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Probing Anisotropic Surface Properties and Interaction Forces of Chrysotile Rods by Atomic Force Microscopy and Rheology Dingzheng Yang, Lei Xie, Erin Bobicki, Zhenghe Xu, qingxia liu, and Hongbo Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5019373 • Publication Date (Web): 12 Aug 2014 Downloaded from http://pubs.acs.org on August 12, 2014

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TOC Graphic Probing Anisotropic Surface Properties and Interaction Forces of Chrysotile Rods by Atomic Force Microscopy and Rheology Dingzheng Yang, Lei Xie, Erin Bobicki, Zhenghe Xu, Qingxia Liu*, and Hongbo Zeng*

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Probing Anisotropic Surface Properties and Interaction Forces of Chrysotile Rods by Atomic Force Microscopy and Rheology Dingzheng Yang, Lei Xie, Erin Bobicki, Zhenghe Xu, Qingxia Liu*, and Hongbo Zeng* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 2V4, Canada *Corresponding authors: [email protected], [email protected]

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Abstract Understanding the surface properties and interactions of non-spherical particles is of both fundamental and practical importance in rheology of complex fluids in various engineering applications. In this work, natural chrysotile, a phyllosilicate composed of 1:1 stacked silica and brucite layers which coil into cylindrical structure, was chosen as a model rod-shape particle. The interactions of chrysotile brucite-like basal or bilayered edge planes and a silicon nitride tip were measured using an atomic force microscope (AFM). The force-distance profiles were fitted using the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which demonstrates anisotropic and pH-dependent surface charge properties of brucite-like basal plane and bilayered edge surface. The points of zero charge (PZC) of the basal and edge planes were estimated to be around pH 10-11 and pH 6-7, respectively. Rheology measurements of 7 vol% chrysotile (with aspect ratio of 14.5) in 10 mM NaCl solution showed pH-dependent yield stress with a local maximum around pH 7-9, which falls between the two PZC values of the edge and basal planes of the rod particles. Based on the surface potentials of the edge and basal planes obtained from AFM measurements, the theoretical analysis of the surface interactions of edge-edge, basal-edge and basal-basal planes of the chrysotile rods suggests the yield stress maximum observed could be mainly attributed to the basal-edge attractions. Our results indicate that the anisotropic surface properties (e.g. charges) of chrysotile rods play an important role in the particle-particle interaction and rheological behaviors, which also provides an insight into the basic understanding of the colloidal interactions and rheology of non-spherical particles. Keywords: suspension rheology, AFM, surface forces, phyllosilicate, chrysotile, anisotropy, nonspherical particle

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Introduction The surface properties and interactions of non-spherical particles have attracted much attentions over the past decades in the fields of colloid science, chemical engineering, bioengineering and mineral engineering.1-8 The anisotropically-charged non-spherical phyllosilicate mineral particles can arrange themselves into various conformations in aqueous media to form various microstructures (i.e. arrangement of particles in the suspensions).9-11 The microstructure development of these non-spherical particles leads to complex rheological phenomena (e.g. yield stress and elasticity).2,12-14 Given the volume fraction and geometry of the particles in the suspension, the microstructure and resulting rheology could be tuned through modulating the water chemistry (i.e. pH, electrolytes and polymers).15-21 For aqueous suspensions of homogeneously charged spherical particles, the interactions of these particles are mainly dominated by the Derjaguin-Landau-Verwey-Overbeek (DLVO) forces and the yield stress of the suspensions are generally pH-dependent, indicating a pH-dependent microstructured particle network which can be controlled through modulating the attractive and repulsive interactions among the particles.22 The maximum yield stress has been generally reported at the PZC of the homogeneously charged spherical particles due to their weakest repulsion (or strongest interparticle attractions) at the PZC.15,22 In contrast, for non-spherical particles (e.g. clays), more complex microstructures such as “house of cards” structures (formed through oppositely charged basal and edge planes) could be formed and were reported.9 High yield stress was observed along with the formation of such a microstructure over a range of pH values, which were reported not to align with the average PZC of anisotropic particles.23-24 The surface force measurements using atomic force microscope on both basal and edge surfaces have been reported to study anisotropic surface properties of several minerals including muscovite25, talc26, and chlorite27. 2 ACS Paragon Plus Environment

