Session Y16: Multivalent and Complex Ions at Charged Interfaces

Characterization of a multivalent charged electrolyte using electrochemical method and electrophoretic NMR

Understanding various transport parameters, for example conductivity, diffusion coefficient and transference member, is important for developing electrolytes for lithium ion battery applications. Theoretically, electrolytes with high transference number are predicted to prevent Li dendrite growth. In this aspect ceramic and solid polymer electrolytes have been studied. However, solid electrolytes can suffer from poor Li-interfacial contact. Additionally, solid polymer electrolytes exhibit lower conductivity at room temperature. On the other hand, liquid electrolytes with bulkier multivalent anion could potentially increase the transference number as mobility of ionic species is inversely proportional to the size of ions. In the present work, we have designed a new electrolyte of a polyanionic lithium salt containing a negative charge of -20, dissolved in ethylene carbonate/dimethyl carbonate mixed solvent. The transport properties of the electrolytes have been fully characterized using various electrochemical techniques. Furthermore, electrophoretic NMR technique was used to investigate the electric-field-induced movement of ions and solvents which in turn provided the transference number for these electrolytes. Thus, the transference number data obtained using two different techniques is compared in this study   

Organic acid anion transport in imidazolium ionomers

Recovering organic acids from processed biomass is important to circular economy tenants. We have observed in electrodialysis and electrodeionization separation studies that imidazolium-type of anion exchange membranes and ionomer binders promote organic acid anion transport rates while also boosting selectivity over inorganic ions. Using 2D NMR (e.g., NOESY) and molecular dynamics simulations, we report imidazolium cations have favorable moiety interactions, such as π-π interactions in the case of aromatic organic acids, that mediate their transport along the polymer backbone. These interactions account for the improved transport rates and uptake when benchmarked against convention quaternary ammonium anion exchange materials. 

    

Controlling complexation between anionic gels and cationic antimicrobials

Self-defensive surfaces that resist bacterial colonization have been increasingly studied over the past decade. We have explored such surfaces using microgels of polyacrylic acid (PAA). They are electrostatically deposited onto a surface and loaded by complexation with cationic antimicrobial peptides. We have shown that contact with bacteria can trigger the release of the antimicrobial, which then kills the challenging bacterium. A key issue in this approach is to ensure stable microgel-antimicrobial complexation under physiological conditions. Colistin, an FDA-approved antibiotic with five positive charges, can be sequestered in PAA microgels in low ionic strength phosphate buffer but is quickly released when exposed to phosphate buffered saline (PBS) with higher ionic strength. We have studied various microgel-antimicrobial combinations to better understand what physicochemical properties most influence the complexation strength. We study this by in situ imaging to establish critical salt concentrations required for release as well as by coarse-grained modeling. In addition to the total net electrostatic charge, we have found, that aromaticity in the microgel (e.g. polystyrene sulfonate) or in the antimicrobial (e.g. polymyxin B) enhances the complexation strength.  Our most recent work concentrates on antimicrobials that self-assemble in solution to form bundles and other supramolecular structures, which bring higher net charges and charge densities to the complexation process.   

    

Ion-ion correlations in concentrated electrolytes using a fluctuating dynamic field theory

Classical models of electrostatic interactions and screening work best when at least one of the objects is dilute. At higher concentrations, correlations due to the long-ranged electrostatic interactions alter the behavior seen. This is relevant for applications such as concentrated electrolyte solutions and in colloidal, polymer, and protein suspensions. In concentrated systems, it is convenient to use a continuum field theory that describes the concentrations of ions instead of explicitly tracking each ion. However, mean field theories cannot easily capture ion-ion correlations and fluctuations. In this talk, we will describe the results of applying a stochastic kinetic theory approach to concentrated charged systems. This method can capture fluctuations and correlations in systems with long-ranged interactions like electrostatic. In particular, we will show how the model produces under-screening in concentrated electrolytes and the role of excluded volume repulsions of the ions due to their finite size.   

Image-charge effects on peptide adsorption near graphene/water interface: A Molecular Dynamics Study

Noncovalent functionalization of graphene by the formation of peptide-graphene complexes have been widely explored for practical applications ranging from biosenor design to drug delivery. Molecular modeling for the protein-graphene interactions and electrostatic interactions across interface is thus not only important for the predictive understanding of peptide adsorption, but also central to the rational design of such complexes. Especially, the image-charge interaction (ICI) induced by the dielectric mismatch near graphene/water interface is expected to play an important role in structural, dynamic, and thermodynamic properties of peptide adsorption. In this work, we study the adsorption of charged peptide ACE-X-NME on graphene/water interface using molecular dynamics simulations, incorporating image charge particles (ICPs) and constant potential method. We focus on elucidating the image-charge effects on binding affinity and conformation of peptide near graphe/water interface, which provide a molecular-level understanding of the interfacial peptide adsorption.   

