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Surface polarity of carbon blacks
SURFACE POLARITY OF CARBON BLACKS S. HAGIWARA,K. TSUTWMI and H. TAKAHASHI Institute of Industrial Science, University of Tokyo 22-1, Roppongi IIchome, Minato-ku, Tokyo 106,Japan
(Receiwd 10Juty 1977) Abstract-Furnace and channel blacks and furnace blacks oxidized by using air, ozone, hydrogen peroxide and nitric acid were examined to the calorimetric study of immersional heats in organic solvents with different dipole moments, from which was derived their surface electrostatic field strength. Two kinds of linear relations were found between the surface electrostatic tiefd strengths and the surface active hydrogen contents of carbon blacks; one is for channel blacks and furnace blacks oxidized by air, having a coefficient of 1.5.0x10’ e.s.u.iequiv.-active hydrogen, and another for furnace blacks and those oxidized by ozone, hydrogen peroxide and nitric acid having a coefficient of 2.9 x 10’.Correlations were also found between the pH of carbon black-water slurries and the surface electrostatic field strengths except for furnace black oxidized by using air. This is in contradiction to a single linear relation holding between the immersional heats into water and the surface active hydrogen contents described in a previous paper18], and suggests that the a~y~ous oxide capable of repeating hy~ati~~ehy~tion, such as tarboxylic acid anhydride, exists on the surface of furnace blacks oxidized by air.
1. INTRODUCTION
carbon blacks containing various amounts of active hydrogen into organic solvents such as n-heptane, nbutylchlo~de and I-uitropropane and average surface electrostatic field strengths of the specimens were calculated. The wetting tendencies of carbon blacks thus obtained were described in relation to the active hydrogen contents measured by Zerewitino~s method and to the pH of slurries. In order to evaluate the effect of chemical modification on the surface properties, the same measurements was extended over furnace black specimens oxidized by using air, ozone, nitric acid, or hydrogen peroxide.
The surface polarity of micr~~s~li~e carbons is an intluential factor on their wettability for other materials. This property closely relates to surface functional groups. Surface carbon atoms in the basal planes of graphite crystal are relatively chemically inactive and hydrophobic, while surface edge carbon atoms on the prism planes are highly reactive. Therefore, surface functional groups are formed on edge carbon atoms. Such polar functional groups as carboxylic acids, hydroxides, quinones and lactones play important roles in enhancing wetting by water, while carbons without polar groups can be easily dispersed into aromatic hydrocarbons and pa&ins. When carbon blacks are heated up to 3WC, the surface of their particles is composed of basal planes. Such carbons can be dispersed in somatic solvents, but not in water. The above phenomena have been extensively investigated by the measurement of immersional heats in various liquids. The immersional heat of carbon blacks is decreased by removal of volatile components such as COz, CO and H, which are generated from oxygenated groups on the blacks during heat treatment f 1-51.Garrett et al.[6] have pointed out that the dispersion of black in linseed oil is improved by an increase in the content of volatile components in the black. Lyon[7] has reported that the ~uidity of ink in which carbon blacks are dispersed is related to the content of volatile components. Immersional heat measurement of carbon blacks is thus a useful method for revealing their surface wettability and, therefore, their applicabilities. The present authors have previously reported on the immersional heats of carbon blacks in water as a function of active hydrogen contents. Based on a linear relation observed between these two properties, it has been suggested that a major part of the immersional heats is due to the formation of oxonium ions by the proton transfer from surface functional groups such as carboxyls or phenolic hydroxyls to water[8]. In the present study, the immersional heat was measured of
2. EXPERIMEN-TALDETAIJS Materials Five co~erci~ carbon blacks were used: ~tsubishi No. 100 (LFC from Mitsubishi Chemical Co.), DiablackH (HAF from Mitsubishi Chemical Co.), Philblack(HAF from Phillips Petroleum Co.), Peerless-155 (LFF from Columbian Carbon Co.) and RCF (from Mitsubishi Chemical Co.). Mitsubishi No. 100 and Diablack-H were treated with pyridine to remove tarry materials on the surface. Some of these two blacks were also graphitized by heating at 3ooo”C in an inert atmosphere for the purpose of complete removal of surface functional groups and the obt~ning of a homogeneous surface. RCF was treated with selected oxidizing agents. It was boiled in 31.2 wt.% hydrogen peroxide at 50°C for 1hr and then dried. In another solution of 4N nitric acid, oxidation was conducted at 60°C for 1 hr, followed by rinsing with water and drying. Oxidation by air was conducted at 250°C for 3 hr or at 400°C for 3 hr. Ozone generated in an air stream by an ozonizer was also applied to the same blacks for oxidation at room temperature until weight increased between 3% and 5% over that of the original value. Each specimen was evacuated under lo-’ Torr at 120°C for 24 hr prior to making measurements. For the calorimetric determination of immersional heats, the 2.1
89
90
S. HAGIW~ etal.
specimen was sealed in a glass ampoule after evacuation prior to measurement. n-Heptane, n-butylchloride and I-nitropropane were of guaranteed grade and further purified by repeated distillation and dehydration.