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Anisotropically-charged rod-like chrysotile was chosen as a model non-spherical particle in this work to investigate its surface properties, interaction forces and how these surface interactions could be correlated to the rheological behaviors. Chrysotile (Mg3Si2O5(OH)4) is a phyllosilicate belonging to the serpentine group of minerals which are major gangue minerals in ultramafic nickel ores.28 Due to its rod-like shape and heterogeneous surface charges, chrysotile increases the yield stress and viscosity of the suspension, and is deleterious to nickel recovery in the processing of ultramafic ores.29 Chrysotile is composed of 1:1 tetrahedral silica and octahedral magnesium hydroxide (or brucite) sheets.30 A natural misfit between the silica and brucite layers results in the curling of the bilayer to form a cylindrical structure with the brucite layer exposed on the outside,31 as shown in Figure 1 (a). Due to the coiled structure of chrysotile, only two surfaces may be exposed in solution: the brucite-like basal plane and the bilayered edge surface. The two surfaces are charged by different mechanisms. Alvarez-Silva et al.32 proposed that the brucite-like basal plane was either electrically neutral or charged due to the vacancies and substitution in the crystal lattice which is expected to be pH-independent. However, a recent study by Yin et al.27 reported pH-dependent surface charges of the chlorite brucite-like basal plane. Ndlovu et al. proposed a pH-dependent brucite-like basal plane for chrysotile based on the correlation between the shear yield stress and the average PZC.33 Nevertheless, the charging mechanism of the chrysotile brucite-like basal plane is still not fully understood.34-35 In contrast, as similar to most phyllosilicate mineral surfaces, the edge surface of chrysotile shows pHdependent surface charge properties due to the protonation and deprotonation of the broken bonds of silica and brucite bilayers.9 Despite the average PZC of chrysotile was reported to be ~pH 8.2 using potentiometric method,33 the understanding of the surface properties of the heterogeneous basal and edge planes of chrysotile and the surface interactions still remain

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unclear. The anisotropic surface properties (e.g. charges) of chrysotile rods are needed to understand the rheological behaviours and interparticle interaction of chrysotile suspensions.

Figure 1 Schematic and crystalline structures of chrysotile sample. (a) Schematic of the two layered structure and morphology of chrysotile; (b) X-ray diffraction (XRD) pattern of the chrysotile rods. C refers to chrysotile (Mg3Si2O5(OH)4) while Ca refers to calcite (CaCO3). In this paper, AFM force measurements and rheological measurements were applied to investigate the surface properties and interactions of chrysotile rods. The surface potentials of the chrysotile brucite-like basal plane and edge surface were determined by fitting the force-distance profiles from AFM measurements using the classical DLVO model. The shear yield stress of a 4 ACS Paragon Plus Environment

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well-defined chrysotile suspension was measured under various pH conditions, in which the volume fraction, aspect ratio and particle length of chrysotile rods and the type and concentration of electrolytes were all fixed. The fitted surface potentials were used to determine the surface interaction energy between the basal and edge surfaces of chrysotile rods, which was applied to correlate the rheological behaviors and possible microstructures of the suspension.

Materials and Methods Materials Chrysotile was received from Minerals Unlimited (Ridgecrest, CA) and used without further purification. X-ray diffraction (XRD) analysis of the raw sample revealed it to be primarily composed of chrysotile with minor calcite (Figure 1b). To obtain the concentrated aqueous suspensions for rheological measurements, the received chrysotile was wet ground at 1000 rpm for one hour in the ball mill which was facilitated in an asbestos hood. The ball mill discharge was washed with Milli-Q water (Millipore Inc.) three times to remove excess ions. The washed suspensions were then condensed into the desired concentration for rheological tests. 10 mM ACS analytical grade NaCl (Sigma-Aldrich) was used as the background electrolyte for both AFM and rheological measurements. Analytical grade HCl and NaOH (Sigma-Aldrich) were used to adjust solution pH values. Milli-Q water with a resistivity of 18.2 MΩcm-1 was used in this study. X-ray Fluorescence (XRF) Spectroscopy X-ray fluorescence (XRF) spectroscopy (Orbis PC Micro-EDXRF Elemental Analyzer, EDAX, Mahwah, NJ, USA) equipped with a microprobe operated at 40 kV and 250 µA and x-ray source 5 ACS Paragon Plus Environment

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of monochromatic Rh Kα radiation was used to determine the elemental composition of the chrysotile sample. The sample was pelletized prior to XRF analysis. Field Emission Scanning Electron Microscope (FE-SEM) The dimensional analysis on the AFM silicon nitride tip (Bruker, Camarillo, CA, USA) was carried out using JAMP-9500F Auger Scanning Election Microprobe (JEOL) at the Alberta Centre for Surface Engineering and Science, University of Alberta. The instrument is equipped with Shottky field emitter that produces electron probe diameter of about 3 to 8 nm on the sample. Based on Figure 2, the height of the tip and the diameter of the spherical cap of the silicon nitride AFM tip are obtained to be about 6.2 µm and 40.8 nm. The angle of the cone is about 52.4°.

