Effective interaction potentials for alkali and alkaline earth metal ions in SPC/E water and polarization model of hydrated ions

In the first paper (J. Phys. Chem. B, 2006, 110, 10878), effective ion-ion potentials in SPC/E water were obtained for Me-Me, Me-Cl-, and Cl(-)-Cl- pairs, where Me is Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ cations. In this second part of the study of effective interionic potentials, ion-ion distribution functions obtained from implicit-water Monte Carlo simulations of electrolyte solution with these potentials have been explored. This analysis verifies the range of applicability of the primitive model of electrolyte. It is shown that this approximation can be applied to monovalent electrolyte solutions in a wide range of concentrations, whereas the nature of ion-ion interactions is notably different for 2:1 electrolytes. An improved model of ions is discussed. The model includes approximations of the ion hydration shell polarization and specific short-range ion-ion interaction. It allows approximation of the potential of mean force acting on ions in strong electric fields of highly charged macromolecules and bilayers.     

Spectroelectrochemistry of EuCl3 in Four Molten Salt Eutectics; 3 LiCl−NaCl, 3 LiCl−2 KCl,LiCl−RbCl,and 3 LiCl−2 CsCl; at 873 K

Key electrochemical properties affecting pyroprocessing of nuclear fuel were examined in four eutectic melts using Eu^3+/2+ as a representative probe. We report the electrochemical and spectroelectrochemical behavior of EuCl₃ in four molten salt eutectics (3 LiCl?NaCl, 3 LiCl?2 KCl, LiCl?RbCl and 3 LiCl?2 CsCl) at 873 K. Cyclic voltammetry was used to determine the reduction potential for Eu^3+/2+ and the applied potentials for spectroelectrochemistry. Single step chronoabsorptometry and thin‐layer spectroelectrochemistry were used to obtain the number of electrons transferred, reduction potentials and diffusion coefficients for Eu³⁺ in each eutectic melt. The reduction potentials determined by thin‐layer spectroelectrochemistry were essentially the same as those obtained using cyclic voltammetry. The diffusion coefficient for Eu³⁺ was the largest in the 3 LiCl?NaCl melt, showed a negative shift in the 3 LiCl?2 KCl melt, and was the smallest in the LiCl?RbCl and 3 LiCl?2 CsCl eutectic melts. The basic one‐electron reversible electron transfer for Eu^3+/2+ was not affected by melt composition.     

磷酸二甲酯阴离子(DMP~-)与一价、二价金属离子 相互作用的理论研究

在B3LYP、HF和MP2水平上运用全电子从头算(AE)和相对论有效实势(RECP)及6-311+G**和LanL2DZ基组计算Ⅰa、Ⅰb、Ⅱa和Ⅱb族金属离子与磷酸二甲酯阴离子(DMP-)的相互作用。 RECP用于除Li+、Be2+外所有的金属离子。 对Na+、K+、Cu+、Mg2+、Ca2+、Zn2+用AE和RECP 2种方法处理。 结果表明:RECP能可靠地用于重金属离子络合物; 二价金属离子络合物(DMP-—M2+)比一价金属离子络合物 (DMP-—M+)稳定;二价金属离子(M2+)可能比一价金属离子(M+)更易使多核苷酸折叠。     

H2O+KCl(饱和)+NaCl+NH4Cl的等压研究

Isopiestic measurements have been carried out at 298.15 K for the quaternary aqueous solution H2O+KCl(sat)+NaCl+NH4Cl saturated with potassium chloride and its ternary sub systems H2O+KCl (sat)+NaCl and H2O+KCl(sat)+NH4Cl. Taking sodium chloride (aq) or calcium chloride (aq) as reference solutions, osm otic coefficients and water activities of the aqueous solution were determined. The experiment results show that the isopiestic actions of the quaternary system related to its ternary sub-systems are in excellent agreement with the ideal like solution model.     