The immersional heats were measured by a twin conduction type microcalorimeter (Tokyo RIKO Co., Ltd.). The microcalorimeter consists of two units of the single type conduction microcalorimeter which are constructed as similarly as possible and placed sy~e~c~y from a mechanical and thermal point of view. Electrical connection is made in such a way as to cancel thermoelectromotive forces of each unit. In order to cancel the heats generated with a breakage of the glass ampoule, an empty ampoule was dipped in a solvent in another cell. Some granules of molecular sieve 3A were added in the solvent to remove water absorbed from the atmosphere. The solvents were kept standing until they reached thermal equilibrium at 25°C. The ampoules were simultaneously broken, the sample was immersed in the solvent, and the liberated heat measured. The active hydrogen content was determined by Zere~tino~s method as modified by Suzuki et a1,[9]. After samples were evacuated in a glass reactor as described above, the reactor was filled with dry argon. Then a 3M solution of methyl magnesium iodide in isoamylether was injected into the specimen through a serum cap. Methane was evolved by reaction of the reagent with the active hydrogen on the carbon black. After sufficient contact time, a certain amount of helium was added as an internal standard, and a portion of the methane-helium-argon mixture was sampled and analysed by gas chromatography. The active hydrogen content of the specimen was calculated from the amount of methane evolved. The pH of the slurries was measured as follows: the specimen was added to distilled water, boiled for 15 min and then cooled to room temperature with a caution to avoid the effect of carbon dioxide in the air. The BET surface area was measured by nitrogen adsorption at - l%“C. The pore size dist~bution was analysed from the nitrogen adsorption data at -l%“C by employing Inkley’s method [lo] and V,-t plots [ 111.
50
L
I I
0
I
2 Otfmle moment, D
I
3
4
Fig. 1. Heats of immersion at 25°Cof O,-oxidizedcarbon blacks as a function of dipole moment of wetting liquids. 0, RCF original; A, RCF O,-oxidized to 3 wt.% increase: 0, RCF 01oxidized to 5 wt.% increase.
with pyridine liberate larger heats than those liberated as a result of surface interaction with the original blacks, probably due to removal of non-polar fragments from the black surface. On the contrary, graphitized Diablack-H showed an extremely low electrostatic field strength, consistent with the homopolar nature on the surface and large dispersion-polarization energies. The electrostatic field strength and the dispersion-~l~ization energies of graphitized ~tsubishi No. X00 were nearly equal to those of HAF grade carbon blacks, presumably due to surface roughness of the original black. Table 1 presents the average surface electrostatic field strengths, dispersion-polarization energies, active hydrogen contents, pH of slurries and specific surface area of various carbon blacks. The immersional heats into water, also presented in this Table, have already been discussed in relation to active hydrogen contents in a short letter[8]. The electrostatic field strengths are correlated with active hydrogen content in two linear modes as shown in Fig. 2. 8 I St
3. RESULTS The immersional heats of carbon blacks into organic
liquids increased linearly with an increase of dipole moment of the liquids (Fig. 1). The organic liquids used have the following dipole moments: OD (n-heptane), 2.05 D (n-butylchloride) and 3.57 D (I-nitropropane). The slope of this diagram gives the average surface electrostatic field strength and the intercept gives dispersion-polarization energies on the surface[ 121. A colour channel black, ~tsubishi No. 100, showed an electrostatic field strength as high as 4.48~ lo* e.s.u./cm2, strongest among the commercial blacks though the active hydrogen content was less and the pH of the slurry was larger than those of Peerless-155 The specimens of Mitsubishi No. 100 and Diablack-H treated
Acttve
hydrogen content, mequWln2 X IO3
Fig. 2. Correlation between average electrostatic field strength and active hydrogen content of various carbon blacks. 0. Furnace black; A, Channel black; 0, Air-oxidized furnace black; A, O,-oxidized furnace black: V, HNO,-oxidized furnace black; q, H,O,-oxidized furnace black.