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Figure 2 Field Emission Scanning Electron Microscope (FE-SEM) for the silicon nitride AFM tip. (a) Height of the tip; (b) enlarged spherical apex. The dimensions of the ground chrysotile particles used in the rheology study were also determined by analyzing SEM images. To obtain the images, the chrysotile suspension was diluted with Milli-Q water and then ultrasonicated (30% power, Sonic Dismembrator Model 500, Fisher) for 20 seconds to disperse the particles well. A drop of the diluted and dispersed particle suspension was placed on the SEM sample holder and allowed to dry in the laminar hood for 12 hrs. Images of the prepared sample were then captured by SEM and a representative image was shown in Figure 3 (a). ImageJ (NIH) software was used to analyze the SEM images, and about 7 ACS Paragon Plus Environment

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1000 particles were measured to get statistically reliable particle dimensions (diameter and rod length). The length of the rods followed a normal distribution and the fitted average length was 825±30 nm. The average diameter of the rods, 2R (R is radius of edge surface of the rod), was 57±20 nm. Therefore, the average aspect ratio of the rods was 14.5.

Figure 3 Particle size distribution of the ground chrysotile rods for rheological measurements (a) Micrograph; (b) particle length distribution. Rheology

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A TA DHR-2 stress-controlled rheometer (TA Instruments, DE) equipped with a Peltier plate was employed for the rheological studies. The concentric cylinder geometry was chosen to minimize the effect of sedimentation of the particles. The surfaces of the geometry were sand blasted (roughness ~84 µm, much larger than the particle size) in order to prevent the wall slip.2 The volume fraction of chrysotile used was 0.07 (16.6 wt%). Such volume fraction was refined and finally selected because the generated network was strong enough to hold the particles from settling over 12 hours. 0.01 M NaCl was used as the background electrolyte. The concentrated particulate suspension was ultrasonicated using a Sonic Dismembrator (Model 500, Fisher) at 30% of the total power for 45 seconds. The stress growth, or stress overshoot method, was used to obtain the shear yield stress by controlling the strain rate.2 A shear strain rate of 0.01 s-1 was used for the tests. Since hydrodynamic and thixotropic effects were not observed at this shear strain rate, it was deemed to be optimal.36 All the samples were pre-sheared for 120 s at 1000 s-1 followed by 120 s rest to rearrange the microstructures. The temperature used in this study was 25 °C. Surface Preparation and Ultramicrotome Cutting Due to the rod-like shape, the orientation of a bundle of chrysotile with centimeter length and millimeter width could be visualized by bare eyes. To generate fresh basal and edge planes, the ultramicrotome cutting technique described by Zhao et al.25 was employed. Firstly, the bundle of chrysotile was embedded into epoxy resin, which was then allowed to solidify over 24 hours. Secondly, the chrysotile bundle enclosed in resin was loaded onto the ultramicrotome sample holder. It was important that the orientation of the chrysotile bundle was as perpendicular or parallel as possible to the cutting direction to expose the edge and brucite-like basal plane,

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respectively. Thirdly, a rough trimming (EM UC 7, Leica Microsystems Inc.) was conducted using a regular diamond knife to expose the target surfaces of the chrysotile sample. The resin around the sample was removed as much as possible to generate a more uniform hardness across the cutting surface. A final delicate cutting with a 45˚ diamond knife (Diatome AG, Biel, Switzerland) was conducted using an ultramicrotome (EM UC 7, Leica Microsystems Inc.) with a low cutting speed. After cutting, to ensure the brucite-like basal plane was exposed, the entire top layer of the embedded chrysotile bundle was peeled off using the sticky tape (3M) to remove all the possibly fractured chrysotile. Samples prepared for analysis of the edge surface were not peeled. The finished sample blocks were glued into a liquid cell with the basal or edge plane of the chrysotile facing upwards and parallel to the base of the substrate. Both basal and edge surfaces were subjected to high pressure nitrogen gas to remove any fine particles or fractures. The prepared surfaces were then rinsed with 0.01 M hydrochloric acid (Sigma-Aldrich), followed by Milli-Q water and ethanol (Sigma-Aldrich) and finally dried with highly purified nitrogen gas (Praxair) prior to use. AFM Imaging and Force Measurements Imaging and surface force measurements on the chrysotile mineral surfaces were carried out in 10 mM NaCl aqueous solutions at various pH values using an Asylum MFP-3D Atomic Force Microscope (Oxford Instrument Company, UK). The topographic imaging of the chrysotile brucite-like basal plane was conducted using a silicon nitride AFM probe (Bruker, Camarillo, CA, USA) in contact mode (Figure 4a-b). After a topographic image was obtained, the highest points along the axial direction of the cylindrical chrysotile basal plane were selected, and the AFM tip was directed to those points. Force-distance profiles were subsequently obtained at the specified locations by approaching and retracting the AFM tip at a spring constant of the 10 ACS Paragon Plus Environment

