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Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agriculture Research Organization (ARO), P.O. Box 6, Bet Dagan 50250, Israel
* Corresponding author (rkeren{at}agri.gov.il)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DDL, diffuse double layer • DH, degree of hydrolysis • MW, molecular weight in units gram per mole • PAM, polyacrylamide • PVA, polyvinyl alcohol • SDS, sodium dodecylsulfate (SDS)
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The stabilization of soil structure is, to a great extent, a result of swelling, flocculation, and dispersion of soil clays. The presence of organic substances such as synthetic polymers may modify the interaction between clay particles in soil because of polymer–clay interaction. This interaction has been extensively studied in terms of adsorption and flocculation (van Olphen, 1977; Theng, 1979). The concept that anionic polymers can flocculate clay suspensions by forming interparticle bonds was suggested by Ruehrwein and Ward (1952). The ability of high MW anionic PAM to promote flocculation of soil clay, kaolinite, and Na-montmorillonite was observed by Michaels (1954), Black et al. (1965), and Aly and Letey (1988). Flocculation depends on polymer properties such as MW and charge density (Michaels, 1954). The higher the MW and charge density, the greater is the probability of interparticle bond formation (Greenland, 1972; Theng, 1979), but optimum flocculation was observed when the DH of PAM was around 30% (Michaels, 1954). On the other hand, the addition of anionic polymers such as polysaccharides and humic acid to montmorillonite suspensions has been found to increase clay dispersivity. The polysaccharide, with a MW in the range of 2 x 105 to 2 x 106, caused flocculation of montmorillonite in suspension, only at concentrations below 50 mg L-1 (Gu and Doner, 1993). It is clear that under certain conditions, polyanions can act as effective flocculating agents of dilute clay suspensions, despite the fact that both the polymer and the suspended particles carry a net negative electrostatic charge.
Since peptizing agents adsorption affect the flocculation value of clay suspension, it is possible that they will affect the interaction between clay platelets. However, the influence of negatively charged polymers on colloidal properties of clays in suspension, such as viscosity and flow behavior, is less well known. In general, the flow behavior of clay suspensions depends on clay concentration, particle size, shape, and the strength of the interactions among clay particles (Brandenburg and Lagaly, 1988; Chen et al., 1990; Keren, 1988, 1989a,b; M'Ewen and Pratt, 1957; Rand et al., 1980). When there is little or no interaction between the clay particles in an aqueous suspension, the flow of the suspension is Newtonian in behavior (the shear stress is proportional to the shear rate). On the other hand, when the clay particles interact, the flow of the suspension is non-Newtonian in behavior. Although there have been extensive studies of flow behavior of various clay suspensions, only a few of them focused on the effects of natural and synthetic polymers in the presence of clay particles. Rheological measurements by Packter (1957) and van der Watt and Bodman (1962) indicated that polyanion adsorption by montmorillonite leads to increased viscosity and yield stress (a certain finite stress above which flow occurs) of suspensions, as a result of interparticle bridging. The viscosity and yield stress of Na-montmorillonite suspension, however, changed with polyvinyl alcohol (PVA) concentration, reaching a maximum at a polymer concentration of 0.5% (Heath and Tadros, 1983). This effect was more pronounced at pH 3 than at pH 7 and 9. In contrast, the extrapolated shear stress and viscosity of montmorillonite suspensions decreased in the presence of Na-humate (Zhang et al., 1991). The yield stress of kaolinite suspension was affected by the presence of anionic surface-active agent (Lagaly, 1989). A non-Newtonian flow was observed for attapulgite suspension in the presence of PVA (Chang et al., 1991). The viscosity of this suspension changed with concentration of the polymer and its MW.
The yield stress and the flow behavior of clay suspensions are both sensitive indicators of clay particle interactions (van Olphen, 1977). They are related to the degree of association between clay platelets, and can be used as an index of particle–particle association strength. The association strength depends on the numbers of particle–particle and particle–polymer–particle linkages, and the energy required to break such linkages. Thus, the role of polymers in this interaction can be assessed from the flow behavior of clay suspension. The objective of the present study was to determine the effects of MW and DH of anionic PAM on the rheological properties of Na-montmorillonite suspensions at various electrolyte concentrations.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Polymer Properties
The various PAMs are commercial products of American Cyanamide Company (Parsippany, NJ) and Aldrich Chemical Company, Inc. (Milwaukee, WI). The anionic PAMs are copolymers of acrylamide and Na-acrylate. The DH of the polymer represents the proportion of acrylate ion on the polymer molecule (Mortimer, 1991). The MW and DH of the various polymers are given in Table 1. Polymer solutions at a concentration of 240 mg L-1 were prepared as stock solutions; their pHs were adjusted to 7.0 ± 0.2 by adding dilute NaOH or HCl solutions.
