Mapping and Quantifying Surface Charges on Clay Nanoparticles

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Mapping and Quantifying Surface Charges on Clay Nanoparticles Jun Liu, Ravi Gaikwad, Aharnish Hande, Siddhartha Das, and Thomas Thundat Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02859 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 16, 2015

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Mapping and Quantifying Surface Charges on Clay Nanoparticles Jun Liu,†, * Ravi Gaikwad,† Aharnish Bhojaraj,† Siddhartha Das,‡ and Thomas Thundat†,* †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta

T6G 2V4, Canada ‡

Department of Mechanical Engineering, University of Maryland, College Park, Maryland

20742, United States

ABSTRACT

Understanding the electrical properties of clay nanoparticles is very important since they play a crucial role in every aspect of oil sands processing, from bitumen extraction to sedimentation in mature fine tailings (MFT). Here, we report the direct mapping and quantification of surface charges on clay nanoparticles using the Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM). The morphology of clean kaolinite clay nanoparticles shows a layered structure, while the corresponding surface potential map shows a layerdependent charge distribution. More importantly, a surface charge density of 25 nC/cm2 was estimated for clean kaolinite layers by using EFM measurements. On the other hand, the EFM measurements show that the clay particles obtained from the tailings demonstrate a reduced

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surface charge density of 7 nC/cm2, which may be possibly attributed to the presence of various bituminous compounds residing on the clay surfaces.

TEXT I. INTRODUCTION Clay minerals are naturally occurring layered materials found abundantly in the soil.1 It has attracted much attention in a wide range of disciplines ranging from geoscience2 and soil electrochemistry3, 4 to nanocomposites5, 6 and oil sands industry7, 8. For example in the oil sands industry, clay nanoparticles play a vital role affecting all aspects of the industry from bitumen extraction, recovery and upgrading to sedimentation in tailing ponds.7 Several parameters such as size, morphology, etc. of clay nanoparticles have been investigated to understand their physical and chemical properties.9, 10, 11, 12 However, one of the most important properties of clay nanoparticles, namely their surface charges, have not been thoroughly investigated or understood. As a matter of fact, surface charge of clay nanoparticles plays the most critical role in their colloidal stability and sedimentation in the tailing.7 Understanding the surface charging of clay nanoparticles, therefore, is of great importance in tailing remediation and removal of residual clay particles from processed bitumen. Clay minerals are layered-structure materials consisting of periodically stacked Si-O tetrahedron sheet (T) and Al-O-OH octahedron sheet (O) as building blocks.7 Under mechanical delamination, the exposed face surface on the clay nanoparticle can either be tetrahedron siloxane surface (Si-O-Si), aluminum hydroxyl surface (Al-OH) or edge surfaces with broken SiO and Al-O bounds.7 The origin of surface charge is rather complicated in terms of surface chemistry, including ion exchange/adsorption, dissociation of surface group, orientation of


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adsorbed molecules with permanent dipole, etc.7, 13 It has been proposed that the overall surface charges of clay are composed of (i) permanent charges and (ii) variable charges. Permanent charges are caused by isomorphous substitution, e.g. Al3+ substituting Si4+ or Mg2+ substituting Al3+. Such substitutions always generate negatively charged sites that in turn can be balanced by interstitial cations like Ca2+, Mg2+, K+ and Na+. Variable charges on the clay surface, on the other hand, can either occur due to the loss of these above-mentioned interstitial cations when the clay sample comes in contact with water, or due to the induction of terminal hydroxyl groups on edge surfaces, which can be either negatively or positively charged determined by the nature of adsorbed metal ions as well as the pH value of the aqueous solution.4, 7, 13 These multiple charging mechanisms will imply that a heterogeneous charge distribution on the surface of these clay nanoparticles is expected. Though several approaches have been utilized to investigate the charged nature of clay, there exist many limitations. One common technique for determination of surface charge is the zeta potential measurement.7, 14 It is based on measuring the electrostatic potential at the shear plane, which is the plane differentiating the Stern and the Gouy-Chapman layers of the Electric Double Layer or EDL. Therefore, this potential is a measure of the surface charge of the object screened by the tightly bound (on the surface itself) inner Stern layer of counter-ions; hence this approach not only fails to provide the accurate value of the real surface charge (or potential) but also becomes completely inapplicable for cases where there is no EDL formation (for example, if one is interested to measure the native surface charge or surface charge in air of an object). Another technique is the ion adsorption-based method; this technique, in addition to being inapplicable in measuring charge/potential in air, also fails to deal with the systems containing both variable and permanent charges (e.g., clay particles), since H+/OH- ions in water always occupy an unknown


