Solvation Shell Of Ions Research Articles

Dissecting ion-specific dielectric spectra of sodium-halide solutions into solvation water and ionic contributions.

Using extensive equilibrium molecular dynamics simulations we determine the dielectric spectra of aqueous solutions of NaF, NaCl, NaBr, and NaI. The ion-specific and concentration-dependent shifts of the static dielectric constants and the dielectric relaxation times match experimental results very well, which serves as a validation of the classical and non-polarizable ionic force fields used. The purely ionic contribution to the dielectric response is negligible, but determines the conductivity of the salt solutions. The ion-water cross correlation contribution is negative and reduces the total dielectric response by about 5%-10% for 1 M solutions. The dominating water dielectric response is decomposed into different water solvation shells and ion-pair configurations, by this the spectral blue shift and the dielectric decrement of salt solutions with increasing salt concentration is demonstrated to be primarily caused by first-solvation shell water. With rising salt concentration the simulated spectra show more pronounced deviations from a single-Debye form and can be well described by a Cole-Cole fit, in quantitative agreement with experiments. Our spectral decomposition into ionic and different water solvation shell contributions does not render the individual contributions more Debye-like, this suggests the non-Debye-like character of the dielectric spectra of salt solutions not to be due to the superposition of different elementary relaxation processes with different relaxation times. Rather, the non-Debye-like character is likely to be an inherent spectral signature of solvation water around ions.

Free Energy Reconstruction from Metadynamics or Adiabatic Free Energy Dynamics Simulations.

In molecular dynamics simulations, most enhanced sampling methods are traditionally associated with one particular estimator to calculate the free energy surface (FES), such as the histogram, the mean force, or the bias potential. Here, we start from the realization that four enhanced sampling methods, metadynamics and well-tempered metadynamics (in their extended Lagrangian form), as well as driven adiabatic free energy dynamics (dAFED) and unified free energy dynamics (UFED), can be used in combination with any of the three above-mentioned FES estimators. We compare the convergence properties of these estimators on the alanine dipeptide and a sodium ion solvation shell. We find that the mean force estimator is superior in all cases. We also show that it can be marginally beneficial to combine information from the histogram and the force, provided that both are of comparable accuracy.

Complex Dynamical Aspects of Organic Electrolyte Solutions

Molecular dynamics of acetone-alkali metal halide (LiBr, LiI) solutions were investigated using depolarized Rayleigh scattering (DRS) and low-frequency Raman spectroscopy in the frequency range from ~0.5 to 200 cm(-1) (~20 GHz to 6 THz). These experiments probe fast dynamical fluctuations of the polarizability anisotropy at picosecond and sub-picosecond time scales that are mainly driven by acetone orientational dynamics. Two distinct contributions were revealed: a fast process (units of picosecond, ps) related to the essentially unperturbed bulk solvent and a slow one (tens of ps) assigned to acetone molecules forming Li(+) solvation shells, decelerated by the motional constraint imposed by the cation. The increase of LiBr and LiI concentration significantly slows down the overall solvent relaxation as a consequence of the increased fraction of acetone molecules involved in the ion solvation shells. The global retardation is larger in LiI than LiBr solutions consistently with viscosity trends. This is explained in terms of ion association (at least ion pairing) more favorably promoted by Br(-) than I(-), with reduced Li(+)-acetone interactions in LiBr than LiI solutions. Anion-induced modulation of the Li(+)···O═C contacts, largely responsible for electrostriction phenomena, also affects the reduced THz-Raman spectral density, ascribed to ultrafast librational motions of acetone molecules. Overall, these findings enlighten the interplay between ion-dipole and ion-ion interactions on the fast solvation dynamics in electrolyte solutions of a typical polar aprotic solvent.

Testing the cation-hydration effect on the crystallization of Ca-Mg-CO3 systems.

