Solid-liquid interfacial free energy out of equilibrium

The properties of the interface between solid and melt are key to solidification and melting, as the interfacial free energy introduces a kinetic barrier to phase transitions. This makes solidification happen below the melting temperature, in out-of-equilibrium conditions at which the interfacial free energy is ill defined. Here we draw a connection between the atomistic description of a diffuse solid-liquid interface and its thermodynamic characterization. This framework resolves the ambiguities in defining the solid-liquid interfacial free energy above and below the melting temperature. In addition, we introduce a simulation protocol that allows solid-liquid interfaces to be reversibly created and destroyed at conditions relevant for experiments. We directly evaluate the value of the interfacial free energy away from the melting point for a simple but realistic atomic potential, and find a more complex temperature dependence than the constant positive slope that has been generally assumed based on phenomenological considerations and that has been used to interpret experiments. This methodology could be easily extended to the study of other phase transitions, from condensation to precipitation. Our analysis can help reconcile the textbook picture of classical nucleation theory with the growing body of atomistic studies and mesoscale models of solidification.

Investigation of the solid-liquid phase transition of carbon at 150 GPa with spectrally resolved X-ray scattering

We have resolved the solid-liquid phase transition of carbon at pressures around 150GPa. High-pressure samples of different temperatures were created by laser-driven shock compression of graphite and varying the initial density from 1.30g/cm³ to 2.25g/cm³. In this way, temperatures from 5700K to 14,500K could be achieved for relatively constant pressure according to hydrodynamic simulations. From measuring the elastic X-ray scattering intensity of vanadium K-alpha radiation at 4.95keVat a scattering angle of 126°, which is very sensitive to the solid-liquid transition, we can determine whether the sample had transitioned to the fluid phase. We find that samples of initial density 1.3g/cm³ and 1.85g/cm³ are liquid in the compressed states, whereas samples close to the ideal graphite crystal density of 2.25g/cm³ remain solid, probably in a diamond-like state.

Polarization Relaxation in an Ionic Liquid Confined between Electrified Walls

The response of a room temperature molten salt to an external electric field when it is confined to a nanoslit is studied by molecular dynamics simulations. The fluid is confined between two parallel and oppositely charged walls, emulating two electrified solid-liquid interfaces. Attention is focused on structural, electrostatic, and dynamical properties, which are compared with those of the nonpolarized fluid. It is found that the relaxation of the electrostatic potential, after switching the electric field off, occurs in two stages. A first, subpicosecond process accounts for 80% of the decay and is followed by a second subdiffusive process with a time constant of 8 ps. Diffusion is not involved in the relaxation, which is mostly driven by small anion translations. The relaxation of the polarization in the confined system is discussed in terms of the spectrum of charge density fluctuations in the bulk.

Estimation of ternary liquid-liquid equilibria for arene/alkane/ionic liquid mixtures using neural networks

Neural network models have been explored for the prediction of the liquid-liquid equilibrium data and aromatic/aliphatic selectivity values. Four ternary systems composed of toluene, heptane, and the ionic liquids 1-ethyl-3-methylimidazolium ethylsulfate, or 1,3-dimethylimidazolium methylsulfate were investigated at 313.2 and 348.2 K.

Use of water in aiding olefin/paraffin (liquid + liquid) extraction via complexation with a silver bis(trifluoromethylsulfonyl)imide salt

This paper describes the extraction of C₅–C₈ linear α-olefins from olefin/paraffin mixtures of the same carbon number via a reversible complexation with a silver salt (silver bis(trifluoromethylsulfonyl)imide, Ag[Tf₂N]) to form room temperature ionic liquids [Ag(olefin)_x][Tf₂N]. From the experimental (liquid + liquid) equilibrium data for the olefin/paraffin mixtures and Ag[Tf₂N], 1-pentene showed the best separation performance while C₇ and C₈ olefins could only be separated from the corresponding mixtures on addition of water which also improves the selectivity at lower carbon numbers like the C₅ and C₆, for example. Using infrared and Raman spectroscopy of the complex and Ag[Tf₂N] saturated by olefin, the mechanism of the extraction was found to be based on both chemical complexation and the physical solubility of the olefin in the ionic liquid ([Ag(olefin)_x][Tf₂N]). These experiments further support the use of such extraction techniques for the separation of olefins from paraffins.

