Probing Adsorption of Polyacrylamide-Based Polymers on Anisotropic

Article pubs.acs.org/Langmuir

Probing Adsorption of Polyacrylamide-Based Polymers on Anisotropic Basal Planes of Kaolinite Using Quartz Crystal Microbalance Lana Alagha,† Shengqun Wang,† Lujie Yan,† Zhenghe Xu,*,†,‡ and Jacob Masliyah† †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

‡

ABSTRACT: Quartz crystal microbalance with dissipation (QCM-D) was applied to investigate the adsorption characteristics of polyacrylamide-based polymers (PAMs) on anisotropic basal planes of kaolinite. Kaolinite basal planes were differentiated by depositing kaolinite nanoparticles (KNPs) on silica and alumina sensors in solutions of controlled pH values. Adsorption of an in-house synthesized organic−inorganic Al(OH)3-PAM (Al-PAM) as an example of cationic hybrid PAM and a commercially available partially hydrolyzed polyacrylamide (MF1011) as an example of anionic PAM was studied. Cationic Al-PAM was found to adsorb irreversibly and preferentially on tetrahedral silica basal planes of kaolinite. In contrast, anionic MF1011 adsorbed strongly on aluminumhydroxy basal planes, while its adsorption on tetrahedral silica basal planes was weak and reversible. Adsorption study revealed that both electrostatic attraction and hydrogen-bonding mechanisms contribute to adsorption of PAMs on kaolinite. The adsorbed Al-PAM layer was able to release trapped water overtime and became more compact, while MF1011 film became more dissipative as backbones stretched out from kaolinite surface with minimal overlapping. Experimental results obtained from this study provide clear insights into the phenomenon that governs flocculation-based solid−liquid separation processes using multicomponent flocculants of anionic and cationic nature.

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INTRODUCTION Understanding adsorption of polymers on clay mineral surfaces has been a scientific subject for a long time1 due to its critical importance in a large variety of systems. In solid−liquid separation as applied to soil stabilization,2,3 water treatment,4 oil recovery,5,6 mud formulation, medical applications,7 nanocomposites processing,8,9 and oil sand tailings management,10−12 for example, polymers are used to flocculate fine clays to enhance particle settling and hence solid−liquid separation. In mineral processing, on the other hand, polymers are used to depress clays (gangue) while floating valuable minerals after selective hydrophobization by specially designed surfactant molecules (collectors). Clays, in general, are platy natural materials, formed by the weathering and decomposition of igneous rocks.13 Clays are known as phyllosilicates and consist mainly of stacks of two-dimensional aluminosilicate layers: silicon dioxide tetrahedral sheets and aluminum oxyhydroxyl octahedral sheets. Tetrahedral and octahedral sheets can be stacked in clay in a variety of manners, leading to different types of clays. The 1:1 clay minerals, for example, are made up of one tetrahedral sheet (T) and one octahedral sheet (O) as an elementary layer. This type of clay sometimes is referred to as T−O clay. Kaolinite is a typical example of T−O type of clays. On the other hand, if an elementary layer of clays consists of an aluminum oxy-hydroxyl octahedral sheet © 2013 American Chemical Society

sandwiched between two silicon dioxide tetrahedral sheets, it is referred to as 2:1 type, which is often referred to as T−O−T clays. Illite, smectite, and montmorillonite are 2:1 type of clays. An overall negative charge on clay basal planes is a result of isomorphic substitution of higher valence metal ions with lower valence metal ions in the lattice structure of clays. This charge is compensated for by cations such as Na+ and Ca2+ that exist in the interlayer region. These cations are exchangeable. The cation exchange capacity of clays depends on many factors, such as type of clays, the degree of isomorphic substitution, and pH of exchange solutions.13−15 Kaolinite, the main mineral of kaolin, has a complex yet interesting surface chemistry because of its anisotropic charge characteristics of edges and basal planes.16,17 Because kaolinite is a 1:1 alumino-silicate (T−O) clay mineral, it has two different basal planes: tetrahedral siloxane surface of Si−O−Si exposure and octahedral aluminum oxy-hydroxyl (Al−O−OH) surface of Al−OH exposure, each having its own charge characteristics. At the edge surfaces of the 1:1 clays, the structure is disrupted with broken bonds that are readily hydrolyzed, exhibiting pH-dependent charging characteristics. Received: December 15, 2012 Revised: February 10, 2013 Published: February 11, 2013 3989

dx.doi.org/10.1021/la304966v | Langmuir 2013, 29, 3989−3998

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Table 1. Polyacrylamide-Based Polymers Used in the Present Study

a

Al content was measured using atomic absorption spectroscopy (AAS).

kaolinite particles on positively charged alumina substrate at the same pH value, alumina basal planes would be exposed while the negatively charged silica basal planes should attach to the positively charged alumina surface. In this way, the anisotropic basal planes of kaolinite can be differentiated and the polymer adsorption on a specific basal plane can be determined. This differentiation allows probing the bonding sites on each basal plane and the interaction mechanisms of different polymers with clay minerals in aqueous systems. In this study, quartz crystal microbalance with dissipation (QCM-D) is used to investigate the adsorption characteristics of polymers on anisotropic basal planes of kaolinite. QCM-D technique can provide both qualitative and quantitative information of polymers adsorbed on solid surfaces. It could also detect conformational changes and adsorption/binding geometry of adsorbates (polymers) at solid/liquid interfaces.28−31 Becauase polyacrylamide (PAM) and its derivatives or copolymers are the most important and commercially available water-soluble polymers used as flocculants,10−12,32−34 two polyacrylamide-based polymers are chosen in the adsorption study (Table 1). An in-house synthesized Al(OH)3-polyacrylamide is used as an example of cationic hybrid PAM, and a commercially available partially hydrolyzed polyacrylamide MF1011 is used as an example of anionic PAM. In Al(OH)3polyacrylamide, also known as (Al-PAM),31,35 positively charged Al(OH)3 colloidal particles serve as cores connected to several PAM chains to form a star-like structure, which was believed to play a major role in improving dewatering characteristics of the polymer in flocculating both “T” and “O” basal planes of kaolinite suspensions.11,35,36 T- and O-basal planes of kaolinite were differentiated by depositing kaolinite nanoparticles on silica and alumina QCM-D sensors at pH 7.8−8. Colloidal forces were measured between silicon−nitride tip and the prepared basal planes in electrolyte solutions using atomic force microscopy (AFM) prior to adsorption tests to confirm the differentiation of basal planes and better understand the effect of interfacial surface charge of kaolinite basal planes on the adsorption process of polymers. Adsorption experiments of Al-PAM and MF1011 on a specific basal plane were conducted with the deposited basal planes.

