- Email: [email protected]
Insulating properties of clay films towards Fe(CN)63− as affected by electrolyte concentration
299
J. Electroanal. Chem., 257 (1988) 299-303 Elsevler Sequoia S.A., Lausanne - Printed
in The Netherlands
Short communication
Insulating properties of clay films towards Fe( CN) i- as affected by electrolyte concentration Alanah Fitch * and Carol L. Faust0 Department (Received
of Chemistry, Loyola Unruersrty of Chicago, 6525 N Sherrdan Rd., Chicago, IL 60626 (U.S.A ) 23 May 1988; in revised form 18 July 1988)
INTRODUCTION
Electrolyte concentration has been observed to cause significant changes in the electrochemical behavior at clay modified electrodes [1,2]. We give results here on the role of electrolyte concentration in determining the insulating character of the clay film with respect to Fe(CN)z-. EXPERIMENTAL
Fe(CN)i(Aldrich) was used as received. Clay (SWY-I, University of Missouri at Colombia, Department of Geology) was purified according to the method of Jackson [3]. Purified clay was re-suspended in distilled/de-ionized water to form a clay solution of 5 g/l. Clay remained in suspension for several months. The cation exchange capacity (CEC) of the SWy-1 clay is 0.74 meq/g [1,4]. The electrochemically active area of the Pt electrode was 5 X 10K3 cm2. The Pt electrode was polished with 1 pm alumina, sonicated in distilled water to remove the alumina, dried with a wipe and then a 1 ~1 solution of 5 g clay/l was dried rapidly at 100°C for 10 min followed by a 5 min cooling period. This results in a clay film whose dry thickness is calculated to be 0.3 pm [l]. An EGG PAR 273 potentiostat/galvanostat and an EGG PAR Model 0091 X-Y recorder were used to obtain the cyclic voltammograms. Scan rates of 50 mV/s were used for all experiments. Potentials were measured against the SCE electrode. The electrode was held in N, purged NaCl solution for 5 min, a background scan taken, and then transferred to a 10 ml N, purged NaCl solution containing 4.2 mM Fe(CN)i- .
l
To whom correspondence
0022-0728/88/$03.50
should
be addressed.
0 1988 Elsevier Sequoia
S.A
300 RESULTS
AND
DISCUSSION
To observe structure effects on ion transport an electrochemically active probe ion must be selected which does not undergo either cation exchange or specific surface adsorption [l] with the clay. Fe(CN)g- was chosen because of its negative charge and because of its relatively small size. The clay modified electrode used was formed by rapid drying of a dilute suspension of clay. The resulting structure behaved as an aerogel (high porosity) in dilute NaCl electrolyte. The film, intact (visibly and as observed by SEM [l]), is very conductive toward Fe(CN)z- (Fig. 1A). Normal peak shaped cyclic voltammograms are observed. The Fe(CN)iis restricted from the total surface of the underlying electrode as demonstrated by a reduction in peak heights. As the NaCl concentration is increased the Fe(CN)zis increasingly excluded and peak heights decrease (Fig. 1B). The electrode eventually becomes nearly insulating with respect to Fe(CN)z-. The shape of the cyclic voltammograms changes from peak shape to sigmoidal. A plot of the ratio of peak height at the modified electrode to peak height at the bare electrode vs. electrolyte concentration is shown in Fig. 2a. For each of the two quasi-plateau regions, a line for the mean is drawn and the area between plus and minus one standard deviation is shaded. A change from nearly insulating behavior to more porous behavior is initiated at p[NaCl] = 0. For comparison, the plot of Rowe11 [5] for uptake of water in oriented (stacked, face to face structure, Fig. 3A) films of Wyoming montmorillonite is reproduced in Fig. 2B. Note that a similar discontinuity is observed. A change in swelling occurs at p[NaCl] = 0.3 (Fig. 2B). Rowe11 refers to this point as the “takeoff point”, which he attributes to a disruption and subsequent osmotic swelling of face to face structure [5,6]. In his study the films eventually “sloughed off” as platelet to platelet distance increased to a minimum of energy interation.