Coordination chemistry of trivalent lanthanide and actinide ions in dilute and concentrated chloride solutions

We have used EXAFS spectroscopy to investigate the inner sphere coordination of trivalent lanthanide (Ln) and actinide (An) ions in aqueous solutions as a function of increasing chloride concentration. At low chloride concentration, the hydration numbers and corresponding Ln,An-O bond lengths are as follows: La3+, N = 9.2, R = 2.54 A; Ce3+, N = 9.3, R = 2.52 A; Nd3+, N = 9.5, R = 2.49 A; Eu3+, N = 9.3, R = 2.43 A; Yb3+, N = 8.7, R = 2.32 A; Y3+, N = 9.7, R = 2.36 A; Am3+, N = 10.3, R = 2.48 A; Cm3+, N = 10.2, R = 2.45 A. In ca. 14 M LiCl, the early Ln3+ ions (La, Ce, Nd, and Eu) show inner sphere Cl- complexation along with a loss of H2O. The average chloride coordination numbers and Ln-Cl bond lengths are as follows: La3+, N = 2.1, R = 2.92 A; Ce3+, N = 1.8, R = 2.89 A; Nd3+, N = 1.9, R = 2.85 A; Eu3+, N = 1.1, R = 2.81 A. The extent of Cl- ion complexation decreases going across the Ln3+ series to the point where Yb3+ shows no Cl- complexation and no loss of coordinated water molecules. The actinide ions, Am3+ and Cm3+, show the same structural effects as the early Ln3+ ions, i.e., Cl- ion replacement of the H2O at high chloride thermodynamic activities. The Clion coordination numbers and An-Cl bond lengths are: Am3+, N = 1.8, R = 2.81 A; Cm3+, N = 2.4, R = 2.76 A. When combined with results reported previously for Pu3+ which showed no significant chloride complexation in 12 M LiCl, these results suggest that the extent of chloride complexation is increasing across the An3+ series. The origin of the differences in chloride complex formation between the Ln3+ and An3+ ions and the relevance to earlier work is discussed.     

Supercooling of aqueous NaCl and KCl solutions under acoustic levitation

The supercooling capability of aqueous NaCl and KCl solutions is investigated at containerless state by using acoustic levitation method. The supercooling of water is obviously enhanced by the alkali metal ions and increases linearly with the augmentation of concentrations. Furthermore, the supercooling depends on the nature of ions and is 2-3 K larger for NaCl solution than that for KCl solution in the present concentration range: Molecular dynamics simulations are performed to reveal the intrinsic correlation between supercoolability and microstructure. The translational and orientational order parameters are applied to quantitatively demonstrate the effect of ionic concentration on the hydrogen-bond network and ice melting point. The disrupted hydrogen-bond structure determines essentially the concentration dependence of supercooling. On the other hand, the introduced acoustic pressure suppresses the increase of supercooling by promoting the growth and coalescence of microbubbles, the effective nucleation catalysts, in water. However, the dissolved ions can weaken this effect, and moreover the degree varies with the ion type. This results in the different supercoolability for NaCl and KCl solutions under the acoustic levitation conditions.     

Molecular simulation analysis and X-ray absorption measurement of Ca2+, K+ and Cl- ions in solution

This paper presents recent advances in the use of molecular simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy, which enable us to understand solvated ions in solution. We report and discuss the EXAFS spectra and related properties governing solvation processes of different ions in water and methanol. Molecular dynamics (MD) trajectories are coupled to electron scattering simulations to generate the MD-EXAFS spectra, which are found to be in very good agreement with the corresponding experimental measurements. From these simulated spectra, the ion-oxygen distances for the first hydration shell are in agreement with experiment within 0.05-0.1 A. The ionic species studied range from monovalent to divalent, positive and negative: K+, Ca2+, and Cl-. This work demonstrates that the combination of MD-EXAFS and the corresponding experimental measurement provides a powerful tool in the analysis of the solvation structure of aqueous ionic solutions. We also investigate the value of electronic structure analysis of small aqueous clusters as a benchmark to the empirical potentials. In a novel computational approach, we determine the Debye-Waller factors for Ca2+, K+, and Cl- in water by combining the harmonic analysis of data obtained from electronic structure calculations on finite ion-water clusters, providing excellent agreement with the experimental values, and discuss how they compare with results from a harmonic classical statistical mechanical analysis of an empirical potential.     