Edges are estimated to occupy approximately 20% of the total kaolinite surface, depending on particle size and the degree of delamination (aspect ratio).18,19 Kaolinite will be the focus of this study due to its practical importance in a wide range of systems such as paper making, ceramic processing, composite materials design, industrial waste management, coal processing, and oil sands extraction. Since the early 1940s, rapid advances have been made to understand the underlying principles of interactions between different types of clays and water-soluble polymers.20 The results from several early studies21,22 suggested that organic polymers such as polyacrylamides interact with basal planes of phyllosilicates through hydrogen bonds formed between polymer moieties and oxygen atoms on siloxane basal planes. However, recent infrared study on clay−organic complexes indicated that oxygen atoms on siloxane basal planes are very weak electron donors and not capable of participating in hydrogen bonding.23,24 Theng25,26 suggested that polymer moieties interact mainly with exchangeable cations on clay surfaces through water bridges. Much of the previous work related to the bonding mechanism between PAMs and clay minerals has been focused mainly on the molecular properties of polymers such as molecular weight and charge density controlled by pH.27 The basic knowledge on interactions between organic polymers and clays such as mode of bonding, active sites, and the conformation of adsorbed species on the clay surface remains to be explored. To fundamentally explain the electrokinetic behavior of clay minerals and thus their interactions with polymers in aqueous systems, it is necessary to study the anisotropic charge properties of their surfaces and examine polymer adsorption on a specific surface such as basal plane and edge surfaces. For this purpose, clay surfaces have to be differentiated. In the case of kaolinite, anisotropic basal planes can be differentiated by depositing kaolinite particles on substrates bearing different charges, such as silica and alumina over a certain pH range, which will result in different arrangement of their basal planes on those substrates.19 For example, when depositing kaolinite particles on silica substrate, at pH ∼8, kaolinite particles would expose their negatively charged silica basal planes while the positively charged alumina basal planes should preferentially attach to silica surface. On the other hand, when depositing 3990


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Figure 1. Scanning electron microscopy (SEM) micrographs of kaolinite particles deposited on silica and alumina QCM-D sensors.

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prepared kaolinite suspension was determined to be ∼5 μm. The kaolinite aqueous suspensions were then left for 2 days to allow the settling of large particles. The remaining supernatant was then filtered through 0.20 μm filter. After the pH was adjusted to ∼7.5, the average size and zeta potential of the kaolinite nanoparticles remained in the supernatant were determined using ZetaPals (Brookhaven, New York) to be 165−170 nm and −35 mV, respectively. The SEM micrographs in Figure 1 show an average area (2D) size of 500 nm. To prepare desired basal planes for QCM-D experiments, ozone treated silica and alumina sensors were placed on a hot plate at ∼60− 70 °C. Two drops of the filtrate with remaining kaolinite nanoparticles (pH 7−7.5) were placed on each QCM-D sensor using a glass pipette and allowed to dry. The fast heating process was used to avoid aggregation of the deposited kaolinite particles during the drying process and to enhance the adhesion of kaolinite particles on QCM-D sensor surfaces. Silica and alumina sensors were then washed with Milli-Q water and blow dried with high-purity nitrogen. Determination of Kaolinite Layer Thickness on QCM-D Sensors. Thickness of kaolinite particles deposited on QCM-D sensors was determined using Q-sense 401 by stitching the data files obtained from measuring sensor oscillations in air, before and after kaolinite particle deposition. In a typical layer thickness measurement, a dry QCM-D sensor, silica or alumina, was mounted in the QCM-D flow chambers, and air was pumped at a flow rate of ∼0.2 mL/min for 10 min. Crystal oscillations were recorded for 5−10 min. After deposition and drying of kaolinite nanoparticles, the same sensor was mounted back to the QCM-D module and oscillations, under the identical conditions were recorded again in air for 5−10 min. The two data files were stitched together using the “stitching data files” function provided in the Q-sense 401 software. The frequency change before and after deposition of kaolinite nanoparticles was determined and converted to mass using Q-tool software. The masses of kaolinite particles deposited on both sensors were determined to be very similar (∼25 mg/m2). This mass value is equivalent to a layer thickness of ∼15 nm. Measurements of Polymer Adsorption by QCM-D. Polymer adsorption on kaolinite was measured using a QCM-D (E4) system from Q-sense (Gothenburg, Sweden). The quartz crystal oscillators (sensors) were AT-cut family with a diameter of 14 mm and a fundamental shear oscillation frequency of 5 MHz. The cell is mounted on a Peltier element, which provides an accurate temperature control (±0.02 °C). Experiments were run at 22 °C and pH 7.8−8.0. The concentration of polymers was fixed at 500 ppm for QCM-D experiments. The changes in frequency and dissipation were measured simultaneously at 5, 15, 25, 35, 45, 55, and 75 MHz. All measurements started by running the background solution (BG), which was Milli-Q water unless otherwise stated, at the same pH as polymer solutions and a constant flow rate of 0.15 mL/min. To determine the accurate amount of mass adsorbed on kaolinite basal planes, modeling of the adsorbed mass was performed using the Voight model,37,38 which is available in QCM-D software (Q-tool). In calculations, the frequency and dissipation energy loss were used as model input, while the

MATERIALS AND METHODS

Materials. Al-PAM used in this work was synthesized in-house by following the procedures described elsewhere.31,35 A partially hydrolyzed PAM, Magnafloc (MF1011), was purchased from Ciba (UK) and used without further purification. Polymer stock solutions were prepared at a concentration of 500 ppm in Milli-Q water two days prior to their use. Silicon dioxide-coated (simulating silica basal surface) and aluminum oxide-coated (simulating aluminum oxyhydroxyl basal surface) QCM-D sensors were purchased from Qsense. High-purity kaolinite was obtained from the University of Utah and used for preparation of kaolinite basal planes by deposition of the particles on QCM-D sensors.19 Reagent-grade hydrochloric acid and sodium hydroxide from Fisher Scientific (Canada) were used as pH modifiers. Reagent grade potassium chloride, also purchased from the Fisher Scientific (Canada), was used for the preparation of electrolyte solutions used in the colloidal force measurements. Deionized water with a resistivity of 18.2 MΩ cm, prepared with an Elix 5 followed by a Millipore-UV Plus water purification system (Millipore Inc., Canada), was used throughout this study, unless otherwise stated. Zeta Potential (ζ) Measurements. Zeta potential measurements were carried out using a Brookhaven ZetaPALS (New York). Zeta potential values were determined using 500 ppm Al-PAM in Milli-Q water suspension, without addition of any electrolyte. The zeta potential value of the Al-PAMs measured at pH 8.5 as shown in Table 1 is a result of overall charges of organic−inorganic hybrid Al-PAM, including contributions of the positively charged Al(OH)3 colloidal cores and dissociated PAM side chains on the Al(OH)3 colloidal cores at an Al to PAM ratio of 0.25 wt %. It should be noted that synthesized Al(OH)3 colloids without PAM have a zeta potential value of 30 mV, which provide strong binding sites with negatively charged tetrahedron silica basal planes. Scanning Electron Microscopy (SEM) Imaging. Scanning electron microscopy micrographs (SEM) of kaolinite deposited on silica and alumina sensors were obtained using a JAMP-9500F field emission Auger Microprobe (JEOL) operating at an accelerating voltage of 10 kV. AFM Colloidal Force Measurements. Agilent 5500 AFM (Agilent Technologies, Inc., Chandler, AZ) was used to measure the interaction forces between the basal planes of kaolinite coated on QCM-D sensor surface and a silicon nitride tip (NP, Veeco Inc., Santa Barbara, CA). Force measurements were carried out in 1 mM KCl solutions of varying pH values. The system was allowed to stabilize for 30 min before any force measurements. To obtain representative results, colloidal forces were measured on at least five locations of a given basal plane surface. All of the experiments were carried out at room temperature (20 ± 1 °C). Preparation of Desired Kaolinite Basal Planes. High-purity kaolinite was used to prepare 0.6 wt % of clay suspensions in Milli-Q water. The pH of the aqueous kaolinite suspensions was adjusted to 10 to efficiently disperse the particles. The prepared kaolinite dispersions were sonicated for 5 min using a sonication bath. The d50 of the 3991