High-pressure phase equilibrium in the {carbon dioxide (1) + 1-chloropropane (2)} binary system

Abstract The current study reports original vapour-liquid equilibrium (VLE) for the system {CO2 (1) + 1-chloropropane (2)}. The measurements have been performed over the entire pressure-composition range for the (303.15, 313.15 and 328.15) K isotherms. The values obtained have been used for comparison of four predictive approaches, namely the equation of state (EoS) of Peng and Robinson (PR), the Soave modification of Benedict–Webb–Rubin (SBWR) EoS, the Critical Point-based Revised Perturbed-Chain Association Fluid Theory (CP-PC-SAFT) EoS, and the Conductor-like Screening Model for Real Solvents (COSMO-RS). It has been demonstrated that the three EoS under consideration yield similar and qualitatively accurate predictions of VLE, which is not the case for the COSMO-RS model examined. Although CP-PC-SAFT EoS exhibits only minor superiority in comparison with PR and SBWR EoS in predicting VLE in the system under consideration, its relative complexity can be justified when taking into account the entire thermodynamic phase space and, in particular, considering the liquid densities and sound velocities over a wider pressure-volume-temperature range.

Evaluation of the ionic liquids 1-alkyl-3-methylimidazolium hexafluorophosphate as a solvent for the extraction of benzene from cyclohexane : ( Liquid + liquid ) equilibria

In the catalytic hydrogenation of benzene to cyclohexane, the separation of unreacted benzene from the product stream is inevitable and essential for an economically viable process. In order to evaluate the separation efficiency of ionic liquids (ILs) as a solvent in this extraction processes, the ternary (liquid + liquid) equilibrium of 1-alkyl-3-methylimidazolium hexafluorophosphate, [Cnmim][PF6] (n = 4, 5, 6), with benzene and cyclohexane was studied at T = 298.15 K and atmospheric pressure. The reliability of the experimentally determined tie-line data was confirmed by applying the Othmer–Tobias equation. The solute distribution coefficient and solvent selectivity for the systems studied were calculated and compared with literature data for other ILs and sulfolane. It turns out that the benzene distribution coefficient increases and solvent selectivity decreases as the length of the cation alkyl chain grows, and the ionic liquids [Cnmim][PF6] proved to be promising solvents for benzene–cyclohexane extractive separation. Finally, an NRTL model was applied to correlate and fit the experimental LLE data for the ternary systems studied.

Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.

Modification of chitin properties for enzymatic deacetylation

Chitins produced via a conventional chemical route as well as from a new biological process were modified to increase the efficiency of enzymatic deacetylation reactions for the production of novel biological chitosan. These modified chitins were reacted for 24h with extracellular fungal enzymes from Colletotrichum lindemuthianum. The chemical and physical properties of the various substrates were analysed and their properties related to the effectiveness in the deacetylation reaction. Modifications of the chitins affected the degree of deacetylation with varied effects. Without further modification to reduce crystallinity and to open up the solid substrate structure, the chitins were found to be poor substrates for the heterogeneous solid-liquid enzymatic catalysis. It was found that the solvent and drying method used in modifying the chitins had significant impact on the final efficiency of the enzymatic deacetylation reaction. The most successful modifications through freeze drying of a colloidal chitin suspension increased the degree of enzymatic deacetylation by 20 fold. These processes reduce the crystallinity of the chitin making it easier for the enzymes to access their internal structure. X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and BET isotherm analysis are employed to characterise the modified chitins to ascertain the degree of crystallinity, porous structure, surface area, and morphology.

