Probing Anisotropic Surface Properties and Surface Forces of Fluorite

Article Cite This: Langmuir 2018, 34, 2511−2521

pubs.acs.org/Langmuir

Probing Anisotropic Surface Properties and Surface Forces of Fluorite Crystals Zhiyong Gao,†,§ Lei Xie,‡,§ Xin Cui,‡ Yuehua Hu,† Wei Sun,*,† and Hongbo Zeng*,‡ †

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, PR China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada

‡

S Supporting Information *

ABSTRACT: Fluorite is the most important mineral source for producing fluorine-based chemicals and materials in a wide range of engineering and technological applications. In this work, atomic force microscopy was employed, for the first time, to probe the surface interactions and adhesion energy of model oleic acid (a commonly used surface modification organics for fluorite) molecules on fluorite surfaces with different orientations in both air and aqueous solutions at different pH conditions. Fitted with the Derjaguin−Landau−Verwey−Overbeek theory, the force results during surface approaching demonstrate the anisotropy in the surface charge of different orientations, with the {111} surface exhibiting a higher magnitude of surface charge, which could be attributed to the difference in the atomic composition. The adhesion measured during surface retraction shows that model oleic acid molecules have a stronger adhesion with the {100} surface than with the {111} surface in both air and aqueous solutions. The anisotropic adhesion energy was analyzed in relation to the surface atom (especially calcium) activity, which was supported by the surface free energy results calculated based on a three-probe-liquid method. Each calcium atom on the {100} surface with four dangling bonds is more active than the calcium atom on the {111} surface with only one dangling bond, supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules present in the ionic form at pH 9 exhibit a higher adhesion energy with fluorite surfaces as compared to their molecular form at pH 6, which was related to the surface activity of different forms. The adhesion energy measured in solution is much lower than that in air, indicating that the solvent exerts an important influence on the interactions of organic molecules with mineral surfaces. The results provide useful information on the fundamental understanding of surface interactions and adhesion energy of organic molecules on mineral surfaces with different orientations, and the methodology can be extended to many other crystal surfaces in various interfacial processes. flotation based on the difference in surface properties of different materials.17 Usually, surface characteristics of a certain material are governed by its most commonly exposed surfaces,7,18−22 such as the {111} and {100} planes for the natural fluorite ore deposit.23 It is of both fundamental and practical importance to understand the surface properties and interaction mechanisms of different orientations of fluorite, ultimately modulating and controlling the adsorption of chemical reagents and flotation performance.

1. INTRODUCTION Surface characteristics of different crystal orientations play important roles in a wide range of engineering applications, such as biopharmacy,1 self-assembly,2,3 photomodulation,4 surface etching,5 corrosion protection,6 and froth flotation.7 The research on anisotropic surface properties can help better control and achieve the desired surface chemistry and functionality of specific surfaces of materials. Fluorite (CaF2) is the most important mineral source for producing fluorinebased chemicals and materials in a variety of industrial, environmental, and medical fields.8−12 Fluorite often coexists with other minerals such as calcite and quartz in ore deposits,13−16 which can be selectively separated by froth © 2018 American Chemical Society

Received: December 6, 2017 Revised: January 17, 2018 Published: January 24, 2018 2511

