Molecular dynamics simulations of the formation for NaCl cluster at the interface between the supersaturated solution and the substrate

Molecular dynamics simulations of supersaturated aqueous NaCl solution including the Pt(100) or NaCl(100) crystal surfaces have been performed at an average temperature of 298 K. The behavior of the NaCl cluster produced in the solution have been studied through the consideration of the water dielectric property near the crystalline surfaces for understanding the role of crystal growth on the surface. The surfaces in the solutions greatly influence heterogeneous nucleation in crystallization process. Density profile of the supersaturated solution and polarization of water molecules was calculated in order to describe the effect of the surfaces on the solution structure at the solid–liquid interfaces. The formation levels of NaCl clusters heavily depended on the water orientation at the interfaces. NaCl clusters were easily formed near the Pt(100) surface compared with the NaCl(100) surface owing to a different construction of water molecules between the platinum and NaCl surface.     

Effect of polymer adsorption on the water structure at the quartz/water interface studied by optical sum frequency generation

The effect of polymers weakly adsorbed on a quartz surface on the structure of the interfacial water molecules was investigated by optical sum frequency (SF) spectroscopy. As polymers, poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), and two types of tri-block copolymer of Pluronic L64 and 17R-4 were used. SF intensity spectra of OH stretching mode of water molecules at the interface between a quartz substrate and aqueous solutions of the polymers were measured. The total SF intensity of the interfacial water of the L64 aqueous solution was smaller than those of other solutions. This result indicates that the L64 aqueous solution has smaller number of oriented interfacial water molecules. It is suggested that the rapid motion of hydrophilic parts of the adsorbed L64 disturbs the average orientation of the interfacial water molecules. On the other hand, the SF intensity from the interfacial water molecules of aqueous solutions with high ion strength did not depend on the species of the polymers in the solutions. The latter result suggests that the hydration of ions determines the structure of the interfacial water molecules.     

Self-diffusion coefficients and orientational correlation times in aqueous NaCl solutions: Complementarity with structural investigations

The dynamical properties of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-6 m) at ambient conditions are characterized by molecular dynamics (MD) simulations. MD simulations are performed with a flexible SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. In this paper, attention has been focused on the self-diffusion coefficients (D_i) of ions and water molecules and on orientational correlation time of water molecules. It is found that the self-diffusion coefficients decrease with ion concentration. Moreover, the self diffusion coefficients of sodium and chloride at higher salt concentrations are very comparable which may be due to the formation of clusters of these ions. The deduced rotational dynamics speeds up as the salt concentration increases. Some complementarities between dynamical properties and structural ones, recently obtained, are carried out.     

Quasi-elastic laser light scattering study of polyacrylamide hydrogel immersed in water and salt solutions

Polyacrylamide (PAAm) hydrogels immersed in water and aqueous NaCl solutions were investigated for their structure and dynamics using static and quasi-elastic laser light scattering (QELS) techniques. Ensemble-averaged electric field correlation function f(q, t) obtained from the non-ergodic analysis of intensity-autocorrelation function for PAAm gel immersed in water and in 5 M NaCl showed an exponential decay to a plateau with an initial decay followed by saturation at long times. The value of the plateau was found to depend on NaCl concentration and was higher than that of water. Collective diffusion coefficient, D, of the polymer network of the hydrogel immersed in water and in different concentrations of NaCl was determined by analysing f(q, t). The measured diffusion coefficient showed linear decrease with increase in concentration of NaCl. The characteristic network parameters were obtained by analyzing f(q, t) with harmonically bound Brownian particle model and from static light scattering studies.     

Electric dipole moment calculations of molecules with sixteen valence electrons

The field gradient fluctuation at the Li⁺ nucleus in dilute aqueous solution is calculated via Monte Carlo simulations of Li⁺ + nH₂O clusters (n = 6, 50 and 150). The intermolecular potentials and the lithium field gradient function, necessary for the simulations, are based on ab initio quantum mechanical calculations. It is found that the dominant contribution to the field gradient fluctuation at the lithium nucleus comes from the water molecules in the first shell. Furthermore, it is the lateral displacement of these molecules that causes the largest fluctuation. The contribution from the rotation of a water molecule is of minor importance. The field gradient at a lithium nucleus arising from a water molecule is badly described in a simple electrostatic model.     

