Electrokinetc Study of Synthetic Smectites by Flat Plate Streaming

of the inner Helmholtz layer capacitance by using the Gouy−Chapman−Stern−Graham (GCSG) model with a single-site dissociation of surface grou...
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Langmuir 2002, 18, 188-193

Electrokinetc Study of Synthetic Smectites by Flat Plate Streaming Potential Technique Satoshi Nishimura,*,† Ken Yao,† Masaya Kodama,† Yusuke Imai,† Kazuya Ogino,‡ and Kenji Mishima‡ National Institute of Advanced Industrial Science and Technology (AIST), Kyushu, 807-1 Shyku, Tosu, Saga 841-0052, Japan, and Department of Chemical Engineering, Fukuoka University, 8-1-19, Nanakuma, Jonan, Fukuoka 814-0180, Japan Received January 23, 2001. In Final Form: September 11, 2001 ζ potentials of synthetic smectites with different locations of lattice charges, i.e., saponite and hectorite, were investigated as functions of pH and NaCl concentration by a flat plate streaming potential technique where two smectite-coated plates were separated by a Teflon gasket to form a slit channel. The lattice charge of saponite originates from the isomorphous substitution of Si4+ by Al3+ ions in tetrahedral silica sheets, whereas the lattice charge of hectorite is caused by the isomorphous substitution of Mg2+ by Li+ ions in octahedral magnesia sheets. The ζ potential for saponite was more sensitive to the pH of electrolyte solution than that for hectorite in acidic solution. Such a difference in pH dependence of the ζ potential could be explained by the difference in the location of the lattice charges between saponite and hectorite as a result of an analysis of the inner Helmholtz layer capacitance by using the Gouy-Chapman-SternGraham (GCSG) model with a single-site dissociation of surface groups. On the other hand, the dependence of the ζ potential on NaCl concentrations could not be fully interpreted by the GCSG model with a single value of the outer Helmholtz layer capacitance, which suggests a shift of the shear plane and/or a change in the dielectric constant of the outer Helmholtz layer with changing NaCl concentration.

Introduction Smectites are swellable 2:1 layer clay minerals in which one alumina- or magnesia-oxygen-hydroxy octahedral sheet shares oxygen atoms with two identical silicaoxygen-hydroxy tetrahedral sheets on each side, and exchangeable Na+ ions are sandwiched between the silicate layers. Exchangeable Na+ ions balance negative lattice change that are produced because of isomorphous substitution of Si4+ by Al3+ ions in the tetrahedral silica sheet or Mg2+ by Li+ ions in the magnesia octahedral sheet.1 Smectites are easily swollen because of the hydration of the exchangeable Na+ ions in the presence of water, and then Na+ ions tend to diffuse away from the surface, whereas they are attracted electrostatically to the negatively charged layer. These opposed effects yield a diffuse electrical double layer on the surface of a smectite particle. Consequently, a colloidal dispersion of negatively charged silicate flakes with a thickness of the order of nanometers is produced. Such a significant feature of smectite suspensions has led to many applications in industry and agriculture and to environmental problems. With the aim of understanding and controlling the behavior of a smectite suspension, the properties of the electrical double layer of smectite have been extensively investigated in the light of swelling pressure,2-8 rheology,9-15 surface charge and cation exchangeability.16-18 In most of the studies on the * Corresponding author. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Fukuoka University. (1) Brindley, G. W.; Brown, G. Crystal Structure of Clay Minerals and Their X-ray Identification; Mineralogical Society: London, 1980; Chapter 1. (2) Norrish, K. Faraday Discuss. Chem. Soc. 1954, 18, 120. (3) Van Olphen, H. J. Colloid Interface Sci. 1962, 17, 660. (4) Davidtz, J. C.; Low, P. F. Clays Clay Miner. 1970, 18, 325. (5) Viani, B. E.; Low, P. F.; Roth, C. B. J. Colloid Interface Sci. 1983, 96, 229.

