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

Abstract

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.

Water-Soluble Synthetic Polymers: Their Environmental Emission Relevant Usage, Transport and Transformation, Persistence, and Toxicity

“…The salinity of the surrounding aqueous environment can affect the diffusion behavior of WSPs by influencing the polymer molecular conformation and the media ionic strength. Generally, the increase in salinity significantly reduces the electrostatic repulsion between negative charges, resulting in a more compressed molecular conformation of the anionic polymer. , This coincides with the results of molecular dynamics simulations, where the hydrodynamic radius and radius of gyration of hydrolyzed polyacrylamide (HPAM) decreased when the salinity increased. , Similarly, for P-AA, the same trend as for HPAM can be observed . However, electrostatic repulsion between negatively charged solid particles and anionic WSPs is reduced as well, leading to more effective sedimentation .…”

“…The effect of enhanced ionic strength on molecular conformation and adsorption is negligible. Among them, PAM and PEO inherently have a highly coiled molecular conformation, which is consistent with the enhanced mobility they exhibit …”

“…67,68 Similarly, for P-AA, the same trend as for HPAM can be observed. 69 However, electrostatic repulsion between negatively charged solid particles and anionic WSPs is reduced as well, leading to more effective sedimentation. 70 Quezada et al 69 showed that the excess salt also caused a decrease in the diffusion coefficient of PAA due to greater adsorption of ions.…”

“…69 However, electrostatic repulsion between negatively charged solid particles and anionic WSPs is reduced as well, leading to more effective sedimentation. 70 Quezada et al 69 showed that the excess salt also caused a decrease in the diffusion coefficient of PAA due to greater adsorption of ions. On the other hand, the cationic and nonionic polymers are insensitive to the increase in salinity.…”

“…Among them, PAM and PEO inherently have a highly coiled molecular conformation, which is consistent with the enhanced mobility they exhibit. 69 Sediment and Soil. The high retention rate of WSPs in sediments and soils is observed in many cases, due to the fact that polymer adsorption implies multisegment interactions.…”