Effect of external electric field on the terahertz transmission characteristics of electrolyte solutions

We fabricated a microfluidic chip with simple structure and good sealing performance, and studied the influence of the electric field on THz absorption intensity of liquid samples treated at different times by using THz time domain spectroscopy system. The tested liquids were deionised water and CuSO₄, CuCl₂, NaHCO₃, Na₂CO₃ and NaCl solutions. The transmission intensity of the THz wave increases as the standing time of the electrolyte solution in the electric field increases. The applied electric field alters the dipole moment of water molecules in the electrolyte solution, which affects the vibration and rotation of the whole water molecules, breaks the hydrogen bonds in the water, increases the number of single water molecules and leads to the enhancement of the THz transmission spectrum.     

The structure and radiophysical properties of solutions of salts in liquid polar dielectrics

Solutions of metal salts in liquid polar dielectrics are treated as systems of interacting charged particles, that is, cations, anions, and water molecules around cations and anions. The size of solvated ion-clusters is estimated in the approximation of the existence of a self-consistent field in solution volume. The behavior of solutions under the action of external electric field and electromagnetic wave is analyzed. The condition under which a solution of a salt in a liquid polar dielectric reflects electromagnetic waves of a certain frequency is found.     

Molecular dynamics in aqueous solutions of β-cyclodextrin

Water molecule mobility in ion-containing and nonionic aqueous solutions of β-cyclodextrin was studied by quasielastic neutron scattering (QENS). The total self-diffusion coefficients and their components corresponding to the contributions from collective (Lagrange type) and single-particle (jump diffusion) parts of molecular motions were determined. From the data obtained, one can conclude that the molecular mobility of free water in nonionic aqueous solutions of β-cyclodextrin with guest molecules (2-aminopyridine) proceeds by a single-particle mechanism. The addition of Pb²⁺ ions into the solution leads to increase in self-diffusion coefficients and growth of a bound water fraction.     

Ratiometric detection of Raman hydration shell spectra

The micro‐structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. However the spectra of hydration shell are difficult to be obtained. Herein, a novel Raman ratio spectra, which is obtained through dividing the Raman spectra of aqueous solutions from the spectrum of water, was applied to deduce the spectra of hydration shell of organic (acetone‐D6) and inorganic compounds (NaNO₃, NaSCN, NaClO₄, Na₂SO₄, NaCl) in water. Those spectra of the hydration shell were employed to study the micro‐structures of the first hydration shells of anions, the number of water molecules in the first hydration shell of free anions and acetone‐D6, and the aggregation behavior of ions in the concentrated aqueous NaNO₃. The number of water molecules in the hydration shell was supported by our molecular dynamic simulations. The Raman ratio spectra can be widely employed to obtain the spectra of the first hydration shell, and it is helpful to understand the micro‐structure of aqueous solutions. Copyright © 2016 John Wiley & Sons, Ltd.     

Water molecules migration at oil–paper interface under the coupling fields of electric and temperature: a molecular dynamics study

Moisture is an important factor affecting the insulation properties of transformers. Due to the limitations of macroscopic experimental methods, the diffusion of water at oil–paper interface cannot be accurately measured. Therefore, molecular dynamics method was used in this work to establish oil–paper layer model of 10⁵ atoms. Through jointly analysing the aggregation degree, diffusion coefficient, free volume as well as radial distribution function of water molecules, the diffusion mechanism of water molecules at oil–paper interface was studied. The results show that when the initial water content in paper was high, water molecules would accumulate at oil–paper interface to form the local high-water region during heating. The polarisation of the electric field strengthened the hydrogen bonding interaction between water molecules and increased the probability of occurrence of the high-water region. Meanwhile, electric field reduced the free volume and diffusion coefficient of water molecules and rendered its diffusion coefficient anisotropic. What’s more, when the electric field was combined with the temperature field, the electric field played a leading role in the diffusion of water molecules while the temperature field was less affected. Diffusion coefficients of water molecules at different temperatures from molecular dynamics simulations were well consistent with experimental results, which verified the rationality of the model.     