Dolomite and magnesite are simple anhydrous calcium and/or magnesium carbonate minerals occurring mostly at Earth surfaces. However, laboratory synthesis of neither species at ambient temperature and pressure conditions has been proven practically possible, and the lack of success was assumed to be related to the strong solvation shells of magnesium ions in aqueous media. Here, we report the synthesis of MgCO3 and MgxCa(1-x)CO3 (0 < x < 1) solid phases at ambient conditions in the absence of water. Experiments were carried out in dry organic solvent, and the results showed that, although anhydrous phases were readily precipitated in the water-free environment, the precipitates' crystallinity was highly dependent on the Mg molar percentage content in the solution. In specific, magnesian calcite dominated in low [Mg(2+)]/[Ca(2+)] solutions but gave way to exclusive formation of amorphous MgxCa(1-x)CO3 and MgCO3 in high-[Mg(2+)]/[Ca(2+)] and pure-Mg solutions. At conditions of [Mg(2+)]/[Ca(2+)] = 1, both nanocrystals of Ca-rich protodolomite and amorphous phase of Mg-rich MgxCa(1-x)CO3 were formed. These findings exposed a previously unrecognized intrinsic barrier for Mg(2+) and CO3(2-) to develop long-range orders at ambient conditions and suggested that the long-held belief of cation-hydration inhibition on dolomite and magnesite mineralization needed to be reevaluated. Our study provides significant insight into the long-standing "dolomite problem" in geochemistry and mineralogy and may promote a better understanding of the fundamental chemistry in biomineralization and mineral-carbonation processes.

Aqueous Mg2+and Ca2+Ligand Exchange Mechanisms Identified with 2DIR Spectroscopy

Biological systems must discriminate between calcium and magnesium for these ions to perform their distinct biological functions, but the mechanism for distinguishing aqueous ions has yet to be determined. Ionic recognition depends upon the rate and mechanism by which ligands enter and leave the first solvation shell surrounding these cations. We present a time-resolved vibrational spectroscopy study of these ligand exchange dynamics in aqueous solution. The sensitivity of the CN-stretch frequency of NCS(-) to ion pair formation has been utilized to investigate the mechanism and dynamics of ligand exchange into and out of the first solvation shell of aqueous magnesium and calcium ions with multidimensional vibrational (2DIR) spectroscopy. We have determined that anion exchange follows a dissociative mechanism for Mg(2+) and an associative mechanism for Ca(2+).

Solvation Structures and Dynamics of the Magnesium Chloride (Mg2+–Cl–) Ion Pair in Water–Ethanol Mixtures

We have performed constrained molecular dynamics simulations of magnesium chloride in water-ethanol mixtures. From the potentials of mean force (PMFs) of the Mg(2+)-Cl(-) ion pair, we notice that, as the mole fraction of ethanol increases, the depths of the minima of the contact ion pair (CIP) and solvent assisted ion pair (SAIP) increase, but the depth of the CIP minimum increases more in comparison to the SAIP minimum. This shows that ion pairing becomes more favorable with an increase in the mole fraction of ethanol. Significant differences in the PMFs between the Mg(2+) and the Cl(-) ion (depending upon whether the second Cl(-) ion is present in the first coordination shell of the Mg(2+) ion or not) seem to have been reported for the first time in this work. The local mole fraction of water molecules in the first solvation shell of ions is generally greater than in the bulk. The diffusional behavior of solvent molecules in solvation shells of the ion-pair indicates that the ions as well as the first solvation shells of the ions diffuse at much slower rates. Also, the diffusion constant of bulk water in the mixtures is greatly reduced compared to the pure solvent value.

