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The Binding of Monovalent Electrolyte Ions on r-Alumina. I. Electroacoustic Studies at High Electrolyte Concentrations Stephen B. Johnson, Peter J. Scales,* and Thomas W. Healy Advanced Mineral Products Special Research Centre, School of Chemistry, University of Melbourne, Parkville, 3052, Australia Received July 14, 1998. In Final Form: December 22, 1998 An electroacoustic technique has been used to monitor the binding of various monovalent inorganic ions on R-alumina at different pHs, nondilute solids, and moderate to high (0.01-1.0 mol dm-3) electrolyte concentrations. The ζ potential versus pH data show that NaNO3, KNO3, CsNO3, KBr, KCl, and KI are indifferent electrolytes for the R-alumina surface. However, LiNO3 causes a significant change in the isoelectric point of R-alumina, indicating that Li+ adsorbs in a specific manner to the surface over the range of concentrations investigated. At high (1.0 mol dm-3) electrolyte concentrations, the monovalent cations bind to the negative R-alumina surface in the order Li+ > Na+ > K+ ≈ Cs+. By contrast, the Br-, Cl-, I-, and NO3- anions adsorb to an almost identical extent over the entire range of concentrations and pH conditions investigated. The cation binding sequence is consistent with the water “structure makingstructure breaking” model first proposed by Gierst et al.1 and Be´rube´ and de Bruyn,2 which is based on the hydration enthalpies of the ions and the heat of immersion of the colloidal substrate. The comparable anion adsorption behavior is believed to arise because of the similar and/or low hydration enthalpies of the anions, which lead to a similar anion-surface interaction in each case.
Introduction The binding of simple inorganic electrolyte ions on colloidal substrates at different pH conditions has been extensively studied and reviewed3-5 in the literature. Typical experimental scenarios involve the addition of low to moderate concentrations of inorganic electrolytes to aqueous suspensions of particles, with the extent of ion binding being determined by either charge titration or electrokinetic methodologies. The charge titration technique involves the gradual addition of potential determining ions (pdi) to a known mass of suspended solid while the solution pH is monitored. The pdi concentration required to achieve each solution condition is then determined and converted to the corresponding relative surface charge. For the surface charge to be calculated in the presence of indifferent electrolyte ions, the charge titration procedure must be repeated for at least one other electrolyte concentration to determine the common intersection point at which the net surface charge is zero. Unfortunately, this procedure is often cumbersome (particularly if workers wish to obtain results at only a single pH condition or electrolyte concentration), and difficulties can arise when attempting to determine whether a common intersection point does in fact exist.6 The electrokinetic techniques provide an alternate option under such circumstances and are more relevant because the * Author for correspondence. Phone: +61-3-93446480. Fax: +613-93446233. E-mail:
[email protected]. (1) Gierst, L.; Vandenberghen, L.; Nicolas, E.; Fraboni, A. J. Electrochem. Soc. 1966, 113, 1025. (2) Be´rube´, Y. G.; de Bruyn, P. L. J. Colloid Interface Sci. 1968, 28, 92. (3) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications, 2nd ed.; Academic Press: London, 1988. (4) Sahai, N.; Sverjensky, D. A. Geochim. Cosmochim. Acta 1997, 61, 2801. (5) James, R. O.; Parks, G. A. In Surface and Colloid Science, Vol. 12; Matijevic, E., Ed.; Plenum Press: New York, 1982. (6) Hunter, R. J. Introduction to Modern Colloid Science; Oxford University Press: Oxford, 1993.
