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MT suspensions
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Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 206–210
Studies on the thixotropic and viscoelastic properties of HTlc/MT suspensions Shu-Ping Li a,b,∗ , Xue-Qin An a , Yin-Yan Zhu a a
Jangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, PR China b Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China Received 16 April 2007; received in revised form 8 July 2007; accepted 10 October 2007 Available online 13 October 2007
Abstract In this paper, we found that the variation content of Fe3+ in the Fe–Al–Mg hydrotalcite-like compounds (HTlc) and the change of R values (R = WHTlc /WMT ) will influence the thixotropic and viscoelastic properties of HTlc/sodium montmorillonite (MT) suspensions. With increasing the content of Fe3+ in the HTlc samples, the storage modulus (G ) and the loss modulus (G ) of the HTlc/MT suspensions will increase gradually, meaning that the structure strength of the HTlc/MT suspension increases with increasing of Fe3+ content in the HTlc samples. Similar results have been found in studying the influence of R values: the structure strength of the suspensions will increase with increasing of R values. Special emphasis has been laid on the phenomenon of thixotropy. With the gradual increase of the structure strength in the HTlc/MT suspension, the thixotropic types will change accordingly. The mechanism has been emphatically discussed in this paper. © 2007 Elsevier B.V. All rights reserved. Keywords: Thixotropy; Viscoelasticity; Rheology
1. Introduction “Thixotropy” was a term first used to describe the timedependent rheological behavior of aluminium hydroxide gels by Freundlich and Bircunshaw [1]. The classical definition of thixotropy is suspension behavior where time-dependent, reversible breakdown of the particulate network structure occurs under shear, followed by structural reformation on resting. The aluminium hydroxide gels investigated by Freundlich and Bircunshaw exhibited a dramatic change from a solid-like (gel) to a liquid-like (sol) state on agitation and back on resting. The breakdown of a thixotropic substance is generally measured by the decay in the shear stress with time at a constant shear rate in a rheometer. Nevertheless, the shear induced structural breakdown of HTlc/MT suspensions is very rapid, almost instantaneous, which makes characterization of the breakdownprocess ∗ Corresponding author at: College of Chemistry and Environment Science, Nanjing Normal University, Nanjing 210097, PR China. Tel.: +86 25 83598280; fax: +86 25 83598678. E-mail address: [email protected] (S.-P. Li).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.10.013
very difficult. Hence, in this paper, thixotropy was studied by monitoring the recovery process as a function of the rest time after intensive shearing. MT compounds, constituted from silica tetrahedral and alumina octahedral sheets have negatively charged sites on the planes owing to the substitution of central Si to Al ions in the crystal lattice. MT suspension is widely used in drilling fluids, paints, cosmetics, soil science and so on. In addition, MT suspensions show shear-thinning, thixotropy and exhibit a yield stress above a certain clay concentration [2]. Hydrotalcite-like compounds (HTlc) belong to a large class of anionic and basic clay, and they are also known as layered double hydroxide. They are composed of positively charged brucite-like [Mg(OH)2 ] layers with trivalent cations substituting for divalent cations at the centers of octahedral sites, and each OH– group is shared by three octahedral cations and points towards the interlayer regions[3,4]. The excess positive charges of HTlcs are compensated by interlamellar anions [3–9]. They can be represented by the general formula of III III [MII 1−x Mx (OH)2 ]x+ An−x/n mH2 O, in which M is the trivalent mental ions, MII the divalent metal ions, A the charge compensating anions, m the number of moles of co-intercalated water
S.-P. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 206–210
per formula weight of compounds; x is the number of moles of MIII per formula weight of compounds, generally ranges from 0.2 to 0.4 [2]. Then, HTlc/MT suspensions are a particular system, consisting of oppositely charged particles, and recently much attention has been focused on this type of system [10,11]. Our previous studies indicated that the suspension of HTlc/MT is easily to form three-dimensional gel-like structure instead of flocculation or coagulation, consequently extensive studies have been reported on their rheological properties [12–15]. In this paper, we emphatically examined the effect of the metal content of the HTlc samples on the rheological properties of the HTlc/MT suspensions, relevant reports have not been found as to the authors known. 