Dynamics of water molecules at liquid–vapour interfaces of aqueous ionic solutions: effects of ion concentration (original) (raw)
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Physical Chemistry Chemical Physics, 2008
We performed a detailed molecular dynamics study of the interfacial structure of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate in order to explain the anomalous dependence of the surface tension on concentration. At low concentrations the surface tension decreases with concentration. At higher concentrations the surface becomes saturated; a plateau is observed in simulations with a non-polarizable force field while a possible increase is detected in simulations with a polarizable force field. The structure is characterized by a surplus of cations at the surface (with hydrophobic butyl chains pointing toward vacuum) which at low concentrations is only partly compensated by the anions because of asymmetric solvation. A more hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate is also simulated for comparison.
Concentration effects in aqueous NaCl solutions. A molecular dynamics simulation
The Journal of Physical Chemistry, 1996
A series of constant-temperature/constant-pressure molecular dynamics simulations of aqueous NaCl solutions at different salt concentrations is carried out to investigate the structure and the dynamical properties. The simulations were performed with the number of molecules ranging from 256 to 2000. The simulations cover several nanoseconds to ensure the convergence of the results and to enable a proper determination of ionion radial distribution functions. The flexible SPC water model is used as the solvent, while the ions are treated as charged Lennard-Jones particles. Only a weak influence of the salt concentration is found on the ion-ion pair correlation functions. The structures of the hydrated shells around ion pairs are studied using three-body correlation functions. The self-diffusion and interdiffusion coefficients are found to decrease with an increase of salt concentration. Molar conductivities are calculated at different salt concentrations. The residence times of water molecules in the hydration shells as well as the residence times of contact and solvent-separated ion configurations are determined.
Molecular dynamics simulation of the behaviour of water in nano-confined ionic liquid–water mixtures
Journal of Physics: Condensed Matter
This work describes the behaviour of water molecules in 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid under nanoconfinement between graphene sheets. By means of molecular dynamics simulations, an adsorption of water molecules at the graphene surface is studied. A depletion of water molecules in the vicinity of the neutral and negatively charged graphene surfaces and their adsorption at the positively charged surface are observed in line with the preferential hydration of the ionic liquid anions. The findings are appropriately described using a two-level statistical model. The confinement effect on the structure and dynamics of the mixtures is thoroughly analyzed using the density and the potential of mean force profiles, as well as by the vibrational densities of states of water molecules near the graphene surface. The orientation of water molecules and the water-induced structural transitions in the layer closest to the graphene surface are also discussed.
Interaction of a sodium ion with the water liquid-vapor interface
Chemical Physics, 1989
Molecular dynamics results are presented for the density profile of a sodium ion near the water liquid-vapor interface at 320 K. These results are compared with the predictions of a simple dielectric model for the interaction of a monovalent ion with this interface. The interfacial region described by the model profile is too narrow and the profile decreases too abruptly near the solution interface. Thus, the simple model does not provide a satisfactory description of the molecular dynamics results for ion positions within two molecular diameters from the solution interface where appreciable ion concentrations are observed. These results suggest that the surfaces associated with dielectric models of ionic processes at aqueous solution interfaces should be located at least two molecular diameters inside the liquid phase. A free energy expense of about 2 kcal/mol is required to move the ion to within two molecular layers of the free water liquid-vapor interface.
The Journal of Chemical Physics, 2004
Molecular-dynamics simulations were performed to model the effect of added salt ions on the liquid-liquid interface in a partially miscible system. Simulations of the interface between saturated phases of a model 1-hexanolϩwater system show a bilayer structure of 1-hexanol molecules at the interface with ϪOH heads of the first layer directed into the water phase and the opposite orientation for the second layer. The alignment of the polar ϪOH groups at the interface stabilizes a charge separation of sodium and chloride ions when salt is introduced into the aqueous phase, producing an electrical double layer. Chloride ions aggregate nearer the interface and sodium ions move toward the bulk water phase, consistent with the explanation that the ϪOH alignment presents a region of partial positive charges to which the hydrated chloride atoms are attracted. Ions near the interface were found to be less solvated than those in the bulk phase. An electric field was also applied to drive ions through the interface. Ions crossing the interface tended to shed water molecules as they entered the hexanol bilayer, leaving a trail of water molecules. Stabilization and facilitated transport of the ion by interactions with the second layer of hexanol molecules appeared to be an important step in the mechanism of sodium ion transport.
