Adsorption of High-Molecular-Weight EOR Polymers on Glass

Apr 10, 2013 - Goshtasp Cheraghian , Seyyed Shahram Khalili Nezhad , Mosayyeb Kamari , Mahmood Hemmati , Mohsen Masihi , Saeed Bazgir. Journal of ...
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Adsorption of High-Molecular-Weight EOR Polymers on Glass Surfaces Using AFM and QCM‑D A. R. Al-Hashmi,*,† P. F. Luckham,‡ J. Y. Y. Heng,‡ R. S. Al-Maamari,† A. Zaitoun,§ H. H. Al-Sharji,∥ and T. K. Al-Wehaibi† †

Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33, 123 Muscat, Sultanate of Oman Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom § Poweltec, 3 rue Paul Heroult, ZAC Rueil 2000, 92500 Rueil, Malmaison, France ∥ Petroleum Development Oman, P.O. Box 81, Muscat, 113 Sultanate of Oman ‡

S Supporting Information *

ABSTRACT: High-molecular-weight (HMW) polyacrylamide and its derivatives are widely used in oilfield applications ranging from drilling fluids, enhanced oil recovery (EOR), and treatment of oil sand tailings. In these applications the adsorption characteristics of these polymers are essential since it would affect their applicability and efficiency. In this study, adsorption of three high-molecular-weight polymers (nonionic (NPAM), partially hydrolyzed (HPAM), and sulfonated (SPAM) polyacrylamides on silica surfaces from 2% KCl) is characterized using a quartz crystal microbalance with dissipation monitoring (QCM-D) and an AFM-based colloidal probe apparatus. QCM-D measurements show that semiequilibrium for adsorption on silica surfaces is reached within 3 h. The adsorbed amount and adsorption rate are highest for NPAM and lowest for SPAM. AFM experiments revealed that after 20 min of incubation in solution, HPAM induced bridging attraction on approach (i.e., compression). On the other hand, only a weak attraction is observed in the NPAM solution. However, SPAM shows only steric repulsion on approach after 20 min of incubation commencing at a separation of around 250 nm. Significant adhesion on retraction was observed after 20 min of incubation in NPAM and HPAM. However, only slight adhesion was observed in SPAM in the same time frame. After incubation in polymer solutions for 20 h, all polymers induced steric repulsion on approach and the absence of adhesion on retraction at different separations, indicating full surface coverage and different effective hydrodynamic layer thickness (EHT). On the basis of the AFM measurements after 20 h of incubation, the EHT of the adsorbed layers in NPAM, HPAM, and SPAM is 125, 30, and 175 nm, respectively. We believe that the results in this study will lead to enhanced understanding of the polymers under investigation with respect to their use in EOR applications. Moreover, this study gives clues on the differences between the three polymers under consideration with respect to their flocculating power, which is employed in the oil sand tailings treatments.

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INTRODUCTION

molecules very slow and hence hinders desorption considerably (i.e., irreversible adsorption).15 In polymer EOR, partially hydrolyzed polyacrylamides (HPAM) are normally used to increase the viscosity of the displacing aqueous phase, thus maximizing the sweep efficiency of the oil bank between the injection and the production wells.8 However, HPAM has its limitation with the increase in temperature, salinity, and hardness. In such cases, sulfonated polyacrylamides (SPAM) can be used due to their tolerance to such conditions. Loss of the injected polymer on the rock surfaces would increase both the technical and the financial cost of the EOR application since the injected slug would be deprived from its viscosifying agent (i.e., the polymer).8 As a result, EOR applies anionic polyacrylamides (HPAM and SPAM) due to their low affinity to the negatively charged sandstone surfaces. The flocculating power of the highmolecular-weight polyacrylamides is also taken advantage of in the extraction of oil from Athabasca oil sand reserves in

High-molecular-weight polyacrylamides have many industrial applications such as in drinking and industrial wastewater treatment,1,2 improvement of soil stability,3−5 and oil sand tailings treatment.6,7 Also, these polymers are widely used in oilfield applications such as drilling fluids and enhanced oil recovery (EOR).8 The optimized application depends greatly on the understanding of the adsorption characteristics,9 which have triggered extensive studies in this area.8,10−13 Nowadays, the adsorbed polymer macromolecule is pictured to consist of a series trains, loops, and two long tails.14 Trains are the directly attached segments of the macromolecule, and so this is the bound fraction of the macromolecule; hence, they act as anchors to the whole macromolecule on the solid surface. In between the trains, loops extend toward the solution away from the solid surface. Tails are the extending parts at both ends of the adsorbed polymer molecule. They are believed to be the longest part of the adsorbed polymer molecule. For large polymer molecules (i.e., high molecular weight), the sum of the adhesion energies for each monomer contributed by the trains’ segments is large enough to make relaxation of the adsorbing © 2013 American Chemical Society