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cantilever about 0.12 N/m. The constant compliance region was usually designated as contact or alternatively the zero of probe-sample separation. For each pH condition, more than 100 measurements were carried out in total at various locations on 3 to 5 fresh sample surfaces to confirm the reproducibility of the data. The topography of chrysotile edge surfaces was also imaged using contact mode (Figure 4c). After the topographic image was obtained, the AFM tip was driven to the most protruding rods for force measurements. The edge of a single rod had to be isolated for force measurement in order to neglect surface forces arising from neighbouring rods. The topographic heights along the 4 different lines shown in Figure 4 (c) were depicted in Figure 4 (d). The roughness of the areas over which force measurements were conducted (most protruding rods) was shown in the inserted graphs. The roughness of these areas is shown to be less than 1 nm.

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Figure 4 AFM images of the chrysotile and the locations used for the force measurements. (a) 2D image of the basal plane; (b) 3-D image of the basal plane; (c) 2-D image of the edge plane; (d) the local roughness of the edge plane. Fitting of Force Curves of AFM Tip and Chrysotile Basal and Edge Planes The pyramid-shaped AFM silicon nitride tip can be considered as the combination of a conical body capped with a spherical top end. The DLVO forces between the tip and an infinite flat substrate have been summarized by Drelich et al.37 In the present case, the chrysotile basal plane is considered as an infinite flat plate. On the other hand, the chrysotile edge surface is considered as a finite flat plate as its diameter (2 R = 57 nm) is comparable with the diameter (2 RT =40.8 nm) of the spherical region of the tip. To determine the van der Waals forces between the tip and 12 ACS Paragon Plus Environment

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the edge surface, the interaction energy between a molecule on the tip and the chrysotile rod with the edge plane oriented perpendicular to the normal force direction was calculated first. Subsequently, the interaction energy between the whole AFM tip and the edge surface were calculated. Finally, van der Waals forces between the tip and the edge surface were obtained by differentiating the calculated interaction energy with respect to the separation distance. Van der Waals force between the AFM tip and the basal plane was obtained using the same method. Detailed derivations are shown in the Supporting Information. Investigation of electrical double layer forces between a nano-sphere and an infinite flat plate by Bhattacharjee et al.38 demonstrated that the Derjaguin Approximation yielded reasonable results when the concentration of the 1:1 electrolyte was higher than 10 mM. Thus, the Derjaguin Approximation was used in this study to determine the interaction force between the tip and different chrysotile surfaces. Because we were more interested in the surface potential, the method derived by Hogg et al.39 for determining electrical double layer forces between two infinite asymmetric plates at the boundary condition of constant potential was used for further integration throughout different geometries of two bodies. Total DLVO force (F) between AFM pyramid-shaped tip and chrysotile basal plane as the function of distance (D) is expressed as DLVO −S −C EDL − S EDL −C F ( D ) TB = F ( D )VDW + F ( D )VDW + F ( D ) TB + F ( D ) TB TB TB

(1)

where the subscripts T and B denote tip and basal plane respectively; the superscript S and C denote the spherical and conical regions of the tip; the superscript VDW and EDL represent van der Waals and electrical double layer forces. Total DLVO force between AFM pyramid-shaped tip and chrysotile edge surface is expressed as

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DLVO −S −C EDL − S EDL −C F ( D ) TE = F ( D )VDW + F ( D )VDW + F ( D ) TE + F ( D ) TE TE TE

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(2)

where subscript E denotes the chrysotile basal plane. Details are shown in the Supporting Information. MATLAB (Math Works Inc.) was used for mathematical calculation of all the equations and for fittings of AFM force curves.