2. Material and method 2.1. Material HTlc was synthesized by the co-precipitation method. The pre-designed metal chloride was dissolved into the deioned water, then NH4 OH was slowly added into the mixed solution to adjust the pH of the solution to about 9.5, so the delicate and small flocs suspended in the solution were formed. Then, the flocs were filtered and washed with the double-distilled water. Finally, the flocs were peptized at about 80 ◦ C in the oven for about 24 h and converted to HTlc sol. The chemical composition of the HTlc samples was made with the 3080E2-type automatic X-ray fluorescent spectrometer (made in Japan), the chemical composition of the samples was listed in Table 1. The MT clay was pretreated according to literature [16]. The clay was first suspended in distilled water with stirring and then decanted leaving the deposit. A certain amount of H2 O2 was added to remove the organic species. The dispersion was heated to ∼70 ◦ C to obtain the clay deposit. Then the deposit was suspended in 1 M NaCl solution for 6 h. Finally, water in the dispersion was replaced for several times with doubledistilled water until its conductivity became as low as about 4 × 10−5 −1 cm−1 , indicating that most of the electrolyte had been removed. The final concentration of the clay can be determined with the evaporation method. A certain amount of MT was mixed with double-distilled water using a high-speed mixer (Model GJ-1, Jiangyin Second Electrical Machinery Plant) for 20 min to ensure that the MT powders were sufficiently dispersed in water. Then after aged in a sealed container for 24 h, MT can be used in the experiments. The mixture of MT and HTlc was stirred by the high-speed mixer for 20 min, and then aged for about 24 h before the
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experiments. The mass ratio of HTlc and MT was defined as R = WHTlc /WMT . The MT content in HTlc/MT suspensions was 3.0 wt.%. The pH values of all the suspensions were adjusted to 9.6 ± 0.2. 2.2. Measurement The thixotropy of the suspension has been defined as the change of the rheological properties with time under constant shear stress or shear rate, following by a gradual recovery when the stress or the shear rate is removed. In this paper, we studied the thixotropy by measuring the viscosity as a function of time after vigorously stirred with reference to experiments described by Heckroodt and Ryan [17]. The HTlc/MT suspension was first presheared under 1000 s−1 for 30 s, and the viscosity (η) of the system at different time was measured at the low shear rate of 10 s−1 . The increase of η with time means positive thixotropy while the decrease of η with time means negative thixotropy; first increase and then decrease of η with time or first decrease and then increase of η with time both means complex thixotropy [11]. Viscoelastic measurements were carried out with a HAAKE RS75 Rheometer (Hakke Co., Germany) at 25 ± 0.2 ◦ C for obtaining G and G . For the measurements of the relationship of G , G with the time, the experiments had to be carried out at sufficiently small shear stress (in the linear viscoelastic region) in order to avoid the disturbance to the recovery process. Therefore, the oscillation conditions were set to f = 1 Hz, τ = 0.1 Pa for the experiments discussed here. 3. Experimental results In this paper, three Fe–Mg–Al HTlc samples were examined. From sample 1 to sample 3, the content of Fe3+ increased gradually while keeping the ratio of Mg to Al remained constant (see Table 1). The re-creation of the equilibrium rest structure after shearing is considered as a fundamental thixotropic process, and this process has been usually investigated by transient oscillatory shear experiments, the condition is simplified if the measurements are performed in the linear viscoelastic region. In this region, no irreversible change of the structure in the suspension has been taken place [13]. Fig. 1 displays the relationship of the logarithms of G and G with the logarithm of the time, and the value of R was all assigned to 0.17. The experiments on other R values have also been performed, and the same tendency has been obtained. It can be observed in Fig. 1 that for each HTlc/MT suspension, G increased with the increasing recovery time, indicating the
Table 1 Chemical Composition of the HTlc samples Sample number
Sample
Raw material molar ratio
Chemical composition
1 2 3
Mg/Al/Fe Mg/Al/Fe Mg/Al/Fe
2:1:0.1 2:1:0.6 2:1:1
[Fe0.034 Mg0.65 Al0.32 (OH)2 ]·Cl0.14 (OH)0.22 [Fe0.21 Mg0.46 Al0.33 (OH)2 ]·Cl0.11 (OH)0.41 [Fe0.29 Mg0.42 Al0.29 (OH)2 ]·Cl0.10 (OH)0.48
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Fig. 1. The plot of the logarithms of G and G with the logarithm of time for the different HTlc/MT suspensions with R = 0.170. (a , a ) Sample 1/MT; (b , b ) sample 2/MT; and (c , c ) sample 3/MT.
