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The viscosity and zeta potential of bentonite dispersions in presence of anionic surfactants
December 2002
Materials Letters 57 (2002) 420 – 424 www.elsevier.com/locate/matlet
The viscosity and zeta potential of bentonite dispersions in presence of anionic surfactants ¨ .I. Ece b,c, N. Gu¨ngo¨r a,* T. Yalcßın a, A. Alemdar a, O a
Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak 80626 Istanbul, Turkey b Department of Geology, Istanbul Technical University, Maslak 80626 Istanbul, Turkey c Research Institute of Materials and Resources, Akita University, 1-1 Tegatagakuen-cho, Akita 010-8502 Akita, Japan Received 4 January 2002; received in revised form 29 March 2002; accepted 3 April 2002
Abstract The influence of surfactants on the flow behaviour of bentonite dispersions (2% w/w) is studied for two anionic surfactants, sodium dodecyl sulfate (SDS) and ammonium lauryl sulfate (ALS). In the range of 10 4 – 10 2 mol/l, the surfactants show different effects on the viscosity and zeta potential of the bentonite suspension. The experimental results are discussed considering changes in the interlayer of the clay minerals. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rheology; Zeta potential; Viscosity; Bentonite – water systems; Anionic surfactant; Ammonium lauryl sulfate; Sodium dodecyl sulfate
1. Introduction The determination of the rheological characteristics such as viscosity and thixotropy of clay – water suspensions plays an important role in the application of clay [1– 3]. These flow properties vary significantly due to the aggregation of clay particles under varying conditions of temperature and electrolytes. In many applications, it becomes necessary to add certain additives or surfactants on the clay particles to adjust the flow behaviour. Many research works have been carried out on the rheological and colloidal properties of bentonite suspensions [4– 10]. These publications are concerned with the effects of factors such as *
Corresponding author. Tel.: +90-212-285-32-15; fax: +90212-285-63-86. E-mail address: [email protected] (N. Gu¨ngo¨r).
concentration, pH [5,11], nature of exchangeable cations [12 – 14] and particle size [15 – 17], additives like salts [8,18,19], polymers and surfactants [20,21]. Surfactants on the surface of the particles have an effect on the electrical double layer interactions and on the Van der Waals interactions. Consequently, they alter the rheological behaviour of clay –water suspensions. Ionic surfactants induce electrostatic interactions, but nonionic surfactants are adsorbed on the surface by steric interactions. Surfactants can increase or decrease the stability of the system. In this study, two types of anionic surfactants, sodium dodecyl sulfate (SDS) and ammonium lauryl sulfate (ALS) are used for addition to a bentonitic suspension in various concentrations. The changes in the flow properties of the modified bentonite – water systems are studied by means of zeta potential and viscosity measurements.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 0 3 - 0
T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
2. Experimental Bentonite sample was collected from Unye area, along the Black Sea coast of Turkey. It is composed mainly of smectite group minerals on the basis of XRD, DTA and IR analysis methods. Its chemical analysis, performed using a Perkin Elmer 3030 model atomic absorption spectrophotometer, gave the following results (wt.%): SiO2, 70.30; Al2O3, 15.00; Fe2O3, 1.10; CaO, 1.60; MgO, 2.30; Na2O, 1.45; K2O, 1.20; TiO2, 0.30. The ignition loss is 6.45%. Such kind of bentonite is used in ceramic industry and for medicine production. Particle size distribution were measured by using Micrometrics Model 5000D sedigraph, for sample dispersed in water with calgon and subjected to magnetic mixing. The average particle size has been found as 0.50 Am. The specific surface areas of the samples were determined by dye absorption technique. The calculated value of the specific surface was 120 m2/g. The most convenient measurements of plasticity of a bentonite –water system are the consistency limits. The liquid limit (LL), water content of the system at the conversion point of the viscous liquid to plastic consistency, and the plastic limit (PL), lowest water content of the system maintaining its plasticity, determine the plasticity quantitatively. The Atterberg plastic limit is the lowest water content, which is expressed as a percentage by weight of oven-dried clay that can be rolled without breaking into pieces. The Atterberg liquid limit is also expressed as a percentage by weight of oven-dried clay – water content for which flow begins. The difference between the liquid limit and plastic limit is the plasticity index, which describes the range of water content in which the clay is plastic. Originally, 30 g dried bentonite is used. The LL and PL are expressed as solid/water (wt.%), and 25 strokes are used as standard in Casegrande instrument. By this approach, the liquid (LL) and plastic limits (PL) of the studied sample are 210% and 88%, respectively. The minimum salt concentration that is needed to cause coagulation of a colloidal dispersion is called critical coagulation concentration, CK. The critical concentration value was determined by test tubes at room temperature. Zeta potential measurements were carried out using a PHOTAL, CSA-600 model (Otsuka Electrokinetic)