measurement provides an indication of the interaction potential of direct relevance to colloid stability. When applied to suspensions of colloidal materials, electrokinetic methods involve the application of an external electric field (electrophoresis or electro-osmosis), pressure gradient (streaming potential), or gravitational force (sedimentation potential) across the suspension to induce particle or fluid motion. The resulting flow is strongly associated with the electrical potential at the surface of shear, a solution-based plane positioned a short distance from the particle surface within which infinite solvent viscosity is assumed. The average potential at the plane of shear is referred to as the ζ potential. Unlike the charge titration method, the electrokinetic techniques provide a direct measure of the absolute ζ potential. Meaningful data can therefore be obtained from singlepoint experiments without the requirement of having to predetermine a zero charge or potential condition. Although they are applicable over a wide range of solution conditions, conventional electrokinetic techniques are restricted in the range of solids concentrations over which they can provide meaningful data. For example, electrophoretic particle motion is typically monitored by microelectrophoresis or laser light scattering techniques. Electrophoresis experiments are therefore generally confined to the study of extremely dilute colloidal suspensions to maintain a satisfactory optical resolution and avoid multiple particle light scattering. Sedimentation potential measurements are also generally restricted to the study of dilute suspensions undergoing free settling because of the difficulty of producing a uniform particle flow at high solids concentrations. By contrast, streaming potential and electro-osmosis methods require particles to be tightly packed into a porous plug to generate the desired solution flow profile. As a result, only suspensions containing very high solids concentrations can be examined. The recent development of instruments based on the electrokinetic sonic amplitude (ESA) electroacoustic phenomenon now allows suspensions of arbitrary solids
10.1021/la980875f CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999
Binding of Monovalent Electrolyte Ions. I. Electroacoustics
concentrations to be studied.7,8 The ESA effect arises when a high frequency alternating voltage is applied across a suspension of a colloidal material, causing charged particles to oscillate with a velocity dependent on their size, charge, and the frequency of the applied field. Provided that the particles are of a different density than the liquid medium, pressure waves arise as particles are driven into and away from the regions comprising the suspension boundaries. The pressure changes then propagate back into the suspension and through the material comprising its boundaries as a so-called ESA signal. For a colloidal suspension of any given solids volume fraction, φ, the ESA may be related to the particle-averaged dynamic mobility, 〈µD〉, by the expression9
∆F 〈µD〉 Z ESA(ω) ) A(ω) φ F
(1)
where ω is the angular frequency of the applied field, A is a constant dependent on the electronics of the measuring instrument, F and (F + ∆F) are the densities of the continuous phase and the colloidal particles, respectively, and Z is a field factor related to the acoustic impedances (ratio of pressure to velocity in an acoustic wave) of the suspension and the delay rods used in the measuring instrument. For spherical particles with a thin electrical double layer at φ < 0.02, the dynamic mobility is related to the ζ potential and the particle radius, a, by the expressions10
〈µD〉 ) and
20ζ (1 + f)G(ωa2/ν) 3η
[
( )]
2∆F iωa2 3+ 9ν F 2 G(ωa /ν) ) 1 ωa2 1 + (1 + i) 2ν
(
(2)
-1
)
1/2
(3)
where and η are the permittivity and viscosity, respectively, of the liquid phase, 0 is the permittivity of a vacuum, ν is the kinematic viscosity () η/F), G(ωa2/ν) is a frequency-dependent inertial term, and f is related to the form of the electric field at the surface. For circumstances in which particles are suspended in an aqueous solution of high electrolyte concentration, the ζ potential is generally low and f approaches 0.5.11 At solids concentrations above φ ) 0.02, the hydrodynamic and electric field disturbances generated by each moving particle are experienced by its near neighbors, retarding their motion. The result is a φ-dependent decrease in the complex dynamic mobility.12-14 To account for these effects, O’Brien has recently developed a (7) Oja, T.; Petersen, G.; Cannon, D. U.S. Patent No. 4497207, 1995. (8) Cannon, D. W. In Proceedings of Electroacoustics for Characterization of Particulates and Suspensions Conference: NIST Special Publication 856; Malghan, S. G., Ed.; National Institute of Standards and Technology: Washington, D. C., 1993. (9) O’Brien, R. W. J. Fluid Mech. 1990, 212, 81. (10) O’Brien, R. W. J. Fluid Mech. 1988, 190, 71. (11) O’Brien, R. W.; Cannon, D. W.; Rowlands, W. N. J. Colloid Interface Sci. 1995, 173, 406. (12) Goetz, R. J.; El-Aasser, M. S. J. Colloid Interface Sci. 1992, 150, 436. (13) O’Brien, R. W.; Rowlands, W. N.; Hunter, R. J. In Proceedings of Electroacoustics for Characterization of Particulates and Suspensions Conference: NIST Special Publication 856; Malghan, S. G., Ed.; National Institute of Standards and Technology: Washington, D. C., 1993. (14) Johnson, S. B.; Russell, A. S.; Scales, P. J. Colloids Surf. A 1998, 141, 119.
