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Adsorption of salicylic acid, 5-sulfosalicylic acid and Tiron at the alumina–water interface
Colloids and Surfaces A: Physicochem. Eng. Aspects 211 (2002) 165 /172 www.elsevier.com/locate/colsurfa
Adsorption of salicylic acid, 5-sulfosalicylic acid and Tiron at the alumina water interface /
Linqin Jiang, Lian Gao *, Yangqiao Liu State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Received 15 February 2002; accepted 30 May 2002
Abstract The influences of three multivalent anionic electrolytes: salicylic acid, 5-sulfosalicylic acid and 1,2-dihydroxy-3,5benzenedisulfonic acid disodium salt (Tiron) on the properties of alumina aqueous suspensions have been investigated in this paper. Zeta potential measurements show that the addition of Tiron results in a more dramatic increase in the absolute zeta potential in the alkaline region, as well as a shift of isoelectric point to the more acidic region than salicylic acid and 5-sulfosalicylic acid. Adsorption of a dispersant is promoted strongly by forming a complex between dispersant and Al atom. The adsorption ability is also related to molecular volume of the dispersant and its number of functional group which creates surface charge. The surface properties of alumina suspensions are examined by using FTIR analysis. The efficiency of the dispersant is characterized by observing the morphology of the sediments using scanning electron microscopy and measuring the sediment volume. The results show that Tiron is more effective than the other two dispersants. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Alumina; Salicylic acid; 5-Sulfosalicylic acid; Tiron; FTIR; Adsorption
1. Introduction Colloidal ceramic suspensions are widely used in wet ceramic processing techniques such as slip casting, tape casting, pressure casting and centrifugal casting [1]. Fine ceramic particles which are in the colloidal range (0/1 mm) usually spontaneously aggregate in the dispersing medium due to the relatively high strength of the interparticle van * Corresponding author. Tel.: /86-21-5241-2718; fax: /8621-52413903 E-mail address: [email protected] (L. Gao).
der Waals attractive forces. Dispersants are widely used to modify the interactions between particles by producing electrostatic repulsion or steric layers, thus overcoming the van der Waals attractive forces. These dispersants are primarily polyelectrolytes or multivalent salts [2 /8]. The dispersant molecules adsorb onto the surface of the ceramic particles and attain an electrical charge due to the dissociation of the chemical functional groups on the dispersant. Recently, a study that describes the effect of the molecular structure of low molecular weight organic dispersants, upon the adsorption and the dynamic electrophoretic
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mobility of alumina suspensions has been reported [7]. However, the adsorption and dispersing ability of small molecule type dispersants are less extensively studied and need further investigation. The intent of this work is to examine the interaction of salicylic acid, 5-sulfosalicylic acid and Tiron with the alumina surface and characterize the effect of these compounds on the stability of aqueous alumina suspensions. In this respect, the adsorption isotherms and electrophoresis mobility of the alumina particles are reported. The compound /alumina interaction mode is characterized using data obtained by FTIR analysis. Additionally, the sediment morphology is observed by using scanning electron microscopy (SEM) and the sedimentation volume is also measured. It is expected that the results can help provide a guideline for choosing multivalent type dispersants. The pH value was maintained at pH 8.5, which was considered a suitable pH to prepare stable suspensions in the presence of anionic dispersants.
2. Experimental 2.1. Materials a-Alumina powder was obtained from Wusong fertilizer factory of Shanghai, China. The BET nitrogen-specific surface area of the powder, determined by a Micromeritics ASAP 2010 surface area analyzer, was 26.27 m2 g 1. Three types of low molecular weight organic compounds were used for the preparation of aqueous suspensions. The salicylic acid was obtained from Shanghai chemical Co. (Shanghai, PR China), with a molecular weight of 138.12 and an active component concentration of E/99% (according to the supplier). The 5-sulfosalicylic acid was also received from Shanghai chemical Co., with a molecular weight of 254.2 and an active component concentration of E/95% (according to the supplier). The Tiron was purchased from Shanghai Sanaisi chemical Co. (Shanghai, PR China), with a molecular weight of 332.22 and an active component concentration of E/95% (according to the supplier). The Tiron and 5-
sulfosalicylic acid are soluble in water in all proportions while the solubility of salicylic acid is 0.16 g/100 g H2O at 4 8C. The molecular structures of these three compounds are shown in Fig. 1. Their dissociation constants are depicted in Table 1 [9]. Distilled water was used in all studies. 2.2. Zeta potential measurements Electrophoretic measurements were carried out with a zetaplus analyzer (Zetaplus, Brookhaven, USA). The ionic strength of a dilute suspension (0.01 vol.%) was maintained at 10 3 M using NaCl. The sample was ultrasonicated for 15 min before measurements. 2.3. Adsorption measurements Suspensions were prepared by adding a specific amount of alumina powder to the aromatic compound solution whose pH had been previously adjusted to 8.5 using analytical grade NaOH. Subsequently, the mixture was ball milled for 24 h. Then the equilibrated suspension was centrifuged at 6000 rpm for 1 h. The sediment obtained was used for FTIR analysis. The clear supernatant was removed and analyzed for residual aromatic compound concentration using a UV /vis spectrophometer (UV /vis 752, Analytical Instrument General Factory, Shanghai). In order to assess the aromatic compound concentration quantitatively, a linear calibration curve was constructed at a wavelength of 304, 302 and 290 nm for salicylic acid, 5-sulfosalicylic acid and Tiron, respectively. The adsorbed amount was determined by calculating the difference between the initial aromatic compound concentration and the residual concentration in the supernatant. The concentrations of
Fig. 1. Molecular structure of the aromatic compounds used.