Bridging the Gap between Ionic Liquids and Molten Salts: Group 1 Metal Salts of the Bistriflamide Anion in the Gas Phase

Fourier transform ion cyclotron resonance mass spectrometry experiments showed that liquid Group 1 metal salts of the bistriflamide anion undergoing reduced-pressure distillation exhibit a remarkable behavior that is in transition between that of the vapor-liquid equilibrium characteristics of aprotic ionic liquids and that of the Group 1 metal halides: the unperturbed vapors resemble those of aprotic ionic liquids, in the sense that they are essentially composed of discrete ion pairs. However, the formation of large aggregates through a succession of ion-molecule reactions is closer to what might be expected for Group I metal halides. Similar experiments were also carried out with bis{(trifluoromethyl)sulfonyl}amine to investigate the effect of H+, which despite being the smallest Group 1 cation, is generally regarded as a nonmetal species. In this case, instead of the complex ion-molecule reaction pattern found for the vapors of Group I metal salts, an equilibrium similar to those observed for aprotic ionic liquids was observed.

Dispersion interactions in room-temperature ionic liquids: Results from a non-empirical density functional

The role of dispersion or van de Waals (VDW) interactions in imidazolium-based room-temperature ionic liquids is studied within the framework of density functional theory, using a recently developed non-empirical functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)], as efficiently implemented in the SIESTA code [G. Roman-Perez and J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009)]. We present results for the equilibrium structure and lattice parameters of several crystalline phases, finding a general improvement with respect to both the local density (LDA) and the generalized gradient approximations (GGA). Similar to other systems characterized by VDW bonding, such as rare gas and benzene dimers as well as solid argon, equilibrium distances and volumes are consistently overestimated by approximate to 7%, compared to -11% within LDA and 11% within GGA. The intramolecular geometries are retained, while the intermolecular distances and orientations are significantly improved relative to LDA and GGA. The quality is superior to that achieved with tailor-made empirical VDW corrections ad hoc [M. G. Del Popolo, C. Pinilla, and P. Ballone, J. Chem. Phys. 126, 144705 (2007)]. We also analyse the performance of an optimized version of this non-empirical functional, where the screening properties of the exchange have been tuned to reproduce high-level quantum chemical calculations [J. Klimes, D. Bowler, and A. Michaelides, J. Phys.: Condens. Matter 22, 074203 (2010)]. The results for solids are even better with volumes and geometries reproduced within 2% of experimental data. We provide some insight into the issue of polymorphism of [bmim][Cl] crystals, and we present results for the geometry and energetics of [bmim][Tf] and [mmim][Cl] neutral and charged clusters, which validate the use of empirical force fields. (C) 2011 American Institute of Physics. [doi:10.1063/1.3652897]

Combined studies of DFT atomistic modelling and in situ FTIR spectroscopy on surface oxidants and CO oxidation at Ru electrodes

We report the combined studies of density functional theory (DFT) calculations and electrochemical in situ FTIR spectroscopy on surface oxidants and mechanisms of CO oxidation at the Ru(0001) electrodes. It is shown that CO can co-adsorb with both O and OH species at lower potential region where a low coverage of the (2 x 2)-O/OH adlayer formed; the oxidation of CO adsorbates takes place at higher potentials where a high coverage of the (1 x 1)-O/OH adlayer formed. Surface O species are not the active oxidants under all coverages studied, due to the high reaction barriers between CO and O (>1 eV). However, surface OH species with higher coverage are identified as the active oxidants, and CO oxidation takes place via a two-steps' mechanism of CO + 3OH -> COOH + 2OH -> CO2 + H2O + OH, in which three nearby OH species are involved in the CO2 formation: CO reacts with OH, forming COOH; COOH then transfers the H to a nearby OH to form H2O and CO2, at the same time, another H in the H2O transfers to a nearby OH to form a weak adsorbed H2O and a new OH. The reaction barrier of these processes is reduced significantly to around 0.50 eV. These new results not only provide an insight into surface active oxidants on Ru, which is directly relevant to fuel cell catalysis, but also reveals the extra complexity of catalytic reactions taking place at solid/liquid electrochemical interface in comparison to the relatively simpler ones at solid/gas phase.