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semiempirical model for relating the reduced dynamic mobility of simple metal oxide suspensions to the ζ potential at high φ conditions.15 The effectiveness of this treatment has been demonstrated for solids volume fractions up to 0.30 provided that electrical double layer overlap is insignificant.14 In addition, an electroacoustic theory that explicitly considers the effects of particle interactions at high φ and low ζ has recently been developed by Ohshima.16 As a result of these advances, an approximate theoretical framework now exists for electroacoustic experiments undertaken over a wide range of solids concentrations. In addition to its application at high solids contents, the ESA electroacoustic phenomenon is useful for examining the surface properties of colloidal particles suspended in very concentrated electrolyte solutions. Measurements at high inorganic salt concentrations have proven particularly difficult using other electrokinetic techniques because of the small particle/fluid displacement (electrophoresis and electro-osmosis) or electrical signal (streaming potential and sedimentation potential) generated by typical applied electric field strengths (electrophoresis and electro-osmosis), pressure gradients (streaming potential), or gravitational forces (sedimentation potential). With electroacoustic methods, however, these effects can be overcome by undertaking measurements at high solids concentrations, leading to a significant ESA signal generated by the simultaneous motion of a large number of particles. For a given colloid-electrolyte system, the total ESA signal is a vector sum of the signals from the suspended particles and the electrolyte ions.17 Extraction of the raw colloidal signal therefore simply requires a measurement of the signal of the background electrolyte alone, and its subtraction from the total ESA response. In addition, however, the presence of an electrolyte solution of high conductivity (>1 S m-1) can cause a number of complications with regard to the application of the alternating electric field. These complications include distortions of the electric field distribution and a substantial increase in the current flowing across the suspension. Experimentally, the latter factor can result in marked decreases in the magnitude of the applied voltage, leading to the calculation of artificially low dynamic mobilities and ζ potentials. Rowlands et al.18 have recently proposed a method through which the aforementioned electric field effects can be negated, allowing accurate determinations of and ζ at high electrolyte concentrations. Their procedure involves the electroacoustic analysis of a solution of CsCl whose conductivity is identical to that of the background electrolyte concentration. The ESA signal of CsCl is well defined18 and the applied electric field strength and form should be the same for systems possessing identical conductivities, so the CsCl signal then provides a standard against which the suspension ESA response can be calibrated. For the first time, therefore, the procedure of Rowlands et al.18 should now allow the ζ potential of colloidal particles to be accurately probed at high electrolyte concentrations as well as large solids contents. The aim of the present study has been to investigate the binding of a number of simple monovalent inorganic cations and anions at high electrolyte concentrations (up (15) The equation of Dr R. W. O’Brien is given in ref 14. (16) Ohshima, H. J. Colloid Interface Sci. 1997, 195, 137. (17) Desai, F. N.; Hammad, H. R.; Hayes, K. F. Langmuir 1993, 9, 2888. (18) Rowlands, W. N.; O’Brien, R. W.; Hunter, R. J.; Patrick, V. J. Colloid Interface Sci. 1997, 188, 325.
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to 1.0 mol dm-3) and the maximum solids concentration at which the complete electroacoustic theory is applicable (φ ) 0.02). All experiments have been conducted over a substantial pH range. The data have then been compared with both the results of previous studies undertaken at lower solids and/or electrolyte concentrations and the predictions of various ion binding models. To the authors’ knowledge, this is the most comprehensive19 study of ion binding on colloidal materials undertaken at both nondilute solids and high electrolyte concentrations. The results are of obvious relevance to a number of practical scenarios, including the behavior of sediments in saline waters and the characterization of concentrated waste tailings produced in the presence of large quantities of electrolyte during the extraction of many minerals from their ores. Experimental Section Apparatus. All electroacoustic measurements were performed using the AcoustoSizer instrument of Matec Applied Sciences (Hopkinton, MA). The AcoustoSizer measurement cell consists of a vessel of volume ca. 400 cm3, a variable