Effect of Tacticity and Degree of Sulfonation of Polystyrene Sulfonate on Calcium-Binding Behavior in the Presence of Dodecyl Sulfate

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.

Studying the Effect of Cross-Linking and Sulfonation on the Calcium-Binding Ability of Polystyrene Sulfonate in the Presence of Dodecyl Sulfate

“…In the case of PSS, the proportion of hydrophobic regions and charge separation can be varied by changing the degree of sulfonation. In our previous study, we observed that lowering the degree of sulfonation increased the PSS-DS association due to greater hydrophobic attraction and the formation of calcium ion bridges between sulfonate and sulfate groups . The effect of cross-linking on such an association remains to be studied.…”

“…The interaction between like-charged polyelectrolytes and surfactants , has received lesser attention than that between oppositely charged or neutral polyelectrolytes and surfactants. − In our previous work, we discussed the possibility of achieving the desired response of PSS-Ca binding to the presence of the anionic surfactant dodecyl sulfate (DS). Additionally, we also investigated the effect of tacticity and the degree of sulfonation on the above response . Both simulation studies considered free PSS chains.…”

“…Our previous studies showed that anionic surfactants associate with the anionic polyelectrolytes via calcium ion bridges or non-electrostatic interactions. 16,17 The number of surfactant ions bound to polyelectrolytes should be significantly less than those that would have formed calcium−surfactant complexes without polyelectrolytes. Otherwise, the number of free surfactant ions will be less even after using the calciumbinding polyelectrolyte.…”

“…In our previous study, we observed that lowering the degree of sulfonation increased the PSS-DS association due to greater hydrophobic attraction and the formation of calcium ion bridges between sulfonate and sulfate groups. 17 The effect of cross-linking on such an association remains to be studied. The present work studies the association between DS ions and cross-linked PSS by varying the degrees of cross-linking and sulfonation.…”

“…Additionally, we also investigated the effect of tacticity and the degree of sulfonation on the above response. 17 Both simulation studies considered free PSS chains. The free PSS chains are soluble in water and easily bind with the calcium ions.…”