Lifting Hofmeister's Curse: Impact of Cations on Diffusion, Hydrogen Bonding, and Clustering of Water

High-capacity water sorbent cycles without hysteresis under dry conditions

Sorbents capable of cycling water vapor under dry conditions are critical for applications such as atmospheric water harvesting, desiccation, and heat pumps; however, few existing sorbents demonstrate both hysteresis-free behavior and cycling stability. Here we show that post-synthetic exchange with lithium, sodium, potassium, magnesium, and tetramethylammonium in the metal-organic framework (MOF) SU-102 ([(CH3)2NH2]2[Zr(HL)2]; H4L = ellagic acid) enables high-capacity water sorption under low humidity ranging from 11.1% to 4.3%. The champion material, Mg-SU-102, exhibits sharp water uptake at 4.3% RH, reaches a high maximum gravimetric capacity of 0.41 g/g (with 0.29 g/g at 15% RH), and displays minimal capacity loss over 500 adsorption-desorption cycles, with essentially no hysteresis. We use vibrational Stark spectroscopy to probe the local electric field environment within each ion-exchanged material and show that the trend in relative humidity follows a Hofmeister-type series in which the cation affects the ability for water to solvate the framework pores. We find strong deviation from this trend for the tetramethylammonium material, as the larger cation does not undergo capillary condensation sorption, suggesting that fine control over pore functionality is necessary. Establishing a correlation between water sorption and a Hofmeister-type series provides foundational principles for the design of porous ionic sorbents.

Bulk Anion Recognition Kinetically Holds Back Interfacial Adsorption

The competition between bulk and interfacial phenomena underlies many key processes in complex chemical phenomena and transport. While competitive processes are often framed in a thermodynamic context, opportunities to leverage transient species found away from equilibrium can provide a kinetic handle to achieve unconventional reaction outcomes. In this work, we outfit an iminoguanidinium headgroup capable of selective SO42- complexation with alkyl tails of varying complexity to probe competitive bulk and interfacial reaction pathways and tune kinetic pathways for selective chemical separations. Using sum frequency generation (SFG) vibrational spectroscopy we unexpectedly find that adsorption of ligands to the air-aqueous interface was dramatically slowed down for species with increasingly hydrophobic tails. Underlying this phenomenon, we show that the formation of bulk colloidal species with differing propensities for SO42- inhibited surface adsorption via a kinetic bottleneck in the exchange of molecular extractants with colloidal aggregates. This kinetic effect could open up avenues to access unconventional selectivity via complexation of strongly coordinating species in the bulk phase, allowing for more weakly coordinating species to transport via interfacial mechanisms. This work broadly probes nonequilibrium phenomena in chemical separations that arise through unexpected interfacial events that are neglected in traditional equilibrium descriptions.

Hydronium Ions Are Less Excluded from Hydrophobic Polymer-Water Interfaces than Hydroxide Ions

The cloud point temperatures of aqueous poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene) oxide (PEO) solutions were measured from pH 1.0 to pH 13.0 at a constant ionic strength of 100 mM. This ionic strength was reached by mixing the appropriate concentration of NaCl with either HCl or NaOH. The phase transition temperature of both polymers was nearly constant between pH 2.0 and 12.0. However, the introduction of 100 mM HCl (pH 1.0) led to an increase in the cloud point temperature, although this value was still lower than the cloud point temperature in the absence of salt. By contrast, the introduction of 100 mM NaOH (pH 13.0) caused a decrease in the cloud point temperature, both relative to adding 100 mM NaCl and adding no salt. Nuclear magnetic resonance (NMR) studies of these systems were performed below the cloud point temperature, and the chemical shifts closely tracked the corresponding changes in the phase transition temperature. Specifically, the introduction of 100 mM HCl caused the 1H chemical shift to move downfield for the CH resonances from both PNIPAM and PEO, while 100 mM NaOH caused the same resonances to move upfield. Virtually no change in the chemical shift was seen between pH 2.0 and 12.0. These results are consistent with the idea that a sufficient concentration of H3O+ led to polymer swelling compared to Na+, while substituting Cl- with OH- reduced swelling. Finally, classical all-atom molecular dynamics (MD) simulations were performed with a monomer and 5-mer corresponding to PNIPAM. The results correlated closely with the thermodynamic and spectroscopic data. The simulation showed that H3O+ ions more readily accumulated around the amide oxygen moiety on PNIPAM compared with Na+. On the other hand, OH- was more excluded from the polymer surface than Cl-. Taken together, the thermodynamic, spectroscopic, and MD simulation data revealed that H3O+ was less depleted from hydrophobic polymer/water interfaces than any of the monovalent Hofmeister metal cations or even Ca2+ and Mg2+. As such, it should be placed on the far-right side of the cationic Hofmeister series. On the other hand, OH- was excluded from the interface and could be positioned in the anionic Hofmeister series between H2PO4- and SO42-.

Qualitative discrimination and quantitative prediction of salt in aqueous solution based on near-infrared spectroscopy

Freshwater resources have been gradually salinized in recent years, dramatically impacting the ecosystem and human health. Therefore, it is necessary to detect the salinity of freshwater resources. However, traditional detection methods make it difficult to check the type and concentration of salt quickly and accurately in solution. This paper uses a portable near-infrared spectrometer to qualitatively discriminate and quantitatively predict the salt in the solution. The study was carried out by adding ten salts of NaCl, KCl, MgCl2, CaCl2, Na2CO3, K2CO3, CaCO3, Na2SO4, K2SO4, MgSO4 to 2 mL of deionized water to prepare a single salt solution (0.02 %-1.00 %) totaling 100 sets. It was found that the Support vector machine (SVM) model was only effective in discriminating the class of salt anions in the solution. The Partial least squares-discriminant analysis (PLS-DA) model, on the other hand, can effectively discriminate the classes of salt in solution, and the accuracies of the optimal model prediction set and the interactive validation set are 98.86 % and 99.66 %, respectively. Furthermore, the Partial least squares regression (PLSR) models can accurately predict the concentration of NaCl, KCl, MgCl2, CaCl2, Na2CO3, K2CO3, CaCO3, Na2SO4, K2SO4, MgSO4 salt solutions. The coefficients of determination R2 of their model interactive validation sets were 0.99, 0.99, 0.99, 0.97, 0.99, 0.99, 0.98, 0.99, 0.98, and 0.98, respectively. This study shows that NIRS can achieve rapid and accurate qualitative and quantitative detection of salts in solution, which provides technical support for the utilization of safe water resources.

Ion effects on minimally hydrated polymers: hydrogen bond populations and dynamics

Compared to bulk water, the effect of ions in confined environments or heterogeneous aqueous solutions is less understood. In this study, we characterize the influence of ions on hydrogen bond populations and dynamics within minimally hydrated polyethylene glycol diacrylate (PEGDA) solutions using Fourier-transform infrared (FTIR) and two-dimensional infrared (2D IR) spectroscopies. We demonstrate that hydrogen bond populations and lifetimes are directly related to ion size and hydration levels within the polymer matrix. Specifically, larger monovalent cation sizes (Li+, Na+, K+) as well as anion sizes (F-, Cl-, Br-) increase hydrogen bond populations and accelerate hydrogen bond dynamics, with anions having more pronounced effects compared to cations. These effects can be attributed to the complex interplay between ion hydration shells and the polymer matrix, where larger ions with diffuse charge distributions are less efficiently solvated, leading to a more pronounced disruption of the local hydrogen bonding network. Additionally, increased overall water content results in a significant slowdown of dynamics. Increased water content enhances the hydrogen bonding network, yet simultaneously provides greater ionic mobility, resulting in a delicate balance between stabilization and dynamic restructuring of hydrogen bonds. These results contribute to the understanding of ion-specific effects in complex partially-hydrated polymer systems, highlighting the complex interplay between ion concentration, water structuring, and polymer hydration state. The study provides a framework for designing polymer membrane compositions with ion-specific properties.

Do Specific Ion Effects on Collective Relaxation Arise from Perturbation of Hydrogen-Bonding Network Structure?

The change in the transport properties (i.e., water diffusivity, shear viscosity, etc.) when adding salts to water has been used to classify ions as either being chaotropic or kosmotropic, a terminology based on the presumption that this phenomenon arises from respective breakdown or enhancement of the hydrogen-bonding network structure. Recent quasi-elastic neutron scattering measurements of the collective structural relaxation time, τC, in aqueous salt solutions were interpreted as confirming this proposed origin of ion effects on the dynamics of water. However, we find similar changes in τC in the same salt solutions based on molecular dynamics (MD) simulations using a coarse-grained water model in which no hydrogen bonding exists, challenging this conventional interpretation of mobility change resulting from the addition of salts to water. A thorough understanding of specific ion effects should be useful in diverse material manufacturing and biomedical applications, where these effects are prevalent, but poorly understood.

Low-Frequency Spectra of Hydrated Ionic Liquids with Kosmotropic and Chaotropic Anions

In this study, we investigated the water concentration dependence of the intermolecular vibrations of two hydrated ionic liquids (ILs), cholinium dihydrogen phosphate ([ch][dhp]) and cholinium bromide ([ch]Br), using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES). The anions of the former and latter hydrated ILs are kosmotropic and chaotropic, respectively. We found that the spectral peak of ∼50 cm-1 shifted to the low-frequency side in hydrated [ch][dhp], indicating the weakening of its intermolecular interactions. In contrast, no change in the peak frequency of the low-frequency band at ∼50 cm-1 was observed with increasing water concentration in hydrated [ch]Br. The vibrational density of states (VDOS) spectra generated from molecular dynamics (MD) simulations were in qualitative agreement with the experimental results. Decomposition analysis of the VDOS spectra for each component revealed that the red shift of the low-frequency band in the hydrated [ch][dhp] upon water addition was essentially due to the contributions of anions and water rather than that of the cholinium cation. We also found from the low-frequency spectra of the two hydrated ILs that they differed in the concentration dependence of the 180 cm-1 band, which is assigned as a hindered translational motion of water molecules combined to form O···O stretching motions. From the relationship between the peak frequency of the low-frequency band and the bulk parameter, which is the square root of the surface tension divided by the density, we found that the peak frequency in the hydrated IL with kosmotropic [dhp]- depends on the bulk parameter, similar to the case for an aqueous solution of the typical deep eutectic solvent reline. However, the peak frequency of the hydrated IL with chaotropic Br- is constant with the bulk parameter.