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ratio of the balanced charge.4 More recently, AFM techniques based on force curve measurement have been applied to investigate the surface charge effect on behavior of clays in electrolyte solutions or processing waters.15, 16, 17 At present, to the best of our knowledge, there exists no direct technique for visualizing and quantifying the surface charge on clay particles, which in turn may reveal their intrinsic charge distribution (i.e., native charge distribution or charge distribution in air) at nanoscale. In this paper, we demonstrate mapping and quantification of surface charges on clay nanoparticles by using Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM). KPFM and EFM are scanning probe-based techniques, which allow investigation of the surface potential distribution and quantification of the local charge density of various materials of interest with high spatial resolution.18, 19, 20. These techniques have been used extensively to study surface electronic/electrical properties of metal, semiconductor, organic materials and biological materials.18, 20 Recently, we used EFM to measure the native asphaltene surface charge or asphaltene charge in air. This finding is specially important to understand issues such as charge-mediated asphaltene stabilization and de-stabilization (e.g., coagulation and flocculation) in organic solvents (i.e., solvents that do not alter the asphaltene native charge significantly)21. Through this study, we established the relevance of EFM measurements in context of heavy and crude oil research. In the present study, we extend this use of EFM by using it to measure the surface charges of clay nanoparticles with massive relevance to a large number of processes in the oil sand industry. In our experiment, pure kaolinite was chosen as the reference clay for KPFM and EFM investigation based on the XRD result. Furthermore, we report on characterizing surface charges of clay nanoparticles from mature fine tailings of oil sands tailing pond located in Athabasca region of Alberta, Canada.


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II. EXPERIMENTAL SECTION Pure kaolinite powders were purchased from Acros Organics. Tailing ponds solution of the Athabasca oil sand sample was obtained from Institute for Oil Sands Innovation (IOSI), University of Alberta. The kaolinite solution was prepared by dissolving 1 mg pure kaolinite powder in 1 mL toluene, followed by a 30 min sonication. Subsequently, a droplet of the solution (20 µL) was drop-casted by pipette on a cleaned silicon oxide wafer (obtained from University wafers, Boston, MA). An estimated SiO2 layer thickness of ~500 nm is measured by ellipsometry. After evaporation of all the toluene, solid kaolinite nanoparticles are deposited on the SiO2 wafer. As for the sample preparation of clays from tailings, a droplet of the solution (20 µL) was taken from the supernatant of tailing ponds solution and drop-casted on the silicon oxide wafer. The SiO2 wafer substrate deposited with clay was mounted on a stage for imaging with Atomic Force Microscope (AFM). A schematic of the procedure is shown in Figure 1. In this configuration, topographic, KPFM and EFM imaging were performed in the “Tapping mode”, “AM-KPFM” and “Interleave (Lift) mode” respectively using Bruker Multimode 8 AFM (Santa Barbara, CA) system. In the KPFM experiment, a lift height of 30 nm was used to avoid the interference from the topography. Electrically conducting SCM-PIT probe (Bruker, Santa Barbara, CA) with resonance frequency of 65 kHz, spring constant of 2.8 N/m, tip of radius of curvature of 20 nm, and a quality factor of approximately 200 was used as the probe. The topography, KPFM and EFM images were processed using Nanoscope analysis software (V1.40, Bruker).


