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The effects of cis–trans configuration of cyclohexane multi-carboxylic acids on colloidal forces in dispersions: steric, hydrophobic and bridging
Colloids and Surfaces A: Physicochemical and Engineering Aspects 160 (1999) 199 – 205 www.elsevier.nl/locate/colsurfa
The effects of cis–trans configuration of cyclohexane multi-carboxylic acids on colloidal forces in dispersions: steric, hydrophobic and bridging A.V.M. Chandramalar a, Y.Y. Lim a, Y.K. Leong b,* b
a Department of Chemistry, Uni6ersity of Malaya, Kuala Lumpur 50603, Malaysia Department of Biotechnology, Ngee Ann Polytechnic, 535 Clementi Rd, Singapore 599489, Singapore
Received 9 April 1998; accepted 6 January 1999
Abstract The effects of cis- and trans-1,2-, trans-1,4-cyclohexanedicarboxylic acid, 95% cis-1,3,5-cyclohexane tricarboxylic acid and cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid on the yield stress – pH behaviour of concentrated ZrO2 dispersions are reported. Adsorbed cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid imparts predominantly steric interactions. It forms a steric barrier keeping the interacting particles apart. Adsorbed cis- and trans-1,2 increase the maximum yield stress and this was attributed to a hydrophobic force resulting from the part of the cyclohexane ring sticking out into the solution which is devoid of charged or hydrophilic group. Adsorbed trans-1,4 increases the maximum yield stress by at least threefold and its configuration favours strong bridging interaction with an adjacent particle. Predominantly, cis-1,3,5 also increases the maximum yield stress but only by 60% at the same additive concentration. This was attributed to a smaller degree of bridging. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Steric, bridging; Hydrophobic forces; Yield stress; pH; Dispersions; Cyclohexane; Carboxylic acids
1. Introduction Chemical additives are so commercially important and so commonly used in the processing of colloidal dispersions [1,2] that their functions and role have to be well understood, and this can only be achieved through extensive research. There is already extensive literature on the role of various * Corresponding author. Tel.: +65-460-6933; fax: + 65467-9109. E-mail address: [email protected] (Y.K. Leong)
chemical additives. However, new important roles are continually being discovered, especially in this era where colloidal and surface forces between distinct bodies containing adsorbed additives are being routinely measured [3–5]. Leong et al. have shown that adsorbed additives can impart a number of forces of interaction between particles in aqueous colloidal dispersions. The type of force clearly depends upon the chemical nature and interfacial molecular configuration of the additives. Steric forces were observed with small hydrophilic molecules [6,7], polyelectrolytes [8],
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bridging forces with bolaform surfactants [9], charged patch attractive forces with strong polyelectrolytes [10,11] and hydrophobic molecules with single head group surfactant [12]. These studies used a combination of surface chemistry and rheological techniques to determine the nature of the forces. An acoustosizer was used to determine the zeta potential and point of zero charge, and a vane yield stress rheometer measured the strength of the flocculated network structure and, hence, indirectly the strength of the interparticle forces. Adsorbed small hydrophilic molecules such as citrate and phosphate form a hard wall steric layer which keeps the interacting particles further apart where the van der Waals interaction is smaller [6,7]. Hence, this results in a smaller yield stress at the pH of zero zeta potential, pHz = 0. Bolaform surfactants form highly directed bridging interactions and these interactions can cause a 10-fold increase in the strength of attraction. The electrostatic bonding between the negative carboxylic acid and a positive site on the surface was calculated to be of the order of 10 kT. Single head group surfactant gives rise to the strong hydrophobic force and a sixfold increase in attraction was recorded [12]. Strong polyelectrolytes such as polystyrene sulphonic acids form charged patches on the particle surface and this charged patch interaction leads to about a threefold increase in attraction [10,11]. Under most conditions, adsorbed large molecules such as polymer can take on a large number of conformations and configurations, giving rise to all type of interactions. As a result, it is quite difficult to identify the predominant force of interaction. However, this problem can be largely overcome by using small molecules as the number of molecular configurations or conformations are very limited. In this paper, the effects of cyclohexanecarboxylic acids containing two, three and six carboxylic acid groups and its cis – trans configuration on the yield stress of concentrated aqueous ZrO2 dispersions are reported. For the purpose of comparison, results for an amphoteric carboxylic acid containing both potentially positive and negatively charged groups are also included.