Molecular-scale experimental characterizations of multivalent ion distributions near electrified graphene

Ion and water organization near charged surfaces is fundamentally interesting and relevant to separation processes such as capacitive deionization. The Poisson-Boltzmann equation can describe the distribution of an ideal, monovalent ion near charged surfaces including the structure of the electrical double layer. However, modeling multivalent ion behavior near electrodes is more complicated and directly relevant to many aqueous systems, including those demonstrating overcharging. In these instances, information about both the cation and anion distributions specifically near the interface are critical. We utilize high resolution x-ray reflectivity and elementally-sensitive resonant anomalous x-ray reflectivity to measure the in situ water and metal ion organization near graphene electrodes. Graphene is promising electrode material because it is atomically smooth and has an ideal surface lacking specific functional groups. We reveal ion distributions with molecular-scale resolution and demonstrate significant trivalent cation overcharging in dilute aqueous systems. Additionally, we consider the effects of anion choice and characterize halide organization likely in response to cation overcharging.   

Recovery of Fatty Acid Monolayers by Divalent Salts Investigated by Sum-Frequency Generation Spectroscopy

Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C₁₄H₂₉COOH (pentadecanoic acid), C₁₅H₃₁COOH (palmitic acid), and C₁₆H₃₃COOH (heptadecanoic acid)) are stable at a neutral pH (pH~6) but become unstable at a high pH (pH ~ 11). The further addition of a small amount of divalent salt in subphase water was found to recover the monolayer at a high pH because binding of the divalent cations to the carboxylic headgroups renders the molecule more stable against dissolution in subphase water. This revival of the monolayer was observed via a pressure-area isotherm measurement and sum-frequency generation spectrum in the CH_x and OH range. Fatty acid with longer alkyl chain needed less amount of MgCl₂ to recover the monolayer at high pH. Much lower concentration of Mg²⁺ as compared to Ca²⁺ is required to revive fatty acid molecules to the surface. Monovalent and trivalent salts were compared with the above divalent salts on the ability to recover the fatty acid monolayers.

    

Ions Salting Out Ions at the Water-Air Interface

In common aqueous systems, including naturally occurring salt water as well as tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific vibrational spectroscopy, i.e., sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in aqueous electrolyte solution. Through the interfacial water’s O-H stretch mode signal, we find that more hydrophobic ions are salted out to the interface in the presence of the hydrophilic ions. This tendency of the salting-out follows the Hofmeister series. Simulations show that the solvation free energy difference between the ions and the intrinsic surface propensity of ions determine the extent of salting-out of ions by other ions. This mechanism provides a unified view of the salting-out of monatomic and polyatomic ions at interfaces of electrolyte solutions with air.

    

Author not Attending

Molecules of solvents with a strong dipole moment have the ability to reorient as a response to an external DC field. At charged interfaces, the topmost layers involve molecules and ions that directly interact with the field source. In consequence, the molecular network at the vicinity of a charged phase boundary reorient within an unclear extent. Furthermore, the DC field towards the bulk is gradually screened by all charged particles in the interfacial region, and the field influence completely vanishes once the bulk is reached. Within this model, the anisotropy of charged interfaces has two components, one related with symmetry breaking by preferential molecular ordering, and the other, that arises from the presence of the field. Stablishing a clear connection between the surface macroscopic observables and the microstructure will open new ways to understand as well as to control relevant interfacial properties and processes. Nevertheless, accessing information that combines depth information and chemical sensitivity is experimentally non-trivial.

With our new depth resolved vibrational spectroscopy tool, in which we combine Sum- and Difference-Frequency Generation spectroscopy (SFG and DFG, respectively), we can extract precise depth information within regions with broken centro-symmetry  and correlate it to specific vibrational features[1]. To explore the effect of the ionic strength on the interfacial depth profiles, we performed measurements on systems with positive and negative charges at the topmost layer, and at relatively high and low concentrations of a monovalent inorganic salt. Based on these measurements we can draw conclusions about the spatial evolution of the electric fields as well as the extents and respective length scale of induced molecular ordering within the interfacial region..

    

X-ray absorption spectroscopy of extractant binding to rare earth ions at aqueous surfaces relevant to solvent extraction

Solvent extraction processes are designed to extract a target species of ion from a multi-component aqueous mixture into an organic solvent, then return the target species to a fresh aqueous phase. Several structural aspects of this process have been studied, including the hydration of ions in water, the local atomic coordination of ion-extractant complexes in the organic phase, and ion-extractant ordering at both the liquid-liquid interface and at the surfaces of model aqueous solutions. Missing from these previous investigations is a study of local atomic coordination of the ion-extractant complex at the interface. However, this interfacial coordination has been shown to play a critical role in the formation of the ion-extractant complexes which are responsible for the transportation of the ions across the interface. This presentation will describe our advancements of synchrotron X-ray absorption spectroscopy to investigate the local coordination of ion-extractant binding at the surface of water, which serves as a model system relevant to the extraction of rare earth ions. These measurements were done at NSF's ChemMatCARS at the Advanced Photon Source, Argonne National Laboratory.   