Effect of NaCl and KCl on phosphatidylcholine and phosphatidylethanolamine lipid membranes: insight from atomic-scale simulations for understanding salt-induced effects in the plasma membrane

To gain a better understanding of how monovalent salt under physiological conditions affects plasma membranes, we have performed 200 ns atomic-scale molecular dynamics simulations of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) lipid bilayers. These two systems provide representative models for the outer and inner leaflets of the plasma membrane, respectively. The implications of cation-lipid interactions in these lipid systems have been considered in two different aqueous salt solutions, namely NaCl and KCl, and the sensitivity of the results on the details of interactions used for ions is determined by repeating the simulations with two distinctly different force fields. We demonstrate that the main effect of monovalent salt on a phospholipid membrane is determined by cations binding to the carbonyl region of a membrane, while chloride anions mostly stay in the water phase. It turns out that the strength and character of the cation-lipid interactions are quite different for different types of lipids and cations. PC membranes and Na+ ions demonstrate strongest interactions, leading to notable membrane compression. This finding was confirmed by both force fields (Gromacs and Charmm) employed for the ions. The binding of potassium ions to PC membranes (and the overall effect of KCl), in turn, was found to be much weaker mainly due to the larger size of a K+ ion compared to Na+. Furthermore, the effect of KCl on PC membranes was found to be force-field sensitive: The binding of a potassium ion was not observed at all in simulations performed with the Gromacs force-field, which seems to exaggerate the size of a K+ ion. As far as PE lipid bilayers are concerned, they are found to be influenced by monovalent salt to a significantly lesser extent compared to PC bilayers, which is a direct consequence of the ability of PE lipids to form both intra- and intermolecular hydrogen bonds and hence to adopt a more densely packed bilayer structure. Whereas for NaCl we observed weak binding of Na+ cations to the PE lipid-water interface, in the case of KCl we witnessed almost complete lack of cation binding. Overall, our findings indicate that monovalent salt ions affect lipids in the inner and outer leaflets of plasma cell membranes in substantially different ways.     

碱金属卤化物与NaA分子筛之间的固态离子交换反应

在低于碱金属卤化物熔点的温度下热处理碱金属卤化物和脱水NaA分子筛干混样品时,一部分分散到分子筛内孔的金属离子能够与脱水分子筛进行程度不同的固态离子交换反应,生成的非挥发性NaCl产物将扩散到分子筛笼外单独结晶或剩余的原卤化物形成溶体.热处理温度愈度,离子交换度愈大;在一定温度下热处理干混样品,存在最大离子交换度.     

Molecular dynamics simulations of LiCl association and NaCl association in water by means of ABEEM/MM

Constrained molecular dynamics simulations have been used to investigate the LiCl and NaCl ionic association in water in terms of atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM). The simulations make use of the seven-site fluctuating charge and flexible ABEEM-7P water model, based on which an ion-water interaction potential has been constructed. The mean force and the potential of mean force for LiCl and NaCl in water, the charge distributions, as well as the structural and dynamical properties of contact ion pair dissociation have been investigated. The results are reasonable and informative. For LiCl ion pair in water, the solvent-separated ion pair configurations are more stable than contact ion pair configurations. The calculated PMF for NaCl in water indicates that contact ion pair and solvent-separated ion pair configurations are of comparable stability.     

Features of Cation Hydration in Concentrated Aqueous Solutions of MeCl (Me = Li, Na, K, Rb): A Study by the Method of Integral Equations

The structural features of concentrated aqueous solutions of MeCl (Me = Li, Na, K, Rb; molar ratio [salt] : [water] = 1 : 15) at 293 K and 0.1 MPa were studied by the method of integral equations. The calculation results show that the disordering effect of the cation on the solvent structure grows in the order NaCl < KCl < RbCl (the number of free water molecules grows, and the content of tetrahedrally ordered molecules decreases). With LiCl, the changes in these parameters are maximal. In the systems containing Li⁺ and Na⁺ ions, the association parameter is lower than in pure water, whereas in the solutions with the K⁺ and Rb⁺ ions it is higher, in agreement with the concept of positive and negative hydration. It was suggested that, with increasing cationic radius, formation of hydrogen bonds between bulk water molecules becomes more preferential, and interactions between the anion and solvent molecules are weakened. On the contrary, the coordination number of the cation increases with its radius. In the examined series of solutions, the probability of formation of contact ion pairs grows considerably, and that of formation of hydration-separated ion pairs decreases.     

