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ζ Potential of capacitor paper in nonaqueous solvents
~"Potential of Capacitor Paper in Nonaqueous Solvents B)kRBEL GOSSE, 1 TOAFIK MECHLIA, ANDRI~ DENAT, AND J. P. GOSSE Laboratoire d'Electrostatique et de Mat~riaux Di£lectriques, CNRS, Avenue des Martyrs, 166 X Centre de Tri, 38042 Grenoble C£dex, France Received November 27, 1984; accepted July 17, 1985
1. I N T R O D U C T I O N
attributed this property to a specific adsorption of ions which leads to a net charging of the paper surface. Since paper becomes charged by "picking up" ions from the impregnating liquid, it is not surprising to observe a dependence of its properties on the nature or the concentration of the electrolyte dissolved in the liquid. This constitutes a major difference with semipermeable membranes which have a constant fixed charge and a semipermeability only slightly dependent on the electrolyte concentration. The aim of the present work is to clarify the influence of different parameters such as the electrolyte nature, the water concentration of the impregnating liquid, and the thermal pretreatment of paper on the paper's semipermeability. Microelectrophoresis was used to determine the ~'potential of various paper-liquid interfaces. From this, the interfacial charge density was deduced. ~ Potential measurements were performed in a polar liquid (propylene carbonate, PC) and in a nonpolar one (cyclohexane, CH). In our study we use capacitor paper, but we also give some data on polyethylene which may be compared to paper.
Electrical properties of solid/liquid interfaces have been the subject of many studies and reviews during the last few years. They were concerned with either the electrical double layer (1) or the charge-transfer reaction (2). In most cases, a metal-aqueous solution interface is investigated. Very little experimental work and few theoretical models are available for nonconducting materials in nonaqueous liquids. It now seems that many electrical phenomena in either liquid or impregnated solid dielectrics, are caused or influenced in a very important and general way by the charged double layer which is always present at an interface. Examples of such phenomena are superficial conductivity (3, 4), ion injection by metal electrodes (5) or by insulating surfaces (6) and ion blocking by paper (7). W e have investigated this particular characteristic of paper, which gives unusual electrical properties to insulating systems containing paper (8). The electrical losses of paper impregnated with an organic liquid have been attributed to the semipermeability of paper (9). Indeed, under the influence of an electric field, ions of one sign migrate through the impreg2. MATERIALS AND METHODS nated paper whereas ions of the opposite sign Capacitor paper, kindly supplied by Bollore remain blocked at the liquid-paper interface. (10), was prepared from a pure Kraft pulp. According to the nature of the liquid or the The a-cellulose content reaches 95%. Paper electrolyte it contains, paper was found peralso contains hemicellulose, residual lignin, meable either to anions or to cations (9). We and traces of mineral compounds such as alkali, iron, and calcium salts. When exposed to the atmosphere, it rapidly reaches an equiTo whom correspondence should be sent. 102 0021-9797/86 $3.00 Copyright© 1986by AcademicPress,Inc. All rightsof reproductionin any formreserved.
Journalof ColloidandInterfaceScience,Vol. 110,No. 1, March 1986
~" P O T E N T I A L
OF CAPACITOR
librium moisture content of about 6 or 7%. By vacuum drying at 130°C for 48 h, a moisture content lower than 0.5% can be easily achieved (11). The paper particles to be studied by electrophoresis were obtained by grinding paper with a turbomixer in the degassed solvent itself. Polyethylene was supplied ground by C d F CHIMIE and the particle radii are in the range 5-50 ~m. It contained some unknown additives, e.g., antioxidants. Propylene carbonate (PC) is a polar solvent (~r = 65 at 20°C) with well-known electrical and electrochemical properties (12). In PC, electrolytes such as LiC104 and AgC104 are fully dissociated at concentrations lower than 10-1 M. On the other hand, LiC1 is an associated electrolyte in PC (association constant ga = 557 (13)). The PC was purified according to the method given by Jasinski (14). Cyclohexane was used without any further treatment. To increase its conductivity and ion content we dissolved triisoamylammonium picrate (TiAP) in it. The electrophoretic mobilities were evaluated using two different techniques. We used the microelectrophoresis method with PC solutions (15). Our flat test cell had a rectangular cross section (0.2 X 10 mm). The motion of the particles was observed in the classical way in different planes parallel to the walls of the cell, and their electrokinetic velocity was obtained in the planes of zero electroosmotic velocity. However, Parreira's cell cannot be used in very resistive solutions, so we measured the ~"potential of the paper particles in cyclohexane in a Teflon cell containing two fiat parallel metal electrodes. The electrode gap was about 3 m m and the electrodes were placed far from the Teflon walls to avoid electroosmosis. Electric fields as high as 100 V cm -1 were applied without any evidence of polarization. The motion of the paper particles in the uniform field between the electrodes was recorded with a video camera and observed on a T V screen.
PAPER
103
The calculation of the ~"potential from the value of the electrokinetic mobility # has been widely discussed (1). We use Smoluchowsky relation ~"= n#/~ (in S.I. units) for the PC case since the ratio R/Lo is greater than 1 (R is the particle radius, Lo the thickness of the diffuse layer). In cyclohexane (R/LD ~ 1) we use Hiickel's relation, ~"= 3 ~#/2e, where n and E are the viscosity and the permittivity of the liquid in S.I. units. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Sign of the Charge of the Paper in Pure PC Cellulose, even when pure, is always partially oxidized and contains carbonyl and carboxyl groups. In commercially available capacitor paper, protons of these groups which would enhance the electrical losses of paper at high temperatures (80-100°C) are replaced by divalent ions such as Zn 2+. The introduction of these ions also increases the thermal stability of the paper. In order to separate the influence of the divalent ions from that of the protons from the carboxylic groups, we measured the ~"potenrials in PC at various water concentrations of paper containing Zn 2+ ions (-0.1%) and of pure cellulose (16) which does not contain such ions (Fig. 1, curves a and b). We observed that paper and cellulose particles were always negatively charged. In aqueous solutions where paper is also found to be negatively charged (17), it is assumed that this charge originates from weakly dissociated carboxylic groups. This assumption seems valid, since the concentration of acid groups measured by titration with methylene blue is found to be in the range 10-4 to 5 X 10 -5 equivalent per gram of paper. These groups, if fully dissociated, would give paper a surface charge density some orders of magnitude higher. In this crude evaluation, we consider that the specific surface of paper is about 103 cm2/g (18).