421
microelectropheresis instrument equipped with a microprocessor. Electrophoretic mobilities were automatically converted into zeta potentials by means of a built-in microprocessor. The flow behaviour of the dispersions was characterized using a Brookfield DVIII+ type low-shear viscometer. The sample was dispersed in water (2% w/w) and shaken intensively for 24 h. An adsorption time of 2 h was adopted for the surfactants. The molecular weights of SDS (C12H25NaO4S, from Fluca Chemica) and ALS (C12H29NO4S, from Fluca Chemica) are 288.83 and 283.43 g/mol, respectively. All chemicals used in this study are pure grade.
3. Results and discussion The surfactants ALS and SDS used in this work are anionic structural surfactants soluble in water. There are three possibilities of interactions between negative charge-carrying clay particles and anionic surfactant. First, it is possible that ion exchange can take place between OH ions on clay surfaces and the anionic part of surfactants. Second, H-bonds can form between clay particles and surfactant molecules. Third, it is possible that Ca2 + cation can establish electrostatic bridges between the anionic part of surfactants and the surface of clay particles. In Fig. 1, the viscosity of the bentonite suspension is plotted as a function of increasing ALS and SDS concentrations. Viscosity values start to separate from each other at the concentration of 2 10 3 mol/l, which is a ‘‘separation point’’. This value is close to the measured value of CK for SDS. A fast increase of viscosity is observed for SDS surfactant above CK = 4 10 3 and above CK = 2 10 2 for ALS. Clay surfaces reach maximum adsorption values at CK level. Also, CK value decreases with an increase in length of alkyl chain. All three possibilities proposed above for the mechanism of adsorption of anionic surfactants by clay surfaces can occur. The viscosity values diminish after initial additions of SDS ( < 4 10 4) and then increase after the addition of 5 10 3 mol/l. Besides, after the initial additions of ALS, the viscosity values increase slightly, and then much faster after the addition of 2 10 2 mol/l. The difference is related to the length of alkyl chains of the
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Fig. 1. The plastic viscosity of Unye bentonite as a function of ALS and SDS concentrations.
two polymers. SDS chain (288.83 g/mol) is slightly longer than ALS (283.43 g/mol). The bridging of solid particles by surfactant causes flocculation. Of course, the mechanical resistance against flowing of the so-obtained clusters of bridging flocculation must be greater.
Zeta potential values, which reflect the stability of the clay solution, are plotted in Fig. 2. As can be seen, zeta potential values decrease in absolute value with increasing the concentration of SDS and ALS surfactants. The initial suspension has 36 mV zeta potential value in natural system, but the same sus-
Fig. 2. The zeta potential of Unye bentonite as a function of ALS and SDS concentrations.
T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
pension becomes more flocculated after addition of the surfactants. This result correlates with the trend of viscosity values after the addition of the additives (Fig. 1). A structural change in the system after the addition of 10 3 mol/l surfactants is observed during both the viscosity and zeta potential measurements (Fig. 2). Beginning with the addition of 10 3 mol/l additive, the system becomes more flocculated. It was expected that zeta potential values should increase, because the screening effects of edge charges of clay minerals with surfactants will make the number of negative charges more important and also similar-charged ions repulse each other. However, zeta potential values decrease as the concentration of surfactants increases. The hydrophobic tails of ALS and SDS molecules, which are attached to the positive edges of clay particles, interact with one another. This creates clusters of bridging flocculation having less mobility compared to small particles, which result in a decrease in zeta potential. Fig. 3 shows the relationship between d(001) spacings data, measured by means of XRD analyses and ALS and SDS concentrations in the sample, which elucidates the swelling properties of the clay minerals. The d(001) spacings data show that ALS and SDS molecules do not enter the interlayers of smectite.
