Sr2+ and Ba2+ salts induced conformational structure of sodium polyacrylate PAA investigated by molecular dynamics simulations

Abstract

The shrinkage of sodium-poly(acrylate) PAA in divalent salt (strontium chloride (SrCl2) and barium chloride (BaCl2) solutions is investigated by atomistic molecular dynamics (MD) simulations to study the salt effect on PAA structure. The salt concentration (Cs) was varied in the range of 0 < Cs < 1 M for fully charged PAA. The PAA radius-of-gyration (Rg) decreases with Cs in the presence of both the divalent salts in qualitative agreement with experiments. The PAA chain stiffness calculated in persistence length (Lp) decreases over the entire range of Cs. At lower Cs, the PAA chains don't form aggregate, while at higher salt concentrations, PAA chains undergo self-association in both the divalent salts. The PAA-Water H-bonds decreases with Cs significantly in the presence of BaCl2 as compared to SrCl2. The PAA-Na+ radial distribution function (RDF) shows a decrease in its coordination number with Cs. The distribution of salt-ions around PAA shows that the Ba2+ condensation onto PAA is more significant than Sr2+ because of a greater coordination number for the former than the latter. Overall, the present study significantly advanced the molecular-level understanding of PAA microstructure in the dilute divalent metal salt solution.