Received: December 22, 2012 Revised: April 9, 2013 Published: April 10, 2013 2437

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Canada, which entails oil sand tailing treatments.16 This has attracted research on the flocculating nature of these polymers using colloidal force7,17,18 and settling measurements.18,19 Polymer EOR and oil sand tailing treatments involve interaction of the polymer with sandstone surfaces, which are predominately silica. Here a quartz crystal microbalance with dissipation monitoring, QCM-D, has been employed to investigate the adsorption of polymers on solid surfaces.20−24 This technique has been complemented by measurements of interaction forces using atomic force microscopy, AFM,25−28 which can be used to obtain information on the adsorption energy and flocculation characteristics of polyacrylamide.12,13,17,18,29−31 This study presents QCM-D and AFM measurements of the adsorption of HMW derivatives of polyacrylamide on silica-dominant surfaces (i.e., silica in QCMD and glass in AFM). The objective of this study is to investigate how different functional groups on the polymer backbone (namely, amide, acrylate, and acrylamido tert-butyl sulfonate) affect the adsorption of polyacrylamide on silica/ glass surfaces. Moreover, the colloidal forces measured using AFM can indicate the role of these polymers as flocculants.18

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Sweden. QCM-D can be used to monitor with time the adsorption of a polymer from its solutions. In this technology, AT-cut quartz crystals are sandwiched between a pair of gold electrodes and excited to produce a shear mode oscillation.33 The resonant frequencies of the crystals (fundamental and up to five harmonics) are recorded in real time. Once excited, the free mechanical oscillation can be monitored by measuring the decaying electric field.34 From the decay curve, the resonance frequency ( f) and dissipation (D) are extracted. If a layer is deposited on the surface of the crystal, changes in the frequency (Δf) and dissipation (ΔD) allow one to characterize the adsorption processes and structural features of the adsorbed layer in real time. If the film is thin and rigid, the decrease in the frequency is proportional to the mass of the film. The mass added to the surface of the crystal (including the solvent incorporated into the layer) can be calculated using the Sauerbrey relation35

Δm = −

vqρq Δf n 2f02 n

=−

C Δf n

where vq is the the speed of sound in the quartz crystal (vq = 3340 m s−1), ρq is the density of the quartz (ρq = 2.65 g cm−3), f 0 is the fundamental frequency ( f 0 = vq/2tq) where tq is the thickness of the quartz plate, C = 17.7 ng Hz−1 cm−2 for a 5 MHz quartz crystal, and n = 1, 3, 5, 7, 9, 13 is the overtone number. The Sauerbrey equation is strictly valid for adsorbed masses that are (i) small, (ii) homogeneous, (iii) rigid, and (iv) coupled with no slip. Hence, it is usually not valid for adsorbed polymer layers, which are viscoelastic in nature (i.e., not rigid). The viscoelasticity of the film will dampen the crystal oscillation due to the energy dissipated (Elost) by the film. Dissipation (D) is a measure of the viscoelasticity of the film and defined as

EXPERIMENTAL SECTION

Materials. Three linear polymers were used in this study: nonionic, partially hydrolyzed, and sulfonated polyacrylamides. Nonionic polyacrylamide (NPAM) is essentially a very high-molecular-weight polymer made of acrylamide monomers (Figure 1a). In practice, there