423
Thus, ALS and SDS cannot affect surface charges too much.
4. Conclusion In the present work, the influence of the addition of anionic surfactants, sodium dodecyl sulfate (SDS) and ammonium lauryl sulfate (ALS), on the flow properties of bentonite – water systems has been studied by means of viscosity and zeta potential mesasurements. Zeta potential values decrease in absolute values, while the viscosity of bentonite suspensions increases for both surfactants above 10 3 mol/l or higher concentrations. This observation is the evidence that surfactants adsorbed by clay particles tend to cause aggregation due to interactions between the hydrophilic tails of the surfactants and the positive edges of the clay particles, which result in the formation of more resistant structures against flowing. In addition, zeta potential studies, which were carried out parallel to the viscosity studies, showed the same results. Both viscosity and zeta potential data indicate that SDS and ALS surfactants above a concentration level of 10 3 mol/l can be used as flocculants.
Fig. 3. The changes in the d(001) interplanar distance of bentonite – water systems with concentrations of ALS and SDS added to dispersions.
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T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
Acknowledgements Dr. Shigeo Hayashi of Akita University is deeply acknowledged for his help in zeta potential measurements. The Research Fund of Istanbul Technical University (project no. 1299) and Akita University, Research Institute of Materials and Resources, Japan supported this project.
References [1] R.E. Grim, Clay Mineralogy, McGraw-Hill, New York, 1968. [2] R.W. Grimshaw, The Chemistry and Physics of Clays, 4th ed., Ernest Benn, London, 1971. [3] H. Van Olphen, Clay Colloid Chemistry, Wiley, New York, 1977. [4] R. Perkins, R. Brace, E. Mattijevic, J. Colloid Interface Sci. 48 (1974) 417. [5] G. Lagaly, Appl. Clay Sci. 4 (1989) 105.
[6] C.W. Angle, H.A. Hamza, Appl. Clay Sci. 4 (1989) 263. [7] Th.F. Tadros, Solid/Liquid Dispersions, Academic Press, New York, 1987. [8] R. Keren, I. Shainberg, E. Klein, Soil Sci. Am. J. 52 (1988) 76. [9] G. Lagaly, M. Reese, S. Abend, Appl. Clay Sci. 83 (1999) 103. [10] G. Lagaly, M. Reese, S. Abend, Appl. Clay Sci. 14 (1999) 279. [11] U. Brandenburg, G. Lagaly, Appl. Clay Sci. 3 (1988) 263. [12] A. Erzan, N. Gu¨ngo¨r, J. Colloid Interface Sci. 176 (1995) 301. [13] L.L. Schramm, J.C.T. Kwak, Clays Clay Miner. 30 (1982) 40. [14] D.C.H. Cheng, J.R. Kruszewski, T.A. Roberts, J. Mater. Sci. 25 (1990) 353. [15] Sß. Dilmacß, N. Gu¨ngo¨r, Powder Handl. Proc. 7 (1995) 251. [16] J.S. Chen, J.H. Cushman, P.H. Low, Clays Clay Miner. 38 (1990) 57. [17] D. Penner, G. Lagaly, Clays Clay Miner. 48 (2000) 246. [18] R. Perkins, R. Brace, E. Mattijevic, J. Colloid Interface Sci. 48 (1974) 417. [19] T. Permien, G. Lagaly, Clay Miner. 29 (1994) 751. [20] N. Gu¨ngo¨r, J. Appl. Polym. Sci. 75 (2000) 107. [21] A. Alemdar, O. Atıcı, N. Gu¨ngo¨r, Mater. Lett. 43 (2000) 57.
