Ion-specific Properties Research Articles

Electrolytes at Uncharged Liquid Interfaces: Adsorption, Potentials, Surface Tension, and the Role of the Surfactant Monolayer.

The article summarizes the results of our research on the behavior of ions at uncharged fluid interfaces, with a focus on moderately to highly concentrated aqueous electrolytes. The ion-specific properties of such interfaces have been analyzed. The ion-specificity series are different for water|air and water|oil; different for surface tension σ, surface Δχ potential and electrolyte adsorption, and they change with concentration. A methodology has been developed that allows to disentangle the multiple factors controlling the ion order. The direct ion-surface interactions are not always the most significant factor behind the observed ion sequences: indirect effects stemming from conjugate bulk properties are often more important. For example, the order of the surface tension with the nature of the anion (σKOH > σKCl > σKNO3 for potassium salts) is often the result of bulk nonideality and follows the order of the bulk activity coefficients (γKOH > γKCl > γKNO3) rather than that of a specific ion-surface interaction potential. The surface Δχ potential of aqueous solutions is, in many cases, insensitive to the ion distribution in the electric double layer but reflects the orientation of water at the surface, through the ion-specific dielectric permittivity ε of the solution. Even the sign of Δχ is often the result of the decrement of ε in the presence of electrolyte. A whole new level of complexity appears when the ions interact with an uncharged surfactant monolayer. A method has been developed to measure the electrolyte adsorption isotherms on monolayers of varying area per surfactant molecule via a combination of experiments-compression isotherms and surface pressure of equilibrium spread monolayers. The obtained isotherms demonstrate that the ions exhibit a maximum in their adsorption on monolayers of intermediate density. The maximum is explained with the interplay between ion-surfactant complexation, volume exclusion and osmotic effects.

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.

Impact of hierarchical water dipole orderings on the dynamics of aqueous salt solutions

Ions exhibit highly ion-specific complex behaviours when solvated in water, which remains a mystery despite the fundamental importance of ion solvation in nature, science, and technology. Here we explain these ion-specific properties by the ion-induced hierarchical dipolar, translational, and bond-orientational orderings of ion hydration shell under the competition between ion-water electrostatic interactions and inter-water hydrogen bonding. We first characterise this competition by a new length λHB(q), explaining the ion-specific effects on solution dynamics. Then, by continuously tuning ion size and charge, we find that the bond-orientational order of the ion hydration shell highly develops for specific ion size and charge combinations. This ordering drastically stabilises the hydration shell; its degree changes the water residence time around ions by 11 orders of magnitude for main-group ions. These findings are fundamental to ionic processes in aqueous solutions, providing a physical principle for electrolyte design and application.

Effects of some ion-specific properties in the electrocoagulation process with aluminum electrodes

The influence of anion specificity on the faradaic efficiency relative to aluminum oxidation during electrocoagulation has been reported. While with chaotrope anions, the faradaic efficiency is higher than 1, it is lower than 1 with kosmotrope anions. The origin of this distinction has been ascribed to the ability of anions to cause pitting through the thin passive film at the aluminum surface, which depends on the varying resistance of the anions to dehydration. The influence of the nature of the cation of the supporting electrolyte on the efficiency of electrocoagulation has been investigated by comparing the same treatment of two characteristic colloidal samples. While removal efficiency is better for a positively charged latex in the presence of sodium chloride, it is better for a negatively charged oil in water emulsion in the presence of ammonium chloride. The results come from different effects of pH and its influence on aluminum species during electrocoagulation.

Zeta Potential Determination from Molecular Simulations

Zeta potential (ZP) is among key physical properties characterizing the behavior of nanoparticles in colloidal solutions. Despite many attempts to calculate and neatly interpret the ZP, full understanding of various factors influencing its values has not been reached yet even for standard metal-oxides, particularly when considering high ionic concentrations and the effect of temperature. This contribution extends our recent work [Langmuir 2016, 32, 10189−10198], where we suggested a direct approach to calculate the ZP from nonequilibrium molecular dynamics (NEMD) simulations. Here we investigate NaCl, RbCl, CaCl2, SrCl2, and Na2C2O4 aqueous solutions interacting with TiO2 and SiO2 surfaces, and show contrasting ZP behavior for these metal-oxides, elucidated by theoretical insights gained by molecular simulations. We show that both surface-specific and ion-specific properties play a key role in the observed electrokinetics. Additionally, we explore concentration and temperature influence on the ZP of selec...

Temperature effects on the spatial distribution of electrolyte mixtures at the aqueous liquid-vapor interface.