Fig. 2. The plot of η with time for the different HTlc/MT suspensions with R = 0.013. (a) Sample 1/MT; (b)sample 2/MT; and (c)sample 3/MT.
recovery of the network structure increased with time. Whereas, for each HTlc/MT suspension, with increasing the content of Fe3+ in the HTlc samples, G and G increased gradually, which illustrated that the structure strength of the HTlc/MT suspensions would be increased with increasing Fe3+ content. Figs. 2 and 3 show the relationship curves of η for the three HTlc/MT suspensions made up of different HTlc samples with the time, and the R values were 0.013 and 0.170, respectively.
Fig. 3. The plot of η with time for the different HTlc/MT suspensions with R = 0.170. (a) Sample 1/MT; (b)sample 2/MT; and (c)sample 3/MT.
Fig. 4. The plot of η with time for sample 1/MT suspension with various R values. R: (a) 0.013; (b) 0.043; (c) 0.085; (d) 0.170; and (e) 0.344.
It can be found in Fig. 2 that when R = 0.013, the three suspensions all showed positive thixotropy, and with increasing the content of Fe3+ in the HTlc samples, the viscosities of the HTlc/MT suspensions increased gradually. It also can be observed in Fig. 3 that when R = 0.170, the system of sample 1/MT behaved as positive thixotropy; while the other systems all showed complex thixotropic behavior. Moreover, with increasing the content of Fe3+ in the HTlc samples, the viscosities of the HTlc/MT suspensions increased earlier then decreased later. Fig. 4 shows the evolution of the viscosities for sample 1/MT suspension with time at different R values. Figs. 5 and 6 also display the relationship of the viscosities with the time for sample 2/MT suspension and sample 3/MT suspension, respectively. It can be concluded that the values of R can obviously influence the thixotropic types of the suspensions. As increasing of R, the thixotropic types of the three suspensions all changed from the positive to the complex. 4. Discussion In any colloidally stable dispersion, the overall particle action is repulsive, therefore, particles repel each other and freely moving primary particles endeavor to place themselves as far from each other as they can. Particles of colloidally unstable dis-
Fig. 5. The plot of η with time for sample 2/MT suspension with various R values. R: (a) 0.013; (b) 0.033; (c) 0.053; (d) 0.100; and (e) 0.170.
S.-P. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 206–210
Fig. 6. The plot of η with time for sample 3/MT suspension with various R values. R: (a) 0.013; (b) 0.060; (c) 0.084; (d) 0.113; and (e) 0.170.