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Rheological Measurements
The rheological measurements for the Na-montmorillonite suspensions were performed with a commercial couette-type viscometer with an outer rotating cylinder and a stationary inner cylinder (Haake Model RV2, Sensor System CV20, Germany). The gap between the outer and the inner cylinders was 0.32 mm. The outer cylinder housing was temperature controlled at 25.0 ± 0.2°C. The viscometer was calibrated against a standard fluid prior to the rheological measurements of the clay–polymer systems. The measurements were carried out at shear rates of 0 to 1000 s-1 during 2 min. The variation among the three rheograms of each treatment were negligible.
Calculations
The apparent viscosity, 
ap, (mPa s) was calculated for several shear rates, D, (s-1) by means of the equation:
 | [1] |
is the shear stress (mPa). |
RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Similar behavior was observed in the clay suspension in the presence of PAM90 polymer (Fig. 1). A plastic flow was observed, however, in the presence of the A185 polymer, with a yield stress of 0.50 N m-2 (Table 2). This flow behavior suggests that the largest polymer, under neutral pH and free electrolyte suspension, formed a three-dimensional structure of clay platelets. Only the largest polymer 
exerted a strong effect on the apparent viscosity of the clay suspension. The viscosity of the suspension with this polymer was much higher than that for the clay alone at any given shear rate. The apparent viscosity dropped from 71.7 to 29.5 mPa s as the shear rate increased from 48 to 260 s-1, and then progressively decreased to 19.5 mPa s at a shear rate of 785 s-1 (Table 2). The drop in viscosity suggests that the shearing energy was enough to break down some of the bonds between the polymer and the clay surface. However, some of the structure remained, since the apparent viscosity of the clay suspension in the presence of this polymer was relatively high.
Newtonian flow was observed when the P35 polymer was present (Fig. 1). The length of the fully extended molecule of this polymer is 12.5 nm whereas the average separation distance between two adjacent platelets is 132 nm (assuming parallel stacking). The Newtonian flow and the absence of yield stress (Table 2), suggest that the adsorbed P35 molecules were too short to bond adjacent platelets into a three-dimensional clay structure. The viscosity of the clay suspension with the polymer of the lowest MW (3.5 x 103) was lower than that of the suspension of clay alone (Table 2). The interaction between the clay platelets in the presence of this polymer was weaker than that in its absence, at any shear rate. The difference between the shear stresses of these two systems, however, decreased as the shear rate increased (Table 2). The lowest viscosity in the presence of P35 can be related to the adsorption of the polymer molecules on the clay platelet edge surfaces, thus preventing edge-to-edge association.
Similar behavior to that of suspensions with P35 has been observed for Na-montmorillonite suspensions in the presence of sodium polymethaphosphate (Keren, 1988) and for anionic surfactants such as Na dodecylsulfate (SDS) (Permien and Lagaly, 1995). These results support the hypothesis of Stutzmann and Siffert (1977) that the polyanion molecules are adsorbed mainly on the clay edge surfaces.
When the NaCl concentration in the suspension was increased to 10 mmolc L-1 (Table 2), a pseudoplastic flow was observed for the suspension of the clay alone. The extremely low yield stress suggests that the attractive forces between clay platelets are very weak in a 10-mmolc L-1 NaCl solution. At NaCl concentration of 10 mmolc L-1, the clay particles were in a dispersed state without any structure (van Olphen, 1977; Heller and Keren, 2001). This conclusion was based on the findings that only a negligible effect of shear rate on Na-montmorillonite suspension viscosity was observed at NaCl concentration of 10 mmolc L-1 (Table 2). The apparent viscosity reached the same minimum value at this electrolyte concentration, regardless of the shear rate and clay particle size (Heller and Keren, 2001).
A pseudoplastic flow was also observed for the clay suspension with the P35 polymer. The apparent viscosity of the suspension in the presence of P35 polymer (MW 3.5 x 103) was close to that of the suspension with the clay alone. As in the case of the suspension without electrolyte, the very low yield stress (0.02 N m-2, Table 2) and viscosity both suggest that the molecules of the P35 polymer (MW 3.5 x 103) were still too small to form a three-dimensional structure of clay platelets in suspension at this low electrolyte concentration.