Journal of Colloid and Interface Science, Vol. 110, No. 1, March 1986
104
GOSSE ET AL. mV) -2
10
10-1
[
1
CH20(M)
I I IIIII
-10 --20 --30 -40 -50 - -
FIG. 1. ~"Potentialofthe CP-paperinterfaceas a functionofthe waterconcentration:curvea--nonpretreated capacitorpaper; curveb--nonpretreated cellulosepowder;curve c--capacitor paper heated at 130°Cunder vacuum, for 48 h.
3.2. Effects of a Thermal Pretreatment of the Paper To verify that the negative charge of paper originates from the dissociation of carboxylic groups, we have modified their concentration by thermal pretreatment at 130°C. Paper oxidation remains a controversial subject (1921). It has been asserted that the heating of paper causes the destruction of the carboxylic acid groups which are not blocked by divalent ions. It also increases the degree of oxidation of cellulose. We observed via IR spectrophotometry that the carbonyl band was enhanced after thermal treatment of the paper under an air blanket. However, aldehyde, ketone, and carboxyl groups absorb very close to the same wavelength (---1730 cm -1) and therefore the carboxyl groups cannot be distinguished from the others (22). Thus we gained no information on the variation of the concentration of these groups after paper oxidation. The ~'potential of the paper heated at 130°C under an air blanket became positive. Its value was + 13 m V after a 2-day heating and +29 mV after 15 days. So, as expected, a thermal treatment of paper decreases its negative charge. However, it actually creates a net positive charge at the surface. The surface groups which dissociate and give a positive net charge Journal of Colloid and Interface Science, V ol. 110, No. 1, March 1986
to the paper may be the carboxylic groups which have fixed divalent ions. The dissociation reaction would be - - C O 0 ZnX - - C O 0 Z + + X-, X - is, for instance, C1-. This assumption is in agreement with our experiments on cellulose powder which does not contain divalent ions. The ~"potential of this powder remained negtive ( - - 5 8 m V at 10-2 M H 2 0 ) after being heated for 2 days at 130°C in air. If the heating is done under vacuum, the absolute value of ~"decreases less than it does in air, and always stays negative (Fig. 1, curve c). This is in agreement with the fact that the thermal degradation of cellulose is weaker in vacuum than in air.
3.3. Effect of the Water Content of PC The influence of the water content in PC on the ~"potential is very complex (Fig. 1). For paper (curve a) and cellulose (curve b), ~-decreased when the water content increased. For paper heated in vacuum at 130°C (curve c), ~ took a maximum value of about - 4 0 mV at a water concentration of 4 × 10-1 M. Thus in all cases, the value of ~'results from two simultaneous effects: --dissociation of carboxylic groups; and --specific adsorption of cations enhanced
~" POTENTIAL OF CAPACITOR PAPER by residual water. These cations might be protons, from the carbonic acid which is always present in PC which absorbs high amounts of CO2 when at equilibrium with air. When the water concentration increases, the activity of the proton of the carbonic acid is increased and adsorption of protons by paper according to the reaction scheme - - C O O H + H + ~- - C O O H f might occur. We have indeed verified this. Bubbling nitrogen in PC with 2.5 M H 2 0 made the ~"potential ~o f paper particles vary from - 10 to - 2 5 mV,
3.4. Effects of Electrolytes In propylene carbonate. First we consider paper without thermal pretreatment and electrolyte solutions in PC with a constant water concentration (3 × 10 -1 M). Figure 2 shows the dependence of the ~"potential on the different electrolyte concentrations (LiC1, LiC104, and AgC104). The ~"potential vs concentration curves for AgCIO4 (curve a) and LiC104 (curve b) are similar, the silver salt producing a more positive potential than the lithium salt. These curves are characteristic of specific adsorptions. The ~"potential, which is negative at very low electrolyte concentrations, reverses its sign at high concentrations showing a strong adsorption ofAg +
105
or Li + ions. On the other hand, the ~potential for the LiC1 electrolyte (curve c) remains negative and its dependence on LiC1 concentration may lead us to suppose that LiC1 behaves as an "indifferent electrolyte." The decrease of ~"when LiC1 concentration increases would be caused by the compression of the diffuse layer. But this assumption is in contradiction with the behavior of LiC104 and the adsorption of Li ÷ on paper. We also measured the amount of electrolyte adsorbed per gram of paper (Fig. 3). We observed that t i f l O 4 and LiC1 were actually adsorbed more strongly when the water content in PC and consequently in paper, was higher. For instance, in a solution of 10 -3 M LiCIO4 in PC, the amount of adsorbed electrolyte (M/ g of paper) was increased by a factor 10 when the water content increased from 1.5 × 10 -2 to 4 X 10 --~ M. In a solution of 10-3 M LiCl in CP, the enhancement factor was about 100. It also appears in Fig. 3 that, at a similar water concentration in CP, LiC1 is adsorbed more than is LiC104. So we conclude that the LiC1 concentration dependence of the ~-potential of paper (Fig. 2, curve c) results from a simultaneous adsorption of Li ÷ and C1-. Since the net charge remains negative, the adsorption of Li ÷ is only slightly preferential.