electrical double layer of clays,19-27 electrophoresis has been used mainly and experimental data on electrophoretic mobility were converted to ζ potential through the use of the Smoluchowski equation and/or the numerical solutions of O’Brien and White.28 However, the behavior of smectite suspension is more complicated than that of most other (6) Leubetkin, S. D.; Middleton, S. R.; Ottewill, R. H. Philos. Trans. R. Soc. London 1984, A311, 133. (7) Spitzer, J. J. Langmuir 1989, 5, 199. (8) Greathouse, J. A.; Feller, S. E.; McQuarrie, A. Langmuir 1994, 10, 2125. (9) Van Olphen, H. Clays Clay Miner. 1956, 4, 204: Van Olphen, H. Clays Clay Miner. 1959, 6, 196. (10) Denis, J. H. J. Colloid Interface Sci. 1991, 39, 35. (11) Tateyama, H.; Scales, P. J.; Ooi, M.; Nishimura, S.; Ree, K.; Healy, T. W. Langmuir 1997, 13, 2440. (12) de Krester R. G.; Scales, P. J.; Boger, D. V. Colloids Surfaces 1998, 137, 307. (13) Adachi, Y.; Nakaishi, K.; Tamaki, M. J. Colloid Interface Sci. 1998, 198, 100. (14) Luckham P. F.; Rossi, S. Adv. Colloid Interface Sci. 1999, 82, 1. (15) Duran J. D. G.; Ramos-Tejade, M. M.; Arroyo, F. J.; GonzalezCaballero, F. J. Colloid Interface Sci. 2000, 229, 107. (16) Van Olphen, H. An Introduction to Clay Colloid Chemistry; WileyInterscience: New York, 1976. (17) Sposito, G. The Surface Chemistry of Soils; Oxford University Press: New York, 1984. (18) Bolt, G. H.; Van Riemsdijk, H. In Aquatic Surface Chemistry; Stum, W. Ed.; Wiley-Interscience: New York, 1987; p 127. (19) Friend, J. P.; Hunter, R. J. Clays Clay Miner. 1970, 18, 275. (20) Swatzen-Allen, S. L.; Matijevic, E. J. Colloid Interface Sci. 1974, 50, 143. (21) Williams, D. J.; Williams, K. P. J. Colloid Interface Sci. 1978, 65, 79. (22) Gonzalez-Caballero, F. J. Colloid Interface Sci. 1985, 113, 203. (23) Pashley, R. Clays Clay Miner. 1985, 3, 193. (24) Horikawa, Y.; Murray, R. S.; Quirk, J. P. Colloids Surfaces A 1988, 32, 181. (25) Miller, S. E.; Low, P. W. Langmuir 1990, 6, 572. (26) Sondi, I.; Biscan, J.; Pravdic, V. J. Colloid Interface Sci. 1996, 178, 514. (27) Thomas, F.; Vantelon, D.; MOntages, E.; Prelot, B.; Cruchaudet, M.; Delon, J. F. Colloids Surfaces A 1999, 159, 351. (28) O’Brien, R. W.; White, L. R. J. Chem. Soc. Faraday Trans. 2 1978, 74, 1607.