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Table 1 Dissociation constants of the aromatic compounds pKa value [9] Salicylic acid 5-Sulfosalicylic acid Tiron
3.00, 12.38 2.49, 12.00 7.66, 12.60
the aromatic compound in this paper are on a dry mass base of the powder (dmb). 2.4. FTIR spectroscopy Sediments were obtained after the suspensions were centrifuged as described above. Then they were washed with NaOH solutions whose pH was the same as that of the suspension. The washed sediments were dried at 80 8C for 12 h and then used for infrared spectroscopy analyses. Infrared measurements were made on an FTIR spectrophotometer (Bio-Rad FTS-185, Hercules, CA). 2.5. Stability evaluation The stability of the 3 vol.% suspension was determined by measuring the sediment volume of 10 ml suspensions after 1 month when the sediment volume did not change.The same suspensions were observed with a SEM. To obtain highquality micrographs, the samples were coated with a thin gold layer prior to measurement.
3. Results and discussion 3.1. Electrophoresis mobility Metal oxide particles (and many other ceramic powders as well) may attain an electrical charge, depending upon the pH of the aqueous suspending medium, due to the association of surface hydroxyl group with H /OH ions [10]. Fig. 2 shows the zeta potential of Al2O3 as a function of pH. The isoelectric point (IEP) of the native alumina powder is approximately pH 8.2, similar to the value reported by others [11,12]. The alumina
Fig. 2. Zeta potential of alumina powder as a function of pH.
surface carries a net positive charge at pH B/8.2 and a net negative charge at pH /8.2. The influence of the aromatic compounds onto eletrokinetic properties of alumina suspensions is also shown in Fig. 2. In order to investigate the influence of single dispersant molecule on the surface charge, the dispersant additions in Fig. 2 are all controlled at 1.3 mmol m 2. It can be seen that the addition of salicylic acid, 5-sulfosalicylic acid and Tiron causes influences on IEP, displacing it to pH 8.1, 6.2 and 4.5, respectively. Electrokinetic properties of the salicylic acid suspension are similar to those of a pure alumina suspension over the wide pH region. Although the ortho position of the hydroxyl group in salicylic acid allows the formation of a chelate complex by coordination of both the hydroxyl group and the carboxyl group to a surface Al(III) atom [7], this complex seems not to create much negative charge which contributes to the negative surface charge on the particles. So, the salicylic acid only causes IEP from 8.2 to 8.1. The addition of 5-sulfosalicylic acid leads to a shift of IEP to lower pH values than salicylic acid, as well as a more significant decrease in zeta potential. The same adsorption mechanism can be used as above for 5sulfosalicylic acid. Apparently, the ionised sulfonate group causes IEP shift and the decrease in zeta potential significantly in addition to the complex. Compared with 5-sulfosalicylic acid, Tiron addition in the suspension leads to a higher density of negative charge. The shift of IEP and the zeta potential reduction caused by Tiron are more pronounced than those caused by the other two. Also, an inner sphere complex is built
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between alcohol groups of Tiron and hydroxyl surface groups. The two ionised sulfonate groups cause the higher negative surface charge significantly. Moreover, the magnitude of the zeta potential at pH 8/10 is as high as 40 mV for the Tiron /adsorbed alumina suspension, compared to the 10 mV for bare alumina suspensions. It can be postulated that the stability of the alumina suspension can be enhanced over the basic pH range of 8 /10 in the presence of Tiron. So, pH 8.5 is compatible to prepare stable suspensions. In the alkaline region, electrostatic repulsion forces exist between the alumina powders and anionic dispersants as they are both negatively charged. Despite the electrostatic repulsion, however, the zeta potential reduction is still as high as 20 and 40 mV at pH 8.5 for 5-sulfosalicylic acid and Tiron, respectively. This must be due to the presence of a specific interaction. The specific free energy of adsorption for the alumina-dispersant system is determined following the explicit relationship between the characteristic shift in IEP and the concentration of aromatic compound provided by Pradip [14]: 0 DpHIEP 1:0396C0 exp( DGsp =RT)
(1)
where DpHIEP is the shift in IEP at the dispersant 0 concentration C0 (mol l 1) and DGsp represents the corresponding specific energy of interaction between the alumina powder surface and the dispersant, R and T are the gas constant and the 0 temperature (K), respectively. The higher DGsp value, the stronger interaction between dispersant and alumina [15]. Using the data presented in Fig. 0 2, the calculated DGsp for salicylic acid, 5sulfosalicylic acid and Tiron absorbed on alumina surface is 2.08, 11.94 and 12.52RT , respectively. 0 So, the higher DGsp value in this study is considered an evidence for the stronger interaction between Tiron and alumina through electrostatic and chemical interactions than the other two.
Fig. 3. Adsorption isotherms, the amount of dispersant adsorbed in 3 vol.% alumina suspension as a function of initial dispersant concentration.
sorbed increases with increasing added amount before the equilibrium can be reached. In addition, the initial part of the isotherms at lower concentration shows a steep increase in adsorption amount. This is characteristic of high affinity adsorption, a behavior which is frequently associated with chemisorption or chelation [15]. It can be seen from Fig. 3 that the affinity of the adsorption isotherm for 5-sulfosalicylic acid is lower than that for salicylic acid, but higher than that for Tiron. To determine the monolayer adsorption of aromatic compound quantitatively, the data in Fig. 3 are analyzed using the Langmuir monolayer adsorption equation [3]: Ceq Ceq k G Gmax Gmax
(2)
The data are plotted as Ceq =G vs. Ceq in Fig. 4, where Ceq (mol l 1) is the equilibrium concentration of absorbate in solution, G (mol m 2) the absorbed amount for a given equilibrium concen-
3.2. Adsorption isotherms Adsorption isotherms for aromatic compounds on alumina at pH 8.5 are presented in Fig. 3. For all of the compounds studied, the amount ad-
Fig. 4. Replot of Fig. 3, based on the Langmuir equation Eq. (2).