persions collide and stick closely together, and so a more or less loose physical network of adhered particles form in the unstable systems, with a large amount of dispersion medium entrapped in the structure. In general, stable suspensions show ideal liquid-like, Newton flow behavior, while the appearance of the non-Newton behavior refers to the network formation of aggregated particles. From our previous studies, the oppositely charged particles of HTlc and MT can form the network structure, and the recovery process of the HTlc/MT suspensions was performed as a monotonic increase of |η*| with time in the linear viscoelastic region, which is attributed to the formation of the three-dimensional network structure [13]. In this paper, the thixotropic behavior was studied under steady shear experiments. The difference between the steady shear experiments and the oscillatory experiments is that the system is disturbed during the measurements in the steady shear experiments, and this disturbance will influence the results. In the different HTlc/MT suspensions, the influence of the disturbance on the recovery process is different. When the structure strength of the system is low, the influence of the disturbance on the recovery process is weak. When the structure strength is high, the influence of the disturbance also becomes distinct. The distinct disturbance will result in the close joint of the oppositely charged HTlc particles and MT particles, in the form of dense floc clusters or sediments. The appearance of the dense floc clusters or sediments will destroy or inhibit the formation of the network, hence, η decreased gradually with the time, as a result, giving rise to the appearance of the complex or negative thixotropy. Whereas, the weak disturbance hardly gives rise to the appearance of dense floc clusters or sediments, hence, η increased gradually with the time, showing positive thixotropy. From our experimental results, the change of R and the change of Fe3+ in the HTlc samples will influence the rheological properties together. When R is low (see Fig. 2), the structure strength of the three suspensions is low as well. As a result, the influence of the shear rate on the recovery process is weak, too. Hence, all the systems behaved as positive thixotropy, and the viscosities of the suspensions increased with Fe3+ in the HTlc samples. While when R is high (see Fig. 3), the structure strength of the three suspensions is also high. Therefore, the influence of
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the disturbance on the recovery process becomes more distinct. Hence, for the system whose structure strength is relatively low (see Fig. 3 line a, the content of Fe3+ is relatively low), the influence of the disturbance is relatively weak, and then η of the system will increase gradually in the recovery process, the suspension behaved as positive thixotropy. For the system whose structure strength is relatively high (see Fig. 3 line b and c), the influence of the disturbance on the recovery process becomes more distinct, and then η of the system will increase earlier then decrease later with time in the recovery process, the suspension showed complex thixotropy. In addition, the formation of the sediments will result in the decrease of the total viscosities. Therefore, the viscosities of the HTlc/MT suspensions with R = 0.170 first increased and then decreased with increasing of Fe3+ . As far as the change of R on the thixotropy of the systems has been concerned, two factors-R values and the change of Fe3+ in the HTlc samples-should be considered together. When the content of Fe3+ in the HTlc sample is low (see Fig. 4), the structure strength of the suspension is low, then the influence of the disturbance on the recovery process is also weak, as a result, the increase of R gives no change to the thixotropic type until the value of R exceeds 0.344. When the content of Fe3+ in the HTlc sample is relatively high (see Figs. 5 and 6), the structure strength becomes higher accordingly, the influence of the shear rate on the recovery process becomes distinct as well, hence, the increase of R gives quick change of thixotropy-from positive to the complex one. 5. Summary It is found that the content of Fe3+ in the HTlc samples and R values of HTlc/MT suspensions will influence the structure strength of the suspensions. The rheological behavior of the HTlc/MT suspensions was studied both under steady shear experiments and under the oscillatory experiments. The difference is that the system is disturbed during the steady shear measurements, and this disturbance will influence the thixotropic types. When the structure strength of the system is low, the influence of the steady shear on the recovery process is weak; when the structure strength is high, the influence of steady shear also becomes distinct. Acknowledgements The authors are grateful for the financial support of National Natural Science Foundation of Jiangsu Province (BK2007223), the Startup Fund for Outstanding Persons in Nanjing Normal University and the Foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education. References [1] V.H. Freundlich, L.L. Bircunshaw, Uber das, Kolloid Z. 40 (1926) 19. [2] S. Albend, G. Lagaly, Appl. Clay Sci. 16 (2000) 201. [3] W.G. Hou, C. Ren, Chin. J. Chem. 24 (2006) 1336.
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[4] Z.L. Jin, W.G. Hou, X.W. Li, D.J. Sun, C.G. Zhang, Chin. Chem. Lett. 16 (2005) 835. [5] W.G. Hou, S.E. Song, J. Colloid Interf. Sci. 269 (2004) 381. [6] W.G. Hou, S.H. Han, D.Q. Li, G.Y. Zhang, Chin. J. Chem. 22 (2004) 268. [7] W.G. Hou, P. Jiang, S.H. Han, J.F. Hu, D.Q. Li, Colloid Polym. Sci. 281 (2003) 738. [8] X.N. Dai, W.G. Hou, H.D. Duan, P. Ni, Colloids Surf. A 295 (2007) 139. [9] Y.N. Jiao, W.G. Hou, Colloids Surf. A 296 (2007) 62. [10] S.P. Li, W.G. Hou, J. Disper. Sci. Technol. 24 (2003) 145. [11] S.P. Li, W.G. Hou, D.J. Sun, C.G. Zhang, Chem. J. Chin. Univ. 22 (2001) 1173.