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'
'
'
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:
FIG. 2. ~"Potential of the CP-paper interfacevs the electrolyteconcentration.Influenceof the nature of the electrolyte.Water concentration of."PC--3 × 10-j M; curve a--AgC104; curve b--LiC104; curve c-LiCI. Journal of Colloid and Interface Science, VoL 110, No. l, March ! 986
106
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FIG. 3. Adsorption isotherms of capacitor paper in PC: curve a--LiC], ].5 X 10-2 M H20; curve a'-LiC1, 4 X 10-' M H20; curve b--LiC]O4, t.5 X 10-2 M H20; CtL~e b'--LiCIO4, 4 X 10-~ M H20.
We have also investigated some other electrolytes such as tetrabutylammonium perchlorate (Bu4NCIO4) and lithium picrate (LiPi) In Bu4NC104 solutions, particles of nontreated paper were always negatively charged, whereas in 10-3 M LiPi solutions, they were positively charged. From these results, we may propose an arrangement of ions in an order of increasing affinity for paper: CIO~ < Bu4N + < C1- < Li +
electric point (~" -- 0) for an AgC104 concentration of 10-4 M ( c u r v e a) and 4.5 × 10 -5 M (curve b). The dependence of the isoelectric point on the water concentration confirms an increase in Ag + adsorption by paper when the residual water concentration increases. The variations of ~"potential with the LiC1 concentration in PC at different water concentrations (Fig. 5, curves a and b) lead to the same conclusion: the ~"potential of paper in PC is deag + . pendent on a strong adsorption of electrolytes It appears that this is also the order of in- by paper, this adsorption being enhanced by creasing affinity of the same ions for the resid- an increase in the water concentration. ual water in PC (23, 24). It has also been exPaper which has been thermally pretreated perimentally demonstrated that in PC, the in air at 130°C, was positively charged in pure cations Li + and Ag + of the electrolytes LiC104, PC. This positive charge was only slightly LiPi, and AgC104 are more solvated by the modified by the presence of an electrolyte in residual water than by the solvent. It is also PC. For instance, the ~ potential of preheated known that in organic solvents, anions are less paper remained positive whatever the AgC104 solvated than cations and more reactive than concentration (Fig. 4, curve c). With the LiC1 their solvated counterions. This is the case for electrolyte, a sign reversal was observed only at a rather high LiCI concentration (10 -2 M, C1- in PC. We measured the ~ potential of paper in which means 3 × 10 -3 M Li + and C1-) (Fig. various electrolytic solutions with different 5, curves c and d). Thus, a thermal pretreatwater concentrations. Figure 4 shows the ment of paper seems to weaken its ion-adAgC104 concentration dependence of the ~" sorption properties. In cyclohexane. In pure cyclohexane, and potential of paper for different water concentrations. The curves "a" (3 × 10 =1 M H20) also in TiAP solutions in CH, paper particles and "b" (7 × 10 -1 M H20) indicate an iso- were found either positively or negatively Journal of Colloid and Interface Science, Vol. 110, No. 1, March 1986
107
~" P O T E N T I A L OF C A P A C I T O R PAPER
3~Oj mV) 20 m
I
i
o
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i i lllill
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-10
-20
-30 FIG. 4. AgC104 concentration dependence of the ~"potential of nonpretreated paper in PC, at various water concentrations: curve a - - 3 X 10-l M H 2 0 ; curve b - - 7 X l0 -t M H 2 0 ; curve c--1.5 X 10- 1 M H 2 0 .
charged. We observed that about the same number of particles traveled to the right and to the left side of the cell with the same speed. In this cell which has a large gap between the electrodes there is no electroosmotic flow. We checked that the motion of the particles was due to electrophoresis, i.e., their velocity was proportional to the applied voltage, and their direction of motion changed at the reversal of the electrode polarity. The ~"potential calculated using Hiickel's expression (the thickness of the diffuse layer is large compared to the particle radius ( ~ 5 ~m)) is about 100 inV. However, its values are rather scattered (Fig.
6) and slowly decrease from 104 to 82 mV when the ionic concentration in the solution increases. The surface charge density a corresponding to ~ ~ 100 mV is about 4 X 10-11 Cm -2, only 100 times smaller than in PC. The carboxylic groups by themselves cannot account for this value of a, since the particles were positively or negatively charged. It also appeared that the presence of the electrolyte TIAP in CH did not influence in an appreciable manner the ~ potential of paper. It seems that paper and its residual water, which constitute a polar dissociating medium behave as an entity independent of the nonpolar sur-
~ j my)
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-30 FIG. 5. Dissociated LiCI concentration dependence of the ~"potential of capacitor paper in PC. Influence of the oxidation of paper: curve a--nonpretreated paper, 3 X 10-1 M H20; curve b---nonpretreated paper, 6 X 10-I M H20; curve c - - t h e r m a l l y oxidized paper (at 130°C for 2 days), 3 × 10-1 M H20; curve d - thermally oxidized paper (at 130°C for 15 days), 2 × 10-1 M H 2 0 . Journal of Colloid and Interface Science, Vol. 110, No. 1, M a r c h 1986
108
GOSSE ET AL. mV) m
m
100
m
J
50
i
L i I ilfl
I
I J IIIJ
I 10-12
10-13
i
i I ILJ4. 0 ( 9 -lm-1)
FIG. 6. ~"Potential of paper and PE particles vs the conductivity of solutions of TiAP in cyclohexane: curve a--nonpretreated paper; curve b - - P E .