A
i\
I
B
,A
J
.,k. I’/ +I
+I
;
EIV Fig. 1. Cyclic voltammograms of 4 mM Fe(CN)zobtained at a bare (upper) and (lower) Pt electrode m (A) 0.05 M and (B) 1.75 M NaCl solutions. The bar is 1 PA.
a clay modified
301
.““.a
I
-1 0
10
00
20
10
-30
’
(0)
8 -20 6
-
4
-10
O-1 0
. 00
10
-0
2.0
-log[NaCI] Fig. 2. (A) A plot of the ratio of cathodic peak he&s for Fe(CN)i- obtained at the clay and the bare electrodes vs. the log of the electrolyte concentration. The shaded area represents f 1 standard deviation from the mean (line). (B) Swelling data for oriented clay platelets (ref. 5). The plot is reproduced as shown in the origmal publication. Also shown is the distance associated with the secondary energy minima for the face to face structure predicted by DLVO theory.
The predicted distance between two face to face platelets can be predicted from Derjaguin, Landau, Verwey and Overbeek (DLVO) theory [7]. A plot of the plate to plate distance for the secondary energy minimum using the CEC for SWy-1 and a Hamaker constant of 2.2 x 10-20 J for mica [8] is shown in Fig. 2B. A straight line
A
B
Fig. 3. (A) Face to face (stacked) and (B) edge fo face (house of cards) orientations of clay.
of 0.5 nm represents the minimum distance between platelets associated with the water of hydration of the cation associated with the surface of the clay. The intercept of the two lines corresponds nicely with both the swelling data and the discontinuity in the plot of cathodic peak height ratio for Fe(CN)i-. These comparisons suggest that in the region of - 0.5 < - log[NaCl] < 0.3, micropores formed from face to face structure affect the transport of Fe(CN)i-. Both the swelling and theoretical data predict a smoothly increasing platelet to platelet distance with dilution of the electrolyte. The transport of Fe(CN)i-, however, reaches a maximum at p[NaCl] = 0.3, and, as such, does not appear to be the result of miropores formed from face to face structure. To explain these results, we note that the method used to form the clay modified electrode are not conducive to the formation of perfectly oriented (face to face or stacked) structure. A dilute electrolyte clay suspension exhibits edge face structure (Fig. 3B) due to the association of positive edges of broken crystals and the negative face surface [9-121. The rapid drying of this house of cards structure can result in the retention of the structure [13]. Subjecting this film to a strong electrolyte will cause a conversion of any edge-face to face-face structure. Subjecting the film to a sufficiently dilute electrolyte will not destroy any edge face structure present. Any swelling which occurs must therefore work against the force of the cross linking which can be strong enough to oppose further swelling [14]. SUMMARY
We show that the structure of a clay film formed by rapid drying on a Pt surface has variable porosity as a function of electrolyte concentration. The porosity is, in part, easily predictable by consideration of the DLVO theory. Two types of structure are observed. In a high electrolyte solution face to face structure is preferred while in a very dilute electrolyte solution the originally formed edge to face a structure is present. The films are most insulating with respect to an anion where collapsed face to face structure is anticipated. The films are most conductive toward an anion where an open cross linked, edge to face, structure is predicted. REFERENCES 1 2 3 4 5 6 7 8 9 10
A. Fitch, A. Lavy-Feder, S.A. Lee and M.T. K&h, J. Phys. Chem., in press. D. Ege, P.K. Ghosh, J.R. White, J.F. Equey and A.J. Bard, J. Am. Chem. Sot., 107 (1985) 5644. O.P. Mehra and M.L. Jackson, Clays Clay Miner., 7 (1960) 317. H. Van Olphen (Ed.), The Data Handbook for Clay Materials and Other Non-Metallic Minerals, Pergamon Press, Oxford, 1979, p. 19. D.L. Rowell, Soil Sci., 100 (1965) 346. D.L. Rowell, Soil Sci., 96 (1963) 368. A.W. Adamson, Physical Chemistry of Surface, Wiley, Chichester, 1982, p. 245. J.N. Jsraelachvili and G.E. Adams, Nature (London), 262 (1976) 774. H. Van Olphen, J. Colloid Sci., 17 (1962) 660. H. Van Olphen, Discuss. Faraday Sot., 11 (1951) 82.
303 11 H. Van Olphen, J. Colloid Sci., 19 (1964) 313. 12 D.J. Cebula, R.K. Thomas, S. Middleton, R.H. Ottewill and J.W. White, Clays Clay Miner., 1979, 27, 39. 13 H. Van Olphen, An Introduction to Clay Colloid Chemistry, 2nd ed., Wiley, New York, 1977, p. 27. 14 K. Norrish and J.A. Rausell-Colom, Clays Clay Mmer., 10 (1963) 123.