[12] S.P. Li, W.G. Hou, C.X. Jia, J.F. Hu, D.Q. Li, Colloids Surf. A 224 (2003) 149. [13] S.P. Li, W.G. Hou, D.J. Sun, P.Z. Guo, C.X. Jia, J.F. Hu, Langmuir 19 (2003) 3172. [14] S.P. Li, W.G. Hou, X.N. Dai, Acta Chim. Sinica 60 (2002) 749. [15] S.-P. Li, K. Liu, Colloids Surface A 304 (2007) 14. [16] D. Heath, T.H.F. Tadors, J. Colloid Interf. Sci. 93 (2) (1983) 307. [17] R.Q. Heckroodt, W. Ryan, Trans. J. Brit. Ceram. Sci. 77 (1978) 180.
Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 206–210
Studies on the thixotropic and viscoelastic properties of HTlc/MT suspensions Shu-Ping Li a,b,∗ , Xue-Qin An a , Yin-Yan Zhu a a
Jangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, PR China b Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China Received 16 April 2007; received in revised form 8 July 2007; accepted 10 October 2007 Available online 13 October 2007
Abstract In this paper, we found that the variation content of Fe3+ in the Fe–Al–Mg hydrotalcite-like compounds (HTlc) and the change of R values (R = WHTlc /WMT ) will influence the thixotropic and viscoelastic properties of HTlc/sodium montmorillonite (MT) suspensions. With increasing the content of Fe3+ in the HTlc samples, the storage modulus (G ) and the loss modulus (G ) of the HTlc/MT suspensions will increase gradually, meaning that the structure strength of the HTlc/MT suspension increases with increasing of Fe3+ content in the HTlc samples. Similar results have been found in studying the influence of R values: the structure strength of the suspensions will increase with increasing of R values. Special emphasis has been laid on the phenomenon of thixotropy. With the gradual increase of the structure strength in the HTlc/MT suspension, the thixotropic types will change accordingly. The mechanism has been emphatically discussed in this paper. © 2007 Elsevier B.V. All rights reserved. Keywords: Thixotropy; Viscoelasticity; Rheology
1. Introduction “Thixotropy” was a term first used to describe the timedependent rheological behavior of aluminium hydroxide gels by Freundlich and Bircunshaw [1]. The classical definition of thixotropy is suspension behavior where time-dependent, reversible breakdown of the particulate network structure occurs under shear, followed by structural reformation on resting. The aluminium hydroxide gels investigated by Freundlich and Bircunshaw exhibited a dramatic change from a solid-like (gel) to a liquid-like (sol) state on agitation and back on resting. The breakdown of a thixotropic substance is generally measured by the decay in the shear stress with time at a constant shear rate in a rheometer. Nevertheless, the shear induced structural breakdown of HTlc/MT suspensions is very rapid, almost instantaneous, which makes characterization of the breakdownprocess ∗ Corresponding author at: College of Chemistry and Environment Science, Nanjing Normal University, Nanjing 210097, PR China. Tel.: +86 25 83598280; fax: +86 25 83598678. E-mail address: [email protected] (S.-P. Li).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.10.013
very difficult. Hence, in this paper, thixotropy was studied by monitoring the recovery process as a function of the rest time after intensive shearing. MT compounds, constituted from silica tetrahedral and alumina octahedral sheets have negatively charged sites on the planes owing to the substitution of central Si to Al ions in the crystal lattice. MT suspension is widely used in drilling fluids, paints, cosmetics, soil science and so on. In addition, MT suspensions show shear-thinning, thixotropy and exhibit a yield stress above a certain clay concentration [2]. Hydrotalcite-like compounds (HTlc) belong to a large class of anionic and basic clay, and they are also known as layered double hydroxide. They are composed of positively charged brucite-like [Mg(OH)2 ] layers with trivalent cations substituting for divalent cations at the centers of octahedral sites, and each OH– group is shared by three octahedral cations and points towards the interlayer regions[3,4]. The excess positive charges of HTlcs are compensated by interlamellar anions [3–9]. They can be represented by the general formula of III III [MII 1−x Mx (OH)2 ]x+ An−x/n mH2 O, in which M is the trivalent mental ions, MII the divalent metal ions, A the charge compensating anions, m the number of moles of co-intercalated water