10.1021/la0101191 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/11/2001

Electrokinetc Study of Synthetic Smectites

sols because of their asymmetrical shape, surface-charge distribution, ion exchangeability, and particle association: (i) The effect of the disk shape of clays has not been fully clarified in the estimation of ζ potential; therefore, it is inevitable to assume that the effect of the disk shape can be neglected or regarded as spherical particles to convert electrophoretic mobility into ζ potential.22,23 (ii) Two types of surface charges, i.e., positively charged broken bonds on the edges and the negative lattice charges arising from isomorphous substitution on the face (basal plane or layer surface), coexist in a platelet, making it difficult to interpret data on the ζ potential. (iii) A uniform medium surrounding the platelets in suspensions cannot necessarily be substantiated, because the variation in mode of particle association, such as face-to-face and edgeto-face aggregations, may cause dissimilar diffuse double layers at the external and internal surface of the clay in aggregations. The electrical double layer of montmorillonite, i.e., natural smectites, has been traditionally of much interest to soil or clay scientists to explain the swelling behavior of smectites in relation to the difference in the density and the location of the lattice charge. However, montomorillonite is generally contaminated with organic and/ or inorganic impurities in nature, making the properties of the electrical double layer somewhat ambiguous. Hence, we focused here on two kinds of synthetic smectites, i.e., saponite and hectorite, in which impurities are substantially reduced compared with natural smectites. Saponite possesses a negative lattice charge originating from the isomorphous substitution of Si4+ by Al3+ ions in the tetrahedral silica sheet, whereas the lattice charge of hectorite is caused by the isomorphous substitution of Mg2+ by Li+ ions in the magnesia octahedral sheet.29 In this study, we examine ζ potential of the face of smectite by means of a flat plate streaming potential technique and discuss the effect of lattice charges on the ζ potential. X-ray diffraction studies30 and atomic force microscopy observations31,32 indicated that the faceoriented film of smectite is obtained by simply casting a dilute dispersion of smectite on a planar substrate due to its own shape of ultrathin platelets. Such a face-oriented film of smectite can be regarded as a macroscopic face of smectite; therefore, the flat plate streaming potential technique for ζ potential measurement of the face-oriented films of smectite facilitates overcoming the abovementioned difficulties: (i) The influence of the disk shape of smectite on hydrodynamics can be neglected as long as the smectite platelets are fixed on a planar substrate as a face-oriented film. (ii) Flow direction of electrolyte solution in a slit-channel of a flat streaming potential cell is parallel to the faces and vertical to the edges of smectite platelets; therefore, the charge transfer of counterions on the edges is much smaller than that on the faces, and then the streaming potential is due to mostly the charge transfer of counterions on the faces of the platelets. Accordingly, the relation between lattice charges and the electrical double-layer properties of smectites can be expected to be clarified by using the flat plate streaming potential for ζ potential measurement of the face-oriented films of smectite. (29) Ross, C. S.; Hendricks, U.S. Geol. Surv. Prof. Pap. 1945, 205B, 23. (30) Brindley, G. W.; Brown, G. Crystal Structure of Clay Minerals and Their X-ray Identification; Mineralogical Society: London, 1980; Chapter 5. (31) Hartman, H.; Sposito, G.; Yang, A.; Manne, S.; Gould, S. A. C.; Hansma, P. K. Clays Clay Miner. 1990, 38, 337. (32) Henderson, G. S.; Vrdoljak, G. A.; Eby, R. K.; Wicks, F. J.; Rachlin, A. L. Colloids Surfaces A 1994, 87, 197.

Langmuir, Vol. 18, No. 1, 2002 189

Figure 1. Schematic diagram of flat plate streaming potential apparatus.

Experimental Section Materials. Two types of synthetic smectites were used for this study; saponite and hectorite were kindly supplied by Kunimine Industries Co., Ltd. and Coop Chemical Co., Ltd., respectively. The particle diameters for saponite and hectorite were approximately 100 nm and 50 nm, respectively. The structural formulas of saponite and hectorite were determined from the chemical compositions: saponite, Na0.4Al2[Si3.6Al0.4]O10OH2; hectorite, Na0.33(Mg2.67Li0.33)Si4O10OH2. The solution of pH was adjusted with HCl and NaOH. NaCl was used as a supporting electrolyte. All reagents were analytical grade and were used as received. The water used in all experiments was obtained using a Milli-Q system. Streaming Potential Measurements. ζ potential of smectite was measured by using a flat plate streaming potential apparatus (Anton Paar, Electrokinetic Analyzer) as functions of pH and NaCl concentration at 25 °C as shown in Figure 1. In this apparatus, two smectite-coated poly(methyl methacrylate) (PMMA) plates, where the faces of the smectite platelets were well oriented, were separated by a Teflon gasket with a thickness of 0.1 mm to form a slit-channel. Electrolyte solution was made to flow through the slit-channel pump. The smectite-coated plates were prepared by dipping a PMMA plate into the dispersion of smectites (0.1 g of smectite in 100 mL of water) on PMMA flat plates(75 × 26 × 1 mm), followed by drying in a laminar flow cabinet at a room temperature. The smectite-coated plates were heated at a temperature of 120 °C for several hours to make smectite platelets glued on the PMMA plate due to the thermoplasticity of PMMA. The smectite-coated plates were rinsed in 10-2 M NaCl aqueous solution to remove excessively stacked platelets from the plates before each measurement. After each measurement, the presence of smectite platelet on the PMMA plate was confirmed by scanning electron microscopic observation with chemical analysis. A potential difference, i.e., streaming potential, ∆E, versus an applied pressure gradient, ∆P, across the slit-channel was converted to ζ-potentials by the HelmholtzSmoluchowski equation (in SI units):

