X-ray Diffraction and Rheology Study of Highly Ordered Clay

The clay was synthesized using talc and Na2SiF6 heated at 800 °C for 2 h in an electric furnace.3 The platelike particle has a high asymmetry in its ...
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Langmuir 1997, 13, 2440-2446

X-ray Diffraction and Rheology Study of Highly Ordered Clay Platelet Alignment in Aqueous Solutions of Sodium Tripolyphosphate Hiroshi Tateyama,† Peter J. Scales,*,‡ Masaru Ooi,§ Satoshi Nishimura,† Kevin Rees,‡ and Thomas W. Healy‡ Kyushu National Industrial Research Institute, Shuku machi, Tosu shi, Saga Prefecture 841, Japan, Advanced Mineral Products Special Research Centre, School of Chemistry, The University of Melbourne, Parkville 3052, Australia, CO-OP Chemicals Co. Ltd., 1-22-3 Chiyoda-ku, Tokyo 110, Japan Received October 15, 1996X Highly ordered parallel alignment of a synthetic clay, referred to herein as an expandable mica (ME) but similar in behavior and physical characteristics to montmorillonite, in sodium tripolyphosphate (STP) solutions has been observed by an X-ray diffraction method. The weight ratio of the clay was 7%, and the ME/STP by weight ratio was varied from 0.1 to 0.4. The X-ray diffraction patterns of the suspensions at high STP concentrations were very sharp, and the basal reflections of the clay gave up to fifth order reflections in some samples. The basal spacing of the clay in the suspensions decreased from 8.70 to 4.82 nm with increasing STP concentrations. The rheological properties of the same suspensions showed nonNewtonian behavior at zero STP addition. The steady shear viscosity of the suspensions decreased dramatically with increasing STP addition at a given shear rate. The total potential energy for particleparticle interaction calculated using classical DLVO theory indicates that the clay particles fall into a secondary minimum and that phosphate anions play an important role in controlling the alignment of the platelets.

Introduction The study of the forces of interaction and swelling behavior between clay platelets such as montmorillonite, vermiculite, and kaolin is extensive and has, over the years, been of great interest to both soil scientists and those interested in fundamental interaction forces. As an example, the swelling behavior of montmorillonite in various 1:1 and 2:1 electrolytes is well documented.1 In particular, the correlation of values of the interparticle distance as calculated from X-ray studies and the predicted separation using classical particle interaction theories such as DLVO theory, based on known surface charge, ionic strength, and ion type in solution, has been extensively debated. Despite the extensive work on swelling behavior, the data are not helpful in many colloidal processing situations because the clays have already delaminated into individual platelets and it is then the nature of the platelet particle interaction and the manipulation of the complex rheological behavior of the resultant suspensions that are of interest. Comparison with the swelling process in clay systems is complicated somewhat by the presence of a heterogeneous ‘face’ and ‘edge’ charge on each particle. This heterogeneity, whilst of little apparent consequence in determining swelling behavior, is crucial in particle interaction in delaminated platelet suspensions. The mechanism by which the surface charge on the clay face is generated, be it as a result of substitution in the octahedral or tetrahedral clay layers, is also of importance, as are both the thickness and lateral dimensions of the clay plates. * Author to whom correspondence should be addressed. Phone: 61-3-93446480. Fax: 61-3-93446233. E-mail: p.scales@ chemistry.unimelb.edu.au. † Kyushu National Industrial Research Institute. ‡ The University of Melbourne. § CO-OP Chemicals Co. Ltd. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed; Wiley: New York, 1977.

