Molecular dynamics simulations of the conformation and diffusion of partially hydrolyzed polyacrylamide in highly saline solutions

Surprising Nanomechanical and Conformational Transition of Neutral Polyacrylamide in Monovalent Saline Solutions

Polyacrylamide (PAM) is one of the most important water-soluble polymers that has been extensively applied in water treatment, drug delivery, and flexible electronic devices. The basic properties, e.g., microstructure, nanomechanics, and solubility, are deeply involved in the performance of PAM materials. Current research has paid more attention to the development and expansion of the macroscopic properties of PAM materials, and the study of the mechanism involved with the roles of water and ions on the properties of PAM is insufficient, especially for the behaviors of neutral amide side groups. In this study, single molecule force spectroscopy was combined with molecular dynamic (MD) simulations, atomic force microscope imaging, and dynamic light scattering to investigate the effects of monovalent ions on the nanomechanics and molecular conformations of neutral PAM (NPAM). These results show that the single-molecule elasticity and conformation of NPAM exhibit huge variation in different monovalent salt solutions. NPAM adopts an extended conformation in aqueous solutions of strong hydrated ion (acetate), while transforms into a collapse globule in the existence of weakly hydrated ion (SCN-). It is believed that the competition between intramolecular and intermolecular weak interactions plays a key role to adjust the molecular conformation and elasticity of NPAM. The competition can be largely influenced by the type of monovalent ions through hydration or a chaotropic effect. Methods utilized in this study provide a means to better understand the Hofmeister effect of ions on other macromolecules containing amide groups at the single-molecule level.

Complex role of chemical nature and tacticity in the adsorption free energy of carboxylic acid polymers at the oil-water interface: molecular dynamics simulations

Scientific understanding of the molecular structure and adsorption of polymers at oil-water liquid interfaces is very limited. In this study the adsorption free energy at the oil (CCl4)-water interface was estimated using umbrella sampling molecular dynamics simulations for six carboxylate type vinyl polymers differing in hydrophobic nature and tacticity: isotactic and syndiotactic poly(acrylic acid) (i-PAA, s-PAA), isotactic and syndiotactic poly(methacrylic acid) (i-PMA, s-PMA), and atactic and syndiotactic poly(ethylacrylic acid) (a-PEA, s-PEA). ΔGads values are in the order i-PMA < a-PEA < s-PEA < s-PAA < i-PAA < s-PMA. The results show the significant and complex influence of the chemical nature as well as tacticity of the polymer on its adsorption free energy as related to hydrogen bonding and orientation of bonds with respect to oil and water phases. The influence of tacticity is found to be the highest for PMA, which is interpreted to occur due to the balance between interactions among side groups and those occurring between side groups and solvent. Interactions between side-groups are crucial for determining the conformation of PAA (most hydrophilic) and the solvation of the side-group in water is crucial for determining the conformation of PEA (most hydrophobic). The adsorption of PMA represents the transition between these two dominating effects. The molecular contributions to the enthalpy of adsorption indicate that adsorption is favored mainly through two interactions: polymer-CCl4 and water-water.

Experimental and Simulation Studies on Hematite Interaction with Na-Metasilicate Pentahydrate

Iron ore is a fundamental pillar in construction globally, however, its process is highly polluting and deposits are becoming less concentrated, making reusing or reprocessing its sources a sustainable solution to the current industry. A rheological analysis was performed to understand the effect of sodium metasilicate on the flow curves of concentrated pulps. The study was carried out in an Anton Paar MCR 102 rheometer, showing that, in a wide range of dosages, the reagent can reduce the yield stress of the slurries, which would result in lower energy costs for transporting the pulps by pumping. To understand the behavior observed experimentally, computational simulation has been used by means of quantum calculations to represent the metasilicate molecule and the molecular dynamics to study the adsorption of metasilicate on the hematite surface. It has been possible to obtain that the adsorption is stable on the surface of hematite, where increasing the concentration of metasilicate increases its adsorption on the surface. The adsorption could be modeled by the Slips model where there is a delay in adsorption at low concentrations and then a saturated value is reached. It was found that metasilicate requires the presence of sodium ions to be adsorbed on the surface by means of a cation bridge-type interaction. It is also possible to identify that it is absorbed by means of hydrogen bridges, but to a lesser extent than the cation bridge. Finally, it is observed that the presence of metasilicate adsorbed on the surface modifies the net surface charge, increasing it and, thus, generating the effect of dispersion of hematite particles which experimentally is observed as a decrease in rheology.

