Probing Anisotropic Surface Properties of Illite by ... - ACS Publications

Subscriber access provided by ROBERT GORDON UNIVERSITY

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Probing Anisotropic Surface Properties of Illite by Atomic Force Microscopy HuaiZhi Shao, Jing CHang, Zhenzhen Lu, Binbin Luo, James Grundy, Guangyuan Xie, Zhenghe Xu, and Qingxia Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00270 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Probing Anisotropic Surface Properties of Illite by Atomic Force Microscopy Huaizhi Shao,† Jing Chang,‡ Zhenzhen Lu,‡ Binbin Luo,‡ James S. Grundy,‡ Guangyuan Xie,† Zhenghe Xu,‡ and Qingxia Liu∗,†,‡ †Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China ‡Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 1H9, Alberta, Canada

* Corresponding author: Dr. Qingxia Liu Emails: [email protected]

1

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: For the purpose of understanding the colloidal behaviors of illite in mineral processing, probing the surface charging property of illite is of great significance. This research explored the edge and basal surfaces of illite using an atomic force microscope (AFM). The interaction forces between Si/Si3N4 probes and illite edge/basal surfaces were measured, respectively, at different pH values in 10 mM KCl solutions. Theoretical Derjaguin-Landau-Verwey-Overbeek (DLVO) forces were matched up with the measured forces to derive the surface potentials of the two surfaces. On illite basal surface, an attractive force occurred at pH 3.0, while repulsive forces dominated from pH 5.0 to 10.0. On the illite edge surface, a slight attractive force was also obtained at pH 3.0. However, the interaction changed into repulsion at pH 5.0, and this repulsive force increased gradually from pH 6.0 to 10.0. Illite basal and edge surfaces were both negatively charged, but the basal surface exhibited more negative charges than the edge surface from pH 3.0 - 10.0. Increasing solution pH from 3.0 to 10.0, there was no detection of the point of zero charge (PZC) of illite basal surface; however, the PZC of illite edge surface should have occurred at a pH slightly lower than 3.0. This is the first time that surface potentials of illite edge and basal surfaces were attained separately by direct force measurements. These findings provide insight into the colloidal behaviors of illite in mineral processing and oil sands extraction. INTRODUCTION Clays, a significant class of industrial minerals, are widely used in many fields, such as cosmetic products, paper making and ceramics.1,2 However, in mineral processing and oil sands extraction, 2


Page 2 of 25

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

clay minerals are difficult to separate from valuable target minerals.3–5 As an important component in clay minerals, illite reduces the recovery of fine coal flotation and presents a challenging problem for sedimentation in the treatment of tailings from oil sands extraction.6–8 In tailings treatment, the challenge mainly comes from recycling the water from a fine clay suspension. Extensive research has illustrated that the lattice structure and anisotropic characteristics of clay minerals can affect their behaviors in suspensions.3,9,10 Thus, it is essential to investigate the anisotropic surface-charging characteristics of illite to better understand the fundamentals of illite particles in suspension. This will ultimately lead to improvements in recovering valuable minerals by flotation and enhanced sedimentation in the treatment of tailings from oil sands extraction. In the literature, surface properties of clay minerals have been studied by different techniques.10– 15

Hussain et al. applied an electrophoresis apparatus to explore the zeta potentials of three clay

minerals.11 Liu et al. also used electrophoresis to obtain the zeta potentials, and Hartley et al. used electrophoresis (for suspensions) and streaming potential (single surface) methods in their work.12,13 Potentiometric titration is another method to obtain the surface charge of clay minerals.16–20 The PZC of talc was determined to occur at about pH 7.7 by potentiometric titration,19 and Motta and Miranda found that the PZC of kaolinite was around pH 4.5.20 It should be noted that, without extensive modeling treatment, both zeta potential measurement and potentiometric titration can only provide the potential of the overall mineral particle at its slipping plane. Recently, atomic force microscope has been extensively adopted to explore physical and chemical

3


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

properties of mineral surface, including surface morphology, surface potential, and surface wettability. 21–34 Using AFM force measurements, the surface potentials of kaolinite basal surfaces were determined.35 Liu et al. obtained very flat kaolinite edge surfaces using an ultramicrotome. Edge surface potential was also determined by force measurements using AFM.36 With the same technique, the anisotropic surface charges of mica, talc, chrysotile rods and molybdenite were determined based on force measurements using AFM.37–39 Illite [K1.5(Al,Fe,Mg)4(Si,Al)8O20(OH)4] is a dioctahedral 2:1 layer structure phyllosilicate.40 One aluminum-oxygen-hydroxyl octahedron sheet is sandwiched between two tetrahedron sheets. Through sharing the apex oxygen atoms of the tetrahedron sheet, the tetrahedron and octahedron sheets are covalently bound together. In the tetrahedron sheet, some silicon atoms are replaced by aluminum atoms, which is called isomorphic substitution. Due to the crystal structure of illite, as shown in Figure 1a, there are two surfaces: the Si-O-Si basal surface and the edge surface with 𝑆𝑖𝑂𝐻 and 𝐴𝑙𝑂𝐻 groups. The two surfaces have totally different charging mechanisms.40 Due to isomorphic substitution, the basal surface, which is pH-independent, takes permanent negative charges. However, due to protonation or deprotonation of the 𝑆𝑖𝑂𝐻 and 𝐴𝑙𝑂𝐻 groups, the charge of edge surface is pH-dependent.