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Figure 1. Schematic of the sample preparation and steps involved in the AFM measurements: (a) A SiO2 wafer (1 cm2 in size) is cleaned with ethanol and dried with N2 gas, (b) a 10 µL drop of kaolinite/tailing ponds solution is deposited on the cleaned SiO2 wafer, (c) toluene/water is allowed to evaporate at room temperature resulting in solid clay nanoparticles on SiO2, and (d) the sample is mounted on a stage for AFM analysis for simultaneous topography and EFM/KPFM data collection. The inset shows the parallel-plate model for the electrostatic interaction between the AFM tip and the clay sample. The sample is assumed to be negatively charged, with total surface charge amount of Q. Image charges (-Q) are induced in the tip as well as in the substrate. For the EFM signal, the AFM is in the lift mode with the tip at a distance z above the sample.


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III. RESULTS AND DISCUSSION Kaolinite, the major contributor of fines in the oil sand, was chosen as the reference clay for study. Pure kaolinite is devoid of surface impurities and contamination from organic materials. Figure 2a shows a topographic image of the pure kaolinite nanoparticles deposited on SiO2 substrate following the evaporation of the toluene drop containing dissolved kaolinite. From the magnified image (Fig. 2b), we can clearly distinguish the layered structure of the single kaolinite nanoparticle, indicating that an effective mechanical exfoliation occurred during the sonication process. Details of the layered structure are further revealed by the cross section analysis of the structure (Fig. 2c) as is marked in Fig.2b. A well-defined three-layer structure with single layer height of 6-7 nm can be identified. Considering the crystalline structure of kaolinite, the unit cell of kaolinite consists of one Si-O tetrahedron sheet (T) linked through oxygen atoms to one Al-OOH octahedron sheet (O).13 Hence, a c-axis preferential orientation of the layered structure is predicted, and we may also estimate 8-9 stacked (O)/(T) sheets in the single layer according to the lattice parameter of kaolinite (c=7.25 Å).7 The estimated number of sheets can vary over a certain range in the presence of intercalated water between the sheets; this is the well-known “swelling effect” in clay minerals.22 It should be noted that these intercalated water molecules are difficult to remove even under high-temperature heating.7 On the center region of the kaolinite particle, amorphous morphology is observed, which is possibly due to the crystal distortion resulting from the sonication process. A more complex situation in terms of crystal may exist in this region where a greater number of surface defects are expected. As demonstrated further, these specific sites turn out to be highly charged.


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Figure 2. AFM topography of (a) kaolinite nanoparticles distribution on SiO2 substrate and (b) single kaolinite nanoparticle (the area marked in (a)). (c) Cross section height analysis corresponding to the dash line in (b). Figure 3a shows a typical topographic image of the kaolinite nanoparticles, and Fig. 3b is the corresponding KPFM surface potential image. A heterogeneous surface potential distribution of the kaolinite nanoparticles can be observed. From the magnified image (Fig. 3c) and its corresponding surface potential image, more detailed information on the clay nanoparticle surface charging can be obtained. It is found that the charge distribution is remarkably layerdependent, and is demonstrated from the overlapped image of topography and surface potential in Fig. 3e. It is found that the layered morphology and surface potential features have a good correlation with each other. Typically, two specific layers marked as regions A and B in Fig. 3c


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are selected for investigation. Figures 3f and 3g provide the cross section analysis of their height (Fig 3c) and surface potential (Fig 3d), respectively. It is quite intriguing to observe a layerdependent surface potential difference on the clay nanoparticles, for example, a typical height of 6.8 nm found on the layer on region B, shows a surface potential difference of -48 mV relative to the substrate while region A exhibits a relatively smaller height of 2.4 nm with a stronger surface potential (-94 mV), which is about two times of that on region B. It is interesting to note that both positive and negative surface potential with respect to the substrate are discernable on kaolinite nanoparticles, as is marked by blue and red rings in Figure 3b, respectively. These surface potential values can be regarded as the intrinsic electrical potential signal on the pure clay nanoparticle surface in solid state. Considering the non-polar property of toluene solvent in our experiment, the dominant charge on the kaolinite surface here should be the permanent charge sites caused by isomorphic substitution, as discussed above.7, 13 The surface potential can be seen to intensify on the center region of the kaolinite nanoparticle, which may arise due to the larger amount of surface defects on these amorphous clusters.