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adsorbed layer was assumed to be uniform in both thickness and density. As recommended, the data from at least two overtones with a good signal-to-noise ratio were used in calculation. In this work, the fifth, seventh, and ninth overtones were used to interpret the adsorption results due to their stable responses. Quartz Crystal Microbalance with Dissipation − Principles. To take full advantage of the capability of quartz crystal microbalance with dissipation, that is, QCM-D, it is important to understand the working principle and data analysis of the instrument. A typical QCM-D consists of a piezoelectric quartz crystal sensor that is used to measure small mass change of deposited/adsorbed materials at nanogram resolution. The thin piezoelectric sensor crystal has gold electrodes on both of its surfaces. Resonance of the quartz crystal occurs when the thickness of the plate (tq) is an odd integer of half wavelengths of the induced wave (tq = n × λ/2). At resonance frequency, an oscillating electric field induces mechanical shear waves in the crystal. The resonance frequency (f) of the quartz crystal is determined by applying an alternating electric field across the crystal.37−41 Any increase in mass (Δm) bound to the quartz crystal surface causes the crystal oscillation frequency to decrease, leading to a negative shift of the resonance frequency (−Δf). The linear relation between Δm and Δf was demonstrated in 1959 by Sauerbrey.42

df = Δf = −

are its basal planes while the steps and kinks are representative of edge surfaces. Unfortunately, the SEM micrographs in Figure 1 do not provide any indication whether the exposed surface is T- or O-basal planes. A recent study by Gupta and Miller19,44 demonstrated that kaolinite particles ordered themselves differently on surfaces carrying different charges over a certain pH range (Figure 2). When deposited on glass substrates

Figure 2. Proposed preferential arrangement of kaolinite nanoparticles on silica and alumina QCM-D sensors. “▲” represents the tetrahedral silica of kaolinite, while “■” represents the octahedral alumina. (A) Alumina basal planes are attached to silica substrate while the silica basal planes are exposed; (B) silica basal planes are attached to alumina substrate while the alumina basal planes are exposed.

2·f 2 f 1 ·Δm = − n 0 = − n· ·Δm tq ·ρq vq ·ρq C

(SiO2), kaolinite particles exposed their silica 001 face with positively charged alumina face preferentially attached to the negatively charged glass surface. On the other hand, when depositing kaolinite particles on positively charged fused alumina substrate (Al2O3), the 001̅ alumina face was exposed with the negatively charged silica face attached to alumina substrate.19,44 To confirm this preferential arrangement when kaolinite nanoparticles were deposited on QCM-D silica and alumina sensors, colloidal forces between a silicon nitride tip and basal planes of kaolinite deposited on silica and alumina sensors were measured by AFM in 1 mM KCl solutions at three pH values of 6, 8, and 10. Although the deposited kaolinite layers were macroscopically rather rough as shown in Figure 1, the basal plane of individually micrometer size clay particles was relatively smooth. When interacting with AFM tips of 20 nm sizes, these micrometer size basal planes provided opportunities to probe their interactions with AFM tip of known surface charge characteristics. The scatter of the measured force profiles was determined to be within 20%, which is within the scatter range of general AFM probe force measurement.45 The most reproducible force curves obtained are shown in Figure 3. It is important to mention that the silicon nitride tip used in this work carries a negative charge over the pH range of interest

where C is the mass sensitivity constant (C = 17.7 ng cm−2 Hz−1 at 5 MHz), while n is the number of overtones (n = 1, 3,...). The condition for the Sauerbrey equation to hold is that the adsorbed film should couple perfectly with the shear oscillation of the sensor’s dead mass. This may not be always the case, in particular when the adsorbed film is viscous. In the case of soft polymers or biofilms, for example, the adsorbed film does not follow sensor’s mechanical oscillation as a dead or rigid mass, but deforms in the shear direction.40 In this case, the frequency change caused by such a viscous film will not only depend on its actual mass but also on its elastic and viscous nature of the mass. In a QCM-D system, the dissipation of sensors’s energy, D, which results from adsorption of a viscous or loose layer, is also measured.39 This dissipation energy is given by: D = 1/Q = E D/2· π ·ES where ED is the energy dissipated during one period of oscillation, and ES is the energy stored in the oscillating system. The measurement of D by QCM-D allows a more accurate estimation of the adsorbed mass in a viscous film by introducing a shear viscosity coefficient and a shear elasticity modulus μ in one of two basic models: (i) Maxwell model and (ii) Voigt model. Both of these models are provided by Q-tools software in the QCM-D instrument. The Maxwell model is usually applied to adsorbed polymers, which exhibit purely liquid-like behavior, in particular at low shear rates. The Voigt model, on the other hand, is applicable to adsorbed polymers, which conserve their shape and do not flow.40,41 The Voigt model is also applicable to polymer systems at high frequency as for the case of physiosorption. Because we cannot ensure purely liquid-like behavior of polymers in our systems, the Voigt model is used in our study. Four physical parameters are often used to describe the viscoelastic behavior of the adsorbed film: film thickness, density, shear modulus, and viscosity.43 For our system, the density and viscosity of bulk fluid are set at 996 kg/m2 and 0.001 kg/ms, respectively.

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RESULTS AND DISCUSSION A. Orientation of Kaolinite Nanoparticles on Silica and Alumina Sensors. Scanning electron microscopy (SEM) micrographs of kaolinite nanoparticles deposited on silica and alumina QCM-D sensors are shown in Figure 1. It is evident that on both sensors, kaolinite nanoparticles are preferentially aligned parallel to substrate surfaces as anticipated. This orientation indicates that the mostly exposed kaolinite surfaces

Figure 3. AFM force curves measured between a silicon nitride tip and basal planes of kaolinite nanoparticles deposited on (A) alumina and (B) silica QCM-D sensors at pH 6, 8, and 10. Solid curves represent the best fit of the force profiles to the classical DLVO theory with the best fitted surface (Stern) potential values given in Table 2. 3992


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(pH > 4) as shown earlier by the AFM colloidal force measurements.46 As shown in Figure 3A, attractive forces dominate interactions between silicon nitride tip and basal planes of kaolinite nanoparticles deposited on alumina sensor at pH 6. Considering a negatively charged silicon nitride tip, the observed attractive forces indicate the basal plane of kaolinite nanoparticles deposited on alumina QCM-D sensor is either positively charged or very weakly negatively charged at this pH. To have a definitive answer, the measured force profile is fitted with the classical DLVO theory. In DLVO fitting, the interaction energy of van der Waals forces per unit area (UvdW) was calculated using UvdW =

