The Binding of Monovalent Electrolyte Ions on α-Alumina. II. The

Advanced Mineral Products Special Research Centre, School of Chemistry, ... showing that the strength of the interparticle network decreases in the sa...
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The Binding of Monovalent Electrolyte Ions on r-Alumina. II. The Shear Yield Stress of Concentrated Suspensions Stephen B. Johnson, George V. Franks, Peter J. Scales,* and Thomas W. Healy Advanced Mineral Products Special Research Centre, School of Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia Received July 14, 1998. In Final Form: December 22, 1998 The shear yield stress properties of R-alumina suspensions have been investigated as a function of pH and different monovalent electrolyte types and concentrations. The results have then been compared with complementary electrokinetic studies of R-alumina under analogous suspension conditions. Over the entire pH range at an electrolyte concentration of 1.0 mol dm-3, the shear yield stress is shown to decrease in the sequence Li+ > Na+ > K+ > Cs+, showing that the strength of the interparticle network decreases in the same order. In addition, use of the shear yield stress model of Scales et al.1 indicates that the interparticle separation in the presence of these species decreases in the sequence Cs+ > K+ > Na+ > Li+. These findings are consistent with the water “structure making-structure breaking” model of Gierst et al.2 and Be´rube´ and de Bruyn,3 which predicts that an entropic attraction will exist between ions and surfaces that promote similar ordering effects in their local aqueous environments. By contrast, almost identical shear yield stress versus pH results are obtained in the presence of Br-, Cl-, I-, and NO3- over the entire range of electrolyte concentrations investigated. The interparticle separation is similarly found to be the same in those cases. These results cannot be predicted using the water “structure making-structure breaking” model, and do not allow the mechanism of anion-surface association to be conclusively resolved.

Introduction Inorganic monovalent electrolyte ions play an important role in controlling the macroscopic behavior of colloidal suspensions. In particular, a prudent choice of indifferent electrolyte concentration allows manipulation of the net interparticle force and through it, such properties as the dispersion stability, 4-6 the flow behavior,7-14 and the consolidation densities7,9,12,15-17 of particulate suspensions. Each of these characteristics is of particular importance in the colloidal processing of ceramic materials, where the use of monovalent electrolytes as flocculation agents * Author for correspondence. Phone: +61-3-93446480. Fax: +61-3-93446233. E-mail: [email protected]. (1) Scales, P. J.; Johnson, S. B.; Healy, T. W.; Kapur, P. C. AIChE J. 1998, 44, 538. (2) Gierst, L.; Vandenberghen, L.; Nicolas, E.; Fraboni, A. J. Electrochem. Soc. 1966, 113, 1025. (3) Be´rube´, Y. G.; de Bruyn, P. L. J. Colloid Interface Sci. 1968, 28, 92. (4) Wiese, G. R.; Healy, T. W. J. Colloid Interface Sci. 1975, 51, 427. (5) Healy, T. W.; Homola, A.; James, R. O.; Hunter, R. J. Faraday Discuss. Chem. Soc. 1978, 65, 156. (6) Healy, T. W.; Homola, A.; James, R. O.; Hunter, R. J. In Polymer Colloids II; Fitch, R. M., Ed.; Plenum: New York, 1980. (7) Velamakanni, B. V.; Chang, J. C.; Lange, F. F.; Pearson, D. S. Langmuir 1990, 6, 1323. (8) Chang, J. C.; Lange, F. F.; Pearson, D. S. J. Am. Ceram. Soc. 1994, 77, 19. (9) Luther, E. P.; Kramer, T. M.; Lange, F. F.; Pearson, D. S. J. Am. Ceram. Soc. 1994, 77, 1047. (10) Yanez, J. A.; Shikata, Y.; Lange, F. F.; Pearson, D. S. J. Am. Ceram. Soc. 1996, 79, 2917. (11) Zaman, A. A.; Moudgil, B. M.; Fricke, A. L.; El-Shall, H. J. Rheol. 1996, 40, 1191. (12) Colic, M.; Franks, G. V.; Fisher, M. L.; Lange, F. F. Langmuir 1997, 13, 3129. (13) Evanko, C. R.; Delisio, R. F.; Dzombak, D. A.; Novak, J. W., Jr. Colloids Surf. A 1997, 125, 95. (14) Santos, L. R. B.; Pulcinelli, S. H.; Santilli, C. V. J. Membr. Sci. 1997, 127, 77. (15) Chang, J. C.; Lange, F. F.; Pearson, D. S.; Pollinger, J. P. J. Am. Ceram. Soc. 1994, 77, 1357. (16) Velamakanni, B. V.; Lange, F. F.; Zok, F. W.; Pearson, D. S. J. Am. Ceram. Soc. 1994, 77, 216. (17) Franks, G. V.; Colic, M.; Fisher, M. L.; Lange, F. F. J. Colloid Interface Sci. 1997, 193, 96.