D=

E lost 2πEstored

where Estored is the energy stored in the oscillator. Dissipation of the crystal can be measured by recording the response of a freely oscillating crystal that has been vibrated at its resonance frequency. Consequently, it is anticipated that thick, loosely bound layers dissipate energy more than thin, rigid ones. Procedure. In this study, fresh silica (SiO2) crystals were used for each experiment. Crystals were sonicated for 10 min in DI water and then rinsed prior to being secured in the flow module. The module was then placed in the temperature-controlled chamber at 25 °C. Afterward, the 2% KCl solvent was injected through the cell at 3 cm3/ h. The baseline was then established by determining the fundamental frequencies ( f 0) and dissipations (D0). Measurements of Δf and ΔD of the different overtones (n = 1, 3, 5, 7, 9, 11, 13) were then monitored during the flow of the solvent. A time period was allowed so that temperature (T), Δf, and ΔD reach constant values. This was found to take different periods ranging from 30 to 60 min. Fundamental values of the frequency ( f 0) and dissipation (D0) were then reset. Measurements of Δf and ΔD during solvent flow were then found to stabilize around zero. This was tested for at least 5 min before injection of the polymer solutions into the flow modules. Afterward, the solvent was replaced by 100 mg/L polymer solution at the same flow rate (i.e., 3 cm3/h). Since the objective is to compare between the three polymers regarding their adsorption on silica, only one QCM-D measurement was conducted using each polymer. Normal Force Measurements. Force−distance measurements were conducted using a colloidal probe apparatus (or a modified AFM) developed in the mid-1990s in Imperial College London.27,36 In this apparatus, a small, soft cantilever is used to sense the force between a particle in the size range of 2−30 μm and a flat surface. The flat surface, which resides on a piezo, approaches and moves away from the particle attached to the cantilever with a predetermined frequency, typically on the order of 1 Hz. The resulting interaction between the particle and the flat surface is measured by monitoring the deflection of the cantilever in response to the interacting force. Deflection is determined using optical beam deflection. A beam from a laser diode is focused onto the end of the cantilever, and the position of the reflected

Figure 1. Chemical structure of the monomers composing the polymer. is usually a finite but small degree of hydrolysis of the amide group into acrylic acid (Figure 1b). NPAM was supplied by Polysciences with a quoted molecular weight of 18 million g/mol. Partially hydrolyzed polyacrylamide (HPAM) has a molecular weight of 18.5 million g/mol and 27.8% hydrolysis, which is the percentage of the acrylate groups in the polymer chain. Sulfonated polyacrylamide (SPAM) is a copolymer of acrylamide and acrylamido tert-butyl sulfonate (ATBS) (Figure 1c) with a quoted molecular weight of 6.5 million g/mol and a sulfonation (i.e., ATBS percentage in the polymer chain) of 25%. HPAM and SPAM are manufactured by SNF Floerger (France) for enhanced oil recovery applications. Three polymers were received as solid granules and expected to have wide polydispersity. Polymers were dissolved in 2% KCl with a pH of around 6.5. Stock solutions of concentration 1000 mg/L were first prepared. Once the desired weights of both the polymer and the solvent were ready, the solvent was put in a beaker and stirred vigorously using a paddle stirrer. Afterward, polymer granules were sprinkled on the shoulder of the well-developed vortex caused by movement of the paddle. After 1 h, the speed of the paddle mixer was lowered and the mixture was stirred overnight to ensure complete dissolution. QCM-D Technology. The quartz crystal microbalance with dissipation monitoring (QCM-D) was manufactured by Q-sense, 2438

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beam is monitored by a position-sensitive detector (PSD). The separation (D) between the particle and the sample surface is controlled by a piezoelectric ceramic of nanometer resolution. Both the piezo element and the PSD are fixed on an antivibration table. The amplified voltage driving the piezo element is generated by a function generator passing through a digital amplifier giving a working voltage range of ±150 V. The approach rate is controlled by the frequency ( f) and applied voltage, while the amount of compression and decompression is controlled by the magnitude of the amplified voltage. Procedure. Glass particles of diameter around 25 μm (Jencons, U.K.) were attached to the cantilevers using a two-part epoxy-based adhesive. Particles were used as supplied without any further treatment. However, these particles are from the same batch as those used in earlier experiments in our laboratory.13,27,31,36 Interactions were measured between these particles on one side and a glass surface on the other. This flat surface was the bottom surface of a glass dish. A fresh glass dish was first rinsed with DI water and then with 2% KCl prior to use in experiments. First, measurements were conducted in 2% KCl as a control and to establish the baseline for the next measurements in polymer solution. Afterward, the solvent was replaced with 100 mg/L polymer solution after which measurements were conducted after 20 min and 20 h of incubation of both surfaces in polymer solutions. All measurements were conducted at a lab temperature of around 20 ± 2 °C. Fresh glass surfaces were used for all experiments (spheres and dishes). The glass dish was covered with PARAFILM laboratory film between measurements to reduce solvent evaporation and minimize contamination. There were at least three approach−retraction profiles recorded at each site and at least three different sites for each time period investigated. All force− distance curves obtained from at least three different surface sites at each incubation time were similar in their form and values. In this study, one representative approach−retraction profile is shown as a typical example of the measurements obtained to compare between adsorption of the three polymers. The variation in onset of interaction point between experiments was on the order of 5%.