J. Electroanal. Chem., 257 (1988) 299-303 Elsevler Sequoia S.A., Lausanne - Printed
in The Netherlands
Short communication
Insulating properties of clay films towards Fe( CN) i- as affected by electrolyte concentration Alanah Fitch * and Carol L. Faust0 Department (Received
of Chemistry, Loyola Unruersrty of Chicago, 6525 N Sherrdan Rd., Chicago, IL 60626 (U.S.A ) 23 May 1988; in revised form 18 July 1988)
INTRODUCTION
Electrolyte concentration has been observed to cause significant changes in the electrochemical behavior at clay modified electrodes [1,2]. We give results here on the role of electrolyte concentration in determining the insulating character of the clay film with respect to Fe(CN)z-. EXPERIMENTAL
Fe(CN)i(Aldrich) was used as received. Clay (SWY-I, University of Missouri at Colombia, Department of Geology) was purified according to the method of Jackson [3]. Purified clay was re-suspended in distilled/de-ionized water to form a clay solution of 5 g/l. Clay remained in suspension for several months. The cation exchange capacity (CEC) of the SWy-1 clay is 0.74 meq/g [1,4]. The electrochemically active area of the Pt electrode was 5 X 10K3 cm2. The Pt electrode was polished with 1 pm alumina, sonicated in distilled water to remove the alumina, dried with a wipe and then a 1 ~1 solution of 5 g clay/l was dried rapidly at 100°C for 10 min followed by a 5 min cooling period. This results in a clay film whose dry thickness is calculated to be 0.3 pm [l]. An EGG PAR 273 potentiostat/galvanostat and an EGG PAR Model 0091 X-Y recorder were used to obtain the cyclic voltammograms. Scan rates of 50 mV/s were used for all experiments. Potentials were measured against the SCE electrode. The electrode was held in N, purged NaCl solution for 5 min, a background scan taken, and then transferred to a 10 ml N, purged NaCl solution containing 4.2 mM Fe(CN)i- .
l
To whom correspondence
0022-0728/88/$03.50
should
be addressed.
0 1988 Elsevier Sequoia
S.A
300 RESULTS
AND
DISCUSSION
To observe structure effects on ion transport an electrochemically active probe ion must be selected which does not undergo either cation exchange or specific surface adsorption [l] with the clay. Fe(CN)g- was chosen because of its negative charge and because of its relatively small size. The clay modified electrode used was formed by rapid drying of a dilute suspension of clay. The resulting structure behaved as an aerogel (high porosity) in dilute NaCl electrolyte. The film, intact (visibly and as observed by SEM [l]), is very conductive toward Fe(CN)z- (Fig. 1A). Normal peak shaped cyclic voltammograms are observed. The Fe(CN)iis restricted from the total surface of the underlying electrode as demonstrated by a reduction in peak heights. As the NaCl concentration is increased the Fe(CN)zis increasingly excluded and peak heights decrease (Fig. 1B). The electrode eventually becomes nearly insulating with respect to Fe(CN)z-. The shape of the cyclic voltammograms changes from peak shape to sigmoidal. A plot of the ratio of peak height at the modified electrode to peak height at the bare electrode vs. electrolyte concentration is shown in Fig. 2a. For each of the two quasi-plateau regions, a line for the mean is drawn and the area between plus and minus one standard deviation is shaded. A change from nearly insulating behavior to more porous behavior is initiated at p[NaCl] = 0. For comparison, the plot of Rowe11 [5] for uptake of water in oriented (stacked, face to face structure, Fig. 3A) films of Wyoming montmorillonite is reproduced in Fig. 2B. Note that a similar discontinuity is observed. A change in swelling occurs at p[NaCl] = 0.3 (Fig. 2B). Rowe11 refers to this point as the “takeoff point”, which he attributes to a disruption and subsequent osmotic swelling of face to face structure [5,6]. In his study the films eventually “sloughed off” as platelet to platelet distance increased to a minimum of energy interation.