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tration and Gmax (mmol m 2) the monolayer adsorption per unit solid surface; k is a constant. Straight lines are observed, which indicates that these isotherms are of Langmuir monolayer-type adsorption. The slopes of the straight lines represent the reciprocal of the monolayer adsorption of dispersants (1/Gmax). The values of Gmax calculated from Fig. 4 are shown in Table 2. The adsorption of a weak electrolyte onto metal oxide is influenced by the pH values because both the particle surface charge and the electrolyte dissociation are pH dependent. At pH 8.5, alumina and aromatic compounds are both negatively charged. The stronger adsorption of salicylic acid must be due to the formation of a chelate complex by coordination of both the hydroxyl group and the carboxyl group to a surface Al(III) atom [7]. For 5-sulfosalicylic acid, the formation of a chelate complex also contributes to adsorption. However, the fifth position of the sulfonate group creates a negative charge, which is repulsed by the negative alumina surface at basic pH. This causes a reduction in the absorbed amount. Compared with 5-sulfosalicylic acid, Tiron is more repulsed by the negative alumina surface with two sulfonate groups at basic pH in addition to the inner sphere complex between the metal ions on the surfaces and the alcohol groups of the molecule [16]. It adsorbs in a smaller amount than 5-sulfosalicylic acid. Another important factor for the adsorption ability is the molecule volume. The molecule volume increases with increasing number of substitute groups on the benzene ring. For a given surface area, it should adsorb more aromatic compounds with small molecule volume. So, salicylic acid adsorbs in the largest amount of the three observed compounds. It can be observed that formation of inner sphere complex onto oxide surface leads to a stronger adsorption.
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3.3. FTIR spectra To determine the adsorption mechanism between alumina surface and the aromatic compounds, FTIR spectra is observed. Curve 5a is the FTIR spectrum of the pure salicylic acid Fig. 5. Curve 5b is that obtained by subtracting the spectrum of pure alumina from the spectrum of the 0.8 dmb% salicylic acid /adsorbed alumina. The strong band at 1658 cm 1 indicates the presence of the /C /O stretching vibration of carboxylic acid. The bands at 1579 and 1326 cm 1 are due to the asymmetrical and symmetrical stretching of CO2 , respectively. 1249 cm 1 is assigned to the phenolic Ph /O stretching vibration. Other bands are due to aromatic C /C ring stretching modes. The bending mode, Ph /O /H, is represented by the doublet at 1384 /1326 cm 1 [13]. From curve 5b, the disappearance of the band at 1658 cm 1 reveals the complete dissociation of CO2 group of salicylic acid. The symmetrical stretching band of CO2 shifts to lower frequencies 1378 cm 1, which suggests that the carboxylate is forming a monodentate mononuclear complex [13]. The doublet bands at 1384 /1326 cm 1 disappear upon adsorption. The band assigned to the phenolic stretching shifts from 1249 to 1264 cm 1, which is due to the coordination of the phenolic oxygen to Al [13]. In addition, the C /C ring stretchings undergo significant shifts upon forming a complex with the surface. These shifts indicate some type of change in the Ph /O bond order. Fig. 8a shows the proposed structure for the most likely surface complex that is formed
Table 2 Saturation coverage determined by fitting the adsorption data in the Langmuir equation Salicylic acid 5-Sulfosalicylic acid Tiron Gmax (mmol m 2) 0.74
0.69
0.60
Fig. 5. FTIR spectra of (a) salicylic acid at pH 8.5 and (b) salicylic acid adsorbed on Al2O3 at pH 8.5.
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by the adsorption of salicylic acid on the alumina surface, which has been suggested by Hidber P.C. et al. derived from the study for salicylic acid adsorption on goethite by IR spectroscopy [7,13]. In this study, the structure for the surface complex is confirmed. The FTIR spectra of pure 5-sulfosalicylic acid and alumina containing adsorbed 5-sulfosalicylic acid are depicted in Fig. 6. The symmetric CO2 band shifts from 1368 cm 1 (curve 6a) to 1380 cm 1 (curve 6b), which suggests that the carboxylate is forming a monodentate mononuclear complex [13]. The phenolic stretching band shifts from 1318 cm 1(curve 6a) to 1355 cm 1(curve 6b), which is due to the coordination of the phenolic oxygen to Al. The band at 1582 cm 1 assigned to the asymmetrical stretching of CO2 shifts to 1613 cm 1. Other C /C ring stretching bands undergo significant shifts upon forming a complex with the surface. Over the observed wavenumber region, the band of SO3 cannot be observed distinctly. Peak et al. has shown that sulfate adsorbs on goethite only as an outer-sphere complex at pH values greater than 6 [17]. Fig. 8b shows the proposed structure for the surface complex. This complex is formed by coordination of both the hydroxyl group and the carboxyl group to surface Al atoms, with ionised sulfonate group outward creating negative charge. The FTIR spectra of pure Tiron and alumina containing adsorbed Tiron are shown in Fig. 7. The adsorption bands of curve e are assigned as follows: 1471, 1438 and 1293 cm 1 to the benzene
Fig. 6. FTIR spectra of (a) 5-sulfosalicylic acid at pH 8.5 and (b) 5-sulfosalicylic acid adsorbed on Al2O3 at pH 8.5.
Fig. 7. FTIR spectra of (a) Tiron at pH 8.5 and (b) Tiron adsorbed on Al2O3 at pH 8.5.
