Specific Ion Interaction Research Articles

ChemFET Anion Sensor Based on MOF Nanoparticles.

Nanoparticles of metal-organic frameworks (nanoMOFs) possess the unusual combination of both internal and external surfaces. While internal surfaces have been the focus of fundamental and applications-based MOF studies, the chemistry of the external surfaces remains scarcely understood. Herein we report that specific ion interactions with nanoparticles of Cu(1,2,3-triazolate)2 (Cu(TA)2) resemble the Hofmeister behavior of proteins and the supramolecular chemistry of synthetic macromolecules. Inspired by these anion-selective interactions, we tested the performance of Cu(TA)2 nanoparticles as chemical field effect transistor (ChemFET) anion sensors. Rather than size-based selectivity, the detection limits of the devices exhibit a Hofmeister trend, with the greatest sensitivity towards anions perchlorate, iodide, and nitrate. These results highlight the importance of the pore-based supramolecular interactions, rather than localized donor-acceptor pairs, in designing MOF-based technologies.

Ion Trios: Cause of Ion Specific Interactions in Aqueous Solutions and Path to a Better pH Definition

Ion Trios: Cause of Ion Specific Interactions in Aqueous Solutions and Path to a Better pH Definition

New thermodynamic insights into pregabalin interactions with H+, Na+, Mg2+, Ca2+, Cu2+, Zn2+: Equilibrium constants, enthalpy changes and sequestering ability

The thermodynamics of the interaction between (S)-(+)-3-aminomethyl-5-methylhexanoic acid (pregabalin) and protons was studied potentiometrically at different temperatures (288.15 ≤ T/K ≤ 310.15), ionic strengths (0.16 ≤ I/mol kg−1(H2O) ≤ 0.97, NaCl), (0.11 ≤ I/mol kg−1(H2O) ≤ 1.11, (C2H5)4NI), (0.10 ≤ I/mol kg−1(H2O) ≤ 1.03, NaClO4, only at T = 298.15 K). The protonation constants at infinite dilution and the corresponding enthalpy change values were determined, as well as their parameters for the dependence on the temperature and ionic strength. The results showed that the protonation reactions are exothermic, and that the entropic contribution is the driving force of the processes.Formation constants of pregabalin (L) with Zn2+, Cu2+, Ca2+, and Mg2+ were determined in NaCl(aq) at different ionic strength values, at 298.15 K. Different speciation models were proposed for the various metal/Pregabalin systems: ZnHL2+, ZnLOH0(aq), CuL+, CuL20(aq), CaL+, CaHL2+, and MgL+, depending on the different acid-base properties of the metals and the possible formation of sparingly soluble species. The modelling of the thermodynamic formation parameters respect to the temperature and ionic strength variation was carried out by using both the Specific Ion Interaction Theory (SIT) and an extended Debye-Hückel type equation.Being Pregabalin an emerging contaminant, it was interesting to investigate its distribution in presence of the investigated metal cations in aqueous solution simulating both biological fluid (urine) and natural water (seawater).

Speciation and formation constants of aqueous lanthanum (III) chloride complexes from 5 to 80 °C by Raman spectroscopy

Lanthanum chloride solutions were studied both computationally and by polarized Raman spectroscopy to determine the aqueous species present and their equilibrium constants. Four species were predicted from Gaussian 16 computations using density functional theory: La(H2O)9 3+, LaCl(H2O)8 2+, LaCl2(H2O)6 2+, and LaCl3(H2O)4 0. Both LaCl(H2O)8 2+ and LaCl2(H2O)6 + were identified in the reduced isotropic solvent-subtracted Raman spectra from their La–Cl vibrational bands at 234 and 221 cm−1, respectively. The spectra did not change significantly with temperature from 5 to 80 °C, and the Raman bands for each species could not be fitted with sufficient accuracy to determine the scattering coefficients directly from the experimental data. Instead, the computed scattering coefficients from Gaussian 16 (B3LYP with the Los Alamos National Laboratory 2-double-zeta effective core potential and 6-311+G(d,p) basis set) were to be used to obtain the equilibrium concentrations of LaCl(H2O)8 2+ and LaCl2(H2O)6 +, from which the concentration of La(H2O)9 3+ was determined. The concentration dependent formation quotients were fitted with the specific ion interaction (SIT) activity coefficient model to determine the stepwise formation constants K0, 1 and K1, 2, from 5 to 80 °C. Both K0, 1 and K1, 2 showed minor increases with temperature.

