Molecular dynamics simulations of the structure and transport properties of tetra-butylphosphonium amino acid ionic liquids

Systematic molecular dynamics simulations are used to study the structure, dynamics and transport properties of the ionic liquids composed of the tetra-butylphosphonium ([TBP](+), or [P(C(4)H(9))(4)](+)) cation with six amino acid ([AA](-)) anions. The structural features of these ionic liquids were characterized by calculating the partial site-site radial distribution functions, g(r), and computing the dihedral angle distribution of n-butyl side chains in the [TBP](+) cations. The dynamics of the ionic liquids are described by studying the velocity autocorrelation function (VACF) and the mean-square displacement (MSD) for the centers of mass of the ions at different temperatures. The ionic diffusion coefficients and the electrical conductivities were evaluated from both the Einstein and Green-Kubo methods. The cross-correlation terms in the electric-current autocorrelation functions, which are an indication of the ion pair correlations, are investigated. The cationic transference numbers were also estimated to study the contributions of the anions and cations to the transport of charge in these ionic liquids. We determined the role of the amino acid anion structures on the dynamical behavior and the transport coefficients of this family of ionic liquids. In general, the MSD and self-diffusion coefficients of the relatively heavier non-planar [TBP](+) cations are smaller than those of the lighter amino acid anions. Introducing polar functional groups (acid or amide) in the side chain of [AA](-) decreases the diffusion coefficient and electrical conductivity of AAILs. The major factors for determining the magnitude of the transport coefficients are the chemical functionality and the length of the alkyl side chain of the [AA](-) anion of these [TBP][AA] ionic liquids.     

Correlation between surface tension and void fraction in ionic liquids

An effort to systematize published and new data on the surface tension gamma of ionic liquids (ILs) is based on the hypothesis that the dimensionless surface tension parameter gamma V v (2/3)/ kT is a function of the void fraction x v = V v/ V m. The void volume V v is defined as the difference between the liquid volume V m occupied by an ion pair (known from cationic and anionic masses and liquid density measurements) and the sum V (+) + V (-) of the cationic and anionic volumes (known from crystal structures), while kT is the thermal energy. Our hypothesis that gamma V m (2/3)/ kT = G( x v) is initially based on cavity theory. It is then refined based on periodic lattice modeling, which reveals that the number N of voids per unit cell (hence the dimensionless surface tension) must depend on x v. Testing our hypothesis against data for the five ILs for which surface tension and density data are available over a wide range of temperatures collapses all of these data almost on a single curve G( x v), provided that slight (4%) self-consistent modifications are introduced on published crystallographic data for V (+) and V (-). An attempt to correlate the surface tension vs temperature data available for inorganic molten salts is similarly successful, but at the expense of larger shifts on the published ionic radii (8.8% for K; 3.3% for I). The collapsed G( x v) curves for ILs and inorganic salts do not overlap anywhere on x v space, and appear to be different from each other. The existence of a relation between gamma and x v is rationalized with a simple capillary model minimizing the energy. Our success in correlating surface tension to void fraction may apply also to other liquid properties.     

Molecular dynamics simulation of dual amino-functionalized imidazolium-based ionic liquids

In this work, the all-atom (AA) force fields were set up for three kinds of dual amino-functionalized imidazolium-based ionic liquids (ILs), composed by cations with different alkyl chain length and amino acid anion [Gly]⁻. The force field was based on our previous work and the default parameters were developed in this study. Molecular dynamics simulations were performed. Validation was carried out by comparing simulation densities with experimental data, and good agreement was obtained. Molar volume and heat capacity at constant pressure were predicted. Mean square displacements for these ILs were computed and these ILs were proved to move very slowly. It may be caused by hydrogen-bonded network between ions and the terminal azyl. To depict the microscopic structures of the ILs, many types of radial distribution functions were investigated. It is interesting to find that not only the cation and anion, but also the anions themselves will form hydrogen bonds.     

Temperature dependence of viscosity and relation with the surface tension of ionic liquids

Viscosities of ionic liquids were correlated with two linear relations. The first one presents the temperature dependence of imidazolium-, pyridinium-, pyrrolidinium-, quaternary ammonium-, and nicotinium-based ionic liquids with high accuracy. The second one is a linear relation between logarithm of surface tension and fluidity involving the characteristic exponent ?, and fits the ionic liquids uniquely with ? = 0.30. Our previously measured surface tension data of ionic liquids and literature's were used in this study. The dependence of surface tension–fluidity relation of the imidazolium-based ionic liquids on the anion type is likely disappeared as alkyl chain length increases.     

Thermodynamic approach to calculating the surface tension of single-component liquids by computer simulations

A thermodynamic method for computing the surface tension at a flat liquid-vapor interface by the Monte Carlo or molecular dynamics methods over a wide temperature range was proposed. The approach is based on the Gibbs separating surface method; it does not require information on the mechanical state of the surface layer.     