ζ ) (ηλ/0r )(∆E/∆P)

(1)

where η is the viscosity of the medium, λ is the conductivity of the capillary, and 0 and r are the permittivity of free space and the relative permittivity of the medium, respectively.

Results pH Dependence of ζ Potentials. In Figure 2, ζ potentials for saponite (a) and hectorite (b) are shown as

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Figure 2. ζ potential and the potential of smectites at the outer Helmholtz plane (OHP potential) as a function of pH. ζ potentials of smectites are shown as open circles. The solid and dotted lines show the OHP potentials calculated by assuming that H+ ion is a specifically adsorbed ion at the inner Helholtz potential (IHP) and a potential determining ion, respectively.

open circles as a function of pH in the presence of 10-3 M NaCl as a supporting electrolyte. ζ potentials for both saponite and hectorite showed a value of ∼-30mV at a natural pH of 5.4-5.8, which was also similar to those of Na montmorillonite presented previously.22,26 ζ potentials for both these smectites tend to be independent of pH in higher pH regions. The ζ potential for saponite is more sensitive to the pH of the electrolyte solution than that for hectorite in the lower pH region; the decrease in the ζ potential for saponite was clearly observed on lowering the pH of the solution, whereas the ζ potential for hectorite was almost independent of pH. One possible explanation for this is that the decrease in the ζ potential for saponite in a lower pH region would be caused by the presence of positively charged edges. The charging behavior of the edges of smectites is expected to be similar to that of Al2O3 (for saponite) or Mg2O (for hectorite) from the chemical composition. The isoelectric points of these metal oxides exist in the region above pH 8.33 So the positive charges on the edges of smectites cause a decrease in the total negative charge in magnitude with decreasing pH. By assuming that smectite platelet is a disk with a thickness of 1 nm and a diameter of 100 nm and 50 nm for saponite and hectorite, the ratio of the surface area of the edge to the face is found to be only 4% and 8% for saponite and hectorite, respectively. According to the Gouy-Chapman diffuse layer theory, i.e., eq 8, an increase in positive charge at the level of 4-8% caused only a ∼1% decrease in the magnitude of potential. In addition, the ζ potential for hectorite should be decreased in magnitude more seriously than that for saponite with lowering pH because of the higher surface area of the positively charged edges for hectorite than that for saponite. This explanation is clearly contrary to the results shown in Figure 2, indicating that the difference in the pH dependence of the ζ potential between saponite and hectorite is not due to the effect of positive charges of (33) Parks, C. A. Chem. Rev. 1965, 15, 177.

Figure 3. ζ potential and OHP potential of smectites as a function of NaCl concentration. ζ potentials of smectites are shown as open circles. The dotted lines show the OHP potentials calculated by assuming that H+ ion is a specifically adsorbed ion at the IHP.

broken bonds on the edges. Another explanation is that the difference in the pH dependence may reflect the locations of lattice charges. In our previous study,34 a similar tendency in ζ potentials of muscovite mica and expandable fluorine mica (EM) was reported. Muscovite mica possesses lattice charges originating from the isomorphous substitution of Si4+ by Al3+ in the tetrahedral silica sheets, whereas the lattice charges for EM are due to lattice defects in Mg2+ in the octahedral magnesia sheets; muscovite mica and EM are equivalent to saponite and hectorite, respectively, in light of the location of lattice charges. Such a difference in the pH dependence of the ζ potential in acidic solution between saponite and hectorite may be closely related to the location of lattice charge as discussed later. The Effect of NaCl Concentration on ζ Potential. In Figure 3, the ζ potentials for saponite (a) and hectorite (b) are shown as open circles as a function of NaCl concentration at a natural pH of 5.4-5.8. The ζ potentials of smectites decreased monotonically with increasing NaCl concentration. The dependence of the ζ potential for saponite on NaCl concentration seemed to be similar to that for hectorite. Contrary to our expectation from the pH dependence of ζ potential, there is no indication of a difference in the structure between saponite and hectorite. Discussion To understand the difference in the ζ potential between saponite and hectorite, smectite surfaces were represented by the Gouy-Chapman-Stern-Graham (GCSG) model (see Figure 4) with a single-site dissociation of surface (34) Nishimura, S.; Kodama, M.; Noma, H.; Inoue, K.; Tateyama, H. Colloids Surfaces A 1998, 143, 1.