is currently generating significant interest. In general, the concentration of electrolyte is adjusted to produce a weakly flocculated particulate suspension in which particle rearrangement is facilitated during the consolidation process.7,18-20 The result is a stable green body of high and regular density that should give rise to ceramic products of sound structural integrity.7,18 Somewhat surprisingly, given its application to colloid science, very few comprehensive studies comparing the influence of different inorganic monovalent electrolyte types on the rheological properties of particulate suspensions currently exist in the literature. For example, although Colic et al.12 and Franks et al.17 investigated the relative effects of LiCl, NaCl, KCl, and CsCl on the flow and consolidation behavior of R-alumina suspensions, their studies were generally restricted to high pH and a single electrolyte concentration. Similarly, Chang et al.8 compared the action of KI and NH4Cl on the flow properties of R-alumina suspensions, but again over a limited range of electrolyte concentrations and only at a single pH condition. By contrast, Leong et al.21 examined the relative effects of NO3- and Cl- on the flow characteristics of zirconia suspensions over a wide pH range. In that case, however, the authors limited their investigations to low electrolyte concentrations. In a preceding study (hereafter referred to as Paper I),22 we demonstrated the effects of a number of different monovalent electrolyte types and concentrations on the electrokinetic properties of R-alumina particles. The aim of the present study was to examine and compare the influences of the same monovalent electrolytes on the shear yield stresses (τy) of concentrated R-alumina suspensions. Formally defined as the minimum applied stress (18) Horn, R. G. J. Am. Ceram. Soc. 1990, 73, 1117. (19) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437. (20) Lopes, R. A.; Segadaes, A. M. J. Eur. Ceram. Soc. 1997, 17, 339. (21) Leong, Y. K.; Boger, D. V.; Parris, D. J. Rheol. 1991, 35, 149. (22) Johnson, S. B.; Scales, P. J.; Healy, T. W. Langmuir 1999, 15, 2836.

10.1021/la9808768 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999

Binding of Monovalent Electrolyte Ions. II. Shear Yield Stress

required to induce flow within a suspension medium, the shear yield stress has traditionally been determined by measuring shear stress versus shear rate flow curves with a rotational viscometer, and then extrapolating the data to the zero shear rate condition. Under such circumstances, the shear yield stress is described by a range of nontrivial hydrodynamic and interparticle force considerations.23,24 Recently, however, it has been demonstrated that, if measured at very low shear rates, τy is simply determined by the total force required to separate the particles present in the yielding cross-section.1,25 In the most simple cases, the net interparticle force, FT, is described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory,26,27 which treats FT as the sum of the attractive van der Waals (FA) and repulsive electrical double layer (FR) forces. Analytical expressions and numerical treatments describing FA and FR over a range of solution conditions are available in the literature,28-32 and allow the form of the shear yield stress to be accurately described as a function of other experimentally derived parameters, such as the pH and the electrokinetic (ζ) potential.1,25 This study begins with an examination of the effects of both pH and different monovalent cation and anion types and concentrations upon the shear yield stress behavior of concentrated R-alumina suspensions. The results are then compared with ion binding predictions arising from complementary electroacoustic analyses of R-alumina particles in the presence of the same electrolyte types and concentrations (Paper I).2 In cases in which the DLVO theory is considered to be applicable, the combined data are modeled with the shear yield stress model of Scales, Johnson, and co-workers,1,25 allowing the net interparticle separation to be probed. The expectations of several conceptual ion binding models are also examined in some detail. To the authors’ knowledge, the combined data comprise the most comprehensive study of the relationship between monovalent ion binding and the shear yield stress that has been undertaken to date. The results are of importance to a number of practical scenarios, in addition to the processing of ceramic materials, including the flow behavior of waste mineral tailings from mining processes (which often contain high solids and high electrolyte concentrations, and must be pumped to suitable disposal areas) and the erosion of cohesive sediments by saline waters. Experimental Section Materials. A well-characterized R-alumina colloid was chosen for study (high purity AKP-30 grade, ex. Sumitomo Chemical Company, Japan). It had a Brunauer-Emmett-Teller (BET) surface area of 7.5 m2 g-1, a density of 3.97 g m-3, and a mean particle diameter (determined with a Coulter LS 130 instrument) of 0.30 µm. Low level impurities specified by the manufacturer included 8 ppm Si, 2 ppm Na, 2 ppm Mg, 1 ppm Cu, and 5 ppm (23) Firth, B. A.; Hunter, R. J. J. Colloid Interface Sci. 1976, 57, 266. (24) Hunter, R. J. Adv. Colloid Interface Sci. 1982, 17, 197. (25) Johnson, S. B.; Russell, A. S.; Scales, P. J. Colloids Surf. A 1998, 141, 119. (26) Derjaguin, B. V.; Landau, L. Acta Physicochim. URSS 1941, 14, 633. (27) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (28) Hunter, R. J. Foundations of Colloid Science, Vol. 1; Clarendon Press: Oxford, 1987. (29) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (30) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. J. Chem. Soc., Faraday Trans. 1966, 62, 1638. (31) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283. (32) McCormack, D.; Carnie, S. L.; Chan, D. Y. C. J. Colloid Interface Sci. 1995, 169, 177.