DOI: 10.1021/acs.langmuir.7b04165 Langmuir 2018, 34, 2511−2521

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Langmuir Over the past few decades, surface characteristics of fluorite surfaces with different orientations have attracted extensive attention and research interest.12,16,20−31 The surface energies of fluorite {111} and {100} surfaces were calculated based on computer simulations,24−26 with a consistent conclusion that {111} has a lower surface energy than {100}. The possible reason could be that {111} has a much lower surface broken bond density (Db) than {100},27 thereby resulting in a lower surface energy and a more stable surface.28−30 The wetting behavior of these two planes was investigated by contact angle measurement and molecular dynamics simulation,27,28 indicating that the {100} surface is more hydrophilic than {111}, mainly contributed from the stronger interaction between interfacial water molecules and the {100} surface that leads to a more ordered arrangement of water molecules.28 Organic molecules, especially oleic acid or sodium oleate, are a commonly used class of collectors to enhance the hydrophobicity and floatability of fluorite minerals. Mielczarski and co-workers observed that bidentate-like bonding is the preferential interaction between carboxyl groups of oleate molecules and calcium atoms on the {111} surface.29−32 However, an atomistic simulation study by de Leeuw and coworkers showed that methanoic acid (a model of oleic acid because of the same functional group) favorably interacts with the fluorite surface by bridging with two adjacent calcium atoms.14 Their simulations also demonstrated different adsorption energies of methanoic acid on three different fluorite surfaces, namely, {310}, {111}, and {110}. Despite the significant progress achieved, the understanding of interaction mechanism between oleate molecules and different fluorite surfaces, particularly at the nanoscale, still remains limited. Atomic force microscopy (AFM) has been widely employed to measure the surface and intermolecular forces in a broad range of material systems at the nanoscale.33−41 Recently, the bubble probe AFM enables direct force measurements between air bubbles and mineral surfaces (e.g., sphalerite and molybdenite) in complex aqueous conditions.42−47 In addition, the chemical force microscopy (CFM) technique provides insights into the understanding of the intermolecular forces and adhesion between AFM tips modified with self-assembled monolayers (SAMs) and solid surfaces.48,49 On the basis of the CFM technique, Xie et al. have successfully mapped the heterogeneous distribution of surface hydrophobicity on the sphalerite mineral surface using the AFM tips functionalized with −CH3 groups because of the nonuniform adsorption of xanthate.50 By modifying AFM tips with model oleic acid molecules, the interaction mechanism between model oleic acid molecules and fluorite surfaces of different orientations can be quantified. The interaction and adhesion on other calciumbased minerals, such as calcite and calcium oxalate, have been studied using AFM force measurements.15,51,52 The adhesion between the calcium dioleate colloidal probe and the calcite surface was found to be pH-dependent, which was much weaker than that between calcium dioleate and the fluorite surface.15,51 The interaction between various functional groups and calcium oxalate surfaces showed the largest adhesion for carboxylate and amidinium groups, and the adhesion measured was sensitive to the structure and composition of crystal faces.52 Despite the significant progress achieved, to date, the surface interactions and adhesion of oleate molecules on fluorite surfaces at the nanoscale, to our knowledge, have not been reported.

In this work, AFM tips functionalized with self-assembled model oleic acid molecules were used to measure the interaction forces with fluorite {100} or {111} surfaces in air or in solution with different pH levels. The measured force profiles during surface approaching were interpreted by the classical Derjaguin−Landau−Vervey−Overbeek (DLVO) model, and the adhesion measured during surface retraction was used to calculate the adhesion energy based on the Derjaguin−Muller−Toporov (DMT) model. Contact angles were measured with three probe liquids on both surfaces, and the Lifshitz−van der Waals and acid−base components of the surface free energy were also calculated. This work provides useful insights into the basic understanding of the anisotropic surface properties and surface forces of fluorite crystals, and the methodology can be extended to many other material systems with anisotropic surface properties.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade 16-mercaptohexadecanoic acid (16-MHA, SH(CH2)15COOH, RCOOH for short) was purchased from Sigma-Aldrich, Canada. Analytical grade sodium oleate was supplied by Guangfu Fine Chemical Research Institute, Tianjing, China. Analytical grade (99% pure) formamide and diiodomethane were supplied by Adamas, Shanghai. Milli-Q water (Millipore deionized, 18.2 MΩ·cm resistivity) was used for all the experiments. Formamide, diiodomethane, and water were employed as three probe liquids for contact angle measurements and surface energy calculation. The pH was adjusted with addition of HCl and NaOH solutions. 2.2. Characterization of Fluorite Crystals. Natural fluorite crystals with dimensions of 8−12 mm were obtained from the Pucheng Mine located in Fujian, China. The commonly exposed surfaces were carefully selected and cleaned with high-purity nitrogen gas. The crystallographic orientations of the exposed surfaces were determined by X-ray diffraction (XRD). The X-ray diffractometer (D8ADVANCE Bruker-AXS) was run in the reflection mode with Cu Kα radiation (λ = 0.15406 nm, tube potential = 40 mV, and tube current = 40 mA) and a goniometer speed of 4°/min (2θ). The single-crystal diffraction patterns with a 0.01° precision of interlayer spacing d measurements were conducted from 5° to 80° (2θ). The XRD analysis was conducted on 10 fluorite crystal samples, and two crystals were selected for the following study. The crystals were mounted inside ∼15 mm diameter disks made of two-part epoxy resin. The orientation of the crystals along either {100} or {111} surface was preserved during mounting. After curing, the mounted crystals were polished with 0.06 μm alumina powder, sonicated, and washed with Milli-Q water. 2.3. AFM Imaging. Nanoscope Multimode IIIa AFM (Veeco, Santa Barbara, CA) was used for imaging the fluorite surfaces. All AFM imaging experiments were carried out in the tapping mode in air at a room temperature of 20 °C, and the scan rate was kept constant at 1.0 Hz. 2.4. Contact Angle Measurements. Contact angle measurements were conducted based on the sessile drop method using a Contact Angle Meter (GBX, France). A given liquid droplet of about 3.5 μL was placed on the fluorite surface, and the readings of contact angles were taken automatically on the left and right sides of the liquid droplet profile by computer software. At least 10 contact angle measurements were made on various locations of at least two independently prepared samples, and the reported results were the average of the above testing values. After each measurement, the fluorite surface was cleaned with Milli-Q water, acetone, and Milli-Q water and then polished again using 0.05 μm alumina powder to restore a fresh surface. For the measurement of the fluorite surface treated with sodium oleate solution, the prepared fluorite sample was first immersed in a sodium oleate solution at a certain concentration in a 150 mL beaker. The solution was under gentle magnetic agitation at a speed of 140 rpm for a complete adsorption of oleate molecules on the fluorite 2512


