Specific Anion Effects Research Articles

Anion Specificity in Dimethyl Sulfoxide-Water Mixtures Exemplified by a Thermosensitive Polymer.

In the present work, we have investigated the anion-specific upper critical solution temperature (UCST) behavior of polymer-supported borinic acid (PBA) in dimethyl sulfoxide-water (DMSO-H2O) mixtures. An inverted V-shaped series CH3COO- < Cl- < salt-free > NO3- > ClO4- > SCN- is observed in terms of the anion-specific UCST of PBA in the DMSO-H2O mixtures. Both direct anion-polymer interactions and indirect solvent-mediated anion-polymer interactions are involved in the specific anion effect on the UCST behavior of PBA. The direct binding of anions to the PBA surface generates a salting-in effect on PBA, causing the UCST for the different types of anions to increase from chaotropic to kosmotropic anions due to the stronger binding of the more chaotropic anions. On the other hand, the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules also produces a salting-in effect on PBA, leading the UCST for the different types of anions to increase from kosmotropic to chaotropic anions because of the stronger capability of the more kosmotropic anions to polarize the hydrogen bonding. Thus, the dominating anion-PBA interactions change from the direct anion binding to the indirect anionic polarization of hydrogen bonding as the anions change from chaotropes to kosmotropes. The observed inverted V-shaped series suggests that the specific anion effect on the UCST behavior of PBA in the DMSO-H2O mixtures is determined by the combined effects of the binding of anions to the PBA surface and the anionic polarization of hydrogen bonding between PBA and DMSO molecules.

Specific anion effects on copper surface through electrochemical treatment: Enhanced photoelectrochemical CO2 reduction activity of derived nanostructures induced by chaotropic anions

Copper derivatives are the most prominent CO2 reduction electrocatalyst. Herein, the metallic copper has been electrochemically treated with some of common ionic salts such as N3¯, HPO2¯, S2¯, F¯, Cl¯, Br¯ and I¯ based on the dissolution of a metallic working electrode in an aqueous solution to derive the surface roughness incorporated with nanostructures. Diverse surface morphology can be obtained when the ionic radii of anions are changed. Surface study reveals various roughness shapes based on the size and polarity of the anions, where the ions with higher ionic radii have higher impact on the Cu surface. In comparison, polyatomic oxyanion such as HPO2¯ even with large ionic radii do not have enough strength to create the surface roughness than that of oxygen-free anions with large ionic radii. The photoelectrochemical behavior of the modified surfaces toward CO2 reduction is studied at a wide potential window in bicarbonate aqueous solution. Based on our investigations, treated surfaces by I¯, Cl¯ and S2¯give a more surface roughness, while I¯ and N3¯ offer higher catalytic activity toward CO2 reduction due to possible complexing ability of these anions with Cu cations, followed by formation of the co-catalyst semiconductor and facilitate electron transfer. This methodology can be applied to investigate the effect of ions on transition metals along with obtaining different surface morphologies tailored to different applications.

Electrolyte solvents for high voltage lithium ion batteries: ion correlation and specific anion effects in adiponitrile.

We studied dynamic and structural properties of two lithium conducting salts in the aprotic organic solvent adiponitrile by a combination of atomistic molecular dynamics (MD) simulations, quantum chemical calculations, and experimental findings. The outcomes of our simulations reveal significant differences between both lithium salts, namely lithium tetrafluoroborate (LiBF4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at various concentrations, which can be mainly attributed to the solvation behavior of the individual anions. The increased tendency of ion complex formation for LiBF4 is reflected by lower values regarding the measured and computed effective ionic conductivities when compared to LiTFSI. All findings highlight the crucial importance of specific anion effects in combination with molecular details of solvation, and advocate the use of adiponitrile as a beneficial solvent in modern lithium ion battery technology with high voltage electrodes.

Specific Anion Effects on Na+ Adsorption at the Aqueous Solution-Air Interface: MD Simulations, SESSA Calculations, and Photoelectron Spectroscopy Experiments.