Langmuir, Vol. 15, No. 8, 1999 2845 Fe. Transmission electron microscopy showed the R-alumina particles to consist more of oblong than of spherical particles, with an average aspect ratio of Na+ > K+ > Cs+ across the entire pH domain. Similar findings have recently been reported by Colic et al.12 for R-alumina surfaces at high pH conditions. To better understand the cation effects just described, the shear yield stress data were modeled in conjunction with previous electrokinetic results (Paper I)22 to obtain information regarding the net interparticle separation distances. Scales et al.1 have recently invoked the DLVO theory26,27 to show that, for shear yield stress measurements made at low shear rates, τy for a suspension of polydisperse spherical particles is given by the expression

τy )

[

0.011φK AH π

12H2

]

- B(H)ζ2

Si

[

Sj

∑j X ∑i j

(Xj + Xi) -

Xi

xX

2 j

]

(4)

+ 2XiXj

where K is the mean coordination number, Si and Sj are the fractions of the total surface area contributed by particles of diameter Xi and Xj, respectively, H is the mean interparticle separation at which the maximum force occurs, AH is the Hamaker constant, and B(H)ζ2 is an appropriate expression for the repulsive electrical double layer force. Equation 4 implicitly assumes that all interparticle links will have reached their elastic response limit, and so will yield simultaneously, when the applied stress is equal to τy. Unfortunately, this assumption proves to be an oversimplification at all but extremely high particle concentrations, and will lead to the calculation of anomalously high shear yield stress values. Scales et al.1 remedied this discrepancy by considering the structural (nonforce) components of eq 4 to be constant as a function of ζ, and then normalized τy by the maximum shear yield stress, τymax. The resulting expression, which is independent of the form of the suspension microstructure, is

τy

12H2 B(H)ζ2 )1τymax AH

Figure 2. The shear yield stress behavior of R-alumina suspensions as a function of both the monovalent cation type and pH. The electrolyte concentration is (a) 0.01 mol dm-3 XNO3 and (b) 1.0 mol dm-3 XNO3. The alumina volume fraction is 0.250 in all cases. Key: (b) X ) Li; (O) X ) Na; (2) X ) K; (4) X ) Cs.

approximate constant potential behavior as a function of H (that is, the surface potential remains constant but the surface charge gradually decreases as the particles approach). For identically charged spherical particles in a monovalent electrolyte solution, eq 5 then becomes

τy τymax

)1-

37

Johnson has recently used a fractal-type model to demonstrate that, for R-alumina suspensions at low (0.01 mol dm-3) electrolyte concentrations, the suspension microstructure is indeed independent of ζ. The quantity B(H)ζ2 can be determined using the “HHF” electrical double layer overlap treatment of Hogg et al.30 provided that ζ is small () 25 mV) and the particles (37) Johnson, S. B. The Relationship Between the Surface Chemistry and the Shear Yield Stress of Mineral Suspensions. Ph.D. Dissertation, University of Melbourne, 1998.

AH (1 + eκH)

(6)

and

κ) (5)

24π0κζ2H2

( ) 2n0E2 0kT

1/2

(7)

where  and 0 are the permittivities of the continuous phase and a vacuum respectively, κ is the so-called DebyeHu¨ckel parameter, n0 is the number concentration of monovalent ions, T is the absolute temperature, k is the Boltzmann constant, and E is the electronic charge. With the exception of H, all parameters in eq 6 and 7 can be readily calculated or experimentally measured. Therefore, H can be determined by generating the best fits of eq 6 to plots of (τy/τymax) versus ζ2, the linear gradients of which are a function of H only.25 At higher ζ potentials and/or for particulate systems that demonstrate nonconstant potential behavior, H can

2848 Langmuir, Vol. 15, No. 8, 1999

Johnson et al. Table 1. Mean Interparticle Separation, H, of r-Alumina Particlesa cation

H (HHF), nm

H (constant ψ), nm

H (constant σ), nm

Li+ Na+ K+ Cs+

1.8 2.1 2.4 2.4

1.8 2.2 2.5 2.5

0.9 1.2 1.4 1.4

a Determined from best fits to the normalized τ versus ζ2 data y using the model of Scales et al.1 (eqs 5-7). The electrolyte -3 concentration is 0.01 mol dm in all cases. The component of the interaction force due to the overlap of the electrical double layers has been calculated using the HHF theory of Hogg et al.,30 and the numerical constant potential (ψ) and constant charge (σ) solutions of Chan et al.31

Figure 3. A comparison of the normalized shear yield stress versus ζ2 behavior of R-alumina suspensions in the presence of the different cationic species investigated in this study. The electrolyte concentration is 0.01 mol dm-3 XNO3. The plotted lines correspond to the best fits obtained using the shear yield stress model of Scales et al.1 (eqs 5-7), and assuming that the HHF30 model of the electrical double layer is applicable. A Hamaker constant of 5.3 × 10-20 J was used in all calculations. Key: (s) H ) 1.8 nm; (- - -) H ) 2.1 nm; (- -) H ) 2.4 nm; (b) X ) Li; (O) X ) Na; (2) X ) K; (4) X ) Cs.

