Solubility isotope effects in aqueous solutions of methane

The isotope effect on the Henry's law coefficients of methane in aqueous solution (H/D and C-12/C-13 substitution) are interpreted using the statistical mechanical theory of condensed phase isotope effects. The missing spectroscopic data needed for the implementation of the theory were obtained either experimentally (infrared measurements), by computer simulation (molecular dynamics technique), or estimated using the Wilson's GF matrix method. The order of magnitude and sign of both solute isotope effects can be predicted by the theory. Even a crude estimation based on data from previous vapor pressure isotope effect studies of pure methane at low temperature can explain the inverse effect found for the solubility of deuterated methane in water. (C) 2002 American Institute of Physics

Screening Methodology for the Efficient Pairing of Ionic Liquids and Carbonaceous Electrodes Applied to Electric Energy Storage

A model is presented that correlates the measured electric capacitance with the energy that comprises the desolvation, dissociation and adsorption energy of an ionic liquid into carbonaceous electrode (represented by single-wall carbon nanotubes). An original methodology is presented that allows for the calculation of the adsorption energy of ions in a host system that does not necessarily compensate the total charge of the adsorbed ions, leaving an overall net charge. To obtain overall negative (favorable) energies, adsorption energies need to overcome the energy cost for desolvation of the ion pair and its dissociation into individual ions. Smaller ions, such as BF4 −, generally show larger dissociation energies than anions such as PF6 − or TFSI−. Adsorption energies gradually increase with decreasing pore size of the CNT and show a maximum when the pore size is slightly greater than the dimensions of the adsorbed ion and the attractive van der Waals forces dominate the interaction. At smaller pore diameters, the adsorption energy sharply declines and becomes repulsive as a result of geometry deformations of the ion. Only for those diameters where the adsorption reaches maximum values is the adsorption energy sufficiently negative to balance the positive dissociation and desolvation energies. We present for each ion (and ionic liquid) what the most adequate electrode pore size should be for maximum capacitance

A thermophysical and structural characterization of ionic liquids with alkyl and perfluoroalkyl side chains

The authors thank FCT/MEC (Portugal) for financial support through grants SFRH/BD/100563/2014 (N. S. M. V.) and SFRH/BPD/94291/2013 (K. S.), FCT Investigator contracts (A. B. P., J. M. M. A., I. M. M. and J. M. S. S. E) and through projects PTDC/CTM-NAN/121274/2010, FCT-ANR/CTM-NAN/0135/2012, PTDC/EQU-FTT/118800/2010, UID/QUI/00100/2013 and UID/Multi/04551/2013. The NMR spectrometers are part of The National NMR Facility, supported by FCT/MEC (RECI/BBB-BQB/0230/2012).This work represents an essential step towards the understanding of the dynamics and thermodynamic characteristics of a novel family of ionic liquids, namely fluorinated ionic liquids based on the combination of 1-alkyl-3-methylimidazolium cations with perfluoroalkylsulfonates or perfluoroalkylcarboxylates anions. The so far scarce information about these fluids constitutes a limiting factor for their potential applications. In this work, we provide detailed evidence on the influence of hydrogenated and fluorinated alkyl chain lengths in the final characteristics of the fluorinated ionic liquids. Different properties, namely, melting point, decomposition temperature, density, dynamic viscosity, ionic conductivity and refractive index, were determined and the experimental results were discussed taking into account the influence of the length of the hydrogenated and fluorinated alkyl chains. Molecular dynamic simulations were also performed to study the nanoscale structure of these novel compounds.authorsversionpublishe

Perfluorinated Alcohols at High Pressure: Experimental Liquid Density and Computer Simulations

The liquid density of five liquid 1H,1H-perfluorinated alcohols (CF3(CF2)n−1CH2OH n = 2, 3, 4, 5, 6) was measured as a function of pressure (0.1−70 MPa) and temperature (293.15−313.15 K). The corresponding isothermal compressibility and isobaric thermal expansivity coefficients were calculated from the experimental data. The results are compared with data from the literature for the equivalent hydrogenated alcohols. Atomistic molecular dynamics simulations were also performed, providing molecular-level insight into the experimental results, in particular about the H-bond network of the perfluorinated alcohols and the effect of pressure on the organization of the liquid

Nano-Segregation and Structuring in the Bulk and at the Surface of Ionic-Liquid Mixtures

Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components, rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This paper presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N] and [C12mim][Tf2N]). Physical property measurements (viscosity, conductivity and density) find that these IL mixtures are not well described by simple mixing laws, suggesting that their structure and dynamics are strongly composition-dependent. Small-angle X-ray and neutron scattering (SAXS and SANS) measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [C12mim][Tf2N], the bulk of the IL is composed of small aggregates of [C12mim]+ ions in a [C2mim][Tf2N] matrix, which is driven by nano-segregation of the long alkyl chains and the polar parts of the IL. As the proportion of [C12mim][Tf2N] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and non-polar domains is formed. Reactive atom scattering-laser-induced fluorescence (RAS-LIF) experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, contrary to previous suggestions, that the [C12mim]+ ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [C12mim][Tf2N]

Nanosegregation and Structuring in the Bulk and at the Surface of Ionic-Liquid Mixtures

Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components, rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This paper presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N] and [C12mim][Tf2N]). Physical property measurements (viscosity, conductivity and density) find that these IL mixtures are not well described by simple mixing laws, suggesting that their structure and dynamics are strongly composition-dependent. Small-angle X-ray and neutron scattering (SAXS and SANS) measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [C12mim][Tf2N], the bulk of the IL is composed of small aggregates of [C12mim]+ ions in a [C2mim][Tf2N] matrix, which is driven by nano-segregation of the long alkyl chains and the polar parts of the IL. As the proportion of [C12mim][Tf2N] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and non-polar domains is formed. Reactive atom scattering-laser-induced fluorescence (RAS-LIF) experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, contrary to previous suggestions, that the [C12mim]+ ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [C12mim][Tf2N]

Molecular dynamics simulation studies of the interactions between ionic liquids and amino acids in aqueous solution

Although the understanding of the influence of ionic liquids (ILs) on the solubility behavior of biomolecules in aqueous solutions is relevant for the design and optimization of novel biotechnological processes, the underlying molecular-level mechanisms are not yet consensual or clearly elucidated. In order to contribute to the understanding of the molecular interactions established between amino acids and ILs in aqueous media, classical molecular dynamics (MD) simulations were performed for aqueous solutions of five amino acids with different structural characteristics (glycine, alanine, valine, isoleucine, and glutamic acid) in the presence of 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imide. The results from MD simulations enable to relate the properties of the amino acids, namely their hydrophobicity, to the type and strength of their interactions with ILs in aqueous solutions and provide an explanation for the direction and magnitude of the solubility phenomena observed in [IL + amino acid + water] systems by a mechanism governed by a balance between competitive interactions of the IL cation, IL anion, and water with the amino acids