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AFM interaction study of α-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions
Journal of Colloid and Interface Science 307 (2007) 116–123 www.elsevier.com/locate/jcis
AFM interaction study of α-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions Huseyin Yilmaz a,b,∗ , Kimiyasu Sato a , Koji Watari a a National Institute of Advanced Industrial Science and Technology (AIST), Anagahora 2266-98, Shimoshidami,
Moriyama-ku, Nagoya-shi, Aichi 463-8560, Japan b Gebze Institute of Technology, Materials Science and Engineering Department, Gebze-Kocaeli, Turkey
Received 25 August 2006; accepted 5 November 2006 Available online 11 December 2006
Abstract Ionic strength dependence of interaction and friction forces between hydrophilic α-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force–distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10−3 M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the α-alumina surfaces at pH 11. © 2006 Elsevier Inc. All rights reserved. Keywords: SPM; AFM; Colloid probe method; Alumina
1. Introduction When oxide surfaces are brought into contact with an aqueous solution, they acquire surface charges depending on the pH of the environment. Surface charge is an important parameter that influences the stability and rheology of dispersions of ceramic oxide particles in aqueous electrolyte solutions. Oxide colloidal particles in an aqueous solution can become charged by adsorption or desorption of potential-determining ions, i.e., protonation and deprotonation reactions [1]. In the case of αAl2 O3 surfaces the reaction occurs between the hydroxylated surface and H+ or OH− species [2]. The interaction of α-Al2 O3 surface can be described as follows [2]: AlOH = AlO− + H+ (basic pH), (acidic pH). AlOH + H+ = AlOH+ 2 It is the magnitude of these surface charges (which can also be viewed as surface potential) that brings stability to colloidal * Corresponding author. Fax: +81 052 736 7405.
E-mail address: [email protected] (H. Yilmaz). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.11.010
systems and it is the source of one of the two forces considered in particle–particle interaction models. Classical DLVO (Derjaguin–Landau–Verway-Overbeek) theory states that in an aqueous solution, two surfaces at close enough separations attract (van der Waals force) and repel (electrostatic force) each other. The van der Waals forces arise between dipoles of atoms or molecules. The other component of the DLVO theory is the electrostatic force and is due to the charges at the surface. The surface charge is balanced by dissolved counterions that redistribute themselves and spread away from the surface, forming an electrical double layer (EDL). Depending on the dominance and extend of these opposing forces, the colloidal stability can be predicted [1]. The EDL thickness, usually named the Debye length, is strongly dependent on the electrolyte concentration. At low ionic strength the Debye length extends far enough to cause repulsion between particles. However, at high ionic strength, due to the compression of the EDL (on the order of ion size), attractive van der Waals forces dominate the slurry behavior and thus flocculation occurs. For low potentials the Debye length is
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calculated as [1] ni∞ e2 zi2 −1/2 −1 κ = , εε0 kT
(1)
i
where κ −1 is the Debye screening length and is determined by the salt concentration, ni∞ is the bulk concentration of the of the ion species i, zi is the valence of the ion i, ε0 is the permittivity of free space, ε is the dielectric constant of water, k is the Boltzmann constant, and T is the temperature. For a monovalent salt solution (such as alkali halides, KCl) the above Eq. (1) simplifies to (in nm) κ −1 =
0.304 , [KCl]
(2)
where [KCl] is the molarity (mol/l) of alkali halide in the bulk solution. The charging of the surfaces in an aqueous solution may also lead to structuring of water molecules nearby [1,3–5]. Such an arrangement was named a hydration layer. Since the structuring is confined to a very thin layer, on the order of the size of the H2 O molecule, it is effective at very short separation distances. It is thought that it would be more difficult to remove such a layer between interfaces before contact, leading to a short-range repulsive interaction called hydration force [1]. The presence of a hydration layer would prevent particles from falling into a deep primary potential minimum by the strong van der Waals forces closer than a few angstroms. From a particle network, where particles are sitting at a shallower potential minimum, a lower viscosity, shear modulus, and yield stress would be expected. Since hydration force was not taken into account in the classical DLVO theory, it was categorized as non-DLVO forces. The structuring of polar H2 O molecules around charged species is known well. H2 O molecules form a hydration layer around a cation or an anion depending on their size [6,7]. Small cations (anion) are known as structure making and large cations (anion) are known as structure-breaking. Therefore, hydration layers around small cation such as Li or Na are dense. The presence of a hydration layer at the solid interface, influenced by the choice of electrolyte, hinders particle approach and has an effect on the rheology of powder compact [8]. Alumina colloids showed unusual stability behavior, which was attributed to the non-DLVO forces operating at distances <5 nm [9]. There is doubt about the origin of this repulsive force. It might be due to the hydration layer or to gel layer formation at the aqueous interface [9]. Depending on the aging conditions, a gel layer thickness ∼15 nm has been reported at pH ∼ 8 [9]. Beattie et al. suggested the presence of charged polymeric Al species at the surface, which were named Keggin ions (Al13 O4 (OH)31−x ), which impart the unusual stability x observed in alumina colloids [10]. The rheology of slurry and powder compact are directly related to the normal and frictional forces between particles [11]. While the normal force quantifies how much the particles in the suspension approach each other, the frictional force quantifies the flow of particles over one another. So the quantification of