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measurement in the frequency and dissipation signals might be attributed to factors such polymer exchange processes among polymers with a wide molecular weight distribution14,15,39 and relaxation of adsorbed polymer molecules.40−42 The adsorption reflected by the decrease in frequency results in the increase in dissipation, which also reaches a semiequilibrium after around 5 h of flow through the cell. The change in frequency and dissipation for the different polymers in 2% KCl is in the following decreasing order: NPAM > HPAM > SPAM. The calculated adsorbed amount obtained using the third overtone is plotted with respect to time in Figure 3. After around 7 h of flow, the adsorbed amount is 2.7

Figure 3. Behavior of the adsorbed amount of 100 mg/L NPAM (●), HPAM (■), and SPAM (▲) on SiO2 surfaces from 2% KCl using the frequency shift of the third overtone of the QCM-D measurements reported in Figure 2.

RESULTS AND DISCUSSION

mg/m2 for NPAM, 2.3 mg/m2 for HPAM, and 1.3 mg/m2 for SPAM. However, these values should be taken with caution since the adsorbed amount here includes the trapped solvent within the adsorbed polymer layer which may result in an overestimate of the adsorbed amount of polymer.33 Even so these values are broadly in line with expected adsorbed amount expected for high-molecular-weight polymers.22,37−39 Here it is interesting to note that McFarlane and co-workers43 obtained the same order of equilibrated adsorbed amounts (i.e., NPAM > HPAM > SPAM) for these polymers on smectite clay. Moreover, our results show that the adsorption rate of the three polymers on the silica surface is in the following decreasing order NPAM > HPAM > SPAM, which is also the same order of adsorption rate of these polymers on smectite and kaolinite clays obtained by McFarlane and co-workers.43 QCM-D results show that adsorption of the polyacrylamide and its derivatives depends on the type of the other functional group grafted along with the acrylamide. For example, the presence of 27.8% acrylate in HPAM results in a decrease of the adsorption by around 15% compared to the similar molecular weight NPAM. This can be attributed to the repulsion between the negatively charged silica surface and the negatively charged acrylate groups. Indeed, one may be surprised that we get any adsorption at all. This is likely to be due to two factors here: first, the very high-molecular-weight of the polymers would encourage adsorption, as any slight tendency for any one monomer to be adsorbed will be greatly increased when one considers the whole polymer molecule. Second, the electrolyte concentration we are adopting here is also very high (0.27 M

QCM-D. The measured normalized frequency (Δf) and dissipation (ΔD) with respect to time in the third overtone are shown in Figure 2. As polymer flows to the cell, the frequency decreases sharply as the polymer starts to adsorb on the silica surface. The polymer seems to continue adsorbing for around 5 h before reaching a semiequilibrium stage. Takahashi and coworkers found that adsorption of HMW polymer can take a period from 3 h37 to sometimes 1 week to reach steady state values.38 This long time required for reaching a steady

Figure 2. Frequency (Δf), solid symbols, and dissipation (ΔD), open symbols, measurements of the third overtone of 100 mg/L NPAM (●), HPAM (■), and SPAM (▲) on SiO2 surfaces from 2% KCl measured at 25 °C. 2439

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KCl), so that the repulsion is screened to very short distances (the double-layer thickness being less than 1 nm). In addition, at these high electrolyte concentrations it is possible that K+ ions will adsorb to the glass surfaces to give anchor points for the polymer.44 It is worth noting that dissipation of the HPAM layer is around 40% that of the NPAM layer, indicating more flexibility of the latter. Apart from being around one-third the molecular weight of NPAM, the presence of the large nonhydrogen-bonding ATBS groups in SPAM decreases the adsorbed amount and dissipation by around 50% and 67%, respectively. Hence, the SPAM layer has the lowest adsorbed amount and highest rigidity. We note that it has been reported previously that the sulfonated polyacrylamides have lower adsorbed amounts compared to nonionic45 and partially hydrolyzed acrylamides46 on silica and kaolinite. Normal Force Measurements. The interaction between the bare glass surfaces in 2% KCl is shown in Figure 4. Silica

Figure 5. Force−distance measurements on approach (solid symbols) and retraction (open symbols) obtained between a 25 μm glass particle and a glass surface in 100 mg/L nonionic polyacrylamide (Mw = 18 × 106 g/mol) in 2 wt % KCl after 20 min of incubation in polymer solution.