In a rather surprising way, PE which is an hydrophobic and nonpolar material, shows an electrokinetic behavior very similar to that of paper which is polar and hydrophilic. We assume that this behavior of PE is due to polar groups brought about by oxidation of its surface. Electrolytes can adsorb on these groups as on paper but to a lower extent. Different analysis of the PE surface by ESCA or ATR for instance (25-27), have confirmed a density of oxygenated groups which increases with the duration or the strength of oxidation.
rounding medium in which the electrolyte dissociation is very small. Polyethylene. We have compared results obtained with capacitor paper and with polyethylene which presents polar groups only as surface defects. In pure PC, PE particles are negatively charged. The dependence of their ~"potential on the electrolyte concentration in PC is given on Fig. 7 for LiCI (curve a) and AgC104 (curve b). In LiC1 solutions the absolute value of ~" decreases by increasing LiC1 content whereas in AgC104 solutions we get an isoelectric point at about 5 × 10-3 M. In cyclohexane, PE particles were positively charged and ~"decreased when the conductivity of the solutions increased (Fig. 6).
CONCLUSIONS
The composition of paper and especially paper used in electrical insulation is very
,,,(mY)
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i 10-5
i
i F i IIi
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C(M)
FIG. 7 . ~"Potential of PE particles vs the electrolyte concentration in propylene carbonate: curve a--LiC1; curve b--AgC104. Journal of Colloid and Interface Science, Vol. 110,No, 1, March 1986
~" POTENTIAL OF CAPACITOR PAPER complex and partially u n k n o w n . However, our work shows that its electrokinetic behavior in a pure polar liquid is similar to that o f pure cellulose. Its ~ potential is attributed to the weak dissociation o f carboxylic groups created by the chemical degradation o f the cellulosic chain. W h e n the nonaqueous solvent contains an electrolyte, the electrokinetic behavior o f paper is determined by a strong electrolyte adsorption, itself dependent on the residual water concentration. F r o m experiments with different electrolytes, we deduce an arrangement o f ions in an order o f increasing affinity for paper. We have noticed that the affinity o f ions for paper is correlated to the preferential solvation o f these ions by the residual water in PC. It also appeared that a thermal pretreatment o f capacitor grade paper in an air blanket ( ~ 130°C) strongly diminishes its adsorption properties. These electrokinetic measurements constitute an easy determination o f the paper surface charge density and o f the influence o f different parameters such as the electrolyte nature, the concentration o f the electrolyte or o f the residual water in the solvent. These results are in agreement with our few p r e v i o u s measurements o f the ion transport n u m b e r in paper which were interpreted by a surface charge density on paper varying with the electrolyte nature and concentration. We n o w have a better knowledge o f the influence o f different parameters on paper semipermeability. Lastly, we observed a polyethylene electrokinetic behavior similar to paper corroborating a strong superficial oxidation o f polyethylene. REFERENCES 1. Hunter, R. J., in "Colloid Science." Academic Press, New York/London, 1981.
109
2. Bockris, O'M., and Reddy, A. K. N., "Modem Electrochemistry." Plenum, New York, 1970. 3. Miranda, F. J., and Gazzana-Priaroggia,P., Proc. lEE 123, 229 (1976). 4. Saad, A., and Tobazeon, R., IEEE Trans. Electr. Insul. E-I 19, 193 (1984). 5. Denat, A., Gosse, B., and Gosse, J. P., J. Electrostatics 7, 205 (1979), 6. Saad, A., and Tobazeon, R., J. Phys. D Appl. Phys. 15, 2505 (1982). 7. Gosse, B., Gosse, J. P., and Tobazeon, R., lEE Proc. A 128, 165 (1981). 8. Parkman, N., IEEE Trans. Electr. Ins. E-I 13, 289 (1978). 9. Gosse, B., Gosse, J. P., and Sauviat, M., IEEE Trans. Electr. Ins. E-I 15, 104 (1980). 10. Bollore, Papeteries Cascadec, 29, Scaer, France. 11. Kelk, E., and Wilson, O., Proc. lEE 112, 602 (1965). 12. Gosse, B., Gosse, J. P., and Felici, N., J. AppL Electrochem. 5, 329 (1975). 13. Mukkerjee, L. M., and Boden, D. P., J. Phys. Chem. 73, 3965 (1969). 14. Jasinski, R. J., and Kirkland, S., Anal. Chem. 1663 (1966). 15. Parreira, H. C., J. Colloid InteoCace Sci. 29, 432 (1969). 16. Avicel pH 101 (a Degremont compound) is a microcrystalline cellulose from wood. 17. Lafaye, J. F., and Jacquelin, G., Peintures, Vernis, Pigments ,15, 313 (1969). 18. Grasset, E., Th&se de Docteur-Ingrnieur, Grenoble University, 1982. 19. Clark, F. M., Trans. Electrochem. Soc. 83, 143 (1943). 20. Viale, F., Samat, Y., and Metzger, G., Rev. G£n. Elect. 81, 746 (1972). 21. Madorsky, S. L., "Thermal Degradation of Organic Polymers." Interscience, New York, 1964. 22. Friedlander, B. I., Dutt, A. S., and Rapson, W. H., Pulp Paper Mag. Canad. 587 (1967). 23. Cogley, D. R., Butler, J. N., and Grunwald, E., J. Phys. Chem. 75, 1477 (1971). 24. L'Her, M., Morin-Bozec, D., and Courtot-Coupez, J., J. ElectroanaL Chem. 55, 133 (1974). 25. Yumoto, M., Takada, T., Sakai, T., and Toriyama, Y., "Ann. Report. Electr. Ins. & Diel. Phenom." National Academy of Sciences, Washington, 1978. 26. Haridoss, S., and Perlman, M. M., J. Appl. Phys. 55, 1332, 1984. 27. Terselius, B., Gedde, U. W., and Jansson J. F., Polym. Eng. Sci. 22, 422 (1982).