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Langmuir surface. After 20 min adsorption, the treated fluorite sample was gently taken out from the beaker and washed with Milli-Q water to remove the residual solution on the surface and then dried at 50 °C for 15 min in a vacuum drying chamber. The water contact angle measurement was conducted as mentioned above. 2.5. AFM Force Measurements. AFM force measurements were conducted in an environmental chamber using a MFP-3D AFM (Asylum Research, Santa Barbara, CA, USA), either in dry air or in aqueous solutions. Prior to the force measurements, gold-coated AFM tips were functionalized with a SAM layer by using an established procedure.33−35 Briefly, gold-coated AFM tips (B Triangular NPG-10, Bruker) with the spring constant of 0.11−0.13 N/m were cleaned by UV/ozone treatment for 10 min and then functionalized by immersing them in an ethanol solution with 0.1 mM 16-MHA overnight. Both oleic acid (C17H33COOH) and 16-MHA (HSC15H30COOH) are the molecules with very similar carbon chain length and are terminated with the −COOH group. In addition, 16-MHA is terminated with −HS on the other end, which can strongly bind to the gold tips through the S−Au bond, forming ordered SAMs of model oleic acid molecules. Thus, 16-MHA was chosen as the model oleic acid molecule to perform the force measurements. The functionalized AFM tips were rinsed with ethanol to remove physisorbed thiols and gently dried using high-purity nitrogen. The 16-MHA-functionalized AFM tips were used to unravel the interaction between model oleic acids and fluorite mineral surfaces in mineral processing as illustrated in Figure 1. During the force measurements, the functionalized AFM tip

histogram of adhesion was plotted and fitted with Gaussian distribution.

3. THEORETICAL ANALYSIS 3.1. Adhesion Energy of Water on the Fluorite Surface. The adhesion energy of a liquid on a solid surface (WSL) could be described by the following equation

WSL = γL(1 + cos θ )

(1)

where γL represents the liquid surface free energy and θ is the contact angle of a liquid on the solid surface. 3.2. Surface Free Energy Calculation. Contact angle measurements of several probe liquids with known surface tension parameters are widely used for evaluating surface wettability, adhesion energy, and surface free energy of solid materials.36 The relation between surface tension (γ) and contact angle (θ) for a solid contacting a liquid is given by the Young equation.

γS − γSL = γL cos θ

(2)

The Lifshitz−van der Waals/Lewis acid−base (LW-AB) approach is widely used for the evaluation of surface tension and its components.37 In the LW-AB approach, surface free energy, γi (i = S (solid) or L (liquid)), is equal to the sum of Lifshitz−van der Waals contribution, γLW i , and (Lewis) acid− AB LW base contribution, γi , such that γi = γi + γAB i . Electron acceptor parameter, γi+, and electron donor parameter, γi−, were taken to satisfy the relation γi AB = 2 γi +γi−

(3)

The free energy of a solid−liquid interface, γSL, can be expressed as

Figure 1. Schematic of force measurements between the AFM tip functionalized with model oleic acids and the fluorite surface.