S0743-7463(96)00995-X CCC: $14.00

The aim of the present work was to reduce the effectiveness of the clay edge charge in a colloidal suspension of a delaminated clay through specific adsorption of the tripolyphosphate anion. The postulated effect would be to allow clay platelet colloidal interactions that were more in line with those experienced in clay swelling and thus allow detailed study of the interaction behavior. X-ray diffraction and shear rheology measurements on the suspensions were chosen as the tools to study the process in detail. The clay chosen for study was a synthetic montmorillonite that was easily delaminated into individual platelets in low ionic strength 1:1 salt solutions. The same process operates in naturally occurring montmorillonite.2 The particles used in this study had a very high specific surface area of 780 m2/g; the plate thickness is 0.95 nm. The clay was synthesized using talc and Na2SiF6 heated at 800 °C for 2 h in an electric furnace.3 The platelike particle has a high asymmetry in its expanded form and is frequently used as a model to study clay swelling by X-ray diffraction. Several X-ray diffraction methods have been developed either for measuring the basal spacing of Na+-montmorillonite and so understanding swelling or for establishing the relationship between the particle interlayer force and the interlayer distance.4-6 Norrish4 used an oriented flake of Na+-montmorillonite in a plastic capillary. Posner and Quirk5 spread a layer of clay paste on a ceramic tile and covered the oriented paste with a thin polyethylene film. Viani et al.6 prepared a highly oriented clay paste by slowly filtering a clay suspension through a membrane (2) Tateyama, H.; Tsunematu, K.; Hirosue, H.; Kimura, K.; Furusawa, T.; Ishida, Y. Proceedings of the 9th International Clay Conference, Strasbourg, 1989; Farmer, V. C., Tardy, Y., Eds.; Universite Louis Pasteur and Centre National de la Recherche Scientifique: Strasbourg, 1989; p 43. (3) Tateyama, H.; Nishimura, S.; Tsunematu, K.; Jinnai, K.; Adachi, Y.; Kimura, M. Clays Clay Miner. 1992, 40, 180. (4) Norrish, K. Discuss. Faraday Soc. 1954, 18, 120. (5) Posner, A. M.; Quirk, J. P. J. Colloid Sci. 1964, 19, 798. (6) Viani, B. E.; Low, P. F.; Roth, C. B.; J. Colloid Interface Sci. 1983, 96, 229.

© 1997 American Chemical Society

Aqueous Solutions of Sodium Tripolyphosphate

Langmuir, Vol. 13, No. 9, 1997 2441

Table 1. Clay Compositions Utilized in This Work sample

STP/ME ratio

[ME] (wt %)

[STP] (wt %)

[STP]a (mol dm-3)

[Na+]b (mol dm-3)

7RAW 7STP-1 7STP-2 7STP-3 7STP-4

0.0 0.1 0.2 0.3 0.4

7.0 7.0 7.0 7.0 7.0

0.0 0.7 1.4 2.1 2.8

0.0 0.020 0.041 0.062 0.084

0.10 0.21 0.31 0.42

a Calculated from the STP concentration in wt %. b Concentration calculated on the assumption that the STP was perfectly dissociated.

filter. The oriented clay paste was then used for measuring the basal spacing of Na+-montmorillonite by X-ray diffraction. The swelling properties of montmorillonite at various water contents were also studied by Fukushima.7 He did not observe a sharp (001) reflection due to regular stacking of Na+-montmorillonite, leading him to propose a zigzag stacking model of Na+-montmorillonite instead of a straight column model. Recently Shang8 reported a transmission X-ray diffraction technique for measuring crystalline swelling of smectites in electrolyte solutions. The X-ray diffraction patterns measured by all these authors showed broad bands and gave only first- or secondorder reflections. A possible reason for this lack of higher order behavior is the influence of edge effects in the stacking of particles. The central purpose of this study was to investigate the arrangement of a synthetic clay in water in the presence of sodium tripolyphosphate by X-ray diffraction and rheology methods, where the role of the polyphosphate is to control and minimize the random stacking nature of the particles and promote straight column stacking. Experimental Section Sample Preparation. The sample used in this study was a pure expandable mica with the physical and swelling characteristics of a montmorillonite. The clay plates had a longitudinal dimension of 1.2 µm and a thickness of 0.95 nm. The chemical composition was SiO2 (58.2%), Al2O3 (0.3%), Fe2O3 (0.06%), MgO (26.4%), CaO (0.10%), Na2O (5.0%), K2O (0.01%), and F (10.0%), and the structural formula was Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96. The material had a, b, and c axis dimensions of 0.524, 0.908, and 0.970 nm, respectively, and a cation exchange capacity of 84.9 mequiv/100 g. The clay is commercially available as ME-100F from CO-OP Chemicals, Tokyo, Japan. Sixteen grams of sample was mixed with 184 cm3 of distilled water for 2 min in a blender to obtain a uniform gel of the clay in water. Sodium tripolyphosphate (STP), Na5P3O10, was a reagent grade chemical. STP solutions were prepared and mixed with the clay gel to adjust the STP and clay concentrations. The compositions of all the suspensions used in this study are listed in Table 1. Each sample was shaken for 30 min in a sealed bottle and allowed to stand for 2 days before rheological measurement and X-ray diffraction analysis. X-ray Diffraction Measurements. A diagram of the X-ray diffraction equipment used in this study and located at KNIRI, Tosu, Japan, is shown schematically in Figure 1 . A 30 mm diameter sample cell was placed on a stainless steel holder which was covered by the thin nickel foil (1 µm) to prevent evaporation. The temperature of the sample cell was controlled at 25 ( 1 °C, and the humidity of the atmosphere in the sample chamber was controlled at 80 ( 1%. The X-ray diffraction pattern of a suspension was measured as shown schematically in Figure 1B. This configuration was used to refine the X-ray diffraction intensity data and investigate the reproducibility of the experiment. X-ray beams were diffracted from the bottom plane of the sample holder. This method avoided any ambiguities in which the diffracted position of the X-ray beams was changed by evaporation. Despite this possibility, X-ray diffraction patterns (7) Fukushima, Y. Clays Clay Miner. 1984, 32, 320. (8) Shang, C.; Thompson, M. L. Clays Clay Miner. 1995, 43, 128.