In the presence of A185 polymer, the shear stress increased sharply when the electrolyte concentration increased from 0 to 10 mmolc L-1 (Table 2). The increase for PAM90 was greater than for A185. This indicates that the influence of highly hydrolyzed polymer on the rheological behavior of Na-montmorillonite suspension depends on the molecular length. The significant increase in shear stress for the shorter molecules suggests that the diffuse double layer (DDL) became thin enough for adjacent platelets to bond at 10 mmolc L-1 NaCl concentration.
The apparent viscosities of the suspensions containing the two largest-molecular-weight polymers (PAM90 and A185), were one order of magnitude greater than that of the free electrolyte suspensions at any given shear rate (Table 2).
Under such conditions the larger polymer molecules 
were long enough to form interparticle bridges. The highly hydrolyzed polyanions become extended-flexible chains with polyfunctional groups at this electrolyte concentration. This flexibility and polyfunctionality enable the polymer to adopt variable conformational states at the clay surface. The attachment of one segment would increase the probability of neighboring segments to be adsorbed, and the number of segments or functional groups per molecule is large (Silberberg, 1968). Thus multiple bonding between one polymer molecule and clay surfaces is favored. Several segments on one molecule can be attached by surface bonds to two or more adjacent plates. Such bridging may form a continuum structure of flocs that is responsible for the change in flow behavior from pseudoplastic to plastic.
The high effectiveness of the two polymers occurred despite the fact that the clay particle–particle association is minimal at this electrolyte concentration (Heller and Keren, 2001). The PAM molecule tends to form itself into an extended chain as the DH increases (Stutzmann and Siffert, 1977). Because of the high hydrolysis (>85%) of the above polymers, the molecules were strongly anionic and formed long extended chains (Mortimer, 1991). At the prevailing pH and electrolyte concentration, the extended molecules may be adsorbed on the edge surfaces of adjacent platelets by a ligand exchange mechanism (Theng, 1982). If this adsorption mechanism is valid, the negative electrostatic field associated with the planar surface may affect the anionic PAM adsorption on the clay edge surfaces. The negative electrical field around clay particles associated with the planar surfaces, may spill over into the edge region. Thus, the negative electric field influences the edges, making them less accessible to approaching negative molecules.
High electrolyte concentrations, however, depress the negative electrical field associated with the planar surfaces, and as a result, the accessibility of the adsorption sites on the edges, to the negative polymer molecules may be enhanced. It is possible, therefore, that the exposed functional groups interact with the adsorption sites on the edge surfaces. The enhanced adsorption of polyanions in the presence of salt (Theng, 1982) supports this hypothesis.
The flow behavior of the clay suspension without polymers changed from pseudoplastic to plastic when the NaCl concentration increased from 10 to 100 mmolc L-1. Similarly, the yield stress increased from 0.04 to 1.35 N m-2 (Table 2).
Norish (1954) and Foster et al. (1955) observed that the basal spacing of Na-montmorillonite in NaCl solution decreases progressively with increasing electrolyte concentration. When the concentration reaches 0.25 molc L-1; it then decreases abruptly from about 4.0 to 1.9 nm. According to the theory of flocculation (Reerink and Overbeek, 1954; Ottewill et al., 1960), the abrupt decrease in basal spacing would not have occurred unless the platelets were in the energy well. Thus Na-montmorillonite does not begin to flocculate in the face-to-face association until the NaCl concentration reaches 0.25 molc L-1. However, Keren et al. (1988) observed a flocculation value of 44 mmolc L-1 for Na-montmorillonite at pH 9.8. The presence of a negative charge on the edge surfaces at this pH, the observed flocculation value of 44 mmolc L-1, and the high gel volume led Keren et al. (1988) to conclude that an open structure with a face-to-face association predominates at specific locations. They assumed that the electrostatic charge is distributed unevenly on the planar surfaces. Thus, face-to-face association between platelets may take place at junction points, in areas having lower specific charge density than the average value of 0.117 C m-2. The calculated thickness of the DDL of Na-montmorillonite at an electrolyte concentration of 100 mmolc L-1 is about 1.0 nm. This electrolyte concentration is enough to compress the DDL. The decrease in the repulsion energy as the electrolyte concentration increases to 100 mmolc L-1, allows the platelets to approach each other closely enough to form face-to-face associations between specific areas on the planar surfaces.