Table 2. Fitted Surface (Stern) Potential Values of Kaolinite Surfaces Deposited on Silica and Alumina QCM-D Sensors in 1 mM KCl at 20 °C Stern potential (mV) kaolinite on alumina

kaolinite on silica

silicon nitride tip

6 8 10

+5 −20 −25

−15 −30 −45

−45 −50 −55

nanoparticles deposited on alumina QCM-D sensors. This value is within the isoelectric point value of pH 6 and 9 reported previously for fused alumina,44,51 confirming that the exposed faces are indeed the octahedral alumina basal planes. As shown in Figure 3B for the basal planes of kaolinite nanoparticles deposited on silica QCM-D sensor, force profiles become progressively more repulsive with increasing pH from 6 to 10, suggesting a negatively charged basal plane of kaolinite nanoparticles deposited on silica QCM-D sensor over this pH range. The Stern potential values of the basal planes of kaolinite nanoparticles deposited on silica sensor, obtained from fitting of the measured force profiles to the classical DLVO theory, decrease from −15 mV for pH 6 to −30 and −45 mV for pH 8 and 10, respectively (Table 2). These values are similar to the values reported in the literature for the similar systems.19 The results suggest that the exposed surfaces of kaolinite nanoparticles deposited on silica QCM-D sensor are the tetrahedral silica basal planes, as anticipated. It should be also noted that the edges of kaolinite sheets, which contain Al−OH and Si− OH groups, also bear a negative charge over the studied pH range of 7.8−8.46 B. Adsorption of Polyacrylamides on Kaolinite. B.1. Interactions of Polyacrylamides with Clays. The bonding mechanism between polyacrylamide polymers and clay minerals has been a subject of much debate. There is a general agreement that the adsorption of cationic polyacrylamides on clays is due to charge neutralization.20,52 However, the nature of interactions between anionic polyacrylamides and negatively charged clays is less certain. On the basis of the results obtained so far, several mechanisms may be at play:52−54 (1) ligand exchange between the surface hydroxyls of the mineral and the carboxylic anions of the polymer; (2) formation of hydrogen bonds between the surface hydroxyls and carbonyl (CO) groups of polymers; (3) bridging of divalent cations between negatively charged sites on the clay surfaces and carboxylate groups of the anionic PAM; (4) generation of protons on the edge surfaces of clay crystallites by the polarization of water molecules in contact with the exchangeable cations of clays, followed by the transfer of hydrogen to amide groups and adsorption of the polymer on the resulting protonated sites; and (5) hydrophobic bonding between the carbon chain of the anionic PAM and the uncharged basal planes of clays. Hydrogen bonding has been previously proposed to be the main driving force for adsorption of nonionic polyacrylamide on aluminosilicate surfaces.52−56 Surfaces such as mica and silicon dioxide contain silanol (−SiOH) groups that can participate in hydrogen bonding. In the case of kaolinite, adsorption of polyacrylamide through hydrogen bonding can occur on both basal planes and edge surfaces. While basal oxygen atoms are poor electron donors and largely incapable of forming H-bonds, free basal silanol groups as well as the formation of Si−OH and Al−OH groups on the clay edge surfaces provide suitable adsorption sites.9,46,49−51 There are

−AH 12πD2

where AH is the Hamaker constant and D is the distance between the two interacting surfaces. The interaction energy arisen from electrostatic double layer force (Uedl) was calculated by applying the Hogg−Healy−Fuerstenau (HHF) model47 to the current conical tip with a spherical cap at its apex interacting with a flat surface using Derjaguin approximation.48,49 For the current system, a mixed boundary condition, that is, constant surface potential for silicon nitride tip and constant surface charge density for the basal plane, was used in calculating electrostatic double layer forces. The corresponding HHF equation for calculating the interaction energy per unit 50 area (Uσ−ψ as: edl ) is given by Kar et al. σ−ψ Uedl =

pH

⎧ ⎛ 4πσ 2 εκψa2 ⎞ 1⎪ b ⎟ ⎨2ψaσb sec h(κD) + ⎜⎜ − 4π ⎟⎠ 2⎪ ⎝ εκ ⎩ ⎫ ⎪ ⎬ × [tanh(κD) − 1]⎪ ⎭

where ψa and σb are the Stern potential of silicon nitride tip and charge density of kaolinite basal planes, respectively; κ is the decay length of electrical double layers; and ε is the dielectric constant of electrolyte solutions. Using the reported surface (Stern) potential value of −45 mV for silicon nitride tip,46 Figure 3 shows an excellent match of the measured force profile with that predicted by DLVO theory (solid line) and Stern potential of +5 mV for the basal planes of kaolinite nanoparticles deposited on alumina. The excellent fit suggests that the basal planes of kaolinite nanoparticles deposited on alumina are positively charged. Because the silica is negatively charged at this pH, the observed positive Stern potential indicates that the exposed basal plane of kaolinite nanoparticles deposited on alumina QCM-D sensor is the aluminum oxyhydroxyl octahedral basal plane. As was also shown in Figure 3, when the pH of the aqueous electrolyte solution is increased to 8 and 10, a progressively increased energy barrier was observed between the silicon nitride tip and basal planes of kaolinite nanoparticles deposited on alumina QCM-D sensor. Using the reported Stern potential values of −50 and −55 mV for silicon nitride tips at these two pHs, the measured force profiles can be well fitted to the classical DLVO theory as shown by the solid lines in the figure. The best fit led to a Stern potential of −20 and −25 mV for the basal plane of kaolinite nanoparticles deposited on alumina QCM-D sensor at pH 8 and 10, respectively (Table 2). The results show a point of zero charge between pH 6 and 8 for the basal planes of kaolinite 3993


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basal planes of kaolinite as revealed from the force curves shown in Figure 3B. In addition to electrostatic attraction as driving force, formation of hydrogen bonds is also possible between the OC−NH2 functional groups in the polyacrylamide backbone and the free silanol groups (Si−OH) on kaolinite T-basal planes and edge surfaces. This strong polymer−clay interaction on both basal planes and edge surfaces via electrostatic attraction and hydrogen bonding is the main reason for the rapid, strong, and irreversible adsorption of Al25PAM600 on kaolinite deposited on silica sensors. The increase of dissipation to ΔD ≈ 10 upon adsorption of Al25-PAM600 on kaolinite deposited on silica sensor suggests a viscous nature of the Al25-PAM600 film adsorbed. Such an increase also gives an indication on conformational change of the adsorbed Al25-PAM600 layer at the solid/liquid interface. In general, polymers can adopt different configurations when adsorbed on solid surfaces as illustrated in Figure 5.58 The

two possible sites as H-bonding acceptor in polyacrylamide molecule: the carbonyl group and the nitrogen atom. Studies have shown that most of the dissociation occurs at the carbonyl group rather than the nitrogen atom site.9,55 At natural pH of kaolinite suspension in water (pH ∼8), ionized SiOH groups (SiO−) at the basal plane also participate in adsorption through electrostatic interactions when polymers carry a positive charge. B.2. Adsorption of Al(OH)3-PAM on Silica Basal Planes of Kaolinite. Adsorption of Al25-PAM600 on T-basal planes of kaolinite deposited on silica sensor is shown in Figure 4. The

Figure 4. Adsorption of Al25-PAM600 on kaolinite deposited on silica sensor: (A) mass uptake and (B) plot of ΔD as a function of Δf. The inset in (A) is the plot of adsorbed mass as a function of square root of adsorption time to illustrate diffusion-controlled mechanism of early stage adsorption.