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both normal and lateral forces is very important in the rheology of ceramic powders. It has been reported that rheology of flocculated alumina suspensions (slurries at pH 9 with and without NH4 Cl were flocked) differs greatly from that of coagulated ones (slurries at pH 4 with NH4 Cl were coagulated) [12]. This was attributed to the modified hydration layer due to the presence of excessively hydrated cations between the particles [12]. Short-range hydration forces have been used to prepare weakly flocculated alumina suspensions with tuned rheological properties [8,13,14]. They were prepared by adjusting the electrolyte concentration in the suspension. The aim was high packing density as well as low viscosities to induce plastic flow. Probing the normal forces as a function of the separation distance between a flat plate and a spherical particle provides valuable information about how colloidal particles interact with each other in a suspension. The convenient thing is that measurements can be done in environments that are close to the real conditions. The technique used to probe the forces is called the colloidal probe technique [15]. In this technique a spherical particle is usually attached (glued) to a scanning probe microscope (SPM) cantilever tip and the force (actually cantilever deflection) is measured as a function of separation distance from either a flat surface or another spherical particle. The results obtained can be analyzed using DLVO theory. Friction is the force between two things that opposes their motion. It is the interaction of surfaces in the lateral direction. So any precaution that modifies the surface has a direct effect on the friction between surfaces. The presence of a hydration layer at the surface determines the closest approach of two surfaces. The more hydrated cations of indifferent electrolyte on the hydrophilic surface (interfaces), the more lubrication effect would be expected. The presence of water molecules tightly bound to ions or ionized surfaces in aqueous electrolytes leads to strong repulsion when they approach each other to within a few nanometers or less [3–5]. At high electrolyte salt concentrations it can dominate the attractive van der Waals and repulsive double-layer forces, especially at high salt concentrations (>0.1 M) [3]. Raviv and Klein questioned the fluidity of such hydration layers bound to ions between mica surfaces. Because such lubrication action has implications for many applications, such as clay or paste plasticity in ceramic science, biological processes that require shear, and displacement of the final subnanometer layers of bound hydration layers [3]. In this study, ionic strength dependence of interaction force and lateral friction force between α-alumina particle and c-sapphire surface (0001) were investigated under basic pH conditions using the SPM method. An atomic force microscope (AFM) cantilever modified with an α-alumina particle was used in measuring the normal and lateral interaction forces between a spherical particle and a flat surface. The normal and frictional forces at low to high electrolyte concentration were measured.
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Table 1 TL-FM 50 colloid probe used in the experiments Normal spring constant (N/m) Torsional spring constant (nN m) Frequency (kHz) Q Length (µm) Width (µm) α-Alumina (µm)
1.9 52.0 71.5 145.0 228.0 34.5 12.8
2. Materials and methods 2.1. Colloid probe preparation The as-received α-alumina (Admatechs Co., Japan; dmean = 9.9 µm) particles were first washed repeatedly in a 1 wt% sodium hexametaphosphate (Na6 P6 O18 ) solution in an ultrasonic bath in PE bottles to separate out unwanted small particles and then resuspended and stored at pH 3.5 for further use [16]. Double-distilled water (Yamato Scientific Auto Still Model WA-72, Japan) that was passed through a reverseosmosis unit (Barnstead/Thermolyne EasyPure, USA), with a resistivity of 18.3 M cm, was used in all cleaning and solution preparation procedures. A colloidal probe was prepared by carefully picking spherical α-alumina particles of size ∼10 µm by a micromanipulator (Model M501-1202-M; Suruga Seiki Co. Ltd., Japan), which is then glued (Araldite ARR30-Nichiban Co. Ltd., Japan) to the tipless AFM cantilever (TL-FM-50; Nanosensors, Switzerland). Table 1 gives the measured specifications of the TL-FM-50 tipless colloid probe. The glued α-alumina particle diameter was 12.8 µm. Just prior to force measurements, the alumina colloid probe the c-sapphire substrate (As One Corporation, Japan), and the SPM liquid cells were UV-plasma (PL16-110D-Sen Light Corp., Japan) treated to remove surface organics and make the surface hydrophilic. KCl was used to adjust the strength of the electrolyte solution. KOH was used to adjust the pH of the electrolyte.
Fig. 1. Optical microscope image of colloid probe used in the force measurement experiments.
2.3. Friction force measurements The same SPM was used to measure lateral friction forces over a trace distance of 500 nm. Lateral friction curves were measured on trace and retrace, thus forming a loop (torsion of the tip during a forward/reverse scan). The loops obtained were used to calculate the average lateral friction forces between two surfaces. The torsional signal on trace and retrace were added and divided by 2 to calculate the average lateral friction signal, which is then converted to force using cantilever torsional spring constant, FL = kL y (where FL is the force, kL is the cantilever spring constant, and y is the torsion of the cantilever measured in distance) [19]. Since the same cantilever was used to do lateral friction force measurements, the SPM cantilever was not characterized for torsional spring constant. It was calculated from normal spring constant using kL = 2kN l 2 /(3(1 + ν)), where l is the cantilever length and ν is the poison ratio of the cantilever which is taken as 0.27 [19]. 3. Results and discussion 3.1. Force curves on approach
2.2. Normal force measurements A scanning probe microscope (SPA 400, Seiko, Japan) with a liquid cell was used. Fig. 1 shows the colloid probe used in the force measurement experiments. What is actually measured in the SPM is the deflection signal from the cantilever as a function of separation distance. The deflection signal is then converted to force (FN ) vs distance (D) curves using cantilever stiffness, FN = kN x (where FN is the force, kN is the cantilever spring constant, and x is the normal deflection of the cantilever measured as a distance). The cantilever stiffness was measured following the protocol of Sader et al. [17]. The α-alumina colloid probe surface and c-sapphire surface were equilibrated in the supernatant for 30 min before any measurement. During force measurements, the approach and retraction speeds were kept constant at 20 nm/s, so that repulsive hydrodynamic forces could be neglected [18]. The total piezo displacement was 100 nm.