instead be determined using the more complex numerical electrical double layer overlap treatment of Chan et al.31 in conjunction with eq 5. The approach of Chan et al.31 allows both constant potential and constant charge (where the surface charge remains constant but the surface potential systematically diminishes as H decreases) boundary conditions to be assumed. In following that treatment, all numerical solutions presented here have been calculated using the “AFM Analysis” program of Ip et al.38 The normalized shear yield stress data for R-alumina suspensions at φ ) 0.250 are plotted against ζ2 in the presence of 0.01 mol dm-3 NaNO3, KNO3, CsNO3, and LiNO3 in Figure 3. The ζ potential data have been obtained from the complementary electroacoustic studies of the same R-alumina substrate presented in Paper I.22 To generate plots in which the data on both the acidic and basic sides of the maximum shear yield stress converged to give a single form, it was necessary to slightly adjust each electrokinetic isoelectric point to coincide exactly with τymax. This procedure, which is likely to be a result of the pH suspension effect (caused by the volume fraction dependence of the measured pH), involved the subtraction of ca. 0.2 pH units from the electroacoustic data. Figure 3 shows each set of normalized τy versus ζ2 data to follow an approximately linear relationship for values of ζ2 less than ca. 1000 mV2. Significant deviations from linearity are obvious at greater values of ζ2, and are likely to correspond to ionic strength and volume fraction uncertainties introduced by both the pH adjustment process and dissolution of the R-alumina surface at high pH. Figure 3 also shows that the normalized τy versus ζ2 data for the NaNO3, KNO3, CsNO3, and LiNO3 electrolyte systems fall into several forms of distinctly different gradient. This effect has been further examined by modeling the data using eq 5 in conjunction with the electrical double layer overlap treatments of Hogg et al.30 (eqs 6 and 7) and Chan et al.31 and a nonretarded Hamaker (38) Ip, L.; Chan, D.; Venters, S. AFM Analysis, Version 2; Department of Mathematics: University of Melbourne, Australia, 1994.

constant of 5.3 × 10-20 J.39 As the ionic strength and volume fraction uncertainties just outlined above are difficult to accurately quantify, modeling has been confined to data falling within the linear portion of the normalized τy versus ζ2 plots. For the purpose of clarity, only the fits obtained using the approximate constant potential treatment of Hogg et al.30 are shown in Figure 3. The results calculated using the numerical treatment of Chan et al.31 are also presented in Table 1. Unsurprisingly, the approximate constant potential solutions of Hogg et al.30 are in good agreement with the numerically derived values of Chan et al.31 when constant potential boundary conditions are assumed. However, the values of H determined assuming constant charge boundary conditions are substantially lower than those calculated when constant potential conditions are used. The sensitivity of the results to the charge regulation characteristics of the surfaces is somewhat disappointing and precludes a meaningful discussion of the precise magnitudes of H. Table 1 does, however, demonstrate a useful qualitative trend; that is, as in all cases, H decreases in the sequence Cs+ > K+ > Na+ > Li+. It is relevant to note that this sequence is the inverse of that followed by the maximum shear yield stresses measured in the presence of the Li+, Na+, K+, and Cs+ cations (Figure 2(b)). Modeling of the (τy/τymax) verses ζ2 data for systems containing electrolyte concentrations in excess of 0.01 mol dm-3 has not been attempted in this study. Three reasons exist for this decision: (i) The treatments of Hogg et al.30 and Chan et al.31 are based on the Gouy-Chapman40,41 description of the electrical double layer. This approach assumes an exponentially decaying distribution of ions away from the charged surface that is governed by the Poisson-Boltzmann equation. Although generally considered to be an accurate model at low ionic strengths, significant deviations from the Poisson-Boltzmann equation are often noted at high (> 0.01 mol dm-3) electrolyte concentrations. These inadequacies have been reviewed in detail by Haydon,42 and are generally attributed to ion size and ion-ion interaction effects. (ii) The plane of shear at which the ζ potential is measured is generally assumed to lie in close proximity with the Stern plane, so that the diffuse layer potential, ψd, and ζ are almost equivalent.43 At high electrolyte (39) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3. (40) Gouy, G. J. Phys. Chem. 1910, 9, 457. (41) Chapman, D. L. Philos. Mag. 1913, 25, 475. (42) Haydon, D. A. In Recent Progress in Surface Science, Vol. 1; Danielli, J. F., Pankhurst, K. G. A., Riddiford, A. C., Eds.; Academic Press: New York, 1964. (43) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: London, 1988.