4


Page 4 of 25

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1. (a) Crystal structure of illite. (b) XRD image of illite.

While AFM has been applied as a tool to investigate anisotropic surface characteristics of many kinds of clays, little research has focused on illite. Hu et al. reported the zeta potential of an illite suspension, but the result was an averaged value of overall surface potential of illite particles, which included both edge and basal surfaces.41 Thus, the impacts that the edge surface charge and basal surface charge had on the overall surface charges could not be distinguished. Long et al. studied the interactions between illite surfaces using a rough illite particle glued on an AFM tip.42 Since the surface of the illite was rough, it resulted in notable variations in force measurements. Because anisotropic surface properties of illite are still not well understood, more detailed investigations on basal and edge surfaces of illite are needed to better understand the interactions between illite particles in an aqueous solution. This study used the ultramicrotome technique to obtain super-flat edge surfaces of illite. To derive the surface potential of illite, the measured forces between AFM tips and illite surfaces at various

5


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

pHs were modelled using the DLVO theory. MATERIALS AND METHODS Materials Illite samples were from Silver Hill in Montana in the United States. Table 1 lists the data of chemical composition, which were provided by the Clay Minerals Society. Table 1. Chemical composition of illite samples

Chemical SiO2 Al2O3 TiO2

Fe2O3

FeO MnO MgO CaO K2O P2O5

composition

Content (%) 49.3

24.25

0.55

7.32

0.55

0.03

2.56

0.43 7.83

0.08

The chemical composition (see Table 1) and X-ray diffraction pattern (see Figure 1b) of illite samples show that the illite contains minor impurities. The illite was used without further purification. Potassium chloride (KCl) solution at 10 mM was used as a background electrolyte. Smooth silica wafers were used to calibrate the AFM tips. The pH was adjusted using hydrochloric acid and potassium hydroxide. All chemicals used were from Sigma-Aldrich at ACS grade. Sample Preparation by Ultramicrotome Cutting To generate the proper particle size, the received illite sample was first ground to particles with

6


Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

particle size less than 5 µm. The illite particles (5 g) were dispersed in 100 mL Milli-Q water using an Ultrasonic Dismembrator (Fisher Model 500) at 20% amplitude for about 1 hr. The resulting slurry was settled for 3 h to remove coarse illite particles. Then, to remove fine illite particles, the supernatant was centrifuged for half an hour at 500 rpm. With the purpose of dispersing the particles completely, the resulting sediment was collected and sonicated for 20 min. The D50 of the resulting illite particles was estimated to be around 1 µm using a Laser Particle Sizer (Malvern Mastersizer-3000). A sapphire substrate was cleaned with a piranha solution for 3 h and sonicated with Milli-Q water. Before being used, the substrate was dried with N2. To obtain the basal surface of illite, one or two drops of the resulting illite suspension were added onto the sapphire substrate and dried on a hot plate in a petri-dish cover. To eliminate the loosely adhered illite particles, the illite-coated sapphire substrate was rinsed with Milli-Q water, then blown dry with N2. To expose illite edge surfaces, the ultramicrotome cutting procedure was applied. First, one or two drops of the resulting illite suspension were added onto the hardened epoxy resin filled in a mould, and the suspension was dispersed over the resin surface. After that, the resin was put on a hot plate（~100 °C）until the illite film was totally dry. With the aim to cover this illite film, another epoxy resin layer was cautiously added, and the resin-illite-resin block was cured for 24 h. After the resin was hardened, a trimer with an optical microscope was employed to trim the top of the resin sample to a rectangular shape. Then, the trimmed block was mounted onto a sample holder of ultramicrotome (EM UC 7, Leica Microsystems Inc.). A glass knife was used to cut the trimmed

7


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

block to obtain a relatively smooth surface. After that, a 45° diamond knife with a very low cutting speed was used to precisely cut, resulting in a fresh and smooth edge surface. Cuts were made perpendicular to the illite layer, and a stream of high pressure N2 was used to blow off the fine debris. After ultramicrotome cutting, the surface of the resin block was glossy, which suggested that the exposed illite edge surfaces were very smooth. Finally, this well-cut sample was glued with epoxy onto a glass substrate with the cut surface facing upward. Before each force measurement by AFM, the prepared samples were rinsed with Milli-Q, then blown dry with N2. Field Emission Scanning Electron Microscope (FE-SEM) As shown in Figure 2, dimensions of the Si3N4 tip and the Si tip (Bruker, Camarillo, CA, USA) were obtained using the FE-SEM (Zeiss Sigma) images of the tips. After analyzing these images, the radii of the tip apex curvatures of the silicon nitride and silicon AFM tips were determined to be about 70 and 30 nm, respectively. Also, the angles of the cones (2β) (noted in Figure 3) of silicon nitride and silicon tips were determined to be 40.6° and 37.6°, respectively. From the values of 2β determined from the FE-SEM images and Figure 3, the values for the angle α used for calculations in this study were 69.7° for the Si3N4 tip and 71.2° for the Si tip.

8


Page 8 of 25

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. FE-SEM images of Si3N4 tip (a) and Si tip (b). Insets are the zoomed-in images of the tip ends.

Zeta Potential Measurements. 0.01 g of ground illite particles were added to 20 mL KCl solution (10 mM). Then, the suspension was equilibrated for 15 minutes after pH adjustment. Zeta potentials of illite were measured by a Zetasizer. The zeta potentials were calculated from electrophoretic mobility. Reported zeta potential values were averaged from at least three independent measurements under each pH condition. AFM Force Measurements. The interactions of AFM tips on substrates were measured at different pH values (3.0, 5.0, 6.0, 8.0 and 10.0) in 10 mM KCl solution using a Dimension Icon atomic force microscope (Bruker, Camarillo, CA, USA). Due to the limited width of the edge surface of the illite, the sharper silicon tip (tip radius about 2 nm) was used for force measurements of the illite edge surfaces to ensure that only tip-edge interaction was probed, whereas the large area of the basal surfaces could be easily probed by the silicon nitride tip (tip radius about 20 nm). Obtained by thermal tune in 9


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NanoScope Analysis software, the spring constants of silicon nitride and silicon cantilevers used in this study were 0.17 - 0.21 N/m and 0.36 - 0.42 N/m, respectively. Before force measurements, a piece of silica wafer (for tip surface potential calibration) was cleaned with piranha solution for 3 h, sonicated with Milli-Q, and blown dry with N2. To stabilize the tip-solution-substrate system, all substrates (basal and edge surfaces of illite, silica wafer) and AFM tips were immersed in each pH environment for more than 30 min. More than 100 approach-retraction curves between the sample surface and the tip for each pH value were obtained at three to five different locations to confirm the reproducibility of the data. The force-separation curves were converted from deflection error-distance data using NanoScope Analysis software. A MATLAB script was employed to find the best-fit DLVO force curves for experimental data. In this work, the values calculated by the DLVO theory were reported. DLVO Theoretical Model The experimental forces between sample surface and tip were fitted with the forces calculated by the DLVO theory. Figure 3 shows the geometry of tip, which at its apex is spherical, although conical describes its main shape. DLVO calculation equations for the AFM tip-flat surface system have been studied previously.43–45 Details are shown in the Supporting Information section.

10


Page 10 of 25

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 3. Schematic of the tip-flat surface system.

RESULTS AND DISCUSSION Images of Illite Basal and Edge Surfaces

Figure 4. SEM image of (a) illite particles layered on sapphire substrate and (b) the embedded edge samples.

As shown in Figure 4a, a SEM image of illite showed that most of the particles are around 1 µm, and they adhered to the sapphire surface with the basal surfaces facing up due to the opposite charges exhibited by the sapphire and basal surfaces. This suggested that the top surface of the

11


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

illite film on the sapphire substrate was mainly composed of basal surfaces that could be directly used for force measurements. The SEM image of the embedded edge samples was showed in Figure 4b, which illustrated that the illite edge surfaces were exposed on the surface of resin. The topographic images of illite basal and edge surfaces are shown in Figure 5. The basal surface was flat, with surface roughness of 1.10 nm over the black dashed rectangle area in Figure 5a. Additionally, cross-sections of Lines 1 and 2 are shown in Figure 5b. The topography of edge surfaces is shown in Figure 5c, and cross-section profiles of two lines on edge surfaces were drawn in Figure 5d. The edge planes were narrower than the basal plane and appeared to line up with one another, indicating that the sheets of illite had indeed been cut perpendicular to the basal plane to expose the illite edges. As marked with a black cross symbol, these flat and smooth areas were selected for force measurements, and the roughness in these locations was less than 1 nm.

Figure 5. AFM images of illite and force measurement positions (black cross symbols): (a) illite basal surface; (b)

12


Page 12 of 25

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

height profile of the cross-section of illite basal surface; (c) illite edge surface; (d) height profile of the cross-section of illite edge surface.

Force Measurements between Silica Wafer and Tip Calibrating the surface potentials of AFM tips at varying pH values is essential before determining the surface potentials of illite. The surface potentials of silica have been extensively studied.37,39,46,47 Therefore, it is feasible to calibrate surface potentials of AFM tips with a silica wafer in an electrolyte solution. It should be noted that a silica layer could form on the outside of a Si3N4 tip and Si tip because of oxidation when the tips are exposed in atmosphere for a period of time.36,37,48 Thus, this silica layer could play a key role in surface interaction.

Figure 6. AFM force curves on a silica wafer with (a) Si3N4 tip, (b) Si tip under different pH conditions in 10 mM KCl solution. Symbols and solid lines represent experimental forces and DLVO fitted forces, respectively.

Figure 6 shows the experimental forces between silica wafer and Si3N4/Si tip from pH 3.0 to 10.0 in 10 mM KCl solution. The tip surface potentials were obtained by fitting classic DLVO theory

13


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with experimental data. Silicon nitride and silicon are both prone to be oxidized to SiO2, which is easily hydrolyzed to Si-OH in an aqueous phase. Due to the protonation reaction (𝑆𝑖𝑂𝐻 + 𝐻 + ⇌ 𝑆𝑖𝑂𝐻2 + ) of the surface Si-OH groups at pH 3.0, both the silicon nitride tip and silicon tip were positive. The surface potentials of the Si3N4 tip determined in this study were +26 mV, -46 mV, 58 mV, -69 mV, and -83 mV at pH 3.0, 5.0, 6.0, 8.0 and 10.0, respectively. Also, surface potential values of the silicon tip were +14 mV, -43 mV, -55 mV, -65 mV, and -77 mV, respectively, for the aforementioned pH values. These potentials were subsequently used as 𝜓𝑇 in a DLVO fitting to obtain the surface potentials of illite surfaces. The fitted surface potentials of tips showed a similar trend with values reported in the literature (see Supporting Information for details).39,46,49 It is evident that the surface potential values of Si3N4 and Si tips by DLVO fitting are consistent with those from the literature. Thus, the fitted surface potentials of both tips are applied for force curve fitting as follows. A discrepancy between the DLVO fitting and experimental data was observed at separation distances of up to about 4 nm, which is possibly because of hydration forces.50–53 To reduce the influence of these short-range forces and improve the fitting of experimental data to long-range van der Waals (VDW) and electric double layer (EDL) forces, only experimental data at separation greater than 5 nm were used for fitting to calibrate the tip surface potentials and subsequently determine the illite surface potentials. Force Measurement between Si3N4 Tip and Illite Basal Surface

14


Page 14 of 25

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 7. DLVO fitting and interaction forces between the illite basal surface and Si3N4 tip at different pH in 10 mM KCl solution. The symbols represent experimental forces. Solid curves are the fitting results using the mixed model.

Figure 7 represents the measured and fitted DLVO force curves between the illite basal surface and a Si3N4 tip from pH 3.0 to 10.0. As seen in the tip calibration, all the force curves fitted well with the DLVO theory at each pH condition for distances longer than about 5 nm. At pH 3.0, the interaction was attractive due to the positively charged Si3N4 tip, while repulsive interactions were detected in the pH range from 5.0 to 10.0. The dominant repulsive interactions from pH 5.0 to 10.0 indicated that repulsive EDL forces overcame the attractive VDW force due to large negative surface charges exhibited by both the Si3N4 tip and illite basal surface. Based on the fitting results, the surface potentials of illite basal surface were -58 mV, -64 mV, -67 mV, -71 mV, and -75 mV at pH 3.0, 5.0, 6.0, 8.0, and 10.0, respectively. This suggests that the basal surface of illite was negatively charged from pH 3.0 to 10.0, and its PZC was less than pH 3.0. Force Measurement between Si Tip and Illite Edge Surface 15


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. DLVO fitting and interaction force curves between the illite edge surface and Si tip at pH 3.0, 5.0, 6.0, 8.0, and 10.0 in 10 mM KCl solution. The symbols represent experimental forces. Solid curves are the fitting results using the mixed model.

The experimental forces between Si tip and illite edge surface from pH 3.0 to 10.0 are shown in Figure 8. The interaction forces were fitted with the DLVO theory and the fitting results agreed well with the experimental data. At pH 3.0, a weak attractive force was observed, and then the force changed to slightly repulsive at pH 5.0. The repulsive interaction increased gradually from pH 6.0 to 10.0, which indicated that an EDL force dominated over the VDW force. The surface potentials of illite edge surfaces were determined to be -7 mV, -28 mV, -42 mV, -51 mV, and -57 mV at pH 3.0, 5.0, 6.0, 8.0, and 10.0, respectively. Zeta Potential vs. Surface Potential of Illite Edge and Basal

16


Page 16 of 25

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 9. Surface potential of the illite edge (red solid sphere) and basal (black solid square) surfaces from AFM force fitting and zeta potential of illite (green solid triangle) measured in 10 mM KCl solutions. Reported zeta potentials (blue hollow reverse triangle) of illite in 10 mM NaCl solution were for comparison.54

Figure 9 summarizes the fitted surface potentials of illite edge/basal surfaces and experimentally determined and literature values for the zeta potentials of illite particles. From pH 3.0 to 10.0, the surface potentials of illite edge decreased sharply, while the surface potentials of illite basal surface decreased progressively. Moreover, the negative charge on illite basal surface was greater than that on edge surface for all pH conditions investigated, probably due to the nature of isomorphic substitution. The fitted surface potentials of the illite basal surface in the 10 mM KCl solution were not sensitive to pH from the pH range 3.0 - 10.0. Over the pH range studied, the PZC of illite basal surface was not found, indicating that the PZCbasal < 3.0. This result is probably caused by the isomorphic

17


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substitution on illite basal surface, which leads to the permanent charge. Figure 9 also shows that the PZC of illite edge should occur at a pH slightly lower than 3.0. According to the findings in this study, the fitted surface potential of illite edge was strongly dependent on pH relative to the basal surface, changing from -7 mV to -57 mV with the pH increasing from 3.0 to 10.0. The change in the charging behavior of the edge surface with pH was probably caused by deprotonation and protonation of the amphoteric 𝑆𝑖𝑂𝐻 and 𝐴𝑙𝑂𝐻 groups on illite edge surfaces. The PZCs of 𝑆𝑖𝑂𝐻 in silica and 𝐴𝑙𝑂𝐻 in gibbsite are about 2 and 9.1, respectively.55,56 This suggests that the 𝑆𝑖𝑂𝐻 groups took a negative charge from pH 3.0 - 10.0 due to a deprotonation reaction (𝑆𝑖𝑂𝐻⇌𝑆𝑖𝑂 ― + 𝐻 + ). With regards to the 𝐴𝑙𝑂𝐻 group, it could be protonated to 𝐴𝑙𝑂𝐻2 + (𝐴𝑙𝑂𝐻 + 𝐻 + ⇌ 𝐴𝑙𝑂𝐻2 + ) at a pH lower than about 9.1 and deprotonated to Al-O- (𝐴𝑙𝑂𝐻 + 𝑂𝐻 ― ⇌𝐴𝑙𝑂 ― + 𝐻2𝑂) at pH 10.0. As the pH changed from 3.0 - 8.0, the deprotonation of 𝑆𝑖𝑂𝐻 probably surpassed the protonation of 𝐴𝑙𝑂𝐻 (less 𝐴𝑙𝑂𝐻2 + ). Thus, the surface potential of illite edge surface was more negative at high pH. When the pH reached 10.0, the surface potential of edge surface continued decreasing because 𝐴𝑙𝑂𝐻 groups started to deprotonate. For comparison, the zeta potentials of illite particles from the same stock in 10 mM KCl solution were also obtained by electrophoretic mobility measurements (EPM). The zeta potentials of illite, which is also pH-dependent, showed a similar trend as the edge surface potentials derived by forcefitting. Moreover, the zeta potential of the whole illite particle was between the fitted surface potentials of illite edge and basal surfaces, which agrees with the notion that the overall surface potential of illite is attributed to a combination of surface potentials of edge and basal surfaces. It

18


Page 18 of 25

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

is worth mentioning that, by definition, the zeta potential is the potential at the slipping plane, and slipping plane is farther away from the solid surface than the Stern plane. Therefore, zeta potential is lower than the potential (obtained by AFM) at the Stern plane. The surface potentials of basal surface are not sensitive to pH, but the surface potentials of edge surface are strongly pH-dependent, which explains why the zeta potentials of the whole particle are as pH-dependent as the edge surface. Thus, it is reasonable that the zeta potentials of the illite particles were closer to the surface potentials of illite edge surface and varied with the solution pH. However, this does not mean that the zeta potentials of illite are attributed to the edge surface based only on the similarity between the curves. The results suggest that 1) force-fitted surface potentials showed a similar trend to the electrophoretic mobility measurements; 2) the surface potentials of illite edge/basal surfaces were separately attained by AFM force measurement, other than mixed as a whole surface in most techniques of surface potential measurements;11,17,19,41 and that 3) the illite basal and edge surfaces possess different surface potential values and different pH dependence due to the anisotropic crystal structure. CONCLUSION The anisotropic surface potentials of illite were researched in this study through the analysis of interaction forces between basal or edge surfaces of illite and AFM tips under different pH conditions in 10 mM KCl solution by fitting the classical DLVO theory. For illite basal surface, an attractive interaction was observed at pH 3.0, while the repulsive interaction dominated from pH 5.0 to 10.0. On illite edge surface, a weak attractive interaction was observed at pH 3.0.

19


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

However, this interaction changed to repulsive after pH 5.0, and it increased gradually up to pH 10.0. Illite basal and edge surfaces both showed negative surface charges, but the basal surface exhibited more negative charge than the edge surface from pH 3.0 - 10.0. The surface potentials of illite basal surface decreased slightly as the pH increased, while the surface potentials of the edge surface decreased sharply. The PZC of the edge surface should occur at a pH slightly lower than 3.0; however, the PZC of basal surface was not detected from pH 3.0 - 10.0. With the help of direct AFM force measurements, this is the first study for which surface potentials of illite edge and basal surfaces were attained separately. A quantitative description of the anisotropic surface charge of illite can provide fundamental insight into charging mechanisms. Our study reveals anisotropic surface charging characteristics of illite, which improves the understanding of colloidal behaviors of illite in mineral processing and oil sands extraction. ACKNOWLEDGMENTS Financial support is gratefully acknowledged from the Natural Science and Engineering Research Council of Canada (NSERC), the Canadian Centre for Clean Coal/Carbon and Mineral Processing Technologies (C5MPT), and the China Scholarship Council (CSC). Thanks to Xiao He and Yong Xiong for their help with this paper. SUPPORTING INFORMATION AVAILABLE Supporting information contains details about the DLVO calculation of systems, comparisons between tip values calibrated in this study, and tip values from the literature.

20


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

REFERENCES (1) (2) (3)

(4) (5) (6)

(7)

(8)

(9) (10)

(11)

(12) (13)

(14)

(15)

Murray, H. H. Overview - Clay Mineral Applications. Appl. Clay Sci. 1991, 5 (5–6), 379– 395. Murray, H. H. Traditional and New Applications for Kaolin, Smectite, and Palygorskite: A General Overview. Appl. Clay Sci. 2000, 17 (5–6), 207–221. Hu, Y.; Wei, S.; Hao, J.; Miller, J. D.; Fa, K. The Anomalous Behavior of Kaolinite Flotation with Dodecyl Amine Collector as Explained from Crystal Structure Considerations. Int. J. Miner. Process. 2005, 76 (3), 163–172. Hu, Y.; Jiang, H.; Wang, D. Electrokinetic Behavior and Flotation of Kaolinite in CTAB Solution. Miner. Eng. 2003, 16 (11), 1221–1223. Hu, Y.; Liu, X. Chemical Composition and Surface Property of Kaolins. Miner. Eng. 2003, 16 (11), 1279–1284. Farrokhpay, S.; Bradshaw, D. Effect of Clay Minerals on Froth Stability in Mineral Flotation: A Review. 26th Int. Miner. Process. Congr. IMPC 2012 Innov. Process. Sustain. Growth - Conf. Proc. 2012, 2 (313), 4601–4611. Zhong, H.; Liu, G.; Xia, L.; Lu, Y.; Hu, Y.; Zhao, S.; Yu, X. Flotation Separation of Diaspore from Kaolinite, Pyrophyllite and Illite Using Three Cationic Collectors. Miner. Eng. 2008, 21 (12–14), 1055–1061. Osacky, M.; Geramian, M.; Ivey, D. G.; Liu, Q.; Etsell, T. H. Influence of Nonswelling Clay Minerals (Illite, Kaolinite, and Chlorite) on Nonaqueous Solvent Extraction of Bitumen. Energy and Fuels 2015, 29 (7), 4150–4159. Miller, J. D.; Nalaskowski, J.; Abdul, B.; Du, H. Surface Characteristics of Kaolinite and Other Selected Two Layer Silicate Minerals. Can. J. Chem. Eng. 2008, 85 (5), 617–624. Yin, X.; Gupta, V.; Du, H.; Wang, X.; Miller, J. D. Surface Charge and Wetting Characteristics of Layered Silicate Minerals. Adv. Colloid Interface Sci. 2012, 179–182, 43–50. Hussain, S. A.; Demirci, Ş.; Özbayoǧlu, G. Zeta Potential Measurements on Three Clays from Turkey and Effects of Clays on Coal Flotation. J. Colloid Interface Sci. 1996, 184 (2), 535–541. Liu, J.; Xu, Z.; Masliyah, J. Role of Fine Clays in Bitumen Extraction from Oil Sands. AIChE J. 2004, 50 (8), 1917–1927. Hartley, P. G.; Larson, I.; Scales, P. J. Electrokinetic and Direct Force Measurements between Silica and Mica Surfaces in Dilute Electrolyte Solutions. Langmuir 1997, 13 (8), 2207–2214. Johnson, S. B.; Franks, G. V; Scales, P. J.; Boger, D. V; Healy, T. W. Surface Chemistry– rheology Relationships in Concentrated Mineral Suspensions. Int. J. Miner. Process. 2000, 58 (1), 267–304. Greenwood, R.; Lapčíková, B.; Surýnek, M.; Waters, K.; Lapčík, L. The Zeta Potential of Kaolin Suspensions Measured by Electrophoresis and Electroacoustics. Chemical Papers. 2007, p 83. 21


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16)

(17)

(18) (19)

(20) (21) (22)

(23)

(24) (25) (26)

(27) (28)

(29) (30)

(31) (32)

Rozalén, M.; Brady, P. V.; Huertas, F. J. Surface Chemistry of K-Montmorillonite: Ionic Strength, Temperature Dependence and Dissolution Kinetics. J. Colloid Interface Sci. 2009, 333 (2), 474–484. Tombácz, E.; Szekeres, M. Surface Charge Heterogeneity of Kaolinite in Aqueous Suspension in Comparison with Montmorillonite. Appl. Clay Sci. 2006, 34 (1–4), 105– 124. Brady, P. V; Cygan, R. T.; Nagy, K. L. Molecular Controls on Kaolinite Surface Charge. J. Colloid Interface Sci. 1996, 183 (2), 356–364. Burdukova, E.; Van Leerdam, G. C.; Prins, F. E.; Smeink, R. G.; Laskowski, J. S. Effect of Calcium Ions on the Adsorption of CMC onto the Basal Planes of New York Talc – A ToF-SIMS Study. Miner. Eng. 2008, 21 (12–14), 1020–1025. Motta, M. M.; Miranda, C. F. Molybdate Adsorption on Kaolinite, Montmorillonite, and Illite: Constant Capacitance Modeling. Soil Sci. Soc. Am. J. 1989, 53, 380–385. Toikka, G.; Hayes, R. A. Direct Measurement of Colloidal Forces between Mica and Silica in Aqueous Electrolyte. J. Colloid Interface Sci. 1997, 191 (1), 102–109. Chen, W.; Xu, Y.; Shi, S.; Cao, Y.; Chen, M. Fast Modification on Wheat Straw Outer Surface by Water Vapor Plasma and Its Application on Composite Material. Sci. Rep. 2018, No. 3, 4–11. Kuwahara, Y.; Uehara, S. AFM Study on Surface Microtopography, Morphology and Crystal Growth of Hydrothermal Illite in Izumiyama Pottery Stone from Arita, Saga Prefecture, Japan. Open Mineral. J. 2008, 2 (1), 34–47. Senden, T. J.; Drummond, C. J. Surface Chemistry and Tip-Sample Interactions in Atomic Force Microscopy. Colloids Surfaces A Physicochem. Eng. Asp. 1995, 94 (1), 29–51. Giessibl, F. J. Advances in Atomic Force Microscopy. Rev. Mod. Phys. 2003, 75 (3), 949– 983. Chen, W.; Zhou, X.; Zhang, X.; Feizbakhshan, M. Progress in Organic Coatings Fast Formation of Hydrophobic Coating on Wood Surface via an Energy-Saving Dielectric Barrier Discharges Plasma. Prog. Org. Coatings 2018, 125 (159), 128–136. Quate, G. B. and C. G. and E. S. and T. R. A. and C. F. Atomic Resolution with Atomic Force Microscope. EPL (Europhysics Lett. 1987, 3 (12), 1281. Hillier, A. C.; Kim, S.; Bard, A. J. Measurement of Double-Layer Forces at the Electrode/Electrolyte Interface Using the Atomic Force Microscope: Potential and Anion Dependent Interactions. J. Phys. Chem. 1996, 100 (48), 18808–18817. Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Direct Force Measurements between Silica and Alumina. Langmuir 1997, 13 (7), 2109–2112. Wallqvist, V.; Claesson, P. M.; Swerin, A.; Schoelkopf, J.; Gane, P. A. C. Interaction Forces between Talc and Pitch Probed by Atomic Force Microscopy. Langmuir 2007, 23 (8), 4248–4256. Chi, R.; Xu, Z.; DiFeo, T.; Finch, J.; Yordan, J. L. Measurement of Interaction Forces between Talc and Toner Particles; 2001; Vol. 27. Wallqvist, V.; Claesson, P. M.; Swerin, A.; Schoelkopf, J.; Gane, P. A. C. Interaction 22


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(33)

(34) (35) (36)

(37)

(38)

(39) (40) (41) (42) (43)

(44)

(45)

(46) (47)

(48)

Forces between Talc and Hydrophobic Particles Probed by AFM. Colloids Surfaces A Physicochem. Eng. Asp. 2006, 277 (1), 183–190. Chen, W.; Zhou, X.; Zhang, X.; Bian, J.; Shi, S. Applied Surface Science Fast Enhancement on Hydrophobicity of Poplar Wood Surface Using Low-Pressure Dielectric Barrier Discharges ( DBD ) Plasma. Appl. Surf. Sci. 2017, 407, 412–417. Ishida, N.; Sakamoto, M.; Miyahara, M.; Higashitani, K. Attraction between Hydrophobic Surfaces with and without Gas Phase. Langmuir 2000, 16 (13), 5681–5687. Gupta, V.; Miller, J. D. Surface Force Measurements at the Basal Planes of Ordered Kaolinite Particles. J. Colloid Interface Sci. 2010, 344 (2), 362–371. Liu, J.; Sandaklie-Nikolova, L.; Wang, X.; Miller, J. D. Surface Force Measurements at Kaolinite Edge Surfaces Using Atomic Force Microscopy. J. Colloid Interface Sci. 2014, 420, 35–40. Yan, L.; Englert, A. H.; Masliyah, J. H.; Xu, Z. S4D23 Determination of Anisotropic Surface Charge Characteristics of Different Phyllosilicates by Direct Force Measurements. 2012, 2012. Yang, D.; Xie, L.; Bobicki, E.; Xu, Z.; Liu, Q.; Zeng, H. Probing Anisotropic Surface Properties and Interaction Forces of Chrysotile Rods by Atomic Force Microscopy and Rheology. Langmuir 2014, 30 (36), 10809–10817. Lu, Z.; Liu, Q.; Xu, Z.; Zeng, H. Probing Anisotropic Surface Properties of Molybdenite by Direct Force Measurements. Langmuir 2015, 31 (42), 11409–11418. Masliyah, J. H.; Czarnecki, J.; Xu, Z. Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands - Volume 1: Theoretical Basis; 2011. Hu, Y.; Liu, X.; Xu, Z. Role of Crystal Structure in Flotation Separation of Diaspore from Kaolinite, Pyrophyllite and Illite. Miner. Eng. 2003, 16 (3), 219–227. Long, J.; Xu, Z.; Masliyah, J. H. Role of Illite-Illite Interactions in Oil Sands Processing. Colloids Surfaces A Physicochem. Eng. Asp. 2006, 281 (1–3), 202–214. Carnie, S. L.; Chan, D. Y. C.; Gunning, J. S. Electrical Double Layer Interaction between Dissimilar Spherical Colloidal Particles and between a Sphere and a Plate: The Linearized Poisson-Boltzmann Theory. Langmuir 1994, 10 (9), 2993–3009. McCormack, D.; Carnie, S. L.; Chan, D. Y. C. Calculations of Electric Double-Layer Force and Interaction Free Energy between Dissimilar Surfaces. J. Colloid Interface Sci. 1995, 169 (1), 177–196. Drelich, J.; Long, J.; Yeung, A. Determining Surface Potential of the Bitumen-Water Interface at Nanoscale Resolution Using Atomic Force Microscopy. Can. J. Chem. Eng. 2008, 85 (5), 625–634. Yin, X.; Yan, L.; Liu, J.; Xu, Z.; Miller, J. D. Anisotropic Surface Charging of Chlorite Surfaces. Clays Clay Miner. 2013, 61 (2), 152–164. Yan, L.; Masliyah, J. H.; Xu, Z. Understanding Suspension Rheology of AnisotropicallyCharged Platy Minerals from Direct Interaction Force Measurement Using AFM. Curr. Opin. Colloid Interface Sci. 2013, 18 (2), 149–156. Yin, X.; Drelich, J. Surface Charge Microscopy: Novel Technique for Mapping Charge23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(49) (50) (51)

(52) (53)

(54) (55)

(56)

Mosaic Surfaces in Electrolyte Solutions. Langmuir 2008, 24 (3), 8013–8020. Binner, J. Characterization of Silicon Carbide and Silicon Powders by XPS. 2001, 123– 126. Ducker, W. A.; Senden, T. J.; Pashley, R. M. Direct Measurement of Colloidal Forces Using an Atomic Force Microscope. Nature 1991, 353, 239. Grabbe, A.; Horn, R. G. Double-Layer and Hydration Forces Measured between Silica Sheets Subjected to Various Surface Treatments. J. Colloid Interface Sci. 1993, 157 (2), 375–383. Yoon, R. H.; Vivek, S. Effects of Short-Chain Alcohols and Pyridine on the Hydration Forces between Silica Surfaces. J. Colloid Interface Sci. 1998, 204 (1), 179–186. Valle-Delgado, J. J.; Molina-Bolívar, J. A.; Galisteo-González, F.; Gálvez-Ruiz, M. J.; Feiler, A.; Rutland, M. W. Hydration Forces between Silica Surfaces: Experimental Data and Predictions from Different Theories. J. Chem. Phys. 2005, 123 (3). Sondi, I.; Bišćan, J.; Pravdić, V. Electrokinetics of Pure Clay Minerals Revisited. J. Colloid Interface Sci. 1996, 178 (2), 514–522. Franks, G. V. Zeta Potentials and Yield Stresses of Silica Suspensions in Concentrated Monovalent Electrolytes : Isoelectric Point Shift and Additional Attraction. 2002, 51, 44– 51. Jodin, M.; Gaboriaud, F.; Humbert, B. Limitations of Potentiometric Studies to Determine the Surface Charge of Gibbsite γ -Al ( OH ) 3 Particles. 2005, 287, 581–591.

24


Page 24 of 25

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

For Table of Contents Use Only:

25


Probing Anisotropic Surface Properties of Illite by ... - ACS Publications

Recommend Documents