Figure 1. Schematic drawing of the X-ray diffraction equipment and mode of operation. measured using the configuration shown in Figure 1A were the same as those for Figure 1B. The A and B assemblies were mounted on the axes of Rigaku (RAD) and Philips (APD) X-ray goniometers respectively to reconfirm the reproducibility of the measurements. These diffractometers were calibrated using fluorophlogopite mica, talc, and silicon metal. Electrophoretic Measurements. The ζ-potential of the clay was measured using a Penkem system 3000. Initially, a very small amount of the clay (1-2 mg) was dispersed in 50 cm3 of a 10-3 mol dm-3 NaCl solution. Further samples were prepared using a STP solution in the range 10-3 to 5 × 10-2 mol dm-3 using the same method. Rheological Measurement. Measurements of the shear stress of a clay suspension as a function of shear rate in the range 1.5-1400 s-1 were made using a Weissenberg (Carri-Med) Rheogoniometer located at the University of Melbourne, with cone and plate fixtures. The cone and plate fixtures were both 75 mm in diameter, the cone angle was 1°, and the gap size between the truncated cone and the plate was 55 µm. The temperature was controlled at 20 ( 1 °C. The interface exposed to the atmosphere was coated with a thin layer of silicon oil to minimize evaporation.

Results and Discussion X-ray Diffraction Analysis. The X-ray diffraction pattern of a 7 wt % solids clay suspension without additives (7RAW) is shown in Figure 2. The data show no Bragg reflections and indicate that water molecules have penetrated between and separated the individual clay sheets. The lack of Bragg reflections indicates that the clay sheets are randomly oriented in suspension after the swelling process. X-ray diffraction patterns of the clay suspensions in the presence of varying concentrations of STP are shown in Figure 3. At low STP addition rates (STP-1), the X-ray diffraction pattern is very diffuse, with only two orders visible, as shown in Figure 3A. The reflections correspond to a basal spacing of the (001) reflection of approximately 8.5 nm and one of the (002) reflection of nearly 4.4 nm. The averaged basal spacing was calculated to be 8.7 nm. In contrast to the case for 7STP-1, the (00l) reflections of 7STP-2 (higher STP addition) were sharp, giving the rational series of (00l) lines at 6.3, 3.16, and 2.11 nm.

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Tateyama et al.

Figure 2. X-ray diffraction pattern of a 7 wt % clay suspension in 10-3 M NaCl (7-RAW).