layer thickness of kaolinite nanoparticles on silica sensor was determined to be ∼15 nm as discussed earlier in this article. As mentioned earlier, for all QCM-D experiments, kaolinitedeposited silica sensors were flushed with background solution (BG) of pH ∼7.8−8 to establish a horizontal baseline. A 500 ppm Al25-PAM600 in Milli-Q water solution of the same pH as the BG was then pumped through the flow module cell at a flow rate of 0.15 mL/min. A rapid decrease in the frequency accompanied with a rapid increase in dissipation was observed upon switching from BG to polymer solution. As shown in Figure 4A, the amount of Al25-PAM600 adsorbed was estimated to be ∼18 mg/m2, and it took less than an hour for the Al25PAM600 to reach the equilibrium state at which polymer chains relaxed with a maximum number of polymer segments attached to the T-basal plane. The inset of Figure 4A shows the adsorbed areal mass of polymer (Γt) plotted as a function of square root of adsorption time √t. The first region of the Γt − t1/2 plot shows a linear correlation between (Γt) and t1/2, indicating that the early stage of Al25-PAM600 adsorption on Tbasal planes of kaolinite is diffusion-controlled. The surface concentration at time (t) obey the Flick’s second law:57

Figure 5. Possible configurations of polymer molecules adsorbed at solid/water interface:52 (A) single point attachment (weak binding); (B) loop adsorption; (C) flat multiple site attachment (strong adsorption); (D) random coil (high molecular weight polymers); (E) nonuniform segment distribution; and (F) multilayer adsorption.

configurations shown in Figure 5 depend on many factors such as the structure of polymer itself, molecular weight, the number and position of active functional groups in the polymer molecules, number of active sites available on the solid surface, the nature of solvent, and the strength of binding between polymer and solids. Adsorption modes (A), (B), (D), and (E) in Figure 5 would result in a thick layer and a large increase in dissipation. In the case of Al25-PAM600 adsorption on kaolinite deposited on silica sensor, despite the negative charge of Tbasal planes of kaolinite and the cationic nature of polymer, Al25-PAM600 does not appear to adopt the well-known pancake configuration59 due to its star-like structure. As a result, adsorption mode (E) rather than (C) is expected. This favorable configuration of Al25-PAM600 at clay/water interface is due to the presence of several active sites, such as positive Al(OH)3 cores and amide groups in the Al25-PAM600 structure that can interact with negatively charged silica basal planes by either electrostatic attraction and/or via H-bonding. A plot of ΔD−Δf for Al25-PAM600 adsorption on kaolinite deposited on silica sensor was constructed for a better understanding of the interfacial behavior and rearrangement

Γ = 2 Γ 0(Dt /π l 2)1/2

where Γ is surface concentration of Al25-PAM600 accumulated over time t (mg/m 2 ), Γ 0 is the equilibrium surface concentration of Al25-PAM600, t is adsorption time, l is the thickness of kaolinite layer and D is the diffusion coefficient. In the current system, accurate value of D for Al25-PAM600 is difficult to obtain since the uptake of polymer determined by QCM-D is the solvated mass of polymer, i.e., polymer with associated water. Washing the adsorbed layer with BG solution after reaching the equilibrium of adsorption resulted in no detectable desorption. The strong irreversible adsorption of Al25-PAM600 on T-basal planes of kaolinite deposited on silica sensor can be attributed to strong electrostatic attraction between the positively charged polymer and the negatively charged T3994


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Figure 6A. After the adsorption reached a steady state, flushing the cell with BG results in a complete desorption of the adsorbed mass as indicated by a mass uptake value of ∼0 mg/ m2. The fast and complete desorption is attributed to the weak binding in the presence of strong electrostatic repulsion between the negatively charged pendants of MF1011 and the negatively charged T-basal planes and edges. Figure 6B shows the change of dissipation in response to the adsorption of MF1011 on T-basal planes of kaolinite. A single stage adsorption process was observed, exhibiting dissipation to mass ratio of ∼0.4. This dissipation to coverage (mass) ratio than the ratio of the Al25-PAM600 adsorption on the same surface over the initial adsorption region I shown in Figure 4B. The observed larger dissipation to mass ratio suggests that the anionic MF1011 adsorbed on negatively charged silica basal planes with a minimal number of segments attached to the surface via hydrogen bonding, which was hindered by the electrostatic repulsion. The low surface coverage of MF1011 adsorption suggests a random mushroom-like configuration of MF1011 on T-basal planes of kaolinite deposited on the silica QCM-D sensor. When polymer adsorbs in such a configuration, tails and loops are predominant. Because of the anionic nature of MF1011 in the good solvent (water), the polymer chains can move freely and protrude into the aqueous phase, which results in a large dissipation to adsorbed mass ratio. A similar adsorption model was proposed for the adsorption of charged high molecular weight organic polymers on surfaces of similar charges.60 B.4. Adsorption of Al(OH)3-PAM on Alumina Basal Planes of Kaolinite. Figure 7 shows the adsorption of Al25-PAM600 on

of Al25-PAM600 molecules adsorbed on kaolinite over time. The results in Figure 4B show three-step adsorption characteristics, indicated by the change of the slope of straight line (ΔD/Δf). Initially in region I, a fast increase in dissipation with decreasing frequency (i.e., increasing coverage or mass) is observed (ΔD/ Δf ≈ 0.25). This region represents a low areal (Γ) coverage. Polymer chains in this region would adsorb with few binding points, leaving loops and tails protruding into the aqueous phase. Because of the low surface coverage at this stage, polyacrylamide arms and Al(OH)3 particles would have sufficient room to move and rearrange at the solid/liquid interface. This adsorption mode results in a larger dissipation to coverage ratio in region I. In region II, dissipation continues to increase with increasing coverage but to a less extent as compared to region I. This observation is indicated by the smaller ΔD/Δf ratio of ∼0.1. As more Al25-PAM600 molecules try to approach the surface, rearrangement of molecules is expected here. Polymer chains tend to stretch out to reduce the steric hindrance. As a result, the layer thickness of adsorbed Al25-PAM600 continues to increase. However, the hydrodynamic interaction between adjacent molecules as well as chain crowding would reduce the movement and slippage of the adsorbed molecules at the solid−liquid interface, which results in a slower change in the dissipation. In region III, ΔD remains constant with continuous increase in the adsorbed mass. At this specific stage, it appears that water molecules start to be squeezed out by incoming Al25-PAM600 molecules, leading to a negligible change in dissipation with further increase in Al25PAM600 adsorption. B.3. Adsorption of MF1011 on Silica Basal Planes of Kaolinite. The adsorption of an anionic polyacrylamide, MF1011, on kaolinite basal planes deposited on silica sensors was also investigated. As shown in Figure 6A, MF1011

Figure 7. Adsorption kinetics of Al-PAM on kaolinite deposited on alumina sensor: (A) mass uptake and (B) ΔD−Δf plot. Figure 6. Adsorption of MF1011 on kaolinite deposited on silica sensor: (A) mass uptake and (B) ΔD−Δf plot.