Figure 6. Force−distance measurements on approach (solid symbols) and retraction (open symbols) obtained between a 25 μm glass particle and a glass surface in 100 mg/L nonionic polyacrylamide (Mw 18 × 106 g/mol) in 2 wt % KCl after 20 h of incubation in polymer solution.

Figure 4. Force−distance measurements on approach (solid circles) and retraction (open circles) obtained between a 25 μm glass particle and a glass surface in 2 wt % KCl prior to polymer addition.

surfaces are negatively charged over a wide range of pH due to the presence of SiO− on the surface.47 The high ionic strength of the solvent used in this study (0.27 M KCl) screens the charge of the surface; hence, double-layer repulsion is not observed in the interaction curves between the bare glass surfaces. Strictly speaking, van der Waals attraction resulted in a jump-in on approach between the bare surfaces at a separation of around 30 nm. Similar profiles were obtained prior to all colloidal force measurements. Figure 5 shows the force−distance curves on approach and retraction between the glass surfaces after 20 min of incubation in 100 mg/L NPAM in 2% KCl. Force on approach shows only a very weak attraction at a separation of around 10 nm. However, the adhesion force required to separate the two surfaces after contact is around 14 nN. On the other hand, measurements conducted after 20 h of incubation in solution as shown in Figure 6 indicate steric repulsion on approach starting at a separation of around 400 nm, upon which the force required for hard contact is around 4 nN. No adhesion on retraction can be observed, but there is a hysteresis between the approach and the retraction curves, which might be due to the fact that the rate of relaxation of the adsorbed layers is lower than the rate of retraction; these results are in line with previous studies of ours for the interaction between adsorbed homopolymers.13,36,48 Another possible explanation for such hysteresis is the requirement of breaking some entanglements

created during hard contact between the adsorbed polymer chains on both surfaces.44 The force−distance curves on approach and retraction after 20 min of incubation in 100 mg/L HPAM in 2% KCl are shown in Figure 7. Now there is a strong attraction on approach starting at a separation of around 60 nm, while an

Figure 7. Force−distance measurements on approach (solid symbols) and retraction (open symbols) obtained between a 25 μm glass particle and a glass surface in 100 mg/L partially hydrolyzed polyacrylamide (Mw = 18.5 × 106 g/mol) in 2 wt % KCl after 20 min of incubation in polymer solution. 2440

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adhesion force of around 10 nN is required to separate the two surfaces after full contact. In an earlier study by Abraham et al, 49 it was found that charge density and polymer concentration affect surface coverage of partially hydrolyzed polyacrylamide on silica surfaces as indicated from AFM measurements conducted after 30 min of incubation in the polymer solutions. For polymer concentration less than 5 ppm, it was found that increasing the degree of hydrolysis above 15% decreased surface coverage and induced bridging attraction on the approach curves.49 Consequently, bridging adsorption was not expected in our AFM measurements of HPAM. However, the molecular weight used here is an order of magnitude higher than the polymer used in the work of Abraham and coworkers,49 which can hinder polymer diffusion toward the surface and subsequent spreading on the surface. This is indeed evident from the QCM-D measurements of the same polymer, which indicates adsorption equilibrium only after 5 h of polymer solution flow on the silica surface. We note that the adhesion obtained in this study between the two silica surfaces after 20 min of incubation was also obtained Bremmell and Scales12 as a result of adsorption of similar molecular weight HPAM after 30 min of incubation. Measurements conducted after 20 h of incubation in the HPAM solution represented in Figure 8 show steric repulsion

Figure 9. Force−distance measurements on approach (solid symbols) and retraction (open symbols) obtained between a 25 μm glass particle and a glass surface in 100 mg/L sulfonated polyacrylamide (Mw = 6.5 × 106 g/mol) in 2 wt % KCl after 20 min of incubation in polymer solution.

