o f the overall gelation reaction of alginate sol with excess Co 2+ ions was determined complexometrically. The mixture of alginate sol a n d Co 2+ ion electrolyte was m a i n t a i n e d at r o o m temperature for a b o u t 48 hr. The unreacted [Co 2+ ] was
1209
1210
R.M. HASSANet al.
/
estimated periodically until it had attained a constant value, i.e. completion of gelation. A stoichiometric ratio of 0.51 +0.02 ([Co 2+]...... ~d/[Alg-]0) was obtained at several initial concentrations of Co :+ ions. The stoichiometry of such a sol-gel transformation can be expressed by the following equation: Co :+ + 2Na-Alg = Co-Alg: + 2Na + . electrolyte
sol
gel
60
(1)
electrolyte
Also, from the kinetic observations of sol-gel transformation on alginates with polyvalent metal ions [11-14], the stoichiometry of the gelation processes may be written generally as
50
,5 el
M "+ + n N a - A l g = M-AIg, + n Na +
40
where M denotes the metal ion and n its valency. The empirical rate law of gelation can be expressed by d[Gel] dt
1 d[Alg- ] n dt
d[M "+ ] dt
30
= Rj [M "+ ] [Alg- ].
(2)
I
I
I
1
2
3
I 4
Time x lO-2/min
Fig. 1. Typical pseudo-first o r d e r plots for C o 2+ a l g i n a t e
[Co 2+] and [Alg - ] dependencies The order with respect to reactants was determined by working under pseudo-first order conditions with Co z÷ in a large excess over alginate sol. The concentration of Co 2+ ions was varied from 0.35 to 0.78 M, whereas [Alg- ] suitable for the kinetic measurements was 0.05 M. Plots o f l n (Coo - Ct) vs time were found to curve significantly at early times but became linear and relatively slow at longer times. These curves indicate that the gelation reaction obeys the expression (Coo - Ct) = Bo e -Rft + Po e-Rs'; Rf and Rs are the first order rate constants of exchange for the fast and slow gelation steps, while B0 and P0 represent the initial concentration of the sol in the two gelation processes, respectively. The rate constants of exchange shown in Table 1 were obtained by drawing a straight line through the fast linear portion (R0 of the first order plot and extrapolating the line back to zero time (B0). The rate constant of exchange, R~, for the slower gelation reaction was obtained from plots of the form In ( C o - C t ) - ( C - C~) vs time. The quantity (C~ - Ct) represents the experimental point and ( C o - C~) represents the extrapolated point at time t' [16]. The pseudo-first order plots are shown in Fig. 1. Temperature dependency The gelation was studied at temperatures from 25 to 40 °. As shown in Fig. 2, the plots of - I n (h/kT) Rj against 1/T of the Eyring equation [17] gave good straight lines. The enthalpy and entropy of activation were calculated from the slopes and the intercepts Table I. Rates of exchange for Co2+-alginate gelation reaction: [Alg- ] = 0.05 M and temp. = 25 ° [Co2+ ] M 0.35 0.46 0.57 0.68 0.78 Average
102 R I min ~
102 R t M - ] min- =
103 R, min t
102 R2 M I min-
1.75 2.41 2.83 3.49 4.02
5.01 5.23 4.97 5.13 5.16
3.51 5.66 6.21 7.89 8.74
1.00 1.23 1.09 1.16 1.12
5. I0
-
I.12
gelation reaction. [Alg-] = 0.05 M, temp = 25°, [Co2+ ] = (O) 0.68, (O) 0.57, (A) 0.47 M. R2
38
J o
~
e
~
°~
37 R1
r~
_._....o-36
o--"
I
/
/
.....o
Y
35 I
I
3.2
3.3 kKT
Fig. 2. E y r i n g plot o f the gelation reaction between C o 2+
and alginate sol. of the straight lines. The kinetic parameters were calculated using the least-squares method and are summarized in Table 2. DISCUSSION
Mechanism As a divalent transition metal ion electrolyte comes in contact with the alginate sol, a primary membrane will be formed on the surface of the sol on immediate contact with the electrolyte. The membrane will separate the sol from the surrounding electrolyte, whereas the macromolecular chains start to distribute themselves statistically on the lower side of the formed membrane. As equilibrium is approached, the metal ions begin to diffuse through the already formed membrane inside the alginate sol. Simul-
Kinetics and mechanism of sol gel transformation
1211
Table 2. Kinetic parameters of the gelation reaction between alginate polyelectrolyte and transition metals: [Alg ] = 0.05 M, [M 2+ ] = 0.57 M and temp. = 2 5 102R min
AS~'I
AH~'/
AG~8/
kJ'kmol~
kJmol i
kJmol ~
Ref.