These results are shown in Figure 3B. The averaged basal spacing was 6.32 nm. In the case of 7STP-3, higher order reflections were observed at 5.4, 2.60, 1.79, 1.35, 1.08, and 0.91 nm, as shown in Figure 3C. The averaged basal spacing was 5.37 nm. The sharpness of the diffraction patterns suggests a regular stacking sequence of the clay platelets in suspension. Figure 3D shows that the (00l) reflections of 7STP-4 were also very sharp and formed the rational series at 4.8, 2.40, 1.61, 1.21, and 0.97 nm. The averaged basal spacing was 4.82 nm. A similar series of experiments was then conducted at a lower solids concentration. Figure 4 shows the X-ray diffraction pattern of 2 wt % clay in a 2.8 wt % STP solution. The (00l) reflections were also very sharp and gave the rational series of reflections. The averaged basal spacing was 5.0 nm. As discussed earlier, a number of authors have already published (00l) reflection data of Na+-montmorillonite in electrolyte solutions. The X-ray diffraction patterns were usually broad and gave only first- or second-order reflections. On the other hand, the present study shows very sharp X-ray diffraction patterns and up to sixth-order reflections in the case of the 7STP-3 suspension. Norrish4 showed that Na+-montmorillonite increased in basal spacing from 1.9 to 4.0 nm as the solution electrolyte concentration decreased below 0.3 M NaCl and that the X-ray diffraction patterns became diffuse in more dilute solutions. He also showed that the basal spacing of Na+-montmorillonite increased continuously below a NaCl concentration (C) of 0.3 M and that the increasing ratio of the basal spacing was linear with respect to C-1/2. If it is assumed that STP is fully dissociated in solution, the concentration of Na+ in the solutions utilized in this study ranged from 0.10 for 7STP-1 to 0.42 for 7STP-4. Therefore, the concentrations of Na+ ions in the present suspensions correspond to the transition region of NaCl solutions in which the basal spacing of montmorillonite changes from 1.9 to 4.0 nm. Figure 5 shows the basal spacing of the clay in STP solutions as a function of the inverse square root of the molar concentration of NaCl (C-1/2). The behavior is similar to that observed in NaCl solutions and had the form

d ) 24.1C-1/2 + 11 where d is the basal spacing in nanometers. The slope of the line was close to that for Li-vermiculite.9 The main (9) Norrish, K.; Raussell-Colom, J. A. Clays Clay Miner. 1963, 10, 123.

Figure 3. X-ray diffraction patterns of 7 wt % clay suspensions mixed with various concentrations of sodium tripolyphosphate.

Aqueous Solutions of Sodium Tripolyphosphate

Figure 4. X-ray diffraction pattern of a 2 wt % clay suspension mixed with sodium tripolyphosphate.

Figure 5. Variation of the basal spacing of the clay plotted against the concentration of electrolyte as C-1/2.

difference between the literature data and that obtained here was the sharpness and order of the reflections, which indicates that the particles stack in columns rather than zigzag structures with the polyphosphate anion as compared to the chloride anion. Electrophoretic Properties. The most obvious point of difference between this and earlier work is the anion type. The results indicate that polyphosphate anions play an important role in the control of the highly regulated stacking sequence of the clay in suspension. The clay has a platelike morphology with negatively charged face planes (due to isomorphous substitution). The charge of the edges is not definitively established but is almost certainly alumina-like in behavior, in which case the edges will be positively charged below a pH of approximately 9. The present study was performed at pH values between 9 and 10 such that the edges would be expected to show only a small positive character. At pH 9, the face planes will repel electrostatically and phosphate anions should be co-ion excluded from the interface. In contrast, the edges of the plate should interact strongly with phosphate anions, and these anions are expected, on the basis of earlier work on surfaces such as zirconia, to be specifically adsorbed.10 To investigate the above postulates, the electrophoretic mobility of the clay was measured and the ζ-potential of the particles was calculated using the Smoluchowski equation. The results of this analysis are shown in Figure (10) Leong, Y. K.; Boger, D. V.; Scales, P. J.; Healy, T. W.; Buscall, R. J. Chem. Soc., Faraday Trans. 1993, 89, 2473.

Langmuir, Vol. 13, No. 9, 1997 2443

Figure 6. ζ-potential of clay at various sodium tripolyphosphate concentrations.

Figure 7. Steady shear viscosity plotted against the shear rate of the 7 wt % clay suspensions.