Interaction Energy of Different Chrysotile Surfaces van der Waals (VDW) Interaction Energy According to the interaction energy between a molecule and infinite flat plate at a distance D, as shown in Eqn. (S3), the interaction energy of two flat edge planes of chrysotile with finite surface areas of πR 2 can be expressed as

W ( D )VDW E −E = −

AR 2 1 D D (2 arctan − π )] [ 2− 3 4 3D R 2R

(3)

where A is Hamaker constant and R is the radius of the edge surface of the chrysotile rod. The interaction energy between the spherical flat edge surface and cylindrical basal plane at the interacting length of 2R can be estimated as follows: 1

W ( D)VDW B−E = −

AR R 6 2D3 / 2

(4)

The van der Waals interaction energy between two parallel cylindrical basal planes at the interacting length of 2R is estimated as: 1

W ( D)VDW B−B = −

AR R 12 D 3 / 2

(5)

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Electrical Double Layer (EDL) Interaction Energy The EDL interaction energy of edge-edge planes for the constant electrical potential case over the area of πR 2 can be obtained using the equation derived by Israelachvili1 for infinite symmetric flat plates:

2 2 W ( D ) EDL E − E = 2πR ε 0 εκψ E

1 − e −κD e κD −e −κD

(6)

where ε 0 and ε are the dielectric constant in the vacuum and the dielectric constant respectively;

κ is the inverse of the Debye length; and ψ is the surface potential. The basal-edge interaction energy for the constant electrical potential case at the interaction length of 2R is: 40

2 W ( D) EDL B − E = 2 πκ R ε 0 ε exp(−κD )[2 2ψ Bψ E −

ψ B2 D+R

exp(−κD) −

ψ E2 R

exp(−κD )]

(7)

The interaction energy between the two parallel cylindrical basal planes for the constant electrical potential case at the length of 2R is: 40

R exp(−2κD)] D+R D + 2R

[ 2 exp(−κD) − 2 2 W ( D) EDL B − B = 4 πκ ε 0 εψ B R

(8)

The total DLVO interaction energy can be expressed as:

W ( D) DLVO = W ( D)VDW + W ( D) EDL

(9)

Results and Discussion AFM Force Curves and Fitted Surface Potentials

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In this section, the AFM force curves and their fittings using the DLVO theory are presented. The measured force curves between the silicon nitride AFM tip and the chrysotile brucite-like basal plane at pH 4, 6, 8 and 11 are shown in Figure 5. 3-5 representative force curves among more than 100 measurements are shown for each pH value and good reproducibility is observed. In the system studied, both van der Waals and electrostatic forces are important during the AFM force measurements beyond the hydration layer.1 The surface force between the tip and brucitelike basal plane is attractive at pH 4 at a separation distance larger than 2-3 nm, below which the repulsive hydration force dominates. Deviation of the force curve from the zero force grid line −1 starts at 10 to15 nm, which is equivalent to 4 to 5 Debye lengths in 10 mM NaCl solution ( κ theory

= 3.0 nm). Hence, there is strong evidence of electrostatic attraction between the tip and brucitelike basal plane at this pH as van der Waals attraction is normally observed at a shorter distance (~5 nm). Electrostatic attractions are observed at both pH 6 and 8. At pH 11, very weak electrostatic repulsion is observed, showing that electrical double layer repulsive force dominates over van der Waals force.

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Figure 5 AFM force curves measured between silicon nitride tip and the chrysotile brucite-like basal plane in 10 mM NaCl solution at (a) pH 4, (b) pH 6, (c) pH 8 and (d) pH 11. (The solid lines are fittings using the DLVO theory.) DLVO theory was employed to fit the experimental force curves between the AFM tip and the chrysotile brucite-like basal plane. The surface potential of the silicon nitride AFM tip was calibrated by measuring the force-distance profiles between the tip and a silica wafer under the same solution conditions (as shown in Figure S1), and listed in the legends of Figure 5. As the curvature of the basal plane (~5 µm in Figure 4b) is much larger than that of the spherical cap of the AFM tip (~40.8 nm in Figure 2b), the basal plane of chrysotile was treated as an infinite flat plate. The theoretical fittings agree reasonably with the experimental AFM force-distance measurements with the R2 ranging from 0.91 to 0.93 (Figure S2). The discrepancy at very short distances (< 2-3 nm) could be due to the other interactions such as hydration forces between the tip and chrysotile surfaces. The fitted Debye lengths agree well with the theoretical values 17 ACS Paragon Plus Environment

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calculated using the ionic strength.1 The solid lines in Figure 5 represent the theoretical predictions using the average surface potential (listed in the legend with the error) of all the measured force curves for each experimental condition. The fitted surface potentials of chrysotile show opposite signs as that of AFM tip at pH 4, 6 and 8, which is consistent with the electrostatic attractions observed. The slightly repulsive force observed at pH 11 also shows consistency with the fitted slightly negative surface potential.