Verification of Solvation Theory of Dilute to Concentrated Solutions of Some Strong 1–1 Electrolytes

Density, ultrasonic velocity and isobaric heat capacity data have been used to study solvation parameters of aqueous solutions of NaCl, NaNO3, and KI. Using correct thermodynamic relations, quantitative solvation parameters have been determined at temperatures from 278.15 to 323.15 K: the hydration numbers h, the molar isentropic compressibilities of the solvation complexes $$ \beta_{S,h} V_{h} , $$ the volume V 1h , the coefficient of isentropic compressibility β 1h of water in the solvation shells of ions, and others. It has been shown that h is independent of the temperature in the range of the investigated conditions, $$ \beta_{S,h} V_{h} $$ is dependent both on the temperature and concentration, whereas the electrostriction compression in the vicinity of ions has a greater effect on its structure than that due to the mere change of pressure.

Toward the Mechanism of Ionic Dissociation in Water

We investigate the solvent effects leading to dissociation of sodium chloride in water. Thermodynamic analysis reveals dissociation to be driven energetically and opposed entropically, with the loss in entropy due to an increasing number of solvent molecules entering the highly coordinated ionic solvation shell. We show through committor analysis that the ion–ion distance is an insufficient reaction coordinate, in agreement with previous findings. By application of committor analysis on various constrained solvent ensembles, we find that the dissociation event is generally sensitive to solvent fluctuations at long ranges, with both sterics and electrostatics of importance. The dynamics of the reaction reveal that solvent rearrangements leading to dissociation occur on time scales from 0.5 to 5 ps or longer, and that, near the transition state, inertial effects enhance the reaction probability of a given trajectory.

Ion disturbance and clustering in the NaCl water solutions

Ion clustering and the solvation properties in the NaCl solutions are explored by molecular dynamics simulations with several popular force fields. The existence of ions has a negligible disturbance to the hydrogen bond structures and rotational mobility of water beyond the first ion solvation shells, which is suggested by the local hydrogen bond structures and the rotation times of water. The potential of mean force (PMF) of ion pair in the dilute solution presents a consistent view with the populations of ion clusters in the electrolyte solutions. The aggregation level of ions is sensitive to the force field used in the simulations. The ion-ion interaction potential plays an important role in the forming of the contact ion pair. The entropy of water increases as the ion pair approaches each other and the association of ion pair is driven by the increment of water entropy according to the results from the selected force fields. The kinetic transition from the single solvent separated state to the contact ion pair is controlled by the enthalpy loss of solution.

Alkali-Ion Microsolvation with Benzene Molecules

The target of this investigation is to characterize by a recently developed methodology, the main features of the first solvation shells of alkaline ions in nonpolar environments due to aromatic rings, which is of crucial relevance to understand the selectivity of several biochemical phenomena. We employ an evolutionary algorithm to obtain putative global minima of clusters formed with alkali-ions (M(+)) solvated with n benzene (Bz) molecules, i.e., M(+)-(Bz)(n). The global intermolecular interaction has been decomposed in Bz-Bz and in M(+)-Bz contributions, using a potential model based on different decompositions of the molecular polarizability of benzene. Specifically, we have studied the microsolvation of Na(+), K(+), and Cs(+) with benzene molecules. Microsolvation clusters up to n = 21 benzene molecules are involved in this work and the achieved global minimum structures are reported and discussed in detail. We observe that the number of benzene molecules allocated in the first solvation shell increases with the size of the cation, showing three molecules for Na(+) and four for both K(+) and Cs(+). The structure of this solvation shell keeps approximately unchanged as more benzene molecules are added to the cluster, which is independent of the ion. Particularly stable structures, so-called "magic numbers", arise for various nuclearities of the three alkali-ions. Strong "magic numbers" appear at n = 2, 3, and 4 for Na(+), K(+), and Cs(+), respectively. In addition, another set of weaker "magic numbers" (three per alkali-ion) are reported for larger nuclearities.

