Contact Ion Pairs in the Bulk Affect Anion Interactions with Poly(-isopropylacrylamide).

Salt effects on the solubility of uncharged polymers in aqueous solutions are usually dominated by anions, while the role of the cation with which they are paired is often ignored. In this study, we examine the influence of three aqueous metal iodide salt solutions (LiI, NaI, and CsI) on the phase transition temperature of poly(-isopropylacrylamide) (PNIPAM) by measuring the turbidity change of the solutions. Weakly hydrated anions, such as iodide, are known to interact with the polymer and thereby lead to salting-in behavior at low salt concentration followed by salting-out behavior at higher salt concentration.

Molecular Mechanism for the Interactions of Hofmeister Cations with Macromolecules in Aqueous Solution.

Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive.

Molecular Scale Solvation in Complex Solutions.

Complex solution environments are ubiquitous in nature. Most life science systems contain hydrated macromolecules whose solubility, function and stability are modulated by several small organic molecules or salts (cosolutes) present simultaneously. This Perspective discusses solvation of mixed cosolutes in water.

Nonadditive Ion Effects Drive Both Collapse and Swelling of Thermoresponsive Polymers in Water.

When a mixture of two salts in an aqueous solution contains a weakly and a strongly hydrated anion, their combined effect is nonadditive. Herein, we report such nonadditive effects on the lower critical solution temperature (LCST) of poly( N-isopropylacrylamide) (PNiPAM) for a fixed concentration of NaSO and an increasing concentration of NaI. Using molecular dynamics simulations and vibrational sum frequency spectroscopy, we demonstrate that at low concentrations of the weakly hydrated anion (I), the cations (Na) preferentially partition to the counterion cloud around the strongly hydrated anion (SO), leaving I more hydrated.

Does an electronic continuum correction improve effective short-range ion-ion interactions in aqueous solution?

Non-polarizable force fields for hydrated ions not always accurately describe short-range ion-ion interactions, frequently leading to artificial ion clustering in bulk aqueous solutions. This can be avoided by adjusting the nonbonded anion-cation or cation-water Lennard-Jones parameters. This approach has been successfully applied to different systems, but the parameterization is demanding owing to the necessity of separate investigations of each ion pair.