Direct calculation of the crystal-melt interfacial free energy via molecular dynamics computer simulation

We review our recent work on the direct calculation of the interfacial free energy, gamma, of the crystal-melt interface via molecular dynamics computer simulation for a number of model systems. The value of gamma as a function of crystal orientation is determined using a thermodynamic integration technique employing moving cleaving walls [Phys. Rev. Lett. 2000, 85, 4751]. The calculation is sufficiently precise to resolve the small anisotropy in gamma, which is crucial in determining the kinetics and morphology of dendritic growth. We report values of gamma for the hard-sphere and Lennard-Jones systems, as well as recent results on the series of inverse-power potentials. For the inverse sixth-, seventh-, and eighth-power systems, we determine gamma for both fcc and bcc crystal structures. For these systems, the bcc-melt gamma is lower than that for fcc crystals by about 25%, consistent with recent experiments and computer simulations on fcc-forming systems that show preferential formation of bcc nuclei in the initial stages of crystallization. In addition, we show that our results give a molecular interpretation of Turnbull's rule, which is the empirical relationship between gamma and the enthalpy of fusion.     

Molecular simulation study of effect of molecular association on vapor-liquid interfacial properties

Vapor-liquid interfacial properties of square-well associating fluids are studied via transition-matrix Monte Carlo simulation. Results for one-site and two-site association models are presented. Coexistence properties, surface tension, cluster distribution, density profile, and orientation profile are presented. Molecular association affects the interfacial properties and cluster fractions more than it affects the bulk densities. We observe that the surface tension exhibits a maximum with respect to association strength. This behavior is in agreement with the recent study of Peery and Evans for one site system using a square-gradient approach.     

Calculation of the crystal-melt interfacial free energy of succinonitrile from molecular simulation

The crystal-metal interfacial free energy for a six-site model of succinonitrile [N triple bond C-(CH(2))(2)-C triple bond N] has been calculated using molecular-dynamics simulation from the power spectrum of capillary fluctuations in interface position. The orientationally averaged magnitude of the interfacial free energy is determined to be (7.0+/-0.4)x10(-3) J m(-2). This value is in agreement (within the error bars) with the experimental value [(7.9+/-0.8)x10(-3) J m(-2)] of Marasli et al. [J. Cryst. Growth 247, 613 (2003)], but is about 20% lower than the earlier experimental value [(8.9+/-0.5)x10(-3) J m(-2)] obtained by Schaefer et al. [Philos. Mag. 32, 725 (1975)]. In agreement with the experiment, the calculated anisotropy of the interfacial free energy of this body-centered-cubic material is small. In addition, the Turnbull coefficient from our simulation is also in agreement with the experiment. This work demonstrates that the capillary fluctuation method of Hoyt et al. [Phys. Rev. Lett. 86, 5530 (2001)] can be successfully applied to determine the crystal-melt interfacial free energy of molecular materials.     

Studies of structural, dynamical, and interfacial properties of 1-alkyl-3-methylimidazolium iodide ionic liquids by molecular dynamics simulation

Bulk and surface properties of the ionic liquids 1-alkyl-3-methyl-imidazolium iodides ([C(n)mim]I) were simulated by classical molecular dynamics using all atom non-polarizable force field (n = 4, butyl; 6, hexyl; 8, octyl). The structure of ionic liquids were initially optimized by density functional theory and atomic charges obtained by CHELPG method. Reduction of partial atomic charges (by 20% for simulation of density and surface tension, and by 10% for viscosity) found to improve the accuracy, while a non-polarizable force field was applied. Additionally, the simulation ensembles approach the equilibrium faster when the charge reduction is applied. By these refined force field parameters, simulated surface tensions in the range of 323-393 k are quite in agreement with the experiments. Simulation of temperature dependent surface tension of [C(4)mim]I well beyond room temperature (up to 700 K) permits prediction of the critical temperature in agreement with that predicted from experimental surface tension data. Simulated densities in the range of 298-450 K for the three ionic liquids are within 0.8% of the experimental data. Structural properties for [C(4)mim]I were found to be in agreement with the results of Car-Parrinello molecular dynamics simulation we performed, which indicates a rather well-structured cation-anion interaction and occurs essentially through the imidazolium ring cation. Diffusion coefficient changes with alkyl chain length in the order of [C(8)mim]I > [C(6)mim]I > [C(4)mim]I for the cation and the anion. Formation of a dense domain in subsurface region is quite evident, and progressively becomes denser as the alkyl chain length increases. Bivariate orientational analysis was used to determine the average orientation of molecule in ionic liquids surface, subsurface, and bulk regions. Dynamic bisector-wise and side-wise movement of the imodazolium ring cation in the surface region can be deduced from the bivariate maps. Atom-atom density profile and bivariate analysis indicate that the imidazolium cation takes a spoon like configuration in the surface region and the tilt of alkyl group is a function length of alkyl chain exposing as linear as possible to the vapor phase.     