2. Materials and methods The zirconia powder (monoclinic ZrO2) used has a BET surface area of 18 m2 g − 1 and an isoelectric point at pH 7.5. The carboxylic acids used as additives are trans-1,4-cyclohexanedicarboxylic acid (Tokyo Kasei), 1,3,5-cyclohexanetricarboxylic acid (Aldrich; 95% cis isomer), disodium salt of ethylenediamine tetraacetate (BDH, AnalaR), 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (Aldrich; all cis), cis-1,2-cyclohexanedicarboxylic acid (Tokyo Kasei), and trans-1,2-cyclohexanedicarboxylic acid (Tokyo Kasei). The following procedure was applied for the preparation of a typical ZrO2 suspension containing an additive. For example, in the preparation of 0.1 dwb% (dwb%, dry weight basis%= g (100 g ZrO2) − 1), trans-1,4-cyclohexanedicarboxylic acid, 35g of zirconia was weighed into a plastic vial. In a separate beaker, 0.035 g of trans-1,4-cyclohexanedicarboxylic acid was added into 31.04 g of distilled water, which was sonicated in a sonicating bath until the additive dissolved completely (if the additive was not completely soluble at its natural pH, then the pH of the mixture was adjusted until complete solubilization was achieved). The additive solution was added into the ZrO2 powder in the plastic vial to give a suspension of 53 wt% ZrO2 with 0.1 dwb% trans1,4-cyclohexanedicarboxylic acid. Sodium hydroxide solution (8 M) was added dropwise to adjust the pH of the suspension to 10 so as to disperse the suspension completely during sonication. The suspension was then sonicated for about 1 min with a sonic probe of a Misonix Ultrasonic Liquid Processor (Model XL2020). The sonifier was operated at 30–40% of the maximum power output of 550 W. The prepared suspension was rested for at least 1 day prior to any measurement. Concentrated nitric acid solutions (1–14.4 M) were used to change the pH of the suspension so as to minimize the extent of dilution. Localized flocculation was observed in the vicinity of the acid droplets. These droplets were redispersed immediately by sonication. After each pH change, the suspension was left to stand for a while and then stirred vigorously with a spatula before conduct-
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ing the pH and yield stress measurements. (After each yield stress measurement, the pH was measured again to confirm the pH of the suspension). The pH was measured with an Orion 410A pH meter and the yield stress with a modified Bohlin VOR vane rheometer.
3. Results and discussion The effects of ethylenediamine tetraacetic (EDTA) acid and cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid on the yield stress – pH behaviour of 53 wt% ZrO2 suspension are shown in Figs. 1 and 2, respectively. It is clear from Figs. 1 and 2 that the pH of ty max are shifted to acidic pH and the extent of shift increases with EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid concentrations, respectively. In the absence of additive, the ZrO2 suspension is dispersed (i.e ty = 0.0) at pH values below 5.0 and above 9.5. At 0.5 dwb% EDTA, the dispersion pH occurs below 2.5 and above 8.0, and the entire yield stress–pH curve is shifted to a lower pH upon addition of EDTA. However, at 1.0 dwb% of EDTA, a
Fig. 1. The effects of EDTA on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant steric effect arising from adsorbed EDTA.
Fig. 2. The effects of all cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid on the yield stress – pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant steric effect arising from adsorbed cyclohexanehexacarboxylic acid.