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Langmuir, Vol. 18, No. 1, 2002 191

siloxane cavity on the face of smectite because of dehydration of the small cations. So it is not so unrealistic if Na+ ions specifically adsorb into the hexagonal cavity. On these bases, this model takes into account the existence of the outer Helmholtz plane (OHP) and inner Helmholtz plane (IHP) as shown in Figure 4. The OHP is the plane of closest approach of hydrated ions to the surface of smectites. The IHP is the plane at which dehydrated Na+ and H+ ions are specifically adsorbed, and then σ0 and Ψ0 are defined as the charge density and the potential at the plane of the lattice charge. The acid dissociation constant, cation-exchange constant, and the activities of surface and bulk species are equated by assuming a Boltzmann distribution of ions.

Figure 4. The electrical double-layer structure and potential profile of the triple-layer model at the face of smectite-solution interface.

groups.35-37 The equilibrium of the surface groups for a smectite surface is assumed as follows:

Ka ) ([S-] [H+]b/[SH]) exp(-eΨi/kT)

(2)

K*Na ) ([SNa] [H+]b)/([SH] [Na+]b)

(3)

where the b represents a bulk species. The condition of double-layer electroneutrality is that;

σ0 ) -e([S-] + [SH] + [SNa]) ) C1(Ψ0 - Ψi) (4) σd ) e([S-]) ) C2(Ψd - Ψi)

(5)

σ 0 + σi + σd ) 0

(6)

SH + Na+ 798 S Na + H+

Ns ) [S-] + [SNa] + [SH]

(7)

where S- is a negatively charged surface site, H+ is a hydrogen ion, Na+ is a sodium cation, and Ka and K*Na are an acid dissociation constant and a cation-exchange constant, respectively. A negative lattice charge of smectite caused by isomorphous substitution in the tetrahedral sheet or octahedral sheet is balanced by an exchangeable Na+ ion. In this model, a surface site (S) is assumed to be a lattice charge. In the presence of water, the exchangeable Na+ ion diffuses away from the face of smectite or exchange for H+ ion, and then unoccupied (negative) and cationoccupied lattice charges are formed. Such an unoccupied lattice charge corresponds to a negatively charged surface site (S-). It is well-known that H+ ions play a role in a potential determining ion for most of metal oxide surfaces. However, ζ potentials for smectites were totally insensitive to the change in pH as noted in Figure 2, except for a low-pH region in saponite, implying that H+ ion for smectites would not behave as a potential determining ion. In this model, we tentatively assume that H+ ions behave as a specifically adsorbed ion at the inner Helmholtz plane rather than a potential determining ion. On the other hand, studies on the fixation of small cations such as Li+ and Mg2+ in the interlayer of smectites39 and atomic force microscopy observations of the basal planes (face) of layer silicates40,41 has supported the belief that the small cations could be stacked in the hexagonal

where σ0, σi, and σd are the charge densities, Ψ0, Ψi, and Ψd are potentials at the plane of the lattice charge (isomorphous substitution) and the IHP and OHP, C1 and C2 are the capacitance of the IHP and OHP, respectively; [S-], [SNa], and [SH] are the activities of surface sites; Ns is the total number of surface sites per unit area. According to the Gouy-Chapman diffuse layer theory,42 the charge, σd, in the diffuse layer is given by:

Ka

-

S H 798 S + H

+

K*Na

(35) Scales, P. J.; Grieser, F.; Healy, T. W. Langmuir 1989, 6, 582. (36) Shubine, V. E.; Kekicheff, P. J. Colloid Interface Sci. 1993, 155, 108. (37) Nishimura, S.; Scales, P. J.; Tateyama, H.; Tsunematsu, K.; Healy, T. W. Langmuir 1994, 10, 291. (38) Pashley, R.; Israelachchvili, J. N. J. Colloid Interface Sci. 1981, 80, 153. (39) Brindley, G. W.; Lemaitre, J. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Mineralogical Society: London, 1987; p 319. (40) Nishimura, S.; Biggs, S. R.; Scales, P. J.; Healy, T. W.; Tsunematsu, K.; Tateyama, H. Langmuir 1994, 10, 4554. (41) Vrdoljak, G. A.; Henderson G. S. Colloids Surfaces A 1994, 87, 187.

σd ) -11.74 Ce1/2 sinh(z e Ψd/2kT) in µC/cm2 (8) where Ce (mol/L) and z are the bulk concentration and valence of electrolyte counterions in the diffuse layer, respectively. By solving the equations above numerically, OHP potential (Ψd) can be calculated by using the input parameters, i.e., Ns, pKa, pK*Na, C1, and C2. The OHP potential was assumed to be consistent with the ζ potential. In Figure 2, the OHP potentials for smectites are shown as solid lines. The surface-site density for smectites has been fixed as the lattice-charge density determined by the structural formulas. We assumed the total number densities of the lattice charge (Ns) to be 8.0 × 1013 cm-2 (40% of muscovite mica) for saponite and 6.6 × 1013 cm-2 (33% of muscovite mica). When the inner layer capacitance of 1000 µF/cm2 (C1) and the outer layer capacitance of 10 µF/cm2 (C2) are assumed for saponite, the OHP potential curve showed a sensitive pH dependence in acidic solutions and fitted well to the ζ potential as shown in Figure 2a. On the other hand, the OHP curve became insensitive to pH and fitted to the ζ potential as shown in Figure 2b when the inner layer capacitance of 5 µF/cm2 (C1) and the outer layer capacitance of 10 µF/cm2 (C2) were assumed for hectorite. These input values of the inner and outer layer capacitance showed good fits of the OHP potential to the ζ potentials for smectites. It should be noticed that (42) Gouy, G. J. Phys. Chem. 1910, 9, 457; Chapman, D. L. Philos. Mag. 1913, 24, 475.

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Table 1. Input Parameters Used in the Calculation of the GCSG Model oxides saponite hectorite expandable fluorine mica beidellite (Put num clay) muscovite mica SiO2 Al2O3 TiO2 iron oxides

C1 (µF/cm2) 1000 5 5 250 800-1000 125-140 90-350 70-250 90-140

C2 (µF/cm2)

Ns (cm-2)

pKa

pK*Na

Reference

10 10 10 20 114-200 4-30 4-20 ∼20 10-20

8.0 × 6.6 × 1013 1.2 ×1014 2.0 × 1014

6.0(6.0) 5.0(5.6) 5.0(5.0) 7.4 3.7-5.6

2.2(2.5) 1.1(3.0) 1.5(3.0) 3.0 2.5-5.5

This study This study This study (Nishimura et al.34) James and Parks43 Scales et al.35, Shubine et al.36, Nishimuta et al.37 James et al.43, Scales et al.44 James et al.43, Smit et al.44 James et al.43, Davis et al.44 James et al.43, Davis et al.44

1013

The values of pKa and pK*Na in parentheses are input parameters for the best fit of OHP potential to ζ potential in the model where H+ ion is assumed to be a potential determining ion.