Materials Letters 57 (2002) 420 – 424 www.elsevier.com/locate/matlet
The viscosity and zeta potential of bentonite dispersions in presence of anionic surfactants ¨ .I. Ece b,c, N. Gu¨ngo¨r a,* T. Yalcßın a, A. Alemdar a, O a
Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak 80626 Istanbul, Turkey b Department of Geology, Istanbul Technical University, Maslak 80626 Istanbul, Turkey c Research Institute of Materials and Resources, Akita University, 1-1 Tegatagakuen-cho, Akita 010-8502 Akita, Japan Received 4 January 2002; received in revised form 29 March 2002; accepted 3 April 2002
Abstract The influence of surfactants on the flow behaviour of bentonite dispersions (2% w/w) is studied for two anionic surfactants, sodium dodecyl sulfate (SDS) and ammonium lauryl sulfate (ALS). In the range of 10 4 – 10 2 mol/l, the surfactants show different effects on the viscosity and zeta potential of the bentonite suspension. The experimental results are discussed considering changes in the interlayer of the clay minerals. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rheology; Zeta potential; Viscosity; Bentonite – water systems; Anionic surfactant; Ammonium lauryl sulfate; Sodium dodecyl sulfate
1. Introduction The determination of the rheological characteristics such as viscosity and thixotropy of clay – water suspensions plays an important role in the application of clay [1– 3]. These flow properties vary significantly due to the aggregation of clay particles under varying conditions of temperature and electrolytes. In many applications, it becomes necessary to add certain additives or surfactants on the clay particles to adjust the flow behaviour. Many research works have been carried out on the rheological and colloidal properties of bentonite suspensions [4– 10]. These publications are concerned with the effects of factors such as *
Corresponding author. Tel.: +90-212-285-32-15; fax: +90212-285-63-86. E-mail address: [email protected] (N. Gu¨ngo¨r).
concentration, pH [5,11], nature of exchangeable cations [12 – 14] and particle size [15 – 17], additives like salts [8,18,19], polymers and surfactants [20,21]. Surfactants on the surface of the particles have an effect on the electrical double layer interactions and on the Van der Waals interactions. Consequently, they alter the rheological behaviour of clay –water suspensions. Ionic surfactants induce electrostatic interactions, but nonionic surfactants are adsorbed on the surface by steric interactions. Surfactants can increase or decrease the stability of the system. In this study, two types of anionic surfactants, sodium dodecyl sulfate (SDS) and ammonium lauryl sulfate (ALS) are used for addition to a bentonitic suspension in various concentrations. The changes in the flow properties of the modified bentonite – water systems are studied by means of zeta potential and viscosity measurements.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 0 3 - 0
T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
2. Experimental Bentonite sample was collected from Unye area, along the Black Sea coast of Turkey. It is composed mainly of smectite group minerals on the basis of XRD, DTA and IR analysis methods. Its chemical analysis, performed using a Perkin Elmer 3030 model atomic absorption spectrophotometer, gave the following results (wt.%): SiO2, 70.30; Al2O3, 15.00; Fe2O3, 1.10; CaO, 1.60; MgO, 2.30; Na2O, 1.45; K2O, 1.20; TiO2, 0.30. The ignition loss is 6.45%. Such kind of bentonite is used in ceramic industry and for medicine production. Particle size distribution were measured by using Micrometrics Model 5000D sedigraph, for sample dispersed in water with calgon and subjected to magnetic mixing. The average particle size has been found as 0.50 Am. The specific surface areas of the samples were determined by dye absorption technique. The calculated value of the specific surface was 120 m2/g. The most convenient measurements of plasticity of a bentonite –water system are the consistency limits. The liquid limit (LL), water content of the system at the conversion point of the viscous liquid to plastic consistency, and the plastic limit (PL), lowest water content of the system maintaining its plasticity, determine the plasticity quantitatively. The Atterberg plastic limit is the lowest water content, which is expressed as a percentage by weight of oven-dried clay that can be rolled without breaking into pieces. The Atterberg liquid limit is also expressed as a percentage by weight of oven-dried clay – water content for which flow begins. The difference between the liquid limit and plastic limit is the plasticity index, which describes the range of water content in which the clay is plastic. Originally, 30 g dried bentonite is used. The LL and PL are expressed as solid/water (wt.%), and 25 strokes are used as standard in Casegrande instrument. By this approach, the liquid (LL) and plastic limits (PL) of the studied sample are 210% and 88%, respectively. The minimum salt concentration that is needed to cause coagulation of a colloidal dispersion is called critical coagulation concentration, CK. The critical concentration value was determined by test tubes at room temperature. Zeta potential measurements were carried out using a PHOTAL, CSA-600 model (Otsuka Electrokinetic)
