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Rheology of Laponite colloidal dispersions modified by sodium polyacrylates
Colloids and Surfaces A: Physicochem. Eng. Aspects 249 (2004) 127–129
Rheology of Laponite colloidal dispersions modified by sodium polyacrylates J. Labanda∗ , J. Llorens Chemical Engineering Department, University of Barcelona, c/ Mart´ı i Franqu`es, 1, 08028 Barcelona, Spain
Abstract The rheological properties of Laponite dispersions were studied in aqueous systems containing an anionic polyelectrolyte. Sodium polyacrylate of 2000 g/mol molecular weight was used as polyelectrolyte. The effect of three variables on lineal viscoelasticity was studied. The variables are Laponite concentration, ionic strength and sodium polyacrylate concentration. All experiments were carried out at constant pH, and the rheology was analysed 6 days after its preparation. The rheological behavior of these dispersions can characterise the physical state of dispersion. Addition of sodium polyacrylate modifies the interactions between particles and the physical state of the dispersion can be changed. In order to determine the physical state, the phase diagram of Laponite dispersions was completed as a function of sodium polyacrylate concentration. The phase diagram shows different sol–gel transitions depending on ionic strength. At constant Laponite concentration and ionic strength, two transition lines were found by varying the sodium polyacrylate concentration. © 2004 Elsevier B.V. All rights reserved. Keywords: Rheology; Viscoelasticity; Laponite; Sodium polyacrylate
1. Introduction Laponite RD is a synthetic hectorite clay which is formed by disk-shaped nanometric-size particles in which the edges and the sides have opposite polarity. Mourchid et al. [1] found that, depending on the volume fraction of the clay and the ionic strength, the phase diagram of Laponite dispersions shows different mechanical responses: stable colloidal solution, elastic gel, plastic paste or solid phase. This phase diagram was completed using optical methods, such as birefringence [2], and a nematic gel was found at a high particle concentration of Laponite. The isotropic gel-nematic transition is due to the orientation of individual particles or small aggregates over macroscopic large length scale [3]. The internal structure of Laponite dispersions was studied using small-angle neutron scattering (SANS) measurements [4,5]. These dispersions show a random aligned internal structure. Under shear, the internal structure breaks down onto aggregates at short length scale, and these aggregates ∗
Corresponding author. E-mail address: [email protected] (J. Labanda).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.063
are aligned in a direction perpendicular to the flow at large length scale [6]. These results are correlated with other studies carried out previously [7,8]. The recovery of the structure after cessation of the shear occurs over two distinct time scales, a short time scale due to the realignment of aggregates and a longer time scale corresponding to an aggregation process that leads to the formation of the fractal network [9]. Nevertheless, more recently it was found that the viscoelastic behavior of the dispersion is not due to the formation of a fractal network [10]. The aggregation process is modified by the presence of atmospheric oxygen that provokes dissolution of cations situated at the surface of Laponite particles [11]. The work aims to study the changes in the rheology of aqueous Laponite dispersions by the addition of different sodium polyacrylate concentrations as a polyelectrolyte. Sodium polyacrylate is able to modify electrostatic interactions between clay particles by increasing their dispersion. This reduces the viscosity of dispersion. Nowadays, sodium polyacrylate is used in industrial ceramic slips as a very effective deflocculant and good alternative to the unenvironmental sodium tripolyphosphate.