Specific ion effects of the large halide anions have been shown to moderate anion adsorption to the air-water interface (AWI), but little quantitative attention has been paid to the behavior of alkali cations. Here we investigate the concentration and local distribution of sodium (Na+) at the AWI in dilute (<1 M) aqueous solutions of NaCl, NaBr, and NaI using a combination of molecular dynamics (MD) and SESSA simulations, and liquid jet ambient pressure photoelectron spectroscopy measurements. We use SESSA to simulate Na 2p photoelectron intensities on the basis of the atom density profiles obtained from MD simulations, and we compare the simulation results with photoelectron spectroscopy experiments to evaluate the performance of a nonpolarizable force field model versus that of an induced dipole polarizable one. Our results show that the nonpolarizable force model developed by Horinek and co-workers (Chem. Phys. Lett. 2009, 479, 173-183) accurately predicts the local concentration and distribution of Na+ near the AWI for all three electrolytes, whereas the polarizable model does not. To our knowledge, this is the first interface-specific spectroscopic validation of a MD force field. The molecular origins of the unique Na+ distributions for the three electrolytes are analyzed on the basis of electrostatic arguments, and shown to arise from an indirect anion effect wherein the identity of the anion affects the strength of the attractive Na+-H2O electrostatic interaction. Finally, we use the photoelectron spectroscopy results to constrain the range of inelastic mean free paths (IMFPs) for the three electrolyte solutions used in the SESSA simulations that are able to reproduce the experimental intensities. Our results suggest that earlier estimates of IMFPs for aqueous solutions are likely too high.

Specific anion effects on the stability of zeolitic imidazolate framework-8 in aqueous solution

In this study, the stability of zeolitic imidazolate framework-8 (ZIF-8) was investigated in 7 common anions aqueous solutions. Experimental results showed that Cl−, SO42−, NO3−, PO43−, and CO32− have insignificant change in the crystallinity and permanent porosity of ZIF-8 by SEM, XRD, nitrogen sorption and dissolution of Zn and 2-methylimidazole (Hmim) characterizations. Nevertheless, ZIF-8 crystalline phase and its porous character are transformed by the presence of HCO3− and HPO42−.The dissolution of Zn and Hmim underwent a process of a giant ascent at the first stage and a following drop until a steady state in the two mediums. On the other hand, the morphology of ZIF-8 and the XRD peaks changes apparently in the both solutions. However, ZIF-8 possesses a higher stability when Hmim is introduced in the solution due to the inhibiting re-dissolution of ZIF-8 in the two solutions.

Specific anion effects on adsorption and packing of octadecylamine hydrochloride molecules at the air/water interface

The spread of collector molecules on bubble surfaces in salt solution is a fundamental phenomenon in the froth flotation of soluble salts, such as KCl, and consideration of ionic effects on adsorption behavior and packing of collector molecules at the air/solution interface is appropriate for study. In this work, specific anion effects on the adsorption and packing of one common collector (octadecylamine hydrochloride, ODA) at the air/water interface were investigated by employing a combination of surface pressure measurements, sum frequency vibrational spectroscopy (SFVS) and molecular dynamics simulations (MDS). The results from experiments and MDS showed that the addition of external anions disturbs the arrangement of ODA molecules due to the penetration of these anions from the water subphase into the ODA monolayer at the air/water interface. During monolayer compression, the penetrated anions might be driven back to the water subphase by electrostatic forces and the added compression force. These anions could screen the charge of NH3 groups resulting in a reduction of the minimum area of the ODA molecule (Amin) for the solid phase state of the monolayer. With the exception of the sulfate anion, the added anions increase hydrogen bonding of interfacial water molecules at the air/water interface. These results allow for a better understanding of the anionic effect on adsorption and packing of ODA molecules at the air/water interface, and might therefore provide new insights into froth flotation of soluble salts and should help to improve soluble salt flotation technology.