Model solvent extraction system confirms enhancement of lanthanide extraction by bilayer structures.

Understanding of the interactions of lanthanide ions with surfactants is essential to the development of improved solvent extraction and separation processes. However, the underlying microscopic mechanisms are still poorly understood. It is known that at the air-water interface, the amphiphilic molecule dihexadecyl phosphate (DHDP) forms bilayer structures with heavier lanthanides upon compression, while monolayers are observed when only light lanthanides are present in the subphase ^[1],[2] . We have studied solvent extraction of lanthanides from aqueous solutions to organic solvents containing model extractants, and correlated the results to the interfacial structures. The selectivity, extraction rates and extraction efficiencies all support the postulate that the formation of hydrophobic bilayers leads to faster and better extraction and potentially to better selectivity in rare earth element separations. A likely reason is the bilayers are hydrophobic and thus more soluble in the organic phase.

 

 

[1] Nayak, Srikanth, et al. “Spontaneous and Ion-Specific Formation of Inverted Bilayers at AIR/Aqueous Interface.” Langmuir, vol. 38, no. 18, 2022, pp. 5617–5625., https://doi.org/10.1021/acs.langmuir.2c00208.

[2] Yoo, Sangjun, et al. “Specific Ion Effects in Lanthanide–Amphiphile Structures at the Air–Water Interface and Their Implications for Selective Separation.” ACS Applied Materials & Interfaces, vol. 14, no. 5, 2022, pp. 7504–7512., https://doi.org/10.1021/acsami.1c24008.

    

Effect of ion-ion correlations on the distribution of halide anions (Cl-, Br-, I-) and Rb+ at the muscovite (001)–water interface under high salinity conditions

While electrical double layer models describing classical ion distributions at charged mineral-water interfaces are well-validated, there is still a gap in understanding non-classical interfacial processes at high ionic strength, which is urgently needed to develop models that describe element transport in environmental systems with high salinity.

This study is an extension of previous work on adsorption of alkali metal Rb⁺ and halides Cl^- and I^- on muscovite (001), demonstrating the onset of nonclassical charge overscreening by adsorbed Rb in high salinity solutions (i.e., [Rb] > 0.1 M). Here, we simultaneously probe cation and anion distribution, i.e., Rb and Br, adsorbed at the mica–water interface by element-specific resonant anomalous X-ray reflectivity (RAXR) to answer the question if and how the charge density of the electrolyte anion controls the positional correlations between monovalent ions at charged interfaces. Our new RAXR measurements reveal that the overscreening leads to the adsorption of 0.6 ± 0.1 Br^-/A_UC. During this process, the adsorption height of Rb⁺ (~2 Å) stays constant, while that of Br^- systematically increases from ~3.7 up to 4 Å with increasing [RbBr] from 1 to 2.7M, which is consistent with the height difference between Rb and Br perpendicular to the (111) plane of RbBr crystals. These results provide molecular scale insights into the transition from classical to nonclassical behavior and the initiation of crystal growth/formation at high ionic strength.

    

Self-consistent theory to describe charge inversion and like-charge attraction in multivalent electrolytes

The standard mean-field Poisson-Boltzmann (PB) theory for electrical double layers does not account for three crucial factors: inhomogeneous ion correlations, spatially varying dielectric permittivity, and finite-size effects of ions and solvent molecules. This does not allow PB to even qualitatively explain the phenomena of charge inversion and like-charge attraction in multivalent electrolytes. Here we present the first rigorous and self-consistent theory to explain them. We capture the surface charge-induced continuous transition from a normal double layer to an overcharged double layer. The strength of charge inversion is decided by ion correlation and excluded volume effects with only a minor contribution from long-range image forces. An increase in the counterion valency and surface charge leads to enhanced charge inversion and even ionic layering and oscillations in ion density profiles near the surface. In quantitative agreement with experiments, the inverted zeta potential behaves non-monotonically with respect to bulk multivalent salt concentration. For two approaching like-charged plates, an increase in surface charge leads to a greater accumulation of multivalent counterions in the double layer and hence a stronger ion correlation effect. The gain in ion correlations overcomes the entropic repulsion and an attractive force is obtained. In agreement with simulations, the strength of the attractive force is again found to depend non-monotonically on bulk salt concentration, highlighting the origins of reentrant condensation in charged colloids. The addition of monovalent salt to a multivalent salt solution cancels charge inversion and reduces the strength of like-charge attraction.