The measured forces between the α-alumina probe and the c-sapphire surface in 10−3 –100 M KCl electrolyte solution at pH 11 are shown in Fig. 2. It is more convenient to express the interaction as normalized force curves (F /2πR) because the area over which the interaction takes place affects the force [20]. It is easier to compare force curves when they are normalized by the radius of the probe. From Fig. 2, it is clear that there were no surface forces between the studied surfaces when the distance was large enough. Only when the separation distance was less than ∼20 nm did a strong dispersion force emerge. The shown normal force–distance curves were averages of five measurements obtained using the same colloid probe at different spots on the c-sapphire surface. Special attention was given to the flatness of the c-sapphire surface and sphericity of the α-alumina particles selected. Furthermore, the same colloid probe was utilized in normal and lateral force measure-
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Fig. 2. Normalized force (FN ) vs distance (H ) curves between 12.8-µm α-alumina particle glued probe and c-sapphire surface in 10−3 –100 M KCl electrolyte solutions at pH 11.
ments to minimize the effect of particle surface imperfections on the reproducibility of the force–distance curves and friction loops. The individual force–distance curves thus obtained were reproducible (data not shown) in terms of start of the interaction separation and forces were repulsive at all separations. Furthermore, the friction loops obtained at different electrolyte concentrations could be quantitatively comparable with each other, since the same α-alumina particle and cantilever were used. The interaction distance of colloidal particles on approach strongly depends on the electrolyte concentration, as seen from Fig. 2. As the electrolyte concentration increased from 10−3 to 100 M the repulsive interaction distance decreased from ∼20 to ∼7 nm. In this study an indifferent electrolyte, namely KCl, was chosen to adjust the Debye length. K ions are relatively large (0.133 nm) [21] and it is considered a structure-breaking ion instead of a structure-making one. The H2 O molecules are weakly bonded to the K cation compared to the small alkali elements such as the Li cation [6,7]. On normal approach repulsive forces dominated the attractive van der Waals forces, either due to the hydration layer or to the gel layer, as reported previously [2,5,13]. Due to the imperfections on the surfaces, it is very difficult to attribute the reason just to the hydration layer between alumina surfaces. No matter what the electrolyte concentration was, the normal forces were always repulsive. The electrostatic interaction force between the probe tip and sapphire surface was modeled using a linearized form of the Poisson–Boltzmann equation, Eqs. (3) and (4), for a symmetric 1:1 electrolyte, in which diffuse layer potential and Debye lengths were used as a fitting parameter. The model used to fit experimental FN distance curves for constant surface charge condition was −2κH 2Ψ1 Ψ2 e−κH FN 2 e + (Ψ − Ψ ) = εε0 κ 1 2 2πR 1 − e−κH 1 − e−2κH
(3)
and
−2κH 2Ψ1 Ψ2 e−κH FN 2 e − (Ψ + Ψ ) = εε0 κ , 1 2 2πR 1 − e−κH 1 − e−2κH
(4)
for constant surface potential condition, where Ψ1 and Ψ2 are surface potentials of the alumina probe and sapphire surface, respectively, R is the probe radius, and H is the separation distance [22]. The retarded van der Waals interaction force (FvdW ) [1] was calculated using an expression for the retarded interaction force, Eqs. (5) and (6), A131 FvdW , =− 2πR 6H 2 5.1 × 10−20 , A131 = (1 + [H /11.82]1.0276 )1/1.0276
(5) (6)
where A131 is the retarded Hamaker constant [23]. The simplifications due to the Derjaguin approximation for sphere–plate geometry allowed us to consider the interaction between two plate geometries [24]. Under all conditions, the separation distances between the sphere and the plate surface were much smaller then the colloid probe radius, satisfying the conditions for the Derjaguin approximation, namely H R. The results in Fig. 3 suggest the DLVO theory describes the interaction forces between surfaces immersed in simple symmetric electrolytes (<0.1 M concentration) at separation distances higher than ∼4 nm. In all electrolyte concentrations a strongly repulsive double-layer force was observed. As the electrolyte concentration was increased, the forces were again totally repulsive but lower in magnitude (Fig. 2). The observed forces were fitted with the constant-charge model described in Eq. (3) and are given in Table 2 along with the fitted Debye lengths. The constants, potential fit models were not shown because the data fits were not satisfactory. The observed Debye lengths were found to be in reasonable agreement with those calculated for the electrolyte level. The isoelectric points of the α-alumina particle and c-sapphire have been reported as pH ∼ 9
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Fig. 3. Logarithm of normalized force (FN ) vs separation distance (H ) curves between 12.8-µm α-alumina particle glued probe and c-sapphire surface in 10−3 –100 M KCl electrolyte solutions at pH 11. Solid lines are DLVO fit to the same data using the constant-charge boundary condition.
Fig. 4. Individual force–distance curves both on approach and retraction in 1 M KCl electrolyte solution at pH 11. Adhesion between two surfaces was measured on retraction. Table 2 Constant-charge boundary condition fitting parameters obtained from fits to interaction force–distance data between an α-alumina particle and a flat c-sapphire substrate immersed in KCl electrolyte solution and calculated Debye lengths from electrolyte concentrations Electrolyte concentration (M)
Ψ (mV)
κ −1 (nm, from fit)
κ −1 (nm, calculated)
10−3 10−2 10−1
−35 −35 −35
9 3 2
9.6 3.04 1
and ∼5, respectively, the reason for which was attributed to the presence of different types of surface hydroxyl groups on the two different types of surfaces [25]. Therefore, at pH 11 both surfaces are negatively charged. The fitted surface potential was found to be −35 mV, confirming that K is not specifically adsorbing onto the alumina surface. The Debye lengths were found to decrease from 9 to 2 nm as the electrolyte concentration increased, as expected.