γSL = γS + γL − 2( γSLWγLLW +

was driven to approach and then retract from the surface with a constant velocity and fixed maximum loading force. The deflection of the cantilever was detected using a laser beam that was reflected from the cantilever to a split photodiode detector, which was further converted to force using the Hooke’s law. To obtain the histogram of adhesion, the mineral surface was imaged in the tapping mode prior to force measurements to select smooth areas to eliminate the influence of surface roughness, and 200 force curves were obtained at over 50 randomly selected positions on the desired regions of two independently prepared samples for the same experimental condition. On the basis of the retraction curves of these 200 force curves, the

γS+γL− +

γS−γL + )

(4)

+ − To obtain the values of γLW S , γS , and γS , it requires contact angles of the solid with an apolar liquid, water, and a polar liquid. On the basis of three equations obtained from eq 4, the energy components can be obtained by eq 5, where L1, L2, and L3 represent three different probe liquids and θ1, θ2, and θ3 represent the contact angle of L1, L2, and L3 on the mineral surface.

Figure 2. (A) Geometry of the conical AFM tip with a spherical cap at the apex, where α is the geometry angle for the spherical cap, R is the radius of the spherical cap, r is the radius of the circle of the tip at a given vertical position, and D is the distance between the tip end and the mineral surface. (B,C) Typical FE-SEM image of AFM tips functionalized with 16-MHA. 2513


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Langmuir ⎧⎡ ⎛ LW ⎞ ⎪⎢ ⎛ γ LW ⎜ γS ⎟ ⎪⎢ ⎜ L1 ⎜ ⎟ ⎪⎢ ⎜ LW ⎜ γS+ ⎟ = ⎨⎢2⎜ γL 2 ⎜ ⎟ ⎪ ⎜ ⎜ γ − ⎟ ⎪⎢ ⎜⎜ LW ⎝ S ⎠ ⎪⎢⎣ ⎝ γL3 ⎩

⎡ D + R(1 − cos α) ⎤ b1 = ⎢R sin α − ⎥ ⎣ ⎦ tan α 1 ⎡⎜⎛ 1 ⎞⎤ + ⎢⎝L1 + ⎟⎠⎥ κ ⎦ tan α ⎣

⎞⎤ γL + ⎟⎥ 1 ⎟⎥ γL + ⎟⎥ 2 ⎟⎥ ⎟⎥ γL + ⎟⎥ 3 ⎠⎦

−1

γL − 1

γL − 2

γL − 3

⎫ ⎛ γ (cos θ1 + 1) ⎞⎪ ⎜ L1 ⎟⎪ ⎜ γ (cos θ + 1)⎟⎪ 2 ⎜ L2 ⎟⎬ ⎜⎜ ⎟⎟⎪ ⎪ γ θ + (cos 1) 3 ⎝ L3 ⎠⎪ ⎭

⎡ D + R(1 − cos α) ⎤ b2 = ⎢R sin α − ⎥ ⎣ ⎦ tan α 1 ⎡⎜⎛ 1 ⎟⎞⎤ + ⎢ L1 + ⎥ tan α ⎣⎝ 2κ ⎠⎦

where A is the Hamaker constant, α and β are the geometrical angles for the spherical cap at the tip end and conical tip with α + β = 90°, D is the distance from the end of the tip to the substrate, L1 is the distance between a differential surface section of the tip and the substrate, r is the radius of the circle of the tip at a given vertical position, R is the radius of the spherical cap at the tip end, ε is the dielectric constant of the solution in this system, ε0 is the permittivity of vacuum, 1/κ is the Debye length, σ is the surface charge density, and subscripts S and T refer to the substrate and the tip, respectively. The adhesion energy per unit area Wadh between 16-MHA and fluorite is correlated to the adhesion measured Fadh by the DMT model.39,54

(5)