Binding of Monovalent Electrolyte Ions. II. Shear Yield Stress

Langmuir, Vol. 15, No. 8, 1999 2849

Figure 4. The shear yield stress behavior of R-alumina suspensions as a function of both pH and the KY concentration. (a) Y ) Br; (b) Y ) Cl; and (c) Y ) I. The alumina volume fraction is 0.250 in all cases. Key: (b) [KY] ) 0.01 mol dm-3; (O) [KY] ) 0.10 mol dm-3; (2) [KY] ) 0.30 mol dm-3; (4) [KY] ) 1.0 mol dm-3.

concentrations, however, even slight differences between these planes may lead to ζ being substantially lower than ψd due to the steepness of the potential drop-off from the surface. The physical relevance of the ζ potential under such circumstances is uncertain. (iii) Mahanty and Ninham44 have indicated that the presence of high electrolyte concentrations can alter the magnitude of AH. Such an occurrence would greatly complicate the analysis undertaken using eqs 5-7. Monovalent Anion Effects. The shear yield stress versus pH behavior of R-alumina suspensions at φ ) 0.250 is shown as a function of KBr, KCl, and KI concentrations in Figures 4(a)-(c). A similar plot with KNO3 as the background electrolyte was shown previously in Figure 1(b). In all cases, the τy versus pH curves broaden significantly and become “bell-like” in form with increasing electrolyte concentration. As was previously observed for the NaNO3, KNO3, and CsNO3 systems, Figures 4(a)-(c) also show that the magnitudes of the maximum shear yield stresses gradually decrease as the electrolyte concentration is raised. In addition, the maximum shear yield stresses occur at pH values that remain relatively constant (9.0-9.3). These pH values are in reasonable agreement with the corresponding electrokinetic isoelectric points (pH ) 9.4-9.6) determined by electroacoustic means in Paper I,22 and indicate that KBr, KCl, and KI act in indifferent manners on the R-alumina surface. (44) Mahanty, J.; Ninham, D. W. Dispersion Forces; Academic Press: New York, 1979.

Figures 5(a) and (b) give a comparison of the shear yield stress versus pH behavior of the R-alumina suspensions in the presence of KBr, KCl, KI, and KNO3 at concentrations of 0.01 and 1.0 mol dm-3, respectively. In both cases, the data obtained for the different electrolyte systems are in close agreement over the entire pH range. This observation is suggestive of an almost identical role played by the Br-, Cl-, I-, and NO3- anions, which is in contrast to the recent studies of Leong et al.21 and Chang et al.,8 both of which demonstrated alterations in the rheological behavior of colloidal alumina and zirconia suspensions with changes in the nature of the monovalent anion. Those cases were, however, complicated by either an imprecise control of the electrolyte concentration21 or a concurrent alteration of the nature of the monovalent cation.8 They are therefore somewhat inconclusive with respect to the role of the monovalent anion. As was also the case for the Li+, Na+, K+, and Cs+ systems previously investigated, the effects of the different monovalent anions have been further investigated by modeling the normalized τy versus ζ2 data obtained at an electrolyte concentration of 0.01 mol dm-3 using eqs 5-7. The ζ potential results have been obtained from complementary electroacoustic studies of R-alumina in the presence of 0.01 mol dm-3 KBr, KCl, KI, and KNO3 (Paper I). 22 Figure 6 shows that the normalized τy versus ζ2 data are almost identical for the KBr, KCl, KI, and KNO3 systems, indicating that the shear yield stress and ζ potential data are insensitive to the nature of the monovalent anion. The data fall into the expected ap-

2850 Langmuir, Vol. 15, No. 8, 1999

Johnson et al.

Figure 6. A comparison of the normalized shear yield stress versus ζ2 behavior of R-alumina suspensions in the presence of the different anionic species investigated in this study. The electrolyte concentration is 0.01 mol dm-3 KY. The plotted lines correspond to the best fits obtained using the shear yield stress model of Scales et al.,1 and calculating the electrical double layer overlap component using the HHF30 treatment (s, H ) 2.4 nm) and the numerical constant potential (- - - , H ) 2.5 nm) and constant charge (- -, H ) 1.4 nm) solutions of Chan et al.31 A Hamaker constant of 5.3 × 10-20 J was used in all calculations. Key: (b) Y ) Br; (O) Y ) Cl; (2) Y ) I; (4) Y ) NO3.

Figure 5. The shear yield stress behavior of R-alumina suspensions as a function of both the monovalent anion type and pH. The electrolyte concentration is (a) 0.01 mol dm-3 KY and (b) 1.0 mol dm-3 KY. The alumina volume fraction is 0.250 in all cases. Key: (b) Y ) Br; (O) Y ) Cl; (2) Y ) I; (4) Y ) NO3.