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J. Labanda, J. Llorens / Colloids and Surfaces A: Physicochem. Eng. Aspects 249 (2004) 127–129
Fig. 1 shows the frequency variation of the storage modulus, G , and loss modulus, G , for a 1.8 wt.% Laponite and 5 × 10−3 M in NaCl as a function of sodium polyacrylate concentration. The weak frequency dependence of the storage modulus, which is higher than the loss modulus, for the dispersion in the absence of sodium polyacrylate suggests that this dispersion is a viscoelastic gel. In the presence of 0.1 wt.% of sodium polyacrylate, the storage and loss moduli
decrease significantly and show the same power law dependence with frequency. At 0.4 wt.% of sodium polyacrylate, the viscoelastic gel behavior reappears and increases both moduli with diminution of frequency dependence. The time required for a gelling dispersion to undergo the transformation from a liquid to a gel is a function of the criteria by which the gel point is defined. The gel point often was determined visually when no flow of the dispersion was observed. Rheologically, in the liquid–gel transition an increase in the elastic behavior of the dispersion is observed as a threedimensional network structure develops. There are theories to find the gel point of polymer blends [12,13]. Chambol et al. [14] associated the gel point with the point where both oscillatory moduli have the same variation with frequency with a power law index value (called relaxation exponent) between 0 and 2. Cocard et al. [15] found a relation between relaxation exponent and fractal dimension and they show that G and G have identical dependence over frequency ranges tested with an empirical relaxation exponent of 0.55 for a 1 wt.% Laponite dispersion. Following these studies, we considered that dispersion at gel point shows the same value of both moduli over frequency with an observed relaxation exponent close to 0.5. For example, Fig. 1 shows that the dispersion formed by 0.1 wt.% of sodium polyacrylate is at gel point with a relaxation exponent equal to 0.51. As it can be deduced from Fig. 1, viscoelastic gel is observed at low and high sodium polyacrylate concentration and viscoelastic liquid at intermediate concentration, so two liquid–gel transitions can be observed as a function of sodium polyacrylate concentration. Similar results were found at different Laponite concentrations and ionic strengths. The gel point is extended to higher sodium polyacrylate concentration when ionic strength is decreased. Fig. 2 shows the phase diagram of aqueous Laponite dispersions as a function of particle, sodium chloride and sodium polyacrylate concentrations. Four transition lines are represented that correspond to the sodium chloride concentrations analysed. Each transition line in the diagram separates a viscous liquid phase domain
Fig. 1. Evolution of storage modulus (filled symbols) and loss modulus (open symbols) over frequency for dispersion formed by 1.8 wt.% of Laponite and 5 × 10−3 M of NaCl at different sodium polyacrylate concentration: () 0 wt.%, () 0.1 wt.% and () 0.4 wt.%.
Fig. 2. Phase diagram of aqueous Laponite dispersions as a function of particle concentration, sodium polyacrylate concentration and ionic strength (0.0001, 0.001, 0.005 and 0.01 M in NaCl).
2. Experimental The substances analysed were aqueous dispersions of the synthetic clay Laponite RD, which is manufactured by Laporte Inc., UK. Its structural formula is Si8 [Mg5.5 Li0.4 H4.0 O24.0 ]0.7− Na0.7 0.7+ , which gives individual particles a disk geometry with a particle density of 2.53 g cm−3 and disk-like particle dimensions of 25–30 nm in diameter and 0.92 nm in thickness. These features cause the formation of colorless and transparent aqueous dispersions. All dispersions analysed were done at a fixed pH of 10. Clay powder was mixed in a solution of demineralized water with a specific quantity of NaCl (in order to fix the ionic strength) and sodium polyacrylate. Dispersions were prepared at room temperature and brisk stirring for 30 min. The sodium polyacrylate (Noramer) was supplied by Rohm and Haas (Verneuil en Halatte, France) with a low molecular weight of 2000 g/mol. Rheological oscillatory measurements were made to study the viscoelastic properties of Laponite dispersion as a function of Laponite concentration, ionic strength and sodium polyacrylate concentration. All experiments were performed in the linear regime where the viscoelastic modulus is independent of the applied stress and they were carried out at 25 ± 0.1 ◦ C with control stress rheometer (TA-Instruments AR-N1000).