3.3. Force Analysis. The classical DLVO model was applied to analyze the force−separation curves measured between functionalized AFM tips and mineral surfaces. The pyramidal geometry of AFM tips was assumed to be conical with a spherical cap at the apex (Figure 2A).53 The geometry of functionalized AFM tips was obtained by the field emission scanning electron microscopy (FE-SEM), and the radius of the spherical cap R (32−40 nm) was determined based on the analysis of the FE-SEM image. For a typical case in Figure 2B,C, R was determined to be ∼36.5 nm. It is worth noting that the radius of the spherical cap of AFM tips used was only 32− 40 nm, which indicated that the contact area between the tip and the fluorite surface was at the nanoscale. At each selected position where the force measurements were performed, the root-mean-square (rms) roughness of the interaction regions was less than 0.5 nm as confirmed by AFM imaging prior to force measurements, much smaller than the size of AFM tips, which thereby makes the influence of surface roughness on the force measurements negligible. The van der Waals forces (FvdW) were modeled according to the following equation F vdW =

Wadh = −

1 Fadh /R 2π

(8)

4. RESULTS AND DISCUSSION 4.1. Surface Characterization. A total of 14 pieces of fluorite crystals were randomly picked up from the crushed samples and measured by XRD. The XRD results revealed nine specimens with the {111} surface and five specimens with

A ⎡ (R + D) − 2L1 R − D⎤ ⎥ ⎢ − 6⎣ L12 D2 ⎦ −

A ⎛ 1.0 ⎜ 3 tan 2 α ⎝ L1

+

R sin α tan α − D − R(1 − cos α) ⎞ ⎟ 2L12 ⎠

(6)

The equation describing the electrical double layer (EDL) force (Fedl, constant surface charge density case) is as follows F edl =

+

4π σ σ (a e−κD − a1 e−κL1) 2 T S 0 ε0εκ 2π (σT 2 + σS2)(a 2 e−2κD − a3 e−2κL1)+ + 2 ε0εκ ⎡ (σ 2 + σS2) −2κL1⎤ 4π ⎥ ⎢b1σTσS e−κL1 + b2 T e 2 ε0 εκ tan α ⎣ ⎦ (7)

where L1 = D + R(1 − cos α), a0 = κR − 1, a1 = κR cos α − 1, a2 = a0 + 0.5, and a3 = a1 + 0.5

Figure 3. XRD patterns of the (A) {111} and (B) {100} surfaces of fluorite crystals (d interlayer spacing, nm). 2514


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Figure 4. AFM topographic images of (A) {111} and (B) {100} polished fluorite surfaces at a scan area of 5 × 5 μm2.

{100}, indicating that the {111} plane is the most commonly exposed cleavage surface for the fluorite crystal, followed by a moderate naturally occurring {100} surface. Only selected XRD patterns for the fluorite crystals are shown in Figure 3. Figure 4 shows the typical morphology of fluorite {111} and {100} surfaces measured using the tapping mode of AFM. The samples of these two orientations both demonstrate the smooth surfaces with the rms roughness of 1.0−1.5 nm over the area of 5 × 5 μm2. 4.2. Surface Energy. The surface energies of fluorite {111} and {100} surfaces were determined by measuring the static contact angles of three probe liquids (i.e., water, formamide, and diiodomethane) on these crystal surfaces, and the results are summarized in Table 1. The contact angle values for three probe liquids on these two fluorite surfaces both follow the order, θW > θF > θD, which agrees well with previously reported values on fluorite plates without determination of the crystallographic orientations.55−57 The θW values are, respectively, measured to be 79.2° and 72.1° for fluorite {111} and

Table 1. Measured Contact Angle Values for Water (θW), Formamide (θF), and Diiodomethane (θD) on Different Fluorite Surfaces fluorite surface

θW (°)

θF (°)

θD (°)

{111} surface {100} surface fluorite plate I55,57 fluorite plate II56

79.2 ± 0.2 72.1 ± 1.1 82.2 67.9

50.2 ± 0.1 44.0 ± 0.1 65.7 52.1

32.7 ± 0.3 33.1 ± 0.8 44.1 44.8

Table 2. Surface Free Energies γS with Lifshitz−van der + Waals γLW S and Electron Acceptor γS and Electron Donor − γS Components for Different Surfaces Calculated by the Acid−Base Theory, mJ/m2 fluorite surface

γS

γLW S

γS+

γS−

γAB S

{111} surface {100} surface fluorite plate I55,57 fluorite plate II56

45.3 47.0 38.88 40.26

43.1 42.9 31.86 32.17

0.4 0.7 0.87 1.01

3.6 6.5 14.18 12.49

2.3 4.1 7.02 8.09

Figure 5. Typical force−separation curves measured between the functionalized AFM tip and fluorite (A) {111} and (B) {100} surfaces in air during the approach. Open symbols are experimental results, and solid curves are theoretical fittings. Histograms and the fitted Gaussian distributions of measured adhesion forces on (C) {111} and (D) {100} surfaces. 2515