plateau is formed below pH 4.0. The high ionic strength of 0.12 M NaNO3 for 1.0 dwb% , EDTA compared with 0.04 M NaNO3 for 0.1 and 0.2 dwb% EDTA has broadened the yield stress–pH curve for 1.0 dwb% EDTA in low pH region. A more interesting feature of Figs. 1 and 2 is the obvious reduction in the maximum yield stress, ty maxwith addition of EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, respectively. The extent of reduction in ty max increases with increasing concentrations of EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid. The 1.0 dwb% of 1,2,3,4,5,6-cyclohexanehexacarboxylic acid has reduced ty maxto about 180 Pa which occurs at pH 2.0. The ty maxvalue in the absence of additive (550 Pa) is 5.5 and three times higher compared with 1.0 dwb% of EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, respectively. The dependence of the magnitude of ty maxwith the amount of additive indicates that the particle–particle interaction at the pH of zero zeta potential is dominated by forces other than global (DLVO) electrostatic forces. The reduction in ty maxis attributed to the adsorbed EDTA and 1,2,3,4,5,6cyclohexanehexacarboxylic acid which provide a
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steric barrier between the ZrO2 particles. A significant difference between Figs. 1 and 2 is the ability of the ZrO2 suspensions to have zero yield stress with EDTA concentrations up to 0.5 dwb% at a very acidic region. At very low pH, EDTA additive can be transformed to a positively charged ion because of the presence of nitrogen atoms from the ethylenediamine group of EDTA. As the ZrO2 particles are also highly positively charged, a strong repulsion in the suspension which results in zero yield stress at a very acidic pH is therefore expected. The yield stress for 1,2,3,4,5,6-cyclohexanehexacarboxylic acid reduces gradually with the increase in concentrations of the additive. Since all the carboxylic groups are positioned at one side of the ring, they are all likely to be adsorbed to one particle only and so steric repulsion between particles occurs. Figs. 3 and 4 show similar yield stress behaviour in which ty maxdecreases initially with the addition of additives, namely cis- and trans-1,2cyclohexanedicarboxylic acid. However, on further addition of additives, the yield stress values are increased considerably. The decrease in yield stress is mainly due to the steric barrier formed by
Fig. 3. The effects of cis-1,2-cyclohexanedicarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant hydrophobic interaction at high additive concentration.
Fig. 4. The effects of trans-1,2-cyclohexanedicarboxylic acid on the yield stress – pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant hydrophobic interaction at high additive concentration.
the negatively charged carboxylic groups which are adsorbed on the oxide particles. The increase is most likely due to the hydrophobic force, which is only applicable at high concentration of additive, from the exposed hydrocarbon chain at one end of the molecule, while the carboxylic groups, which are at the other end, are adsorbed onto the oxide surface. Since cis- and trans-1,2-cyclohexanedicarboxylic acid are anionic additives, the isoelectric point of the ZrO2 suspension with these additives also shift towards the acidic region and the shift of the pH of ty max increases with the concentration of the additives added. The effects of trans-1,4-cyclohexanedicarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 suspension are shown in Fig. 5. All suspensions with the additive were initially dispersed at a pH value of 10.0–11.0. The pH was decreased gradually with 5 M nitric acid solution during the yield stress measurement. The flocculation of the suspension with 0.1 and 0.2 dwb% of additive was observed to start at pH 9.0 and was characterized by low yield stress. With further decrease in pH, the yield stress increases to a maximum and then decreases. The trans-1,4-cy-
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Fig. 5. The effects of trans-1,4-cyclohexanedicarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show significant bridging interactions at low additive concentrations.
clohexanedicarboxylic acid increases the yield stress very significantly, as evidenced by a large increase in the maximum yield stress, ty max. The addition of 0.1 dwb% of additive increases the ty maxto 1000 Pa from 550 Pa for a ZrO2 suspension without any additive (i.e. approximately a twofold increase). The increase in ty maxis threefold with the addition of 0.2 dwb% additive in the suspension. To account for this large effect, an
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additional attractive force or forces other than the Van der Waals force must be present. Since the carboxylic groups of this additive are at the opposite ends of the cyclohexane ring, it is plausible that by adsorption onto the oxide surfaces, the additive can form a bridge between two ZrO2 particles (Fig. 6) and hence bind them together, with a concomitant increase in the yield stress of the suspension. This observation shows that a high molecular weight polymeric additive [13,14] is not the only one that can enhance bridging force. Hydrophobic force is unlikely to be important here as there are no long hydrocarbon chains in this additive. Upon increasing the concentration of additive to 0.8 dwb%, ty max decreases to 500 Pa. This is likely due to the saturation of the adsorbed additives on the surface of ZrO2 particles, which results in same surface charges on the particles and steric force to occur between ZrO2 particles. The ionic strength at ty max is about 0.07 M NaNO3, for suspensions containing 0.0–0.2 dwb% of trans-1,4-cyclohexanedicarboxylic acid. This increases to 0.15 M NaNO3 for 0.8 dwb% additive. Unlike the suspension containing a lower concentration of additive, there is no complete dispersed region for 0.8 dwb% additive at high pH. The flocculation at pH11 is most probably due to ionic strength effect because the negatively charged trans-1,4-cyclohexanedicarboxylate is unlikely to be adsorbed on a highly negative charged ZrO2 particle at this region. The isoelectric point of the suspension with adsorbed
Fig. 6. A representation of particle bridging by adsorbed trans-1,4-cyclohexanedicarboxylic acid molecules.
