Studying the effect of cross-linking and sulfonation on the calcium-binding ability of polystyrene sulfonate in the presence of dodecyl sulfate

Abstract

Calcium ions in hard water deteriorate the cleaning performance of detergents by interacting with the anionic surfactants and inducing their precipitation. Polystyrene sulfonate is a common component of cation-exchange resins that adsorb hard ions such as Ca+2 and Mg+2, which adhere to the sulfonate groups. Therefore, understanding the calcium-binding ability of polystyrene sulfonate in the presence of dodecyl sulfate can help in designing novel water-softening agents for applications related to detergency. Studying the association between polystyrene sulfonate and dodecyl sulfate via calcium ion bridges is also important. Since resins contain cross-linked polystyrene sulfonate, degrees of cross-linking and sulfonation are two important parameters. We use molecular dynamics simulations to study the effect of the above parameters on the calcium-binding ability of isotactic polystyrene sulfonate in the presence of dodecyl sulfate. We observe negligible dependence of calcium-ion binding of polystyrene sulfonate on the degrees of cross-linking and sulfonation. The formation of calcium ion bridges between cross-linked polystyrene sulfonate and dodecyl sulfate is strongly affected by the degree of sulfonation. Such bridges are observed more for the intermediate degrees of sulfonation. This is because, at a very low degree of sulfonation, dodecyl sulfate ions align closer to the cross-linking aromatic groups than the sulfonated ones. On the other hand, at high degrees of sulfonation, stronger electrostatic repulsion disfavors the proximity between dodecyl sulfate and polystyrene sulfonate.