Ion-Specific Interactions Engender Dynamic and Tailorable Properties in Biomimetic Cationic Polyelectrolytes.

Biomaterials such as spider silk and mussel byssi are fabricated by the dynamic manipulation of intra- and intermolecular biopolymer interactions. Organisms modulate solution parameters, such as pH and ion co-solute concentration, to effect these processes. These biofabrication schemes provide a conceptual framework to develop new dynamic and responsive abiotic soft material systems. Towards these ends, the chemical diversity of readily available ionic compounds offers a broad palette to manipulate the physicochemical properties of polyelectrolytes via ion-specific interactions. In this study, we show for the first time that the ion-specific interactions of biomimetic polyelectrolytes engenders a variety of phase separation behaviors, creating dynamic thermal- and ion-responsive soft matter that exhibits a spectrum of physical properties, spanning viscous fluids to viscoelastic and viscoplastic solids. These ion-dependent characteristics are further rendered general by the merger of lysine and phenylalanine into a single, amphiphilic vinyl monomer. The unprecedented breadth, precision, and dynamicity in the reported ion-dependent phase behaviors thus introduce a broad array of opportunities for the future development of responsive soft matter; properties that are poised to drive developments in critical areas such as chemical sensing, soft robotics, and additive manufacturing.

Thermodynamic properties and revised Helgeson-Kirkham-Flowers equations of state parameters of the hydrated and dehydrated monomeric silica species at t = 0.01–600oC, P = 1–3000 bars, ρH2O = 0.35–1.1 g cm−3, and Im= 0 m

The thermodynamic properties and revised Helgeson-Kirkham-Flowers equations of state (r-H-K-F EoS) parameters of the hydrated (Si(OH)4(aq), SiO(OH)3– and SiO2(OH)22−) and corresponding dehydrated (SiO2(aq), HSiO3− and SiO32−) monomeric silica species are important to describe the pH, composition, temperature, and pressure dependence of formation/breakdown reactions of all silicon-bearing compounds globally. Experimental log10 equilbrium constant, K values describing the formation reactions of these hydrated and dehydrated monomeric silica species were therefore compiled from the literature, extrapolated to zero ionic strength by specific ion interaction theory as required and used to derive their thermodynamic properties and r-H-K-F EoS parameters.Consideration of all formation reactions in the same study provides a collective, internally consistent update to the thermodynamic properties and r-H-K-F EoS parameters of the monomeric silica species that are able to provide satisfactory matches to the available experimental log10 K values at t = 0.01–600oC, P = 1–3000 bars, ρH2O = 0.35–1.1 g cm−3, and zero ionic strength. These temperature and pressure limits comfortably bracket t = 0.01–100oC and P = 1–270 bars relevant to the geological disposal of radioactive wastes at depths of up to 1 km.Updates to the thermodynamic properties of silicon-bearing compounds in all of the available geochemical thermodynamic databases are necessary, especially if reaction properties are used or given. Internal consistency between the hydrated and dehydrated species means that the hydrated species alone can be used as entries in geochemical thermodynamic databases.

Effect of novel surfactant protic ethanolamine based ionic liquids on the aqueous solubility and thermodynamic properties of acetaminophen