C1 for hectorite was substantially lower than that for saponite. Nonetheless, a mild dilemma exists about the assumption that H+ ion plays a role in a specifically adsorbed ion rather than a potential determining ion. For comparison, a similar model calculation was performed on the assumption that H+ ion behaves as a potential determining ion. The OHP potentials were calculated instead of eqs 2-4 by:

Ka ) ([S-] [H+]b/[SH]) exp(-eΨ0/kT)

(9)

K*Na ) {([SNa][H+]b)/([SH][Na+]b)} × exp{e(Ψi - Ψ0)/kT} (10) σ0 ) -e([S-] + [SNa]) ) C1(Ψ0 - Ψi)

(11)

In Figure 2, the best fits of the OHP potentials to ζ potentials are shown as dotted lines. The input parameters for the best fit are the same values as for the former model except for the pK*Na. The fits were not as good as those calculated by using the former model as noted in Figure 2. This provides further support for the assumption that the H+ ion plays a role in a specifically adsorbed ion rather than a potential determining ion. All input parameters to obtain good fits for smectites are listed in Table 1 with those given for other layer silicates presented in the previous studies.34,35-37 For comparison, C1 and C2 reported for typical metal oxides are also listed in Table 1.43-47 The values of C2 obtained for smectites in this study were of the same order as those for metal oxides except for muscovite mica. The values of C1 for saponite and hectorite are in good agreement with those for muscovite mica and EM, respectively. We previously reported that the electrical double-layer potential was dominated by the inner Helmholtz layer capacitance rather than the lattice-charge density, and the inner layer capacitance reflects the location of lattice charge from the comparison of electrical double layer potential between mica and EM.34 The inner and outer layer capacitance is given as a simple expression for a condenser:

C ) 0/d

(12)

where 0 is the dielectric constant in a vacuum,  is the relative dielectric constant in the inner or outer layer, (43) James, R. O.; Parks, G. A. In Surfaces and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1982; Vol. 12, p 119. (44) Scales, P. J.; Grieser, F.; Healy, T. W.; Chan, D. Y. C.; White, L. R. Langmuir 1992, 7, 965. (45) Smit, W.; Holten, L. M. J. Colloid Interface Sci. 1980, 78, 1; Smit, W. J. Colloid Interface Sci. 1986, 109, 295. (46) Davis, J. A.; James, R. O.; Leckie, J. O. J. Colloid Interface Sci. 1978, 63, 480. (47) Davis, J. A.; Leckie, J. O. J. Colloid Interface Sci. 1978, 67, 90.

and d is the distance between the plane of the lattice charge and IHP or between IHP and OHP. The lattice charges of hectorite are located in the octahedral magnesia layer at a depth of 0.5 nm from the crystal surface, corresponding to the structure of EM, whereas saponite possesses its own lattice charge at a depth of ∼0.1 nm in the tetrahedral silica sheet like muscovite mica.48 In addition, the value of  for hectorite seems to be lower than that of saponite because a siloxane tetrahedral layer, which has a much lower value of (4∼7) than the water phase (∼80), intervenes between the plane of the lattice charges and IHP for hectorite. Such a difference in the location of lattice charges is consistent with the result that C1 values for both hectorite and EM are much lower than those for saponite and muscovite mica, because a lower dielectric constant in the inner layer and/or a larger distance between the plane of the lattice charge and IHP yields a lower value of C1 according to eq 12. The value of C1 for beidellite (Putnum clay) is close to that for saponite and muscovite mica rather than hectorite and EM. The lattice charges of beidellite arise from the isomorphous substitution of Si4+ by Al3+ ions in the tetrahedral silica sheet like muscovite mica and saponite. This may provide further support for the intimate relation between C1 and the location of the lattice charge. H+ ions adsorbed at the face of saponite more specifically (higher pKa value) than at the face of hectorite. In the model calculation where H+ ion was assumed to be a potential determining ion, the binding of H+ ions for beidellite is higher than that for EM. Taking into account the difference in isomorphous substitution, it can be said that the binding of H+ ions for smectites with a tetrahedral charge is higher than that with a octahedral charge. On the other hand, it is inappropriate to simply compare the values of K*Na including different value of Ka. Instead of a cation exchange equilibrium, a simple equilibrium of Na+ ion dissociation is given by: KNa

SNa 9 7 8 S- + Na+ The equilibrium constant is then;

KNa ) ([S-] [Na+]b/[SNa+]) exp(-eΨi/kT)

(13)

Values of pKNa for saponite and hectorite are predicted to be 3.8 and 3.9, respectively. This is consistent with the fact that there is no difference in the dependence of the ζ potential for saponite on NaCl concentration. The value of pKNa for EM is predicted to be 3.5 by using the model. These values are in the same order of 3.5 for muscovite mica (SFA)49 and 4.4 for beidellite. The dissociation of (48) Bleam, R. J. Clays Clay Miner. 1990, 38, 527. (49) Pashley, R. J. Colloid Interface Sci. 1981, 83, 531.