Fig. 8. The proposed structures of the surface complex of (a) salicylic acid; (b) 5-sulfosalicylic acid and (c) Tiron with Al atom.
ring characteristic vibrations; 1380 cm 1 to the OH bending vibration in plane; and 1236 cm 1 to the C /O stretching vibration. From curve 7b, the band at 1380 cm 1 divided into many small peaks is deformation of /OH [18]. The difference between the two curves in the range of 1236 / 1300 cm 1 indicates the change of the deformation of C /O stretching vibration [18]. This is due to the coordination of the two oxygen atoms of a Tiron molecule to a Al3 ion. Similar behavior is observed in Fe2O3 /Tiron system [19]. The groups of SO3 are unlikely to coordinate with Al atom due to electrostatic repulsion and weak force [17]. The proposed structure is shown in Fig. 8C. From these three proposed structures in Fig. 8, it can be concluded that the presence of a hydroxyl group or a carboxyl group in the ortho-position enables the formation of a chelate complex with metal atom, which results in an increase in the amount adsorbed. The additional functional groups of the aromatic compound act as an outer sphere, which results in an increase of surface charge significantly.
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3.4. The stability of the suspension The 20 000-fold micrograph of sediment obtained from alumina suspensions without and with 0.8 dmb% aromatic compounds is shown in Fig. 9. Large differences can be observed when comparing these micrographs. Fig. 9a for pure alumina shows the presence of large flocs composed of many associating particles. The dispersing states of particles in the sediment are improved with dispersant addition (Fig. 9b /d), however, the Tiron-stabilized Al2O3 suspension exhibits the best dispersion. This can be interpreted by the traditional DLVO theory. Without dispersants, the zeta potential of alumina suspension is only /2 mV, which is too low to overcome the attracting van der Waals force between particles. As the alumina particles collide with one
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another in the suspension, flocs are built up before undertaking any significant settling. Then the large aggregates settle fast under gravity. As a result, large flocs dominate the sediment surface. The addition of Tiron significantly increases the magnitude of zeta potential of powders and enhances the stability of the suspension via electrostatic forces than salicylic acid and 5-sulfosalicylic acid. Thus, independent particles or aggregates have the time to fall downward and accumulate before they could aggregate with each other to build a large floc. So, particles of the resultant sediment are relatively independent without associating with one another. Fig. 10 illustrates sediment volume versus different aromatic compound amounts for 3 vol.% alumina suspensions after one month for salicylic acid, 5-sulfosacylic acid and Tiron, respectively.
Fig. 9. SEM micrographs of 3 vol.% alumina suspensions without dispersants (a) pure Al2O3 and with dispersants; (b) salicylic acid; (c) 5-sulfosalicylic acid and (d) Tiron.
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Fig. 10. Sediment volume of 3 vol.% suspensions vs dispersant concentration.
By comparing the three isotherms (Fig. 3) and the sediment curve (Fig. 10), the sediment volume of three dispersants does not change significantly when the added amount exceeds 0.8 dmb%, which is lower than the saturation dispersant concentration. This is result of the two conflicting aspects of increasing dispersant amount. From one hand, more dispersants can slightly increase the surface charge of alumina, which is favorable to the stability of alumina suspension. From another hand, the addition of more dispersants will inevitably increase the ionic strength in the suspension, which can reduce the electrostatic repulsion between particles due to the screening effect of counterions. So, the sediment volume is comparatively constant over a specific concentration range. From the zeta potential /pH curves in the presence of 1.3 mmol m 2 aromatic compounds, the addition of Tiron causes more dramatic influences on IEP than salicylic acid and 5-sulfosalicylic acid. The sediment volume exhibit similar trends. The suspensions doped with Tiron result in the most closely packed sediments. Tiron was found to be an efficient dispersant for the alumina suspension investigated. This shows that a dispersant could be efficient if the molecule possesses both functional groups which permit strong adsorption and groups which create the surface charge.
addition of salicylic acid, 5-sulfosalicylic acid and Tiron displaces IEP from pH 8.2 to pH 8.1, 6.2 and 4.5, respectively. For the Tiron /adsorbed alumina suspension, the maximum magnitude of the zeta potential is as high as 40 mV. High adsorption ability can be enhanced by forming a chelate complex. The additional ionised groups of the dispersant result in a increase of surface charge significantly. The proposed structures are confirmed by FTIR spectra. Large difference exists between the morphology of sediments formed from suspensions without and with the three aromatic compounds. It is found that particles are well dispersed in the Tiron /Al2O3 suspension. A dispersant can be efficient if the molecule possesses both functional groups which permit strong adsorption and groups which create surface charge. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
4. Conclusions Three aromatic compounds containing a benzene ring and different substituents are investigated as dispersants for alumina suspensions. The
[17] [18] [19]
F.F. Lange, J. Am. Ceram. Soc. 72 (1989) 3. V.A. Hackley, J. Am. Ceram. Soc. 80 (1997) 2315. J.H. Jean, H.R. Wang, J. Am. Ceram. Soc. 81 (1998) 1589. ¨ hlen, H. Lewandowski, H.D. Narres, M.J. K. VermO Schwuger, Colloids Surfaces A 163 (2000) 45. Y.Q. Liu, L. Gao, J.K. Guo, Colloids Surfaces A 174 (2000) 349. B.J. Briscoe, A.U. Khan, P.F. Luckham, J. Euro. Ceram. Soc. 18 (1998) 2141. P.C. Hidber, T.J. Graule, L.J. Gauckler, J. Euro. Ceram. Soc. 17 (1997) 239. R. Ashcraft, G. Bohnsack, R. Kleimstueck, S. Storp, Mater. Perform. 27 (1998) 31. J.A. Dean, Lange’s Handbook of Chemistry, vol. 11, 1973, p. 13. J. Wernet, D.L. Feke, J. Am. Ceram. Soc. 77 (1994) 2693. J. Cesarano, I.A. Aksay, J. Am. Ceram. Soc. 71 (1988) 250. P.C. Hidber, T.J. Graule, L.J. Gauckler, J. Am. Ceram. Soc. 79 (1996) 1857. C. Yost, M. Tejedor-Tejedor, M.A. Anderson, Environ. Sci. Technol. 24 (1990) 822. R.S. Pradip, Trans. Indian Inst. Metals 41 (1988) 15. Y.Q. Liu, L. Gao, L. Yu, J.K. Guo, J. Colloid Interface Sci. 227 (2000) 164. R. Laucournet, C. Pagnoux, T. Chartier, J.F. Baumard, J. Euro. Cera. Soc. 21 (2001) 869. D. Peak, R.G. Ford, D.L. Sparks, J. Colloid Interface Sci. 218 (1999) 289. H.A. Szymanski, Interpreted Infrared Spectra, Plenum, New York, 1967, p. 3. T. Sugimoto, H. Itoh, T. Mochida, J. Colloid Interface Sci. 205 (1998) 42.