3. Results and discussion
J. Labanda, J. Llorens / Colloids and Surfaces A: Physicochem. Eng. Aspects 249 (2004) 127–129
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persion formed by 2 wt.% of Laponite, 10−3 M in NaCl and 0.2 wt.% of sodium polyacrylate is a viscoelastic liquid. 4. Conclusions
Fig. 3. Phase diagram of aqueous Laponite dispersions as a function of Laponite, NaCl and total Na+ concentration. The latter is calculated as the sum of Na+ added via NaCl and sodium polyacrylate. NaCl concentrations are: 0.0001, 0.001, 0.005 and 0.01 M. Black squares correspond to transition line in the absence of sodium polyacrylate.
from a viscoelastic gel phase. The liquid phase is situated on the left-hand side of the transition line and the gel phase takes place at higher Laponite concentrations. The increase of ionic strength reduces the thickness of the electric double layer (following DLVO theory) and it reduces the swelling pressure due to the modification of the salt concentration in the suspension and the network [16]. Thus, the liquid–gel transition is shifted to a lower Laponite concentration. The addition of sodium polyacrylate involves an increase of Na+ cations in the bulk of dispersions. Fig. 3 shows the phase diagram of aqueous Laponite dispersions as a function of Laponite, NaCl and total Na+ concentration. The total concentration is calculated by the sum of Na+ added via NaCl and sodium polyacrylate. The black squares represent the liquid–gel transition line of Laponite dispersions in the absence of sodium polyacrylate. The sodium polyacrylate is correlated with the phase diagram found by Mourchid et al. [1]. In the presence of sodium polyacrylate, the sol–gel transition line is assigned by the corresponding NaCl concentration. The effect of sodium polyacrylate on the rheology of dispersions is shown in Fig. 3. For example, the dispersion formed by 2 wt.% of Laponite, 10−2 M in NaCl and 0.0 wt.% of sodium polyacrylate is a viscoelastic gel, while the dis-
Laponite particle dispersions, at moderate ionic strengths and high pH, aggregate from dense microdomains into macroaggregates, which themselves form a network that controls the mechanical properties of dispersions. The presence of sodium polyacrylate modifies the electrostatic interactions between Laponite particles and, therefore, their viscoelastic properties. The physical state of these dispersions depends on Laponite, sodium chloride and sodium polyacrylate concentration, and two sol–gel transitions are observed as a function of sodium polyacrylate concentration. Acknowledgement It is a pleasure to thank Dr. R. Hughes and all the members of the Bristol Colloid Center for helpful and enlightening discussions on rheology. This study received financial support from the Project CICYT (PPQ2002-04115-C02-02). References [1] A. Mourchid, A. Delville, J. Lambard, P. Levitz, Langmuir 11 (1995) 1942. [2] A. Mourchid, E. Lecolier, H. van Damme, P. Levitz, Langmuir 14 (1998) 4718. [3] J.O. Fossum, Physica A 270 (1999) 270. [4] J.D.F. Ramsay, J. Colloid Interface Sci. 109 (1986) 441. [5] J.D.F. Ramsay, P. Lidner, J. Chem. Soc. Faraday Trans. 89 (1993) 4207. [6] F. Pignon, A. Magnin, J.M. Piau, Phys. Rev. Lett. 79 (1997) 4689. [7] M. Kroon, W.L. Vos, G.H. Wegdam, Phys. Rev. E 57 (1998) 1962. [8] F. Pignon, A. Magnin, J.M. Piau, J. Rheol. 40 (1996) 573. [9] F. Pignon, A. Magnin, J.M. Piau, J. Rheol. 42 (1998) 1349. [10] D. Bonn, H. Kellay, H. Tanaka, G. Wegdam, J. Meunier, Langmuir 15 (1999) 7534. [11] A. Mourchid, P. Levitz, Phys. Rev. E 57 (1998) R4887. [12] W.H. Stockmayer, J. Chem. Phys. 11 (1943) 45. [13] D. Stauffer, A. Coniglio, M. Adam, Adv. Polym. Sci. 44 (1982) 74. [14] F. Chambon, H.H. Winter, Polym. Bull. 13 (1985) 499. [15] S. Cocard, J.F. Tassin, T. Nicolai, J. Rheol. 44 (2000) 585. [16] A. Delville, Langmuir 10 (1994) 395.