Figure 6 AFM force curves measured between silicon nitride tip and the chrysotile edge plane in 10 mM NaCl solution at (a) pH 4, (b) pH 6, (c) pH 8 and (d) pH 11. (The solid lines are fittings using the DLVO theory.) AFM force curves and fittings by the DLVO theory (Eqn. 2) on the chrysotile edge surface at pH 4, 6, 8 and 11 are shown in Figure 6. The surface force between the tip and the edge is pHdependent, indicating a pH-dependent surface potential for the edge surface. At pH 4 and 6, weak attractions between the tip and chrysotile edge surface are observed, and positive surface 18 ACS Paragon Plus Environment

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potentials of 36 mV and 5 mV are obtained through the theoretical fittings respectively. The surface force turns repulsive at pH 8 and the repulsion becomes stronger at pH 11 where a more negative surface potential was fitted. The fitted surface potentials of the chrysotile brucite-like basal and edge planes are shown in Figure 7. For the chrysotile brucite-like basal plane, the surface potential is observed to be pHdependent and to decrease from positive to negative values over the pH range from 4 to 11. The PZC of the brucite-like basal plane is roughly between pH 10 and 11. It is seen in Figure 7 that the surface potential of brucite-like basal plane is comparable to the zeta potential of brucite measured by Pokrovsky et al.41 at pH 9-11 and fitted surface potential of brucite-like layer of chlorite obtained by Yin et al. at pH 5.6-9. The pH-dependent brucite-like basal plane is most likely caused by the protonation and deprotonation of the brucite.27, 42 Analysis of the mineral sample by XRF confirmed the Mg:Si atomic ratio was only 1.25 compared to the ideal value of 1.5 (Table 1). Thus, there are a high number of vacancies in the octahedral layer of the chrysotile sample used in this study. It is likely that the vacancies provided more extra spaces for H+ or OHto hydrolyze the brucite.

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Figure 7 Fitted surface potentials of basal and edge planes of chrysotile, and the comparison with the surface potential of silicon dioxide24 (▲) and chloride brucite-like plane27 (◄) in 1 mM KCl, and zeta potential of brucite in 10 mM NaCl41 (▼) adapted from the literature (The continuous lines are used to guide the eyes). The surface potential of the chrysotile edge surface was also found to be pH-dependent with a PZC. The edge surface of the chrysotile consists of both octahedral magnesium and tetrahedral silica groups. Surface charge developed on the edge surface of phyllosilicate minerals is mainly caused by the protonation and deprotonation of the broken bonds. The PZC of silica is about 1.83.543, while that of brucite is about 11.41 Therefore, it is reasonable that the surface potential of the edge surface of chrysotile shows pH-dependent and that the magnitude is approximately the average of the surface potential of brucite and silica across the pH range studied (Figure 7). At pH 4, the dominant effect of protonated brucite layer over the silica layer leads to a total positive charge on the edge surface. With the increase of pH values, the fitted surface potentials decrease and turn from positive to negative at about pH 6 to 7. It is noted that at pH 11 a certain degree of selective dissolution of chrysotile might occur but it would not influence the determination of the edge PZC. The PZC of the chrysotile edge plane is a little lower than that observed for the talc edge surface (pH 8.1)26 and the chlorite edge surface (pH 8.5)27. While chrysotile consists of 1:1 silica and brucite layers, talc is a 2:1 clay mineral consisting of a brucite layer sandwiched between two silica layers26 and chlorite is a 2:1:1 clay mineral consisting of brucite or gibbsite sheets sandwiched between mica-like trilayers.24 Since the chrysotile edge surface should contain relatively more magnesium groups than either talc or chlorite, and since the PZC of brucite is higher than that of both silica and gibbsite,27 the chrysotile edge PZC should theoretically be

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higher than that of talc and chlorite. The lower PZC obtained for the chrysotile edge surface in this study could be attributed to the following several reasons.