Ab initio molecular dynamics simulations of a binary system of ionic liquids

This work presents first insights into the structural properties of a binary mixture of ionic liquids from the perspective of ab initio molecular dynamics simulations. Simulations were carried out for a one-to-one mixture of 1-ethyl-3-methyl-imidazolium thiocyanate and 1-ethyl-3-methyl-imidazolium chloride and compared to pure 1-ethyl-3-methyl-imidazolium thiocyanate.     

Surface tension of ionic liquids and ionic liquid solutions

Some of the most active scientific research fronts of the past decade are centered on ionic liquids. These fluids present characteristic surface behavior and distinctive trends of their surface tension versus temperature. One way to explore and understand their unique nature is to study their surface properties. This critical review analyses most of the surface tension data reported between 2001 and 2010 (187 references).     

Molecular dynamics simulations of nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces

We have studied the self-assembly of hydrophobic nanoparticles at ionic liquid (IL)-water and IL-oil (hexane) interfaces using molecular dynamics (MD) simulations. For the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)])/water system, the nanoparticles rapidly approached the IL-water interface and equilibrated more into the IL phase although they were initially in the water phase. In contrast, when the nanoparticles were dispersed in the hexane phase, they slowly approached the IL-hexane interface but remained primarily in the hexane phase. Consequently, the IL-hexane interface was rather undisturbed by the nanoparticles whereas the IL-water interface changed significantly in width and morphology to accommodate the presence of the nanoparticles. The equilibrium positions of the nanoparticles were also supported and explained by potential of mean force (PMF) calculations. Interesting ordering and charge distributions were observed at the IL-liquid interfaces. At the IL-hexane interface, the [BMIM] cations preferentially oriented themselves so that they were immersed more in the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributed to a heightened IL density and a slightly positive charge at the IL-hexane interface. In contrast, the cations at the IL-water interface were oriented isotropically unless in the presence of nanoparticles, where the cations aligned across the nanoparticle surfaces.     

The surface structure of ionic liquids: comparing simulations with x-ray measurements

The surface-normal electron density profile of an ionic liquid, [bmim][PF6], derived from x-ray reflectivity measurements, is compared with two independent molecular-dynamics simulations. It is shown that a meaningful comparison requires a detailed accounting for both thermal and nonthermal surface roughening effects. The former is due to thermally excited capillary waves, and the latter is due to the molecular zero-point motion and form. These quantities influence very significantly, but differently, the simulated and measured density profiles. Stripping off these effects from both types of profiles yields the intrinsic structure factor of the surface. The simulated intrinsic structure factors are found to deviate considerably from the measured one. The introduction of additional ad hoc surface roughness to the simulated profiles greatly reduces the deviation, however, no physical origin for this effect can be identified. The method employed in this study should prove useful for simulation-experiment comparisons of other liquid surfaces, provided they obey capillary-wave theory, as do almost all liquid surfaces studied to date by x-ray reflectivity.     

Influence of the anion on the surface tension of 1-ethyl-3-methylimidazolium-based ionic liquids

The air–liquid interfacial tensions of eight ionic liquids, from (298 to 343) K, are presented in this work. The studied ionic liquids are formed by the fixed 1-ethyl-3-methylimidazolium cation combined with the anions acetate, dicyanamide, dimethylphosphate, methylphosphonate, methanesulfonate, thiocyanate, tosylate, and trifluoromethanesulfonate. The selected ionic liquids allowed a comprehensive study through the influence of the anion nature on the surface tension and on their surface ordering. A slight dependence of the surface tension with the ionic liquid molar volume was identified. The surface thermodynamic functions are mainly controlled by the anion which constitutes a given ionic liquid. The hypothetical critical temperatures of all ionic liquids were estimated by means of the Eötvos and Guggenheim correlations and are presented.     

Molecular dynamics simulations of charged nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces

Nanoparticle self-assembly at liquid-liquid interfaces can be significantly affected by the individual nanoparticle charges. This is particularly true at ionic liquid (IL) based interfaces, where Coulombic forces play a major role. Employing 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) as a model IL, we have studied the self-assembly of hydrophobic nanoparticles with different surface charges at the IL/water and IL/oil (hexane) interfaces using molecular dynamics simulations. In the IL/water system, the nanoparticles were initially dispersed in the water phase but quickly equilibrated at the interface, somewhat in favor of the IL phase. This preference was lessened with increased nanoparticle charge. In the IL/hexane system, all charged nanoparticles interacted with the IL to some extent, whereas the uncharged nanoparticles remained primarily in the hexane phase. Potential of mean force calculations supported the observations from the equilibrium studies and provided new insights into the interactions of the nanoparticles and ionic liquid based interfaces.     