It was Hunter who first quantitatively related interparticle forces and rheological properties of colloidal dispersions [26]. Based on the force–distance curves shown in Fig. 2 and using DLVO theory, it is not possible to give an account of the rheological behavior of alumina suspensions as being only electrostatically stabilized. Because 25 vol% α-alumina suspensions at various electrolyte concentrations showed yield stress and shear thinning behavior that are not consistent with the repulsive forces observed at all separations in the AFM study [8]. In rheology, the yield stress and shear-thinning behavior of a suspension are explained by weakly attracting particle interactions. In Fig. 4, individual force–distance curves both on approach and on retraction in 1 M KCl electrolyte solution are shown. The force curve on approach is repulsive, as already stated. However, giving a closer look to the retraction curve, the presence of adhesion between the α-alumina particle and c-sapphire is a clear indication of attractive van der Waals forces. That is, forces that are repulsive on approach do not necessarily mean that the particles stay dispersed in the slurry at all times. At con-
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Fig. 5. Friction loops measured at four different KCl electrolyte concentrations at applied normal force 250 nN over a trace distance of 500 nm.
tact the α-particle adheres to the c-sapphire surface. Adhesion forces, as well known by everybody, scatter a lot, due to the measurement conditions, surface roughness, contact area and time, etc. Thus it is very difficult to draw conclusions on rheological properties of alumina suspensions based on adhesion forces. Similar force–distance curves were obtained at different electrolyte concentrations. The macroscopic properties of alumina suspensions are under study.
Table 3 Exponential decay lengths (λ1 ) and pre-exponential constants obtained (C1 ) from the force–distance curves fitted to the empirical double exponential function Electrolyte concentration (M)
C1 (mN/m)
λ1 (nm, from fit)
10−3 10−2 10−1 100
0.15 0.24 0.31 0.31
1.1 1.1 1.1 1.1
3.2. Short range forces Although the classical DLVO theory fits the measured force– distance curves very well down to separations of ∼4 nm, the discrepancy between the theory and the experimental data becomes more and more significant at very close separations. Since the classic DLVO theory considers only two components, i.e., van der Waals force and electrostatic force, this discrepancy may come from another force that exists only at short range. Since no surfactant but electrolyte was added to the system, this extra force may be a hydration force. Having this in mind, the experimental data were fitted to the extended DLVO theory by including a third force component, hydration force, into the classical DLVO theory. Table 3 gives the exponential decay lengths (λ1 ) and preexponential constants obtained (C1 ) from the force–distance curves fitted to the empirical double exponential function [1]: H H + C2 exp − . FH = C1 exp − (7) λ1 λ2 Since only short-distance structural force is assumed, the second terms were neglected. The fits to the curves were better than R 2 > 0.97. The exponential decay lengths (λ1 ) did not change with ionic strengths and stayed constant as expected, whereas the preexponential factor (C1 ) calculated as 0.15 mN/m in 10−3 M
KCl electrolyte solution increased and saturated at 0.31 mN/m in 10−1 M one. The pre-exponential constants given in Table 3 for the 10−1 and 100 M electrolyte cases did not differ much. The reason was that the force–distance curves for the two cases were almost identical, as can be seen from Fig. 2. Actually, the short-range exponential decay lengths compare well with the ones given in Ref. [27], in which electrolyte-species-dependent hydration forces between silica surfaces were reported. It is hard to make the same comment about the repulsion parameters, which do not compare well with the values mentioned in the same reference. The discrepancy might be related to the nature of the hydration layer and/or the surface structure between silica and alumina. 3.3. Friction forces In this study, shear interaction between the α-alumina particle and the c-sapphire surface (0001) were investigated under the same conditions as stated above, namely between 10−3 and 100 M KCl electrolyte concentrations at pH 11. Fig. 5 shows that the friction loops obtained at 250 nN applied normal force at four different electrolyte concentrations. The friction loops were smooth over a trace distance of 500 nm; they were analyzed as described in Section 3.1. The observed asymmetry
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Fig. 6. The lateral friction (FL ) vs applied normal force (FN ) curves between a 12.8-µm α-alumina particle attached probe and a c-sapphire surface in 10−3 –100 M KCl electrolyte solutions at pH 11. Solid lines are linear fit to the data point (Amantons law). Table 4 Friction coefficients calculated from the slopes of the curves given in Fig. 6
4. Conclusions
Electrolyte concentration (M)
μeff
10−3 10−2 10−1 100
0.54 0.49 0.36 0.27
Normal and lateral friction forces between an α-alumina colloid particle and a flat c-sapphire surface in KCl electrolyte solution at pH 11 were measured using a colloid probe technique. Repulsive forces were observed that were well described by DLVO theory at all separation distances greater than 4 nm. A surface potential of −35 mV and a Debye length of 9 nm were calculated for the 10−3 M KCl solution. The calculated Debye lengths were in very good agreement with the data observed from the linear Poisson–Boltzmann equation. An exponential force law with the second term omitted was used to fit the hydration force. The fitted short decay length (λ1 ) for the hydration force was estimated to be 1.1 nm. Analysis of friction behavior was done assuming that Amantons law was obeyed. Solid surfaces sliding over each other across aqueous electrolyte solutions exhibited different behavior depending on the degree of population of the interface with hydrated ions. Concentrated KCl electrolyte solutions exhibited a low friction force resulting in a small friction coefficient compared to the dilute ones.
in the lateral friction loops indicates that there was a contribution to the friction force from surface roughness [19]. In order to eliminate the orientation dependence of friction force, all measurements were averaged in both scan directions. The lateral friction (FL ) vs applied normal (FN ) force curves at four different electrolyte concentrations are given in Fig. 6. The friction behavior was analyzed assuming that the friction force between two surfaces was proportional to the normal force. At a fixed velocity, lateral force increases with applied load in a linear fashion, obeying the Amantons law. The linear nature of friction curves was an indication of the presence of multiple asphericity. The microscale friction coefficient between alumina surfaces by the lateral frictional curve measurement obtained by fitting the Amantons law to the friction force vs normal force data. An effective friction coefficient μeff is defined as μeff =
FL . FN
(8)
Table 4 shows the friction coefficients calculated from the slopes of the curves given in Fig. 6. At 10−3 M KCl electrolyte concentration the effective friction coefficient was calculated as 0.54, whereas at 100 M KCl the friction coefficient was 0.27. The friction curves at higher electrolyte concentration showed better fits to the Amantons law. A significant lubrication effect was clear for solutions of high electrolyte concentration, which is consistent with the data given in [14]. Similar work has been done by Higashitani’s group for silica surfaces, in which the lubrication effect of hydrated cations was studied [28,29].