Specific Ion Effects of Salt Solutions on Colloidal Properties of Octadecylamine Hydrochloride

AbstractFlotation continues to be a major technique for the production of potash from low grade, complex systems containing a variety of ores and ions. The specific ion effects on the behavior of the flotation collector in the KCl flotation system becomes important to the interpretation of the flotation mechanism. In this work, specific ion effects on the turbidity, surface tension and aggregation behavior of the common collector in KCl flotation, octadecylamine hydrochloride (ODA), have been investigated. The results from turbidity and surface tension studies of ODA solutions show that both cations and anions can affect the colloidal properties of ODA. Molecular dynamics simulations show that the binding that energy barrier between ODA headgroups and anions is principally responsible for the specific anion effect of γCAC of ODA solutions. In addition, the ion effect on the ODA aggregation particle size was also studied by using transmission electron micrographs (TEM) and dynamic light scattering. The data reveal that the presence of salt ions can induce the formation of larger ODA colloidal particles, on which the cation effect is more significant. This work provides detailed information of specific ion effect on colloidal properties of ODA, which may promote a further understanding of the flotation mechanism and help to improve the flotation of KCl from brine sources.

Hofmeister Anion Effects on Thermodynamics of Caffeine Partitioning between Aqueous and Cyclohexane Phases.

Specific anion effects on the thermodynamics of caffeine partitioning between aqueous and cyclohexane phases were studied in the presence of 11 sodium salts by utilizing UV-vis spectroscopy. It is observed that weakly hydrated anions such as ClO4-, SCN-, and I- salt caffeine into the aqueous phase and increase the standard Gibbs free energy for caffeine transfer. On the other hand, well-hydrated anions such as CO32- and SO42- salt caffeine molecules out of the aqueous solution and promote the transfer process. Results suggest that weakly hydrated anions associate with the hydrophobic patches of caffeine including three methyl groups and a flat heteroatomic ring to solvate caffeine molecules. Well-hydrated anions are excluded from the caffeine surface to salt caffeine molecules out of aqueous solution. Moreover, the enthalpy and entropy of caffeine transfer were obtained by measuring the standard Gibbs free energy for caffeine transfer at varied temperatures. The transfer of caffeine from the aqueous to cyclohexane phase was an endothermic process driven by the entropy of caffeine transfer. However, the trend in standard Gibbs free energy across the Hofmeister series was determined by the enthalpy of caffeine transfer. These results provide an enthalpic origin to explain the Hofmeister trends in aqueous solution.

Corrosion behavior of superhydrophobic aluminum alloy in concentrated potassium halide solutions: When the specific anion effect is manifested

The comparative analysis of evolution of electrochemical parameters and wettability for two types of superhydrophobic coatings (SHCs) at prolonged contact with 3M potassium halide solutions revealed various aspects of a complex corrosion protection provided by SHCs on aluminum alloy. The difference in the pitting processes of Al alloy with SHCs in concentrated potassium halide solutions becomes essential after prolonged contact with the electrolyte. Long-term contact of SHCs of both types with halide solutions shows that the corrosion currents and the rate of hydrolysis of hydrophobic molecules follow the Hofmeister series I−<Br−<Cl−.

Configuration, Anion-Specific Effects, Diffusion, and Impact on Counterions for Adsorption of Salt Anions at the Interfaces of Clay Minerals

Interfacial interactions of clay minerals with salt solutions are ubiquitous and play a crucial role in a wide range of fields, where salt cations are the focus while anions are generally regarded as spectators. Here, molecular dynamics simulations show that the various anions are strongly adsorbed on the surfaces of clay minerals, and the resulting anion-specific effects are pronounced. Although constructing only H-bonds, anions form stable inner- and outer-sphere complexes with clay minerals, and F– and OH– can result in even more stable complexes than metal ions. The underlying anion-specific effects abide by the sequence OH– > F– > Cl– > I– and show apparent enhancements with increase of salt concentrations. OH– is particular at relatively high concentrations, forming clusters and capturing metal ions at octahedral AlO6 surfaces and approaching tetrahedral SiO4 surfaces with help of metal ions in addition to the monodispersive inner- and outer-sphere species at octahedral AlO6 surfaces that are simila...