speed stirrer, a heat exchange thermocouple to maintain thermal equilibrium, builtin pH and conductivity probes, and a dual titrator syringe module for precise reagent additions. The AcoustoSizer utilizes two parallel plate electrodes embedded in opposite walls of the measurement cell for the generation of pulsed alternating electric fields in the megahertz frequency domain. Each electrode is attached to a glass rod, the longer of which serves to differentiate the ESA signal from electrical “cross talk” arising from the initial voltage application, and the shorter of which assists in the acoustic impedance calibration (Z in eq 1). In practice, the acoustic impedance is automatically determined by redirecting the applied voltage pulse down the shorter glass rod and observing the amplitude of the sound wave signal reflected at the rodsuspension boundary. The ESA response is then corrected by dividing the raw signal by the quantity (Sa - Ss), where Sa and Ss are the Fourier transforms of the reflected signal in the presence of air and a suspension sample, respectively.11 The calculation of the instrument constant (A(ω) in eq 1) involves the measurement of the ESA signal for an electrolyte system, ESAe(ω), in which φ, ∆F, F, 〈µD〉, and (Sa - Ss) are all known quantities. The value of A(ω) is given by the formulas11
A(ω) )
ESAe(ω)
(4)
γ(Sa - Ss)
and
∑(φ F 〈µ 〉) 2
γ)
∆F
(5)
D
j)1
j
where γ allows for contributions to the ESA signal from both the anionic and cationic species. For a two species electrolyte, γ can be expressed in terms of the solution conductivity, Ks, so that18
γ)
Ks ez-(1 + q)
[
(m- - Fυ-) +
z(m - Fυ+)q z+ +
]
(6)
where z-,+, m-,+, and υ-,+ are the ionic valences, atomic weights, and partial molar volumes of the anion and cation, respectively, and e is the electronic charge. The parameter q is the magnitude (19) Kosmulski and Rosenholm (Kosmulski, M.; Rosenholm, J. B. J. Phys. Chem. 1996, 100, 11681.) have electroacoustically studied monovalent ion adsorption on titania and zirconia under nondilute electrolyte and solids concentrations. However, the majority of their investigations were undertaken only at high pH values. In addition, their study preceded the high salt calibration procedure of Rowlands et al.,18 so the data obtained at high suspension conductivities (> 1 S m-1) are unrealistically low.
of the ratio of the anion to the cation mobility, and is assumed to be independent of frequency. For solution conductivities up to 1.0 S m-1, a potassium R-dodecatungstosilicate solution was used as a calibration standard. In this case, O’Brien et al.11 have shown γ to be given by
γ ) 3.02 Ks
(7)
At low conductivities ( K+ ≈ Cs+. This sequence follows the decreasingly negative hydration enthalpies of the cationic species60 (Table 1), indicating that the cations with the higher affinities for their hydration shells also exhibit the greatest association with the charged surface. By contrast, the secondary hydration model of Pashley and Israelachvili61-64 predicts that ionic binding to a colloidal surface should increase (60) Burgess, J. Ions in Solution: Basic Principles of Chemical Interactions; Ellis Horwood: Chichester, 1988. (61) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (62) Pashley, R. M. Adv. Colloid Interface Sci. 1982, 16, 57. (63) Israelachvili, J. N. Adv. Colloid Interface Sci. 1982, 16, 31. (64) Pashley, R. M.; Israelachvili, J. N. J. Colloid Interface Sci. 1984, 97, 446.
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Figure 4. The ζ potential behavior of R-alumina 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. In (b), data points beyond pH ) 9 were calculated using a high-frequency analogue of the Smoluchowski approximation.11 The alumina volume fraction is 0.020 in all cases. Key: (b) Y ) Br; (O) Y ) Cl; (2) Y ) I; (4) Y ) NO3. Table 1. Hydration Enthalpies of Various Monovalent Cations and Anions at 298 Ka 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
Reference 60.
as the hydration enthalpy decreases due to the barrier to adsorption presented by a tightly held hydration shell. Although in accordance with previous monovalent cation binding studies on mica61,65,66 and silica substrates,67,68 the secondary hydration model is clearly inconsistent with the cation adsorption sequence obtained on R-alumina in the present study. An alternative description of ionic binding to colloidal surfaces is provided by the water “structure makingstructure breaking” model first proposed by Gierst et al.1 and Be´rube´ and de Bruyn.2 In this treatment, structure making and structure breaking ions are defined as those (65) Lyons, J. S.; Furlong, D. N.; Healy, T. W. Aust. J. Chem. 1981, 34, 1177. (66) Scales, P. J.; Grieser, F.; Healy, T. W. Langmuir 1990, 6, 582. (67) Tadros, T. F.; Lyklema, J. J. Electroanal. Chem. 1968, 17, 267. (68) Abendroth, R. P. J. Colloid Interface Sci. 1970, 34, 591.
