Abstract:
The hardness-causing ions like calcium are detrimental to the performance of anionic surfactants like dodecyl sulfate. Polystyrene sulfonate is an essential component of ion exchange resins that are used to adsorb such ions. Therefore, understanding the binding behavior of calcium ions to polystyrene sulfonate in the presence of dodecyl sulfate is of industrial relevance. Two important structural parameters of polystyrene sulfonate are known to influence its behavior: (i) The arrangement of aromatic groups around the carbon backbone (tacticity) and (ii) the degree of sulfonation. We use molecular dynamics simulations to study the effect of the above two parameters on the calcium-binding to polystyrene sulfonate in the presence of dodecyl sulfate. We observe that the calcium-binding ability and its response to the dodecyl sulfate are highly sensitive to the arrangement of aromatic groups around the backbone. We identify one atactic structure of 100% sulfonated polystyrene sulfonate and another of 20% sulfonated polystyrene sulfonate that shows a significant increase in calcium ion binding upon the addition of dodecyl sulfate ions. The increase in the 100% sulfonated structure is due to the formation of calcium ion bridges between the sulfonate groups located on one side of the backbone. The increase in the 20% sulfonated structure is due to the increase in the sulfonate-Ca-Cl type of ion bridges. We also observe a stronger association between dodecyl sulfate and low sulfonated polystyrene sulfonate because of the hydrophobic interaction between polystyrene sulfonate's backbone and the hydrocarbon tail of dodecyl sulfate. However, aligning all the aromatic groups on the same side of the backbone (isotactic structure) sharply weakens the association between the two species. Our results can aid in achieving the desired water softening capacity of ion exchange resins in the presence of anionic surfactants via altering the tacticity and degree of sulfonation.