S.-P. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 206–210
per formula weight of compounds; x is the number of moles of MIII per formula weight of compounds, generally ranges from 0.2 to 0.4 [2]. Then, HTlc/MT suspensions are a particular system, consisting of oppositely charged particles, and recently much attention has been focused on this type of system [10,11]. Our previous studies indicated that the suspension of HTlc/MT is easily to form three-dimensional gel-like structure instead of flocculation or coagulation, consequently extensive studies have been reported on their rheological properties [12–15]. In this paper, we emphatically examined the effect of the metal content of the HTlc samples on the rheological properties of the HTlc/MT suspensions, relevant reports have not been found as to the authors known. 2. Material and method 2.1. Material HTlc was synthesized by the co-precipitation method. The pre-designed metal chloride was dissolved into the deioned water, then NH4 OH was slowly added into the mixed solution to adjust the pH of the solution to about 9.5, so the delicate and small flocs suspended in the solution were formed. Then, the flocs were filtered and washed with the double-distilled water. Finally, the flocs were peptized at about 80 ◦ C in the oven for about 24 h and converted to HTlc sol. The chemical composition of the HTlc samples was made with the 3080E2-type automatic X-ray fluorescent spectrometer (made in Japan), the chemical composition of the samples was listed in Table 1. The MT clay was pretreated according to literature [16]. The clay was first suspended in distilled water with stirring and then decanted leaving the deposit. A certain amount of H2 O2 was added to remove the organic species. The dispersion was heated to ∼70 ◦ C to obtain the clay deposit. Then the deposit was suspended in 1 M NaCl solution for 6 h. Finally, water in the dispersion was replaced for several times with doubledistilled water until its conductivity became as low as about 4 × 10−5 −1 cm−1 , indicating that most of the electrolyte had been removed. The final concentration of the clay can be determined with the evaporation method. A certain amount of MT was mixed with double-distilled water using a high-speed mixer (Model GJ-1, Jiangyin Second Electrical Machinery Plant) for 20 min to ensure that the MT powders were sufficiently dispersed in water. Then after aged in a sealed container for 24 h, MT can be used in the experiments. The mixture of MT and HTlc was stirred by the high-speed mixer for 20 min, and then aged for about 24 h before the
207
experiments. The mass ratio of HTlc and MT was defined as R = WHTlc /WMT . The MT content in HTlc/MT suspensions was 3.0 wt.%. The pH values of all the suspensions were adjusted to 9.6 ± 0.2. 2.2. Measurement The thixotropy of the suspension has been defined as the change of the rheological properties with time under constant shear stress or shear rate, following by a gradual recovery when the stress or the shear rate is removed. In this paper, we studied the thixotropy by measuring the viscosity as a function of time after vigorously stirred with reference to experiments described by Heckroodt and Ryan [17]. The HTlc/MT suspension was first presheared under 1000 s−1 for 30 s, and the viscosity (η) of the system at different time was measured at the low shear rate of 10 s−1 . The increase of η with time means positive thixotropy while the decrease of η with time means negative thixotropy; first increase and then decrease of η with time or first decrease and then increase of η with time both means complex thixotropy [11]. Viscoelastic measurements were carried out with a HAAKE RS75 Rheometer (Hakke Co., Germany) at 25 ± 0.2 ◦ C for obtaining G and G . For the measurements of the relationship of G , G with the time, the experiments had to be carried