that promote order and disorder, respectively, in the local aqueous medium. The structure-inducing behavior of each ionic species is closely related to its hydration enthalpy, such that for monovalent cations, water order is promoted in the sequence Li+ > Na+ > K+ > Cs+.2,69,70 The sequence of cation binding to a surface is then critically dependent on the water structuring properties of the surface itself. An ion and a surface will be entropically attracted when they exert a similar structural effect on their surrounding water environment.1,2 The sequence of cation binding determined in this study is therefore consistent with the water structuring model only if the negative R-alumina surface can be classed as a “structure maker”. The characterization of a surface as a “structure maker” or “structure breaker” has traditionally been a somewhat arbitrary process, being determined according to the result required to make sense of experimental ion binding observations. In an alternative approach, Dumont and co-workers69,70 have suggested that, because the water ordering properties of ions are closely related to their hydration enthalpies, the water structuring capabilities of surfaces should similarly increase as a function of the magnitude of their heats of immersion. Their investigations have found good agreement between this expectation and ion binding sequences observed for a range of different materials. A number of calorimetry studies have demonstrated that the R-alumina surface possesses a large heat of immersion compared with other inorganic substrates.69-72 Griffiths and Fuerstenau73 have additionally demonstrated the heat of immersion to be greatest for R-alumina powders of low surface area (the AKP-30 alumina surface area is 7.5 m2 g-1), and to rise as the magnitude of the surface charge increases. Each of these findings suggests that the negative R-alumina surface examined in this study should strongly promote water structure in its vicinity. The monovalent cation binding sequence determined by electroacoustic means (Li+ > Na+ > K+ ≈ Cs+) is therefore in good agreement with the model of Gierst et al.1 and Be´rube´ and de Bruyn.2 It is interesting to note that, as substrates such as titania and hematite generally possess quite high heats of immersion,69-72 the order of ion binding often observed in those cases (i. e., Li+ > Na+ > K+ > Cs+2,43,52,54,56,57) is also consistent with the model of Gierst et al.1 and Be´rube´ and de Bruyn.2 Furthermore, the water “structure makingstructure breaking” model can be used to rationalize the reverse Cs+ > K+ > Na+ ) Li+ binding sequence promoted by substrates such as mica61,65,66 and silica.67,68 The heat of immersion of silica has been experimentally demonstrated to be low compared with other metal oxide materials.69-72 In addition, although no direct calorimetric data are available, the heat of immersion of mica is anticipated to be small by virtue of the position of its isoelectric point,72 which, in the absence of typical dissolution processes, should occur at pH < 3.65,66 The silica and mica surfaces are therefore predicted to be “structure breaking” in nature, and so will most strongly associate with ions that similarly disrupt their neighboring water environment (i.e., Cs+ > K+ > Na+ > Li+). (69) Dumont, F.; Dang Van Tan; Watillon, A. J. Colloid Interface Sci. 1976, 55, 678. (70) Dumont, F.; Warlus, J.; Watillon, A. J. Colloid Interface Sci. 1990, 138, 543. (71) Guderjahn, C. A.; Paynter, D. A.; Berghausen, P. E.; Good, R. J. J. Phys. Chem. 1959, 63, 2066. (72) Healy, T. W.; Fuerstenau, D. W. J. Colloid Sci. 1965, 20, 376. (73) Griffiths, D. A.; Fuerstenau, D. W. J. Colloid Interface Sci. 1981, 80, 271.
