Transferable, Polarizable Force Field for Ionic Liquids Goloviznina, Kateryna; Canongia Lopes, José N; Costa Gomes, Margarida ...
Journal of chemical theory and computation,
11/2019, Volume:
15, Issue:
11
Journal Article
Peer reviewed
A general, transferable, polarizable force field for molecular simulation of ionic liquids (ILs) and their mixtures with molecular compounds is developed. This polarizable model is derived from the ...widely used CL&P fixed-charge force field that describes most families of ILs, in a form compatible with OPLS-AA, one of the major force fields for organic compounds. Models for ILs with fixed, integer-ionic charges lead to pathologically slow dynamics, a problem that is corrected when polarization effects are included explicitly. In the model proposed here, Drude-induced dipoles are used with parameters determined from atomic polarizabilities. The CL&P force field is modified upon inclusion of the Drude dipoles to avoid double-counting of polarization effects. This modification is based on first-principles calculations of the dispersion and induction contributions to the van der Waals interactions using symmetry-adapted perturbation theory (SAPT) for a set of dimers composed of positive, negative, and neutral fragments representative of a wide variety of ILs. The fragment approach provides transferability, allowing the representation of a multitude of cation and anion families, including different functional groups, without the need to reparametrize. Because SAPT calculations are expensive, an alternative predictive scheme was devised, requiring only molecular properties with a clear physical meaning, namely, dipole moments and atomic polarizabilities. The new polarizable force field, CL&Pol, describes a broad set of ILs and their mixtures with molecular compounds and is validated by comparisons with experimental data on density, ion diffusion coefficients, and viscosity. The approaches proposed here can also be applied to the conversion of other fixed-charge force fields into polarizable versions.
It is widely believed that a defining characteristic of ionic liquids (or low-temperature molten salts) is that they exert no measurable vapour pressure, and hence cannot be distilled. Here we ...demonstrate that this is unfounded, and that many ionic liquids can be distilled at low pressure without decomposition. Ionic liquids represent matter solely composed of ions, and so are perceived as non-volatile substances. During the last decade, interest in the field of ionic liquids has burgeoned, producing a wealth of intellectual and technological challenges and opportunities for the production of new chemical and extractive processes, fuel cells and batteries, and new composite materials. Much of this potential is underpinned by their presumed involatility. This characteristic, however, can severely restrict the attainability of high purity levels for ionic liquids (when they contain poorly volatile components) in recycling schemes, as well as excluding their use in gas-phase processes. We anticipate that our demonstration that some selected families of commonly used aprotic ionic liquids can be distilled at 200-300 °C and low pressure, with concomitant recovery of significant amounts of pure substance, will permit these currently excluded applications to be realized.
A new comprehensive Molecular Dynamics study using large simulation boxes has been performed in order to complete and extend the structural analysis on the mesoscopic segregation observed in the ...ionic liquids of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide homologous series, C n C1imNtf2 (2 ≤ n ≤ 10). The analysis includes the discussion along the whole family of the corresponding structure factors, S(q), in the low-q range (1.6 ≤ q/nm–1 ≤ 20); the confirmation of the periodicity of the polar network of the ionic liquid and its intermediate low-q peak equivalence; and the introduction of five statistical functions that probe the existence and characterize the polar network and the nonpolar aggregates that are formed along the C n C1imNtf2 series. The later functions comprise aggregate size distributions, average number of contact neighbors within an aggregate, neighbor distributions, distributions of aggregate maximum length, and distributions of aggregate volume.
Laser spectroscopy of muonic deuterium Pohl, Randolf; Nez, François; Fernandes, Luis M. P. ...
Science (American Association for the Advancement of Science),
08/2016, Volume:
353, Issue:
6300
Journal Article
Peer reviewed
Open access
The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius rd and the polarizability serve as important ...benchmarks for understanding the nuclear forces and structure. Muonic deuterium μd is the exotic atom formed by a deuteron and a negative muon μ⁻. We measured three 2S-2P transitions in μd and obtain rd = 2.12562(78) fm, which is 2.7 times more accurate but 7.5σ smaller than the CODATA-2010 value rd = 2.1424(21) fm. The μd value is also 3.5σ smaller than the rd value from electronic deuterium spectroscopy. The smaller rd, when combined with the electronic isotope shift, yields a "small" proton radius rp, similar to the one from muonic hydrogen, amplifying the proton radius puzzle.