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Fig. 7. The effects of 95% cis-1,3,5-cyclohexanetricarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show significant bridging interactions at low additive concentrations.
additive is shifted towards the more acidic region, namely pH 4.0 – 5.0 for all the concentrations. A similar effect on the yield stress behaviour is displayed by cis-1,3,5-cyclohexanetricarboxylic acid, and the results are shown in Fig. 7. However, the increase in ty max for 0.1 and 0.2 dwb% of additive is not as significant as the effect from trans-1,4-cyclohexanedicarboxylic acid. The additions of 0.1 and 0.2 dwb% of 1,3,5-cyclohexanetricarboxylic acid increase the ty maxto 600 and 800 Pa, respectively. Since all the carboxylic groups are placed in equatorial positions [15], two of the carboxylic groups can be adsorbed onto one ZrO2 particle and the third carboxylic group to another ZrO2 particle, bridging the two ZrO2 particles together. Hence, bridging force also acts here as the additional attractive force. When the concentration of the additive is 0.5 dwb%, ty max is reduced to 580 Pa; however, there is still a slight increase in the yield stress compared with the ZrO2 suspension without any additive. Upon increasing the concentration to 1.0 dwb%, ty maxis reduced further to 220 Pa. The reduction of the yield stress at high concentration of additive is due to the increasing adsorption of the additive
on the surface of the ZrO2 particles. At 0.5 dwb%, the surfaces of the ZrO2 particles are not adsorbed by the additive completely, so there is still some bridging force existing. However, when the concentration of the additive has reached 1.0 dwb%, the surfaces of the ZrO2 particles are fully covered by additives causing the particles to repel each other due to steric effects, resulting in the reduction of the yield stress. The pH of ty maxis also shifted to the acidic region, i.e to pH 5.5, 4.5, 4.0 and 3.8 for 0.1, 0.2, 0.5 and 1.0 dwb% of additives, respectively, which is expected for an anionic additive. The ionic strength at ty max for 0.1 and 0.2 dwb% are very low compared with the ionic strength at ty max for 0.5 and 1.0 dwb% of additive, i.e 0.4 M NaNO3. It is possible that flocculation which takes place at high pH for high concentration of additive suspension is again due to the ionic strength effect.
4. Conclusion 1. Adsorbed cis-1,2,3,4,5,6-cyclohexanhexacarboxylic acid decreases the maximum yield stress and this is attributed to the adsorbed additives keeping the interacting zirconia further apart by forming a steric barrier. 2. Both adsorbed cis- and trans-1,2-cyclohexanedicarboxylic acid increase the maximum yield stress and this is attributed to the hydrophobic forces arising from the adsorbed additives. 3. Adsorbed trans-1,4-cyclohexanedicarboxylic acid increases the maximum yield stress by threefold at 0.2 dwb% and this is attributed to bridging interactions. 4. Adsorbed cis-1,3,5-cyclohexanetricarboxylic acid also increases the maximum yield stress but only by 60% at 0.2 dwb% and this is attributed to a smaller degree of bridging.
Acknowledgements This work was supported by IRPA grant number 09-02-03-0374.
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References [1] R. Moreno, Am. Ceram. Soc. Bull. 71 (1992) 1521. [2] R. Moreno, Am. Ceram. Soc. Bull. 71 (1992) 1647. [3] J.N. Israelachvili, R.M. Pashley, J. Colloid Interface Sci. 98 (1984) 500. [4] J.N. Israelachvili, Intermolecular and Surface Forces, 2nd edn, Academic Press, New York, 1992. [5] S. Biggs, P.J. Scales, Y.K. Leong, T.W. Healy, J. Chem. Soc. Faraday Trans. 91 (1995) 2921. [6] Y.K. Leong, P.J. Scales, T.W. Healy, D.V. Boger, R. Buscall, J. Chem. Soc. Faraday Trans. 89 (1993) 2473. [7] Y.K.Leong, D.V. Boger, P.J. Scales, T.W. Healy, R. Buscall, Chem Commun. (1993) 639–641.