91
Surface polarity of carbon blacks
Table 1. Selected properties of carbon blacks and their water slurries
Surface area Specimens --
W/g)
Active hydrogen content ( IO3mequiv./m’)
Average electrostatic field strength (W
e.s.u./cm’)
Dispersionpolarization
Heats of immersion
energy (erg/cm*)
into water (erg/cm’)
pH of Slurry
87.4 96.0 30.0 90.8 121.3 193.0 336 203.6 137.6
7.18 7.50 7.65 4.45 4.18 2.83 4.18
Diablack-H (HAF) extracted with pyridine heat-treated at 3000°C Philblack(HAF) Mitsubishi No. 100 (LFC) extracted with pyridine heat-treated at 3000°C Peerless-155 (LFF) extracted with pyridine
82.0 79.6 65.9 82.7 94.8 95.4 65.7 129.7 133.0
0.137 0.227 0.147 0.319 0.378 0.974 0.712
0.26 0.25 0 0.15 4.48 6.39 0.28 2 55 1.48
116.0 1177 121.2 101.5 88.1 78.4 116.9 103.9 102.4
RCF HNO,-oxidized at 60°C H,O,-oxidized at 50°C air-oxidized at 250°C air-oxidized at 400°C O,-oxidized to 3 wt.% increase O,-oxidized to 5 wt.% increase
99.5 111.6 107.8 127.7 366.8 99.6 92.2
0.023 0.400 0.455 0.082 0.209 0.643 1.616
0.58 0.90 1.54 1.57 1.99 1.65 4.50
111.7 119.8 111.6 102.7 105.9 111.2 118.0
Channel blacks and air-oxidized furnace blacks show a steep increase of electrostatic field strength with an increase of active hydrogen content, which are designated as the first group in the figure. Original furnace blacks and those oxidized by ozone, hydrogen peroxide and nitric acid show a smaller increase, being designated as the second group. The slopes of these relations were 15.0x IO’ and 2.9 x 10’ e.s.u./equiv.-active hydrogen,
respectively. Most of the active hydrogen detected by Zerewitinoff’s method on carbon black surfaces are considered to have acidic character, either strong or weak, and the acidity of carbon black slurries would be ascribed to them. Figure 3 presents the pH of slurries as a function of the electrostatic field strength. These plots seem to correspond to the two groups present in Fig. 2, except that furnace blacks oxidized by air were classified to the 1st group in Fig. 2 and to the 2nd group in Fig. 3. A correlation between the surface density of active hydrogen and the pH of slurries is shown in Fig. 4, where, channel blacks and furnace blacks oxidized by ozone, hydrogen peroxide and nitric acid show higher
66.6 -
7.81 3.40 3.50 4.59 2.98 3.22 2.75
OH
0 Acwe
I
I
I
05
IO
15
hydrogen
content,
mequ~v /m2
i
x IO3
Fir. 4. pH Value as a function of active hydrogen content of various carbon blacks. 0, Furnace black: A. Channel black. 0, Air-oxidized furnace black: A. O,-oxidized furnace black: 0. HNO,-oxidized furnace black; 0, H,O?-oxidized furnace black.
surface densities of active hydrogen and moderate acidities of slurries. On the other hand, furnace blacks oxidized by using air show stronger acidity of slurries in spite of a lower surface density of active hydrogen. The oxidation of RCF thus results in different changes of properties in accordance with the nature of the oxidizing agents. It is noted that the specific surface area is more greatly increased, and the surface density of active hydrogen is less greatly increased by oxidation with air than by other oxidizing agents. The pH of the slurry was lowered to an equal extent by all oxidizing agents. 4. DISCUSSION
Fig. 3. pH Value as a function of average electrostatic field stength of various carbon blacks. 0, Furnace black; A, Channel black; l, Aii-oxidized furnace black; A, 0,oxidized furnace black: V. HNO,-oxidized furnace black: 0. H?O,-oxidized furnace black.
According to the theory proposed by Zettlemoyer et al. [12], the correlating coefficient between the immersional heats of solids in organic liquids and the dipole moments of liquids is defined as the average surface electrostatic field strength of solids. This definition is based on the assumption that the experimentally determined heats per unit area can be expressed the sum of a