Rheology of Laponite colloidal dispersions modified by sodium polyacrylates J. Labanda∗ , J. Llorens Chemical Engineering Department, University of Barcelona, c/ Mart´ı i Franqu`es, 1, 08028 Barcelona, Spain
Abstract The rheological properties of Laponite dispersions were studied in aqueous systems containing an anionic polyelectrolyte. Sodium polyacrylate of 2000 g/mol molecular weight was used as polyelectrolyte. The effect of three variables on lineal viscoelasticity was studied. The variables are Laponite concentration, ionic strength and sodium polyacrylate concentration. All experiments were carried out at constant pH, and the rheology was analysed 6 days after its preparation. The rheological behavior of these dispersions can characterise the physical state of dispersion. Addition of sodium polyacrylate modifies the interactions between particles and the physical state of the dispersion can be changed. In order to determine the physical state, the phase diagram of Laponite dispersions was completed as a function of sodium polyacrylate concentration. The phase diagram shows different sol–gel transitions depending on ionic strength. At constant Laponite concentration and ionic strength, two transition lines were found by varying the sodium polyacrylate concentration. © 2004 Elsevier B.V. All rights reserved. Keywords: Rheology; Viscoelasticity; Laponite; Sodium polyacrylate
1. Introduction Laponite RD is a synthetic hectorite clay which is formed by disk-shaped nanometric-size particles in which the edges and the sides have opposite polarity. Mourchid et al. [1] found that, depending on the volume fraction of the clay and the ionic strength, the phase diagram of Laponite dispersions shows different mechanical responses: stable colloidal solution, elastic gel, plastic paste or solid phase. This phase diagram was completed using optical methods, such as birefringence [2], and a nematic gel was found at a high particle concentration of Laponite. The isotropic gel-nematic transition is due to the orientation of individual particles or small aggregates over macroscopic large length scale [3]. The internal structure of Laponite dispersions was studied using small-angle neutron scattering (SANS) measurements [4,5]. These dispersions show a random aligned internal structure. Under shear, the internal structure breaks down onto aggregates at short length scale, and these aggregates ∗
Corresponding author. E-mail address: [email protected] (J. Labanda).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.063
are aligned in a direction perpendicular to the flow at large length scale [6]. These results are correlated with other studies carried out previously [7,8]. The recovery of the structure after cessation of the shear occurs over two distinct time scales, a short time scale due to the realignment of aggregates and a longer time scale corresponding to an aggregation process that leads to the formation of the fractal network [9]. Nevertheless, more recently it was found that the viscoelastic behavior of the dispersion is not due to the formation of a fractal network [10]. The aggregation process is modified by the presence of atmospheric oxygen that provokes dissolution of cations situated at the surface of Laponite particles [11]. The work aims to study the changes in the rheology of aqueous Laponite dispersions by the addition of different sodium polyacrylate concentrations as a polyelectrolyte. Sodium polyacrylate is able to modify electrostatic interactions between clay particles by increasing their dispersion. This reduces the viscosity of dispersion. Nowadays, sodium polyacrylate is used in industrial ceramic slips as a very effective deflocculant and good alternative to the unenvironmental sodium tripolyphosphate.