Figure 10. Force−distance measurements on approach (solid symbols) and retraction (open symbols) obtained between a 25 μm glass particle and a glass surface in 100 mg/L sulfonated polyacrylamide (Mw = 6.5 × 106 g/mol) in 2 wt % KCl after 20 h of incubation in polymer solution.

adhesion at a separation of around 100 nm, which is probably due to the entanglement between the tails and the loops of the adsorbed polymer layers.44 The force on hard contact after 20 min and 20 h in the SPAM solution is about 4 and 5 nN, respectively. Further Remarks. It is generally agreed that polyacrylamide adsorbs to silica surfaces through hydrogen bonding between the CO group on the amide and the proton donors on the oxide surfaces (mainly OH groups).9,47,50 Adsorption of NPAM after 20 min of incubation resulted in the absence of jump-in on approach otherwise observed in measurements between the bare glass surfaces. However, adhesion on retraction was similar, perhaps slightly larger than that between the bare surfaces. On the other hand, the jump-in distance on approach in HPAM after 20 min of incubation occurred at about 60 nm. This indicates partial surface coverage, allowing the furthermost polymer segments from the surfaces (i.e., mainly tails) to induce bridging attraction on approach.31,51 Contrary to NPAM and HPAM, obvious steric repulsion on approach appears after 20 min of incubation in SPAM starting at a separation of 250 nm. Since colloidal forces measured after different incubation times is an indication of the polymer surface coverage,13,26,27,31,36,49 the above observations may suggest that SPAM is faster in providing full surface coverage than NPAM and the HPAM. However, this is not the case as

Figure 8. Force−distance measurements on approach (solid symbols) and retraction (open symbols) obtained between a 25 μm glass particle and a glass surface in 100 mg/L partially hydrolyzed polyacrylamide (Mw = 18.5 × 106 g/mol) in 2 wt % KCl after 20 h of incubation in polymer solution.

on approach starting at around 60 nm, and a force of around 4 nN is required for hard contact between the interacting surfaces. Moreover, there is no adhesion on retraction and no hysteresis between the approach and the retraction curves. The distance at which steric repulsion occurred in our measurements is similar to that obtained by Long and co-workers18 using similar molecular-weight HPAM with 22% hydrolyzed polyacrylamide. AFM measurements conducted after 20 min of incubation in 100 mg/L SPAM in 2% KCl are shown in Figure 9. At this early time of incubation, steric repulsion on approach starting at around 250 nm can be observed with only a slight adhesion on retraction of about 2 nN. This adhesion might be due to formation in entanglements between the adsorbed polymer molecules on the interacting surfaces.44 As shown in Figure 10, the steric repulsion starts at around 400 nm on approach after 20 h of incubation in solution. Hysteresis can be observed between the approach and the retraction curves with slight 2441

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invistigated in this study. Similar results were obtained in our recent publication,44 in which the adsorbed amount is lower for the polymer−solvent−surface system that has shown longer range steric repulsion in the AFM measurements. It should be noted that flow can promote55 and/or demote56 adsorption kinetics, which depends on the flow regime under consideration. The present study highlights important results that can have some practical implications in polymer EOR and oil sand tailings treatment. In EOR, retention of the polymer is a major concern as noted above. However, our results show that lower retention reflected by the adsorbed amounts does not necessarily indicate lower EHT of the adsorbed polymer layer. There is an inverse relationship between polymer retention and the EHT of the adsorbed polymer layer. Use of the high-molecular-weight polyacrylamides in oil sand tailings treatment involves employing the flocculating power of these polymers. Since no flocculation (settling) experiments were conducted for this study, AFM measurements can be used as an indication only, as demonstrated by AFM and settling experiments of Abraham and co-workers.49 In the short time frame of the settling applications (i.e., minutes), HPAM seems to be the best candidate since it induced bridging attraction on approach in AFM measurements conducted after 20 min of incubation. Indeed, the settling measurements by McFarlane and co-workers43 obtained settling rates for the three polymers using smectite and kaolinite dispersions in the following decreasing order: SPAM > HPAM > NPAM. The flocculation superiority of HPAM over NPAM was also obtained by McGuire and co-workers after their settling experiments using iron oxide dispersion.54 Moreover, HPAM was found to be a better flocculant than NPAM in papermaking suspensions.57 It is worth noting that the polymer concentration used in our AFM experiments is very high compared to the concentrations used in the settling studies. However, the ability of SPAM to form more extended polymer layers on the solid surfaces compared to NPAM and HPAM might give it this flocculation efficiency. The extended chains of the adsorbed polymer layer play an effective role in interparticle bridging adsorption, hence giving SPAM more advantage as a flocculant at short time frames prior to full surface coverage.