92
S. HAGGIWARA ef al.
dispersion force contribution, a polarization force contribution and a contribution due to the interaction between the permanent dipole on the solid surface and the dipole of the liquid. The cross-sectional area of the liquid molecules is supposed to be 25 AZ. It is apparent that the average surface electrostatic field strength becomes zero when a furnace black is heat-treated at 3000°C, while the dispersion and polarization energies increase. This result could be interpreted by assuming that the molecules of solvents are oriented so as to direct the paraflinic ends toward the surface of the black, because the homopolar graphitized surface should have a larger ai%nity for the partinic ends than for the functionated ends. The electrostatic field strength of the channel black graphitized under the same conditions was nearly equal to those of HAF grade carbon blacks, though it had been decreased as compared with those of the original black. Such a phenomenon would result from the fact that the surface is not completely homogenized by graphitization due to the surface roughness of the original black. The surface electrostatic field strength of the carbon black is considered to originate from the surface functional groups including active hydrogens. The most significant point in the results is that the dependence of the surface electrostatic field strength on the surface density of active hydrogen can be classified into two categories, i.e. a relation with the correlating c~~cient of 15.0x 10’ e.s.u./equiv.-active hydrogen for channel blacks and furnace blacks oxidized by using air, and another one with the coefficient of 2.9 x 10’ e.s.u./equiv.active hydrogen for furnace blacks and furnace blacks oxidized by ozone, hydrogen peroxide and nitric acid. A furnace black graphitized has no active hydrogen and has the least surface electrostatic field strength; it is situated at the intercept of the two relations in Fig. 2. Two groups are also found in the relation between the pH of the slurries and the surface electrostatic field strength. These phenomena would relate to the kind of functional groups on the surface. The acidic groups in the pH range below 7.0 would be mainly carboxylic acids and not phenols, because the latter groups have generally weaker acidic sites than & 7.0 and they do not dissociate in that pH range. On the other hand, the active hydrogen originates from either carboxylic acids, phenols and or alcohols. From these considerations, channel blacks which showed weaker acidity in the slurry, stronger surface electrostatic field strength and less active hydrogen content than others would become polar owing to an existence of non-acidic or weakly acidic surface functional groups such as ketones, quinones, phenols and a small amount of strongly acidic groups. The carbon blacks in the second group would have more strongly acidic groups, but less non-acidic or weakly acidic polar groups. These relations were in contrast with a linear dependence of the immersional heats of carbon blacks into water on the surface densities of active hydrogens. This discrepancy would be caused by the difference in the heat evolution mechanisms. The immersional heats in water are produced by the protonation of water molecules induced by active hydrogens, while the heats in organic solvents
are generated by an electrostatic dipole-dipole interaction taking place between the solvent molecules and the carbon surface. The behavior of furnace blacks oxidized by air (shown in Figs. 24, is very peculiar, and suggests that the functional groups in carboxyls contribute to the surface acidity through hydration, having an electrostatic field such as carboxylic acid anhydride on the surface. The pore size dis~bution of RCF and of its airoxidized speciments obtained by using Inkley’s method and V,-t plots are shown in Figs. 5 and 6. As shown there, the higher the temperature of oxidation, the more the surface roughness. The specific surface area of the 250°C~air-oxidized specimen is larger than that of the original sample. The roughness of the former is, nevertheless, nearly equal to that of the latter, and hence the roughness is not responsible for the peculiar behavior mentioned above. The effects of oxidation on the surface properties of carbon blacks differ in accordance with the nature of oxidizing agents used, probably due to differences in the mechanism of the oxidation reactions. The results of oxidations will be described in detail in the near future.
Pore diameter, 8
Fig. 5. Pore size distribution of air-oxidizedcarbon blacks by Inkley’s procedure. -, Original RCF; ----, RCF air-oxidized at 250°C for 3hr: -----, RCF air-oxidized at 4OO*Cfor 3 hr.
Fig, 6. V,-f plot of air-oxidized carbon blacks. -. Original RCF air-oxidized at 250°C for 3 hr; -----, RCF RCF; ----, air-oxidized at 400°Cfor 3 hr.
Surface polarity of carbon blacks
93
The immersional heats of carbon blacks into organic solvents thus exhibit the surface electrostatic nature which is related to the chemical states of the surface. The present study has shown the characteristic difference between channel blacks and furnace blacks and the difference in the oxidation effects of various
3. W. H. Wade, J. Colloid Interface Sci. 31, 111(1%9). 4. S. S. Barton and B. H. Harrison,Carbon 10,245 (1972).
agents on the surface properties of the furnace black.
8. S. Hagiwara, K. Tsutsumi and H. Takahashi, Carbon 9,693 (1971). 9. S. Suzuki and K. Miyazaki, Nippon Kagaku Zasshi 87, 1303
REFERENCES
1. G. J. Kraus, Phys. Chem. 57, 343 (1954). 2. B. R. Puri, D. D. Singh and L. R. Sharma, J. Phys. Chem. 62, 756 (1958).
5. S. S. Barton, G. L. Boultonand B. H. Harrison, Carbon 10, 391(1972). 6. M. D. Garrett and H. J. Zaborsky, Am. Ink Maker 47(3),24 (1%9). 7. F. Lyon and W. Stoy, Paint Manuf. 36(4), 36 (1966).
(1966). 10. R. W. Cranston and F. A. Jnkley, Adu. Cot. 9, 143(1957). 11. J. H. de Boer, B. G. Linsen, Th. van der Plass and G. J. Zondervan, J. Cat. 4,649 (1%5). 12. A. C. Zettlemoyer, Ind. Engng Chem. 57(2), 27 (1%5).