basal planes of kaolinite deposited on alumina sensor. As compared to the adsorption of ∼18 mg/m2 on basal planes of kaolinite deposited on silica sensor shown in Figure 4A, the maximum mass uptake of 10 mg/m2 on kaolinite deposited on alumina as shown in Figure 7A is much smaller. Furthermore, unlike adsorption on kaolinite deposited on silica, the mass uptake of Al25-PAM600 on basal planes of kaolinite deposited on alumina sensor reduced from the maximum value of ∼10 mg/ m2 to an equilibrium value of ∼8 mg/m2 with prolonged adsorption. Such a distinct difference between the two cases suggests a dominant effect of clay basal planes rather than edge surfaces on the adsorption of Al25-PAM600. The difference in the adsorption behavior between the two cases is attributed to different arrangement of kaolinite nanoparticles when deposited on different substrates as mentioned earlier in section A of the Results and Discussion. When deposited on alumina substrate, the kaolinite nanoparticles exposed their octahedral alumina basal planes due to electrostatic attraction of positively charged alumina sensor surfaces and negatively charged T-basal planes of kaolinite, which was schematically shown in Figure 2. At pH

exhibited little adsorption on T-basal planes of kaolinite at pH 7.8−8 (∼3 mg/m2). The adsorption of MF1011 on kaolinite T-basal planes deposited on silica sensor (∼3 mg/m2) is much weaker than the adsorption of Al25-PAM600 (20 mg/ m2) on the same substrate at the same pH. As discussed earlier in this Article, the T-basal plane of kaolinite deposited on silica sensor over the pH range studied is negatively charged. Therefore, strong electrostatic repulsion between negatively charged SiO− sites on tetrahedral silica sheets and negatively charged moieties in MF1011 molecules is anticipated. MF1011 molecules in this case would adsorb through hydrogen-bonding mechanism between amide groups in the polymer chains and silanol groups on kaolinite basal planes only after overcoming the electrostatic repulsion. On the other hand, the van der Waals forces may also contribute to bringing the MF1011 molecules to the negatively charged tetrahedral silica sheets. In this case, the adsorption of MF1011 is reversible as shown in 3995


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driving force is the hydrogen bonding between amide groups in the MF1011 molecules and hydroxyl groups on O-basal planes of kaolinite. Desorption of MF1011 after reaching mass uptake maximum did occur, which resulted in an overall mass drop from 15 to 11 mg/m2. This desorption can be attributed to repulsive forces between the adsorbed anionic MF1011 molecules and relatively weak binding of the MF1011 molecules on alumina basal planes, which are slightly negative at pH 8 as shown by the measured force curves. In addition, the edges of kaolinite layer also carry a net negative charge at the pH of the experiment.46 The repulsive interactions are also possible between the edges of kaolinite layers and anionic MF1011 molecules. As shown in Figure 8B, the ΔD−Δf plot reveals two different adsorption regimes prior to desorption of adsorbed MF1011. When compared to the adsorption of Al25-PAM600 on silica basal planes, an opposite behavior is observed. Initially in region I at lower surface coverage, the adsorbed MF1011 molecules are far apart from each other to allow maximum number of trains attached to the surface. Because of the high molecular weight and the linear structure of MF1011, most of the molecular segments at this stage are in train configuration as indicated by the low dissipation to coverage ratio (ΔD/Δf = 0.17). This adsorption mode has been mentioned in the literature as “pancake-like”, and it is always observed when polymer adsorbs on the surfaces of opposite charges,59 suggesting that MF1011 in the current system occupies mainly positively charged sites at low surface coverage. With increasing the surface coverage, the adsorbed MF1011 molecules start to touch each other. Because adsorption is running in a good solvent, which is water in this case, and due to the anionic nature of MF1011, negatively charged groups start to repel each other to avoid overlap due to electrostatic repulsive forces. At this point, the strongly adsorbed groups will replace the weakly adsorbed backbones, pushing these polymer backbones away from the surface into the aqueous phase. This rearrangement would lead to the formation of more extended MF1011 layer at the solid−water interface, resulting in an increase in dissipation to coverage ratio to ΔD/Δf = 0.4, region II in Figure 8B. Eventually, the repulsive interaction and competitive occupancy lead to desorption of adsorbed MF1011 molecules after reaching its maximum mass uptake of 15 mg/m2. Accompanying this decrease in areal coverage to 11 mg/m2 is the drop of dissipation value. C. Robust Flocculation Process Using Dual Polymers. Results obtained from the adsorption experiment of cationic hybrid Al25-PAM600 and anionic MF1011 on silica and alumina basal planes of kaolinite shed light on the mechanism of flocculation of clay suspensions using a dual polymer system consisting of a cationic and an anionic polyacrylamide-based polymer.5,61,62 According to data obtained from this particular work, cationic hybrid Al25-PAM600 is anticipated to adsorb preferentially on T-basal planes while the anionic MF1011 on O-basal planes as proposed in Figure 9. The adsorption of Al25-PAM600 on T-basal planes occurs via strong electrostatic attractive forces and hydrogen bonds, which is anticipated to result in the formation of compact flocs, while the high molecular weight anionic MF1011 adsorbs on O-basal planes mainly through hydrogen bond, bridging smaller flocs into larger aggregates. Therefore, more effective flocculation can be achieved by extending the one polymer system to multi polymer flocculation system using for example MF1011 and Al25-

between 7.8 and 8, the surface potential of O-basal planes is slightly negative with the majority of surface sites being neutral Al−OH groups. As a result, weaker electrostatic attraction between slightly negatively charged O-basal planes of kaolinite and cationic Al25-PAM600 is expected, as compared to the case of T-basal planes. The adsorption of Al25-PAM600 on kaolinite deposited on alumina surface occurs mainly through hydrogen bonding, enhanced by specific adsorption between Al−OH in Al25-PAM600 and neutral Al−OH groups on O-basal planes to form Al−O−Al linkage. However, it is possible that few positively charged species in the form of (Al−OH2+) are present on the O-basal plane of kaolinite deposited on alumina sensor at the pH close to the PZC of alumina basal planes. As a result, there might be a small electrostatic barrier for Al25PAM600 adsorption on kaolinite deposited on alumina, which could lead to the observed slower adsorption process and lower overall adsorbed mass shown in Figure 7A. The adsorption of Al25-PAM600 on O-basal planes of kaolinite deposited on alumina appears to follow the adsorption mode (B) shown in Figure 5. Al25-PAM600 molecules adsorb through H-bonding facilitated by weak electrostatic attractive forces. This adsorption mechanism would result in a significant increase in layer thickness, accompanied by an increase in dissipation shift to ∼6. The ΔD−Δf plot for Al25-PAM600 adsorption on kaolinite deposited on alumina sensor shown in Figure 7B reveals a single adsorption mode. Over the entire adsorption period, the ΔD/Δf ratio was determined to be 0.25. This invariant ΔD/Δf ratio indicates that the dissipation of the adsorbed Al25-PAM600 layer continuously increases with increasing coverage without any further rearrangement (densification) or relaxation of the adsorbed Al25-PAM600 molecules at the solid−liquid interface. This interesting observation is basically due to the absence of steric hindrance or overlapping of Al25-PAM600 chains as a result of limited adsorption under weaker binding forces, as compared to the case of Al25-PAM600 adsorption on T-basal planes. B.5. Adsorption of MF1011 on Alumina Basal Plane of Kaolinite. Figure 8 shows the adsorption characteristics of

Figure 8. Adsorption kinetics of MF1011 on kaolinite deposited on alumina sensor: (A) mass uptake and (B) ΔD−Δf plot.