421
microelectropheresis instrument equipped with a microprocessor. Electrophoretic mobilities were automatically converted into zeta potentials by means of a built-in microprocessor. The flow behaviour of the dispersions was characterized using a Brookfield DVIII+ type low-shear viscometer. The sample was dispersed in water (2% w/w) and shaken intensively for 24 h. An adsorption time of 2 h was adopted for the surfactants. The molecular weights of SDS (C12H25NaO4S, from Fluca Chemica) and ALS (C12H29NO4S, from Fluca Chemica) are 288.83 and 283.43 g/mol, respectively. All chemicals used in this study are pure grade.
3. Results and discussion The surfactants ALS and SDS used in this work are anionic structural surfactants soluble in water. There are three possibilities of interactions between negative charge-carrying clay particles and anionic surfactant. First, it is possible that ion exchange can take place between OH ions on clay surfaces and the anionic part of surfactants. Second, H-bonds can form between clay particles and surfactant molecules. Third, it is possible that Ca2 + cation can establish electrostatic bridges between the anionic part of surfactants and the surface of clay particles. In Fig. 1, the viscosity of the bentonite suspension is plotted as a function of increasing ALS and SDS concentrations. Viscosity values start to separate from each other at the concentration of 2 10 3 mol/l, which is a ‘‘separation point’’. This value is close to the measured value of CK for SDS. A fast increase of viscosity is observed for SDS surfactant above CK = 4 10 3 and above CK = 2 10 2 for ALS. Clay surfaces reach maximum adsorption values at CK level. Also, CK value decreases with an increase in length of alkyl chain. All three possibilities proposed above for the mechanism of adsorption of anionic surfactants by clay surfaces can occur. The viscosity values diminish after initial additions of SDS ( < 4 10 4) and then increase after the addition of 5 10 3 mol/l. Besides, after the initial additions of ALS, the viscosity values increase slightly, and then much faster after the addition of 2 10 2 mol/l. The difference is related to the length of alkyl chains of the
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T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
Fig. 1. The plastic viscosity of Unye bentonite as a function of ALS and SDS concentrations.
two polymers. SDS chain (288.83 g/mol) is slightly longer than ALS (283.43 g/mol). The bridging of solid particles by surfactant causes flocculation. Of course, the mechanical resistance against flowing of the so-obtained clusters of bridging flocculation must be greater.
Zeta potential values, which reflect the stability of the clay solution, are plotted in Fig. 2. As can be seen, zeta potential values decrease in absolute value with increasing the concentration of SDS and ALS surfactants. The initial suspension has 36 mV zeta potential value in natural system, but the same sus-
Fig. 2. The zeta potential of Unye bentonite as a function of ALS and SDS concentrations.
T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
pension becomes more flocculated after addition of the surfactants. This result correlates with the trend of viscosity values after the addition of the additives (Fig. 1). A structural change in the system after the addition of 10 3 mol/l surfactants is observed during both the viscosity and zeta potential measurements (Fig. 2). Beginning with the addition of 10 3 mol/l additive, the system becomes more flocculated. It was expected that zeta potential values should increase, because the screening effects of edge charges of clay minerals with surfactants will make the number of negative charges more important and also similar-charged ions repulse each other. However, zeta potential values decrease as the concentration of surfactants increases. The hydrophobic tails of ALS and SDS molecules, which are attached to the positive edges of clay particles, interact with one another. This creates clusters of bridging flocculation having less mobility compared to small particles, which result in a decrease in zeta potential. Fig. 3 shows the relationship between d(001) spacings data, measured by means of XRD analyses and ALS and SDS concentrations in the sample, which elucidates the swelling properties of the clay minerals. The d(001) spacings data show that ALS and SDS molecules do not enter the interlayers of smectite.