Co:+
R~ = 4.97 R 2 = 1.09
- 176,64 - 188.53
37.8 38.03
90.44 94.21
This work
Ni2+
R I = 2.34 R2 = 0.64
-224.49 -235.70
25,29 25.34
92.19 95,58
[14]
Cu2~
R I =2.13 R2 = 0.39
-260.89 -275.23
14.77 14.68
92.5 96.7
[27]
M
taneously, the counter ions resulting from the dissociation of the sol, Na ÷, start to diffuse through the membrane into the outside metal ion electrolyte. The net process of the exchange reaction leads to the formation of the gel membrane complex with definite structure and orientation. When the metal ions are allowed to diffuse upward, non-capillary gel membranes are formed. This method of gel preparation is known as the ascending technique. When the metal ions diffused in the downward direction, there are capillaries that are straight, parallel and nearly identical to each other. The process of diffusion takes place stepwise to give capillary ionotropic polymembranes. Under microscopic investigation, these capillaries appear as fine pores of the same radius in a transverse section [9]. This method of gel preparation is termed the descending technique. In these ionotropic gels, the metal ion chelates the functional groups of the macromolecular chains. A sort of bridge between two carboxylates and one or two pairs of hydroxyl groups is formed, depending on the coordination number of the metal ion. These chelates may be either intermolecular (I) [18] when the carboxylate and hydroxyl groups belong to different chains.
%~O m H
o~
~o 0,~
0
II
/0
H 0
HI IH H H
0 (1)
O/ H H
or intramolecular (II) [9] in which the functional groups belong to the same chain thus %C~0-J"--'Q
/o,, /h
H
//
/
,~"
M--O~C ~*0 "\ HAI--'--O H 0
"-,I/~,.
H
H
(Ii)
\1
g
In exchange reactions, two possible rate determining steps have been considered. These are the counter ion exchange across the interface between the sol and the electrolyte [19] and the actual exchange reaction at the fixed ionic groups [20-22]. Rate control by ion exchange across the interface is very unlikely [23]. The fast initial part of the sigmoidal curve, Rr, can be explained by the formation of primary membrane, whereas the slow part of the curve shown at longer times, R s, is attributed to the steady exchange of the interdiffused counter ions, Na + and Co 2+, through the already formed membrane. The slow process includes some other factors such as the penetration power of Co -'+ ions and the orientation of the macromolecular chains and solvent molecules toward the chelated Co 2+ ion. It is worthwhile to consider the exchange of the interdiffused ions in either the fast or the slow gelation steps as rate-controlling. It is noticed that the rate constants of gelation were in the order Co > Ni > Cu alginates as shown in Table 2. The magnitude depends on the ionic radii of the transition metal ions and on the diameter of the formed capillaries. Cobalt (II) ion is the largest [24] and has the greatest rate as experimentally observed. The width of the formed capillaries will enhance the exchange process between the interdiffused metal ions. The diameters of capillaries lie in the order Co > Ni > Cu alginates [8] in good accord with the experimental observation of rate of exchange. Furthermore, the large negative entropy of activation can be explained by the necessity for the small metal ions to penetrate the large carboxylate groups in order to bring them together and form the ionotropic gel complexes. It was found that the orientations of the macromolecular chains and solvent molecules towards the metal ions strongly influence the kinetic parameters of sol-gel transformation and tend to affect the entropy of activation. Since the entropy of activation increases with decreasing orientation factor, the magnitude of orientation should be in the order Co < Ni < Cu alginates [7, 25]. This is consistent with the values of entropy of activation obtained for these gel complexes. The kinetic data indicate that the stability of the gel complexes lies in the order Cu > Ni > Co alginates in good agreement with that reported elsewhere [10, 26]. In spite of the variety of the transition metal ions used, it seems that the free energy of activation remained unaltered. This fact may suggest that the mechanisms of the sol-gel transformation for these transition metal ion alginates are nearly similar.
1212
R.M. HASSANet al.
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