6. It should be noted at this point that the electrokinetic potential for the platelike particles may not be truly represented by these ζ-potential values because the equation utilized assumes a spherical rather than platelike particle geometry. The ζ-potential of the particles shows an increase in magnitude as the STP concentration was increased from zero to 5 × 10-3 mol dm-3. The potential then decreased slightly in magnitude as the STP concentration and ionic strength increased further. The trend of the results in Figure 6 is consistent with a change in the potential of the edges from positive or near neutral to negative caused by polyphosphate adsorption. A qualitative assessment of the surface area for adsorption and of the level of polyphosphate present shows the polyphosphate ion to be in excess at all concentrations. Rheological Properties. Steady shear viscosity against shear rate for 7 wt % clay suspensions at various STP concentrations is shown in Figure 7. The flow behavior of the suspension without additives (7-RAW) exhibits non-Newtonian flow behavior, and a hysteresis loop was obtained when readings were taken at increasing and then decreasing rates of shear. Further tests showed that the steady shear viscosity gradually decreased at a constant shear stress. The reduction of the steady shear viscosity as a function of time indicates that particle links in the network structure are gradually being broken down or that a reorientation/particle alignment is taking place. Tests showed the viscosity shear rate behavior could be represented by a common curve after two ascendant and

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descendant shear viscosity runs. No hysteresis was observed after this point, and the system was assumed to be shear equilibrated. The high viscosity of the 7 wt % suspension (7RAW) is explained both in terms of the clay forming a weak “house of cards” structure due to edge-to-face interaction between negatively charged plate planes and the positively charged edge surfaces and in terms of the fully expanded nature of the particles. The volume fraction of particles is obviously constant, but the volume occupied/swept out by an individual particle is maximized under these conditions of random plate orientation, and the result is that the apparent volume fraction of the suspension is also a maximum. Literature explanations of the cause of the enhanced viscosity of fully dissociated clay suspensions are numerous. Rand et al.11 reported that dilute suspensions of Na+montmorillonite were Newtonian liquids and showed no tendency to coagulate over the pH range 4-11 at NaCl concentrations below 5 × 10-3 mol dm-3. This is in contrast to the non-Newtonian nature of kaolinite suspensions under the same conditions.12,13 Ottewill14 showed that the electrostatic attraction between edge and face was small for montmorillonite compared to the repulsion between two faces because of the small area of the edge. This goes a long way to understanding the difference in rheology of suspensions of thin plates of montmorillonite and the thicker kaolinite plate system. Secor et al.15 reached the same conclusion by solving the PoissonBoltzmann equation on the basis of a cylindrical model. At low indifferent electrolyte concentrations the negative electrostatic field emanating from the face plane spills over into the edge surfaces, and as a result the positively charged edge is swamped by the negative electrostatic field. Recently Chang et al.16 applied modified Gouy-Chapman theory to a disk model and solved the PoissonBoltzmann equation numerically. They found that the barrier for adsorption of anions to the edge surface is mainly controlled by particle thickness. They also concluded that the presence of a positively charged edge surface affects clay-electrolyte interactions most significantly at low electrolyte concentrations. Therefore, the network structure of the condensed clay suspensions studied here may not be simply explained on the basis of a simple “house of cards” structure model. Indeed the time dependence of the rheology may simply represent a reorientation in the shear field causing a lowering in the apparent volume occupied by each particle and hence a lowering of the viscosity of the system as clay platelets align. Rheological data for 7 wt % clay suspensions in the presence of STP are shown in Figure 7. Rheological measurements at low STP concentrations show that the flow curve of 7STP-1 is still non-Newtonian, but the steady shear viscosity was small compared with that of 7RAW. The steady shear viscosities of the suspensions decreased with increasing STP concentration at high shear rate. As expected, the relative viscosity increased as the volume fraction of solids increased. For an expanding clay, the relative volume of the swollen particles can be calculated from the concentration of the (11) Rand, B.; Pekenc, E.; Goodwin, J. W.; Smith, R. W. J. Chem. Soc., Faraday Trans. 1 1980, 76, 225. (12) Schofield, R. K.; Samson, H. R, Discuss. Faraday Soc. 1954, 18, 135. (13) van Olphen, H. Dicuss. Faraday Soc. 1951, 11, 82. (14) Ottewill, R. H. J. Colloid Interface Sci. 1977, 58, 357. (15) Secor, R. B.; Radke, C. J. J. Colloid Interface Sci. 1985, 103, 237. (16) Chang, F. C.; Sposito, G. J. Colloid Interface Sci. 1994, 163, 19.