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Figure 3. (a) AFM topography and (b) the corresponding KPFM surface potential distribution of single kaolinite nanoparticle. (c) AFM topography and (b) the corresponding KPFM surface potential of the layered structure. (e) Overlapped image of topographic and surface potential images for a better visualization of the matched features. (f) and (g) are the cross section analyses of height and surface potential in (c) and (d), respectively. For KPFM measurement, the lift height is set to z=30 nm in order to avoid any interference from topography. Figure 4 shows another example of the surface potential distribution on kaolinite nanoparticle. A two-layer structure is clearly revealed by the cross section height analysis in Fig. 4c. The height of the layers is about 6.4 and 6.5 nm, respectively. It is shown that relative to the substrate, the


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first and the second layers have negative surface potentials of –16.5 mV and –24.4 mV, respectively (Fig. 4d). A step-like surface potential “jump” on the boundary between these two layers is distinguishable in the KPFM image; this may result from the heterogeneous charge distribution on the interface. Such phenomena are observed for other materials where the step height may cause a change in the distribution of charges due to the movement of the tip.23, 24

Figure 4. (a) AFM topography and (b) the corresponding KPFM surface potential distribution of the kaolinite nanoparticle with double-layered structure. (c) and (d) are the cross section analyses of height and surface potential in (a) and (b), respectively. For KPFM measurement, the lift height is set to z=30 nm. In an attempt to quantify the charge density on kaolinite, the specific layer with a height of 2.5 nm (as discussed previously in Fig. 3) was chosen for EFM investigation. Figure 5a shows the topographic image, and (b) is the corresponding image of EFM frequency change in lift mode. In the EFM measurement, the AFM tip detects the surface morphology during the trace, and detects the EFM signal during the retrace at a user-defined constant lift height (z). During the retrace, the conducting tip is sensitive to the surface charges, which is proportional to the force gradient in


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the vertical direction F'(z) ≡∂F/ ∂z.20 Consequently, the frequency shift ∆f with respective to the resonance frequency of cantilever f0 can be expressed by20 ∆f= -f0 F'(z)/ (2k),

(1)

where f0=65 KHz and k=2.8 N/m is the spring constant of the cantilever in our experiment. As for the origin of force F (z), it is considered to include Coulomb interactions of the charge on kaolinite layer, its corresponding image charges in the tip and Si substrate, and the induced charges of VEFM.25 By examining the EFM frequency change in the target region (Fig. 5b), a flat plateau can be observed in the cross section analysis (Fig. 5c), indicating an even distribution of charge on this specific layer. Accordingly, we consider the layer as a charged plate with a charge density of . By using a simple parallel-plate model25, 26, the force F (z) can be given by25

()

F z =

2  (h* )2σ 2 2h*VEFMσ ε 0VEFM  S × − + +  , * * 2 * 2 * 2  ε (z + h / ε )  2ε 0 (ε )

(2)

where h * = hSiO + hClay is the thickness of the dielectric layer, ε * the effective dielectric constant 2

of dielectric layer, z the tip-sample separation, VEFM the voltage applied, the charge density and S the surface area of the charged region. Combining Eq. (1) and (2), we can write the frequency shift ∆f as:

∆f =

2  ( h* ) 2 σ 2 2 h*VEFMσ ε 0VEFM  f0 S × − + +  , * * 3 * 2 * ε k ( z + h / ε )  2ε 0 (ε ) 2 

(3)

Figure 5d shows the plot of z-dependent ∆f as well as the fitted curve predicted by the Eq. (3). The error bar results from the different f value chosen from the plateau area. As the tip-sample separation increases, ∆f decreases due to the weaker electrostatic force gradient in the vertical


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direction. By using f0 =65 KHz, k=2.8 N/m, h * = 510 nm ( hSiO 2 =500 nm, hClay =10 nm for the clay double layer), ε*=10.2, ε0=8.85×10-12, S=104 nm2 (estimated area of the charged clay layer) and VEFM=0 V, a charge density of 25 nC/cm2 can be obtained according to equation (1). The coefficient of determination parameter ( CD R 2 ) of the fitting is 0.98, indicating the precise prediction of the EFM data via using the theoretical model. As disused above, this estimated value may be ascribed to the permanent charge on this kaolinite layer.