MF1011 on O-basal planes of kaolinite deposited on alumina sensor at pH 7.8−8. As shown in Figure 8A, the amount of MF1011 uptake reached a maximum value of 15 mg/m2 before the film started to desorb until it reached an equilibrium value of about 11 mg/m2. Two possible mechanisms could be involved in the adsorption process: short-range (weak) electrostatic attraction and hydrogen-bonding formation. Electrostatic attraction exists between the negatively charged moieties in polymer backbones and positively charged sites on aluminum hydroxide basal planes, which might exist in the form of (AlOH2+) because the adsorption test was performed at the pH near the PZC of alumina basal planes. However, the major 3996


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for this work from NSERC (Natural Sciences and Engineering Research Council of Canada) Industrial Research Chair in Oil Sands Engineering is gratefully acknowledged. The partial support (Z.X.) by the National Science Foundation of China (Grant number 51274129) for this work is also acknowledged.

Figure 9. Proposed robust flocculation process using cationic Al25PAM600 and anionic MF1011. Progression from A to C of preferential adsorption of Al25-PAM600 and MF1011 on T- and O-basal planes of kaolinite, respectively, would result in more effective flocculation.

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REFERENCES

(1) Schloesing, Th. Determination of clay in arable soils. Acad. Sci. Paris 1874, 78, 1276. (2) Nadler, A.; Perfect, E.; Kay, B. D. Effect of polyacrylamide application on the stability of dry and wet aggregates. Soil Sci. Am. J. 1996, 60, 555. (3) Zhang, X. C.; Miller, W. P. Polyacrylamide effect on in filtration and erosion in furrows. Soil Sci. Am. J. 1996, 60, 866. (4) Letterman, R. D.; Pero, R. W. Contaminants in polyelectrolytes used in water treatment. J. Am. Water Work Assoc. 1990, 82, 87. (5) Stutzmann, T.; Siffert, B. Contribution to the adsorption mechanism of acetamide and polyacrylamide on to clays. Clays Clay Miner. 1977, 25, 392. (6) Pefferkorn, E. Polyacrylamide at solid/liquid interfaces. J. Colloid Interface Sci. 1999, 216, 197. (7) Williams, L.; Holland, M.; Eberl, D.; Brunet, T.; De Courrsou, L. Killer clays! Natural antibacterial clay minerals. Miner. Soc. Bull. 2004, 139, 3. (8) Ray, S.; Okamoto, M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. Polym. Sci. 2003, 28, 1539. (9) Liu, P. Polymer modified clay minerals: A review. Appl. Clay Sci. 2007, 38, 64. (10) Li, H.; Long, J.; Xu, Z.; Masliyah, J. Novel polymer aids for lowgrade oil sand ore processing. Can. J. Chem. Eng. 2008, 86, 168. (11) Wang, X.; Feng, X.; Xu, Z.; Masliyah, J. Polymer aids for settling and filtration of oil sands tailings. Can. J. Chem. Eng. 2010, 88, 403. (12) Li, H.; Long, J.; Xu, Z.; Masliyah, J. H. Synergetic role of polymer flocculant in low-temperature bitumen extraction and tailings treatment. Energy Fuels 2005, 19, 936. (13) Anderson, R.; Ratcliffe, I.; Greenwell, H.; Williams, P.; Cliffe, S.; Coveney, P. Clay swelling - A challenge in the oilfield. Earth-Sci. Rev. 2010, 98, 201. (14) Pinnavaia, T. Intercalated clay catalysts. Science 1983, 220, 365. (15) Chen, B. Polymer-clay nanocomposites: An overview with emphasis on interaction mechanisms. Br. Ceram. Trans. 2004, 103, 241. (16) Tombacz, E.; Szekeres, M. Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite. Appl. Clay Sci. 2006, 34, 105. (17) Brady, P.; Cygan, R.; Nagy, J. Molecular controls on kaolinite surface charge. J. Colloid Interface Sci. 1996, 183, 356. (18) Ayadi, A.; Pagnoux, C.; Baklouti, S. Kaolin-poly(methacrylic) acid interaction: Polymer conformation and rheological behaviour. C. R. Chim. 2011, 14, 456. (19) Gupta, V.; Miller, J. Surface force measurements at the basal planes of ordered kaolinite particles. J. Colloid Interface Sci. 2010, 344, 362. (20) Deng, Y.; Dixon, J.; While, N. Adsorption of polyacrylamide on smectite, Illite, and kaolinite. Soil Sci. Am. J. 2006, 70, 297. (21) Emerson, W. Complexes of calcium-montmorillonite with polymers. Nature 1960, 186, 573.

PAM600. This dual polymer system would help to form a larger, higher density and more compact floccules of the largest settling rate and lowest supernatant turbidity. Such a dual polymer system for more effective flocculation of oil sands tailings has been explored.63

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CONCLUSIONS Understanding the microstructure of adsorbed polymer layers at kaolinite/water interface using quartz crystal microbalance with dissipation provides enhanced insights into the phenomenon that governs polymer flocculation-based solid−liquid separation processes. In the current work, the anisotropic basal planes of kaolinite were differentiated by depositing kaolinite nanoparticles on silica and alumina sensors at desired pHs. The different basal planes of kaolinite on QCM-D sensors were confirmed by surface force measurement using the scanning probing technique. The results from polymer adsorption on a specific basal plane revealed that both electrostatic attractions and hydrogen bonding contribute to the adsorption of Al25-PAM600 and MF1011 on kaolinite basal planes. MF1011 adsorbed preferentially on O-basal planes through weak electrostatic attractions and hydrogen bonds, while Al25-PAM600 adsorbed strongly on T- basal planes via both strong and long-range electrostatic attractive forces and hydrogen bonding. Al25-PAM600 was found to adsorb irreversibly on T-silica basal planes, while the MF1011 layer reproducibly underwent a rapid and total desorption on the same surface. Partial desorption was detected when adsorption of MF1011 or AlPAM was measured on O-alumina basal planes due to their weak binding with the O-basal planes at the pH studied and intermolecular repulsion among the charged moieties of polymers adsorbed on the basal planes. Cationic Al25-PAM600 formed a more compact adsorbed film than did the anionic MF1011 layer as shown by a smaller dissipation to frequency (ΔD/Δf) ratio, although both polymers adsorbed with the availability of loops and tails at the interface for efficient bridging flocculation. Moreover, holistic improvement in settling of fine solids in flocculation-based solid−liquid separation processes can be achieved by using dual polymer flocculant systems consisting of cationic hybrid Al25-PAM600 and anionic high molecular weight MF1011. 3997