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AFM interaction study of α-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions Huseyin Yilmaz a,b,∗ , Kimiyasu Sato a , Koji Watari a a National Institute of Advanced Industrial Science and Technology (AIST), Anagahora 2266-98, Shimoshidami,
Moriyama-ku, Nagoya-shi, Aichi 463-8560, Japan b Gebze Institute of Technology, Materials Science and Engineering Department, Gebze-Kocaeli, Turkey
Received 25 August 2006; accepted 5 November 2006 Available online 11 December 2006
Abstract Ionic strength dependence of interaction and friction forces between hydrophilic α-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force–distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10−3 M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the α-alumina surfaces at pH 11. © 2006 Elsevier Inc. All rights reserved. Keywords: SPM; AFM; Colloid probe method; Alumina
1. Introduction When oxide surfaces are brought into contact with an aqueous solution, they acquire surface charges depending on the pH of the environment. Surface charge is an important parameter that influences the stability and rheology of dispersions of ceramic oxide particles in aqueous electrolyte solutions. Oxide colloidal particles in an aqueous solution can become charged by adsorption or desorption of potential-determining ions, i.e., protonation and deprotonation reactions [1]. In the case of αAl2 O3 surfaces the reaction occurs between the hydroxylated surface and H+ or OH− species [2]. The interaction of α-Al2 O3 surface can be described as follows [2]: AlOH = AlO− + H+ (basic pH), (acidic pH). AlOH + H+ = AlOH+ 2 It is the magnitude of these surface charges (which can also be viewed as surface potential) that brings stability to colloidal * Corresponding author. Fax: +81 052 736 7405.
E-mail address: [email protected] (H. Yilmaz). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.11.010
systems and it is the source of one of the two forces considered in particle–particle interaction models. Classical DLVO (Derjaguin–Landau–Verway-Overbeek) theory states that in an aqueous solution, two surfaces at close enough separations attract (van der Waals force) and repel (electrostatic force) each other. The van der Waals forces arise between dipoles of atoms or molecules. The other component of the DLVO theory is the electrostatic force and is due to the charges at the surface. The surface charge is balanced by dissolved counterions that redistribute themselves and spread away from the surface, forming an electrical double layer (EDL). Depending on the dominance and extend of these opposing forces, the colloidal stability can be predicted [1]. The EDL thickness, usually named the Debye length, is strongly dependent on the electrolyte concentration. At low ionic strength the Debye length extends far enough to cause repulsion between particles. However, at high ionic strength, due to the compression of the EDL (on the order of ion size), attractive van der Waals forces dominate the slurry behavior and thus flocculation occurs. For low potentials the Debye length is
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calculated as [1] ni∞ e2 zi2 −1/2 −1 κ = , εε0 kT
(1)
i
where κ −1 is the Debye screening length and is determined by the salt concentration, ni∞ is the bulk concentration of the of the ion species i, zi is the valence of the ion i, ε0 is the permittivity of free space, ε is the dielectric constant of water, k is the Boltzmann constant, and T is the temperature. For a monovalent salt solution (such as alkali halides, KCl) the above Eq. (1) simplifies to (in nm) κ −1 =
0.304 , [KCl]
(2)
where [KCl] is the molarity (mol/l) of alkali halide in the bulk solution. The charging of the surfaces in an aqueous solution may also lead to structuring of water molecules nearby [1,3–5]. Such an arrangement was named a hydration layer. Since the structuring is confined to a very thin layer, on the order of the size of the H2 O molecule, it is effective at very short separation distances. It is thought that it would be more difficult to remove such a layer between interfaces before contact, leading to a short-range repulsive interaction called hydration force [1]. The presence of a hydration layer would prevent particles from falling into a deep primary potential minimum by the strong van der Waals forces closer than a few angstroms. From a particle network, where particles are sitting at a shallower potential minimum, a lower viscosity, shear modulus, and yield stress would be expected. Since hydration force was not taken into account in the classical DLVO theory, it was categorized as non-DLVO forces. The structuring of polar H2 O molecules around charged species is known well. H2 O molecules form a hydration layer around a cation or an anion depending on their size [6,7]. Small cations (anion) are known as structure making and large cations (anion) are known as structure-breaking. Therefore, hydration layers around small cation such as Li or Na are dense. The presence of a hydration layer at the solid interface, influenced by the choice of electrolyte, hinders particle approach and has an effect on the rheology of powder compact [8]. Alumina colloids showed unusual stability behavior, which was attributed to the non-DLVO forces operating at distances <5 nm [9]. There is doubt about the origin of this repulsive force. It might be due to the hydration layer or to gel layer formation at the aqueous interface [9]. Depending on the aging conditions, a gel layer thickness ∼15 nm has been reported at pH ∼ 8 [9]. Beattie et al. suggested the presence of charged polymeric Al species at the surface, which were named Keggin ions (Al13 O4 (OH)31−x ), which impart the unusual stability x observed in alumina colloids [10]. The rheology of slurry and powder compact are directly related to the normal and frictional forces between particles [11]. While the normal force quantifies how much the particles in the suspension approach each other, the frictional force quantifies the flow of particles over one another. So the quantification of
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both normal and lateral forces is very important in the rheology of ceramic powders. It has been reported that rheology of flocculated alumina suspensions (slurries at pH 9 with and without NH4 Cl were flocked) differs greatly from that of coagulated ones (slurries at pH 4 with NH4 Cl were coagulated) [12]. This was attributed to the modified hydration layer due to the presence of excessively hydrated cations between the particles [12]. Short-range hydration forces have been used to prepare weakly flocculated alumina suspensions with tuned rheological properties [8,13,14]. They were prepared by adjusting the electrolyte concentration in the suspension. The aim was high packing density as well as low viscosities to induce plastic flow. Probing the normal forces as a function of the separation distance between a flat plate and a spherical particle provides valuable information about how colloidal particles interact with each other in a suspension. The convenient thing is that measurements can be done in environments that are close to the real conditions. The technique used to probe the forces is called the colloidal probe technique [15]. In this technique a spherical particle is usually attached (glued) to a scanning probe microscope (SPM) cantilever tip and the force (actually cantilever deflection) is measured as a function of separation distance from either a flat surface or another spherical particle. The results obtained can be analyzed using DLVO theory. Friction is the force between two things that opposes their motion. It is the interaction of surfaces in the lateral direction. So any precaution that modifies the surface has a direct effect on the friction between surfaces. The presence of a hydration layer at the surface determines the closest approach of two surfaces. The more hydrated cations of indifferent electrolyte on the hydrophilic surface (interfaces), the more lubrication effect would be expected. The presence of water molecules tightly bound to ions or ionized surfaces in aqueous electrolytes leads to strong repulsion when they approach each other to within a few nanometers or less [3–5]. At high electrolyte salt concentrations it can dominate the attractive van der Waals and repulsive double-layer forces, especially at high salt concentrations (>0.1 M) [3]. Raviv and Klein questioned the fluidity of such hydration layers bound to ions between mica surfaces. Because such lubrication action has implications for many applications, such as clay or paste plasticity in ceramic science, biological processes that require shear, and displacement of the final subnanometer layers of bound hydration layers [3]. In this study, ionic strength dependence of interaction force and lateral friction force between α-alumina particle and c-sapphire surface (0001) were investigated under basic pH conditions using the SPM method. An atomic force microscope (AFM) cantilever modified with an α-alumina particle was used in measuring the normal and lateral interaction forces between a spherical particle and a flat surface. The normal and frictional forces at low to high electrolyte concentration were measured.