Ab initio calculation of molecular chiroptical properties

This review describes the first-principles calculation of chiroptical properties such as optical rotation, electronic and vibrational circular dichroism, and Raman optical activity. Recent years have witnessed a flurry of activity in this area, especially in the advancement of density-functional and coupled cluster methods, with two ultimate goals: the elucidation of the fundamental relationship between chiroptical properties and detailed molecular structure, and the development of a suite of computational tools for the assignment of the absolute configurations of chiral molecules. The underlying theory and the basic principles of such calculations are given for each property, and a number of representative applications are discussed.     

Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation I. Effects on chain conformation and interfacial properties

Particle-based Monte Carlo simulations were employed to examine the effects of bonding density on molecular structure in reversed-phase liquid chromatography. Octadecylsilane stationary phases with five different bonding densities (1.6, 2.3, 2.9, 3.5, and 4.2 mumol/m(2)) in contact with a water/methanol (50/50 mol%) mobile phase were simulated at a temperature of 323 K. The simulations indicate that the alkyl chains become more aligned and form a more uniform alkyl layer as coverage is increased. However, this does not imply that the chains are highly ordered (e.g., all-trans conformation or uniform tilt angle), but rather exhibit a broad distribution of conformations and tilt angles at all bonding densities. At lower densities, significant amounts of the silica surface are exposed leading to an enhanced wetting of the stationary phase. At high densities, the solvent is nearly excluded from the bonded phase and persists only near the residual silanols. An enrichment in the methanol concentration and a disruption in the mobile phase's hydrogen bond network are observed at the interface as bonding density is increased.     

Computation of interfacial properties via grand canonical transition matrix Monte Carlo simulation

We examine two free-energy-based methods for studying the wetting properties of a fluid in contact with a solid substrate. Application of the first approach involves examination of the adsorption behavior of a fluid at a single substrate, while the second technique requires investigation of the properties of a system confined between two parallel substrates. Both of the techniques rely upon computation and analysis of the density dependence of a system's surface free energy and provide the contact angle and solid-vapor and solid-liquid interfacial tensions for substrate-fluid combinations within the partial wetting regime. Grand canonical transition matrix Monte Carlo simulation is used to obtain the required free-energy curves. The methods examined within this work are general and are applicable to a wide range of molecular systems. We probe the performance of the methods by computing the interfacial properties for two systems in which an atomistic fluid interacts with a fcc crystal. For both of the systems studied we find good agreement between our results and those obtained via the mechanical definition of the interfacial tension.     

Generation of a basis set for the calculation of molecular magnetic properties

Completeness conditions have been applied in the theory of molecular magnetic properties to determine the best gauge for a given basis set. They can similarly be used to optimise the basis for a fixed gauge origin. Test results indicate that the gauge dependence of the calculated properties can be significantly reduced in this way.     

Rosenbluth-sampled nonequilibrium work method for calculation of free energies in molecular simulation

We present methods that introduce concepts from Rosenbluth sampling [M. N. Rosenbluth and A. W. Rosenbluth, J. Chem. Phys. 23, 356 (1955)] into the Jarzynski nonequilibrium work (NEW) free-energy calculation technique [C. Jarzynski, Phys. Rev. Lett. 78, 2690 (1997)]. The proposed hybrid modifies the way steps are taken in the NEW process. With it, each step is selected from a range of alternatives, with bias given to steps that contribute the least work. The definition of the work average is modified to account for the bias. We introduce two variants of this method, lambda-bias sampling and configuration-bias sampling, respectively; a combined lambda- and configuration-bias method is also considered. By reducing the likelihood that large nonequilibrated work values enter the ensemble average, the Rosenbluth sampling aids in remedying problems of inaccuracy of the calculation. We demonstrate the performance of the proposed methods through a model system of N independent harmonic oscillators. This model captures the difficulties involved in calculating free energies in real systems while retaining many tractable features that are helpful to the study. We examine four variants of this model that differ qualitatively in the nature of their phase-space overlap. Results indicate that the lambda-bias sampling method is most useful for systems with entropic sampling barriers, while the configuration-bias methods are best for systems with energetic sampling barriers. The Rosenbluth-sampling schemes yield much more accurate results than the unbiased nonequilibrium work method. Typically the accuracy can be improved by about an order of magnitude for a given amount of sampling; this improvement translates into two or more orders of magnitude less sampling required to obtain a given level of accuracy, owing to the generally slow convergence of the NEW calculation when the inaccuracy is large.     