Figure 5. (a) AFM topography and the corresponding (b) EFM frequency distribution of the kaolinite layered structure at a lift height of 30 nm. (c) The cross section analysis of the


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frequency shift marked in region (b). (d) Plot of experimental data for the variation of frequency shift with lift height z in the marked region. Red curve represents prediction of the parallel-plate EFM model with f0 =65 KHz, k=2.8 N/m, h * = 510 nm, ε * =10.2, ε 0 =8.85×10-12, S=104 nm2 and VEFM=0 V. A charge density =25 nC/cm2 can be obtained accordingly. The error bar results from the different f value chosen from the plateau area. Surface charge of the clay nanoparticles impedes their coagulation, which in turn lays the foundation of the consolidated tailings technology in oil sand industry. Through Figs. 3-5, we have demonstrated the use of the KPFM and the EFM for determining the surface charging of pure clay nanoparticles; this motivated us to further study the surface charging of the clay nanoparticles from mature fine tailings sample obtained from oil sands tailing. In order to determine the type of clay in the tailing ponds solution, we performed the X-ray diffraction (XRD) analysis on the drop cast-deposited sample of the tailing ponds solution (Fig. 6d). Results confirm the presence of kaolinite and illite, which are the two dominant clay minerals in Athabasca oil sand.7 Figure 6a shows a typical topographic image of the nanoparticles from the same solution, whereas Fig. 6b provides the corresponding adhesion image acquired by Quantitative nanomechanical mode (QNM) mode. As shown in Fig. 6(c), all of these nanoparticles exhibit a sheet-like morphology with layer height of 6-7 nm (Fig. 6c), which is consistent with the typical height of kaolinite as demonstrated above. This specific morphology correlates with the proposed clay nanoparticle shape in literature14; this, along with the XRD result, indicate the very possibility of the presence of these clay nanoparticles in tailing ponds.


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Figure 6. (a) AFM topography and the corresponding (b) adhesion distribution of the clay nanoparticles obtained from the tailing ponds. (c) Cross section height analyses of the marked lines in (a). The red square marks highlight a specific region with higher adhesion (d) XRD pattern of the dried out tailing ponds solution. It is interesting to see from Fig. 6b that the adhesion signal of the marked layer is relatively higher than that of other clay nanoparticles. Meanwhile, the cross section analysis reveals a relative higher topography (7.3 nm) of the specific layer compared with the typical layer height (6~7 nm). We hypothesize that the clay nanoparticles in tailing pond solution are partially covered with some organic materials composed of some bituminous species.7 These viscous materials can very likely give rise to the increased height of clay nanoparticles in our experiment (marked in Fig 6b). It should be noted that AFM being a surface characterization technique is


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unable to distinguish the different types of clay but the XRD spectra clearly prove the presence of kaolinite and illite species in the tailing pond solution. However, this does not impede us from understanding the overall morphology of these clay nanoparticles as well as the charge behavior on their surface. Figure 7 shows the EFM study of a typical clay nanosheet from tailing ponds. Eq. (3) is used to fit the experimental results on the z-dependent EFM frequency change (Fig. 7a). It is found that the intensity of EFM signal depends strongly on the bias voltage VEFM in this specific experiment (Supporting Information, Fig. S1), which should be ascribed to the capacitance force manipulation.27 Therefore, the VEFM= -2 V was applied to the system in order to enhance the EFM signal contrast. A charge density of =7 nC/cm2 is obtained from the fit. The coefficient of determination parameter ( CD R 2 ) for the fitting is 0.94. It should be noted here that the electric surface properties of the clays from tailing ponds is much more complicated as compared to that of the pure kaolinite, stemming from a much more involved surface chemistry. As a matter of fact, besides the permanent charge sites on the structure, exchanged ions and various adsorbates such as water, bitumen and other organic spices in the tailing ponds could contribute to the overall surface charge to certain extent; this may explain the charge density difference between the pure kaolinite nanoparticles and clay nanoparticles from tailings ponds .7