Table 1 Elemental composition (major elements only) of chrysotile sample as determined by xray fluorescence (XRF) spectroscopy. Oxide

MgO

SiO2

CaO

Al2O3

Fe2O3

Abundance (wt %)

43.16

51.84

2.04

1.40

1.01

Firstly, as mentioned, while the ideal Mg:Si atomic ratio for chrysotile is 1.5 (as indicated by its chemical formula, Mg3Si2O5(OH)4), the Mg:Si atomic ratio of the chrysotile used in this study is 1.25 as shown from XRF results in Table 1. Although the PZC on the chrysotile edge cannot simply average that of brucite and silica, it is reasonable to suggest that the less amounts of magnesium in brucite layer can reduce the overall PZC on chrysotile edge surface. Secondly, isomorphic substitution of iron for magnesium in the chrysotile octahedral layer can further reduce the PZC of the chrysotile edge as iron hydroxides have a lower PZC values ranging from pH 6 to 8.44 FTIR spectra collected for the chrysotile sample suggests that iron substitution is present (Figure S3).45-47

Interaction Energy of Edge-Edge, Basal-Edge and Basal-Basal Planes The fitted surface potentials of basal and edge planes were further utilized to calculate the interaction energy between three pair surfaces, including edge-edge, basal-edge and basal-basal surfaces using Eqn. (9), as shown in Figure 8. In Figure 8 (a), it is seen that at pH 4 the electrical double layer force dominates the van der Waals force for chrysotile edge-edge interaction. With the solution pH increases to 8, the surface potential of chrysotile edge reverses from positive to 21 ACS Paragon Plus Environment

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negative as found in Figure 7. The lower magnitude of surface potential at pH 8 than pH 4 generates a lower repulsive force and energy barrier that brings the two edge surfaces into close proximity. Compared to pH 4 and 8, the repulsion and energy barrier become higher at pH 11 due to its higher magnitude of surface potential. The interaction energy between the brucite-like basal plane and bilayered edge surface is shown in Figure 8 (b). At pH 4, both basal and edge surfaces are strongly positively charged, resulting in a strong repulsion with high energy barrier. With the increase of solution pH to 8, the surface potential of chrysotile edge turns to negative while the surface potential of basal plane remains positive. Thus, a strong electrostatic attraction between the negatively charged edge surface and positively charged basal plane is observed at such a pH condition. At pH 11, although the surface potential of edge surface is -42 mV, since the basal plane is weakly negatively charged (-14 mV), weak repulsion and low energy barrier are observed. The brucite-like basal plane exhibits a decreasing surface potential with a PZC at pH 10-11 as shown in Figure 7. Therefore, from pH 4 to 8, the electrical double layer repulsions decrease but still dominate the van der Waals attractions, resulting in a decrease of total energy barrier. When further increasing the solution pH to 11 which is close to the PZC of the brucite-like basal plane, van der Waals force is more dominant compared to the electrical double layer force, leading to an attraction between the two parallel basal planes. The interaction energy of different surfaces reveals the preferred conformations of chrysotile pair surfaces. It can be seen that at pH 4, the interaction energy of all the three possible surface pairs is repulsive, which suggests any two pair tends to separate with each other. At pH 8, the edgeedge and basal-basal interactions are repulsive at the short range due to overlap of the electrical

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double layers, but basal-edge interactions are attractive. Therefore, to minimize the overall interaction energy of two chrysotile rods, brucite basal plane and bilayered edge surface will preferentially attract together. At pH 11, while the edge-edge and basal-edge interactions are repulsive, the basal-basal interactions are attractive. Two chrysotile rods will prefer to attract each other side by side.

Figure 8 Interaction energy of different chrysotile surfaces at various pH values. (a) Edge-edge planes, (b) basal-edge planes and (c) basal-basal planes.

Rheology and Microstructure The rheology of 7 vol% chrysotile suspension was studied at various pH conditions. In Figure 9 (a), the shear stress-time profiles are shown and used to obtain the shear yield stress (i.e., stress growth method).2 At the initial stage of shearing, the shear stress grows due to the elastic 23 ACS Paragon Plus Environment