Adsorption of salicylic acid, 5-sulfosalicylic acid and Tiron at the alumina water interface /
Linqin Jiang, Lian Gao *, Yangqiao Liu State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Received 15 February 2002; accepted 30 May 2002
Abstract The influences of three multivalent anionic electrolytes: salicylic acid, 5-sulfosalicylic acid and 1,2-dihydroxy-3,5benzenedisulfonic acid disodium salt (Tiron) on the properties of alumina aqueous suspensions have been investigated in this paper. Zeta potential measurements show that the addition of Tiron results in a more dramatic increase in the absolute zeta potential in the alkaline region, as well as a shift of isoelectric point to the more acidic region than salicylic acid and 5-sulfosalicylic acid. Adsorption of a dispersant is promoted strongly by forming a complex between dispersant and Al atom. The adsorption ability is also related to molecular volume of the dispersant and its number of functional group which creates surface charge. The surface properties of alumina suspensions are examined by using FTIR analysis. The efficiency of the dispersant is characterized by observing the morphology of the sediments using scanning electron microscopy and measuring the sediment volume. The results show that Tiron is more effective than the other two dispersants. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Alumina; Salicylic acid; 5-Sulfosalicylic acid; Tiron; FTIR; Adsorption
1. Introduction Colloidal ceramic suspensions are widely used in wet ceramic processing techniques such as slip casting, tape casting, pressure casting and centrifugal casting [1]. Fine ceramic particles which are in the colloidal range (0/1 mm) usually spontaneously aggregate in the dispersing medium due to the relatively high strength of the interparticle van * Corresponding author. Tel.: /86-21-5241-2718; fax: /8621-52413903 E-mail address: [email protected] (L. Gao).
der Waals attractive forces. Dispersants are widely used to modify the interactions between particles by producing electrostatic repulsion or steric layers, thus overcoming the van der Waals attractive forces. These dispersants are primarily polyelectrolytes or multivalent salts [2 /8]. The dispersant molecules adsorb onto the surface of the ceramic particles and attain an electrical charge due to the dissociation of the chemical functional groups on the dispersant. Recently, a study that describes the effect of the molecular structure of low molecular weight organic dispersants, upon the adsorption and the dynamic electrophoretic
0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 2 ) 0 0 2 7 6 - 5
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mobility of alumina suspensions has been reported [7]. However, the adsorption and dispersing ability of small molecule type dispersants are less extensively studied and need further investigation. The intent of this work is to examine the interaction of salicylic acid, 5-sulfosalicylic acid and Tiron with the alumina surface and characterize the effect of these compounds on the stability of aqueous alumina suspensions. In this respect, the adsorption isotherms and electrophoresis mobility of the alumina particles are reported. The compound /alumina interaction mode is characterized using data obtained by FTIR analysis. Additionally, the sediment morphology is observed by using scanning electron microscopy (SEM) and the sedimentation volume is also measured. It is expected that the results can help provide a guideline for choosing multivalent type dispersants. The pH value was maintained at pH 8.5, which was considered a suitable pH to prepare stable suspensions in the presence of anionic dispersants.
2. Experimental 2.1. Materials a-Alumina powder was obtained from Wusong fertilizer factory of Shanghai, China. The BET nitrogen-specific surface area of the powder, determined by a Micromeritics ASAP 2010 surface area analyzer, was 26.27 m2 g 1. Three types of low molecular weight organic compounds were used for the preparation of aqueous suspensions. The salicylic acid was obtained from Shanghai chemical Co. (Shanghai, PR China), with a molecular weight of 138.12 and an active component concentration of E/99% (according to the supplier). The 5-sulfosalicylic acid was also received from Shanghai chemical Co., with a molecular weight of 254.2 and an active component concentration of E/95% (according to the supplier). The Tiron was purchased from Shanghai Sanaisi chemical Co. (Shanghai, PR China), with a molecular weight of 332.22 and an active component concentration of E/95% (according to the supplier). The Tiron and 5-
sulfosalicylic acid are soluble in water in all proportions while the solubility of salicylic acid is 0.16 g/100 g H2O at 4 8C. The molecular structures of these three compounds are shown in Fig. 1. Their dissociation constants are depicted in Table 1 [9]. Distilled water was used in all studies. 2.2. Zeta potential measurements Electrophoretic measurements were carried out with a zetaplus analyzer (Zetaplus, Brookhaven, USA). The ionic strength of a dilute suspension (0.01 vol.%) was maintained at 10 3 M using NaCl. The sample was ultrasonicated for 15 min before measurements. 2.3. Adsorption measurements Suspensions were prepared by adding a specific amount of alumina powder to the aromatic compound solution whose pH had been previously adjusted to 8.5 using analytical grade NaOH. Subsequently, the mixture was ball milled for 24 h. Then the equilibrated suspension was centrifuged at 6000 rpm for 1 h. The sediment obtained was used for FTIR analysis. The clear supernatant was removed and analyzed for residual aromatic compound concentration using a UV /vis spectrophometer (UV /vis 752, Analytical Instrument General Factory, Shanghai). In order to assess the aromatic compound concentration quantitatively, a linear calibration curve was constructed at a wavelength of 304, 302 and 290 nm for salicylic acid, 5-sulfosalicylic acid and Tiron, respectively. The adsorbed amount was determined by calculating the difference between the initial aromatic compound concentration and the residual concentration in the supernatant. The concentrations of
Fig. 1. Molecular structure of the aromatic compounds used.