out at sufficiently small shear stress (in the linear viscoelastic region) in order to avoid the disturbance to the recovery process. Therefore, the oscillation conditions were set to f = 1 Hz, τ = 0.1 Pa for the experiments discussed here. 3. Experimental results In this paper, three Fe–Mg–Al HTlc samples were examined. From sample 1 to sample 3, the content of Fe3+ increased gradually while keeping the ratio of Mg to Al remained constant (see Table 1). The re-creation of the equilibrium rest structure after shearing is considered as a fundamental thixotropic process, and this process has been usually investigated by transient oscillatory shear experiments, the condition is simplified if the measurements are performed in the linear viscoelastic region. In this region, no irreversible change of the structure in the suspension has been taken place [13]. Fig. 1 displays the relationship of the logarithms of G and G with the logarithm of the time, and the value of R was all assigned to 0.17. The experiments on other R values have also been performed, and the same tendency has been obtained. It can be observed in Fig. 1 that for each HTlc/MT suspension, G increased with the increasing recovery time, indicating the
Table 1 Chemical Composition of the HTlc samples Sample number
Sample
Raw material molar ratio
Chemical composition
1 2 3
Mg/Al/Fe Mg/Al/Fe Mg/Al/Fe
2:1:0.1 2:1:0.6 2:1:1
[Fe0.034 Mg0.65 Al0.32 (OH)2 ]·Cl0.14 (OH)0.22 [Fe0.21 Mg0.46 Al0.33 (OH)2 ]·Cl0.11 (OH)0.41 [Fe0.29 Mg0.42 Al0.29 (OH)2 ]·Cl0.10 (OH)0.48
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Fig. 1. The plot of the logarithms of G and G with the logarithm of time for the different HTlc/MT suspensions with R = 0.170. (a , a ) Sample 1/MT; (b , b ) sample 2/MT; and (c , c ) sample 3/MT.
Fig. 2. The plot of η with time for the different HTlc/MT suspensions with R = 0.013. (a) Sample 1/MT; (b)sample 2/MT; and (c)sample 3/MT.
recovery of the network structure increased with time. Whereas, for each HTlc/MT suspension, with increasing the content of Fe3+ in the HTlc samples, G and G increased gradually, which illustrated that the structure strength of the HTlc/MT suspensions would be increased with increasing Fe3+ content. Figs. 2 and 3 show the relationship curves of η for the three HTlc/MT suspensions made up of different HTlc samples with the time, and the R values were 0.013 and 0.170, respectively.
Fig. 3. The plot of η with time for the different HTlc/MT suspensions with R = 0.170. (a) Sample 1/MT; (b)sample 2/MT; and (c)sample 3/MT.
Fig. 4. The plot of η with time for sample 1/MT suspension with various R values. R: (a) 0.013; (b) 0.043; (c) 0.085; (d) 0.170; and (e) 0.344.
It can be found in Fig. 2 that when R = 0.013, the three suspensions all showed positive thixotropy, and with increasing the content of Fe3+ in the HTlc samples, the viscosities of the HTlc/MT suspensions increased gradually. It also can be observed in Fig. 3 that when R = 0.170, the system of sample 1/MT behaved as positive thixotropy; while the other systems all showed complex thixotropic behavior. Moreover, with increasing the content of Fe3+ in the HTlc samples, the viscosities of the HTlc/MT suspensions increased earlier then decreased later. Fig. 4 shows the evolution of the viscosities for sample 1/MT suspension with time at different R values. Figs. 5 and 6 also display the relationship of the viscosities with the time for sample 2/MT suspension and sample 3/MT suspension, respectively. It can be concluded that the values of R can obviously influence the thixotropic types of the suspensions. As increasing of R, the thixotropic types of the three suspensions all changed from the positive to the complex. 4. Discussion In any colloidally stable dispersion, the overall particle action is repulsive, therefore, particles repel each other and freely moving primary particles endeavor to place themselves as far from each other as they can. Particles of colloidally unstable dis-
Fig. 5. The plot of η with time for sample 2/MT suspension with various R values. R: (a) 0.013; (b) 0.033; (c) 0.053; (d) 0.100; and (e) 0.170.