proximately linear form below ca. 1000 mV2. Marked deviations from linearity are observed beyond this point and are presumably caused by ionic strength and volume fraction uncertainties generated by both adjustment of the suspension pH and dissolution of R-alumina at high pH.1 Modeling of the normalized τy versus ζ2 data using eqs 5-7 has been undertaken using a nonretarded Hamaker constant of 5.3 × 10-20 J39 and, for simplicity, has been restricted to data falling within the linear portion of Figure 6. Given the close correlation of the τy versus ζ2 data for the different electrolyte systems, it is unsurprising that the combined results can be modeled using a single value of H for each electrical double layer expression. Figure 6 gives the calculated interparticle separation as 2.4 nm when the data is fitted with the approximate constant potential treatment of Hogg et al.,30 and 2.5 and 1.4 nm, respectively, when the numerical constant potential and constant charge solutions of Chan et al.31 are used instead. The actual value of H is likely to fall within the limits set by the constant potential and constant charge boundary conditions. Discussion With the exception of the LiNO3/R-alumina systems, Figures 1 and 4 show that all τy versus pH data curves gradually broaden and become “bell-like” in form as the electrolyte concentration increases. At high electrolyte

concentrations, the increased values of τy observed well away from the maximum shear yield stress conditions are indicative of an increase in the net attractive interparticle force caused by a reduction of the electrical double layer repulsion (i.e., a decrease in the magnitude of the ζ potential). They are therefore consistent with a number of previous electrokinetic studies on R- and γ-alumina surfaces, that demonstrate marked decreases in |ζ| as the monovalent electrolyte concentration increases.4,22,45,46 The transformation from parabolic τy versus pH curves at 0.01 mol dm-3 to bell-shaped plots at 0.10, 0.30, and 1.0 mol dm-3 is also consistent with these electrokinetic results, which typically show a systematic reduction in the gradient of ζ potential versus pH plots as the pH is moved progressively further away from the isoelectric point. Importantly, the pHs at which the maximum shear yield stress data in Figures 1 and 4 occur are in close agreement with the corresponding zero ζ potential conditions shown in Paper I.22 This finding is predicted by eqs 5 and 6, and confirms that a maximum in the shear yield stress is generated when the electrical double layer repulsion is at its minimum. For the LiNO3/R-alumina systems, many of the properties just discussed are also observed on the acidic side of the maximum shear yield stress condition. At high pH, however, the shapes of the τy versus pH curves are radically altered from a parabolic to an almost linear form (constant τy) as the LiNO3 concentration increases. The latter finding is believed to indicate a rise in the degree of specific Li+ binding to the net negative R-alumina surface. Specific Li+ adsorption will be further discussed with respect to τymax later in this section. Again, with the exception of the LiNO3/R-alumina systems, all aqueous electrolyte/R-alumina suspensions studied here demonstrate a significant decrease in the magnitude of the maximum shear yield stress as the electrolyte concentration increases. Similar electrolyte(45) Sprycha, R. J. Colloid Interface Sci. 1989, 127, 1. (46) Smit, W.; Holten, C. L. M. J. Colloid Interface Sci. 1980, 78, 1.

Binding of Monovalent Electrolyte Ions. II. Shear Yield Stress

Langmuir, Vol. 15, No. 8, 1999 2851

Table 2. Hydration Enthalpies of Monovalent Cations and Anionsa cation

hydration enthalpy, (kJ mol-1)

anion

hydration enthalpy, kJ mol-1

Li+ Na+ K+ Cs+

-515 -405 -321 -263

NO3ClBrI-

-328 -364 -337 -296

a

All data determined at 298 K (according to Burgess51).

induced reductions in the viscosities of R-alumina and silicon nitride suspensions have been reported in the literature,7-9 and were attributed to a decrease in the interparticle force generated by short-range secondary hydration interactions. The secondary hydration force model of Pashley and Israelachvili47-50 assumes that the strength of the short-range repulsion will rise as the hydration enthalpies of the surface-associated ions increase. This prediction is a result of the barrier to interparticle approach imposed by tightly held water molecules. Based on the hydration enthalpies presented in Table 2,51 the maximum shear yield stresses of the LiNO3, NaNO3, KNO3, and CsNO3/R-alumina suspensions should decrease in the order Cs+ > K+ > Na+ > Li+. This sequence is the opposite of that experimentally determined in the present study, and therefore demonstrates that the shear yield stress results are inconsistent with the secondary hydration force model. In their recent rheological study, Colic et al.12 found that the viscosity and shear yield stress data for concentrated R-alumina suspensions decreased in the order Li+ > Na+ > K+ > Cs+ at high pH conditions. The authors also discounted the secondary hydration model, and instead rationalized their results using the water “structure making-structure breaking” model of Gierst et al.2 and Be´rube´ and de Bruyn.3 This treatment predicts that an entropic attraction will exist between ions and surfaces that promote a similar ordering effect in their local aqueous environments. The structure-inducing behavior of ionic species closely correlates with their hydration enthalpies, with monovalent cations promoting water order in the sequence Li+ > Na+ > K+ > Cs+.3,52,53 Similarly, Dumont and co-workers52,53 have shown that surfaces possessing high heats of immersion will most strongly promote water structure in their vicinity. R-Alumina substrates possess a large heat of immersion compared with other common colloidal materials.52-55 Consequently, monovalent cations are predicted to be attracted to the R-alumina surface in the order Li+ > Na+ > K+ > Cs+, an expectation that is also supported by the electroacoustic data obtained on the negatively charged R-alumina surface in Paper I.22 Colic et al.12 also assumed R-alumina to be a structurepromoting entity that would therefore result in “structure making” species (such as Li+ and Na+) approaching the surface more closely than “structure breaking” ions (such as K+ and Cs+). The end effect, shown schematically in Figure 7, was predicted to be a smaller physical barrier (47) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (48) Pashley, R. M. Adv. Colloid Interface Sci. 1982, 16, 57. (49) Israelachvili, J. N. Adv. Colloid Interface Sci. 1982, 16, 31. (50) Pashley, R. M.; Israelachvili, J. N. J. Colloid Interface Sci. 1984, 97, 446. (51) Burgess, J. Ions in Solution: Basic Principles of Chemical Interactions; Ellis Horwood: Chichester, 1988. (52) Dumont, F.; Dang Van Tan; Watillon, A. J. Colloid Interface Sci. 1976, 55, 678. (53) Dumont, F.; Warlus, J.; Watillon, A. J. Colloid Interface Sci. 1990, 138, 543. (54) Guderjahn, C. A.; Paynter, D. A.; Berghausen, P. E.; Good, R. J. J. Phys. Chem. 1959, 63, 2066. (55) Healy, T. W.; Fuerstenau, D. W. J. Colloid Sci. 1965, 20, 376.