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clay expressed as grams of dry clay in 100 cm3 suspensions and the volume percent of water in the expanded clay as derived from the increase of the d-spacing upon hydration. This formula is based on the assumption that particle interaction between clay domains is absent. The decrease in the steady shear viscosity from 7STP-1 to 7STP-4 can be explained on this basis because the d-spacing of the suspension decreased with increasing STP concentration. However, at the very low shear rate, the steady shear viscosity increased with increasing STP concentrations. This result indicates that the clay column structures in suspension are very shear sensitive despite the expected increase in the interparticle attractive force with increasing STP and hence electrolyte concentration. Particle Interactions. Norrish4 first attributed that, at low NaCl concentrations, the increasing tendency of the basal spacing of Na+-montmorillonite is in accordance with diffuse double-layer theory. The total potential energy (VT) was calculated on the basis of DLVO theory such that

VT )

AF(VR + VT) kT

(1)

where AF (m2) is the area of the face of a platelike particle, VR (J/m2) is the repulsive energy, VA (J/m2) is the attractive energy, k is the Boltzmann constant, and T is the absolute temperature. To calculate the energy of interaction for the clay suspension in this study, VR was calculated on the basis of the potential energy equation for plate-plate heterocoagulation17 such that

VR )

κ 2 [(φ1 + φ22)(1 - coth(2 κh)) + 2 2φ1φ2 cos ech(2 κh)] (2)

where  is the dielectric constant, 1/κ is the inverse Debye length, φ is the Stern plane or zeta potential, and 2h is the distance between plates. The attractive component of the energy of interaction (VA) was calculated using

VR )

[

]

Η 1 1 2 + 48π h2 (h + δ)2 (h + δ/2)2

(3)

where H is the Hamaker constant and δ is the plate thickness. The Stern plane potential of the clay was taken as -45 mV in accord with the ζ-potential data in Figure 6. The thickness of the clay plate was δ ) 0.95 nm, and the Hamaker constant was taken as H ) 2.2 × 10-20 (J).18 Figure 8 shows the total potential energy of interaction between two plate surfaces calculated for a 1:1 electrolyte solution in the concentration regime of interest. The results indicate that there is a very high potential barrier to close particle approach such that the clay sheets cannot fall into a plate-to-plate primary minimum. Using these calculated data, the particles are predicted to fall into a deep secondary minimum. The calculations show that the depth of the secondary minimum increases with increasing STP concentration. The distances between two plate surfaces (2h) also decrease from 7STP-1 to 7STP-4. On the basis of entrapment in the secondary minimum, the calculated interparticle spacing is predicted to be 10.4, 5.9, 4.4, and 3.4 nm, respectively. The 2h distances (17) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. J. Chem. Soc., Faraday Trans. 1966, 62, 1638. (18) Israelachvili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press, London, 1991.

Aqueous Solutions of Sodium Tripolyphosphate

Figure 8. Total potential energy between plates calculated on the basis of the DLVO theory.

Figure 9. Variation of the distance between two particles (2h) plotted against C-1/2.

obtained from X-ray diffraction data were 7.7, 5.3, 4.4, and 3.8 nm with increasing STP concentration. The calculated distances between particles based on DLVO theory are smaller than those measured by the X-ray diffraction method at high STP concentrations (7STP-4) but are larger at low STP concentrations. The deviation appears significant for STP-1 but in fact probably reflects the very shallow nature of the secondary minimum at this concentration. Indeed, a variation of 10kT in the depth of the minimum results in a change of 1.2 nm in the calculated interparticle spacing. Therefore, the predicted interparticle plate spacing based on the location in the secondary minimum has a reasonably large error in the case of low STP concentrations. The errors associated with the X-ray data are also higher at low STP concentrations due to the more diffuse nature of the spectra. Figure 9 shows the interplanar distances calculated using an assumption of a 1:1 electrolyte and STP ionic strengths plotted against C-1/2. The interplate spacing (2h) values calculated on the basis of a 1:1 electrolyte solution were comparable to the observed X-ray data. The 2h values calculated using the theoretical ionic strengths of the STP solutions were smaller than the observed distances. These calculations do not take account of the role of the edge charge and only include electrostatic and van der Waals contributions to the interaction from the face planes of the particles. At high STP concentrations, the calculations predict that the van der Waals force would overcome the repulsive force between two plate surfaces and cause the surfaces