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the adhesion energy is, the more hydrophilic the surface is.59 According to eq 1, the adhesion energy of water on the {100} surface is calculated to be 95.18 mJ/m2, which is larger than 86.43 mJ/m2 on {111}. Table 2 shows surface free energies and their components of {111} and {100} surfaces. The surface energy for the {111} surface is calculated to be 45.3 mJ/m2, slightly lower than that for the {100} surface (∼47.0 mJ/m2). This trend is consistent with the theoretically calculated surface energy results reported previously.24−26 The difference in the surface energy of different orientations could be most likely attributed to the smaller density of surface broken bonds (∼15.48 nm−2) on the {111} plane as compared to those on the {100} plane (∼26.81 nm−2)27 or the change of Ca−F bond length before and after relaxation during the surface generation.24 For both fluorite surfaces, the polar components γAB contributed less to their total surface energies. This result might be due to the fact that in the generation of {111} and {100} surfaces, the loss of bonding does not obviously lead to enhanced reactivity of surface calcium and fluorine ions. 4.3. AFM Force Measurements in Air. To further unravel the interaction mechanism between oleate molecules and fluorite surfaces, 16-MHA with a structure similar to oleic acid was self-assembled on AFM tips and used to measure its intermolecular forces with fluorite surfaces. Figure 5 shows the interaction forces between 16-MHA (model oleic acid)functionalized AFM tips and fluorite {111} and {100} surfaces in air. The measured force curves (open symbols) in Figure 5A,B clearly show the attraction at a separation distance of ∼5 nm as the tip was driven to approach fluorite surfaces. The measured attraction arises from the van der Waals interaction, which is shown as the red curve to interpret the measured force results. The fitted Hamaker constant for functionalized tip− fluorite was determined to be 3.0 × 10−19 J. The discrepancy at

Figure 6. Structures and atom bonding states of (A) {111} and (B) {100} surfaces of fluorite. Green = Ca, light blue = F. (A) {111} (3 unit cells; every unit cell has one Ca atom; every Ca has one broken bond). (B) {100} (3 unit cells; every unit cell has one Ca atom; every Ca has four broken bonds).

{100} surfaces, indicating the natural hydrophobicity of fluorite surfaces.58 Table 1 also shows that the static contact angle of water on the {111} surface is larger than that on {100}, and the same trend was reported by Zhang et al.28 This difference in wettability can be interpreted by the adhesion energy of water on the fluorite surface, with the understanding that the higher

Figure 7. Force−separation curves between the functionalized AFM tip and the fluorite {111} surface at (A) pH 6 and (B) pH 9 and the fluorite {100} surface at (C) pH 6 and (D) pH 9 in 1 mM NaCl solution during the approach. Open symbols are experimental results, and solid curves are theoretical fittings. 2516


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Figure 8. Histograms and fitted Gaussian distributions of measured adhesion forces on the fluorite surface {111} at (A) pH 6 and (B) pH 9 and on the fluorite surface {100} at (C) pH 6 and (D) pH 9 in 1 mM NaCl solution.