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Table 1 TL-FM 50 colloid probe used in the experiments Normal spring constant (N/m) Torsional spring constant (nN m) Frequency (kHz) Q Length (µm) Width (µm) α-Alumina (µm)
1.9 52.0 71.5 145.0 228.0 34.5 12.8
2. Materials and methods 2.1. Colloid probe preparation The as-received α-alumina (Admatechs Co., Japan; dmean = 9.9 µm) particles were first washed repeatedly in a 1 wt% sodium hexametaphosphate (Na6 P6 O18 ) solution in an ultrasonic bath in PE bottles to separate out unwanted small particles and then resuspended and stored at pH 3.5 for further use [16]. Double-distilled water (Yamato Scientific Auto Still Model WA-72, Japan) that was passed through a reverseosmosis unit (Barnstead/Thermolyne EasyPure, USA), with a resistivity of 18.3 M cm, was used in all cleaning and solution preparation procedures. A colloidal probe was prepared by carefully picking spherical α-alumina particles of size ∼10 µm by a micromanipulator (Model M501-1202-M; Suruga Seiki Co. Ltd., Japan), which is then glued (Araldite ARR30-Nichiban Co. Ltd., Japan) to the tipless AFM cantilever (TL-FM-50; Nanosensors, Switzerland). Table 1 gives the measured specifications of the TL-FM-50 tipless colloid probe. The glued α-alumina particle diameter was 12.8 µm. Just prior to force measurements, the alumina colloid probe the c-sapphire substrate (As One Corporation, Japan), and the SPM liquid cells were UV-plasma (PL16-110D-Sen Light Corp., Japan) treated to remove surface organics and make the surface hydrophilic. KCl was used to adjust the strength of the electrolyte solution. KOH was used to adjust the pH of the electrolyte.
Fig. 1. Optical microscope image of colloid probe used in the force measurement experiments.
2.3. Friction force measurements The same SPM was used to measure lateral friction forces over a trace distance of 500 nm. Lateral friction curves were measured on trace and retrace, thus forming a loop (torsion of the tip during a forward/reverse scan). The loops obtained were used to calculate the average lateral friction forces between two surfaces. The torsional signal on trace and retrace were added and divided by 2 to calculate the average lateral friction signal, which is then converted to force using cantilever torsional spring constant, FL = kL y (where FL is the force, kL is the cantilever spring constant, and y is the torsion of the cantilever measured in distance) [19]. Since the same cantilever was used to do lateral friction force measurements, the SPM cantilever was not characterized for torsional spring constant. It was calculated from normal spring constant using kL = 2kN l 2 /(3(1 + ν)), where l is the cantilever length and ν is the poison ratio of the cantilever which is taken as 0.27 [19]. 3. Results and discussion 3.1. Force curves on approach
2.2. Normal force measurements A scanning probe microscope (SPA 400, Seiko, Japan) with a liquid cell was used. Fig. 1 shows the colloid probe used in the force measurement experiments. What is actually measured in the SPM is the deflection signal from the cantilever as a function of separation distance. The deflection signal is then converted to force (FN ) vs distance (D) curves using cantilever stiffness, FN = kN x (where FN is the force, kN is the cantilever spring constant, and x is the normal deflection of the cantilever measured as a distance). The cantilever stiffness was measured following the protocol of Sader et al. [17]. The α-alumina colloid probe surface and c-sapphire surface were equilibrated in the supernatant for 30 min before any measurement. During force measurements, the approach and retraction speeds were kept constant at 20 nm/s, so that repulsive hydrodynamic forces could be neglected [18]. The total piezo displacement was 100 nm.