Water adsorption in ion-bearing nanopores

Grand canonical Monte Carlo simulations are used to examine the adsorption of water into cylindrical nanopores containing single ions. The isotherms for water adsorbing into nanopores with radii of 0.44, 0.54, 0.64, and 0.74 nm and containing Na+, K+, Ca2+, Cl-, or F- at 298 K are computed. In all cases the nanopores are found to fill at reservoir chemical potentials below the chemical potential of saturated water vapor at 298 K. The threshold chemical potential is found to be sensitive to both the size of the channel and the ion species, with the anion-bearing pores filling at lower chemical potentials. Additionally, the filling threshold chemical potential is found to decrease as the radius of the pores is decreased. Pores with K+ and Cl- are compared, and the Cl- pores are found to exhibit higher water densities in the filled states and a more energetically favorable water structure while yielding lower per particle entropies. Sample simulation configurations are also examined and indicate that at low chemical potentials, the adsorbed water forms a cluster around the ion. Finally, the influence of the choice of water model on the adsorption isotherms is examined.     

Influence of electrolytes on the microenvironment of F127 triblock copolymer micelles: a solvation and rotational dynamics study of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements of coumarin 153 (C153) and coumarin 151 (C151) as fluorescence probes have been carried out to understand the influence of electrolytes (NaCl and LiCl) on the hydration behavior of aqueous (ethylene oxide)100-(propylene oxide)70-(ethylene oxide)100 (EO100-PO70-EO100, F127) block copolymer micelles. A small blue shift in the fluorescence spectra of C153 has been observed in presence of electrolytes due to the dehydration of the oxyethylene chains in the PEO-PPO region, although fluorescence spectra of C151 remain unaltered. The close vicinity of bulk water for C151 probably negates the effect of dehydration in the PEO region. Fluorescence anisotropy measurements indicate a gradual increase in microviscosity with electrolyte concentrations. The partial collapse of copolymer blocks in the presence of electrolytes has been suggested as a reason for the increase in microviscosity along with the strong hydration of ions in the corona region. The interplay between the ion hydration and the mechanically trapped water content, and specific interaction of ions, such as complexation of Li+ ions with the copolymer block, is found to control solvation dynamics in the corona region. In addition to that, it has been established that Na+ ions reside deep into the corona region whereas Li+ ions prefer to reside closer to the surface. Owing to its higher lyotropicity, LiCl influences the corona hydration to a greater extent than NaCl and sets in micelle-micelle interaction above the 2 M LiCl concentration, as reflected in the saturation of solvation time constants. The formation of larger clusters of F127 micelles above 2 M LiCl has been confirmed by dynamic light scattering measurements; however, such cluster formation is not evident with NaCl.     

Influence of major cations of sea water on the desorption of137Cs from marine sediments

Desorption studies of¹³⁷Cs from marine sediments by artificial sea water and artificial sea water devoid of individual major cations such as Na, K, Ca, Mg and Sr indicated that only Na and K were effective in the desorption of¹³⁷Cs. Studies with various ionic strengths ranginf from 0.01 to 1.6M KCl and NaCl solutions showed that KCl desorbs constantly about 45%¹³⁷Cs at and above an ionic strength of 0.1. In case of NaCl, the percent desorption increases linearly with ionic strength. The difference in desorption by K and Na is attributed to the contraction of the clay mineral layers by K ion and expansion of the layers by Na ion.     