A
i\
I
B
,A
J
.,k. I’/ +I
+I
;
EIV Fig. 1. Cyclic voltammograms of 4 mM Fe(CN)zobtained at a bare (upper) and (lower) Pt electrode m (A) 0.05 M and (B) 1.75 M NaCl solutions. The bar is 1 PA.
a clay modified
301
.““.a
I
-1 0
10
00
20
10
-30
’
(0)
8 -20 6
-
4
-10
O-1 0
. 00
10
-0
2.0
-log[NaCI] Fig. 2. (A) A plot of the ratio of cathodic peak he&s for Fe(CN)i- obtained at the clay and the bare electrodes vs. the log of the electrolyte concentration. The shaded area represents f 1 standard deviation from the mean (line). (B) Swelling data for oriented clay platelets (ref. 5). The plot is reproduced as shown in the origmal publication. Also shown is the distance associated with the secondary energy minima for the face to face structure predicted by DLVO theory.
The predicted distance between two face to face platelets can be predicted from Derjaguin, Landau, Verwey and Overbeek (DLVO) theory [7]. A plot of the plate to plate distance for the secondary energy minimum using the CEC for SWy-1 and a Hamaker constant of 2.2 x 10-20 J for mica [8] is shown in Fig. 2B. A straight line
A
B
Fig. 3. (A) Face to face (stacked) and (B) edge fo face (house of cards) orientations of clay.
of 0.5 nm represents the minimum distance between platelets associated with the water of hydration of the cation associated with the surface of the clay. The intercept of the two lines corresponds nicely with both the swelling data and the discontinuity in the plot of cathodic peak height ratio for Fe(CN)i-. These comparisons suggest that in the region of - 0.5 < - log[NaCl] < 0.3, micropores formed from face to face structure affect the transport of Fe(CN)i-. Both the swelling and theoretical data predict a smoothly increasing platelet to platelet distance with dilution of the electrolyte. The transport of Fe(CN)i-, however, reaches a maximum at p[NaCl] = 0.3, and, as such, does not appear to be the result of miropores formed from face to face structure. To explain these results, we note that the method used to form the clay modified electrode are not conducive to the formation of perfectly oriented (face to face or stacked) structure. A dilute electrolyte clay suspension exhibits edge face structure (Fig. 3B) due to the association of positive edges of broken crystals and the negative face surface [9-121. The rapid drying of this house of cards structure can result in the retention of the structure [13]. Subjecting this film to a strong electrolyte will cause a conversion of any edge-face to face-face structure. Subjecting the film to a sufficiently dilute electrolyte will not destroy any edge face structure present. Any swelling which occurs must therefore work against the force of the cross linking which can be strong enough to oppose further swelling [14]. SUMMARY
We show that the structure of a clay film formed by rapid drying on a Pt surface has variable porosity as a function of electrolyte concentration. The porosity is, in part, easily predictable by consideration of the DLVO theory. Two types of structure are observed. In a high electrolyte solution face to face structure is preferred while in a very dilute electrolyte solution the originally formed edge to face a structure is present. The films are most insulating with respect to an anion where collapsed face to face structure is anticipated. The films are most conductive toward an anion where an open cross linked, edge to face, structure is predicted. REFERENCES 1 2 3 4 5 6 7 8 9 10
A. Fitch, A. Lavy-Feder, S.A. Lee and M.T. K&h, J. Phys. Chem., in press. D. Ege, P.K. Ghosh, J.R. White, J.F. Equey and A.J. Bard, J. Am. Chem. Sot., 107 (1985) 5644. O.P. Mehra and M.L. Jackson, Clays Clay Miner., 7 (1960) 317. H. Van Olphen (Ed.), The Data Handbook for Clay Materials and Other Non-Metallic Minerals, Pergamon Press, Oxford, 1979, p. 19. D.L. Rowell, Soil Sci., 100 (1965) 346. D.L. Rowell, Soil Sci., 96 (1963) 368. A.W. Adamson, Physical Chemistry of Surface, Wiley, Chichester, 1982, p. 245. J.N. Jsraelachvili and G.E. Adams, Nature (London), 262 (1976) 774. H. Van Olphen, J. Colloid Sci., 17 (1962) 660. H. Van Olphen, Discuss. Faraday Sot., 11 (1951) 82.
303 11 H. Van Olphen, J. Colloid Sci., 19 (1964) 313. 12 D.J. Cebula, R.K. Thomas, S. Middleton, R.H. Ottewill and J.W. White, Clays Clay Miner., 1979, 27, 39. 13 H. Van Olphen, An Introduction to Clay Colloid Chemistry, 2nd ed., Wiley, New York, 1977, p. 27. 14 K. Norrish and J.A. Rausell-Colom, Clays Clay Mmer., 10 (1963) 123.