Electrokinetc Study of Synthetic Smectites

Na+ ions for layer silicates seems to be insensitive to the difference in the location and density of the lattice charge. The smectite surface was also modeled by using the GCSG model as a function of NaCl concentration as shown in Figure 3 by the dotted lines. It was recognized that any acceptable fit to the data, except for the potentials at 10-3 M NaCl, cannot be obtained by using the same parameters as in the analysis of pH dependence of the ζ potential. The fit was overestimated at lower concentrations of NaCl and underestimated at higher concentrations of NaCl as compared with the experimental data for the ζ potential. Scales et al.37 reported that a reasonable fit of the OHP potential to the ζ potential of silica could be produced by increasing C2 as a function of KCl concentration from a value of 4 µF/cm2 at 10-4 M KCl to 30 µF/cm2 at 10-1 M KCl, suggesting that the value of C2 tends to increase with increasing NaCl concentration. Thus we attempt to fit the OHP and ζ potentials to the ζ potential by changing the outer layer capacitance, C2, from 2 to 200 µF/cm2. This is equivalent to shifting the position of the shear plane and/or inducing a change in the dielectric constant of the outer Helmholtz layer. The OHP potential curve shifts to a higher concentration of NaCl with increasing value of C2; the concentrations of NaCl where the OHP potential curves intersect the ζ potential curve shift to a higher region with increasing the value of C2. These observations are common to both saponite and hectorite. Pashley indicated that a short-range repulsion, i.e., “secondary hydration force” caused by adsorption of Na+ ions appeared at a separation less than 5 nm with increasing electrolyte concentration.49 This implies that the plane of diffuse layer potential that corresponds to the OHP potential shifts outward with increasing NaCl concentration, although there is no indication to justify the change in the dielectric constant. Conclusions In this study, ζ potentials of saponite and hectorite were investigated as functions of pH and NaCl concentration by the flat plate streaming potential technique where two

Langmuir, Vol. 18, No. 1, 2002 193

smectite-coated plates (face oriented smectites) were separated by a Teflon gasket to form a slit channel. The ζ potential for saponite was more sensitive to the pH of solution than that for hectorite in lower pH regions. The ζ potentials of smectites decreased monotonically with increasing NaCl concentration. The dependence of the ζ potential for saponite on NaCl concentration was quite similar to that for hectorite, contrary to our expectation from the pH dependence of ζ potential. According to the GCSG model, the inner Helmholtz layer capacitance, C1, of 1000 µF/cm2 for saponite and 5 µF/cm2 for hectorite, were required to produce a good fit of the outer Helmholtz potential to the ζ potential obtained in acidic solutions. A lower dielectric constant in the inner layer and/or a larger distance between the plane of the lattice charge and IHP yields a lower value of C1; therefore, it can be said that the values of C1 clearly reflect the difference in the location of lattice charges for smectites. In addition, the fits of the OHP potential to ζ potential were improved by assuming that H+ ion plays a role in a specifically adsorbed ion rather than a potential determining ion. On the other hand, no reasonable fit for the dependence of NaCl concentration on the ζ potential could be produced with a single value of the outer layer capacitance, C2, using the same parameters as for the analysis of the pH dependence of the ζ potential. The concentration of NaCl where the OHP potential curve intersected the ζ potential curve shifted to a higher region with the increasing value of C2, indicating the sift of share plane and/or a change in the dielectric constants of the outer Helmholtz layer. Acknowledgment. S.N. thanks Dr. H. Tateyama, Vice Director of the Institute for Structural and Engineering Materials, for valuable discussions. K.O. thanks Dr. E. Abe, Chief Senior Researcher in the National Institute of Advanced Science and Technology, Kyushu, for his continuous encouragement and for providing the opportunity to do this work at the National Institute of Advanced Science and Technology. LA0101191