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Table 1 Dissociation constants of the aromatic compounds pKa value [9] Salicylic acid 5-Sulfosalicylic acid Tiron
3.00, 12.38 2.49, 12.00 7.66, 12.60
the aromatic compound in this paper are on a dry mass base of the powder (dmb). 2.4. FTIR spectroscopy Sediments were obtained after the suspensions were centrifuged as described above. Then they were washed with NaOH solutions whose pH was the same as that of the suspension. The washed sediments were dried at 80 8C for 12 h and then used for infrared spectroscopy analyses. Infrared measurements were made on an FTIR spectrophotometer (Bio-Rad FTS-185, Hercules, CA). 2.5. Stability evaluation The stability of the 3 vol.% suspension was determined by measuring the sediment volume of 10 ml suspensions after 1 month when the sediment volume did not change.The same suspensions were observed with a SEM. To obtain highquality micrographs, the samples were coated with a thin gold layer prior to measurement.
3. Results and discussion 3.1. Electrophoresis mobility Metal oxide particles (and many other ceramic powders as well) may attain an electrical charge, depending upon the pH of the aqueous suspending medium, due to the association of surface hydroxyl group with H /OH ions [10]. Fig. 2 shows the zeta potential of Al2O3 as a function of pH. The isoelectric point (IEP) of the native alumina powder is approximately pH 8.2, similar to the value reported by others [11,12]. The alumina
Fig. 2. Zeta potential of alumina powder as a function of pH.
surface carries a net positive charge at pH B/8.2 and a net negative charge at pH /8.2. The influence of the aromatic compounds onto eletrokinetic properties of alumina suspensions is also shown in Fig. 2. In order to investigate the influence of single dispersant molecule on the surface charge, the dispersant additions in Fig. 2 are all controlled at 1.3 mmol m 2. It can be seen that the addition of salicylic acid, 5-sulfosalicylic acid and Tiron causes influences on IEP, displacing it to pH 8.1, 6.2 and 4.5, respectively. Electrokinetic properties of the salicylic acid suspension are similar to those of a pure alumina suspension over the wide pH region. Although the ortho position of the hydroxyl group in salicylic acid allows the formation of a chelate complex by coordination of both the hydroxyl group and the carboxyl group to a surface Al(III) atom [7], this complex seems not to create much negative charge which contributes to the negative surface charge on the particles. So, the salicylic acid only causes IEP from 8.2 to 8.1. The addition of 5-sulfosalicylic acid leads to a shift of IEP to lower pH values than salicylic acid, as well as a more significant decrease in zeta potential. The same adsorption mechanism can be used as above for 5sulfosalicylic acid. Apparently, the ionised sulfonate group causes IEP shift and the decrease in zeta potential significantly in addition to the complex. Compared with 5-sulfosalicylic acid, Tiron addition in the suspension leads to a higher density of negative charge. The shift of IEP and the zeta potential reduction caused by Tiron are more pronounced than those caused by the other two. Also, an inner sphere complex is built
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between alcohol groups of Tiron and hydroxyl surface groups. The two ionised sulfonate groups cause the higher negative surface charge significantly. Moreover, the magnitude of the zeta potential at pH 8/10 is as high as 40 mV for the Tiron /adsorbed alumina suspension, compared to the 10 mV for bare alumina suspensions. It can be postulated that the stability of the alumina suspension can be enhanced over the basic pH range of 8 /10 in the presence of Tiron. So, pH 8.5 is compatible to prepare stable suspensions. In the alkaline region, electrostatic repulsion forces exist between the alumina powders and anionic dispersants as they are both negatively charged. Despite the electrostatic repulsion, however, the zeta potential reduction is still as high as 20 and 40 mV at pH 8.5 for 5-sulfosalicylic acid and Tiron, respectively. This must be due to the presence of a specific interaction. The specific free energy of adsorption for the alumina-dispersant system is determined following the explicit relationship between the characteristic shift in IEP and the concentration of aromatic compound provided by Pradip [14]: 0 DpHIEP 1:0396C0 exp( DGsp =RT)
(1)
where DpHIEP is the shift in IEP at the dispersant 0 concentration C0 (mol l 1) and DGsp represents the corresponding specific energy of interaction between the alumina powder surface and the dispersant, R and T are the gas constant and the 0 temperature (K), respectively. The higher DGsp value, the stronger interaction between dispersant and alumina [15]. Using the data presented in Fig. 0 2, the calculated DGsp for salicylic acid, 5sulfosalicylic acid and Tiron absorbed on alumina surface is 2.08, 11.94 and 12.52RT , respectively. 0 So, the higher DGsp value in this study is considered an evidence for the stronger interaction between Tiron and alumina through electrostatic and chemical interactions than the other two.