Ion-specific effects on the kinetics of mineral dissolution

Specific effects of background electrolytes on mineral growth and dissolution can be interpreted on the basis of the ability of ions to modify solute hydration, in a similar way to the systematic effects of inorganic ions on precipitation, structure and function of organic macromolecules (i.e., the Hofmeister effect). Here, the effect of a range of background electrolytes (sodium and chloride salts) on dolomite (Ca 0.5Mg 0.5CO 3) reactivity was investigated as a model system by measuring dissolution rates using in-situ Atomic Force Microscopy. The systematic trends found for the different ions are interpreted in terms of characteristic parameters of background ions such as effective hydrated radii. Entropic effects associated with the ordering of solvent molecules induced by constituting cations from the crystal ultimately dictate how electrolytes affect dissolution rates. In dilute electrolyte solutions, ion–ion interactions dominate and the stabilisation of the solvation shell of ions constituting the crystal, by counter-ions present in solution enhances the unfavourable entropic effect on dolomite dissolution. The tendency for electrolytes to form ion pairs in solution reduces such an effect, thus leading to an inverse correlation between dissolution rates and background ion separation in solution. On the other hand, in concentrated saline solutions the interaction between background ions and water molecules determines the hydration of a constituent ion immersed in an electrolyte solution. In this case, dissolution rates correlate with the mobility of background ions and, therefore, with their effective hydration radii. The observed effects of background ions on growth and dissolution could be applicable for other inorganic systems where the Hofmeister effect has been reported.

Structures and energetics of helium cluster cations: Equilibrium geometries revisited through the genetic algorithm approach

Equilibrium geometries and dissociation energies of He(N)(+) clusters have been calculated for N=3-35 using an extended genetic algorithm approach and a semiempirical model of intracluster interactions [P. J. Knowles, J. N. Murrell, and E. J. Hodge, Mol. Phys. 85, 243 (1995)]. A general aufbau principle is formulated for both ionic cores and neutral solvation shells, and the results are thoroughly compared with other theoretical data available for helium cluster cations in literature.

Comparison of the Mobilities of Negative and Positive Ions in Nonpolar Solutions

The mobilities of organic radical ions of different molecular volumes have been determined in squalane and hexane solutions to study the influence of the ion charge sign on the ionic mobility in a weakly polar liquid. The relative mobility of geminate radical ions was measured using the method of time-resolved electric field effect in the recombination fluorescence. To determine the mobility of cations and anions separately, a trend in the value of the relative mobility was analyzed by varying the mobility of one of the geminate partners. The ratios between the mobilities of the anion and the cation of the same molecules were found to be about 1.1. It was shown that in liquid alkanes, the solvent electrostriction was the main factor determining a decrease in the mobility of an ion as compared to the parent neutral molecule. The strong dependence of the electrostrictive effect on the radius of the ionic solvation shell allows the observed difference between negative and positive charge carriers by a small but systematic difference in the effective radii of the ions to be explained.

Low-Frequency Modes of Aqueous Alkali Halide Solutions: Glimpsing the Hydrogen Bonding Vibration

The solvation of ions in aqueous media is a fundamental process in biology and chemistry. Here, we report direct time-domain observations of the hydrogen bond vibrational mode formed between a halide ion (chloride, bromide, or iodide) and the surrounding water molecules. The frequency of the hydrogen bond mode is sensitive to both the atomic weight and the concentration of the ion. The peak frequencies fall in the 125 to 175 wave-number range, a spectral region accessed through time-domain polarization-resolved coherent Raman scattering using a diffractive optic method. The polarized Raman response observed is discussed in terms of the structure of the anion's solvation shell and modeled through calculations on water chloride clusters.