Some effects of structural disorder on the optical properties of organic molecular crystals

We investigate the effects of structural disorder on the absorption line shapes of the lowest exciton states in molecular crystals at zero temperature by the application of the average t-matrix approximation to a system characterized by a gaussian distribution of site excitation energies. The line shape exhibits asymmetric line broadening with the high energy portion being close to lorentzian, while the low energy edge is sharp. The energy dependence of the low energy tail is determined by the dimensionality of the exciton band structure.     

An ab initio calculation of the molecular structure and properties of hypofluorous acid

An ab initio LCAO MO SCF calculation has been carried out on hypofluorous acid (HOF) in order to study the electronic structure, geometry and other one-electron properties of this molecule. The minimum energy geometry was found to be R_OF = 1.450 Å, R_OH = 1.080 Å, and θ = 100.8°, which is in good agreement with experiment. In general, the bonds in HOF appear to be similar to the analogous bonds in H₂O and F₂O. The first three ionization potentials 13.97, 14.94, and 17.16 eV compare well with recent photoelectron spectroscopic data.     

Comparison of the capillary wave method and pressure tensor route for calculation of interfacial tension in molecular dynamics simulations

We have studied the calculation of surface and interfacial tension for a variety of liquid–vapor and liquid–liquid interfaces using molecular dynamics (MD) simulations. Because of the inherently small scale of MD systems, large pressure fluctuations can cause imprecise calculations of surface tension using the pressure tensor route. The capillary wave method exhibited improved precision and stability throughout all of the simulated systems in this study. In order to implement this method, the interface was defined by fitting an error function to the density profile. However, full mapping of the interface from coordinate files produced enhanced accuracy. Upon increasing the system size, both methods exhibited higher precision, although the capillary wave method was still more reliable. © 2013 Wiley Periodicals, Inc.     

Correct procedures for the calculation of heats of adsorption for heterogeneous adsorbents from molecular simulation

Several procedures for calculating the heat of adsorption from Monte Carlo simulations for a heterogeneous adsorbent are presented. Simulations have been performed to generate isotherms for nitrogen at 77 K and methane at 273.15 K in graphitic slit pores of various widths. The procedures were then applied to calculate the heat of adsorption of an activated carbon with an arbitrary pore size distribution. The consistency of the different procedures shows them to be correct in calculating interaction energy contributions to the heat of adsorption. The currently favored procedure for this type of calculation, from the literature, is shown to be incorrect and in serious error when calculating the heat of adsorption of activated carbon.     

The calculation of polymer adsorption by TiO2 pigment from interfacial measurements

Summary A method for calculating the adsorption of polymer onto pigmentary forms of titania from solution is described. It is based en the assumption that titania surfaces, coated. or not, can be treated as water surfaces. From a knowledge of interfacial parameters and the surface area of the pigment the amount of polymer adsorbed from solution can be calculated.The experimentally determined polymer adsorption isotherms were found to lie within the two predicted polymer adsorption isotherms based on (a) surface area of pigment from nitrogen adsorption. (b) surface area of pigment from electron micrographs.

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Zusammenfassung Es wird eine Methode zur Berechnung der Adsorption von Polymeren aus Lösungen an Titandioxyd-Pigmenten beschrieben. Sie basiert auf der Annahme, daß Titandioxyd-Oberflächen, sowohl beschichtet als auch nicht beschichtet, wie Wasseroberflächen behandelt werden können. Wenn die Grenzflächenparameter und die Abmessungen der Oberfläche des Pigments bekannt sind, kann die aus der Lösung adsorbierte Polymerisatmenge berechnet werden.Es wurde festgestellt, daß die experimentell bestimmten Polymeradsorptionsisothermen innerhalb der zwei vorhergesagten Polymeradsorptionsisothermen liegen, die auf (a) der Pigmentoberfläche aus der Stickstoff Adsorption, (b) der Pigmentoberfläche aus Elektronen-Mikrographien basieren.

