No abstract

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 Na2SO4 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 (SO4 2–), leaving I– more hydrated. However, upon further increase in the NaI concentration, this weakly hydrated anion is forced out of solution to the polymer/water interface by sulfate. Thus, the LCST behavior of PNiPAM involves competing roles for ion hydration and polymer–iodide interactions. This concept can be generally applied to mixtures containing both a strongly and a weakly hydrated anion from the Hofmeister series.

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. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.

eakly hydrated anions, such as I − , SCN − and ClO 4 − , weaken the hydrophobic effect in aqueous solutions. These large, polarizable anions denature proteins, inhibit supramolecular complexation and dissolve surfactant micelles [1][2][3] . At the molecular level, weakly hydrated anions partially shed their hydration shells and adsorb to nonpolar interfaces, thereby inhibiting hydrophobic assembly [4][5][6][7][8][9][10] . The adsorption of these anions to amide-rich polymers has been characterized by submolar to molar equilibrium dissociation constants, K D = 0.05-1.60 M (refs. 11-14 ). Even tighter adsorption has been observed at macroscopic surfaces, such as the air/water interface (K D = 0.03-0.26 M) and in the concave pockets of cavitands and proteins (K D = 0.003-0.09 M) 2,15-22 . By stark contrast, anions are repelled from small molecules, like N-methyl acetamide and tert-butyl alcohol (K D = 4-8 M) 23,24 . As a consequence, weakly hydrated anions precipitate small non-ionic solutes out of aqueous solutions, including acetone and diacetone alcohol 25 . The dramatic range of anion affinity for chemically similar aliphatic binding sites exposes a critical gap in our knowledge of the mechanisms for anion-specific effects.The surface curvature of nonpolar solutes is known to influence solubility because of the distinct local hydration of curved and flat interfaces 26,27 . The water hydrogen-bonding network can wrap around small and convex solutes to maintain its bulk-like structure. Large solutes, however, have a flatter topography that disrupts the hydrogen bonds between water molecules 27 . As such, small solutes can be incorporated into the water network, while larger ones with broken hydrogen bonds associate with each other and release water molecules into the bulk solution. The cartoon in Fig. 1 depicts a simple model for a polymer chain. The termini are highly curved due to their half-spherical geometry, while the centre of the chain is flatter because it has a cylindrical-like structure. Experimental and computational studies of nonpolar solutes with varying chain lengths have shown that this topography disorders water at the centre of the chain more than at the termini 28,29 .Here, the role of surface curvature on anion-specific effects is explored by systematically measuring the interactions of NaSCN with polyethylene oxides (PEO) of varying molecular weights, ranging from monomers to polymers. The results indicate that SCN − is repelled from monomers but attracted to oligomers of increasing chain length. These interactions are distinct because SCN − binds selectively to the centre of oligomer chains, as opposed to their termini (Fig. 1). Investigations of polyether hydration shells reveal that the water structure at the centre of the chain is more disordered than at the termini. Together, these findings imply that SCN − interacts with low-curvature interfaces to displace water at sites of hydrogen-bonding defects. The correlation of binding affinity and water structure measurements at specific locati...

The anomalously high mobility of hydroxide and hydronium ions in aqueous solutions is related to proton transfer and structural diffusion. The role of counterions in these solutions, however, is often considered to be negligible. Herein, we explore the impact of alkali metal counter cations on hydroxide solvation and mobility. Impedance measurements demonstrate that hydroxide mobility is attenuated by lithium relative to sodium and potassium. These results are explained by ab initio molecular dynamics simulations and experimental vibrational hydration shell spectroscopy, which reveal substantially stronger ion pairing between OH– and Li+ than with other cations. Hydration shell spectra and theoretical vibrational frequency calculations together imply that lithium and sodium cations have different effects on the delocalization of water protons donating a hydrogen bond to hydroxide. Specifically, lithium leads to enhanced proton delocalization compared with sodium. However, proton delocalization and the overall diffusion process are not necessarily correlated.