Journal of Colloid and Interface Science, Vol. 110, No. 1, March 1986
1. I N T R O D U C T I O N
attributed this property to a specific adsorption of ions which leads to a net charging of the paper surface. Since paper becomes charged by "picking up" ions from the impregnating liquid, it is not surprising to observe a dependence of its properties on the nature or the concentration of the electrolyte dissolved in the liquid. This constitutes a major difference with semipermeable membranes which have a constant fixed charge and a semipermeability only slightly dependent on the electrolyte concentration. The aim of the present work is to clarify the influence of different parameters such as the electrolyte nature, the water concentration of the impregnating liquid, and the thermal pretreatment of paper on the paper's semipermeability. Microelectrophoresis was used to determine the ~'potential of various paper-liquid interfaces. From this, the interfacial charge density was deduced. ~ Potential measurements were performed in a polar liquid (propylene carbonate, PC) and in a nonpolar one (cyclohexane, CH). In our study we use capacitor paper, but we also give some data on polyethylene which may be compared to paper.
Electrical properties of solid/liquid interfaces have been the subject of many studies and reviews during the last few years. They were concerned with either the electrical double layer (1) or the charge-transfer reaction (2). In most cases, a metal-aqueous solution interface is investigated. Very little experimental work and few theoretical models are available for nonconducting materials in nonaqueous liquids. It now seems that many electrical phenomena in either liquid or impregnated solid dielectrics, are caused or influenced in a very important and general way by the charged double layer which is always present at an interface. Examples of such phenomena are superficial conductivity (3, 4), ion injection by metal electrodes (5) or by insulating surfaces (6) and ion blocking by paper (7). W e have investigated this particular characteristic of paper, which gives unusual electrical properties to insulating systems containing paper (8). The electrical losses of paper impregnated with an organic liquid have been attributed to the semipermeability of paper (9). Indeed, under the influence of an electric field, ions of one sign migrate through the impreg2. MATERIALS AND METHODS nated paper whereas ions of the opposite sign Capacitor paper, kindly supplied by Bollore remain blocked at the liquid-paper interface. (10), was prepared from a pure Kraft pulp. According to the nature of the liquid or the The a-cellulose content reaches 95%. Paper electrolyte it contains, paper was found peralso contains hemicellulose, residual lignin, meable either to anions or to cations (9). We and traces of mineral compounds such as alkali, iron, and calcium salts. When exposed to the atmosphere, it rapidly reaches an equiTo whom correspondence should be sent. 102 0021-9797/86 $3.00 Copyright© 1986by AcademicPress,Inc. All rightsof reproductionin any formreserved.
Journalof ColloidandInterfaceScience,Vol. 110,No. 1, March 1986
~" P O T E N T I A L
OF CAPACITOR
librium moisture content of about 6 or 7%. By vacuum drying at 130°C for 48 h, a moisture content lower than 0.5% can be easily achieved (11). The paper particles to be studied by electrophoresis were obtained by grinding paper with a turbomixer in the degassed solvent itself. Polyethylene was supplied ground by C d F CHIMIE and the particle radii are in the range 5-50 ~m. It contained some unknown additives, e.g., antioxidants. Propylene carbonate (PC) is a polar solvent (~r = 65 at 20°C) with well-known electrical and electrochemical properties (12). In PC, electrolytes such as LiC104 and AgC104 are fully dissociated at concentrations lower than 10-1 M. On the other hand, LiC1 is an associated electrolyte in PC (association constant ga = 557 (13)). The PC was purified according to the method given by Jasinski (14). Cyclohexane was used without any further treatment. To increase its conductivity and ion content we dissolved triisoamylammonium picrate (TiAP) in it. The electrophoretic mobilities were evaluated using two different techniques. We used the microelectrophoresis method with PC solutions (15). Our flat test cell had a rectangular cross section (0.2 X 10 mm). The motion of the particles was observed in the classical way in different planes parallel to the walls of the cell, and their electrokinetic velocity was obtained in the planes of zero electroosmotic velocity. However, Parreira's cell cannot be used in very resistive solutions, so we measured the ~"potential of the paper particles in cyclohexane in a Teflon cell containing two fiat parallel metal electrodes. The electrode gap was about 3 m m and the electrodes were placed far from the Teflon walls to avoid electroosmosis. Electric fields as high as 100 V cm -1 were applied without any evidence of polarization. The motion of the paper particles in the uniform field between the electrodes was recorded with a video camera and observed on a T V screen.
PAPER
103
The calculation of the ~"potential from the value of the electrokinetic mobility # has been widely discussed (1). We use Smoluchowsky relation ~"= n#/~ (in S.I. units) for the PC case since the ratio R/Lo is greater than 1 (R is the particle radius, Lo the thickness of the diffuse layer). In cyclohexane (R/LD ~ 1) we use Hiickel's relation, ~"= 3 ~#/2e, where n and E are the viscosity and the permittivity of the liquid in S.I. units. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Sign of the Charge of the Paper in Pure PC Cellulose, even when pure, is always partially oxidized and contains carbonyl and carboxyl groups. In commercially available capacitor paper, protons of these groups which would enhance the electrical losses of paper at high temperatures (80-100°C) are replaced by divalent ions such as Zn 2+. The introduction of these ions also increases the thermal stability of the paper. In order to separate the influence of the divalent ions from that of the protons from the carboxylic groups, we measured the ~"potenrials in PC at various water concentrations of paper containing Zn 2+ ions (-0.1%) and of pure cellulose (16) which does not contain such ions (Fig. 1, curves a and b). We observed that paper and cellulose particles were always negatively charged. In aqueous solutions where paper is also found to be negatively charged (17), it is assumed that this charge originates from weakly dissociated carboxylic groups. This assumption seems valid, since the concentration of acid groups measured by titration with methylene blue is found to be in the range 10-4 to 5 X 10 -5 equivalent per gram of paper. These groups, if fully dissociated, would give paper a surface charge density some orders of magnitude higher. In this crude evaluation, we consider that the specific surface of paper is about 103 cm2/g (18).