(Receiwd 10Juty 1977) Abstract-Furnace and channel blacks and furnace blacks oxidized by using air, ozone, hydrogen peroxide and nitric acid were examined to the calorimetric study of immersional heats in organic solvents with different dipole moments, from which was derived their surface electrostatic field strength. Two kinds of linear relations were found between the surface electrostatic tiefd strengths and the surface active hydrogen contents of carbon blacks; one is for channel blacks and furnace blacks oxidized by air, having a coefficient of 1.5.0x10’ e.s.u.iequiv.-active hydrogen, and another for furnace blacks and those oxidized by ozone, hydrogen peroxide and nitric acid having a coefficient of 2.9 x 10’.Correlations were also found between the pH of carbon black-water slurries and the surface electrostatic field strengths except for furnace black oxidized by using air. This is in contradiction to a single linear relation holding between the immersional heats into water and the surface active hydrogen contents described in a previous paper18], and suggests that the a~y~ous oxide capable of repeating hy~ati~~ehy~tion, such as tarboxylic acid anhydride, exists on the surface of furnace blacks oxidized by air.
1. INTRODUCTION
carbon blacks containing various amounts of active hydrogen into organic solvents such as n-heptane, nbutylchlo~de and I-uitropropane and average surface electrostatic field strengths of the specimens were calculated. The wetting tendencies of carbon blacks thus obtained were described in relation to the active hydrogen contents measured by Zerewitino~s method and to the pH of slurries. In order to evaluate the effect of chemical modification on the surface properties, the same measurements was extended over furnace black specimens oxidized by using air, ozone, nitric acid, or hydrogen peroxide.
The surface polarity of micr~~s~li~e carbons is an intluential factor on their wettability for other materials. This property closely relates to surface functional groups. Surface carbon atoms in the basal planes of graphite crystal are relatively chemically inactive and hydrophobic, while surface edge carbon atoms on the prism planes are highly reactive. Therefore, surface functional groups are formed on edge carbon atoms. Such polar functional groups as carboxylic acids, hydroxides, quinones and lactones play important roles in enhancing wetting by water, while carbons without polar groups can be easily dispersed into aromatic hydrocarbons and pa&ins. When carbon blacks are heated up to 3WC, the surface of their particles is composed of basal planes. Such carbons can be dispersed in somatic solvents, but not in water. The above phenomena have been extensively investigated by the measurement of immersional heats in various liquids. The immersional heat of carbon blacks is decreased by removal of volatile components such as COz, CO and H, which are generated from oxygenated groups on the blacks during heat treatment f 1-51.Garrett et al.[6] have pointed out that the dispersion of black in linseed oil is improved by an increase in the content of volatile components in the black. Lyon[7] has reported that the ~uidity of ink in which carbon blacks are dispersed is related to the content of volatile components. Immersional heat measurement of carbon blacks is thus a useful method for revealing their surface wettability and, therefore, their applicabilities. The present authors have previously reported on the immersional heats of carbon blacks in water as a function of active hydrogen contents. Based on a linear relation observed between these two properties, it has been suggested that a major part of the immersional heats is due to the formation of oxonium ions by the proton transfer from surface functional groups such as carboxyls or phenolic hydroxyls to water[8]. In the present study, the immersional heat was measured of
2. EXPERIMEN-TALDETAIJS Materials Five co~erci~ carbon blacks were used: ~tsubishi No. 100 (LFC from Mitsubishi Chemical Co.), DiablackH (HAF from Mitsubishi Chemical Co.), Philblack(HAF from Phillips Petroleum Co.), Peerless-155 (LFF from Columbian Carbon Co.) and RCF (from Mitsubishi Chemical Co.). Mitsubishi No. 100 and Diablack-H were treated with pyridine to remove tarry materials on the surface. Some of these two blacks were also graphitized by heating at 3ooo”C in an inert atmosphere for the purpose of complete removal of surface functional groups and the obt~ning of a homogeneous surface. RCF was treated with selected oxidizing agents. It was boiled in 31.2 wt.% hydrogen peroxide at 50°C for 1hr and then dried. In another solution of 4N nitric acid, oxidation was conducted at 60°C for 1 hr, followed by rinsing with water and drying. Oxidation by air was conducted at 250°C for 3 hr or at 400°C for 3 hr. Ozone generated in an air stream by an ozonizer was also applied to the same blacks for oxidation at room temperature until weight increased between 3% and 5% over that of the original value. Each specimen was evacuated under lo-’ Torr at 120°C for 24 hr prior to making measurements. For the calorimetric determination of immersional heats, the 2.1
89
90
S. HAGIW~ etal.
specimen was sealed in a glass ampoule after evacuation prior to measurement. n-Heptane, n-butylchloride and I-nitropropane were of guaranteed grade and further purified by repeated distillation and dehydration.