128
J. Labanda, J. Llorens / Colloids and Surfaces A: Physicochem. Eng. Aspects 249 (2004) 127–129
Fig. 1 shows the frequency variation of the storage modulus, G , and loss modulus, G , for a 1.8 wt.% Laponite and 5 × 10−3 M in NaCl as a function of sodium polyacrylate concentration. The weak frequency dependence of the storage modulus, which is higher than the loss modulus, for the dispersion in the absence of sodium polyacrylate suggests that this dispersion is a viscoelastic gel. In the presence of 0.1 wt.% of sodium polyacrylate, the storage and loss moduli
decrease significantly and show the same power law dependence with frequency. At 0.4 wt.% of sodium polyacrylate, the viscoelastic gel behavior reappears and increases both moduli with diminution of frequency dependence. The time required for a gelling dispersion to undergo the transformation from a liquid to a gel is a function of the criteria by which the gel point is defined. The gel point often was determined visually when no flow of the dispersion was observed. Rheologically, in the liquid–gel transition an increase in the elastic behavior of the dispersion is observed as a threedimensional network structure develops. There are theories to find the gel point of polymer blends [12,13]. Chambol et al. [14] associated the gel point with the point where both oscillatory moduli have the same variation with frequency with a power law index value (called relaxation exponent) between 0 and 2. Cocard et al. [15] found a relation between relaxation exponent and fractal dimension and they show that G and G have identical dependence over frequency ranges tested with an empirical relaxation exponent of 0.55 for a 1 wt.% Laponite dispersion. Following these studies, we considered that dispersion at gel point shows the same value of both moduli over frequency with an observed relaxation exponent close to 0.5. For example, Fig. 1 shows that the dispersion formed by 0.1 wt.% of sodium polyacrylate is at gel point with a relaxation exponent equal to 0.51. As it can be deduced from Fig. 1, viscoelastic gel is observed at low and high sodium polyacrylate concentration and viscoelastic liquid at intermediate concentration, so two liquid–gel transitions can be observed as a function of sodium polyacrylate concentration. Similar results were found at different Laponite concentrations and ionic strengths. The gel point is extended to higher sodium polyacrylate concentration when ionic strength is decreased. Fig. 2 shows the phase diagram of aqueous Laponite dispersions as a function of particle, sodium chloride and sodium polyacrylate concentrations. Four transition lines are represented that correspond to the sodium chloride concentrations analysed. Each transition line in the diagram separates a viscous liquid phase domain
Fig. 1. Evolution of storage modulus (filled symbols) and loss modulus (open symbols) over frequency for dispersion formed by 1.8 wt.% of Laponite and 5 × 10−3 M of NaCl at different sodium polyacrylate concentration: () 0 wt.%, () 0.1 wt.% and () 0.4 wt.%.
Fig. 2. Phase diagram of aqueous Laponite dispersions as a function of particle concentration, sodium polyacrylate concentration and ionic strength (0.0001, 0.001, 0.005 and 0.01 M in NaCl).
2. Experimental The substances analysed were aqueous dispersions of the synthetic clay Laponite RD, which is manufactured by Laporte Inc., UK. Its structural formula is Si8 [Mg5.5 Li0.4 H4.0 O24.0 ]0.7− Na0.7 0.7+ , which gives individual particles a disk geometry with a particle density of 2.53 g cm−3 and disk-like particle dimensions of 25–30 nm in diameter and 0.92 nm in thickness. These features cause the formation of colorless and transparent aqueous dispersions. All dispersions analysed were done at a fixed pH of 10. Clay powder was mixed in a solution of demineralized water with a specific quantity of NaCl (in order to fix the ionic strength) and sodium polyacrylate. Dispersions were prepared at room temperature and brisk stirring for 30 min. The sodium polyacrylate (Noramer) was supplied by Rohm and Haas (Verneuil en Halatte, France) with a low molecular weight of 2000 g/mol. Rheological oscillatory measurements were made to study the viscoelastic properties of Laponite dispersion as a function of Laponite concentration, ionic strength and sodium polyacrylate concentration. All experiments were performed in the linear regime where the viscoelastic modulus is independent of the applied stress and they were carried out at 25 ± 0.1 ◦ C with control stress rheometer (TA-Instruments AR-N1000).
