title: Effect of salt valency and concentration on structure and thermodynamic behavior of anionic polyelectrolyte Na+-polyethacrylate aqueous solution.
abstract: The intermolecular structure and solvation enthalpy of anionic polyelectrolyte atactic Na+-polyethacrylate (PEA) in aqueous solution, as a function of added salt concentration Cs(dilute to concentrated) and valency (NaCl versus CaCl2), were investigated via molecular dynamics simulations with explicit-ion-solvent and atomistic polymer description. An increase in Csleads to a decrease in α, which stabilizes to a constant value beyond critical Cs. A significant reduction in Rgin the presence of CaCl2salt was observed, due to ion bridging of PEA by Ca2+ions, in agreement with results available in literature on other similar polycarboxylates. An increase in salt valency reduces the value of critical Csfor the onset of stabilization of the overall size and shape of the polymer chain. The critical Csratio for the divalent to monovalent salt case is in excellent agreement with results of Langevin dynamics studies on model systems available in the literature. PEA-water H-bond half-life increases with Csfor CaCl2, but no appreciable effect is seen for NaCl. The hydration of PEA becomes stronger in the presence of divalent salt. The strength of H-bond interaction energy is greater for cations as compared to anions of the salt. The salt cation effect in displacing water molecules from the vicinity of PEA, with increase in Cs, is greater for NaCl solution. The decrease in water coordination to PEA carboxylate groups, due to increased Cs, is more pronounced in NaCl solution. The nature of the behavior of the solvation enthalpy of PEA and the type of intermolecular interactions contributing to it, is in agreement with experimental observations from the literature. The hydration enthalpy of PEA in divalent CaCl2aqueous salt solution is more exothermic compared to monovalent NaCl salt solution, in agreement with experimental data. The solvation of PEA is thermodynamically more favorable in the case of CaCl2solution. The exothermic solvation enthalpy, H-bond lifetime, number of H-bonds and H-bond interaction energy are greater in magnitude in CaCl2aqueous solution.
authors: Macromolecular Modeling and Simulation Laboratory, Department of Chemical Engineering, Indian Institute of Technology (IIT) Madras, Chennai, 600036, India.
authors: Praveenkumar
authors: P
authors: Sappidi
authors: Macromolecular Modeling and Simulation Laboratory, Department of Chemical Engineering, Indian Institute of Technology (IIT) Madras, Chennai, 600036, India. unatarajan@iitm.ac.in.
authors: Upendra
authors: U
authors: Natarajan
keywords: Dimensions; Enthalpy; Hydration; Molecular dynamics; Polyelectrolyte; Salt; Valency