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[8] Y.K. Leong, P.J. Scales, T.W. Healy, D.V. Boger, Colloids Surf. A: Physicochem. Eng. Aspects 95 (1995) 43. [9] Y.K. Leong, J. Chem. Soc. Faraday Trans. 93 (1997) 105. [10] Y.K. Leong, in: L.X. Zhou, X.F. Li (Eds.), Multiphase Fluid, Non-Newtonian Fluid and Physicochemical Fluid Flows, International Academic Publishers, Beijing, 1997, pp. 5 – 91. [11] Y.K. Leong, Colloid Polym. Sci. 277 (1999) 299 – 305. [12] Y.K. Leong, D.V. Boger, P.J. Scales, T.W. Healy, J. Colloid Interface Sci. 181 (1996) 605. [13] D.H. Napper, Polymeric Stabilisation of Colloidal Dispersions, Academic Press, London, 1983. [14] Y. Otsubo, K. Watanabe, Colloids Surf. 50 (1990) 341. [15] D.S. Kemp, K.S. Petrakis, J. Org. Chem. 46 (1981) 5140.
The effects of cis–trans configuration of cyclohexane multi-carboxylic acids on colloidal forces in dispersions: steric, hydrophobic and bridging A.V.M. Chandramalar a, Y.Y. Lim a, Y.K. Leong b,* b
a Department of Chemistry, Uni6ersity of Malaya, Kuala Lumpur 50603, Malaysia Department of Biotechnology, Ngee Ann Polytechnic, 535 Clementi Rd, Singapore 599489, Singapore
Received 9 April 1998; accepted 6 January 1999
Abstract The effects of cis- and trans-1,2-, trans-1,4-cyclohexanedicarboxylic acid, 95% cis-1,3,5-cyclohexane tricarboxylic acid and cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid on the yield stress – pH behaviour of concentrated ZrO2 dispersions are reported. Adsorbed cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid imparts predominantly steric interactions. It forms a steric barrier keeping the interacting particles apart. Adsorbed cis- and trans-1,2 increase the maximum yield stress and this was attributed to a hydrophobic force resulting from the part of the cyclohexane ring sticking out into the solution which is devoid of charged or hydrophilic group. Adsorbed trans-1,4 increases the maximum yield stress by at least threefold and its configuration favours strong bridging interaction with an adjacent particle. Predominantly, cis-1,3,5 also increases the maximum yield stress but only by 60% at the same additive concentration. This was attributed to a smaller degree of bridging. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Steric, bridging; Hydrophobic forces; Yield stress; pH; Dispersions; Cyclohexane; Carboxylic acids
1. Introduction Chemical additives are so commercially important and so commonly used in the processing of colloidal dispersions [1,2] that their functions and role have to be well understood, and this can only be achieved through extensive research. There is already extensive literature on the role of various * Corresponding author. Tel.: +65-460-6933; fax: + 65467-9109. E-mail address: [email protected] (Y.K. Leong)
chemical additives. However, new important roles are continually being discovered, especially in this era where colloidal and surface forces between distinct bodies containing adsorbed additives are being routinely measured [3–5]. Leong et al. have shown that adsorbed additives can impart a number of forces of interaction between particles in aqueous colloidal dispersions. The type of force clearly depends upon the chemical nature and interfacial molecular configuration of the additives. Steric forces were observed with small hydrophilic molecules [6,7], polyelectrolytes [8],
0927-7757/00/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 0 2 8 - X
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bridging forces with bolaform surfactants [9], charged patch attractive forces with strong polyelectrolytes [10,11] and hydrophobic molecules with single head group surfactant [12]. These studies used a combination of surface chemistry and rheological techniques to determine the nature of the forces. An acoustosizer was used to determine the zeta potential and point of zero charge, and a vane yield stress rheometer measured the strength of the flocculated network structure and, hence, indirectly the strength of the interparticle forces. Adsorbed small hydrophilic molecules such as citrate and phosphate form a hard wall steric layer which keeps the interacting particles further apart where the van der Waals interaction is smaller [6,7]. Hence, this results in a smaller yield stress at the pH of zero zeta potential, pHz = 0. Bolaform surfactants form highly directed bridging interactions and these interactions can cause a 10-fold increase in the strength of attraction. The electrostatic bonding between the negative carboxylic acid and a positive site on the surface was calculated to be of the order of 10 kT. Single head group surfactant gives rise to the strong hydrophobic force and a sixfold increase in attraction was recorded [12]. Strong polyelectrolytes such as polystyrene sulphonic acids form charged patches on the particle surface and this charged patch interaction leads to about a threefold increase in attraction [10,11]. Under most conditions, adsorbed large molecules such as polymer can take on a large number of conformations and configurations, giving rise to all type of interactions. As a result, it is quite difficult to identify the predominant force of interaction. However, this problem can be largely overcome by using small molecules as the number of molecular configurations or conformations are very limited. In this paper, the effects of cyclohexanecarboxylic acids containing two, three and six carboxylic acid groups and its cis – trans configuration on the yield stress of concentrated aqueous ZrO2 dispersions are reported. For the purpose of comparison, results for an amphoteric carboxylic acid containing both potentially positive and negatively charged groups are also included.