Fig. 3. Adsorption isotherms, the amount of dispersant adsorbed in 3 vol.% alumina suspension as a function of initial dispersant concentration.
sorbed increases with increasing added amount before the equilibrium can be reached. In addition, the initial part of the isotherms at lower concentration shows a steep increase in adsorption amount. This is characteristic of high affinity adsorption, a behavior which is frequently associated with chemisorption or chelation [15]. It can be seen from Fig. 3 that the affinity of the adsorption isotherm for 5-sulfosalicylic acid is lower than that for salicylic acid, but higher than that for Tiron. To determine the monolayer adsorption of aromatic compound quantitatively, the data in Fig. 3 are analyzed using the Langmuir monolayer adsorption equation [3]: Ceq Ceq k G Gmax Gmax
(2)
The data are plotted as Ceq =G vs. Ceq in Fig. 4, where Ceq (mol l 1) is the equilibrium concentration of absorbate in solution, G (mol m 2) the absorbed amount for a given equilibrium concen-
3.2. Adsorption isotherms Adsorption isotherms for aromatic compounds on alumina at pH 8.5 are presented in Fig. 3. For all of the compounds studied, the amount ad-
Fig. 4. Replot of Fig. 3, based on the Langmuir equation Eq. (2).
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tration and Gmax (mmol m 2) the monolayer adsorption per unit solid surface; k is a constant. Straight lines are observed, which indicates that these isotherms are of Langmuir monolayer-type adsorption. The slopes of the straight lines represent the reciprocal of the monolayer adsorption of dispersants (1/Gmax). The values of Gmax calculated from Fig. 4 are shown in Table 2. The adsorption of a weak electrolyte onto metal oxide is influenced by the pH values because both the particle surface charge and the electrolyte dissociation are pH dependent. At pH 8.5, alumina and aromatic compounds are both negatively charged. The stronger adsorption of salicylic acid must be due to the formation of a chelate complex by coordination of both the hydroxyl group and the carboxyl group to a surface Al(III) atom [7]. For 5-sulfosalicylic acid, the formation of a chelate complex also contributes to adsorption. However, the fifth position of the sulfonate group creates a negative charge, which is repulsed by the negative alumina surface at basic pH. This causes a reduction in the absorbed amount. Compared with 5-sulfosalicylic acid, Tiron is more repulsed by the negative alumina surface with two sulfonate groups at basic pH in addition to the inner sphere complex between the metal ions on the surfaces and the alcohol groups of the molecule [16]. It adsorbs in a smaller amount than 5-sulfosalicylic acid. Another important factor for the adsorption ability is the molecule volume. The molecule volume increases with increasing number of substitute groups on the benzene ring. For a given surface area, it should adsorb more aromatic compounds with small molecule volume. So, salicylic acid adsorbs in the largest amount of the three observed compounds. It can be observed that formation of inner sphere complex onto oxide surface leads to a stronger adsorption.
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3.3. FTIR spectra To determine the adsorption mechanism between alumina surface and the aromatic compounds, FTIR spectra is observed. Curve 5a is the FTIR spectrum of the pure salicylic acid Fig. 5. Curve 5b is that obtained by subtracting the spectrum of pure alumina from the spectrum of the 0.8 dmb% salicylic acid /adsorbed alumina. The strong band at 1658 cm 1 indicates the presence of the /C /O stretching vibration of carboxylic acid. The bands at 1579 and 1326 cm 1 are due to the asymmetrical and symmetrical stretching of CO2 , respectively. 1249 cm 1 is assigned to the phenolic Ph /O stretching vibration. Other bands are due to aromatic C /C ring stretching modes. The bending mode, Ph /O /H, is represented by the doublet at 1384 /1326 cm 1 [13]. From curve 5b, the disappearance of the band at 1658 cm 1 reveals the complete dissociation of CO2 group of salicylic acid. The symmetrical stretching band of CO2 shifts to lower frequencies 1378 cm 1, which suggests that the carboxylate is forming a monodentate mononuclear complex [13]. The doublet bands at 1384 /1326 cm 1 disappear upon adsorption. The band assigned to the phenolic stretching shifts from 1249 to 1264 cm 1, which is due to the coordination of the phenolic oxygen to Al [13]. In addition, the C /C ring stretchings undergo significant shifts upon forming a complex with the surface. These shifts indicate some type of change in the Ph /O bond order. Fig. 8a shows the proposed structure for the most likely surface complex that is formed
Table 2 Saturation coverage determined by fitting the adsorption data in the Langmuir equation Salicylic acid 5-Sulfosalicylic acid Tiron Gmax (mmol m 2) 0.74
0.69
0.60
Fig. 5. FTIR spectra of (a) salicylic acid at pH 8.5 and (b) salicylic acid adsorbed on Al2O3 at pH 8.5.
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by the adsorption of salicylic acid on the alumina surface, which has been suggested by Hidber P.C. et al. derived from the study for salicylic acid adsorption on goethite by IR spectroscopy [7,13]. In this study, the structure for the surface complex is confirmed. The FTIR spectra of pure 5-sulfosalicylic acid and alumina containing adsorbed 5-sulfosalicylic acid are depicted in Fig. 6. The symmetric CO2 band shifts from 1368 cm 1 (curve 6a) to 1380 cm 1 (curve 6b), which suggests that the carboxylate is forming a monodentate mononuclear complex [13]. The phenolic stretching band shifts from 1318 cm 1(curve 6a) to 1355 cm 1(curve 6b), which is due to the coordination of the phenolic oxygen to Al. The band at 1582 cm 1 assigned to the asymmetrical stretching of CO2 shifts to 1613 cm 1. Other C /C ring stretching bands undergo significant shifts upon forming a complex with the surface. Over the observed wavenumber region, the band of SO3 cannot be observed distinctly. Peak et al. has shown that sulfate adsorbs on goethite only as an outer-sphere complex at pH values greater than 6 [17]. Fig. 8b shows the proposed structure for the surface complex. This complex is formed by coordination of both the hydroxyl group and the carboxyl group to surface Al atoms, with ionised sulfonate group outward creating negative charge. The FTIR spectra of pure Tiron and alumina containing adsorbed Tiron are shown in Fig. 7. The adsorption bands of curve e are assigned as follows: 1471, 1438 and 1293 cm 1 to the benzene
Fig. 6. FTIR spectra of (a) 5-sulfosalicylic acid at pH 8.5 and (b) 5-sulfosalicylic acid adsorbed on Al2O3 at pH 8.5.