Adsorption of CO32-/HCO3- on a quartz surface: cluster formation, pH effects, and mechanistic aspects

Soluble inorganic carbon is an important component of a soil carbon pool, and its fate in soils, sediments, and underground water environments has great effects on many physiochemical and geological processes. However, the dynamical processes, behaviors and mechanism of their adsorption by soil active components, such as quartz, are still unclear. The aim of this work is to systematically address the anchoring mechanism of CO₃^2- and HCO₃^- on a quartz surface at different pH values. Three pH values (pH 7.5, pH 9.5 and pH 11) and three carbonate salt concentrations (0.07, 0.14 and 0.28 M) are considered, and molecular dynamics methods are used. The results indicate that the pH value regulates the adsorption behavior of CO₃^2- and HCO₃^- on the quartz surface by affecting the CO₃^2-/HCO₃^- ratio and the surface charge of quartz. In general, both HCO₃^- and CO₃^2- ions were able to adsorb on the quartz surface and the adsorption capacity of CO₃^2- is higher than that of HCO₃^-. HCO₃^- ions tended to uniformly distribute in an aqueous solution and contact the quartz surface in the form of single molecules instead of clusters. In contrast, CO₃^2- ions were mainly adsorbed as clusters which became larger as the concentration increased. Na⁺ ions were essential for the adsorption of HCO₃^- and CO₃^2-, because some of the Na⁺ and CO₃^2- ions spontaneously associated together to form clusters, promoting the clusters to be adsorbed on the quartz surface through cationic bridges. The local structures and dynamics trajectory of CO₃^2- and HCO₃^- showed that the anchoring mechanism of carbonate solvates on quartz involved H-bonds and cationic bridges, which changed in relation to the concentration and pH values. However, the HCO₃^- ions mainly adsorbed on the quartz surface via H-bonds while the CO₃^2- ions tended to be adsorbed through cationic bridges. These results may help in understanding the geochemical behavior of soil inorganic carbon and further the processes of the Earth's carbon chemical cycle.

Rheological Performance of High-Temperature-Resistant, Salt-Resistant Fracturing Fluid Gel Based on Organic-Zirconium-Crosslinked HPAM

Development of low-cost, high-temperature-resistant and salt-resistant fracturing fluids is a hot and difficult issue in reservoir fluids modification. In this study, an organic zirconium crosslinker that was synthesized and crosslinked with partially hydrolyzed polyacrylamide (HPAM) was employed as a cost-effective polymer thickener to synthesize a high-temperature-resistant and salt-resistant fracturing fluid. The rheological properties of HPAM in tap water solutions and 2 × 10⁴ mg/L salt solutions were analyzed. The results demonstrated that addition of salt reduced viscosity and viscoelasticity of HPAM solutions. Molecular dynamics (MD) simulation results indicated that, due to electrostatic interaction, the carboxylate ions of HPAM formed an ionic bridge with metal cations, curling the conformation, decreasing the radius of rotation and thus decreasing viscosity. However, optimizing fracturing fluids formulation can mitigate the detrimental effects of salt on HPAM. The rheological characteristics of the HPAM fracturing fluid crosslinking process were analyzed and a crosslinking rheological kinetic equation was established under small-amplitude oscillatory shear (SAOS) test. The results of a large-amplitude oscillation shear (LAOS) test indicate that the heating effect on crosslinking is stronger than the shear effect on crosslinking. High-temperature-resistant and shear-resistant experiments demonstrated good performance of fracturing fluids of tap water and salt solution at 200 °C and 180 °C.

Phase Behavior of Ion-Containing Polymers in Polar Solvents: Predictions from a Liquid-State Theory with Local Short-Range Interactions

The thermodynamic phase behavior of charged polymers is a crucial property underlying their role in biology and various industrial applications. A complete understanding of the phase behaviors of such polymer solutions remains challenging due to the multi-component nature of the system and the delicate interplay among various factors, including the translational entropy of each component, excluded volume interactions, chain connectivity, electrostatic interactions, and other specific interactions. In this work, the phase behavior of partially charged ion-containing polymers in polar solvents is studied by further developing a liquid-state (LS) theory with local shortrange interactions. This work is based on the LS theory developed for fully-charged polyelectrolyte solutions. Specific interactions between charged groups of the polymer and counterions, between neutral segments of the polymer, and between charged segments of the polymer are incorporated into the LS theory by an extra Helmholtz free energy from the perturbed-chain statistical associating fluid theory (PC-SAFT). The influence of the sequence structure of the partially charged polymer is modeled by the number of connections between bonded segments. The effects of chain length, charge fraction, counterion valency, and specific short-range interactions are explored. A computational App for salt-free polymer solutions is developed and presented, which allows easy computation of the binodal curve and critical point by specifying values for the relevant model parameters.