Journal of Colloid and Interface Science, Vol. 110, No. 1, March 1986
104
GOSSE ET AL. mV) -2
10
10-1
[
1
CH20(M)
I I IIIII
-10 --20 --30 -40 -50 - -
FIG. 1. ~"Potentialofthe CP-paperinterfaceas a functionofthe waterconcentration:curvea--nonpretreated capacitorpaper; curveb--nonpretreated cellulosepowder;curve c--capacitor paper heated at 130°Cunder vacuum, for 48 h.
3.2. Effects of a Thermal Pretreatment of the Paper To verify that the negative charge of paper originates from the dissociation of carboxylic groups, we have modified their concentration by thermal pretreatment at 130°C. Paper oxidation remains a controversial subject (1921). It has been asserted that the heating of paper causes the destruction of the carboxylic acid groups which are not blocked by divalent ions. It also increases the degree of oxidation of cellulose. We observed via IR spectrophotometry that the carbonyl band was enhanced after thermal treatment of the paper under an air blanket. However, aldehyde, ketone, and carboxyl groups absorb very close to the same wavelength (---1730 cm -1) and therefore the carboxyl groups cannot be distinguished from the others (22). Thus we gained no information on the variation of the concentration of these groups after paper oxidation. The ~'potential of the paper heated at 130°C under an air blanket became positive. Its value was + 13 m V after a 2-day heating and +29 mV after 15 days. So, as expected, a thermal treatment of paper decreases its negative charge. However, it actually creates a net positive charge at the surface. The surface groups which dissociate and give a positive net charge Journal of Colloid and Interface Science, V ol. 110, No. 1, March 1986
to the paper may be the carboxylic groups which have fixed divalent ions. The dissociation reaction would be - - C O 0 ZnX - - C O 0 Z + + X-, X - is, for instance, C1-. This assumption is in agreement with our experiments on cellulose powder which does not contain divalent ions. The ~"potential of this powder remained negtive ( - - 5 8 m V at 10-2 M H 2 0 ) after being heated for 2 days at 130°C in air. If the heating is done under vacuum, the absolute value of ~"decreases less than it does in air, and always stays negative (Fig. 1, curve c). This is in agreement with the fact that the thermal degradation of cellulose is weaker in vacuum than in air.
3.3. Effect of the Water Content of PC The influence of the water content in PC on the ~"potential is very complex (Fig. 1). For paper (curve a) and cellulose (curve b), ~-decreased when the water content increased. For paper heated in vacuum at 130°C (curve c), ~ took a maximum value of about - 4 0 mV at a water concentration of 4 × 10-1 M. Thus in all cases, the value of ~'results from two simultaneous effects: --dissociation of carboxylic groups; and --specific adsorption of cations enhanced
~" POTENTIAL OF CAPACITOR PAPER by residual water. These cations might be protons, from the carbonic acid which is always present in PC which absorbs high amounts of CO2 when at equilibrium with air. When the water concentration increases, the activity of the proton of the carbonic acid is increased and adsorption of protons by paper according to the reaction scheme - - C O O H + H + ~- - C O O H f might occur. We have indeed verified this. Bubbling nitrogen in PC with 2.5 M H 2 0 made the ~"potential ~o f paper particles vary from - 10 to - 2 5 mV,
3.4. Effects of Electrolytes In propylene carbonate. First we consider paper without thermal pretreatment and electrolyte solutions in PC with a constant water concentration (3 × 10 -1 M). Figure 2 shows the dependence of the ~"potential on the different electrolyte concentrations (LiC1, LiC104, and AgC104). The ~"potential vs concentration curves for AgCIO4 (curve a) and LiC104 (curve b) are similar, the silver salt producing a more positive potential than the lithium salt. These curves are characteristic of specific adsorptions. The ~"potential, which is negative at very low electrolyte concentrations, reverses its sign at high concentrations showing a strong adsorption ofAg +
105
or Li + ions. On the other hand, the ~potential for the LiC1 electrolyte (curve c) remains negative and its dependence on LiC1 concentration may lead us to suppose that LiC1 behaves as an "indifferent electrolyte." The decrease of ~"when LiC1 concentration increases would be caused by the compression of the diffuse layer. But this assumption is in contradiction with the behavior of LiC104 and the adsorption of Li ÷ on paper. We also measured the amount of electrolyte adsorbed per gram of paper (Fig. 3). We observed that t i f l O 4 and LiC1 were actually adsorbed more strongly when the water content in PC and consequently in paper, was higher. For instance, in a solution of 10 -3 M LiCIO4 in PC, the amount of adsorbed electrolyte (M/ g of paper) was increased by a factor 10 when the water content increased from 1.5 × 10 -2 to 4 X 10 --~ M. In a solution of 10-3 M LiCl in CP, the enhancement factor was about 100. It also appears in Fig. 3 that, at a similar water concentration in CP, LiC1 is adsorbed more than is LiC104. So we conclude that the LiC1 concentration dependence of the ~-potential of paper (Fig. 2, curve c) results from a simultaneous adsorption of Li ÷ and C1-. Since the net charge remains negative, the adsorption of Li ÷ is only slightly preferential.
~ ~,(mY)
20-10--
,0-3
0 10_5
'
'
'
-10 - -20 - -30 ~ - -
:
FIG. 2. ~"Potential of the CP-paper interfacevs the electrolyteconcentration.Influenceof the nature of the electrolyte.Water concentration of."PC--3 × 10-j M; curve a--AgC104; curve b--LiC104; curve c-LiCI. Journal of Colloid and Interface Science, VoL 110, No. l, March ! 986
106
GOSSE ET AL. -4
~63
J
-®
/
10-4
I
16e _ _
1~ 7
z" + /" " 10-6
/
(
l~ ~ o~ 10-5
--10-4
.... 10-3
- - I?DI 0 -2
C(M)
FIG. 3. Adsorption isotherms of capacitor paper in PC: curve a--LiC], ].5 X 10-2 M H20; curve a'-LiC1, 4 X 10-' M H20; curve b--LiC]O4, t.5 X 10-2 M H20; CtL~e b'--LiCIO4, 4 X 10-~ M H20.