The solubility of poorly soluble drugs in aqueous media remains an obstacle in the pharmaceutical development process. Ionic liquids (ILs), with their remarkable solvent properties, have gained considerable attention as potential solubility enhancers for such drugs. In this context, surfactant protic ethanolamine-based ILs have emerged as a particularly promising class of compounds due to their unique dual functionality. This study explores the significant role of these novel solvents in improving drug solubility, focusing on their potential applications in pharmaceutical formulations. Acetaminophen (ACP), a widely used analgesic and antipyretic drug, often encounters solubility limitations that hinder its effective pharmaceutical applications. In the past few years, there has been a growing recognition of the potential of ILs (ionic liquids) as effective solvents. They offer promising prospects for enhancing the solubility of pharmaceutical substances in water-based solutions. This study investigates the enhancement of ACP solubility through the utilization of ILs. The solubilization behavior of ACP in the attendance of mono, bis and tris-(2-hydroxyethylammonium) octanoate systems is explored, highlighting the significant impact of different number of hydroxyethyl groups combinations on its solubility profile. Experimental results reveal that these ILs exhibit a remarkable ability to improve the solubility of ACP, making them attractive candidates for pharmaceutical formulations. The mechanisms underlying this enhancement are discussed, encompassing factors such as the ion-specific interactions, polarity, and hydrogen bonding capabilities of ionic liquids. Furthermore, the potential implications of utilizing ILs for enhancing the bioavailability and formulation versatility of ACP are considered. To accurately correlate the solubility data, various thermodynamic models were employed. Furthermore, to examine the observable thermodynamic characteristics of dissolution in the studied systems Van't Hoff and Gibbs equations were used. The findings indicated that the dissolution process in all co-solvents used was characterized by an endothermic nature and driven by enthalpy.

Speciation studies of a Bisphosphonate Drug. New thermodynamic insights of Sodium Alendronate with Ca2+ and Mg2+ in NaCl aqueous solution (0.10 ≤ I/mol kg-1 ≤ 1.02)

The thermodynamics of interaction of a second-generation bisphosphonate drug, 4-amino-1-hydroxy-1-phosphonobutyl phosphonic acid mono sodium (sodium alendronate) with H+, Ca2+ and Mg2+ has been investigated. The protonation constants were determined in NaCl and (C2H5)4NI aqueous solutions at different ionic strengths (0.10 ≤ I/mol kg-1(H2O) ≤ 1.02) and temperatures (288.15 ≤ T/K ≤ 310.15) by using the ISE-[H+] potentiometry. The differences of the log KiH in the two ionic media were interpreted in terms of activity coefficient variations and formation of Na+ weak complexes. The interactions with Mg2+ and Ca2+ were studied in NaCl(aq) (0.10 ≤ I/ mol kg-1 (H2O) ≤ 1.02) and T = 298.15 K. Two different speciation models were obtained; Ca2+: CaH2L, CaHL, CaL. Mg2+: MgH3L, MgH2L, MgHL, MgL. The dependence of the thermodynamic parameters on the ionic strength and temperature was modelled by means of the Specific Ion Interaction Theory (SIT) and a modified Debye-Hückel equation including a term for the dependence on T/K. The first two protonation equilibria are exothermic, and the reactions are not favoured by an increase of the temperature whilst the last two are endothermic. The entropic contribution resulted to be the driving force of the reactions. The sequestering ability of alendronate towards the two metal cations was quantified by means of the pL0.5 parameter calculated at different pHs and ionic strengths. It resulted that alendronate has a higher sequestration toward Mg2+ with respect to Ca2+. A simulation of the alendronate behaviour in human blood plasma was carried out.

Impact of ion-specific interactions on laser-induced liquid microjet generation

Jet velocity and jet power are the crucial parameters that govern the release profile of drugs/vaccines in needle-free injection systems, where salts are considered as one of the essential constituents of drugs/vaccines as adjuvants. However, the impact of salts on the generation of laser-induced liquid microjets that have shown tremendous potential for drug delivery has remained unexplored. Here, we have investigated the influence of Hofmeister ions on the velocity of laser-induced liquid jets in rhodamine 6G (Rh6G) dye solution. A comprehensive experimental approach is adopted to find out the values of the liquid microjet velocity by varying the parameters, such as input energy of the laser beam, concentrations of the dye, and specific Hofmeister ions in the aqueous solution. From our experimental findings, it is observed that the presence of ions significantly influences the jet velocity that follows the Hofmeister series: SO42−>I−>SCN−. This, in turn, governs the ion-specific penetration depth of liquid jets as simulated for the model soft tissues. The observed Hofmeister trend in the jet velocity values showcases the possible specific-ion interactions between anions and Rh6G molecules in the aqueous solution. Our detailed experimental findings on ion-specific jet velocity and jet power could be beneficial in designing needle-free drug delivery systems with controlled penetration depths.

Evaluation of Thermodynamic Model of Pd(II) Complex Formation with Isosaccharinic Acid

Palladium-107 is one of the selected radionuclides in the safety assessment of geological disposal of radioactive waste. Although isosaccharinic acid (ISA) forms strong complexes with many elements and enhances element solubility, the thermodynamic evaluation of the complex of Pd with ISA has not been conducted. In this study, the solubility of Pd(OH)2 at pH 8.5–12.5 in the presence of ISA was investigated under inert gas (N2) atmosphere. Furthermore, the coordination state of aqueous Pd and ISA was investigated by X-ray absorption and Fourier transform infrared spectroscopy, respectively. According to experimental results, the number of OH ligands in the mixed complex depends on pH. The thermodynamic model and conditional equilibrium constants of the Pd-ISA complex were estimated by slope analyses of solubility experiments at different pH levels and ionic strengths based on the specific ion interaction theory. Hence, the impact of complexation with ISA on Pd(II) solubility under disposal conditions could be quantified using the proposed thermodynamic models in this study.

Comparative analysis of polyamide nanofiltration membranes resistance to different acids: Insights from experiments and density functional theory simulations

Recent research has focused on the acid resistance of polyamide nanofiltration (NF) membranes, a critical aspect for their stability during practical applications. However, the impact of different acids, particularly the role of various anions, on NF membrane acid resistance remains unexplored. Our research systematically evaluates NF membrane performance and property alterations under various acids at identical pH levels, supplemented by density functional theory (DFT) simulations. Results reveal that membranes treated with H₃PO₄ retain desirable NF performance, in stark contrast to those treated with H₂SO₄ and HCl, which undergo significant degradation. Specifically, H₃PO₄-treated membranes showed only a 4.2 % decline in Na₂SO₄ rejection, compared to a dramatic 83.3 % decline for HCl-treated membranes under identical pH. This degradation is linked to the acids infiltrating the polyamide structure. The membrane's enhanced resistance to H₃PO₄ is primarily due to the strong negative charge of phosphate ions, influenced by the Donnan effect, which prevents deep penetration into the membrane. This is further supported by DFT simulations, which reveal that the interaction energy between the membrane surface and phosphate ions is over twice as high as with chlorine ions, highlighting the importance of ion-specific interactions. This research highlights the crucial role of anions in acidic environments for the maintenance and operation of NF membranes in industrial applications. It provides key insights for enhancing membrane efficiency and longevity under acidic conditions.

Post-Synthetic Ensembling Design of Hierarchically Ordered FAU-type Zeolite Frameworks for Vacuum Gas Oil Hydrocracking.

Zeolites hold importance as catalysts and membranes across numerous industrial processes that produce most of the world's fuels and chemicals. In zeolite catalysis, the rate of molecular diffusion inside the micropore channels defines the catalyst's longevity and selectivity, thereby influencing the catalytic efficiency. Decreasing the diffusion pathlengths of zeolites to the nanoscopic level by fabricating well-organized hierarchically porous architecture can efficiently overcome their intrinsic mass-transfer limitations without losing hydrothermal stability. We report a rational post-synthetic design for synthesizing hierarchically ordered FAU-type zeolites exhibiting 2D-hexagonal (P6mm) and 3D-cubic (Ia d) mesopore channels. The synthesis involves methodical incision of the parent zeolite into unit-cell level zeolitic fragments by in situ generated base and bulky surfactants. The micellar ensembles formed by these surfactant-zeolite interactions are subsequently reorganized into various ordered mesophases by tuning the micellar curvature with ion-specific interactions (Hofmeister effect). Unlike conventional crystallization, which offers poor control over mesophase formation due to kinetic constraints, crystalline mesostructures can be developed under dilute, mild alkaline conditions by controlled reassembly. The prepared zeolites with nanometric diffusion pathlengths have demonstrated excellent yields of naphtha and middle-distillates in vacuum gas oil hydrocracking with decreased coke deposition.

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions.

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Advances in iron(III) hydrolysis studies. Effect of the metal concentration, ionic medium and ionic strength

The hydrolytic behaviour of iron(III) was studied in absence and in presence of different salt solutions, in the ionic strength range: 0 < I/moldm−3 ≤ 1.0 in NaNO3(aq) and NaClO4(aq), and 0 < I/moldm−3 ≤ 3.0 in NaCl(aq), at T = 298.15 ± 0.15 K, by means of potentiometry and UV–Vis spectrophotometry (only in NaClO4). The two techniques allowed to perform the study in a wide range of iron(III) concentration: 2.5 ≤ cFe3+/mmoldm−3 ≤ 195.6 for the first, while 0.04 ≤ cFe3+/mmoldm−3 ≤ 0.25, for the second one. The results obtained by potentiometric measurements in NaNO3(aq) and NaCl(aq), allowed to determine the speciation model: Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+, Fe3(OH)45+ and Fe12(OH)342+. In the case of the spectrophotometric titrations in NaClO4(aq), the best results were obtained considering the species: Fe(OH)2+, Fe(OH)2+ and Fe(OH)3(aq); for each one, the molar absorptivity values were calculated at the various experimental conditions investigated. The dependence on the ionic strength of the hydrolytic constants was modelled by means of Debye-Hückel type equation and of the Specific ion Interaction Theory. The differences of the hydrolytic constants in NaCl(aq) and NaNO3(aq) were interpreted in terms of different interactions of the Fe3+ with the anions of the supporting electrolyte. This interaction can be neglected when a not interacting medium as NaNO3(aq) is used. This consideration allowed to extrapolate the formation constants at infinite dilution of the weak FeCl2+ and FeCl2+ complexes; by using experimental data, it was also possible to calculate the equilibrium constant of the mixed FeOHCl+ species.

Evaluation of Polymers as a Strategy to Reduce Asphaltene Adsorption on Rock Surface

Summary One of the most extensively studied flow assurance issues in the petroleum industry is the precipitation and deposition of asphaltenes. This is in part because of the molecular structure’s intricacy and the interconnected factors that influence and regulate its activity. The injection of inhibitors and dispersants, which affects the economics of crude oil production, is now the most successful strategy for preventing asphaltene problems. Throughout the crude oil supply chain, from the reservoir through the tubing and refinery systems, asphaltene is a concern. However, the area closest to the wellbore, where the highest pressure drop is seen, is the most prone to asphaltene adsorption and deposition. Thus, the goal of this study is to investigate the use of sacrificial fluids to reduce asphaltene adsorption and deposition around the wellbore. To prevent asphaltene from interacting with the rock surface and shifting the asphaltene problem into tubing, where its impact on wettability is low, polymers with functional capabilities are investigated. The performance test (adsorption inhibition capacity), binding energy analysis, adsorption experiments (adsorption affinity, configuration, and mechanism), and fluid characterization (salinity tolerance, surface energy, and interfacial tension) of the selected novel fluids for asphaltene adsorption mitigation are presented. The investigation of ion-specific rock-fluid interactions offers great potential in the search for an effective answer to the asphaltene problem, according to the results. This was proved by the fluid levels of binding energy to carbonate rock samples and their capacity to prevent interactions between asphaltene molecules and the rock surface. These findings provide a fresh perspective on the creation of an economic strategy to deal with asphaltene issues and their effects. This study is the first to investigate a long-term fix for wettability changes caused by asphaltene adsorption on rock minerals. The findings revealed that an optimal concentration exists for the polymers under study, at which the asphaltene interaction is mitigated. More so, surface energy evaluation is observed to be a critical tool that can help to screen polymers for this application. Furthermore, the method of implementation, which could be either squeeze operation or continuous injection, is critical to the success of the remediation.

Characterization of thermodynamic properties on Al3+/dopamine system

The interactions of dopamine (Dop−) with aluminum were studied in NaCl(aq) at different ionic strengths (0.15 ≤ I/mol dm−3 ≤ 1.0) and temperatures (288.15 ≤ T/K ≤ 310.15). The investigations were carried out by using ISE-[H+] potentiometry, UV–Vis spectrophotometry and 1H NMR. The results obtained from the different techniques were in good agreement, both in terms of speciation model and stability constants of the complexes. The speciation model is featured by mononuclear complexes, namely: AlDop; AlDopOH; AlDop(OH)2 and AlDop(OH)3 (charges omitted). From 1H NMR investigations, it resulted that both the amino and the phenolic groups in meta position of aromatic ring are involved in the protonation equilibria, while the Al3+ complexation occurs from the catechol side of the ligand. The dependence on the ionic strength and temperature of the complex formation constants was modelled by a modified extended Debye-Hückel equation, containing a term for the dependence on T/K. The SIT approach allowed the calculation of the specific ion interaction coefficients of the aluminum:dopamine complexes. From the enthalpy change values obtained, it resulted that the formation of the species is exothermic in nature and that the entropy is the driving force of the processes. The sequestering ability of dopamine towards Al3+ was evaluated by the calculation of pL0.5 parameter at different ionic strengths, pHs and temperatures; the dependence of the pL0.5 on these variables was modelled by using an empirical predictive equation that allows the calculation of the effective sequestering ability of dopamine at desired conditions. The precipitates collected at the end of the potentiometric titrations were studied by means of thermogravimetric (TGA) analysis.

Ion-Specific Interactions at a Mineral-Water Interface Revealed by Surface-Sensitive Spectroscopy under Flow Conditions

Ion-Specific Interactions at a Mineral-Water Interface Revealed by Surface-Sensitive Spectroscopy under Flow Conditions

Application of machine learning for modeling brønsted-guggenheim-scatchard specific ion interaction theory (SIT) coefficients

Machine learning methodologies can provide insight into Brønsted-Guggenheim-Scatchard specific ion interaction theory (SIT) parameter values where experimental data availability may be limited. This study develops and executes machine learning frameworks to model the SIT interaction coefficient, ε. Key findings include successful estimations of ε via artificial neural networks using clustering and value prediction approaches. Applicability to other chemical parameters is also assessed briefly. Models developed here provide support for a use-case of machine learning in geologic nuclear waste disposal research applications, namely in predictions of chemical behaviors of high ionic strength solutions (i.e., subsurface brines).

Study of the Specific Adsorption of Calcium Ions on the Surface of Heterogeneous and Homogeneous Cation-Exchange Membranes to Increase Their Selectivity towards Singly Charged Ions

Ion-exchange membranes with high specific selectivity towards singly charged ions are in demand in various industries. One of the ways to increase the specific selectivity can be formation of a thin layer on the membrane surface with a charge opposite to the charge of its fixed groups. The possibility of forming such a layer due to the specific interaction of calcium ions with the sulfonate groups of the membrane during treatment with a high-intensity electric current in a CaCl2 solution is studied. The ability of heterogeneous (MK-40, Ralex CMH) and homogeneous (CMX, CJMC-5) sulfocationite membranes for specific adsorption of calcium ions on their surface is investigated. It is shown that the CMX membrane exhibits this ability to the greatest extent, which is due to the higher density of \( - {\text{SO}}_{3}^{ - }\) groups on its surface compared to other studied membranes. It is found that the formation of a thin positively charged layer on the surface of the CMX membrane increases the permselectivity coefficient of the membrane, \({{P}_{{{{{\text{N}}{{{\text{a}}}^{{\text{ + }}}}} \mathord{\left/ {\vphantom {{{\text{N}}{{{\text{a}}}^{{\text{ + }}}}} {{\text{C}}{{{\text{a}}}^{{{\text{2 + }}}}}}}} \right.} {{\text{C}}{{{\text{a}}}^{{{\text{2 + }}}}}}}}}},\) by 69%. At the same time, the presence of such a layer does not lead to an increase in undesirable water splitting, which occurs when applying widely used as modifiers polyelectrolytes with amino groups.

Ion-Specific Water–Macromolecule Interactions at the Air/Aqueous Interface: An Insight into Hofmeister Effect

The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH2-groups, which follows the Hofmeister series: SO42- > Cl- > NO3- > Br- > ClO4- > SCN-. The minimal orientation angle of CH2-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO4-, and SCN-. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH2-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.