At 298 K, the surface tension of ionic liquids (ILs) of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series, C n C1ImNTf2, ranges from around 35 mN·m–1 for C2C1ImNTf2 to just ...below 30 mN·m–1 for C12C1ImNTf2. However, the decrease rate along the series is not constant: a large decrease from C2C1ImNTf2 to C8C1ImNTf2 is followed by almost constant values from C8C1ImNTf2 to C12C1ImNTf2. Such behavior is hard to interpret from a molecular point of view without suitable information about the free-surface structure of the different ILs. In this work, we have successfully used the Langmuir principle in combination with structural data obtained from angle-resolved X-ray photoelectron spectroscopy experiments and molecular dynamics simulations, to predict the correct surface tension trend along the IL series. The concepts unveiled for this particular homologous IL family can be easily extended to other systems.
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 article 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 (C2mimTf2N and C12mimTf2N). Physical property measurements (viscosity, conductivity, and density) reveal that these IL mixtures are not well described by simple mixing laws, implying that their structure and dynamics are strongly composition dependent. Small-angle X-ray and neutron scattering measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of C12mimTf2N, the bulk of the IL is composed of small aggregates of C12mim+ ions in a C2mimTf2N matrix, which is driven by nanosegregation of the long alkyl chains and the polar parts of the IL. As the proportion of C12mimTf2N 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 nonpolar domains is formed. Reactive atom scattering-laser-induced fluorescence 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, in contrast 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 C12mimTf2N.
Candida albicans is a commensal yeast fungus of the human oral, gastrointestinal, and genital mucosal surfaces, and skin. Antibiotic-induced dysbiosis, iatrogenic immunosuppression, and/or medical ...interventions that impair the integrity of the mucocutaneous barrier and/or perturb protective host defense mechanisms enable C. albicans to become an opportunistic pathogen and cause debilitating mucocutaneous disease and/or life-threatening systemic infections. In this review, we synthesize our current knowledge of the tissue-specific determinants of C. albicans pathogenicity and host immune defense mechanisms.
Understanding physicochemical properties of ionic liquids is important for their rational use in extractions, reactions, and other applications. Ionic liquids are not simple fluids: their ions are ...generally asymetric, flexible, with delocalized electrostatic charges, and available in a wide variety. It is difficult to capture their subtle properties with models that are too simplistic. Molecular simulation using atomistic force fields, which describe structures and interactions in detail, is an excellent tool to gain insights into their liquid-state organization, how they solvate different compounds, and what molecular factors determine their properties. The identification of certain ionic liquids as self-organized phases, with aggregated nonpolar and charged domains, provides a new way to interpret the solvation and structure of their mixtures. Many advances are the result of a successful interplay between experiment and modeling, possible in this field where none of the two methodologies had a previous advance.
Nanometer-scale structuring in room-temperature ionic liquids is observed using molecular simulation. The ionic liquids studied belong to the 1-alkyl-3-methylimidazolium family with ...hexafluorophosphate or with bis(trifluoromethanesulfonyl)amide as the anions, C n mimPF6 or C n mim(CF3SO2)2N, respectively. They were represented, for the first time in a simulation study focusing on long-range structures, by an all-atom force field of the AMBER/OPLS_AA family containing parameters developed specifically for these compounds. For ionic liquids with alkyl side chains longer than or equal to C4, aggregation of the alkyl chains in nonpolar domains is observed. These domains permeate a tridimensional network of ionic channels formed by anions and by the imidazolium rings of the cations. The nanostructures can be visualized in a conspicuous way simply by color coding the two types of domains (in this work, we chose red = polar and green = nonpolar). As the length of the alkyl chain increases, the nonpolar domains become larger and more connected and cause swelling of the ionic network, in a manner analogous to systems exhibiting microphase separation. The consequences of these nanostructural features on the properties of the ionic liquids are analyzed.
Melanoma treatment in review Domingues, Beatriz; Lopes, José Manuel; Soares, Paula ...
ImmunoTargets and therapy,
01/2018, Volume:
7
Journal Article
Peer reviewed
Open access
Melanoma represents the most aggressive and the deadliest form of skin cancer. Current therapeutic approaches include surgical resection, chemotherapy, photodynamic therapy, immunotherapy, ...biochemotherapy, and targeted therapy. The therapeutic strategy can include single agents or combined therapies, depending on the patient's health, stage, and location of the tumor. The efficiency of these treatments can be decreased due to the development of diverse resistance mechanisms. New therapeutic targets have emerged from studies of the genetic profile of melanocytes and from the identification of molecular factors involved in the pathogenesis of the malignant transformation. In this review, we aim to survey therapies approved and under evaluation for melanoma treatment and relevant research on the molecular mechanisms underlying melanomagenesis.
PDF