At 100 mmolc L-1 NaCl, a platelet network structure may form in the suspension. This can occur because the individual platelets are flexible (Tessier and Pedro, 1981). The increases in yield stress as well as the change in flow pattern for the clay without polymers, indicate that such a strong clay particle association (face-to-face) exists at this high electrolyte concentration as described by Keren et al. (1988).
The results for the three polymers (P35, A185, and PAM90), under electrolyte concentration of 100 mmolc L-1 (Table 2), suggest that the adsorbed polymers stabilized the clay flocs that formed at such a high electrolyte concentration, and that the bridging forces between flocs increased in the order: PAM90 > A185 > P35.
Whereas the effect of polymer P35 was negligible at NaCl concentrations of 0 and 10 mmolc L-1, the yield stress and viscosity were much higher at 100 mmolc L-1. These results suggest that the average thickness of the DDL was small enough for two adjacent platelets to be bonded, even by this short polymer molecule (12.5 nm in its extended condition). The results may support the hypothesis that the light weight polymers can be adsorbed on the edge surfaces of the platelets by a ligand exchange mechanism, as suggested by Theng (1982).
When the electrolyte concentration in suspension increased from 10 to 100 mmolc L-1, the suspension viscosity and the yield stress decreased in the presence of PAM90 and A185 polymers (Table 2). Lower apparent viscosity and sharp reduction as the shear stress increased, occurred in the presence of the two heaviest polymers. Because the polymers were added to the suspension after the addition of the electrolyte, the adsorbed polymers stabilized the existing platelet flocs. These flocs, which were connected by the adsorbed polymers, form a network of flocs throughout the suspension. The polymers, however, were adsorbed more tightly on the platelets within the flock than between the flocs. This hypothesis is supported by the finding that the flocculation value for clay is lower when the electrolyte is added before the polyanion than when the order of addition is reversed (Theng, 1979).
Hydrolysis Rate Effect
The flow behavior of Na-montmorillonite suspensions as a function DH of three PAM polymers (at similar MW) in the absence of free electrolyte is given in Fig. 2
. The suspension flow behavior changed from pseudoplastic for the clay alone to plastic in the presence of the three high MW polymers, regardless of the DH. The shear stress increased with the hydrolysis at any given shear rate (Table 2). While the A185 polymer showed a pronounced effect on the flow behavior, the A130 (DH 33%) and the A110 (DH 15%) polymers showed less effect on the flow behavior. In the absence of free electrolytes the polymers showed a strong effect on suspension viscosity, and this effect increased with increasing DH.
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2. The electrostatic repulsive forces between the negatively charged clay surface and the anionic polymer. These forces increase with increasing hydrolysis, so reducing the ability of the polymer molecule to approach the adsorption sites.
The flow behavior of the free electrolyte suspension (Fig. 2) and the results of the suspension viscosity (Table 2) suggest that the polymer conformation factor has a greater impact on clay structure stabilization in suspension than the electrostatic repulsion effect. The clay structure stability was increased with increasing polymer electrostatic negative charge for similar MW.
The flow behavior, the suspension viscosity, and the yield stress of 0.5 N m-2 (Table 2) indicated that the polymer with the highest electrostatic charge (A185) was able to link clay particles and to stabilize the weak structure of edge-to-edge associations to a certain degree. The lesser effects of the A130 (DH 33%) and the A110 (DH 15%) polymers are seen on the flow behavior and the yield stress (0.26 and 0.24 N m-2, respectively, Table 2). Their effects resulted from the coiled configuration of the polymer molecules since the number of charge sites along the polymer molecule, whose mutual repulsion would extend the molecule, decreased at lower hydrolysis levels.
When the NaCl concentration in suspension increased to 10 mmolc L-1, the shear stress increased with increasing hydrolysis at any given shear rate. Whereas a pseudoplastic flow was observed for the suspension of clay alone and in the presence of the A110 polymer, a plastic flow was observed in the presence of the A130 and A185 polymers. Yield stresses of 2.40 and 5.20 N m-2 (Table 2) were observed for the A130 and A185 polymers systems, respectively.