Specific Anion Effects on the Kinetics of Iodination of Acetone.

Specific ion effects on the kinetics of iodination of acetone in an acidic medium are investigated by UV/Vis spectrophotometry as a function of nature of the acid and temperature. The results indicate that the order of the reaction with respect to acetone is practically unaffected by the composition of the acid while the value of the mixed constant k1 K increases according to the sequence HBr<HCl<HClO4 <HF<H2 SO4 <H3 PO4 . The results are discussed in terms of the hydration free energy and size of the anion and of the interaction between the cationic intermediate and the anion.

Hofmeister effect of anions on calcium translocation by sarcoplasmic reticulum Ca(2+)-ATPase.

The occurrence of Hofmeister (specific ion) effects in various membrane-related physiological processes is well documented. For example the effect of anions on the transport activity of the ion pump Na+, K+-ATPase has been investigated. Here we report on specific anion effects on the ATP-dependent Ca2+ translocation by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Current measurements following ATP concentration jumps on SERCA-containing vesicles adsorbed on solid supported membranes were carried out in the presence of different potassium salts. We found that monovalent anions strongly interfere with ATP-induced Ca2+ translocation by SERCA, according to their increasing chaotropicity in the Hofmeister series. On the contrary, a significant increase in Ca2+ translocation was observed in the presence of sulphate. We suggest that the anions can affect the conformational transition between the phosphorylated intermediates E1P and E2P of the SERCA cycle. In particular, the stabilization of the E1P conformation by chaotropic anions seems to be related to their adsorption at the enzyme/water and/or at the membrane/water interface, while the more kosmotropic species affect SERCA conformation and functionality by modifying the hydration layers of the enzyme.

Interactions between Polyelectrolyte Brushes and Hofmeister Ions: Chaotropes versus Kosmotropes.

We have investigated the interactions between the positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes and the Hofmeister anions and the interactions between the negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brushes and the Hofmeister cations using a combination of quartz crystal microbalance with dissipation and spectroscopic ellipsometry. A V-shaped anion series is observed in terms of the ion-specific interactions between the PMETAC brushes and the Hofmeister anions. We have found that the chaotropic and kosmotropic anions interact with the PMETAC brushes in different manners. The ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the direct interactions between the anions and the positively charged quaternary ammonium group via ion pairing mediated by ionic hydration strength or polarizability, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules between the anions and the brushes. The ion-specific interactions between the PMETAC brushes and the anions have significant influences on both the hydration and the conformation of the brushes. The cations exhibit weaker specific ion effects on the PSPMA brushes in comparison with the specific anion effects on the PMETAC brushes.

A Colloidal Description of Intermolecular Interactions Driving Fibril-Fibril Aggregation of a Model Amphiphilic Peptide.

We apply a kinetic analysis platform to study the intermolecular interactions underlying the colloidal stability of dispersions of charged amyloid fibrils consisting of a model amphiphilic peptide (RADA 16-I). In contrast to the aggregation mechanisms observed in the large majority of proteins and peptides, where several elementary reactions involving both monomers and fibrils are present simultaneously, the system selected in this work allows the specific investigation of the fibril-fibril aggregation process. We examine the intermolecular interactions driving the aggregation reaction at pH 2.0 by changing the buffer composition in terms of salt concentration, type of ion as well as type and concentration of organic solvent. The aggregation kinetics are followed by dynamic light scattering, and the experimental data are simulated by Smoluchowski population balance equations, which allow to estimate the energy barrier between two colliding fibrils in terms of the Fuchs stability ratio (W). When normalized on a dimensionless time weighted on the Fuchs stability ratio, the aggregation profiles under a broad range of conditions collapse on a single master curve, indicating that the buffer composition modifies the aggregation kinetics without affecting the aggregation mechanism. Our results show that the aggregation process does not occur under diffusion-limited conditions. Rather, the reaction rate is limited by the presence of an activation energy barrier that is largely dominated by electrostatic repulsive interactions. Such interactions could be reduced by increasing the concentration of salt, which induces charge screening, or the concentration of organic solvent, which affects the dielectric constant. It is remarkable that the dependence of the activation energy on the ionic strength can be described quantitatively in terms of charge screening effects in the frame of the DLVO theory, although specific anion and cation effects are also observed. While anion effects are mainly related to the binding to the positive groups of the fibril surface and to the resulting decrease of the surface charge, cation effects are more complex and involve additional solvation forces.