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Figure 7. (a) Plot of experimental data for the variation of frequency shift with lift height z on the clay nanosheet from tailing ponds. Red curve represents prediction of the parallel-plate EFM model with f0 =65 KHz, k=2.8 N/m, h * = 505 nm, ε * =10.1, ε 0 =8.85×10-12, S=104 nm2 and VEFM= -2 V. A charge density =7 nC/cm2 can be obtained accordingly. The error bar results from cross section analysis at different locations on the nanosheet. (b) AFM topographic image and its corresponding (c) EFM frequency distribution of the clay nanosheet from tailing ponds at a lift height of 25nm. (d) and (e) shows the cross section analysis of the marked line in (b) and (c), respectively. By providing the direct visualization of surface potential distribution, we make a significant step forward for the understanding of electrical surface property on the clay nanoparticles, which plays vital role on the bitumen recovery and tailing ponds management in the oil sand industry. It can be expected that more Scanning Probe Microscopy (SPM)-based studies about the influence of external stimuli (pH value, organic additives etc.) on the surface charge behavior of clay nanoparticles may be conducted in the future, aiding in processes and techniques for tailing ponds remediation.


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IV. CONCLUSIONS We have presented direct mapping of surface charges on clay nanoparticles using KPFM and EFM technique. It was shown that a heterogeneous surface potential distribution exist on the pure kaolinite nanoparticles, which is remarkably layer-dependent. In our EFM study, a parallelplate model was used to predict the surface charge density on the specific clay layer, giving the estimated value of 25 nC/cm2 of the pure kaolinite sample. Moreover, we extended our EFM experiment to the clay nanoparticle from tailing ponds solution obtaining a surface charge density of 7 nC/cm2 by using the same model, which provides us with a direct perception of the electric surface charge density on these clay nanoparticles. The charge detection can help the oil sands industry to develop new processes and techniques for rapid settling of clay in MFT’s thereby paving the way for cleaner production of bitumen from oil sands. ASSOCIATED CONTENT Supporting Information Bias-dependent EFM images of clay nanoparticle. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * Jun Liu, E-mail: [email protected] * Thomas Thundat, E-mail: [email protected]


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Canada Excellence Research Chair (CERC) program for the Oil sands at the University of Alberta. We would also like to thank members of our Nanointerfaces and Molecular Engineering (NIME) group for fruitful discussions. REFERENCES (1) Bergaya, F.; Lagaly, G. General introduction: clays, clay minerals, and clay science. Developments in clay science 2006, 1, 1-18. (2) Hochella, M. F.; Lower, S. K.; Maurice, P. A.; Penn, R. L.; Sahai, N.; Sparks, D. L.; Twining, B. S. Nanominerals, mineral nanoparticles, and earth systems. Science 2008, 319 (5870), 1631-1635. (3) Vane, L. M.; Zang, G. M. Effect of aqueous phase properties on clay particle zeta potential and electro-osmotic permeability: Implications for electro-kinetic soil remediation processes. Journal of Hazardous Materials 1997, 55 (1), 1-22. (4) Hou, J.; Li, H.; Zhu, H.; Wu, L. Determination of clay surface potential: a more reliable approach. Soil Science Society of America Journal 2009, 73 (5), 1658-1663. (5) Okamoto, M.; Nam, P. H.; Maiti, P.; Kotaka, T.; Nakayama, T.; Takada, M.; Ohshima, M.; Usuki, A.; Hasegawa, N.; Okamoto, H. Biaxial flow-induced alignment of silicate layers in polypropylene/clay nanocomposite foam. Nano letters 2001, 1 (9), 503-505. (6) Fu, X.; Qutubuddin, S. Polymer–clay nanocomposites: exfoliation of organophilic montmorillonite nanolayers in polystyrene. Polymer 2001, 42 (2), 807-813.


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Mapping and Quantifying Surface Charges on Clay Nanoparticles

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