The measured forces between the α-alumina probe and the c-sapphire surface in 10−3 –100 M KCl electrolyte solution at pH 11 are shown in Fig. 2. It is more convenient to express the interaction as normalized force curves (F /2πR) because the area over which the interaction takes place affects the force [20]. It is easier to compare force curves when they are normalized by the radius of the probe. From Fig. 2, it is clear that there were no surface forces between the studied surfaces when the distance was large enough. Only when the separation distance was less than ∼20 nm did a strong dispersion force emerge. The shown normal force–distance curves were averages of five measurements obtained using the same colloid probe at different spots on the c-sapphire surface. Special attention was given to the flatness of the c-sapphire surface and sphericity of the α-alumina particles selected. Furthermore, the same colloid probe was utilized in normal and lateral force measure-
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Fig. 2. Normalized force (FN ) vs distance (H ) curves between 12.8-µm α-alumina particle glued probe and c-sapphire surface in 10−3 –100 M KCl electrolyte solutions at pH 11.
ments to minimize the effect of particle surface imperfections on the reproducibility of the force–distance curves and friction loops. The individual force–distance curves thus obtained were reproducible (data not shown) in terms of start of the interaction separation and forces were repulsive at all separations. Furthermore, the friction loops obtained at different electrolyte concentrations could be quantitatively comparable with each other, since the same α-alumina particle and cantilever were used. The interaction distance of colloidal particles on approach strongly depends on the electrolyte concentration, as seen from Fig. 2. As the electrolyte concentration increased from 10−3 to 100 M the repulsive interaction distance decreased from ∼20 to ∼7 nm. In this study an indifferent electrolyte, namely KCl, was chosen to adjust the Debye length. K ions are relatively large (0.133 nm) [21] and it is considered a structure-breaking ion instead of a structure-making one. The H2 O molecules are weakly bonded to the K cation compared to the small alkali elements such as the Li cation [6,7]. On normal approach repulsive forces dominated the attractive van der Waals forces, either due to the hydration layer or to the gel layer, as reported previously [2,5,13]. Due to the imperfections on the surfaces, it is very difficult to attribute the reason just to the hydration layer between alumina surfaces. No matter what the electrolyte concentration was, the normal forces were always repulsive. The electrostatic interaction force between the probe tip and sapphire surface was modeled using a linearized form of the Poisson–Boltzmann equation, Eqs. (3) and (4), for a symmetric 1:1 electrolyte, in which diffuse layer potential and Debye lengths were used as a fitting parameter. The model used to fit experimental FN distance curves for constant surface charge condition was −2κH 2Ψ1 Ψ2 e−κH FN 2 e + (Ψ − Ψ ) = εε0 κ 1 2 2πR 1 − e−κH 1 − e−2κH
(3)
and
−2κH 2Ψ1 Ψ2 e−κH FN 2 e − (Ψ + Ψ ) = εε0 κ , 1 2 2πR 1 − e−κH 1 − e−2κH
(4)
for constant surface potential condition, where Ψ1 and Ψ2 are surface potentials of the alumina probe and sapphire surface, respectively, R is the probe radius, and H is the separation distance [22]. The retarded van der Waals interaction force (FvdW ) [1] was calculated using an expression for the retarded interaction force, Eqs. (5) and (6), A131 FvdW , =− 2πR 6H 2 5.1 × 10−20 , A131 = (1 + [H /11.82]1.0276 )1/1.0276
(5) (6)
where A131 is the retarded Hamaker constant [23]. The simplifications due to the Derjaguin approximation for sphere–plate geometry allowed us to consider the interaction between two plate geometries [24]. Under all conditions, the separation distances between the sphere and the plate surface were much smaller then the colloid probe radius, satisfying the conditions for the Derjaguin approximation, namely H R. The results in Fig. 3 suggest the DLVO theory describes the interaction forces between surfaces immersed in simple symmetric electrolytes (<0.1 M concentration) at separation distances higher than ∼4 nm. In all electrolyte concentrations a strongly repulsive double-layer force was observed. As the electrolyte concentration was increased, the forces were again totally repulsive but lower in magnitude (Fig. 2). The observed forces were fitted with the constant-charge model described in Eq. (3) and are given in Table 2 along with the fitted Debye lengths. The constants, potential fit models were not shown because the data fits were not satisfactory. The observed Debye lengths were found to be in reasonable agreement with those calculated for the electrolyte level. The isoelectric points of the α-alumina particle and c-sapphire have been reported as pH ∼ 9
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Fig. 3. Logarithm of normalized force (FN ) vs separation distance (H ) curves between 12.8-µm α-alumina particle glued probe and c-sapphire surface in 10−3 –100 M KCl electrolyte solutions at pH 11. Solid lines are DLVO fit to the same data using the constant-charge boundary condition.
Fig. 4. Individual force–distance curves both on approach and retraction in 1 M KCl electrolyte solution at pH 11. Adhesion between two surfaces was measured on retraction. Table 2 Constant-charge boundary condition fitting parameters obtained from fits to interaction force–distance data between an α-alumina particle and a flat c-sapphire substrate immersed in KCl electrolyte solution and calculated Debye lengths from electrolyte concentrations Electrolyte concentration (M)
Ψ (mV)
κ −1 (nm, from fit)
κ −1 (nm, calculated)
10−3 10−2 10−1
−35 −35 −35
9 3 2
9.6 3.04 1
and ∼5, respectively, the reason for which was attributed to the presence of different types of surface hydroxyl groups on the two different types of surfaces [25]. Therefore, at pH 11 both surfaces are negatively charged. The fitted surface potential was found to be −35 mV, confirming that K is not specifically adsorbing onto the alumina surface. The Debye lengths were found to decrease from 9 to 2 nm as the electrolyte concentration increased, as expected.
It was Hunter who first quantitatively related interparticle forces and rheological properties of colloidal dispersions [26]. Based on the force–distance curves shown in Fig. 2 and using DLVO theory, it is not possible to give an account of the rheological behavior of alumina suspensions as being only electrostatically stabilized. Because 25 vol% α-alumina suspensions at various electrolyte concentrations showed yield stress and shear thinning behavior that are not consistent with the repulsive forces observed at all separations in the AFM study [8]. In rheology, the yield stress and shear-thinning behavior of a suspension are explained by weakly attracting particle interactions. In Fig. 4, individual force–distance curves both on approach and on retraction in 1 M KCl electrolyte solution are shown. The force curve on approach is repulsive, as already stated. However, giving a closer look to the retraction curve, the presence of adhesion between the α-alumina particle and c-sapphire is a clear indication of attractive van der Waals forces. That is, forces that are repulsive on approach do not necessarily mean that the particles stay dispersed in the slurry at all times. At con-
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Fig. 5. Friction loops measured at four different KCl electrolyte concentrations at applied normal force 250 nN over a trace distance of 500 nm.
tact the α-particle adheres to the c-sapphire surface. Adhesion forces, as well known by everybody, scatter a lot, due to the measurement conditions, surface roughness, contact area and time, etc. Thus it is very difficult to draw conclusions on rheological properties of alumina suspensions based on adhesion forces. Similar force–distance curves were obtained at different electrolyte concentrations. The macroscopic properties of alumina suspensions are under study.