S.-P. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 206–210
Fig. 6. The plot of η with time for sample 3/MT suspension with various R values. R: (a) 0.013; (b) 0.060; (c) 0.084; (d) 0.113; and (e) 0.170.
persions collide and stick closely together, and so a more or less loose physical network of adhered particles form in the unstable systems, with a large amount of dispersion medium entrapped in the structure. In general, stable suspensions show ideal liquid-like, Newton flow behavior, while the appearance of the non-Newton behavior refers to the network formation of aggregated particles. From our previous studies, the oppositely charged particles of HTlc and MT can form the network structure, and the recovery process of the HTlc/MT suspensions was performed as a monotonic increase of |η*| with time in the linear viscoelastic region, which is attributed to the formation of the three-dimensional network structure [13]. In this paper, the thixotropic behavior was studied under steady shear experiments. The difference between the steady shear experiments and the oscillatory experiments is that the system is disturbed during the measurements in the steady shear experiments, and this disturbance will influence the results. In the different HTlc/MT suspensions, the influence of the disturbance on the recovery process is different. When the structure strength of the system is low, the influence of the disturbance on the recovery process is weak. When the structure strength is high, the influence of the disturbance also becomes distinct. The distinct disturbance will result in the close joint of the oppositely charged HTlc particles and MT particles, in the form of dense floc clusters or sediments. The appearance of the dense floc clusters or sediments will destroy or inhibit the formation of the network, hence, η decreased gradually with the time, as a result, giving rise to the appearance of the complex or negative thixotropy. Whereas, the weak disturbance hardly gives rise to the appearance of dense floc clusters or sediments, hence, η increased gradually with the time, showing positive thixotropy. From our experimental results, the change of R and the change of Fe3+ in the HTlc samples will influence the rheological properties together. When R is low (see Fig. 2), the structure strength of the three suspensions is low as well. As a result, the influence of the shear rate on the recovery process is weak, too. Hence, all the systems behaved as positive thixotropy, and the viscosities of the suspensions increased with Fe3+ in the HTlc samples. While when R is high (see Fig. 3), the structure strength of the three suspensions is also high. Therefore, the influence of
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the disturbance on the recovery process becomes more distinct. Hence, for the system whose structure strength is relatively low (see Fig. 3 line a, the content of Fe3+ is relatively low), the influence of the disturbance is relatively weak, and then η of the system will increase gradually in the recovery process, the suspension behaved as positive thixotropy. For the system whose structure strength is relatively high (see Fig. 3 line b and c), the influence of the disturbance on the recovery process becomes more distinct, and then η of the system will increase earlier then decrease later with time in the recovery process, the suspension showed complex thixotropy. In addition, the formation of the sediments will result in the decrease of the total viscosities. Therefore, the viscosities of the HTlc/MT suspensions with R = 0.170 first increased and then decreased with increasing of Fe3+ . As far as the change of R on the thixotropy of the systems has been concerned, two factors-R values and the change of Fe3+ in the HTlc samples-should be considered together. When the content of Fe3+ in the HTlc sample is low (see Fig. 4), the structure strength of the suspension is low, then the influence of the disturbance on the recovery process is also weak, as a result, the increase of R gives no change to the thixotropic type until the value of R exceeds 0.344. When the content of Fe3+ in the HTlc sample is relatively high (see Figs. 5 and 6), the structure strength becomes higher accordingly, the influence of the shear rate on the recovery process becomes distinct as well, hence, the increase of R gives quick change of thixotropy-from positive to the complex one. 5. Summary It is found that the content of Fe3+ in the HTlc samples and R values of HTlc/MT suspensions will influence the structure strength of the suspensions. The rheological behavior of the HTlc/MT suspensions was studied both under steady shear experiments and under the oscillatory experiments. The difference is that the system is disturbed during the steady shear measurements, and this disturbance will influence the thixotropic types. When the structure strength of the system is low, the influence of the steady shear on the recovery process is weak; when the structure strength is high, the influence of steady shear also becomes distinct. Acknowledgements The authors are grateful for the financial support of National Natural Science Foundation of Jiangsu Province (BK2007223), the Startup Fund for Outstanding Persons in Nanjing Normal University and the Foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education. References [1] V.H. Freundlich, L.L. Bircunshaw, Uber das, Kolloid Z. 40 (1926) 19. [2] S. Albend, G. Lagaly, Appl. Clay Sci. 16 (2000) 201. [3] W.G. Hou, C. Ren, Chin. J. Chem. 24 (2006) 1336.
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