Fig. 7. FTIR spectra of (a) Tiron at pH 8.5 and (b) Tiron adsorbed on Al2O3 at pH 8.5.
Fig. 8. The proposed structures of the surface complex of (a) salicylic acid; (b) 5-sulfosalicylic acid and (c) Tiron with Al atom.
ring characteristic vibrations; 1380 cm 1 to the OH bending vibration in plane; and 1236 cm 1 to the C /O stretching vibration. From curve 7b, the band at 1380 cm 1 divided into many small peaks is deformation of /OH [18]. The difference between the two curves in the range of 1236 / 1300 cm 1 indicates the change of the deformation of C /O stretching vibration [18]. This is due to the coordination of the two oxygen atoms of a Tiron molecule to a Al3 ion. Similar behavior is observed in Fe2O3 /Tiron system [19]. The groups of SO3 are unlikely to coordinate with Al atom due to electrostatic repulsion and weak force [17]. The proposed structure is shown in Fig. 8C. From these three proposed structures in Fig. 8, it can be concluded that the presence of a hydroxyl group or a carboxyl group in the ortho-position enables the formation of a chelate complex with metal atom, which results in an increase in the amount adsorbed. The additional functional groups of the aromatic compound act as an outer sphere, which results in an increase of surface charge significantly.
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3.4. The stability of the suspension The 20 000-fold micrograph of sediment obtained from alumina suspensions without and with 0.8 dmb% aromatic compounds is shown in Fig. 9. Large differences can be observed when comparing these micrographs. Fig. 9a for pure alumina shows the presence of large flocs composed of many associating particles. The dispersing states of particles in the sediment are improved with dispersant addition (Fig. 9b /d), however, the Tiron-stabilized Al2O3 suspension exhibits the best dispersion. This can be interpreted by the traditional DLVO theory. Without dispersants, the zeta potential of alumina suspension is only /2 mV, which is too low to overcome the attracting van der Waals force between particles. As the alumina particles collide with one
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another in the suspension, flocs are built up before undertaking any significant settling. Then the large aggregates settle fast under gravity. As a result, large flocs dominate the sediment surface. The addition of Tiron significantly increases the magnitude of zeta potential of powders and enhances the stability of the suspension via electrostatic forces than salicylic acid and 5-sulfosalicylic acid. Thus, independent particles or aggregates have the time to fall downward and accumulate before they could aggregate with each other to build a large floc. So, particles of the resultant sediment are relatively independent without associating with one another. Fig. 10 illustrates sediment volume versus different aromatic compound amounts for 3 vol.% alumina suspensions after one month for salicylic acid, 5-sulfosacylic acid and Tiron, respectively.
Fig. 9. SEM micrographs of 3 vol.% alumina suspensions without dispersants (a) pure Al2O3 and with dispersants; (b) salicylic acid; (c) 5-sulfosalicylic acid and (d) Tiron.
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Fig. 10. Sediment volume of 3 vol.% suspensions vs dispersant concentration.
By comparing the three isotherms (Fig. 3) and the sediment curve (Fig. 10), the sediment volume of three dispersants does not change significantly when the added amount exceeds 0.8 dmb%, which is lower than the saturation dispersant concentration. This is result of the two conflicting aspects of increasing dispersant amount. From one hand, more dispersants can slightly increase the surface charge of alumina, which is favorable to the stability of alumina suspension. From another hand, the addition of more dispersants will inevitably increase the ionic strength in the suspension, which can reduce the electrostatic repulsion between particles due to the screening effect of counterions. So, the sediment volume is comparatively constant over a specific concentration range. From the zeta potential /pH curves in the presence of 1.3 mmol m 2 aromatic compounds, the addition of Tiron causes more dramatic influences on IEP than salicylic acid and 5-sulfosalicylic acid. The sediment volume exhibit similar trends. The suspensions doped with Tiron result in the most closely packed sediments. Tiron was found to be an efficient dispersant for the alumina suspension investigated. This shows that a dispersant could be efficient if the molecule possesses both functional groups which permit strong adsorption and groups which create the surface charge.
addition of salicylic acid, 5-sulfosalicylic acid and Tiron displaces IEP from pH 8.2 to pH 8.1, 6.2 and 4.5, respectively. For the Tiron /adsorbed alumina suspension, the maximum magnitude of the zeta potential is as high as 40 mV. High adsorption ability can be enhanced by forming a chelate complex. The additional ionised groups of the dispersant result in a increase of surface charge significantly. The proposed structures are confirmed by FTIR spectra. Large difference exists between the morphology of sediments formed from suspensions without and with the three aromatic compounds. It is found that particles are well dispersed in the Tiron /Al2O3 suspension. A dispersant can be efficient if the molecule possesses both functional groups which permit strong adsorption and groups which create surface charge. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
4. Conclusions Three aromatic compounds containing a benzene ring and different substituents are investigated as dispersants for alumina suspensions. The
[17] [18] [19]
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