Characterizing Ion-Polymer Interactions in Aqueous Environment with Electric Fields

Abstract

Polymers make the basis of highly tunable materials that could be designed and optimized for metal recovery from aqueous environments. While experimental studies show that this approach has potential, it suffers from a limited knowledge of the detailed molecular interaction between polymers and target metal ions. Here, we propose to calculate intrinsic electric fields from polarizable force field molecular dynamics simulations to characterize the driving force behind Eu 3+ motion in the presence of poly(ethylenimine methylenephosphonate), a specifically designed metal chelating polymer. Focusing on the metal chelation initiation step (i.e., before binding), we can rationalize the role of each molecule on ion dynamics by projecting these electric fields along the direction of ion motion. We find that the polymer functional groups act indirectly, and the polymer-metal ion interaction is actually mediated by water. This result is consistent with the experimental observation that metal sequestration by these polymers is entropically driven. This study suggests that electric field calculations can help the design of metal chelating polymers, for example, by seeking to optimize polymer−solvent interactions rather than polymer-ion interactions.

Polymer Tacticity Effects in Polymer–Lanthanide Chelation Thermodynamics

“…Each polymer–REE interaction was endothermic, and we observed relatively large values for |− T Δ S| . These entropy–driven interactions are largely the result of releasing bound water molecules surrounding the polymer and REE ion, consistent with previous observations in both polymer–metal and protein–metal binding thermodynamics. ,− Interestingly, we observed larger |Δ H | and |− T Δ S | values for the more isotactic PMAAs (Figures and A,B), with only slight variations in Δ G across the series.…”

“…Taken together, these experimental and computational results suggest a multifaceted role of tacticity in polymer–metal binding. Polymer conformation unquestionably plays a role, as has been previously observed both experimentally and in simulations in previous publications, ,, but perturbations in water structure are likely central in the changes in thermodynamics we observed. These two factors, however, are challenging to deconvolute, as changes in polymer conformation produce changes in water structure with both ultimately influencing polymer–metal interactions.…”

Modulating ion-binding at macromolecular interfaces during (bio)mineralization: A snapshot review for calcium carbonate and calcium phosphate systems

“…The anionic nature of these molecules at physiological pH has naturally led to the idea that they concentrate cations through coulombic interactions. While electrostatics play a role in drawing cations near the macromolecule, the intensity and longevity of the interaction are typically stabilized by entropic effects [28][29][30]. CaCO 3 (and other Ca mineral) ion pair formation is also water/entropy motivated [31][32][33].…”

Predicting Ion Sequestration in Charged Polymers with the Steepest-Entropy-Ascent Quantum Thermodynamic Framework

“…This technique has been used to correlate thermodynamic properties to the sequestration performance of a variety polymer systems [ 2 , 7 , 18 , 19 ]. The underlying thermodynamic driving force for sequestration has been studied computationally [ 20 , 21 ]. Isothermal titration calorimetry indicates a reduction of entropy drives ion sequestration [ 2 ].…”