Kinetics of crystal nucleation in ionic solutions: Electrostatics and hydration forces

Abstract The heat of precipitation, the mean crystal size and the broadness of crystal size distribution of barium sulfate precipitating in aqueous solutions of different background electrolytes (KCl, NaCl, LiCl, NaBr or NaF), was shown to vary at constant thermodynamic driving force (supersaturation) and constant ionic strength depending on the salt present in solution. The relative inversion in the effect of respective background ions on the characteristics of barite precipitate was observed between two studied supersaturation ( Ω ) and ionic strength (IS) conditions. The crystal size variance ( β 2 ) increased in the presence of background electrolytes in the order LiCl Ω = 10 3.33 and IS = 0.03 M and KCl Ω = 10 3.77 and IS = 0.09 M. At a given Ω and IS the respective size of barite crystals decreased with increasing β 2 in chloride salts of different cations and remained constant in sodium salts of different anions. We suggest that ionic salts affect the kinetics of barite nucleation and growth due to their influence on water of solvation and bulk solvent structure. This idea is consistent with the hypothesis that the kinetic barrier for barium sulfate nucleation depends on the frequency of water exchange around respective building units that can be modified by additives present in solution. In electrolyte solution the relative switchover between long range electrostatic interactions and short range hydration forces, which influence the dynamics of solvent exchange between an ion solvation shell and bulk fluid, results in the observed inversion in the effect of differently hydrated salts on nucleation rates and the resulting precipitate characteristics.

Changes in the composition of the solvation shell of lithium ions in γ-butyrolactone upon addition of 15-crown-5 as studied by quantum chemistry

Quantum chemical modeling of Li+ ion transfer from the solvation shell of γ-butyrolactone (GBL) as the solvent to the cavity of 15-crown-5 (15C5) macrocyclic ligand was carried out. Calculations were performed using the PBE nonempirical density functional and an extended basis for the SBK pseudopotential. The solvation energy was included in the framework of the polarizable continuum model. The calculated geometric parameters of GBL and 15C5 molecules are in good agreement with experimental X-ray data. The energies and structures of the Li(GBL)n+ (n = 1–5) complexes and Li(GBL)m(15C5)+ (m = 0–3) mixed complexes were calculated. The binding energy of the fifth GBL molecule is low; therefore, the Li+ ion is mainly surrounded by four GBL molecules. The formation of mixed complexes by consecutive displacement of GBL molecules from the solvation shell of the lithium ion leads to structures with the coordination number 5. The equilibrium constants of these processes were used to determine the dependence of the composition of the solvation complexes on the concentration of 15C5 in the system. The concentrations of the Li(15C5)+ and Li(GBL)(15C5)+ complexes appeared to be comparable. The revealed structural features of the Li+ solvation complexes in the GBL-15C5 system were used to analyze the operating efficiency of lithium power sources.

Solvation of KSCN in Water

The results of a neutron diffraction experiment performed on aqueous solutions of KSCN are analyzed and discussed in comparison with similar data for KCl and KOH solutions. The effect of the different ions on the structure of water and the differences and similarities among the ion solvation shells are discussed in detail. In particular it is shown that the effect of KSCN on the structure of water is visible as a shift and a broadening of the second peak of the oxygen-oxygen radial distribution function, which corresponds to a lower number of interstitial water molecules, compared to pure water. The hydration shell of the cation is similar to that found in the case of KCl solutions, and more interestingly the hydration shell of SCN- is asymmetric, with water molecules H-bonded to the N site and weakly correlated to the S site. These results provide a reasonable microscopic description of the mechanism for the high efficiency of thiocyanate in crystallizing basic proteins.