Lysozyme stability and amyloid fibrillization dependence on Hofmeister anions in acidic pH.

We have explored an effect of Hofmeister anions, Na2SO4, NaCl, NaBr, NaNO3, NaSCN and NaClO4, on stability and amyloid fibrillization of hen egg white lysozyme at pH 2.7. The stability of the protein was analyzed by differential scanning calorimetry. The Hofmeister effect of the anions was assessed by the parameter dT trs/d[anion] (T trs, transition temperature). We show that dT trs/d[anion] correlates with anion surface tension effects and anion partition coefficients indicating direct interactions between anions and lysozyme. The kinetic of amyloid fibrillization of lysozyme was followed by Thioflavin T (ThT) fluorescence. Negative correlation between dT trs/d[anion] and the nucleation rate of fibrillization in the presence of monovalent anions indicates specific effect of anions on fibrillization rate of lysozyme. The efficiency of monovalent anions to accelerate fibrillization correlates with inverse Hofmeister series. The far-UV circular dichroism spectroscopy and atomic force microscopy findings show that conformational properties of fibrils depend on fibrillization rate. In the presence of sodium chloride, lysozyme forms typical fibrils with elongated structure and with the secondary structure of the β-sheet. On the other hand, in the presence of both chaotropic perchlorate and kosmotropic sulfate anions, the fibrils form clusters with secondary structure of β-turn. Moreover, the acceleration of fibril formation is accompanied by decreased amount of the formed fibrils as indicated by ThT fluorescence. Taken together, our study shows Hofmeister effect of monovalent anions on: (1) lysozyme stability; (2) ability to accelerate nucleation phase of lysozyme fibrillization; (3) amount, and (4) conformational properties of the formed fibrils.

Specific anion effects in Artemia salina

The specific anion effect on the vitality of Artemia salina was investigated by measuring the Lethal Time LT50 of the crustaceans in the presence of different sodium salts solutions at room temperature and at the same ionic strength as natural seawater. Fluoride, thiocyanate and perchlorate are the most toxic agents, while chloride, bromide and sulfate are well tolerated. The rates of oxygen consumption of brine shrimps were recorded in mixed NaCl+NaF or NaCl+NaSCN solutions as a function of time. The results are discussed in terms of the Hofmeister series, and suggest that, besides the biochemical processes that involve F−, SCN− and ClO4−, the different physico-chemical properties of the strong kosmotropic and chaotropic anions may contribute in determining their strong toxicity for A. salina.

Anion-specific effects on the behavior of pH-sensitive polybasic brushes.

The anion-specific solvation and conformational behavior of weakly basic poly(2-dimethylamino)ethyl methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (poly(DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly(DPA)) brushes, with correspondingly increasing inherent hydrophobicity, have been investigated using in situ ellipsometric and quartz crystal microbalance with dissipation (QCM-D) measurements. In the osmotic brush regime, as the initial low concentration of salt is increased, the brushes osmotically swell by the uptake of solvent as they become charged and the attractive hydrophobic inter- and intrachain interactions are overcome. With increased ionic strength, the brushes move into the salted brush regime where they desolvate and collapse as their electrostatic charge is screened. Here, as the brushes collapse, they transition to more uniform and rigid conformations, which dissipate less energy, than similarly solvated brushes at lower ionic strength. Significantly, in these distinct regimes brush behavior is not only ionic strength dependent but is also influenced by the nature of the added salt based on its position in the well-known Hofmeister or lyotropic series, with potassium acetate, nitrate, and thiocyanate investigated. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Increasing thiocyanate concentration causes the brushes to collapse at lower ionic strength than for the other two anions. This study of weak polybasic brushes demonstrates the importance of all ion, solvent, and polymer interactions.