2. Materials and methods The zirconia powder (monoclinic ZrO2) used has a BET surface area of 18 m2 g − 1 and an isoelectric point at pH 7.5. The carboxylic acids used as additives are trans-1,4-cyclohexanedicarboxylic acid (Tokyo Kasei), 1,3,5-cyclohexanetricarboxylic acid (Aldrich; 95% cis isomer), disodium salt of ethylenediamine tetraacetate (BDH, AnalaR), 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (Aldrich; all cis), cis-1,2-cyclohexanedicarboxylic acid (Tokyo Kasei), and trans-1,2-cyclohexanedicarboxylic acid (Tokyo Kasei). The following procedure was applied for the preparation of a typical ZrO2 suspension containing an additive. For example, in the preparation of 0.1 dwb% (dwb%, dry weight basis%= g (100 g ZrO2) − 1), trans-1,4-cyclohexanedicarboxylic acid, 35g of zirconia was weighed into a plastic vial. In a separate beaker, 0.035 g of trans-1,4-cyclohexanedicarboxylic acid was added into 31.04 g of distilled water, which was sonicated in a sonicating bath until the additive dissolved completely (if the additive was not completely soluble at its natural pH, then the pH of the mixture was adjusted until complete solubilization was achieved). The additive solution was added into the ZrO2 powder in the plastic vial to give a suspension of 53 wt% ZrO2 with 0.1 dwb% trans1,4-cyclohexanedicarboxylic acid. Sodium hydroxide solution (8 M) was added dropwise to adjust the pH of the suspension to 10 so as to disperse the suspension completely during sonication. The suspension was then sonicated for about 1 min with a sonic probe of a Misonix Ultrasonic Liquid Processor (Model XL2020). The sonifier was operated at 30–40% of the maximum power output of 550 W. The prepared suspension was rested for at least 1 day prior to any measurement. Concentrated nitric acid solutions (1–14.4 M) were used to change the pH of the suspension so as to minimize the extent of dilution. Localized flocculation was observed in the vicinity of the acid droplets. These droplets were redispersed immediately by sonication. After each pH change, the suspension was left to stand for a while and then stirred vigorously with a spatula before conduct-
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ing the pH and yield stress measurements. (After each yield stress measurement, the pH was measured again to confirm the pH of the suspension). The pH was measured with an Orion 410A pH meter and the yield stress with a modified Bohlin VOR vane rheometer.
3. Results and discussion The effects of ethylenediamine tetraacetic (EDTA) acid and cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid on the yield stress – pH behaviour of 53 wt% ZrO2 suspension are shown in Figs. 1 and 2, respectively. It is clear from Figs. 1 and 2 that the pH of ty max are shifted to acidic pH and the extent of shift increases with EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid concentrations, respectively. In the absence of additive, the ZrO2 suspension is dispersed (i.e ty = 0.0) at pH values below 5.0 and above 9.5. At 0.5 dwb% EDTA, the dispersion pH occurs below 2.5 and above 8.0, and the entire yield stress–pH curve is shifted to a lower pH upon addition of EDTA. However, at 1.0 dwb% of EDTA, a
Fig. 1. The effects of EDTA on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant steric effect arising from adsorbed EDTA.
Fig. 2. The effects of all cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid on the yield stress – pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant steric effect arising from adsorbed cyclohexanehexacarboxylic acid.
plateau is formed below pH 4.0. The high ionic strength of 0.12 M NaNO3 for 1.0 dwb% , EDTA compared with 0.04 M NaNO3 for 0.1 and 0.2 dwb% EDTA has broadened the yield stress–pH curve for 1.0 dwb% EDTA in low pH region. A more interesting feature of Figs. 1 and 2 is the obvious reduction in the maximum yield stress, ty maxwith addition of EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, respectively. The extent of reduction in ty max increases with increasing concentrations of EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid. The 1.0 dwb% of 1,2,3,4,5,6-cyclohexanehexacarboxylic acid has reduced ty maxto about 180 Pa which occurs at pH 2.0. The ty maxvalue in the absence of additive (550 Pa) is 5.5 and three times higher compared with 1.0 dwb% of EDTA and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, respectively. The dependence of the magnitude of ty maxwith the amount of additive indicates that the particle–particle interaction at the pH of zero zeta potential is dominated by forces other than global (DLVO) electrostatic forces. The reduction in ty maxis attributed to the adsorbed EDTA and 1,2,3,4,5,6cyclohexanehexacarboxylic acid which provide a
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steric barrier between the ZrO2 particles. A significant difference between Figs. 1 and 2 is the ability of the ZrO2 suspensions to have zero yield stress with EDTA concentrations up to 0.5 dwb% at a very acidic region. At very low pH, EDTA additive can be transformed to a positively charged ion because of the presence of nitrogen atoms from the ethylenediamine group of EDTA. As the ZrO2 particles are also highly positively charged, a strong repulsion in the suspension which results in zero yield stress at a very acidic pH is therefore expected. The yield stress for 1,2,3,4,5,6-cyclohexanehexacarboxylic acid reduces gradually with the increase in concentrations of the additive. Since all the carboxylic groups are positioned at one side of the ring, they are all likely to be adsorbed to one particle only and so steric repulsion between particles occurs. Figs. 3 and 4 show similar yield stress behaviour in which ty maxdecreases initially with the addition of additives, namely cis- and trans-1,2cyclohexanedicarboxylic acid. However, on further addition of additives, the yield stress values are increased considerably. The decrease in yield stress is mainly due to the steric barrier formed by
Fig. 3. The effects of cis-1,2-cyclohexanedicarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant hydrophobic interaction at high additive concentration.
Fig. 4. The effects of trans-1,2-cyclohexanedicarboxylic acid on the yield stress – pH behaviour of a 53 wt% ZrO2 dispersion. The results show a significant hydrophobic interaction at high additive concentration.
the negatively charged carboxylic groups which are adsorbed on the oxide particles. The increase is most likely due to the hydrophobic force, which is only applicable at high concentration of additive, from the exposed hydrocarbon chain at one end of the molecule, while the carboxylic groups, which are at the other end, are adsorbed onto the oxide surface. Since cis- and trans-1,2-cyclohexanedicarboxylic acid are anionic additives, the isoelectric point of the ZrO2 suspension with these additives also shift towards the acidic region and the shift of the pH of ty max increases with the concentration of the additives added. The effects of trans-1,4-cyclohexanedicarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 suspension are shown in Fig. 5. All suspensions with the additive were initially dispersed at a pH value of 10.0–11.0. The pH was decreased gradually with 5 M nitric acid solution during the yield stress measurement. The flocculation of the suspension with 0.1 and 0.2 dwb% of additive was observed to start at pH 9.0 and was characterized by low yield stress. With further decrease in pH, the yield stress increases to a maximum and then decreases. The trans-1,4-cy-
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Fig. 5. The effects of trans-1,4-cyclohexanedicarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show significant bridging interactions at low additive concentrations.
clohexanedicarboxylic acid increases the yield stress very significantly, as evidenced by a large increase in the maximum yield stress, ty max. The addition of 0.1 dwb% of additive increases the ty maxto 1000 Pa from 550 Pa for a ZrO2 suspension without any additive (i.e. approximately a twofold increase). The increase in ty maxis threefold with the addition of 0.2 dwb% additive in the suspension. To account for this large effect, an
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additional attractive force or forces other than the Van der Waals force must be present. Since the carboxylic groups of this additive are at the opposite ends of the cyclohexane ring, it is plausible that by adsorption onto the oxide surfaces, the additive can form a bridge between two ZrO2 particles (Fig. 6) and hence bind them together, with a concomitant increase in the yield stress of the suspension. This observation shows that a high molecular weight polymeric additive [13,14] is not the only one that can enhance bridging force. Hydrophobic force is unlikely to be important here as there are no long hydrocarbon chains in this additive. Upon increasing the concentration of additive to 0.8 dwb%, ty max decreases to 500 Pa. This is likely due to the saturation of the adsorbed additives on the surface of ZrO2 particles, which results in same surface charges on the particles and steric force to occur between ZrO2 particles. The ionic strength at ty max is about 0.07 M NaNO3, for suspensions containing 0.0–0.2 dwb% of trans-1,4-cyclohexanedicarboxylic acid. This increases to 0.15 M NaNO3 for 0.8 dwb% additive. Unlike the suspension containing a lower concentration of additive, there is no complete dispersed region for 0.8 dwb% additive at high pH. The flocculation at pH11 is most probably due to ionic strength effect because the negatively charged trans-1,4-cyclohexanedicarboxylate is unlikely to be adsorbed on a highly negative charged ZrO2 particle at this region. The isoelectric point of the suspension with adsorbed
Fig. 6. A representation of particle bridging by adsorbed trans-1,4-cyclohexanedicarboxylic acid molecules.
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Fig. 7. The effects of 95% cis-1,3,5-cyclohexanetricarboxylic acid on the yield stress–pH behaviour of a 53 wt% ZrO2 dispersion. The results show significant bridging interactions at low additive concentrations.
additive is shifted towards the more acidic region, namely pH 4.0 – 5.0 for all the concentrations. A similar effect on the yield stress behaviour is displayed by cis-1,3,5-cyclohexanetricarboxylic acid, and the results are shown in Fig. 7. However, the increase in ty max for 0.1 and 0.2 dwb% of additive is not as significant as the effect from trans-1,4-cyclohexanedicarboxylic acid. The additions of 0.1 and 0.2 dwb% of 1,3,5-cyclohexanetricarboxylic acid increase the ty maxto 600 and 800 Pa, respectively. Since all the carboxylic groups are placed in equatorial positions [15], two of the carboxylic groups can be adsorbed onto one ZrO2 particle and the third carboxylic group to another ZrO2 particle, bridging the two ZrO2 particles together. Hence, bridging force also acts here as the additional attractive force. When the concentration of the additive is 0.5 dwb%, ty max is reduced to 580 Pa; however, there is still a slight increase in the yield stress compared with the ZrO2 suspension without any additive. Upon increasing the concentration to 1.0 dwb%, ty maxis reduced further to 220 Pa. The reduction of the yield stress at high concentration of additive is due to the increasing adsorption of the additive
on the surface of the ZrO2 particles. At 0.5 dwb%, the surfaces of the ZrO2 particles are not adsorbed by the additive completely, so there is still some bridging force existing. However, when the concentration of the additive has reached 1.0 dwb%, the surfaces of the ZrO2 particles are fully covered by additives causing the particles to repel each other due to steric effects, resulting in the reduction of the yield stress. The pH of ty maxis also shifted to the acidic region, i.e to pH 5.5, 4.5, 4.0 and 3.8 for 0.1, 0.2, 0.5 and 1.0 dwb% of additives, respectively, which is expected for an anionic additive. The ionic strength at ty max for 0.1 and 0.2 dwb% are very low compared with the ionic strength at ty max for 0.5 and 1.0 dwb% of additive, i.e 0.4 M NaNO3. It is possible that flocculation which takes place at high pH for high concentration of additive suspension is again due to the ionic strength effect.
4. Conclusion 1. Adsorbed cis-1,2,3,4,5,6-cyclohexanhexacarboxylic acid decreases the maximum yield stress and this is attributed to the adsorbed additives keeping the interacting zirconia further apart by forming a steric barrier. 2. Both adsorbed cis- and trans-1,2-cyclohexanedicarboxylic acid increase the maximum yield stress and this is attributed to the hydrophobic forces arising from the adsorbed additives. 3. Adsorbed trans-1,4-cyclohexanedicarboxylic acid increases the maximum yield stress by threefold at 0.2 dwb% and this is attributed to bridging interactions. 4. Adsorbed cis-1,3,5-cyclohexanetricarboxylic acid also increases the maximum yield stress but only by 60% at 0.2 dwb% and this is attributed to a smaller degree of bridging.
Acknowledgements This work was supported by IRPA grant number 09-02-03-0374.
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