Langmuir, Vol. 13, No. 9, 1997 2445

to fall into a primary minimum. It is our postulate that the disagreement between the observed and calculated data in STP solutions using the ionic strength calculated including the fully dissociated phosphate anion indicates that the phosphate anions are co-ion excluded from between the clay face surfaces. In lieu of the high negative charge and large ionic sizes as compared with other monovalent anions present in solution, this postulate seems reasonable. Subsequent work on the same clay system using pyrophosphate instead of tripolyphosphate at the same concentrations does induce collapse to the primary minimum.19 In force balance measurements between mica surfaces (where particle edge effects are of no relevance), Pashley and Quirk20 showed that, in dilute NaCl electrolyte solutions (below about 0.01 M), the van der Waals force caused the force between two surfaces to be net attractive at short separations. This is indeed the expected result on the basis of DLVO theory, but at higher electrolyte concentrations, an additional repulsive force arose which completely negated the van der Waals attraction at short range. This force was assumed to be due to a layer of cations. The lack of three-dimensional order in swelling studies using 1:1 electrolytes may well be attributed to this same phenomenon although the observed interparticle spacing is often considerably larger than the measured range of the short range repulsive force. It is unlikely on this basis that this is the reason for the lack of threedimensional order in swelling. Alternatively, the disordering effect of the presence of an electrostatically attractive force between the positive edges and negative faces of the particles may be the main contributor to the lack of three-dimensional order. This effect would be of little consequence in crystalline swelling experiments typical of those conducted by Norrish4 since edge-face interaction was minimized and three-dimensional order was present by default in this case. In swelling experiments of noncrystalline clays and in swelling of clay pastes, this would not be the case, and it is interesting to note that three-dimensional order is not normally observed in such experiments. It is also interesting that the extra attractive force associated with the interaction of the clay edges and faces is not taken into account in theoretical comparisons. Noncompliance of experiment with DLVO theory is often associated with an extra attractive component of the force of interaction. The phosphate anions utilized in the present study are expected to negate the edge charge and, as such, the disordering effect of edges and faces of opposite charge. Under high phosphate conditions, good three-dimensional order is observed as is, indeed, good agreement with DLVO theoretical predictions under the same conditions. The low steady shear viscosity of the clay suspensions in the presence of STP is consistent with coagulation to the secondary minimum as explained above. The parallel alignment of the faces of individual clay sheets results in domains having a column structure in the suspension. The increase in ionic strength reduces the thickness of the diffuse double layer and decreases the basal spacing of the clay. Each effect will decrease the volume of the domain. The small domain size of the clay at high STP concentrations translates into a lower apparent clay volume per plate and a significant reduction in viscosity. Therefore, the rheology data are entirely consistent with the X-ray diffraction data. (19) Tateyama, H.; Scales, P. J.; Ooi, M.; Johnson, S. B.; Rees, K.; Boger, D. V.; Healy, T. W. Langmuir, submitted. (20) Pashley, R. M.; Quirk, J. P. Colloids Surf. 1984, 9, 1.

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Conclusions The parallel alignment of clay suspensions has been studied by X-ray diffraction, ζ-potential, and rheological measurements in the presence of STP. The degree of the clay stacking in suspension becomes more regular with increasing STP concentration, and up to sixth order reflections were observed in some samples. The steady shear viscosity of the clay suspensions decreased both as three-dimensional clay domains were formed and further with decreasing the basal spacing in the clay domains from 8.7 to 4.82 nm. Theoretical calculations indicate that phosphate anions may be excluded from the region between plates at high STP concentration. Electrophoresis data indicate that the phosphate anions are adsorbed

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to the edge surfaces of the clay plate. Rheological data indicate that the size of the clay domains and the reduction in apparent volume fraction caused by platelet stacking in domains are the most important parameters in reducing the suspension rheology at a given shear rate. Acknowledgment. Financial support for this work was provided by the Advanced Mineral Products Special Research Centre, a Special Research Centre of the Australian Research Council and the Kyushu National Industrial Research Institute, Tosu, Japan. The scientific input of Professors David Boger, Jim Quirk, and Derek Chan is gratefully acknowledged. LA960995S