Specific Anion Effects for Aggregation of Colloidal Minerals: A Joint Experimental and Theoretical Study

In this work, dynamic light scattering experiments and density functional calculations were combined to demonstrate the specific anion effects for the aggregation of negatively charged colloidal minerals. Although the aggregation kinetics is dominated by electrolyte cations, anions also play a significant role. The critical coagulation concentrations and activation energies indicated a clear Hofmeister series for the various anions as H2PO4– < Cl– < NO3– < SO42– < HPO42– < PO43–. Moreover, interaction energies of anions with cations and proton affinities of anions were explored as the influencing factors for anion specificities, which were corroborated by measurement of surface charge densities. Owing to the largest interactions with cations, PO43– causes the most pronounced inhibition effect to the aggregation kinetics and corresponds to the strongest anion specificity. Proton exchange from H2PO4– reduces the negative charges of minerals and accelerates the aggregation process, thus resulting in an inferior anion specificity than NO3–. Density functional calculations indicated that proton transfer from minerals to OH– can occur facilely and increase the negative charges of minerals, as confirmed by charge density measurements and dilution experiments. This further adds to the aggregation difficulty and causes OH– to show distinctly stronger anion specificity than other univalent anions.

Adaptability of monoglyceride-induced crystallization of K2SO4: effect of various anions and lipid chain splay.

Understanding the interaction of a surfactant assembly with surrounding ionic compositions is critical in controlling the nanostructure formed by amphiphilic systems. In this study, we investigated the influence of specific anions upon the character of the crystallization process of a model inorganic salt (K2SO4) in an aqueous microdroplet (10-100 μm) in the presence of monolayers of monoelaidin (ME), monovaccenin (MV), monoolein (MO), and monolinolein (ML) assembled at a water-decanol interface. In particular, control monolayers of each monoglyceride at the water-oil interface did not accelerate nucleation (i.e., act as a template) of the crystallization of K2SO4. However, in the presence of 1-10 mM exogenous anions, certain types of monoglycerides did induce nucleation. The influence of the anions was consistent with the Hofmeister series. Monoglyceride monolayers in combination with chaotropic anions, such as SCN(-), imparted a euhedral crystal habit with a relatively lower C(onset), while kosmotropic anions showed less notable changes in the crystallization characteristics. However, this specific anion effect was seen only for monolayers of ME, MV, and MO but not for ML. This enhancement of nucleation-inducing ability upon addition of chaotropic anions appears to be related to the degree of chain splay: the larger the chain splay, the less anion efficiency in enhancing the templating capability. Our findings indicate that the improved nucleation-enhancing capabilities may originate from mutual interactions between anions and the monolayer, allowing the monoglyceride monolayers to adapt to a modulated self-assembled structure resulting in the greatest degree of control over K2SO4 crystallization.

Photoelectron spectroscopy and theoretical studies of anion-π interactions: binding strength and anion specificity.

Proposed in theory and then their existence confirmed, anion-π interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion-π interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl(-), Br(-), I(-), linear thiocyanate SCN(-), trigonal planar nitrate NO3(-), pyramidic iodate IO3(-), and tetrahedral sulfate SO4(2-)). The binding energies of the resultant gaseous 1 : 1 complexes (1·Cl(-), 1·Br(-), 1·I(-), 1·SCN(-), 1·NO3(-), 1·IO3(-) and 1·SO4(2-)) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl(-), NO3(-), IO3(-) with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol(-1), but only about 40% of that between 1 and SO4(2-). Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion-π binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion-π binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion-π interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions.