Specific Ion Interaction Research Articles

Modeling solubility and acid-base properties of some polar side chain amino acids in NaCl and (CH3)4NCl aqueous solutions at different ionic strengths and temperatures

The aim of the paper is to perform a critical analysis of the thermodynamic properties of five L-α-amino acids, namely arginine, aspartic acid, glutamic acid, glutamine and histidine. New potentiometric measurements to determine protonation constants and solubility (not for arginine) in NaCl and (CH3)4NCl aqueous solutions are reported, in order to investigate the medium effect. A preliminary analysis of the state of art evidenced the considerable amount of data in the ionic strength range 0 < I/mol kg−1 ≤ 0.1 and a lack of data (especially for arginine and glutamine) above 0.1 mol kg−1. For this reason, this paper deals with an analysis of both the literature and the new experimental data, assigning a weight to each datum as a function of its reliability. The ionic strength dependence of the protonation constants was interpreted using the EDH (Extended Debye-Hückel), the SIT (Specific ion Interaction Theory) and the Pitzer approaches. Simultaneous analysis of solubility data and protonation constants allowed us to obtain the Setschenow and the activity coefficients of all the species involved in the equilibria. The differences between the two media were also interpreted considering the formation of weak complexes between the ions of the supporting electrolytes and the ligands according to the complex formation model.Finally, recommended, tentative or provisional data for protonation constants and protonation enthalpy changes of the five amino acids at different temperatures and ionic strengths are provided.

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.

Barium and Radium Complexation with Ethylenediaminetetraacetic Acid in Aqueous Alkaline Sodium Chloride Media

The speciation of Ra2+ and Ba2+ with EDTA was investigated at 25 °C in aqueous alkaline NaCl media as a function of ionic strength (0.2–2.5 mol·L−1) in two pH regions where the EDTA4− and HEDTA3− species dominate. The stability constants for the formation of the [BaEDTA]2− and [RaEDTA]2− complexes were determined using an ion exchange method. Barium-133 and radium-226 were used as radiotracers and their concentrations in the aqueous phase were measured using liquid scintillation counting and gamma spectrometry, respectively. The specific ion interaction theory (SIT) was used to account for [NaEDTA]3− and [NaHEDTA]2− complex formation, and used to extrapolate the logarithms of the apparent stability constants (log10K) to zero ionic strength (BaEDTA2−: 9.86 ± 0.09; RaEDTA2−: 9.13 ± 0.07) and obtain the Ba2+ and Ra2+ ion interaction parameters: [ε(Na+, BaEDTA2−) = − (0.03 ± 0.11); ε(Na+, RaEDTA2−) = − (0.10 ± 0.11)]. It was found that in the pH region where HEDTA3− dominates, the reaction of Ba2+ or Ra2+ with the HEDTA3− ligand also results in the formation of the BaEDTA2− and RaEDTA2− complexes (as it does in the region where the EDTA4− ligand dominates) with the release of a proton. Comparison of the ion interaction parameters of Ba2+ and Ra2+ strongly indicates that both metal ions and their EDTA complexes have similar activity coefficients and undergo similar short-range interactions in aqueous NaCl media.

Thermodynamics of neptunium(V) complexation with sulfate in aqueous solution

The complexation of NpO2+ with sulfate is studied in aqueous solution using absorption spectroscopy. The total ligand concentration (Na2SO4), ionic strength (NaClO4), and temperature (T = 20–85 °C) is varied, yielding detailed information on the thermodynamics of the complexation reaction. A single complex species (NpO2(SO4)−) is identified by peak deconvolution of the absorption spectra. The molar fraction of the complex species increases with increasing temperature. The conditional stability constants log β′1(T) are calculated and extrapolated to zero ionic strength for each temperature condition with the specific ion interaction theory (SIT). The determined log β01(T) values increase by approximately one logarithmic unit in the studied temperature range. Furthermore, the log β01(T) values are linearly correlated with the reciprocal temperature. Thus, fitting the data according to the integrated Van’t Hoff equation yields the standard reaction enthalpy (ΔrH0m) and entropy (ΔrS0m) of the formation of the NpO2(SO4)− complex. The results show an endothermic reaction which is solely driven by the reaction entropy. In addition, the binary SIT ion-ion interaction coefficient ε(Na+, NpO2(SO4)−) of the complex species is determined.

Hydration and ion-binding of glycine betaine: How they may be involved into protection of proteins under abiotic stresses

Glycine betaine (GB) is a widespread natural osmolyte, which accumulates in the cell cytoplasm to protect structure and function of various proteins and enzymes under osmotic, salinity, or temperature stresses. It is assumed that the hydration of GB is playing an important role in the protective mechanism. However, the details of GB interactions with water surroundings are far from being fully understood. In this contribution, the hydration structure of GB under effects of concentration, temperature, and NaCl/KCl additions, as well as ion-binding of this zwitterionic compound was studied to gain information on the functioning of GB in particular and of osmolytes in general. The data were obtained by the integral equation method in the framework of the 1D- and 3D-RISM (Reference Interaction Site Model) approaches. An attempt was made to link the structural features of hydration and ion-binding of GB to its biological role. It was found that under osmolyte concentration effect, as well as under temperature and salinity stresses, GB molecules are strongly hydrated, revealing the strong ability of the osmolyte to bind a significant amount of water. These results are a confirmation for the stabilizing effect of GB on proteins under abiotic stresses via an indirect mechanism. Moreover, the salt additions stimulate the formation of direct ion-molecular associates between the charged groups of GB and inorganic ions as a result of ion-specific interactions. In our opinion, the ability of GB to retain the inorganic ions around itself is directly related to the protection of proteins from salting-out.

Electrolyte effects on enzyme electrochemistry

Summary The electrolytes used in enzymatic biosensors or biofuel cells have always been considered to be inert. However, recent studies have demonstrated that this assumption is not correct and that the nature of the electrolyte needs to be considered. Ion-specific interactions can occur with the faradaic response observed in both direct and mediated electron transfer being modulated by the nature of the salt used in solution. Specific ion effects arise from the Hofmeister series, which is well established in studies of protein systems but not in electrochemical studies of redox enzymes. Recent experimental and theoretical work on explaining the Hofmeister effect is described.

Phase Behaviour and Miscibility Studies of Collagen/Silk Fibroin Macromolecular System in Dilute Solutions and Solid State.

Miscibility is an important issue in biopolymer blends for analysis of the behavior of polymer pairs through the detection of phase separation and improvement of the mechanical and physical properties of the blend. This study presents the formulation of a stable and one-phase mixture of collagen and regenerated silk fibroin (RSF), with the highest miscibility ratio between these two macromolecules, through inducing electrostatic interactions, using salt ions. For this aim, a ternary phase diagram was experimentally built for the mixtures, based on observations of phase behavior of blend solutions with various ratios. The miscibility behavior of the blend solutions in the miscible zones of the phase diagram was confirmed quantitatively by viscosimetric measurements. Assessing the effects of biopolymer mixing ratio and salt ions, before and after dialysis of blend solutions, revealed the importance of ion-specific interactions in the formation of coacervate-based materials containing collagen and RSF blends that can be used in pharmaceutical, drug delivery, and biomedical applications. Moreover, the conformational change of silk fibroin from random coil to beta sheet, in solution and in the final solid films, was detected by circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR), respectively. Scanning electron microscopy (SEM) exhibited alterations of surface morphology for the biocomposite films with different ratios. Surface contact angle measurement illustrated different hydrophobic properties for the blended film surfaces. Differential scanning calorimetry (DSC) showed that the formation of the beta sheet structure of silk fibroin enhances the thermal stability of the final blend films. Therefore, the novel method presented in this study resulted in the formation of biocomposite films whose physico-chemical properties can be tuned by silk fibroin conformational changes by applying different component mixing ratios.

Neptunium sorption and redox speciation at the illite surface under highly saline conditions

Neptunium (Np) uptake on illite is investigated in 1 and 3.2 molal (m) NaCl solutions under inert (Ar) atmosphere for 4<pHm<10 (pHm=−log mH+) and 5×10−8<[Np(V)]tot<3×10−4M. In agreement with a previous study in 0.1m NaCl solutions (Marsac et al., 2015a), Np(V) is the prevailing oxidation state in the aqueous solution, but Np uptake by illite is affected by surface induced reduction. The extent of Np(V) reduction to Np(IV) follows the measured redox potential (or the pe=−log ae−), which is influenced by the introduced Np(V) amount, because of the low redox capacity of the illite. The presence of Np(IV) on the solid phase is verified by X-ray Absorption Near Edge Spectroscopy (XANES). We can conclude that Np uptake by illite is not significantly affected by the variation of mNaCl from 0.1 to 3.2m and thus is in agreement with reports on tetravalent actinide and Np(V) sorption to clays at high ionic strength. The combination of (i) the two site protolysis non-electrostatic surface complexation and cation exchange model, (ii) the specific ion interaction theory to calculate activity coefficients for dissolved species and (iii) by accounting for redox equilibria and the stability of surface Np species, the overall Np uptake by illite can be simulated as a function of pHm, pe and mNaCl using a single set of parameters. The present experimental and modeling results are particularly important in the context of deep geological nuclear waste disposal since many sedimentary rocks or clay formations that are deemed suitable for this purpose exhibit highly saline porewaters.

Thermodynamic model of Ni(II) solubility, hydrolysis and complex formation with ISA

AbstractThe solubility of β–Ni(OH)2(cr) was investigated atT=(22±2)°C in the absence and presence of α-isosaccharinic acid (ISA), the main degradation product of cellulose under alkaline pH conditions. Batch solubility experiments were performed from undersaturation conditions under inert gas (Ar) atmosphere. Solubility experiments in the absence of ISA were conducted in 0.5 and 3.0 M NaCl–NaOH solutions at 7.5≤ pHm≤13 (with pHm=–log10[H+]). XRD analyses of selected solid phases collected after completing the solubility experiments (≈300 days) confirmed that β–Ni(OH)2(cr) remains as solid phase controlling the solubility of Ni(II) in all investigated conditions. Based on the slope analysis (log10[Ni] vs. pHm) of the solubility data and solid phase characterization, the equilibrium reactions β–Ni(OH)2(cr)+2 H+⇔Ni2++2 H2O(l) and β–Ni(OH)2(cr)⇔Ni(OH)2(aq) were identified as controlling the solubility of Ni(II) within the investigated pHmregion. The conditional equilibrium constants determined from the solubility experiments at different ionic strengths were evaluated with the specific ion interaction theory (SIT). In contrast to the current thermodynamic selection in the NEA–TDB, solubility data collected in the present work does not support the formation of the anionic hydrolysis species Ni(OH)3−up to pHm≤13.0. Solubility experiments in the presence of ISA were conducted in 0.5 M NaCl–NaOH–NaISA solutions with 0.01 M≤[NaISA] ≤0.2 M and 9≤ pHm≤13. XRD analyses confirmed that β–Ni(OH)2(cr) is also the solid phase controlling the solubility of Ni(II) in the presence of ISA. Solubility data of all investigated systems can be properly explained with chemical and thermodynamic models including the formation of the complexes NiOHISA(aq), Ni(OH)2ISA−and Ni(OH)3ISA2−. The reported data confirm the low solubility (&lt;10−7M) of Ni(II) in hyperalkaline pH conditions representative of cementitious environments (10≤ pH ≤13), which increases to up to 10−5M in the presence of 0.2 M NaISA. These results significantly improve source term estimations for Ni(II) in environments relevant for the disposal of low and intermediate level radioactive waste (L/ILW). The chemical and thermodynamic models derived in this work can be implemented in geochemical models/calculations, and provide further confidence in the safety analysis of repositories for the disposal of L/ILW.

Crystal nucleation initiated by transient ion-surface interactions at aerosol interfaces.

Particle collisions are a common occurrence in the atmosphere, but no empirical observations exist to fully predict the potential effects of these collisions on air quality and climate projections. The current consensus of heterogeneous crystal nucleation pathways relevant to the atmosphere dictates that collisions with amorphous particles have no effect on the crystallization relative humidity (RH) of aqueous inorganic aerosols because there is no stabilizing ion-surface interaction to facilitate the formation of crystal nuclei. In contrast to this view of heterogeneous nucleation, we report laboratory observations demonstrating that collisions with hydrophobic amorphous organic aerosols induced crystallization of aqueous inorganic microdroplets at high RH, the effect of which was correlated with destabilizing water-mediated ion-specific surface interactions. These same organic aerosols did not induce crystallization once internally mixed in the droplet, pointing toward a previously unconsidered transient ion-specific crystal nucleation pathway that can promote aerosol crystallization via particle collisions.

A novel conformation of the LC3-interacting region motif revealed by the structure of a complex between LC3B and RavZ

LC3-family member proteins play a critical role in autophagy, a cellular process responsible for the degradation of massive cellular components including intracellular pathogens. A variety of molecules involved in the autophagic pathway engage in specific interactions with a unique sequence motif referred to as the LIR (LC3-interacting region) motif. Although identification of conserved structural features of LIR motifs in complex with LC3-family members has established a canonical LIR motif, atypical conformations of LIR motifs have recently been revealed. Here, we determined the three-dimensional crystal structures of LC3B in complex with three different LIR motifs of RavZ from Legionella pneumophila, an intracellular pathogen that can manipulate the host autophagy system. The tandem LIR motifs located in the N-terminal region of RavZ adopt a novel β-sheet conformation and thus provide specific ionic interactions with LC3B in addition to canonical hydrophobic plugged-in interactions. Consequently, these motifs possess higher binding affinity to LC3-family members than canonical LIR motifs, although the tandem repeats can only bind to one LC3 molecule. These findings broaden our understanding of the functional repertoire of LIR motifs in autophagy.

The thermodynamic formation constants for iron(III) thiocyanate complexes at zero ionic strength

The successive thermodynamically relevant standard formation constants for Fe(SCN)2+(K10) and Fe(SCN)2+(K20) have been determined at 25°C using absorbance spectra at time zero to reduce the impact of kinetic instability. Average ionic strengths of 0.0581M, 0.0955 M, 0.1756 M, 0.2506 M, 0.5004 M, and 1.0011 M were used in the study to enable extrapolation to zero ionic strength. Extrapolation to zero ionic strength was supported by the application of the Specific Ion Interaction theory (SIT) developed by Brønsted, Guggenheim and Scatchard. Application to the iron(III) thiocyanate system led to a log K10 value of (2.85 ± 0.08) and a log K20 value of (1.51 ± 0.13). The change in the empirical specific ion interaction coefficients associated with K10, Δε1, is (−0.29 ± 0.16), and that associated with K20, Δε2, is (−0.18 ± 0.25). The log K10 value is lower than those published in the literature due to the fact that, for this study, the equilibrium data were based on initial spectra captured before kinetic instability significantly interfered.

Distribution of Znx(OH)yx−2y species and morphology of ZnO microflowers

Controlling the genesis of ZnO micro-particles is desirable for tuning their morphologies. The OH-/Zn2+ ratio has been correlated with morphologies showed in past research, but for different experimental procedures several morphologies have been synthesized for the same ratios. Then, the initial species distribution could serve better for predicting the final morphology than the OH-/Zn2+ ratio. In this work, synthesis of ZnO micro-particles varying OH-/Zn2+ ratios were conducted, and the initial species distributions were estimated based on the specific ion interaction theory. A significant correlation was observed between the initial species distribution and the final morphology in both, the current experimental results and results from the literature.

Stability Constants and Spectroscopic Properties of Thorium(IV)–Arsenazo III Complexes in Aqueous Hydrochloric Medium

The complexation characteristics of thorium–arsenazo III in the range of 1–6 mol·L−1 hydrochloric acid media were investigated by UV–Vis absorption spectroscopy and computational analysis. The chemical equilibrium model of thorium–arsenazo III complexation was established including the species distribution of arsenazo III, the formation of thorium chloride species, and the release of protons from thorium–arsenazo III complexes. In the spectra of thorium–arsenazo III complexes, two characteristic absorption peaks were observed at 610 and 660 nm, and the latter peak showed a tendency to shift about 4 nm to higher wavelength as the acidity of the hydrochloric acid media increased from 1 to 6 mol·L−1. Analysis of the experimental data indicates that the molar absorptivities of both 1:1 and 1:2 complexes (thorium to arsenazo III) steadily increase as the acidity of medium increases. The determined stability constants of 1:1 and 1:2 complexes at various concentrations of hydrochloric acid were extrapolated to zero ionic strength, based on the specific ion interaction theory (SIT) approach. The limiting stability constants were determined to be $$ { \log }_{10} \beta_{11}^{\text{o}} $$ = 8.56 ± 0.13 and $$ {\log}_{10} \beta_{12}^{\text{o}} $$ = 15.17 ± 0.18 with ion interaction coefficients of Δe 11 = –0.57 ± 0.02 kg·mol−1 and Δe 12 = –0.60 ± 0.04 kg·mol−1, respectively.

Solubility of Zr(OH)4(am) and the Formation of Zr(IV) Carbonate Complexes in Carbonate Solutions Containing 0.1–5.0 mol·dm−3 NaNO3

The solubility of amorphous zirconium hydroxide [Zr(OH)4(am)] was investigated in carbonate solutions containing various concentrations of sodium nitrate. The observed dependences of Zr(IV) solubility on the hydrogen ion concentration (pHc) and carbonate concentration suggested the formation of $$ {\text{Zr}}({{\text{CO}}}_{3} )_{4}^{4 - } $$ , $$ {\text{Zr}}({{\text{CO}}}_{3} )_{5}^{6 - } $$ , and $$ {\text{Zr(OH)}}_{ 2} ( {{\text{CO}}}_{3} )_{2}^{2 - } $$ as the dominant species in the neutral to weakly alkaline pH regions. The solubility of Zr(IV) at certain pHc values and carbonate concentrations was observed to increase slightly with increasing ionic strength, while the solid phase was determined to be Zr(OH)4(am) at all ionic strengths by using thermal analysis. By applying the specific ion interaction theory, the solubility data at different pHcs, carbonate concentrations, and ionic strengths were analyzed to determine the formation constants of the Zr(IV) carbonate complexes and their ion interaction coefficients. The obtained values explain well the solubility data, which are discussed in comparison with those of analogous tetravalent actinide carbonates.

Beyond the Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions.

Ions differ in their ability to salt out proteins from solution as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting out/salting in idea to include enzyme activities and the crystallization of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its molecular origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion-specific behavior in various environments, rather than to reach a molecular level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solutions. In this Feature Article we show that the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in solution. At the same time, we demonstrate the limitations of separating Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, we outline the route beyond Hofmeister chemistry in the direction of understanding specific roles of ions in various biological functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.

Thermodynamic Parameters for the Interaction of Amoxicillin and Ampicillin with Magnesium in NaCl Aqueous Solution, at Different Ionic Strengths and Temperatures

In this paper, the sequestering ability of amoxicillin and ampicillin toward Mg2+ in NaCl aqueous solutions at different ionic strengths I = (0 to 1.0) mol·kg–1 and temperatures of T = (288.15 to 318.15) K was investigated by potentiometry (ISE-H+, glass electrode). The complex formation constants determined at different ionic strengths and temperatures were modeled by means of the Debye–Hückel equation and the Specific ion Interaction Theory (SIT). From the results, a weak ability of the two penicillins to bind the metal ion can be observed; in fact, the stability constants of the ML species (M = Mg2+ and L = amoxicillin or ampicillin) are log β = 4.348 and 3.242 at infinite dilution and T = 298.15 K, respectively. The dependence of the formation constants on the temperature was modeled by means of a van’t Hoff equation, which allowed us to calculate the enthalpy and entropy change values of formation of each species. The sequestering ability of amoxicillin and ampicillin toward Mg2+ in the different experimental conditions (pH, ionic strength, temperature) was quantified by means of a sigmoid equation and of the pL0.5 parameter. The pL0.5 values reflect the low stability constant values of the species; as an example at I = 0.15 mol·kg–1, pH = 7.4, and T = 298.15 K, we have pL0.5 = 2.52 and 2.78, for Mg2+/amox2− and Mg2+/amp− systems, respectively.

Modeling metal ion-humic substances complexation in highly saline conditions

Because highly saline groundwaters are found at potential repository sites for nuclear waste, geochemical models should predict the speciation of relevant radionuclides in brines, including their complexation with substances such as humic acids (HA). In this study, available experimental radionuclide-HA complexation data in high 1:1 background electrolyte solutions (0.01 < mNaCl/NO3/ClO4 < 4 molal, m) are reviewed. Discrepancies in the amplitude of ionic strength effects on radionuclide-HA complexation are observed, which might depend on the nature of the interacting radionuclide or on the origin of HA. However, significant differences in the experimental conditions and calculations applied to determine conditional metal ion-HA complexation constants hamper direct comparison between these datasets. To clarify whether metal ion-HA binding in saline solutions can be described, two sophisticated humic-ion binding models (Model VII and NICA-Donnan) are presently used. This is the first time that Model VII and NICA-Donnan are applied to predict metal ion-HA binding at high ionic strength (I > 1 m). The advantage of these models, compared to more simple ones (e.g., the polyelectrolyte or the charge neutralization models), is that both electrostatic and chemical contributions to the overall metal ion-HA binding are explicitly taken into account. Model VII and NICA-Donnan are shown to produce very similar results. Trends in conditional metal ion-HA binding constants and in the maximum metal ion uptake by HA (e.g., the loading capacity) with I agree with experiments. The present data evaluation suggests that most of the apparent discrepancies between various experimental datasets arise from differences in the experimental conditions. Both Model VII and NICA-Donnan predict that the specific ion interaction theory (SIT) parameters for metal ion-HA systems, which are required for high ionic strength with more simple models, vary with pH and metal loading. Overall, Model VII and NICA-Donnan are able to account for various mechanisms involved in metal ion-HA complexation, including the metal loading effects and cation competition, and might be helpful predictive tools for performance safety assessment up to highly saline conditions.

Protonation of Pyridine Monocarboxylate‐N‐Oxides – Determination of Thermodynamic, Absorbance and Ion Interaction Parameters

AbstractThe thermodynamic parameters (pKa, ΔH, ΔG, ΔS) for the protonation of three isomeric pyridine monocarboxylate‐N‐oxides (PCNO) namely picolinate‐N‐oxide (PANO) nicotinate‐N‐oxide (NANO) isonicotinate‐N‐oxide (IANO) were determined in the present study. The pKa determined by potentiometry follow the order: PANO &gt; IANO &gt; NANO at all ionic strengths. The calorimetric results revealed that the PANO protonation was endothermic and the other are exothermic, indicating the entropy driven protonation of PANO while the other two has minor contribution of enthalpy as well. The ion interaction parameters for PCNOs are obtained by determining the pKa for the same at various ionic strengths (I=0.1 −3.0 M) and the data is extrapolated to find pKa0 (pKa at I=0) by specific ion interaction theory (SIT). The pKa / pKa0 determined by absorption spectra are also in line with potentiometric results. The energetics and variations in charge densities on individual atoms on protonation were calculated theoretically. PCNOs protonation are compared with its base moieties: pyridine monocarboxylates to interpret the effect of the N‐oxide moiety on the protonation thermodynamics.

Ability of the Poisson-Boltzmann equation to capture molecular dynamics predicted ion distribution around polyelectrolytes.

Here, we examine polyelectrolyte (PE) and ion chemistry specificity in ion condensation via all-atom molecular dynamics (MD) simulations and assess the ability of the Poisson-Boltzmann (PB) equation to describe the ion distribution predicted by the MD simulations. The PB model enables the extraction of parameters characterizing ion condensation. We find that the modified PB equation which contains the effective PE radius and the energy of the ion-specific interaction as empirical fitting parameters describes ion distribution accurately at large distances but close to the PE, especially when strongly localized charge or specific ion binding sites are present, the mean field description of PB fails. However, the PB model captures the MD predicted ion condensation in terms of the Manning radius and fraction of condensed counterions for all the examined PEs and ion species. We show that the condensed ion layer thickness in our MD simulations collapses on a single master curve for all the examined simple, monovalent ions (Na+, Br+, Cs+, Cl-, and Br-) and PEs when plotted against the Manning parameter (and consequently the PE line charge density). The significance of this finding is that, contrary to the Manning radius extracted from the mean field PB model, the condensed layer thickness in the all atom detail MD modelling does not depend on the PE chemistry or counterion type. Furthermore, the fraction of condensed counterions in the MD simulations exceeds the PB theory prediction. The findings contribute toward understanding and modelling ion distribution around PEs and other charged macromolecules in aqueous solutions, such as DNA, functionalized nanotubes, and viruses.