Molecular Dynamics Simulations of the Interactions between a Hydrolyzed Polyacrylamide with the Face and Edge Surfaces of Molybdenite

Process water used in mineral processing operations corresponds to water recovered from the thickeners and tailings dams, containing residual reagents such as hydrolyzed polyacrylamides (HPAMs). These polymers depress the flotation of different minerals, and their effect on molybdenite has been experimentally demonstrated. The objective of this work was to study the interactions between a segment of a HPAM with the face and edge of molybdenite. The sigma profile, the radial distribution functions of the HPAM, and the orientation and atomic density profiles of water molecules on the face and edge surfaces of molybdenite were calculated. The results obtained from molecular dynamics simulations showed that the interactions between the HPAM and molybdenite are mainly explained by the interactions of the amide group with the faces and edges of the mineral. Molecular dynamics simulations also showed that the HPAM molecule rearranges in such a way that the amide group moves towards the molybdenite face or edge, and the carboxylate group moves away from the mineral surface. The results obtained in the simulations showed that the interactions of the HPAM with the molybdenite edge are slightly stronger than the interaction of this molecule with the mineral face. Simulations demonstrated that the presence of the sodium and hydroxide ions reduces the concentration of HPAM around the face and edge surfaces, which is expected to affect HPAM adsorption on molybdenite. The conclusions obtained through molecular dynamics simulations are in line with the results obtained in previous studies carried out at a macroscopic scale, which reported that HPAMs adsorb onto molybdenite particles and reduce their hydrophobicity.

A review of flocculants as an efficient method for increasing the efficiency of municipal sludge dewatering: Mechanisms, performances, influencing factors and perspectives

Mechanical sludge dewatering is one of the stages of the municipal wastewater treatment process, which allows the amount of generated sludge and the cost of its transport and management to be reduced. Achieving a high degree of dewatering is possible thanks to the use of flocculation technology. The article presents issues related to the theory of flocculation, sewage sludge, and its dewatering. The main mechanisms of flocculation, the kinetics of the process, the division of flocculants, and flocculation in dual systems are discussed. The influence of particular parameters on the efficiency of flocculation and the dewatering of sewage sludge was analyed. The assessed parameters are: pH, the presence of salt, the mixing process, the structure and ionicity of chains, and the dose. The results of experimental studies on the dewatering of various types of sludge were compared. The literature review included in the paper helps to better understand the process of flocculation and sludge dewatering, and presents the progress to date and the possible directions for further development in this field.

Molecular Dynamics Study of the Conformation, Ion Adsorption, Diffusion, and Water Structure of Soluble Polymers in Saline Solutions

Polymers have interesting physicochemical characteristics such as charge density, functionalities, and molecular weight. Such attributes are of great importance for use in industrial purposes. Understanding how these characteristics are affected is still complex, but with the help of molecular dynamics (MD) and quantum calculations (QM), it is possible to understand the behavior of polymers at the molecular level with great consistency. This study was applied to polymers derived from polyacrylamide (PAM) due to its great use in various industries. The polymers studied include hydrolyzed polyacrylamide (HPAM), poly (2-acrylamido-2-methylpropanesulfonate) (PAMPS), polyacrylic acid (PAA), polyethylene oxide polymer (PEO), and guar gum polysaccharide (GUAR). Each one has different attributes, which help in understanding the effects on the polymer and the medium in which it is applied along a broad spectrum. The results include the conformation, diffusion, ion condensation, the structure of the water around the polymer, and interatomic polymer interactions. Such characteristics are important to selecting a polymer depending on the environment in which it is found and its purpose. The effect caused by salinity is particular to each polymer, where polymers with an explicit charge or polyelectrolytes are more susceptible to changes due to salinity, increasing their coiling and reducing their mobility in solution. This naturally reduces its ability to form polymeric bridges due to having a polymer with a smaller gyration radius. In contrast, neutral polymers are less affected in their structure, making them favorable in media with high ionic charges.

Polyacrylic Acid to Improve Flotation Tailings Management: Understanding the Chemical Interactions through Molecular Dynamics

Molecular dynamic simulations of polyacrylic acid polyelectrolyte (PAA) analyzed its interaction with the main minerals that make up characteristic tailings of the mining industry, in this case, quartz, kaolinite, and montmorillonite. The simulations were carried out with the package Gromacs 2020.3. The interaction potentials used were General AMBER Force Field (GAFF) for PAA and CLAYFF-MOH for mineral surfaces. The SPC/E model described water molecules and Lennard-Jones 12-6 parameters adjusted for SPC/E model were used for Na+ and Cl− ions. The studied systems were carried out at pH 7, obtaining stable adsorption between the PAA and the studied surfaces. Interestingly, the strongest adsorptions were for montmorillonite at both low and high salt concentrations. The effect of salinity differs according to the system, finding that it impairs the absorption of the polymer on montmorillonite surfaces. However, a saline medium favors the interaction with quartz and kaolinite. This is explained because montmorillonite has a lower surface charge density and a greater capacity to adsorb ions. This facilitated the adsorption of PAA. It was possible to identify that the main interaction by which the polymer is adsorbed is through the hydroxyl of the mineral surface and the COO−Na+ complexes. Molecular dynamics allows us to advance in the understanding of interactions that define the behavior of this promising reagent as an alternative for sustainable treatment of complex tailings in highly saline environments.