When the A110 polymer (DH 15%) was present, it was observed that: (i) the shear stresses and the apparent viscosity (Table 2) were only slightly above the values for the clay alone; and (ii) the yield stress was 0.01 N m-2 (Table 2). These findings may indicate that this polymer, whose molecule is coiled under these conditions (Mortimer, 1991), has only a small effect on the clay particle arrangement in suspension. For the more highly hydrolyzed polymers (33 and 92%), however, a significant effect was observed. This indicates that the molecules of the A185 polymer, which have a high electrostatic charge, remained in their stretched mode (Mortimer, 1991) at this electrolyte concentration. The structure of the molecules of the A130 polymer, which have lower electrostatic charge, appears to have changed somewhat to the coiled mode, as indicated by the suspension shear stress being lower than with the A185 polymer at any given shear rate. As concluded above, the polymer conformation has a greater impact on clay structure stability in suspension than the electrostatic repulsion effect.
When the NaCl concentration in the suspension was increased to 100 mmolc L-1, a plastic flow was observed for clay suspensions in the absence or presence of the polymers, regardless of the DH. Yield stresses of 1.35, 1.71, 2.13, and 3.64 N m-2 (Table 2) were observed for the suspensions of clay alone, and in the presence of A185, A130, and A110 polymers, respectively. Similarly, the apparent viscosity was the highest for clay suspension with the A110 polymer and it decreased as the DH increased, at any given shear rate (Table 2). The values were lower than those observed for the systems under 10 mmolc L-1 NaCl concentration. In contrast to the systems with 10 mmolc L-1 NaCl concentration, the shear stress and the yield stress decreased with increasing DH when the NaCl concentration in the suspension was 100 mmolc L-1.
The increase in polymer effectiveness with decreasing DH can be explained as follows. In the presence of this high electrolyte concentration, the DDL of the clay platelets was compressed and the clay platelets in suspension formed flocs. The electrolytes in solution cause the highly charged polyelectrolyte molecule to coil up (Mortimer, 1991). Thus, the specific electrostatic charge density on the external surface of the coiled molecule increases with increasing hydrolysis. The repulsive forces between the coiled polymer molecule and the external surfaces of the clay flocs increase with increasing polymer DH. In addition, the number of functional groups on the coiled molecule exposed to clay platelets is smaller than that on the extended molecule. Thus, the relative effectiveness of the polymers in bridging between adjacent clay flocs under high electrolyte concentration increases with decreasing DH.
It is important to note that the effect of the A110 polymer (which has relatively low specific electrostatic charge) on suspension viscosity was insignificant when the NaCl concentration was 10 mmolc L-1 but became significant when the electrolyte concentration was increased to 100 mmolc L-1 (Table 2). The results indicate that this polymer was not active in bridging between individual clay platelets in a well-dispersed suspension, despite its high MW. At NaCl concentration of 10 mmolc L-1, the molecule shape is in its coiled form. Therefore, the repulsive forces between it and the clay platelets were greater than those prevailing at the higher electrolyte concentration. At NaCl concentration of 100 mmolc L-1, two phenomena occurred: (i) the forces between the coiled polymer and the clay platelets decreased significantly; and (ii) the high electrolyte concentration depressed the negative electrical field associated with the planar surfaces. Thus, the accessibility of the adsorption sites on the edges to the negative polymer molecules was enhanced. The stability of the network structure increased because of the interaction between the exposed functional groups on the polymer molecule and the edge surfaces of the clay flocs.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In the free-electrolyte clay suspension, only the largest polymer (A185) could form a three-dimensional network, while the smallest P35 polymer prevented edge-to-edge association. The strong effect of the high MW polymers (A110, A130, and A185) increased with increasing DH (15% < 33% < 92%).
The greatest influence of the polymers was obtained in clay suspensions with electrolyte concentration of 10 mmolc L-1 (at which the attractive forces between clay platelets are very weak, Heller and Keren, 2001). At this electrolyte concentration, the effectiveness of the polymers increased with increasing MW and DH.
At electrolyte concentration of 100 mmolc L-1, the adsorbed polymers stabilized the flocs that were already formed at such a high electrolyte concentration, and the bridging forces between flocs increased in the order P35 < A185 < PAM90. The polymers effectiveness in bridging between adjacent clay flocs under high electrolyte concentration increased with decreasing DH (15% > 33% > 92%).
Rheological measurements were found to be a useful tool in determining the effectiveness of anionic PAM in modifying the interactions between clay particles in suspensions. The effectiveness of the polymers was depended upon their MW and DH. These results suggest that high MW and high DH of negative PAM together with low electrolyte concentration in soil solution could be more effective in soil aggregate stabilization than those with lower MW and DH.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication September 8, 2000.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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85%) on the rheology behavior of Na-montmorillonite suspensions at concentration of 20 g L-1 and pH 7, in the absence of free electrolytes. NP = absence of PAM.