Experimental evaluation of single ion activities

When an ion-exchange membrane separates two electrolyte solutions having two different co-ions and a common counterion a bi-ionic potential (bi-co-ionic potential) appears across the membrane. If the membrane is ideally permselective for counterions and the activities of counterions on both sides of the membrane are equal to each other, the membrane potential Δψ becomes zero. We selected KCl as a reference electrolyte because of a symmetrical electrolyte in aqueous solutions. Then, the mean activity of ions in KCl may be assumed to be equal to each single ion activity at low molalities. Single ion activity in a test electrolyte containing K⁺ ion or Cl⁻ ion was estimated from the molality of KCl at Δψ=0 by interpolating bi-co-ionic potential to zero for KCl/membrane/LiCl, NaCl, C₆H₅COOK, or p-CH₂CHC₆H₄SO₃K systems. The values of single ion activity coefficients estimated in this work were fairly different not only from the mean activity coefficients of ions but also from the single ion activity coefficients estimated by Debye–Hückel formula.     

肌醇在碱金属氯化物溶液中的体积性质

使用精密数字密度计测定了298.15和308.15 K肌醇在不同浓度的LiCl-H2O、NaCl-H2O或KCl-H2O溶液中的密度, 计算了肌醇的表观摩尔体积Vφ和极限偏摩尔体积Vθφ , 得到了其由纯水溶剂转移至混合溶剂中的迁移偏摩尔体积⊿trsVθ椎 .结果表明, LiCl, NaCl和KCl在溶液中对肌醇的体积性质影响显著, 极限偏摩尔体积Vθφ和极限迁移偏摩尔体积⊿trsVθφ都随盐浓度的增大而增加;温度对肌醇的极限偏摩尔体积和极限迁移偏摩尔体积只有轻微影响. 从分子-离子间的相互作用角度对实验结果进行了讨论.     

Preparation and evaluation of effective fixed charge density of titanium phosphate membranes for uni-univalent electrolytes in aqueous solutions

The preparation of polystyrene-based titanium phosphate membranes at different pressures with varying amounts of material has been explained. The membrane potentials of inorganic membranes were measured with uni-univalent electrolytes (KCl, NaCl and LiCl) solution using saturated calomel electrodes (SCEs). The TMS method was used for the evaluation of the effective fixed charge density of these membranes with increasing pressure, and the surface charge density of membrane appeared to be increased due to gradually diminution in surface opening channels. The order of surface charge density for electrolytes used is found to be KCl > NaCl > LiCl. In addition to the effective fixed charge density, distribution coefficient, transport numbers, charge effectiveness and other related parameters were calculated for characterizing the ion exchange membranes by utilizing the TMS method. The theoretical prediction is consistent well with the experimental data. The SEM of these membranes at various pressures has been presented.     

Validating affinities for ion-lipid association from simulation against experiment

Understanding biological membranes at physiological conditions requires comprehension of the interaction of lipid bilayers with sodium and potassium ions. These cations are adsorbed at palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers as indicated from previous studies. Here we compare the affinity of Na(+) and K(+) for POPC in molecular dynamics (MD) simulations with recent data from electrophoresis experiments and isothermal calorimetry (ITC) at neutral pH. NaCl and KCl were described using GROMOS or parameters matching solution activities on the basis of Kirkwood-Buff theory (KBFF), and K(+) was also described using parameters by Dang et al., all in conjunction with the Berger parameters for the lipids and the SPC water model. Apparent binding constants of GROMOS-Na(+) and KBFF-K(+) are the same within error and in good agreement with values from ITC. Although these force fields yield the same number of bound ions per number of lipids for Na(+) and K(+), they give a larger number of Na(+) ions per surface area compared to K(+), in agreement with the electrophoresis experiments, because Na(+) causes a stronger reduction in the area per lipid than K(+). The intrinsic binding constants, on the other hand, are reproduced by Dang-K(+) but overestimated by GROMOS-Na(+) and KBFF-K(+). That no ion force field reproduces the intrinsic and the apparent binding constant simultaneously arises from the fact that in MD simulations, implicitly meant to mimic neutral pH, pure PC is usually modeled with zero surface charge. In contrast, POPC at neutral conditions in experiment carries a low but significant negative surface charge and is uncharged only at acidic pH as indicated from electrophoretic mobilities. Implications for future simulation and experimental studies are discussed.