氯化钠对葡萄糖水溶液近红外光谱的影响

人体内钠盐的含量影响血糖代谢且与糖尿病具有较高的相关性。因此,在进行血糖的近红外光谱无创检测时,不仅要考虑血液中大颗粒及大分子物质对光谱的吸收和散射影响,也应从分子结构层面上分析小分子物质对葡萄糖分子结构及其特征吸收的影响。基于声光可调谐滤波器(AOTF)的高精度近红外光谱采集系统,测量并研究了在水溶液环境下氯化钠(NaCl)对葡萄糖分子结构及其近红外特征吸收的影响。首先,测量含有不同NaCl含量的葡萄糖水溶液透射光谱,分别采用纯水和同浓度 NaCl 样本进行背景修正,实验表明,在水溶液环境中 NaCl会改变水分子和葡萄糖分子特征吸收峰的位置和强度;对不含NaCl和含有NaCl的糖水样本分别扣除纯水和同浓度NaCl样本后进行二维相关光谱分析,同步谱的切线谱显示NaCl减弱了葡萄糖分子在1 400和1 520～1 700 nm处的特征吸收。最后,通过偏最小二乘回归模型定量分析NaCl对葡萄糖预测精度的影响,发现模型的预测均方根误差随NaCl含量的增加而增大,并且含NaCl的样本与不含NaCl的样本对葡萄糖浓度预测值之差的平均值与加入的NaCl含量近似为线性关系。结果表明,在水溶液环境下NaCl分子会改变葡萄糖分子键状态并影响其特征吸收,从而降低模型的预测精度。若将NaCl含量作为变量因子,有助于提升血糖的近红外光谱无创检测精度。     

A computer simulation study of the separation of aqueous solutions using thin zeolite membranes

A recently developed molecular simulation scheme for studying solutions undergoing osmosis and reverse osmosis was used to study the separation of aqueous solutions using thin zeolite membranes. This method allows for the preservation of the atomic roughness of the membranes, while the molecules that constitute the membranes are also allowed to vibrate. In the simulations, two thin membranes cut from a cubic cell of ZK-4 zeolite were used as the semi-permeable membranes to separate water from aqueous NaCl solutions. Both osmosis and reverse-osmosis phenomena were observed. The study showed that ZK-4 zeolite membranes show promise for use in membrane-based separation of aqueous electrolyte solutions, as well as other similar systems.     

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.     

The effect of ultrasound treatment on the interaction of brine with pork meat proteins

The objective of the study is to elucidate the effect of ultrasound treated salt solution on curing of pork meat. The interactions of salt (NaCl) solutions of 3 and 25% with the proteins of pork meat are studied. High intensity ultrasound operating at 20 kHz was used. The differential scanning calorimetry (DSC), NMR spin-spin relaxation time, unfrozen water and water diffusion coefficient measurements were carried out in meat cured with ultrasound treated and untreated salt solutions. The effect of ultrasonication was most evident from measured spin-spin relaxation times T₂₁, the rate of chemical exchange of water protons k and the amount of unfrozen water W_unf in the meat. The measured diffusion coefficient of water D_w in meat cured with ultrasound treated and control salt solution did not show significant difference. The nuclear magnetic resonance (NMR) relaxation data, differential scanning calorimetry (DSC) and the diffusion coefficient data reliably show that the possible action of ultrasound to salt solution was manifested on the first 2 days of the experiment with a 3% salt solution.     

Dielectric properties of pyridine N-oxide aqueous solution under the static electric field and microwave field

Molecular dynamics simulations of the pyridine N-oxide aqueous solution have been performed in the canonical ensemble macroscopic canonical ensemble (NVT) both in the absence and presence of an external electromagnetic field. It extracts the radial distribution function for each concentration solution, dielectric constant and other information on dielectric properties. Analysing the microscopic dielectric information of the aqueous solution under the static electric field (0–3×10⁹ V/m) and microwave frequencies (2.45G, 0–3×10⁹ V/m), and comparing the dielectric information between the different concentrations and different field strengths, we can get the dielectric properties of two kinds of polar aqueous solution under microwave irradiation. Thus, this project can provide the data of the sample to other correlation studies.     