The immersional heats were measured by a twin conduction type microcalorimeter (Tokyo RIKO Co., Ltd.). The microcalorimeter consists of two units of the single type conduction microcalorimeter which are constructed as similarly as possible and placed sy~e~c~y from a mechanical and thermal point of view. Electrical connection is made in such a way as to cancel thermoelectromotive forces of each unit. In order to cancel the heats generated with a breakage of the glass ampoule, an empty ampoule was dipped in a solvent in another cell. Some granules of molecular sieve 3A were added in the solvent to remove water absorbed from the atmosphere. The solvents were kept standing until they reached thermal equilibrium at 25°C. The ampoules were simultaneously broken, the sample was immersed in the solvent, and the liberated heat measured. The active hydrogen content was determined by Zere~tino~s method as modified by Suzuki et a1,[9]. After samples were evacuated in a glass reactor as described above, the reactor was filled with dry argon. Then a 3M solution of methyl magnesium iodide in isoamylether was injected into the specimen through a serum cap. Methane was evolved by reaction of the reagent with the active hydrogen on the carbon black. After sufficient contact time, a certain amount of helium was added as an internal standard, and a portion of the methane-helium-argon mixture was sampled and analysed by gas chromatography. The active hydrogen content of the specimen was calculated from the amount of methane evolved. The pH of the slurries was measured as follows: the specimen was added to distilled water, boiled for 15 min and then cooled to room temperature with a caution to avoid the effect of carbon dioxide in the air. The BET surface area was measured by nitrogen adsorption at - l%“C. The pore size dist~bution was analysed from the nitrogen adsorption data at -l%“C by employing Inkley’s method [lo] and V,-t plots [ 111.
50
L
I I
0
I
2 Otfmle moment, D
I
3
4
Fig. 1. Heats of immersion at 25°Cof O,-oxidizedcarbon blacks as a function of dipole moment of wetting liquids. 0, RCF original; A, RCF O,-oxidized to 3 wt.% increase: 0, RCF 01oxidized to 5 wt.% increase.
with pyridine liberate larger heats than those liberated as a result of surface interaction with the original blacks, probably due to removal of non-polar fragments from the black surface. On the contrary, graphitized Diablack-H showed an extremely low electrostatic field strength, consistent with the homopolar nature on the surface and large dispersion-polarization energies. The electrostatic field strength and the dispersion-~l~ization energies of graphitized ~tsubishi No. X00 were nearly equal to those of HAF grade carbon blacks, presumably due to surface roughness of the original black. Table 1 presents the average surface electrostatic field strengths, dispersion-polarization energies, active hydrogen contents, pH of slurries and specific surface area of various carbon blacks. The immersional heats into water, also presented in this Table, have already been discussed in relation to active hydrogen contents in a short letter[8]. The electrostatic field strengths are correlated with active hydrogen content in two linear modes as shown in Fig. 2. 8 I St
3. RESULTS The immersional heats of carbon blacks into organic
liquids increased linearly with an increase of dipole moment of the liquids (Fig. 1). The organic liquids used have the following dipole moments: OD (n-heptane), 2.05 D (n-butylchloride) and 3.57 D (I-nitropropane). The slope of this diagram gives the average surface electrostatic field strength and the intercept gives dispersion-polarization energies on the surface[ 121. A colour channel black, ~tsubishi No. 100, showed an electrostatic field strength as high as 4.48~ lo* e.s.u./cm2, strongest among the commercial blacks though the active hydrogen content was less and the pH of the slurry was larger than those of Peerless-155 The specimens of Mitsubishi No. 100 and Diablack-H treated
Acttve
hydrogen content, mequWln2 X IO3
Fig. 2. Correlation between average electrostatic field strength and active hydrogen content of various carbon blacks. 0. Furnace black; A, Channel black; 0, Air-oxidized furnace black; A, O,-oxidized furnace black: V, HNO,-oxidized furnace black; q, H,O,-oxidized furnace black.