Figure 7. A diagrammatic representation of the aqueous electrolyte/R-alumina interface in the presence of various monovalent cations according to the scheme of Colic et al.12 The most hydrated cations are able to more closely approach the R-alumina surface and therefore give rise to lower interparticle separations (diagram is not to scale).

to primary minimum contact in the presence of ions that promote water structure, and therefore a smaller intersurface separation. A greater interparticle attraction would then result. This expectation is consistent with the results presented in the present study, which demonstrate that the interparticle separation decreases in the sequence Cs+ > K+ > Na+ > Li+ (Figure 3) and, at high electrolyte concentrations, show that all τy data decrease in the opposite order (Figure 2 (b)). This prediction is also in agreement with the calculations of Kallay et al.56 who found that the ion-surface separation for a “structure making” titania colloid decreases in the sequence Cs+ > K+ > Li+. The model of Gierst et al.2 and Be´rube´ and de Bruyn3 therefore appears to provide a realistic description of the cation-surface association in this case. It is interesting to note that although the charge of the R-alumina surface varies in both sign and magnitude as the pH changes, significant shear yield stress differences exist between the Li+, Na+, K+, and Cs+/R-alumina systems over the entire pH domain. The presence of a substantial cation concentration in the intersurface region is therefore indicated under both net negative and, surprisingly, net positive surface charge conditions. The recent potentiometric titration studies of Contescu and co-workers57,58 (56) Kallay, N.; Colic, M.; Fuerstenau, D. W.; Jang, H. M.; Matijevic, E. Colloid Polym. Sci. 1994, 272, 554. (57) Contescu, C.; Jagiello, J.; Schwarz, J. A. Langmuir 1993, 9, 1754.

2852 Langmuir, Vol. 15, No. 8, 1999

provide a possible explanation for this behavior in demonstrating the existence of several nonequivalent binding sites on alumina substrates when in solution. The result is predicted to be a surface that, although possessing a net positive charge at pH values less than the isoelectric point, will still maintain a number of negatively charged sites to which cations will be electrostatically attracted. In considering the role of monovalent anions in coagulation studies involving titania and hematite substrates, Dumont and co-workers52,53,59 found that the Br-, Cl-, I-, and NO3- species possessed net water “structure breaking” properties. As such, the model of Gierst et al.2 and Be´rube´ and de Bruyn3 predicts that these anions will be entropically rejected away from the “structure making” R-alumina surface to an extent related to their hydration enthalpies (shown in Table 2). Contrary to this finding, however, the shear yield stress results obtained at all KBr, KCl, KI, and KNO3 concentrations and pHs investigated in this study were insensitive to the nature of the monovalent anion (Figure 5). In addition, the modeling results presented in Figure 6 indicate an identical interparticle separation for the KBr, KCl, KI, and KNO3/R-alumina systems. Two explanations appear possible for these results. First, the “structure breaking” potassium cation may be more vigorously rejected away from the R-alumina surface than are any of the Br-, Cl-, I-, or NO3- anions, such that it governs the interparticle separation and therefore the interparticle force in all cases. Second, despite their different hydration enthalpies, the anions may bind in a similar manner to the R-alumina surface. Our electroacoustic results presented in Paper I,22 which showed that the Br-, Cl-, I-, and NO3- ions adsorbed to an almost identical extent on the R-alumina surface, support the latter conclusion. However, a combination of both explanations could be correct. Clearly, further work is required on other monovalent electrolyte systems and colloidal substrates to better resolve the mechanism underlying the surface-anion association. Such studies are presently being undertaken in this laboratory. The discussion to date has been applicable to cases in which the presence of electrolyte ions generates a decrease in the magnitude of the maximum shear yield stress. For the LiNO3/R-alumina suspensions investigated in this study, however, a significant rise in the maximum shear yield stress is observed as a function of increasing LiNO3 concentration (Figure 1 (d)). This effect cannot be predicted by either the secondary hydration force or the water “structure making-structure breaking” models, and therefore requires further consideration. Shubin and Ke´kicheff60 have recently observed a nonDLVO increase in the attractive interaction between mica surfaces at small separations in the presence of LiNO3 solutions. They attributed this additional adhesive force to the Li+ ions sharing their hydration water with the two mica surfaces in what the authors termed a hydration bridging effect. Although a possibility for the molecularly smooth mica substrate, hydration bridging is a more tenuous explanation for the maximum shear yield stress increase observed in this study given that the surface roughness of the R-alumina colloids is likely to be substantially greater than the size of the hydrated Li+ ion. An increase in the attractive interparticle force has also recently been predicted for heterogeneously charged particles in which equal proportions of positive and (58) Contescu, C.; Hu, J.; Schwarz, J. A. J. Chem. Soc., Faraday Trans. 1993, 89, 4091. (59) Dumont, F.; Watillon, A. Discuss. Faraday Soc. 1971, 52, 352. (60) Shubin, V. E.; Ke´kicheff, P. J. Colloid Interface Sci. 1993, 155, 108.