3. Results and discussion
J. Labanda, J. Llorens / Colloids and Surfaces A: Physicochem. Eng. Aspects 249 (2004) 127–129
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persion formed by 2 wt.% of Laponite, 10−3 M in NaCl and 0.2 wt.% of sodium polyacrylate is a viscoelastic liquid. 4. Conclusions
Fig. 3. Phase diagram of aqueous Laponite dispersions as a function of Laponite, NaCl and total Na+ concentration. The latter is calculated as the sum of Na+ added via NaCl and sodium polyacrylate. NaCl concentrations are: 0.0001, 0.001, 0.005 and 0.01 M. Black squares correspond to transition line in the absence of sodium polyacrylate.
from a viscoelastic gel phase. The liquid phase is situated on the left-hand side of the transition line and the gel phase takes place at higher Laponite concentrations. The increase of ionic strength reduces the thickness of the electric double layer (following DLVO theory) and it reduces the swelling pressure due to the modification of the salt concentration in the suspension and the network [16]. Thus, the liquid–gel transition is shifted to a lower Laponite concentration. The addition of sodium polyacrylate involves an increase of Na+ cations in the bulk of dispersions. Fig. 3 shows the phase diagram of aqueous Laponite dispersions as a function of Laponite, NaCl and total Na+ concentration. The total concentration is calculated by the sum of Na+ added via NaCl and sodium polyacrylate. The black squares represent the liquid–gel transition line of Laponite dispersions in the absence of sodium polyacrylate. The sodium polyacrylate is correlated with the phase diagram found by Mourchid et al. [1]. In the presence of sodium polyacrylate, the sol–gel transition line is assigned by the corresponding NaCl concentration. The effect of sodium polyacrylate on the rheology of dispersions is shown in Fig. 3. For example, the dispersion formed by 2 wt.% of Laponite, 10−2 M in NaCl and 0.0 wt.% of sodium polyacrylate is a viscoelastic gel, while the dis-
Laponite particle dispersions, at moderate ionic strengths and high pH, aggregate from dense microdomains into macroaggregates, which themselves form a network that controls the mechanical properties of dispersions. The presence of sodium polyacrylate modifies the electrostatic interactions between Laponite particles and, therefore, their viscoelastic properties. The physical state of these dispersions depends on Laponite, sodium chloride and sodium polyacrylate concentration, and two sol–gel transitions are observed as a function of sodium polyacrylate concentration. Acknowledgement It is a pleasure to thank Dr. R. Hughes and all the members of the Bristol Colloid Center for helpful and enlightening discussions on rheology. This study received financial support from the Project CICYT (PPQ2002-04115-C02-02). References [1] A. Mourchid, A. Delville, J. Lambard, P. Levitz, Langmuir 11 (1995) 1942. [2] A. Mourchid, E. Lecolier, H. van Damme, P. Levitz, Langmuir 14 (1998) 4718. [3] J.O. Fossum, Physica A 270 (1999) 270. [4] J.D.F. Ramsay, J. Colloid Interface Sci. 109 (1986) 441. [5] J.D.F. Ramsay, P. Lidner, J. Chem. Soc. Faraday Trans. 89 (1993) 4207. [6] F. Pignon, A. Magnin, J.M. Piau, Phys. Rev. Lett. 79 (1997) 4689. [7] M. Kroon, W.L. Vos, G.H. Wegdam, Phys. Rev. E 57 (1998) 1962. [8] F. Pignon, A. Magnin, J.M. Piau, J. Rheol. 40 (1996) 573. [9] F. Pignon, A. Magnin, J.M. Piau, J. Rheol. 42 (1998) 1349. [10] D. Bonn, H. Kellay, H. Tanaka, G. Wegdam, J. Meunier, Langmuir 15 (1999) 7534. [11] A. Mourchid, P. Levitz, Phys. Rev. E 57 (1998) R4887. [12] W.H. Stockmayer, J. Chem. Phys. 11 (1943) 45. [13] D. Stauffer, A. Coniglio, M. Adam, Adv. Polym. Sci. 44 (1982) 74. [14] F. Chambon, H.H. Winter, Polym. Bull. 13 (1985) 499. [15] S. Cocard, J.F. Tassin, T. Nicolai, J. Rheol. 44 (2000) 585. [16] A. Delville, Langmuir 10 (1994) 395.