91
Surface polarity of carbon blacks
Table 1. Selected properties of carbon blacks and their water slurries
Surface area Specimens --
W/g)
Active hydrogen content ( IO3mequiv./m’)
Average electrostatic field strength (W
e.s.u./cm’)
Dispersionpolarization
Heats of immersion
energy (erg/cm*)
into water (erg/cm’)
pH of Slurry
87.4 96.0 30.0 90.8 121.3 193.0 336 203.6 137.6
7.18 7.50 7.65 4.45 4.18 2.83 4.18
Diablack-H (HAF) extracted with pyridine heat-treated at 3000°C Philblack(HAF) Mitsubishi No. 100 (LFC) extracted with pyridine heat-treated at 3000°C Peerless-155 (LFF) extracted with pyridine
82.0 79.6 65.9 82.7 94.8 95.4 65.7 129.7 133.0
0.137 0.227 0.147 0.319 0.378 0.974 0.712
0.26 0.25 0 0.15 4.48 6.39 0.28 2 55 1.48
116.0 1177 121.2 101.5 88.1 78.4 116.9 103.9 102.4
RCF HNO,-oxidized at 60°C H,O,-oxidized at 50°C air-oxidized at 250°C air-oxidized at 400°C O,-oxidized to 3 wt.% increase O,-oxidized to 5 wt.% increase
99.5 111.6 107.8 127.7 366.8 99.6 92.2
0.023 0.400 0.455 0.082 0.209 0.643 1.616
0.58 0.90 1.54 1.57 1.99 1.65 4.50
111.7 119.8 111.6 102.7 105.9 111.2 118.0
Channel blacks and air-oxidized furnace blacks show a steep increase of electrostatic field strength with an increase of active hydrogen content, which are designated as the first group in the figure. Original furnace blacks and those oxidized by ozone, hydrogen peroxide and nitric acid show a smaller increase, being designated as the second group. The slopes of these relations were 15.0x IO’ and 2.9 x 10’ e.s.u./equiv.-active hydrogen,
respectively. Most of the active hydrogen detected by Zerewitinoff’s method on carbon black surfaces are considered to have acidic character, either strong or weak, and the acidity of carbon black slurries would be ascribed to them. Figure 3 presents the pH of slurries as a function of the electrostatic field strength. These plots seem to correspond to the two groups present in Fig. 2, except that furnace blacks oxidized by air were classified to the 1st group in Fig. 2 and to the 2nd group in Fig. 3. A correlation between the surface density of active hydrogen and the pH of slurries is shown in Fig. 4, where, channel blacks and furnace blacks oxidized by ozone, hydrogen peroxide and nitric acid show higher
66.6 -
7.81 3.40 3.50 4.59 2.98 3.22 2.75
OH
0 Acwe
I
I
I
05
IO
15
hydrogen
content,
mequ~v /m2
i
x IO3
Fir. 4. pH Value as a function of active hydrogen content of various carbon blacks. 0, Furnace black: A. Channel black. 0, Air-oxidized furnace black: A. O,-oxidized furnace black: 0. HNO,-oxidized furnace black; 0, H,O?-oxidized furnace black.
surface densities of active hydrogen and moderate acidities of slurries. On the other hand, furnace blacks oxidized by using air show stronger acidity of slurries in spite of a lower surface density of active hydrogen. The oxidation of RCF thus results in different changes of properties in accordance with the nature of the oxidizing agents. It is noted that the specific surface area is more greatly increased, and the surface density of active hydrogen is less greatly increased by oxidation with air than by other oxidizing agents. The pH of the slurry was lowered to an equal extent by all oxidizing agents. 4. DISCUSSION
Fig. 3. pH Value as a function of average electrostatic field stength of various carbon blacks. 0, Furnace black; A, Channel black; l, Aii-oxidized furnace black; A, 0,oxidized furnace black: V. HNO,-oxidized furnace black: 0. H?O,-oxidized furnace black.
According to the theory proposed by Zettlemoyer et al. [12], the correlating coefficient between the immersional heats of solids in organic liquids and the dipole moments of liquids is defined as the average surface electrostatic field strength of solids. This definition is based on the assumption that the experimentally determined heats per unit area can be expressed the sum of a