Binding of Monovalent Electrolyte Ions. I. Electroacoustics
The ζ potential data obtained over the entire range of electrolyte concentrations investigated in this study indicate that the Br-, Cl-, I-, and NO3- anions bind in a fashion similar to the R-alumina surface (Figure 4). Dumont and co-workers69,70,74 have compared the coagulation characteristics of a large number of anions by studying their ability to induce rapid aggregation. The authors found that the order of anion binding to the positive surfaces of “structure making” hematite and titania hydrosols decreases in the sequence NO3f Cl- > Br- ≈ I-, whereas for some other positive titania surfaces, I- ≈ Br- ≈ Cl- ≈ NO3-.70 Neither of these scenarios bears any obvious resemblance to the hydration enthalpies of the anionic species60 (Table 1). In the latter case,70 the authors rationalized the similar binding properties of the anions by considering the positive titania surfaces to be “practically neutral from the structuring point of view”. To make such a statement in the present study would, however, conflict with the findings of Griffiths and Fuerstenau73 that the heat of immersion of R-alumina substrates is at its greatest when the surfaces are positively charged, indicating particularly strong structure promoting qualities. A substantial entropic interaction is therefore predicted to exist between the anions and the positive R-alumina surface, which, based on the anionic hydration enthalpies, should lead to binding in the sequence Cl- > Br- > NO3f I-. Clearly, such an expectation is inconsistent with the ζ potential data presented in this study. One possible explanation for almost identical binding properties of the Br-, Cl-, I-, and NO3- species is the tight range (68 kJ mol-1) within which all of the anionic hydration enthalpies fall (Table 1). This range is not only substantially smaller than that spanned by the hydration enthalpies of the Li+, Na+, K+, and Cs+ species (252 kJ mol-1), but is also lower than the hydration enthalpy difference between all but the K+ and Cs+ cations () 58 kJ mol-1). Significantly, K+ and Cs+ possess very similar binding properties in this study. Perhaps not coincidently, K+, Cs+, Cl-, Br-, I-, and NO3- also possess the smallest hydration enthalpies of the ions investigated here. These findings appear to indicate that for different monovalent ions possessing small and/or similar (ca. ( 35 kJ mol-1) hydration enthalpies, the relative adsorption strengths on R-alumina are insensitive to the hydration enthalpy sequence. To further investigate this proposition, other workers in our laboratory have recently examined the binding characteristics of monovalent anions such as F-, IO3-, and BrO3-.75 These species possess hydration enthalpies that are of significantly higher magnitude than those that have been investigated here. Preliminary results indicate that in all cases, the F-, IO3-, and BrO3(74) Dumont, F.; Watillon, A. Faraday Discuss. Chem. Soc. 1971, 52, 352. (75) Franks, G. V., unpublished data, 1998.
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ions exhibit binding properties that differ markedly from those of Cl-, Br-, I-, and NO3-. In addition, the relative binding strengths closely follow the predictions of the model of Gierst et al.1 and Be´rube´ and de Bruyn.2 A full analysis of these investigations, which have been undertaken on both R-alumina and zirconia particles, will be presented in a later paper. Summary An electroacoustic method has been used to study the effects of cation and anion binding on an R-alumina colloid over a wide range of pH conditions at nondilute solids (φ ) 0.02) and moderate to high electrolyte (0.01-1.0 mol dm-3) concentrations. The resulting ζ potential versus pH data indicate that NaNO3, KNO3, CsNO3, KBr, KCl, and KI are indifferent electrolytes for R-alumina. By contrast, the isoelectric point of R-alumina rises significantly as the LiNO3 concentration increases, showing that Li+ is binding in a specific chemical manner to the surface. These findings are in good agreement with previous electrokinetic and charge titration experiments performed at lower electrolyte and/or solids concentrations on both alumina and a range of other metal oxide materials. At the highest electrolyte concentration studied (1.0 mol dm-3), monovalent inorganic cations bound to the negative R-alumina surface in the order Li+ > Na+ > K+ ≈ Cs+. This sequence is inconsistent with the secondary hydration model of Pashley and Israelachvili.61-64 However, based on the hydration enthalpies of the cations and the large heat of immersion of the R-alumina surface, the results are in good qualitative agreement with the water “structure making-structure breaking” model of Gierst et al.1 and Be´rube´ and de Bruyn.2 Over the entire range of electrolyte concentrations investigated in this study, the Br-, Cl-, I-, and NO3- anions bound in an almost identical manner to the R-alumina surface. These results do not appear to be well described by the model of Gierst et al.1 and Be´rube´ and de Bruyn.2 We propose that the comparable binding behavior of the Br-, Cl-, I-, and NO3- anions (and the K+ and Cs+ cations) is due to their low and/or similar hydration enthalpies. Comparable water structuring qualities should then result, giving rise to a similar interaction between each ion and the “structure making” R-alumina surface. Acknowledgment. This study was conducted using funding provided by the Advanced Mineral Products Research 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 Scholarship. The authors thank Dr. G. V. Franks for providing details of his preliminary anion binding results on R-alumina and zirconia. LA980875F