All-atom and united-atom simulations of guanidinium-based ionic liquids

Ionic liquids (ILs) have been widely used in separation, catalysis, electrochemistry, etc., and one of the most outstanding characteristics is that ILs can be tailored and tuned for specific tasks. In order to design and make better use of ionic liquids, the structures and properties relationship is indispensable. Both molecular dynamics and Monte Carlo simulations have been proved useful to understand the behavior of molecules at the microscale and the properties of the system. However, the quality of such simulations depends on force field parameters describing the interactions between atoms. All-atom (AA) or the united-atom (UA) force fields will be chosen because of the demand for more exact results or the lower computational cost, respectively. In order to make a systematic comparison of the two force fields, molecular simulations for four kinds of acyclic guanidinium-based ionic liquids (cations: (R2N)2C=N+<, anion: nitric or perchloric acid) were performed based on the AA and the UA force fields in this work. AA force field parameters were derived from our previous work (Fluid Phase Equilib., 2008, 272: 1-7), and the UA parameters were proposed in this work. Molecular dynamics simulation results for the AA and UA force fields were compared. Simulation densities are very similar to each other. Center of mass radial distribution functions (RDFs), site to site RDFs and spatial distribution functions (SDFs) were also investigated to depict the microscopic structures of the ILs.     

The surface tension of polymer liquids

A brief review of the surface tension of polymer liquids is presented. A strong emphasis is placed on recent measurements of surface tensions of homologous liquid series up to high-molecular-weight polymers, and the thermodynamic liquid properties of these same homologous series obtained from sources such as pressure-volume-temperature (PVT) data. The accuracy and limitations of the thermodynamic information which are used as input to many of the theories applied to the surface properties of polymer molecules are discussed. By scaling the surface tension data using a true measure of the cohesive energy density of the liquid state, we can clearly observe the entropic contribution to the surface tension caused by the conformational restriction of a large molecule at the liquid-vapor interface. The scaling implies the existence of a corresponding states principle for both polymer liquids and for low-molecular-weight liquids. The ramifications of the existence of a corresponding states principle for the surface tension of polymer melts are discussed. One consequence of the corresponding states principle is that it allows us to use surface tension measurements to compute the cohesive energy density of polymer melts using PVT data.     

Molecular simulation of guanidinium-based ionic liquids

A new systematic all-atom force field was developed for cyclic guanidinium-based ionic liquids (ILs) based on the AMBER force field. Optimized molecular geometries and equilibrium bond lengths and angles were obtained by ab initio calculations, and charges were allocated to each atom center by fitting the ab initio electrostatic potential. Molecular dynamics simulations were performed for eleven kinds of ILs that are comprised of NO3(-) anions and cyclic guanidinium-based cations. Validation was carried out by comparing our simulated densities with experimental and calculated data from the literature. Transport properties such as self-diffusion coefficients, viscosities, and conductivities were calculated by molecular dynamic simulation, and their dependence on the length of the alkyl chains of cyclic guanidinium-based cations are discussed. Radial distribution functions and spatial distribution functions were investigated to depict the microscopic structures of the ILs, and the relationship between their properties and microstructures is also discussed.     

Interactions and dynamics in ionic liquids

Precise dielectric spectra have been determined at 25 degrees C over the exceptionally broad frequency range of 0.1 相似文献   

Molecular dynamics study of the surface tension of a binary immiscible fluid

The planar interface between two liquids having two degrees of affinity to mix has been studied by molecular dynamics simulations. The surface tension is calculated from the normal, PN, and transverse, PT, components of the pressure tensor P for a wide range of temperatures. An unusual increase in surface tension with increasing temperature is attributed to a pressure induced void transfer mechanism that is justified by basic thermodynamic arguments. This effect is diminished on the addition of a modest attractive potential between the two species, and there is a turnover point at higher temperatures beyond which the surface tension decreases with increasing temperature. An order parameter is identified as the gradient of the mole fraction distribution through the interfacial region. An additional effect is the dramatic inversion of the kinetic and potential contributions to the PN profile as the temperature is varied. It is found that a commonly used approximation for P, the Irving-Kirkwood 1 or IK1 method, results in a relatively modest unphysical variability in PN that weakly violates the condition of local mechanical stability. However, this artifact does not prevent the IK1 method from producing an interfacial tension which is nearly identical to that derived from the complete IK formula with no additional approximations.     

Aqueous solutions of ionic liquids: study of the solution/vapor interface using molecular dynamics simulations

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.     

Ultrafast dynamics of pyrrolidinium cation ionic liquids

We have investigated the ultrafast molecular dynamics of five pyrrolidinium cation room temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P14+/NTf2-), N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bromide P1EOE+, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P1EOE+/DCA-). For comparing dynamics among the five ionic liquids, we categorize the ionic liquids into two groups. One group of liquids comprises the three pyrrolidinium cations P14+, P1EOM+, and P1EOE+ paired with the NTf2- anion. The other group of liquids consists of the P1EOE+ cation paired with each of the three anions NTf2-, Br-, and DCA-. The overdamped relaxation for time scales longer than 2 ps has been fit by a triexponential function for each of the five pyrrolidinium ionic liquids. The fast ( approximately 2 ps) and intermediate (approximately 20 ps) relaxation time constants vary little among these five ionic liquids. However, the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range from 0 to 750 cm(-1) are quite similar for the group of three pyrrolidinium ionic liquids paired with the NTf2- anion. The intermolecular vibrational line shapes between 0 and 150 cm(-1) are fit to a multimode Brownian oscillator model; adequate fits required at least three modes to be included in the line shape.