Adsorption Of Polyacrylamide Research Articles

Investigation of the Pore Characteristics and Capillary Forces in Shale before and after Reaction with Supercritical CO2 and Slickwater

CO2–slickwater hybrid fracturing technology is an essential part of shale gas recovery and CO2 geo-storage. However, the exposure to supercritical CO2 (ScCO2) and slickwater can result in potential changes of the pore structures and surface wetting behavior, which affect the gas transportation and CO2 sequestration security in shale reservoirs. Therefore, in this paper, X-ray diffraction (XRD), low-pressure nitrogen gas adsorption (N2GA), mercury intrusion porosimetry (MIP), and fractal analysis were used to describe the pore characteristics of shale before and after ScCO2–slickwater coupling treatments. Shale’s surface wettability was confirmed by contact angle measurements. After the ScCO2–slickwater treatments, the number of micropores (<3.5 nm) and mesopores (3.5–50 nm) increased, while that of macropores (>50 nm) declined based on the N2GA and MIP experiments. Combined with fractal analysis, we argue that the pore connectivity diminished and the pore structure became more complicated. By analyzing the results of XRD, shale pore changes occurring after the ScCO2–slickwater treatment can be explained by the adsorption of polyacrylamide (PAM). Contact angle measurement results showed that the shale’s surface treated by ScCO2 and slickwater was more hydrophilic than that treated by ScCO2 and water, and indirectly prove our argument above. Hence, the coupling using effect of ScCO2 and slickwater can impair the negative effect of CO2 on the shale capillary force to improve shale gas productivity, but it can negatively affect the security of CO2 sequestration in shale reservoirs.

Effect of PAM on Surface Hydrophobicity of Montmorillonite and Difference of Interface Adsorption: An Experimental and Simulation Study.

How to realize efficient treatment of coal slime generated by a coal washing operation is an urgent problem to be solved in this industry. The presence of clay minerals, especially highly hydrophilic montmorillonite (MMT), is the key to the poor treatment effect of coal slime. Polyacrylamide (PAM) is very popular as a polymer agent to improve the treatment of coal slime. However, when it is used to treat coal slime with a high content of MMT, the selection of PAM type and the mechanism of action are still lacking. In this study, the effects of different types of PAM on the treatment of coal slime water containing MMT are considered by sedimentation and press filtration tests. The interaction mechanism of PAM on the MMT surface is studied by using ζ-potential, Brunauer-Emmett-Teller (BET) analysis, low-field nuclear magnetic resonance, density functional theory (DFT), and molecular dynamics (MD) simulations. The results show that the three PAM can improve the sedimentation and filtration effect of coal slime water, and the performance is CPAM > NPAM > APAM. The ζ-potential of the MMT (001) surface increases under the action of three PAM, and the effect of CPAM is the most significant. The adsorption of PAM on the MMT (001) surface has the ability to neutralize the surface charge of MMT. The flocculation of MMT particles under PAM results in an increase of particle size and a decrease of specific surface area. Meanwhile, the pore volume of MMT decreases, and the average pore size increases. In addition, PAM mainly removes vicinal water on the MMT surface. The active sites of the MMT surface and PAM are calculated by DFT. The adsorption of three PAM structural units on the MMT Na-001 surface and non-001 surface is nonbonding interaction, and the adsorption energy of CPAM is the largest. And the left shift of εp of the O atom on the MMT surface is conducive to the stable adsorption of CPAM. The MD results show that the concentration of water molecules on the surface of MMT Na-001 decreases after PAM is adsorbed on the MMT Na-001 surface, indicating that PAM can keep water molecules away from the surface of MMT, which means that the hydrophobicity of the MMT surface is enhanced. This study has guiding significance for the selection of PAM and the development of new flocculants in the treatment of coal slime with a high content of MMT.

Atomic Force Microscopy and Molecular Dynamic Simulation of Adsorption of Polyacrylamide with Different Chemistries onto Calcium Carbonate.

This study investigates the interaction of polyacrylamide (PAM) of different functional groups (sulfonate vs. carboxylate) and charge density (30% hydrolysed vs. 10% hydrolysed) with calcium carbonate (CaCO3) via atomic force microscopy (AFM) and partly via molecular dynamic (MD) simulations. The PAM used were F3330 (30% hydrolysed), AN125 (25% sulfonated), and AN910 (% hydrolysed). A total of 100 ppm of PAMs was prepared in 0.1% NaCl, 3% NaCl, and 4.36% NaNO3 to be employed in AFM experiments, while oligomeric models (30 repeating units) of hydrolysed polyacrylamide (HPAM), sulfonated polyacrylamide (SPAM), and neutral PAM (NPAM) were studied on a model calcite surface on MD simulations. AFM analysis indicated that F3330 has a higher average adhesion and interaction energy with CaCO3 than AN125 due to the bulky sulfonate side group of AN125 interfering with SPAM adsorption. Steric repulsion of both PAMs was similar due to their comparable molecular weights and densities of the charged group. In contrast, AN910 showed lower average adhesion and interaction energy, along with slightly longer steric repulsion with calcite than F3330, suggesting AN910 adopts more loops and tails than the slightly flatter F3330 configuration. An increase in salt concentration from 0.1% to 3% NaCl saw a reduction in adhesion and interaction energy for F3330 and AN125 due to charge screening, while AN910 saw an increase, and these values increased further at 4.36% NaNO3. MD simulations revealed that the salt ions in the system formed salt bridges between PAM and calcite, indicating that the adhesion and interaction energy observed from AFM are likely to be the net balance between PAM charged group screening and salt bridging by the salt ions present. Salt ions with larger bare radii and smaller hydrated radii were shown to form stronger salt bridges.

Adsorption of polyacrylamides on mineral oxides: Effect of solution pH and polymer molecular weight

Acrylamide-based polymers such as polyacrylamide (PAM) and their derivatives are widely used as additives to enhance solid-liquid separation in the mineral processing industry. Thus, an investigation of polyacrylamide behaviour at oxide-aqueous solution interfaces is of vital importance. In this work, the adsorption of different molecular weight polyacrylamides onto iron oxides and silica was quantified at different pH values. Mineral oxides were characterised by zeta potential, particle size distribution, scanning electron microscopy images and X-ray diffraction. The Sauter mean D[3,2] diameter of the hematite, magnetite and silica were measured to be 0.8, 1.9, and 2.0 µm respectively. An anionic polyacrylamide (PAM A, MW 520 kg/mol) exhibited strong variation in adsorbed amount with pH, while the adsorption of two neutral polyacrylamides (PAM N, MW 40 & 5500 kg/mol) had a weak dependence on pH. All three polyacrylamides had similar adsorption capacity on the iron oxides particles, thus minimal influence of molecular weight was observed. The adsorption isotherms were fit to the Langmuir model to facilitate interpretation of the adsorption mechanism. The highest adsorption plateau density of 11.4 mg/m2 on magnetite and 7.4 mg/m2 on hematite for PAM A was found to be at lower pH. At the same pH, high molecular weight PAM N shows the highest adsorption plateau density of 7.8 mg/m2 and at unadjusted pH, low molecular weight PAM N adsorbs to 5.2 mg/m2 on magnetite. All PAM samples selectively adsorb on the iron oxide particles compared to silica particles. These quantified adsorbed amounts are concordant with the current use of high molecular weight PAM as an effective flocculant.

Characteristics and influencing factors of interaction between hydrophobic surface and drag-reducing polymer: A MC and MD simulation study

Hydrophobic surfaces and polymer additives are both desirable for effectively reducing drag when carrying fluids, and the synergy of these two has naturally attracted a great deal of attention. The interaction between a polyurethane surface with different raw material compositions and drag-reducing polymers such as polyacrylamide (PAM), polyethylene oxide (PEO), xanthan gum (XG), and guar gum (GG) was investigated using molecular simulation methods such as molecular dynamics and Monte Carlo. The effects of polymerization degree (DP), temperature, and water molecule adsorption on the interaction were further examined. The findings demonstrate that rigid polymers (XG and GG) have a higher capacity for adsorption on the polyurethane surface than do flexible polymers (PAM and PEO). Furthermore, the DP clearly affects the PEO interacts with surfaces. Under a fixed water molecule load, the influence of surface type on water molecule adsorption ability is greater than that of polymer type but weaker than that of DP. Temperature clearly influences the interaction of PAM and surfaces, whereas PEO has the opposite effect. Additionally, PEO's deformation energy contributes more to the adsorption energy than others, whereas PAM's adsorption energy is higher because its longer molecular chains occupy the adsorption site on the surface.

Adsorption of Different Ionic Types of Polyacrylamide on Montmorillonite Surface: Insight from QCM-D and Molecular Dynamic Simulation.

This study investigates the interaction between montmorillonite and polyacrylamide (PAM) with different ionic types using quartz crystal microbalance with dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations. The goal was to understand the effect of ionicity and ionic type on polymer deposition on montmorillonite surfaces. The results of the QCM-D analysis showed that a decrease in pH led to an increase in the adsorption of montmorillonite on the alumina surface. The ranking of adsorption mass on alumina and pre-adsorbed montmorillonite alumina surfaces was found to be cationic polyacrylamide (CPAM) > polyacrylamide (NPAM) > anionic polyacrylamide (APAM). The study also found that CPAM had the strongest bridging effect on montmorillonite nanoparticles, followed by NPAM, while APAM had a negligible bridging effect. The MD simulations showed that ionicity had a significant influence on the adsorption of polyacrylamides. The cationic functional group N(CH3)3+ had the strongest attraction interaction with the montmorillonite surface, followed by the hydrogen bonding interaction of the amide functional group CONH2, and the anionic functional group COO- had a repulsive interaction. The results suggest that at high ionicity levels, CPAM can be adsorbed on the montmorillonite surface, while at low ionicity levels, APAM may still be adsorbed with a strong coordination trend.

Describing the adsorption of PAM on coal/kaolinite surface in aquatic by combining experiments and MD simulation

Hydration is an important factor influencing the adsorption of reagent molecules at the mineral interface. Understanding the micro-scale characteristics of hydration and the molecular adsorption at the interface is fundamental to improving the effectiveness of solid–liquid separation. In this paper, the flocculation processes of coal sludge in water and in kaolinite solutions were investigated, and the surface properties of the settling materials were analysed using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that as the concentration of polyacrylamide (PAM) increased, the flocculation effect showed a trend of first increasing and then decreasing. The kaolinite particles covered the coal surface, reducing the exposed area of the coal and the amount of PAM molecules adsorbed. Under the same conditions, the settling time of particles in kaolinite solutions is 60% longer than that in water. Further, molecular dynamics (MD) simulation was used to study the distribution of water molecules on the surface of coal and kaolinite, and the micro-scale configurations of PAM at the coal-water and kaolinite-water interfaces were analysed. The results showed that the coal surface was weakly hydrated, and PAM molecules can replace some of the water molecules near the coal surface. Kaolinite surface was strongly hydrated and the maximum density of the hydration layer is close to that of the kaolinite. PAM cannot cross the hydration layer with three layers of water molecules at the aluminium-oxygen octahedra (001) surface of kaolinite. However, there was only one layer of water molecules between PAM and silicon-oxygen tetrahedra (00–1) surface. When the kaolinite covered the coal surface, it was difficult for the PAM to replace the water at the surface, the 00–1 surface was the key dissociation surface to promote flocculation. This study illustrated microscopic insights into the effect of mineral hydration on the adsorption of flocculant molecules and may enrich the theory of mineral flocculation and sedimentation.

Effect of Sand Grain on Adsorption of Xanthan Gum and Polyacrylamide

Polymer flooding is an Enhanced oil recovery (EOR) method using chemicals, such as xanthan gum and polyacrylamide which are widely used in the field. Polymer flooding can improve the mobility of the injection fluid, but in the reservoir the polymer has an interaction with sand known as adsorption which causes some of the polymer to be bound to the surface of the pore of sand, so that it has an impact on the ineffectiveness of the injection. The purpose of this study want to determine and compare the effect of sand grain on the adsorption of xanthan gum and polyacrylamide polymer solutions. Based on the experiment results, it was found that the increase in adsorption was proportional to the increase in polymer concentration. From two polymers used, xanthan gum showed lower adsorption than polyacrylamide. Based on the sand grain, the adsorption that occurs is greater in the grain size for large sand compare small.

A Molecular Dynamics Simulation of Polymers' Interactions with Kaolinite (010) Surfaces in Saline Solutions.

The search for polymers that meet the demands of the water recovery process in mining is a contingent challenge. Both the presence of clays and saline waters can impair water recovery from tailings when conventional flocculants are used. In this work, the adsorption of polyacrylamide (PAM), hydrolyzed polyacrylamide (HPAM), poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS), polyacrylic acid (PAA), polyethylene oxide (PEO), and guar gum (GUAR) on a kaolinite surface (010) was investigated using classical molecular dynamics. The results show that the presence of sodium chloride modifies the affinities of the polymers with kaolinite (010). At low salt concentrations, the PAM and GUAR polymers generally show higher adsorption due to the formation of hydrogen bridges. However, the highest adsorptions occur in salt solutions in the presence of HPAM by cationic bridging with sodium ions as a mediator. This high affinity of HPAM is not efficient for flocculation because it re-disperses the particles, but it is promising for the design of new additives produced by grafting HPAM groups onto advanced polymers.

Effects of inorganic cations on the steric force between polyacrylamide layer physically adsorbed on SiO2 wafer and poly (ethylene oxide) layer

The physical adsorption of polyacrylamide on the SiO2 wafer in high salty solutions was investigated by an improved AFM steric force measurement. The colloid probe was modified by poly (ethylene oxide) layer and used to determine the steric force between adsorbed polyacrylamide and poly (ethylene oxide) layer in NaCl, KCl, MgCl2 and CaCl2 solutions. Assuming the adsorbed polyacrylamide complies with the self-similar model, the surface excesses, the thicknesses of the proximal regions, and "free energies of sticking" were estimated. Take the case of 5000 mg/L NaCl for example. The concentration profile was deduced from the corresponding steric force profile. The polyacrylamide concentration next to the SiO2 wafer was estimated to be about 147 g/L and the average concentration in the proximal region was estimated to be about 100 g/L. Based on the above estimations, interaction strength between the adsorbed cations and polyacrylamide was established, Mg2+ > Ca2+ > K+ > Na+. Moreover, the conformations of the adsorbed polyacrylamide were changed from "lying flat" to folded ones by the stronger interactions between the adsorbed cations and polyacrylamide.”

Desert silty sand modified by anionic PAM and ordinary portland cement: Microfabric reinforcement and durability

The modification of desert silty sands has been a great challenge due to its relative uniform particle dimension and non-cohesion. This paper presents a study on utilizing a new additive, anionic polyacrylamide (PAM) with ordinary Portland cement (OPC) to modify the desert silty sand. Unconfined compression strength (UCS) tests were conducted to investigate the strength and durability (resistance to cyclic wet-dry and freeze–thaw damage) of modified sands at the maximum dry density; the scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy tests were carried out for microstructural analysis. Results reveal that the PAM fraction has a significant influence on the mechanical performance of the modified desert silty sand, and a threshold PAM dosage exists, beyond which adding excessive PAM will inhibit the strength and durability further. The microstructural analysis reveals that the physical stick-up of the PAM-to-sand particles leads to the formation of the spatial cobweb-like reticular structure. The Ca2+ cation released by OPC served as a cationic bridge, which in turn enhance the PAM adsorption efficiency. However, excessive PAM would play a negative role, chiefly because of the excessive accumulation between the hydrated production and sand particles. This study indicates that the combination of cement and PAM has the virtues of both cement-stabilized and PAM microfabric-reinforced sand. This binary PAM–OPC modification approach plays a more efficient role if a rational PAM incorporation (0.1–0.3 % in this study) is selected.

Interaction between PAM and kaolinite (0 0 1) surface in aqueous solution, insights from DFT and MD simulations

Molecular dynamics (MD) simulations and density functional theory (DFT) calculations were performed to investigate the adsorption of cationic polyacrylamide (CPAM), nonionic polyacrylamide (NPAM), and anionic polyacrylamide (APAM) on the kaolinite (001) surface. DFT calculations revealed that O atoms are the active sites for the amide group and anionic group, while the active sites of the cationic group are O atoms and the quaternary ammonium group. The adsorption behaviors have been studied in terms of relative concentration distribution, interaction energy, number density, and self-diffusion coefficient. MD simulations indicated that the order of the adsorption strength and bridging performance is CPAM > NPAM > APAM. However, NPAM is easier to form hydrogen bonds with the atoms on the kaolinite (001) surface compared with CPAM and APAM. Moreover, the electrostatic interaction is the main driving force for the adsorption of the three polyacrylamides on the kaolinite (001) surface. Analysis of the dynamic properties of water indicated that the effect of polyacrylamide adsorption on the wettability of the kaolinite (001) surface showed identical trends as adsorption strength. The simulation results are in good agreement with the experimental data.

Rheology and Thixotropy of Cement Pastes Containing Polyacrylamide

In this article, the rheological and thixotropic properties of fresh cement pastes (fcps) with nonionic polyacrylamide (PAM) are investigated through the coaxial rotary method. The B-HB model is resorted to fit the shear stress versus shear rate diagram, from which the slurry plastic viscosity is particularly discussed. The results show that with the development of PAM, the plastic viscosity increases first, then decreases, and then increases again, exhibiting the “up-down-up” trend, which is consistent with the fcps fluidity. The adsorption, lubrication, and entanglement mechanisms of PAM are successfully used to interpret this phenomenon. Combined with the suspension density, the relationship among plastic viscosity, flowability, and density is successfully established follower via a multivariate linear regression method. The Durbin-Watson coefficient, variance inflation factor, μ significance, ρ significance, and R 2 are 2.122, 1.024, 0.014, 0.004, and 0.776, respectively, demonstrating the feasibility of fitting formula. Besides that, the PAM containing slurry which exhibits the explicit thixotropy is also found according to the appeared hysteresis curve during one shearing cycle. With the increase of PAM dosage, the thixotropic indexes including Th1 and Th2 decreased, until the PAM exceeded 0.5%-0.6% dosage, both of them yielded negative values. The phenomenon that the fcps final shear stress exceeds its initial value occurs.

Adsorption and Corrosion Behavior of Polyacrylamide and Polyvinylpyrollidone as Green Coatings for Mg–Al–Zn–Mn Alloy: Experimental and Computational Studies

Adsorption and Corrosion Behavior of Polyacrylamide and Polyvinylpyrollidone as Green Coatings for Mg–Al–Zn–Mn Alloy: Experimental and Computational Studies

Control of the Micro-Defects on the Surface of Silicon Wafer in Chemical Mechanical Polishing

The final polishing of silicon results in the irresistible formation of micro-defects (i.e., particle residues and scratches) on the surface. In view of this problem, the synergistic effect of surfactants and water-soluble polymers in inhibiting the micro-defects on the silicon surface was studied in this paper to improve the wettability of the slurry and reduce the micro-flocculation of abrasive particles. The results showed that the total number of residual particles (≥0.06 μm) on the polished surface was reduced from 24,784 to 277 with the adsorption of cationic polyacrylamide (CPAM) and fatty alcohol polyoxyethylene ether (AEO-9). The water-soluble polyvinylpyrrolidone (PVP) polymer could coat on the SiO2 abrasives, inhibit the flocculation of abrasive particles, avoid scratches on the silicon surface and further reduce the number of residual particles (≥0.06 μm) to 67 on the polished surface. Furthermore, a contact angle analyzer was used to characterize the wettability of the components in the slurry, and a large particle counter was used to analyze the changes in the number of large particles in the slurry component. Finally, a mechanism of surfactants and a water-soluble polymer combined system was proposed to suppress the micro-defects on the surface of the silicon wafer.

Pore changes of slickwater-containing shale under supercritical CO2 treatment

Application of liquid CO2-slickwater hybrid fracturing technology can significantly improve the energy-enhancing efficiency and fracturing fluid flowback efficiency of shale reservoirs. However, supercritical CO2 (ScCO2) fluid and slickwater are inevitably retained in reservoirs after fracturing, resulting in limited understanding of the changes of shale pore structure, further impacting the evaluation of the gas adsorption and transportation behaviors as well as CO2 sequestration capacity in shale reservoirs. Therefore, the influences of the ScCO2-slickwater coupling effect on shale pore structure was analyzed by using X-ray diffraction, low-pressure nitrogen gas adsorption (N2GA), mercury intrusion porosimetry (MIP), field emission-scanning electron microscopy (FE-SEM), and fractal analysis. After ScCO2-slickwater treatment, the calcite and albite contents decreased, while the quartz and clay minerals contents increased in shale. The increase in number of micropores and mesopores and the reduction in number of macropores were observed by a combination of N2GA and MIP, which led to the total specific surface area (TSSA) and total pore volume (TPV) of the shale enlarged, while the average pore size reduced. Meanwhile, the fractal dimensions calculated by N2GA (D1, D2) and MIP (D3) were raised, indicating that pore surface roughness and structural complexity were strengthened. D1, D2 and D3 can roughly reflect fractal characteristic of micropores (<3.5 nm), mesopores (3.5–50 nm) and macropores (>50 nm), respectively. Thus, D1 and D2/D3 can be used to evaluate the gas adsorption and flow behaviors of shale, respectively. By analyzing the control experimental results of ScCO2-water and He-slickwater treatment and the FE-SEM images, shale pore changes after ScCO2-slickwater treatment can be explained by CO2 extraction and dissolution, secondary mineral precipitation and polyacrylamide adsorption. After treatment, the increase of micropores and mesopores as well as TSSA and TPV makes the gas-adsorption capacity of shale improved, showing that the coupling effect of ScCO2-slickwater can not only improve shale gas productivity, but also benefit geological storage of CO2.

Activation mechanisms of apatite by purifying reagent polyacrylamide for backwater from mineral processing in flotation of rare earths

This study revealed activation mechanisms of apatite by polyacrylamide (PAM) during flotation of rare earth minerals in Bayan Obo Ore. This activation leads to high phosphorus content in rare earth concentrate. Ultraviolet (UV) absorption measurements show that adsorption of PAM on the surfaces of apatite and rare earths follows a pseudo-second-order reaction kinetic model. The isothermal adsorption process follows the Freundlich adsorption model, indicating that the adsorption of PAM on apatite is a multi-layer, non-uniform process, that is, physical and chemical adsorption occurs simultaneously. The measured zeta potential shows that under weakly alkaline conditions, amide groups with a negative charge are adsorbed on the surface of apatite, increasing the electronegativity, so that the potential is shifted negatively; however, the negative shift of the potential on the surface of rare earths is insignificant. Fourier transform infrared (FT-IR) test and X-ray photoelectron spectroscopy (XPS) results imply that PAM is chemically adsorbed on the surface of apatite, while 506E undergoes stronger chemical adsorption than PAM on the surface of rare earth minerals. It is found that multi-layer, non-uniform adsorption of PAM combining physical and chemical adsorption occurs on the surface of apatite and the process is dominated by physical adsorption: the adsorption of PAM on surfaces of apatite is stronger than that on the surface of rare earth minerals. Such adsorption activates the apatite, causing it to float during the flotation of rare earths, which is the reason for the high phosphorus content in the rare earth concentrate.

Adsorption mechanism of polyacrylamide on kaolinite surface in the presence of Ca2+: Insights from DFT calculation

The microscopic adsorption mechanism of polyacrylamide (PAM) on kaolinite surface in the presence of Ca2+ was investigated by using density functional theory (DFT). The PAM model was constructed firstly, and the optimal adsorption configurations and adsorption mechanism were examined, with adsorption configurations, bond populations and adsorption energy analyzed. Calculation results demonstrated that the adsorption of PAM on kaolinite (001) surface was dominated by three strong hydrogen bonds between the amide groups of PAM and hydroxyl groups of kaolinite (001) surface. And the PAM adsorbed on the kaolinite (00–1) surface through non-bonding interaction. Ca2+ was selected to explore the influence of metal cations on the adsorption of PAM on kaolinite surface. The results suggested that the adsorption of PAM on the kaolinite (001) surface was promoted in the presence of Ca2+, while the adsorption of PAM on the kaolinite (00–1) surface was inhibited.

Enhanced Adsorption of Anionic Polymer on Montmorillonite by Divalent Cations and the Effect of Salinity.

Adsorption of polymers from an aqueous solution onto clay minerals is of great interest to many applications such as water purification and soil conditioning. Molecular dynamics simulations were performed to study the adsorption of anionic polyacrylamide (APAM) on anionic montmorillonite, in an aqueous solution containing monovalent or divalent salts. Compared with monovalent salts (NaCl), the enhancement of APAM adsorption brought by divalent salts (CaCl2) was significant, which could not be explained by the Poisson-Boltzmann theory alone. Each solvated Ca2+ was coordinated by 4-6 water oxygens in its first coordination shell. One to two of these water molecules were displaced when APAM formed a complex with Ca2+. Ca2+ ions in the adsorbed Ca2+-APAM complexes did not serve as bridges sandwiched between APAM and Mt; instead, the complexes carried a residual positive charge and were subsequently attracted to montmorillonite. The number of adsorbed Ca2+-APAM complexes changed with salinity in a nonmonotonic manner, due to the modulation of apparent charges of montmorillonite and APAM by Ca2+. Increasing adsorption of Ca2+-APAM complexes also promoted APAM adsorption through direct hydrogen bonding with montmorillonite. The findings provided new molecular insights into the long-standing debates on the role of divalent ions in promoting polymer adsorption on like-charged solid surfaces.

Polyacrylamide adsorption on (1 0 1) quartz surfaces in saltwater for a range of pH values by molecular dynamics simulations

The adsorption of partially hydrolyzed polyacrylamide (HPAM) on quartz plays a key role in solid-liquid separation processes that seek to recover water for processing. In this work, computational molecular dynamics is used to study the water-quartz interface in the presence of HPAM under conditions of high salt and wide pH range. The salts used are mostly those of seawater. Small water-structure maker cations are preferentially adsorbed onto the polymer rather than onto the quartz surfaces. In freshwater, the adsorption of HPAM on quartz is reduced to very few contacts, however in saltwater the adsorption contacts of HPAM increase although decrease with surface charge, mainly due to electrostatic repulsion. In the presence of monovalent cations and for any surface charge on the quartz, above the isoelectric point, the adsorption contacts of HPAM decreases as the cation size increases, which is expected considering that smaller cations are more effective in neutralizing both quartz and HPAM. The results show that the adsorption of HPAM occurs exclusively via salt bridges. One way of describing the conformation of an HPAM chain adsorbed onto quartz is to reduce it to the skeleton of carbon atoms of the polymer chain and then determine the fraction of these atoms in loops, trains, and tails. In general, the adsorption contacts of HPAM are few, the carbon fractions in trains and loops are tiny, and therefore most of the carbon atoms in HPAM remain in long tails ready to get entangled with other tails thus favoring the flocculation process. The results are expected to contribute to sustainable water management through the right choice of flocculant based on molecular aspects.