We have also investigated some other electrolytes such as tetrabutylammonium perchlorate (Bu4NCIO4) and lithium picrate (LiPi) In Bu4NC104 solutions, particles of nontreated paper were always negatively charged, whereas in 10-3 M LiPi solutions, they were positively charged. From these results, we may propose an arrangement of ions in an order of increasing affinity for paper: CIO~ < Bu4N + < C1- < Li +
electric point (~" -- 0) for an AgC104 concentration of 10-4 M ( c u r v e a) and 4.5 × 10 -5 M (curve b). The dependence of the isoelectric point on the water concentration confirms an increase in Ag + adsorption by paper when the residual water concentration increases. The variations of ~"potential with the LiC1 concentration in PC at different water concentrations (Fig. 5, curves a and b) lead to the same conclusion: the ~"potential of paper in PC is deag + . pendent on a strong adsorption of electrolytes It appears that this is also the order of in- by paper, this adsorption being enhanced by creasing affinity of the same ions for the resid- an increase in the water concentration. ual water in PC (23, 24). It has also been exPaper which has been thermally pretreated perimentally demonstrated that in PC, the in air at 130°C, was positively charged in pure cations Li + and Ag + of the electrolytes LiC104, PC. This positive charge was only slightly LiPi, and AgC104 are more solvated by the modified by the presence of an electrolyte in residual water than by the solvent. It is also PC. For instance, the ~ potential of preheated known that in organic solvents, anions are less paper remained positive whatever the AgC104 solvated than cations and more reactive than concentration (Fig. 4, curve c). With the LiC1 their solvated counterions. This is the case for electrolyte, a sign reversal was observed only at a rather high LiCI concentration (10 -2 M, C1- in PC. We measured the ~ potential of paper in which means 3 × 10 -3 M Li + and C1-) (Fig. various electrolytic solutions with different 5, curves c and d). Thus, a thermal pretreatwater concentrations. Figure 4 shows the ment of paper seems to weaken its ion-adAgC104 concentration dependence of the ~" sorption properties. In cyclohexane. In pure cyclohexane, and potential of paper for different water concentrations. The curves "a" (3 × 10 =1 M H20) also in TiAP solutions in CH, paper particles and "b" (7 × 10 -1 M H20) indicate an iso- were found either positively or negatively Journal of Colloid and Interface Science, Vol. 110, No. 1, March 1986
107
~" P O T E N T I A L OF C A P A C I T O R PAPER
3~Oj mV) 20 m
I
i
o
I
i i lllill
i0-4
I I Illl%
10-3
C(M)
-10
-20
-30 FIG. 4. AgC104 concentration dependence of the ~"potential of nonpretreated paper in PC, at various water concentrations: curve a - - 3 X 10-l M H 2 0 ; curve b - - 7 X l0 -t M H 2 0 ; curve c--1.5 X 10- 1 M H 2 0 .
charged. We observed that about the same number of particles traveled to the right and to the left side of the cell with the same speed. In this cell which has a large gap between the electrodes there is no electroosmotic flow. We checked that the motion of the particles was due to electrophoresis, i.e., their velocity was proportional to the applied voltage, and their direction of motion changed at the reversal of the electrode polarity. The ~"potential calculated using Hiickel's expression (the thickness of the diffuse layer is large compared to the particle radius ( ~ 5 ~m)) is about 100 inV. However, its values are rather scattered (Fig.
6) and slowly decrease from 104 to 82 mV when the ionic concentration in the solution increases. The surface charge density a corresponding to ~ ~ 100 mV is about 4 X 10-11 Cm -2, only 100 times smaller than in PC. The carboxylic groups by themselves cannot account for this value of a, since the particles were positively or negatively charged. It also appeared that the presence of the electrolyte TIAP in CH did not influence in an appreciable manner the ~ potential of paper. It seems that paper and its residual water, which constitute a polar dissociating medium behave as an entity independent of the nonpolar sur-
~ j my)
------L. o
20-10-0 10_5
I
I
I IIIII
[0-4
I
I
I I IIII
I
[Jl!~ c(M)
-10 -i-
-20-r--
-30 FIG. 5. Dissociated LiCI concentration dependence of the ~"potential of capacitor paper in PC. Influence of the oxidation of paper: curve a--nonpretreated paper, 3 X 10-1 M H20; curve b---nonpretreated paper, 6 X 10-I M H20; curve c - - t h e r m a l l y oxidized paper (at 130°C for 2 days), 3 × 10-1 M H20; curve d - thermally oxidized paper (at 130°C for 15 days), 2 × 10-1 M H 2 0 . Journal of Colloid and Interface Science, Vol. 110, No. 1, M a r c h 1986
108
GOSSE ET AL. mV) m
m
100
m
J
50
i
L i I ilfl
I
I J IIIJ
I 10-12
10-13
i
i I ILJ4. 0 ( 9 -lm-1)
FIG. 6. ~"Potential of paper and PE particles vs the conductivity of solutions of TiAP in cyclohexane: curve a--nonpretreated paper; curve b - - P E .