atoms on the {111} surface, and each surface calcium atom has one broken (dangling) bond. In contrast, no fluorine exists on the {100} surface, and each surface calcium atom has four dangling bonds (Figure 6B). The higher adhesion energy of model oleic acid on the {100} surface in air is attributed to more surface calcium atoms that are available to bridge the −COOH groups of model oleic acid as well as higher reactivity due to more dangling bonds.63,64 4.4. AFM Force Measurements in Aqueous Solution. The interaction between the model oleic acid and fluorite surfaces in aqueous solution was further evaluated to better understand the influence of the aqueous environment on the adhesion energy of model oleic acid on fluorite surfaces in mineral flotation. The predominant species of oleic acid in solution are dependent on the critical pH for the formation of acid−soap (RCOOH−RCOO−) complexes.65,66 The critical pH was found to fall in the range of pH 6.5−8.8 at the oleate concentration of 10−6 to 10−3 mol/L.67 Hence, the model oleic acid exists predominantly in the molecular form of RCOOH at pH 6 and in the ionic form of RCOO− at pH 9. Figure 7 shows the interaction force profiles between 16MHA-functionalized AFM tips and fluorite {111} and {100} surfaces in 1 mM NaCl solution at pH 6 and 9. For both fluorite surfaces, the measured force curves (open symbols) show the stronger and longer-range repulsion during the approach at pH 9 (Figure 7B,D) as compared to that at pH 6 (Figure 7A,C), which is attributed to the ionization of model oleic acid and the adsorption of OH− on the interacting surfaces that strengthen the EDL repulsion. The Hamaker constant for the functionalized tip−water−fluorite was calculated to be 6.8 × 10−20 J based on the combining relation of Hamaker constants for functionalized tip−fluorite, fluorite− fluorite, and water−water.54 By fitting the measured force results between the functionalized tips and fluorite surfaces with the classical DLVO theory (red curve), the surface potentials of fluorite could be determined. As shown in Figure 7A,B, the surface potential of the fluorite {111} surface is determined to be −35 mV at pH 6 and increases to −63 mV at pH 9. The {100} surface in Figure 7C,D shows a lower magnitude of surface potential (−5 mV at pH 6 and −33 mV at pH 9). The

Figure 9. Water contact angle values on fluorite {100} and {111} surfaces treated with oleate solutions at different concentrations at pH 9.

a separation distance below 2 nm is mainly due to the effects of surface roughness.60 The histograms of adhesion measured during retraction (the typical retraction force curves are shown in Figure S1) and the fitted Gaussian distributions (solid curves) in Figure 5C,D show that the adhesion of model oleic acid on the {111} surface is measured to be 716.9 mN/m, which is much weaker than that on the {100} surface (1146.3 mN/m). On the basis of the DMT model (eq 8), the effective adhesion energies of model oleic acid on fluorite {111} and {100} surfaces in air are calculated to be Wadh ∼114.1 and ∼182.4 mJ/m2, respectively, which indicates that the carboxyl group of model oleic acid has a stronger interaction with the {100} surface than the {111} surface in air. It has been found that the most favorable adsorption configuration of −COOH on the calciumcontaining mineral surface is in the bridged form followed by a bidentate or unidentate coordination.61,62 It is noted that the adhesion was measured in dry air, and the water contact angle of the fluorite {111} surface (79.2°) was similar to that of the fluorite {100} surface (72.1°). Therefore, the influence of air humidity and surface moisture on the comparison of adhesion on different fluorite surfaces could be neglected here. It is also noted that the adhesion measured on the {111} surface was much lower than that on the {100} surface. As shown in Figure 6A, the fluorine atom exists between two adjacent calcium 2517


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Figure 10. Topographic AFM images (1 × 1 μm2) of fluorite (A) {100} and (B) {111} surfaces treated with oleate solution at a concentration of 5 × 10−5 mol/L at pH 9.

the fluorite {111} surface were calculated to be −292.8830 and −104 kJ/mol, respectively.68 4.5. Adsorption of Oleic Acid on Fluorite Surfaces. At pH 9, sodium oleate has a much stronger adhesion force with two fluorite surfaces. Thus, contact angle measurements were also carried out to investigate the wettability of the fluorite {100} and {111} surfaces treated with oleate solution at different concentrations at pH 9.0, as shown in Figure 9. The concentration from 1 × 10−6 to 5 × 10−5 mol/L was chosen in this study, given that the adsorption of oleate on the fluorite surface gradually reached a full monolayer with the increase of the concentration studied.69 Figure 9 shows that the contact angle values (or hydrophobicity) increase steadily for the two surfaces as the oleate concentration increases. The contact angle value of the fluorite {100} surface is about 5° higher than that of the {111} surface. This also indicates that a larger adsorption density and a stronger interaction of COO− on the {100} surface can lead to a more hydrophobic {100} surface when treated with oleate solution. The surface topographies of fluorite {100} and {111} treated with oleic acid solution at pH 9 were also captured, as shown in Figure 10. We chose the concentration of 5 × 10−5 mol/L to treat fluorite surfaces in oleate (COO−) solution because the adsorption of COO− reached a full monolayer on the fluorite surface at this concentration.69 The adsorption of oleate acid on the fluorite {100} surface is relatively compact and smooth (Figure 10A), while pronounced aggregates could be observed on the fluorite {111} surface after adsorption (Figure 10B), indicating a larger adsorption density and a stronger interaction of COO− on the {100} surface.