Microscopic dynamics in water and ionic aqueous solutions: velocity cross-correlations

The microscopic momentum transfer between distinct particles in water and ionic aqueous solutions has been investigated by molecular dynamics simulation. The time correlation functions between the initial velocity of a central molecule and later velocities of molecules within different coordination shells were calculated for liquid water. Special attention was paid to the influence of the intermolecular hydrogen bonds on the exchange of momentum among water molecules and on the power spectra of the velocity autocorrelation functions. The velocity cross-correlations between ions in an aqueous NaCl solution also were calculated. It was observed that the difference between the results for the Na⁺ and Cl^? ions are associated mainly with the different sizes of the ions.     

Dehydration process in NaCl solutions under various external electric fields

Ionic motions at solid-liquid interface in supersaturated NaCl solutions have been investigated by molecular dynamics (MD) simulation for understanding crystal growth processes. The density profile in the vicinity of the interfaces between NaCl(100) and the supersaturated NaCl solution was calculated. Diffusion coefficients of water molecules in the solution were estimated as a function of distance from the crystal interface. It turned out that the structure and dynamics of the solution in the interfaces was different from those of bulk solution owing to electric fields depending on the surface charge. Therefore, the electric field was applied to the supersaturated solutions and dehydration phenomenon occurring in the process of the crystal growth was discussed. As the electric field increased, it was observed that the Na⁺ keeping strongly hydration structure broke out by the electric force. In supersaturated concentration, the solution structure is significantly different from that of dilution and has a complicated structure with hydration ions and clusters of NaCl. If the electric fields were applied to the solutions, the breakout of hydration structure was not affected with increasing the supersaturated ratio. This reason is that the cluster structures are destroyed by the electric force. The situation depends on the electric field or crystal surface structure.     

Study of the effect of a low-frequency magnetic field on the spin-lattice relaxation in water and in aqueous solutions

The spin-lattice relaxation times in water and NaCl aqueous solutions in a low-frequency magnetic field have been measured on natural-concentration ¹⁷O nuclei. The activation energies have been calculated. The differences in the temperature dependences of the spin-lattice relaxation time and the changes in the relaxation with the magnetic field frequency are established.     

Microwave heating and non-thermal effects of sodium chloride aqueous solution

ABSTRACT

The dielectric thermal and non-thermal properties of sodium chloride aqueous solution under the microwave region have been estimated. The dielectric properties, hydrogen bonding, transport properties, energy distribution and local structure have been evaluated by classical molecular dynamics method. In the process of microwave energy distribution, the direct coupling of rotational motion, vibration and redirection is revealed. Microwave energy is converted into kinetic energy and interaction energy between two molecules. A mechanism for exploring the effects of microwaves on the non-thermal effects of brine systems over a longer simulation time and a wider microwave range is proposed. The increase in field intensity is usually accompanied by local damage to the water structure near the hydrated ions. More specifically, above the field threshold, the residence time of water molecules near the ions significantly decreases.

Highlights

1. Microwave energy is transferred to the kinetic energy and the energy between the molecules.

2. The increase in field intensity is usually accompanied by local damage to the water structure near the hydrated ions.

3. The larger electric field strength amplifies the effect of frequency.

4. The residence time of water molecules near the ions significantly decreases.

     

Aqueous electrolyte surfaces in strong electric fields: molecular insight into nanoscale jets and bridges

Exposing aqueous surfaces to a strong electric field gives rise to interesting phenomena, such as formation of a floating water bridge or an eruption of a jet in electrospinning. In an effort to account for the phenomena at the molecular level, we performed molecular dynamics simulations using several protocols on both pure water and aqueous solutions of sodium chloride subjected to an electrostatic field. All simulations consistently point to the same mechanisms which govern the rearrangement of the originally planar surface. The results show that the phenomena are primarily governed by an orientational reordering of the water molecules driven by the applied field. It is demonstrated that, for pure water, a sufficiently strong field yields a columnar structure parallel to the field with an anisotropic arrangement of the water molecules with their dipole moments aligned along the applied field not only in the surface layer but over the entire cross section of the column. Nonetheless, the number of hydrogen bonds per molecule does not seem to be affected by the field regardless of its strength and molecule’s orientation. In the electrolyte solutions, the ionic charge is able to overcome the effect of the external field tending to arrange the water molecules radially in the first coordination shell of an ion. The ion–water interaction interferes thus with the water–electric field interaction, and the competition between these two forces (i.e., strength of the field versus concentration) provides the key mechanism determining the stability of the observed structures.