Ultrafast Dynamics of Hydrogen Bond Exchange in Aqueous Ionic Solutions

The structural and dynamical properties of aqueous ionic solutions influence a wide range of natural and biological processes. In these solutions, water has the opportunity to form hydrogen bonds with other water molecules and anions. Knowing the time scale with which these configurations interconvert represents a key factor to understanding the influence of molecular scale heterogeneity on chemical events in aqueous ionic solutions. We have used ultrafast IR spectroscopy and Car-Parrinello molecular dynamics (CPMD) simulations to investigate the hydrogen bond (H-bond) structural dynamics in aqueous 6 M sodium perchlorate (NaClO4) solution. We have measured the H-bond exchange dynamics between spectrally distinct water-water and water-anion H-bond configurations with 2DIR spectroscopy and the orientational relaxation dynamics of water molecules in different H-bond configurations with polarization-selective IR pump-probe experiments. The experimental H-bond exchange time correlates strongly with the experimental orientational relaxation time of water molecules. This agrees with prior observations in water and aqueous halide solutions, and has been interpreted within the context of an orientational jump model for the H-bond exchange. The CPMD simulations performed on aqueous 6 M NaClO4 solution clearly demonstrate that water molecules organize into two radially and angularly distinct structural subshells within the first solvation shell of the perchlorate anion, with one subshell possessing the majority of the water molecules that donate H-bonds to perchlorate anions and the other subshell possessing predominantly water molecules that donate two H-bonds to other water molecules. Due to the high ionic concentration used in the simulations, essentially all water molecules reside in the first ionic solvation shells. The CPMD simulations also demonstrate that the molecular exchange between these two structurally distinct subshells proceeds more slowly than the H-bond exchange between the two spectrally distinct H-bond configurations. We interpret this to indicate that orientational motions predominantly dictate the rate of H-bond exchange, while translational diffusion must occur to complete the molecular exchange between the two structurally distinct subshells around the perchlorate anions. The 2DIR measurements observe the H-bond exchange between the two spectrally distinct H-bond configurations, but the lifetime of the hydroxyl stretch precludes the observation of the slower molecular exchange. Our 2DIR experiments and CPMD simulations demonstrate that orientational motions predominantly equilibrate water molecules within their local solvation subshells, but the full molecular equilibration within the first solvation shell around the perchlorate anion necessitates translational motion.

Electrical conductivity and ionic association of lithium and sodium perchlorates in tetrahydrofuran

The total limiting molar electrical conductivities of ions and triads of ions and the association constants of ions with the formation of ion pairs and triads of ions were calculated from the concentration dependences of the electrical conductivity of solutions of lithium and sodium perchlorates in tetrahydrofuran at 278.15–318.15 K with the use of the method specially developed earlier. The experimental total limiting electrical conductivities were used to calculate the limiting molar electrical conductivities and attraction friction factors of separate ions (Li+, Na+, ClO 4 − , Li2ClO 4 + , Na2ClO 4 + , Li(ClO4) 2 − , and Na(ClO4) 2 − ). The constants of ion association into ion pairs were used to calculate the Gibbs energy of non-Coulomb interionic interaction (ΔG*+−), and the constants of association into triads of ions, to determine the a 3 distance parameter between the centers of the ion and the dipole of the ion pair. Positive ΔG*+−), values and deviations of the experimental a 3 value from the distance parameter calculated theoretically (a 3 0 ) for the triad of ions (Δa 3 = a 3 − a 3 0 ) were related to non-Coulomb repulsion in the region of overlap of the solvation shells of ions and the influence of temperature and ion charge density on this repulsion.

Does Saltwater Wobble?

When ionic salts dissolve in water, the resultant solution tends to become more viscous than the pure liquid. This observation can be broadly understood based on a picture in which the web of hydrogen bonds holding the water molecules together rigidifies around solvated ions. However, the molecular details appear much more complicated, because time-resolved vibrational spectroscopy has suggested that most individual water molecules continue to rotate freely regardless of how much dissolved salt is present. Turton et al. explore this discrepancy using two related spectroscopic techniques to probe the molecular structure of aqueous salt solutions. The first, dielectric relaxation spectroscopy, is sensitive to individual molecules' orientations, and confirms the rotational freedom previously observed. The second technique, optical Kerr effect (OKE) spectroscopy, reflects polarizability and so is more sensitive to intermolecular changes that stem from translation. The OKE data reveal increasing translational restriction with rising salt concentration, as the ion solvation shells crowd against one another. The authors note that such a decoupling of rotation and translation is analogous to the jamming that occurs during transitions from a liquid to a glass upon supercooling. — JSY