anisotropy in the surface charge of different orientations could be mainly attributed to the difference in the atomic composition, with fluorine exposed on the {111} surface that could enhance its electronegativity. Zeta potential was reported to be around −28 mV at pH 6 and −43 mV at pH 9 for the fluorite {111} surface,15 whereas the zeta potential of the fluorite {100} surface was found to be more positive,28 which were consistent with the surface potential values obtained from AFM force measurements in this work. The histograms of adhesion measured during separation (the typical retraction force curves are shown in Figure S1) and the fitted Gaussian distributions (solid curves) in 1 mM NaCl in Figure 8 show that the adhesion of 16-MHA on the fluorite {111} surface is weaker than that on the {100} surface, which could be attributed to the combined effects of the more stable binding mode, more active surface calcium atoms, and weaker EDL repulsion for the {100} surface. The effective adhesion energy between the model oleic acid and the fluorite {111} surface is estimated to be ∼10.0 mJ/m2 at pH 6 and ∼11.3 mJ/ m2 at pH 9, whereas the adhesion energy on the {100} surface is relatively higher with the value of 20.9 mJ/m2 at pH 6 and 24.0 mJ/m2 at pH 9. For both fluorite {111} and {100} surfaces, the adhesion energy of RCOOH at pH 6 is slightly weaker than that of RCOO− at pH 9. With pH increased from 6 to 9, the carboxyl groups of model oleic acid are deprotonated and become more negatively charged, which could enhance the electrostatic repulsion between the model oleic acid and fluorite surfaces. However, RCOO− was found to exhibit stronger surface activity as compared to RCOOH,63 thereby leading to a stronger interaction between RCOO− and calcium atoms on fluorite surfaces, which could be the origination of relatively higher adhesion energy during the separation of model oleic molecules and fluorite surfaces at pH 9. These results are supported by previously reported computational simulation results.30,68 The adsorption energies of COO− and COOH with

5. CONCLUSIONS In this work, the surface properties of fluorite surfaces with different orientations and the interaction forces of model oleic acid molecules with these fluorite surfaces were investigated at the nanoscale both in air and in aqueous solutions at different 2518


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pHs using AFM. The force results demonstrate the difference in the intermolecular forces during the approach of model oleic acid molecules to fluorite {111} or {100} surfaces, which can be well-fitted with the DLVO theory. The theoretical fitting of AFM force profiles show that the surface potential of the {111} plane was −35 mV at pH 6 and −63 mV at pH 9, with a higher magnitude than that of the {100} plane (−5 mV at pH 6 and −33 mV at pH 9) because of different atomic compositions. The adhesion energy of model oleic acid molecules on the {111} plane was found to be weaker than the {100} case both in air and in 1 mM NaCl solutions, indicating the strong anisotropic surface atom activity of different orientations. More specifically, each calcium atom has four dangling bonds on the {100} surface while only one dangling bond on the {111} surface. Consequently, the calcium atom on the {100} surface is more active, which was supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules show a higher adhesion energy with fluorite surfaces at pH 9 than that at pH 6, which could be attributed to the higher activity of the ionic form of model oleic acid molecules in the alkaline condition to form effective adhesion bonding with the fluorite surfaces. This work shows a useful methodology for probing the anisotropic surface properties and interaction forces of organic molecules on mineral surfaces with different orientations at the nanoscale. This methodology can be readily extended to many other interfacial processes.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b04165. Typical retraction force curves measured between the functionalized AFM tip and fluorite surfaces in air and in 1 mM NaCl solutions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.S.). *E-mail: [email protected]. Phone: +1-780-492-1044 (H.Z.). ORCID

Hongbo Zeng: 0000-0002-1432-5979 Author Contributions §

Z.G. and L.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

The authors acknowledge financial support from the National Natural Science Foundation of China (51774328 and 51404300), the Innovation-Driven Program of Central South University of China (2017CX007), Young Elite Scientists Sponsorship Program By CAST (2017QNRC001), the National 111 Project (B14034), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and the Canada Research Chairs program (H.Z.). 2519


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Probing Anisotropic Surface Properties and Surface Forces of Fluorite

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