Johnson et al.

negative charges are present as patches on the surface.61 In such circumstances, should regions of positive charge on one surface generally align with patches of negative charge on another, the result will be an electrostatic attraction between the particles. The “patch” mechanism of Miklavic et al.61 is a more likely explanation for the LiNO3/R-alumina shear yield stress behavior observed here, particularly at high Li+ concentrations and high pH, where the ζ potential is low and a large quantity of localized positive charges (in the form of Li+ ions) are specifically adsorbed to a net negatively charged underlying surface.22 Provided that the resulting electrostatic attraction is sufficient to outweigh the ionic hydration effects previously discussed in this section, the result will be an increase in the net attractive interparticle force, and therefore a rise in the magnitude of the maximum shear yield stress. Conclusions The effects of pH and the monovalent electrolyte type and concentration on the shear yield stress behavior of R-alumina suspensions have been examined. In the presence of 0.01-1.0 mol dm-3 NaNO3, KNO3, CsNO3, KBr, KCl, and KI, the pHs at which the maximum shear yield stresses occur are quite constant and are in close agreement with the corresponding electrokinetic isoelectric points, indicating that these electrolytes are acting in an indifferent manner on the R-alumina surface. By contrast, the pH of the maximum shear yield stress increases substantially when in the presence of LiNO3, which is consistent with specific Li+ binding to the R-alumina surface at high pH conditions. With the exception of the LiNO3/R-alumina system, all τy versus pH plots show a marked decrease in the magnitude of the maximum shear yield stress as the electrolyte concentration increases. In addition, at the highest studied electrolyte concentration (1.0 mol dm-3), all shear yield stress data decrease in the sequence Li+ > Na+ > K+ > Cs+, whereas modeling of normalized τy versus ζ2 data shows that the net interparticle separation decreases in the opposite order. These findings are inconsistent with the secondary hydration force model of Pashley and Israelachvili.47-50 However, based on the hydration enthalpies of the cations and the large heat of immersion of the R-alumina surface, they are in good agreement with the water “structure making-structure breaking” model proposed by Gierst et al.2 and Be´rube´ and de Bruyn.3 An increase in the magnitude of the maximum shear yield stress observed as a function of LiNO3 concentration is believed to be caused by an additional electrostatic interaction between specifically adsorbed Li+ “patches” on some surfaces and bare negatively charged regions on others. In contrast to the differences found between the LiNO3, NaNO3, KNO3, and CsNO3/R-alumina suspensions, the τy versus pH properties of R-alumina suspensions have been demonstrated to be insensitive to the nature of the monovalent anion. In addition, modeling of the normalized τy versus ζ2 data shows that the interparticle separation is identical for the KBr, KCl, KI, and KNO3 systems. These results are a little unexpected but can be rationalized in terms of the very small difference in the hydration enthalpies of the Br-, Cl-, I-, and NO3- species relative to the cations considered in this study. Acknowledgment. The work presented in this study was funded by the Advanced Mineral Products Research (61) Miklavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. J. Phys. Chem. 1994, 98, 9022.

Binding of Monovalent Electrolyte Ions. II. Shear Yield Stress

Centre, a Special Research Centre of the Australian Research Council. S.B.J. gratefully acknowledges the receipt of an Australian Post-Graduate Award and a Melbourne University Post-Graduate Writing-Up Schol-

Langmuir, Vol. 15, No. 8, 1999 2853

arship. The authors thank Prof. D. V. Boger for his support of this study. LA9808768