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deformation of the fluid. Once the shear deformation exceeds a critical point, the fluid begins to yield. The local maximum shear stress is defined as the shear yield stress, as labelled on the stress growth curve at pH 4 in Figure 9 (a). Beyond the yield stress the shear stress decreases due to the breakdown of the original structured network. A steady state will eventually be reached as indicated by a plateau of shear stress. The stress “overshoot” peak was found to be apparent at pH 4. However, at pH 8 and 11, the shear stress increase with time and reach plateaux which are one order of magnitude higher than the shear stress plateau at pH 4. In Figure 9 (b), the shear yield stress of the chrysotile suspension is shown to be pH-dependent with the maximum yield stress occurring around pH 7-9, which falls between the PZC of chrysotile basal plane (pH 1011) and edge surface (pH 6-7). Based on the interaction energy of different surfaces shown in Figure 8 and the yield stress of the suspension shown in Figure 9 (b), 3-D microstructures of the chrysotile are proposed at various pH as shown in Figure 9 (c). At pH 4, the observed much lower yield stress than at pH 8 and 11 is probably attributed to a repulsive force driven system formed as shown in Figure 9 (c1). At pH 8, an attractive rod network is suggested to govern the relatively high yield stress compared to pH 4. As the basal and edge planes carry opposite charge at pH 7-9 as shown in Figure 7, it is reasonable to expect the maximum basal-edge DLVO force (attractive) over such a range of pH values. Therefore, it is very likely that a microstructure of 3-D basal-edge attractive network of chrysotile rods (Figure 9 c2) is responsible for the observed maximum yield stress at pH 7-9. Similar model, such as “house of cards” model, has been proposed by van Olphen9 on platy clay minerals. At pH 11, based on the interaction energy shown in Figure 8 it has been proposed that the two parallel basal planes prefer to attract together. It is reasonable to make a further suggestion that the basal-basal attraction can induce a band-like structure throughout the entire 24 ACS Paragon Plus Environment

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volume of the suspension, as shown in Figure 9 (c3). The attractive basal-basal interaction energy at pH 11 is smaller than that of basal-edge interaction energy at pH 8 as shown in Figure 8. Therefore, it takes more external stress to break down the basal-edge attractive conformation at pH 8 than the basal-basal attractive conformation at pH 11. This is consistent with the magnitude of the observed yield stress at the two pH values (Figure 9 b). The above results and discussion have demonstrated that the surface interactions of anisotropically-charged chrysotile rods can be tuned between attractive and repulsive by changing pH values, leading to a change of interactive microstructure network of the rods and the magnitude of yield stress of the suspension.

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Figure 9 pH dependent shear yield stress. (a) Stress growth curves; (b) yield stress at various pH values; (c) proposed microstructures at (c1) pH 4, (c2) pH 8 and (c3) pH 11. (The solid line in Figure b is used to guide the eyes.)

Conclusions The surface properties and interaction forces of anisotropically-charged chrysotile rods have been studied using atomic force microscopy and rheology. Surface potentials of chrysotile 26 ACS Paragon Plus Environment

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brucite-like basal plane and bilayered edge surface were obtained by fitting the AFM force curves using the classical DLVO theory, which shows pH-dependent with the PZC of the basal and edge planes around pH 10-11 and pH 6 -7, respectively. The fitted surface potentials were further applied to predict the interaction energy of chrysotile edge-edge, basal-edge and basalbasal planes, which suggests the preferred interaction configurations of different surfaces over a range of pH values. Rheology studies on 7 vol% chrysotile in 10 mM NaCl solution shows that the shear yield stress is pH-dependent with a local maximum around pH 7-9, which locates between PZC values of chrysotile basal and edge planes. By correlating the yield stress with surface interaction energy, various microstructures of chrysotile rod suspension were proposed to develop at different pH conditions, viz. from repulsion dominated network at pH 4, to basal-edge and basal-basal attraction dominated network at pH 8 and pH 11 respectively. Our results demonstrated that surface interactions and suspension rheology of non-spherical and anisotropic colloids can be manipulated by tuning pH conditions, providing an effective approach in engineering complex fluids.

Acknowledgement The authors acknowledge the financial support from Teck, Vale and NSERC (Natural Sciences and Engineering Research Council of Canada), Canada Foundation for Innovation and the Alberta Advanced Education & Technology Small Equipment Grants Program (AET/SEGP).

Supporting Information Available Further details on the Hamaker constant, AFM tip calibration, FTIR results and DLVO interaction calculations and fittings on force curves. This information is available free of charge via the Internet at http://pubs.acs.org/. 27 ACS Paragon Plus Environment

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TOC Graphic Probing Anisotropic Surface Properties and Interaction Forces of Chrysotile Rods by Atomic Force Microscopy and Rheology Dingzheng Yang, Lei Xie, Erin Bobicki, Zhenghe Xu, Qingxia Liu*, and Hongbo Zeng*

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