The microscopic picture of ions at the aqueous liquid-vapor interface continues to be an important and active area of research. Both experiments and simulations have shown that certain ions, such as Br- and I-, prefer to adsorb at the interface, but there is not yet a consensus as to the relative importance of various ion-specific properties that influence surface solvation. In a previous study, we systematically explored the effect of ion size on determining whether or not a monovalent ion would adsorb at the surface, and found that, for electrolyte mixtures represented by non-polarizable models, the larger/smaller anions are enriched/depleted at the interface. Here, we extend that study to include temperature effects enabling a van't Hoff analysis of the enthalpic and entropic contributions. We perform configurational-bias Monte Carlo simulations in the Gibbs ensemble to investigate the partitioning of mixtures of differently sized ions at the aqueous liquid-vapor interface from 284.09 K to 347.22 K at a pressure of 1 atm. Ions are represented using our own previously developed models that vary only in size (i.e., the Lennard-Jones σ parameter changes, while all other parameters are held constant across ion types). System properties studied include surface tension, interfacial width, ion surface excess, number density profiles, z-dependent transfer free energy, enthalpy, entropy, and anion-cation coordination numbers.

Switching selectivity of carboxylic acids and associated physico-chemical changes with pH during electrodialysis of ternary mixtures

Abstract Electrodialytic (ED) transport of carboxylic acids from their ternary mixtures (acetic, malic and citric) of different compositions were investigated through anion exchange membrane (AEM). Solution pH played crucial role in selectivity and ion transport (expressed by limiting current density, LCD). These observations were analysed based on flow hydrodynamics, ion specific properties and ionic interaction with AEM influencing water uptake and membrane resistance. Fractional hydrolysis of weak acids generated complex mixture of multiple anions of varying charges which strongly influenced LCD. Theoretically estimated LCD (from effective diffusivity, Deff and Sh number based effective mass transfer coefficient, keff) could closely match experimental LCD (±4–8% deviation). The nature of solute species, concentration and their interaction with AEM strongly influenced ion transport. Estimation of Deff, transport number, ion exchange equilibrium and membrane resistance could satisfactorily explain the observed LCD. At low pH, LCD was proportional to available free ions while, at pH > 5.2 component specific properties/interactions dominated. Water uptake indicated variation in micro-structure and ion selectivity with different compositions of ternary mixture. This was attributed to shrinkage/swelling of membrane due to ion-membrane interactions. Stronger hydrophobic interaction of acetic acid caused shrinkage of micro-structure resulted in low water uptake but got reversed at higher pH.

Collins’s rule, Hofmeister effects and ionic dispersion interactions

Models based solely on electrostatics cannot explain ion specific properties of electrolyte solutions. We give calculations of dispersion interactions of ions with other ions and with water molecules. These are done via ab initio Symmetry Adapted Perturbation Theory with Density Functional Theory (DFT–SAPT). The calculations establish the substantial, specific contribution of dispersion interactions to ionic interactions. They explain several puzzling properties of electrolyte solutions: the variation in solvation energy among ions of the same size, the small repulsion of iodide from the air–water interface, and the affinity of large ions for each other in water embodied in Collins’s rules.

Aqueous NaCl and CsCl Solutions Confined in Crystalline Slit-Shaped Silica Nanopores of Varying Degree of Protonation

All-atom molecular dynamics simulations were conducted to study the dynamics of aqueous electrolyte solutions confined in slit-shaped silica nanopores of various degrees of protonation. Five degrees of protonation were prepared by randomly removing surface hydrogen atoms from fully protonated crystalline silica surfaces. Aqueous electrolyte solutions containing NaCl or CsCl salt were simulated at ambient conditions. In all cases, the ionic concentration was 1 M. The results were quantified in terms of atomic density distributions within the pores, and the self-diffusion coefficient along the direction parallel to the pore surface. We found evidence for ion-specific properties that depend on ion-surface, water-ion, and only in some cases ion-ion correlations. The degree of protonation strongly affects the structure, distribution, and the dynamic behavior of confined water and electrolytes. Cl(-) ions adsorb on the surface at large degrees of protonation, and their behavior does not depend significantly on the cation type (either Na(+) or Cs(+) ions are present in the systems considered). The cations show significant ion-specific behavior. Na(+) ions occupy different positions within the pore as the degree of protonation changes, while Cs(+) ions mainly remain near the pore center at all conditions considered. For a given degree of protonation, the planar self-diffusion coefficient of Cs(+) is always greater than that of Na(+) ions. The results are useful for better understanding transport under confinement, including brine behavior in the subsurface, with important applications such as environmental remediation.

Ion-specific thermodynamical properties of aqueous proteins

Ion-specific interactions between two colloidal particles are calculated using a modified Poisson-Boltzmann (PB) equation and Monte Carlo (MC)simulations. PB equations present good results of ionic concentration profiles around a macroion, especially for salt solutions containing monovalent ions. These equations include not only electrostatic interactions, but also dispersion potentials originated from polarizabilities of ions and proteins. This enables us to predict ion-specific properties of colloidal systems. We compared results obtained from the modified PB equation with those from MC simulations and integral equations. Phase diagrams and osmotic second virial coefficients are also presented for different salt solutions at different pH and ionic strengths, in agreement with the experimental results observed Hofmeister effects. In order to include the water structure and hydration effect, we have used an effective interaction obtained from molecular dynamics of each ion and a hydrophobic surface combined with PB equation. The method has been proved to be efficient and suitable for describing phenomena where the water structure close to the interface plays an essential role. Important thermodynamic properties related to protein aggregation, essential in biotechnology and pharmaceutical industries, can be obtained from the method shown here.