Table 3 Exponential decay lengths (λ1 ) and pre-exponential constants obtained (C1 ) from the force–distance curves fitted to the empirical double exponential function Electrolyte concentration (M)
C1 (mN/m)
λ1 (nm, from fit)
10−3 10−2 10−1 100
0.15 0.24 0.31 0.31
1.1 1.1 1.1 1.1
3.2. Short range forces Although the classical DLVO theory fits the measured force– distance curves very well down to separations of ∼4 nm, the discrepancy between the theory and the experimental data becomes more and more significant at very close separations. Since the classic DLVO theory considers only two components, i.e., van der Waals force and electrostatic force, this discrepancy may come from another force that exists only at short range. Since no surfactant but electrolyte was added to the system, this extra force may be a hydration force. Having this in mind, the experimental data were fitted to the extended DLVO theory by including a third force component, hydration force, into the classical DLVO theory. Table 3 gives the exponential decay lengths (λ1 ) and preexponential constants obtained (C1 ) from the force–distance curves fitted to the empirical double exponential function [1]: H H + C2 exp − . FH = C1 exp − (7) λ1 λ2 Since only short-distance structural force is assumed, the second terms were neglected. The fits to the curves were better than R 2 > 0.97. The exponential decay lengths (λ1 ) did not change with ionic strengths and stayed constant as expected, whereas the preexponential factor (C1 ) calculated as 0.15 mN/m in 10−3 M
KCl electrolyte solution increased and saturated at 0.31 mN/m in 10−1 M one. The pre-exponential constants given in Table 3 for the 10−1 and 100 M electrolyte cases did not differ much. The reason was that the force–distance curves for the two cases were almost identical, as can be seen from Fig. 2. Actually, the short-range exponential decay lengths compare well with the ones given in Ref. [27], in which electrolyte-species-dependent hydration forces between silica surfaces were reported. It is hard to make the same comment about the repulsion parameters, which do not compare well with the values mentioned in the same reference. The discrepancy might be related to the nature of the hydration layer and/or the surface structure between silica and alumina. 3.3. Friction forces In this study, shear interaction between the α-alumina particle and the c-sapphire surface (0001) were investigated under the same conditions as stated above, namely between 10−3 and 100 M KCl electrolyte concentrations at pH 11. Fig. 5 shows that the friction loops obtained at 250 nN applied normal force at four different electrolyte concentrations. The friction loops were smooth over a trace distance of 500 nm; they were analyzed as described in Section 3.1. The observed asymmetry
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Fig. 6. The lateral friction (FL ) vs applied normal force (FN ) curves between a 12.8-µm α-alumina particle attached probe and a c-sapphire surface in 10−3 –100 M KCl electrolyte solutions at pH 11. Solid lines are linear fit to the data point (Amantons law). Table 4 Friction coefficients calculated from the slopes of the curves given in Fig. 6
4. Conclusions
Electrolyte concentration (M)
μeff
10−3 10−2 10−1 100
0.54 0.49 0.36 0.27
Normal and lateral friction forces between an α-alumina colloid particle and a flat c-sapphire surface in KCl electrolyte solution at pH 11 were measured using a colloid probe technique. Repulsive forces were observed that were well described by DLVO theory at all separation distances greater than 4 nm. A surface potential of −35 mV and a Debye length of 9 nm were calculated for the 10−3 M KCl solution. The calculated Debye lengths were in very good agreement with the data observed from the linear Poisson–Boltzmann equation. An exponential force law with the second term omitted was used to fit the hydration force. The fitted short decay length (λ1 ) for the hydration force was estimated to be 1.1 nm. Analysis of friction behavior was done assuming that Amantons law was obeyed. Solid surfaces sliding over each other across aqueous electrolyte solutions exhibited different behavior depending on the degree of population of the interface with hydrated ions. Concentrated KCl electrolyte solutions exhibited a low friction force resulting in a small friction coefficient compared to the dilute ones.
in the lateral friction loops indicates that there was a contribution to the friction force from surface roughness [19]. In order to eliminate the orientation dependence of friction force, all measurements were averaged in both scan directions. The lateral friction (FL ) vs applied normal (FN ) force curves at four different electrolyte concentrations are given in Fig. 6. The friction behavior was analyzed assuming that the friction force between two surfaces was proportional to the normal force. At a fixed velocity, lateral force increases with applied load in a linear fashion, obeying the Amantons law. The linear nature of friction curves was an indication of the presence of multiple asphericity. The microscale friction coefficient between alumina surfaces by the lateral frictional curve measurement obtained by fitting the Amantons law to the friction force vs normal force data. An effective friction coefficient μeff is defined as μeff =
FL . FN
(8)
Table 4 shows the friction coefficients calculated from the slopes of the curves given in Fig. 6. At 10−3 M KCl electrolyte concentration the effective friction coefficient was calculated as 0.54, whereas at 100 M KCl the friction coefficient was 0.27. The friction curves at higher electrolyte concentration showed better fits to the Amantons law. A significant lubrication effect was clear for solutions of high electrolyte concentration, which is consistent with the data given in [14]. Similar work has been done by Higashitani’s group for silica surfaces, in which the lubrication effect of hydrated cations was studied [28,29].
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