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(22) Greenland, D. Adsorption of poly(vinyl alcohols) by montmorillonite. J. Colloid Sci. 1963, 18, 647. (23) Farmer, V. Characterization of adsorption bonds in clays by infrared spectroscopy. Soil Sci. 1971, 112, 62. (24) Susko, F. An FT-IR study of calcium-exchanged montmorillonite treated with polyacrylamide and poly(ethylene oxide). Miner. Metall. Process. 1990, 7, 206. (25) Theng, B. The Chemistry of Clay-Organic Reactions; John Wiley & Sons Inc.: London, 1974. (26) Theng, B. Formation and Properties of Clay-Polymer Complexes; Elsevier Science: Amsterdam, 1979. (27) Laird, D. Bonding between polyacrylamide and claymineral surfaces. Soil Sci. 1997, 162, 826. (28) Cooper, M.; Singletom, V. A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions. J. Mol. Recognit. 2007, 20, 154. (29) Kown, K. D.; Green, H.; Bjoorn, P.; Kubicki, J. D. Model bacterial extracellular polysaccharide adsorption onto silica and alumina: Quartz crystal microbalance with dissipation monitoring of dextran adsorption. Environ. Sci. Technol. 2006, 40, 7739. (30) Palmquist, L.; Holmberg, K. Dispersant adsorption and viscoelasticity of alumina suspensions measured by quartz crystal microbalance with dissipation monitoring and in situ dynamic rheology. Langmuir 2008, 24, 9989. (31) Alagha, L.; Wang, S.; Xu, Z.; Masliah, J. Adsorption kinetics of a novel organic-inorganic hybrid polymer on silica and alumina studied by quartz crystal microbalance. J. Phys. Chem. C 2011, 115, 15390. (32) Xu, Y.; Hamza, H. Thickening and disposal of oil sand tailings. Min. Eng. (Littleton, CO, U.S.) 2003, 55, 33. (33) Cymerman, G.; Kwong, T.; Lord, E.; Hamza, H.; and Xu, Y. Polymers in Mineral Processing. Proceedings of the 3rd UBC-McGill BiAnnual International Symposium on Fundamentals of Mineral Processing; Quebec City, QC, Canada, Aug 22−26, 1999; p 605. (34) Sanford, E. C. Processibility of athabasca oil sand: Interrelationship between oil sand fine solids, process aids, mechanical energy and oil sand age after mining. Can. J. Chem. Eng. 1983, 61, 554. (35) Yang, W.; Qian, J.; Shen, Q. A novel flocculant of Al(OH)3polyacrylamide ionic hybrid. J. Colloid Interface Sci. 2004, 273, 400. (36) Sun, W.; Long, J.; Xu, Z.; Masliyah, J. Study of Al(OH)3polyacrylamide-induced pelleting flocculation by single molecule force spectroscopy. Langmuir 2008, 24, 14015. (37) Weber, N.; Wendel, H. P.; Kohn, J. Formation of viscoelastic protein layers on polymeric surf aces relevant to platelet adhesion. J. Biomed. Mater. Res., Part A 2005, 72A, 420. (38) Rodahl, M.; Hook, F.; Kasemo, B. QCM operation in liquids: An explanation of measured variations in frequency and Q factor with liquid conductivity. Anal. Chem. 1996, 68, 2219. (39) Reviakine, I.; Morozov, A.; Rossetti, F. Effects of finite crystal size in the quartz crystal microbalance with dissipation measurement system: implications for data analysis. J. Appl. Phys. 2004, 95, 7712. (40) Voinova, M.; Rodahl, M.; Jonson, M.; Kasemo, B. Viscoelastic acoustic response of layeredpolymer films at fluid-solid interfaces: continuum mechanics approach. Phys. Scr. 1999, 59, 391. (41) Neyra, M. Interactions between titanium surfaces and biological components. Ph.D. Thesis Dissertation, Departament de Ciència dels Materials i Enginyeria Metallúrgica, E.T.S. d’Enginyeria Industrial de Barcelona, Universitat Politècnica de Catalunya, 2009. (42) Sauerbrey, G. The use of quartz oscillators for weighing thin layers and for microweighing. Z. Phys. 1959, 155, 206. (43) Munro, J.; Frank, C. Polyacrylamide adsorption from aqueous solutions on gold and silver surfaces monitored by the quartz crystal microbalance. Macromolecules 2004, 37, 925. (44) Gupta, V.; Hampton, M.; Nguen, A.; Miller, J. Crystal lattice imaging of the silica and alumina faces of kaolinite using atomic force microscopy. J. Colloid Interface Sci. 2010, 352, 75. (45) Butt, H. J.; Cappella, B.; Kappl, M. Force measurements with the atomic force microscope: Technique, interpretation and applications. Surf. Sci. Rep. 2005, 59, 1.

(46) Yan, L.; Englert, A.; Masliyah, J.; Xu, Z. Determination of anisotropic surface characteristics of different phyllosilicates by direct force measurements. Langmuir 2011, 27, 12996. (47) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Mutual coagulation of colloidal dispersions. Trans. Faraday Soc. 1966, 62, 1628. (48) 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. 2007, 85, 625. (49) Yin, X.; Drelich, J. Surface charge microscopy: Novel technique for mapping charge-mosaic surfaces in electrolyte solutions. Langmuir 2008, 24, 8013. (50) Kar, G.; Chander, S.; Mika, T. S. Potential energy of interaction between dissimilar electrical double layers. Colloid Interface Sci. 1973, 44, 347. (51) Kusmolski, M. Chemical Properties of Material Surfaces: Surfactant Science Series; Marcel Dekker, Inc.: New York, 2001. (52) Theng, B. K. Clay-polymer interactions: summary and perspectives. Clays Clay Miner. 1982, 30, 1. (53) Hadar, H.; Rami, K. Anionic polyacrylamide polymer adsorption by pyrophyllite and montmorillonite. Clays Clay Miner. 2003, 51, 334. (54) Nabzar, I.; Carroy, A.; Pefferkorn, F. Formation and properties of the kaolinite-polyacrylamide complex in aqueous media. Soil Sci. 1986, 141, 113. (55) Konan, K.; Peyratout, C.; Bonnet, J.; Smith, A.; Jacquet, A.; Magnoux, P.; Ayrault, P. Surface properties of kaolin and Illite suspensions in concentrated calcium hydroxide medium. J. Colloid Interface Sci. 2007, 307, 101. (56) Theng, B. Interactions of clay minerals with organic polymers: Some practical applications. Clays Clay Miner. 1970, 18, 357. (57) Ward, A.; Tordai, L. Time dependence of boundary tensions of solutions. I. The role of diffusion in time effects. J. Chem. Phys. 1946, 14, 435. (58) Sato, T.; Ruch, R. Stabilization of Colloidal Dispersions by Polymer Adsorption: Surfactant Science Series; Marcel Dekker, Inc.: New York and Basel, 1980; Vol. 9. (59) Ou-yang, H. D.; Gao, Z. A pancake-to-brush transition in polymer adsorption. J. Phys. II 1991, 1, 1375. (60) Liu, G.; Yan, L.; Chen, X.; Zhang, G. Study of the kinetics of mushroom-to-brush transition of charged polymer chains. Polymer 2006, 47, 3157. (61) O’Gorman, J. V.; Kitchener, J. A. Flocculation and de-watering of kimberlite clay slimes. Int. Miner. Process. 1974, 1, 33. (62) Gregory, J.; Barany, S. Adsorption and flocculation by polymers and polymer mixtures. Adv. Colloid Interface Sci. 2011, 169, 1. (63) Yuan, X. S.; Shaw, W. Novel processes for treatment of Syncrude fine transition and marine ore tailings. Can. Metall. Q. 2007, 46, 265.

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