Journal of Physical Chemistry B, 2019
Surface properties of room temperature ionic liquids (RTILs) consisting of half neutralized diamine cations (H 2 N−(CH 2) n −NH 3 + , n = 2, 4) and triflate anions have been investigated by molecular dynamics simulations, based on an empirical atomistic force field. Planar slabs periodically repeated in 2D have been considered, and the temperature range 260 ≤ T ≤ 360 K has been covered, extending from below the melting and glass point to the equilibrium liquid range of the diamine compounds under investigation. Addition of water at 1% weight concentration allowed us to investigate the kinetics of water absorption through the RTIL surface, and to characterize the structural and dynamical properties of subsurface water. Animations of the simulation trajectory highlight the quick absorption of water molecules, progressing downhill in free energy and taking place without apparent intermediate kinetic stages. To verify and quantify these observations, a variant of the umbrella sampling algorithm has been applied to compute the variation of excess free energy upon displacing a water molecule along the normal to the surface, from the center of the slab to the vapor phase. The results provide a comprehensive picture of the thermodynamic properties underlying the kinetics of water absorption and evaporation through the surface, and they also provide the ratio of the equilibrium density of water in the vapor and liquid phase at the average concentration considered by simulations. A variety of properties such as the surface energy, the 90−10% width of the profile, the layering of different species at the interface, and the electrostatic double layer at the surface are computed and discussed, focusing on the effect of water contamination on all of them.
The Journal of Physical Chemistry B, 2003
The equilibrium and dynamical properties of liquid-vapor interfaces of electrolyte solutions are investigated by employing a simple model where the ions are represented by charged Lennard-Jones particles and the dipolar solvent molecules are characterized by the so-called Stockmayer potential. The technique of molecular dynamics simulation is employed to calculate the density profiles and the orientational structure of the interfaces, the surface tension, the translational and rotational diffusion coefficients, and also the dipole orientational relaxation times of both interfacial and bulk molecules. It is found that the ions prefer to stay in the interior of the solutions and tend to avoid the surfaces. The dipole vectors of the interfacial solvent molecules tend to align parallel to the surfaces. The surface tension shows an increasing trend with increase of ion concentration, and this increase of the surface tension is found to be the net outcome of a decrease of the solvent-solvent contribution and an increase of the ion-solvent and ion-ion contributions. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. The solvent molecules at the interfaces rotate and translate in the parallel direction at a faster rate than that of bulk molecules. The perpendicular diffusion, however, occurs at a slower rate for the interfacial molecules. Also, on increase of ion concentration of the solutions, the dynamical properties of the interfaces are found to change differently from those of the bulk solutions. The equilibrium and dynamical results of the present interfacial solutions are also compared with the results of liquid-vapor interfaces of aqueous NaCl solutions that were reported in an earlier study (Chem.
A Molecular Dynamics Study of Aqueous Solutions
Zeitschrift für Naturforschung A, 1976
Results of a molecular dynamics study of a 0.55 molal aqueous NaCl solution are reported. The basic periodic box contained 200 water molecules, 2 sodium ions and 2 chloride ions. The calculated properties of this solution are compared with those obtained previously for a 2.2 molal NaCl solution. The formation of second hydration shells, an increase of the number of water molecules in the first hydration shells, and a release of internal pressure are the main changes connected with a decrease of the concentration.
Surface tension at the liquid-vapor interface of screened ionic mixtures
The liquid-vapor interface of binary mixtures of charged particles is studied using molecular dynamics (MD) simulations. The interaction between parti-cles is given by a short-range repulsive potential plus an attractive/repulsive Yukawa term, which models screened electrostatic interactions. To obtain the components of the pressure tensor two methods were used: a hybrid MD method which combines the hard sphere and continuous forces and a standard continuous MD method where the hard sphere was replaced by a soft interaction. We show that both models give essentially the same results. As the range of interaction decreases, the critical temperature and surface tension increase. The comparison with the restricted primitive mod-el of ionic fluids is discussed.
What does an ionic liquid surface really look like? Unprecedented details from molecular simulations
Physical Chemistry Chemical Physics, 2011
We present the first intrinsic analysis of the surface of the [bmim][PF 6 ] room-temperature ionic liquid. Our detailed analysis reveals unprecedented details about the structure of the interface by providing the relative prevalence of different molecular orientations. These results suggest that 10 experimental data should be reinterpreted considering a distribution of molecular arrangements.