92
S. HAGGIWARA ef al.
dispersion force contribution, a polarization force contribution and a contribution due to the interaction between the permanent dipole on the solid surface and the dipole of the liquid. The cross-sectional area of the liquid molecules is supposed to be 25 AZ. It is apparent that the average surface electrostatic field strength becomes zero when a furnace black is heat-treated at 3000°C, while the dispersion and polarization energies increase. This result could be interpreted by assuming that the molecules of solvents are oriented so as to direct the paraflinic ends toward the surface of the black, because the homopolar graphitized surface should have a larger ai%nity for the partinic ends than for the functionated ends. The electrostatic field strength of the channel black graphitized under the same conditions was nearly equal to those of HAF grade carbon blacks, though it had been decreased as compared with those of the original black. Such a phenomenon would result from the fact that the surface is not completely homogenized by graphitization due to the surface roughness of the original black. The surface electrostatic field strength of the carbon black is considered to originate from the surface functional groups including active hydrogens. The most significant point in the results is that the dependence of the surface electrostatic field strength on the surface density of active hydrogen can be classified into two categories, i.e. a relation with the correlating c~~cient of 15.0x 10’ e.s.u./equiv.-active hydrogen for channel blacks and furnace blacks oxidized by using air, and another one with the coefficient of 2.9 x 10’ e.s.u./equiv.active hydrogen for furnace blacks and furnace blacks oxidized by ozone, hydrogen peroxide and nitric acid. A furnace black graphitized has no active hydrogen and has the least surface electrostatic field strength; it is situated at the intercept of the two relations in Fig. 2. Two groups are also found in the relation between the pH of the slurries and the surface electrostatic field strength. These phenomena would relate to the kind of functional groups on the surface. The acidic groups in the pH range below 7.0 would be mainly carboxylic acids and not phenols, because the latter groups have generally weaker acidic sites than & 7.0 and they do not dissociate in that pH range. On the other hand, the active hydrogen originates from either carboxylic acids, phenols and or alcohols. From these considerations, channel blacks which showed weaker acidity in the slurry, stronger surface electrostatic field strength and less active hydrogen content than others would become polar owing to an existence of non-acidic or weakly acidic surface functional groups such as ketones, quinones, phenols and a small amount of strongly acidic groups. The carbon blacks in the second group would have more strongly acidic groups, but less non-acidic or weakly acidic polar groups. These relations were in contrast with a linear dependence of the immersional heats of carbon blacks into water on the surface densities of active hydrogens. This discrepancy would be caused by the difference in the heat evolution mechanisms. The immersional heats in water are produced by the protonation of water molecules induced by active hydrogens, while the heats in organic solvents
are generated by an electrostatic dipole-dipole interaction taking place between the solvent molecules and the carbon surface. The behavior of furnace blacks oxidized by air (shown in Figs. 24, is very peculiar, and suggests that the functional groups in carboxyls contribute to the surface acidity through hydration, having an electrostatic field such as carboxylic acid anhydride on the surface. The pore size dis~bution of RCF and of its airoxidized speciments obtained by using Inkley’s method and V,-t plots are shown in Figs. 5 and 6. As shown there, the higher the temperature of oxidation, the more the surface roughness. The specific surface area of the 250°C~air-oxidized specimen is larger than that of the original sample. The roughness of the former is, nevertheless, nearly equal to that of the latter, and hence the roughness is not responsible for the peculiar behavior mentioned above. The effects of oxidation on the surface properties of carbon blacks differ in accordance with the nature of oxidizing agents used, probably due to differences in the mechanism of the oxidation reactions. The results of oxidations will be described in detail in the near future.
Pore diameter, 8
Fig. 5. Pore size distribution of air-oxidizedcarbon blacks by Inkley’s procedure. -, Original RCF; ----, RCF air-oxidized at 250°C for 3hr: -----, RCF air-oxidized at 4OO*Cfor 3 hr.
Fig, 6. V,-f plot of air-oxidized carbon blacks. -. Original RCF air-oxidized at 250°C for 3 hr; -----, RCF RCF; ----, air-oxidized at 400°Cfor 3 hr.
Surface polarity of carbon blacks
93
The immersional heats of carbon blacks into organic solvents thus exhibit the surface electrostatic nature which is related to the chemical states of the surface. The present study has shown the characteristic difference between channel blacks and furnace blacks and the difference in the oxidation effects of various
3. W. H. Wade, J. Colloid Interface Sci. 31, 111(1%9). 4. S. S. Barton and B. H. Harrison,Carbon 10,245 (1972).
agents on the surface properties of the furnace black.
8. S. Hagiwara, K. Tsutsumi and H. Takahashi, Carbon 9,693 (1971). 9. S. Suzuki and K. Miyazaki, Nippon Kagaku Zasshi 87, 1303
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1. G. J. Kraus, Phys. Chem. 57, 343 (1954). 2. B. R. Puri, D. D. Singh and L. R. Sharma, J. Phys. Chem. 62, 756 (1958).
5. S. S. Barton, G. L. Boultonand B. H. Harrison, Carbon 10, 391(1972). 6. M. D. Garrett and H. J. Zaborsky, Am. Ink Maker 47(3),24 (1%9). 7. F. Lyon and W. Stoy, Paint Manuf. 36(4), 36 (1966).
(1966). 10. R. W. Cranston and F. A. Jnkley, Adu. Cot. 9, 143(1957). 11. J. H. de Boer, B. G. Linsen, Th. van der Plass and G. J. Zondervan, J. Cat. 4,649 (1%5). 12. A. C. Zettlemoyer, Ind. Engng Chem. 57(2), 27 (1%5).