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With 12 figures and 1 table     

Some geometric properties of quantum phases and calculation of phase formulas

An introduction to several geometrical ideas which are of use to quantum mechanics is presented. The Aharonov‐Anandan phase is introduced and without reference to any dynamical equation, this phase is formulated by defining an appropriate connection on a specific fiber bundle. The holonomy element gives the phase. By introducing another connection, the Pancharatnam phase formula is derived following a different procedure. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

不同稀释剂中HDEHP的界面性质研究

用滴体积法研究了HDEHP在不同稀释剂-0.05mol.dm^-^3(N2, Na2)SO4(pH=2.40)体系中的界面性质, 认为吸附于液-液界面的是单体HDEHP分子, 得到了各体系中HDEHP的Cmin, Tmax, Ai以及△Gad等界面吸附参数。HDEHP在不同稀释剂体系中的界面活性顺序为: 脂肪烃>芳香烃>氯仿>甲基异丁基酮, 这种变化主要是在体相中和界面上稀释剂与萃取剂、界面上的萃取剂及稀释剂与界面层水之间分子间相互作用的结果。同时讨论了HDEHP在不同稀释剂中的萃取动力学机理。     

Properties of free surface of water-methanol mixtures. Analysis of the truly interfacial molecular layer in computer simulation

Molecular dynamics simulations of the vapor-liquid interface of water-methanol mixtures of five different compositions were performed on the canonical (N,V,T) ensemble at 298 K. In addition, the vapor-liquid interface of the two neat systems was simulated, as well. The obtained configurations were analyzed by means of the novel identification of truly interfacial molecules method, which provides a full list of the molecules that are right at the surface (i.e., at the boundary of the two phases). The molecular level roughness of the surface, the adsorption of the methanol molecules at the surface layer, the orientation of the surface molecules, the residence time of the molecules at the surface layer, as well as the surface aggregation of the molecules were analyzed in detail. Both the frequency and the amplitude of the surface roughness were found to become larger with an increasing methanol content. This effect was found to be stronger for the amplitude, which falls in the range of 2-4 A, depending on the composition of the system. Methanol was found to be adsorbed at the surface layer, being preferentially at the humps of the molecularly rough surface. Surface methanol prefers to orient in such a way that the O-CH(3) bond remains perpendicular to the macroscopic plane of the surface, pointing the methyl group to the vapor phase. The main orientational preference of the water molecules is to lie parallel to the surface. Methanol was found to remain considerably longer at the surface layer of the mixed systems than water. Thus, contrary to the fact that the residence times of the two molecules were found to be rather similar to each other at the surface of their neat liquids, the residence time of the methanol molecules was an order of magnitude larger than that of water molecules at the surface of their mixtures. A strong lateral microscopic segregation of the molecules was observed at the surface layer; the minor component of the system (irrespective of whether it was water or methanol) was found to form two-dimensional aggregates, leaving the rest of the surface empty for the major component. The effect of the vicinity of the vapor phase on the properties of the molecules was found to vanish very quickly: the composition of the second layer as well as the properties of the molecules of this layer (e.g., dynamics and orientation) did not differ considerably from those in the bulk liquid phase.     

DNA strand break: structural and electrostatic properties studied by molecular dynamics simulation

Due to their lethal consequences and a relatively high probability of introduction of repair errors and mutations, single and double strand breaks are among the most important and dangerous DNA lesions. However, the mechanisms of their recognition and repair processes are only poorly known at present. This work defines and analyzes a DNA with single strand break as a template study for future complex analyses of biologically serious double strand break damage and its enzymatic repair mechanisms. Besides a non-damaged DNA serving as a reference system with no surprising results, system with open valences of the atoms at the strand break ends as well as a system with filled valences were simulated. In both cases during the first few nanoseconds the broken ends of strand breaks are significantly exposed to the outside of the molecule. However, with increasing time, the system with single strand break with open valences is partially disrupted. On the contrary, the system with filled valences shows stable conformation with newly created hydrogen bond between the two strand break endings. Moreover, these endings are steadily situated in the inner part of the molecule, thus making the recognition and docking process of a repair enzyme more complicated in the case of filled valences.