QCM-D results presented in Figure 3 show the opposite trend with lower adsorption rate and adsorbed amount of SPAM adsorbing after 20 min compared to both HPAM and NPAM. A more likely answer is the configuration of the adsorbed polymer with both HPAM and NPAM adsorbing faster but adopting a flatter configuration on the glass surfaces than SPAM, even though SPAM has a lower molecular weight than the other two polymers. In an earlier study we investigated the adsorption behavior of a lower molecular weight nonionic polyacrylamide on glass surfaces using 2% KCl.44 However, SPAM provides more rapid and better surface coverage compared to the similar molecular weight nonionic polyacrylamide. SPAM in this study also formed a thicker adsorbed polymer layer indicated by the steric repulsion at longer separation (250 nm) compared to the nonionic polyacrylamide studied by Al-Hashmi et al.44 which after 20 min had formed a layer only 25 nm thick. After 20 h incubation, all approach−retraction curves for the three polymers indicate full surface coverage reflected by the steric repulsion on approach and absence of any significant adhesion on retraction.26,36 The effective hydrodynamic thickness (EHT) of the adsorbed polymer layer can be estimated using AFM measurements.13,52 Table 1 summarizes Table 1. Effective Hydrodynamic Layer Thickness (EHT) of the Adsorbed Polymer Layers on Glass Surfaces Using AFM Measurements Conducted after 20 h of Incubation in 100 mg/L Polymer Solution polymer

EHT (nm)

NPAM HPAM SPAM

125 30 175

the EHT values obtained after 20 h of incubation in polymer solutions for NPAM, HPAM, and SPAM. Inclusion of 27% of acrylate groups (i.e., the HPAM) to NPAM resulted in a decrease of the adsorbed layer thickness from 125 to 30 nm. This indicates that HPAM adsorbs flatter on the glass surface. This might be caused by the stronger anchoring of the polymer by the shorter acrylate groups on the polymer backbone by hydrogen bonds, hence bringing the whole macromolcule nearer to the surface. The double-layer repulsion between the negatively charged acrylate groups and the surface is screened by the relatively high electrolyte concentrations which we used in this study.53On the other hand, the presence of longer and larger ATBS groups in SPAM lowers the adsorption energy of this polymer to the silica surface because of the non-hydrogenbonding ATBS groups.45 This would allow the polymer (i.e., SPAM) to further extend away from the surface by forming longer loops and tails. QCM-D measurements show that the adsorption rate and adsorbed amount are highest for NPAM and lowest for SPAM. The higher adsorption rate and adsorbed amount of NPAM compared to HPAM might be attributed to the more contracted conformation of the adsorbed NPAM molecules, hence allowing more NPAM molecules to adsorb to the surface.54 Indeed, this is reflected by formation of NPAMadsorbed layer with higher thickness than that of HPAM as indicated by our AFM measurements after 20 h of adsorption. It is suggested here that the longer loops and tails prevailing in AFM measuremnt of SPAM prevents further adsorption of the polymer from the bulk of the solution in the time frame

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CONCLUSIONS Adsorption of high-molecular-weight nonionic, partially hydrolyzed, and sulfonated polyacrylamides from 2% KCl and at pH of around 6.5 on silica surfaces has been characterized using a quartz crystal microbalance with dissipation monitoring and a colloidal force apparatus. After 5 h, the semiequilibrium adsorbed amount reflected by the QCM-D measurements was highest for nonionic polyacrylamide and lowest for the sulfonated one. However, measurements of colloidal force probe apparatus showed full surface coverage of SPAM, leading to the appearance of steric repulsion at a separation of around 250 nm on approach after 20 min of incubation. On the other hand, strong bridging attraction was observed after 20 min of incubation in the HPAM solution. After 20 h of incubation, all polymers induced steric repulsion on approach without significant adhesion on retraction, indicating full surface coverage. However, steric repulsion occurred at different separations, giving rise to EHT of 125, 30, and 175 nm for NPAM, HPAM, and SPAM, respectively. Hence, we conclude that inclusion of larger non-hydrogen-bonding side chains (i.e., ATBS) to the polyacrylamide can reduce the adsorbed amount 2442

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but would also increase the EHT of the adsorbed polymer layer on silica-dominated surfaces and vice versa.

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ASSOCIATED CONTENT

S Supporting Information *

Description and figures of QCM-D and AFM devices. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +968 24 14 25 44. Fax: +968 24 14 13 54. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Petroleum Development Oman (PDO) for their financial support. REFERENCES

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