423
Thus, ALS and SDS cannot affect surface charges too much.
4. Conclusion In the present work, the influence of the addition of anionic surfactants, sodium dodecyl sulfate (SDS) and ammonium lauryl sulfate (ALS), on the flow properties of bentonite – water systems has been studied by means of viscosity and zeta potential mesasurements. Zeta potential values decrease in absolute values, while the viscosity of bentonite suspensions increases for both surfactants above 10 3 mol/l or higher concentrations. This observation is the evidence that surfactants adsorbed by clay particles tend to cause aggregation due to interactions between the hydrophilic tails of the surfactants and the positive edges of the clay particles, which result in the formation of more resistant structures against flowing. In addition, zeta potential studies, which were carried out parallel to the viscosity studies, showed the same results. Both viscosity and zeta potential data indicate that SDS and ALS surfactants above a concentration level of 10 3 mol/l can be used as flocculants.
Fig. 3. The changes in the d(001) interplanar distance of bentonite – water systems with concentrations of ALS and SDS added to dispersions.
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T. Yalcßın et al. / Materials Letters 57 (2002) 420–424
Acknowledgements Dr. Shigeo Hayashi of Akita University is deeply acknowledged for his help in zeta potential measurements. The Research Fund of Istanbul Technical University (project no. 1299) and Akita University, Research Institute of Materials and Resources, Japan supported this project.
References [1] R.E. Grim, Clay Mineralogy, McGraw-Hill, New York, 1968. [2] R.W. Grimshaw, The Chemistry and Physics of Clays, 4th ed., Ernest Benn, London, 1971. [3] H. Van Olphen, Clay Colloid Chemistry, Wiley, New York, 1977. [4] R. Perkins, R. Brace, E. Mattijevic, J. Colloid Interface Sci. 48 (1974) 417. [5] G. Lagaly, Appl. Clay Sci. 4 (1989) 105.
[6] C.W. Angle, H.A. Hamza, Appl. Clay Sci. 4 (1989) 263. [7] Th.F. Tadros, Solid/Liquid Dispersions, Academic Press, New York, 1987. [8] R. Keren, I. Shainberg, E. Klein, Soil Sci. Am. J. 52 (1988) 76. [9] G. Lagaly, M. Reese, S. Abend, Appl. Clay Sci. 83 (1999) 103. [10] G. Lagaly, M. Reese, S. Abend, Appl. Clay Sci. 14 (1999) 279. [11] U. Brandenburg, G. Lagaly, Appl. Clay Sci. 3 (1988) 263. [12] A. Erzan, N. Gu¨ngo¨r, J. Colloid Interface Sci. 176 (1995) 301. [13] L.L. Schramm, J.C.T. Kwak, Clays Clay Miner. 30 (1982) 40. [14] D.C.H. Cheng, J.R. Kruszewski, T.A. Roberts, J. Mater. Sci. 25 (1990) 353. [15] Sß. Dilmacß, N. Gu¨ngo¨r, Powder Handl. Proc. 7 (1995) 251. [16] J.S. Chen, J.H. Cushman, P.H. Low, Clays Clay Miner. 38 (1990) 57. [17] D. Penner, G. Lagaly, Clays Clay Miner. 48 (2000) 246. [18] R. Perkins, R. Brace, E. Mattijevic, J. Colloid Interface Sci. 48 (1974) 417. [19] T. Permien, G. Lagaly, Clay Miner. 29 (1994) 751. [20] N. Gu¨ngo¨r, J. Appl. Polym. Sci. 75 (2000) 107. [21] A. Alemdar, O. Atıcı, N. Gu¨ngo¨r, Mater. Lett. 43 (2000) 57.