In a rather surprising way, PE which is an hydrophobic and nonpolar material, shows an electrokinetic behavior very similar to that of paper which is polar and hydrophilic. We assume that this behavior of PE is due to polar groups brought about by oxidation of its surface. Electrolytes can adsorb on these groups as on paper but to a lower extent. Different analysis of the PE surface by ESCA or ATR for instance (25-27), have confirmed a density of oxygenated groups which increases with the duration or the strength of oxidation.
rounding medium in which the electrolyte dissociation is very small. Polyethylene. We have compared results obtained with capacitor paper and with polyethylene which presents polar groups only as surface defects. In pure PC, PE particles are negatively charged. The dependence of their ~"potential on the electrolyte concentration in PC is given on Fig. 7 for LiCI (curve a) and AgC104 (curve b). In LiC1 solutions the absolute value of ~" decreases by increasing LiC1 content whereas in AgC104 solutions we get an isoelectric point at about 5 × 10-3 M. In cyclohexane, PE particles were positively charged and ~"decreased when the conductivity of the solutions increased (Fig. 6).
CONCLUSIONS
The composition of paper and especially paper used in electrical insulation is very
,,,(mY)
20-10
-
t -
0
r-
-t0 -2O --
I
i 10-5
i
i F i IIi
i ,0-4
~
I J [llf 10-3
C(M)
FIG. 7 . ~"Potential of PE particles vs the electrolyte concentration in propylene carbonate: curve a--LiC1; curve b--AgC104. Journal of Colloid and Interface Science, Vol. 110,No, 1, March 1986
~" POTENTIAL OF CAPACITOR PAPER complex and partially u n k n o w n . However, our work shows that its electrokinetic behavior in a pure polar liquid is similar to that o f pure cellulose. Its ~ potential is attributed to the weak dissociation o f carboxylic groups created by the chemical degradation o f the cellulosic chain. W h e n the nonaqueous solvent contains an electrolyte, the electrokinetic behavior o f paper is determined by a strong electrolyte adsorption, itself dependent on the residual water concentration. F r o m experiments with different electrolytes, we deduce an arrangement o f ions in an order o f increasing affinity for paper. We have noticed that the affinity o f ions for paper is correlated to the preferential solvation o f these ions by the residual water in PC. It also appeared that a thermal pretreatment o f capacitor grade paper in an air blanket ( ~ 130°C) strongly diminishes its adsorption properties. These electrokinetic measurements constitute an easy determination o f the paper surface charge density and o f the influence o f different parameters such as the electrolyte nature, the concentration o f the electrolyte or o f the residual water in the solvent. These results are in agreement with our few p r e v i o u s measurements o f the ion transport n u m b e r in paper which were interpreted by a surface charge density on paper varying with the electrolyte nature and concentration. We n o w have a better knowledge o f the influence o f different parameters on paper semipermeability. Lastly, we observed a polyethylene electrokinetic behavior similar to paper corroborating a strong superficial oxidation o f polyethylene. REFERENCES 1. Hunter, R. J., in "Colloid Science." Academic Press, New York/London, 1981.
109
2. Bockris, O'M., and Reddy, A. K. N., "Modem Electrochemistry." Plenum, New York, 1970. 3. Miranda, F. J., and Gazzana-Priaroggia,P., Proc. lEE 123, 229 (1976). 4. Saad, A., and Tobazeon, R., IEEE Trans. Electr. Insul. E-I 19, 193 (1984). 5. Denat, A., Gosse, B., and Gosse, J. P., J. Electrostatics 7, 205 (1979), 6. Saad, A., and Tobazeon, R., J. Phys. D Appl. Phys. 15, 2505 (1982). 7. Gosse, B., Gosse, J. P., and Tobazeon, R., lEE Proc. A 128, 165 (1981). 8. Parkman, N., IEEE Trans. Electr. Ins. E-I 13, 289 (1978). 9. Gosse, B., Gosse, J. P., and Sauviat, M., IEEE Trans. Electr. Ins. E-I 15, 104 (1980). 10. Bollore, Papeteries Cascadec, 29, Scaer, France. 11. Kelk, E., and Wilson, O., Proc. lEE 112, 602 (1965). 12. Gosse, B., Gosse, J. P., and Felici, N., J. AppL Electrochem. 5, 329 (1975). 13. Mukkerjee, L. M., and Boden, D. P., J. Phys. Chem. 73, 3965 (1969). 14. Jasinski, R. J., and Kirkland, S., Anal. Chem. 1663 (1966). 15. Parreira, H. C., J. Colloid InteoCace Sci. 29, 432 (1969). 16. Avicel pH 101 (a Degremont compound) is a microcrystalline cellulose from wood. 17. Lafaye, J. F., and Jacquelin, G., Peintures, Vernis, Pigments ,15, 313 (1969). 18. Grasset, E., Th&se de Docteur-Ingrnieur, Grenoble University, 1982. 19. Clark, F. M., Trans. Electrochem. Soc. 83, 143 (1943). 20. Viale, F., Samat, Y., and Metzger, G., Rev. G£n. Elect. 81, 746 (1972). 21. Madorsky, S. L., "Thermal Degradation of Organic Polymers." Interscience, New York, 1964. 22. Friedlander, B. I., Dutt, A. S., and Rapson, W. H., Pulp Paper Mag. Canad. 587 (1967). 23. Cogley, D. R., Butler, J. N., and Grunwald, E., J. Phys. Chem. 75, 1477 (1971). 24. L'Her, M., Morin-Bozec, D., and Courtot-Coupez, J., J. ElectroanaL Chem. 55, 133 (1974). 25. Yumoto, M., Takada, T., Sakai, T., and Toriyama, Y., "Ann. Report. Electr. Ins. & Diel. Phenom." National Academy of Sciences, Washington, 1978. 26. Haridoss, S., and Perlman, M. M., J. Appl. Phys. 55, 1332, 1984. 27. Terselius, B., Gedde, U. W., and Jansson J. F., Polym. Eng. Sci. 22, 422 (1982).
Journal of Colloid and Interface Science, Vol. 110, No. 1, March 1986