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, [CnC1im][Ntf2] (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 [CnC1im][Ntf2] 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.

Several structural features of aqueous solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate were analyzed in the entire concentration range using molecular dynamics simulation results. Different analysis tools developed in-house were applied to describe the size and connectivity of different water and ion aggregates as a function of the solution concentration. Four concentration ranges-x(H(2)O)<0.5, 0.50.95-with four distinct structural regimes--isolated water molecules, chain-like water aggregates, bicontinuous system, and isolated ions or small ion clusters--were identified and discussed, including two different percolation limits: that of water in the ionic liquid network (around x(H(2)O)=0.8) and that of the ionic liquid in water (around x(H(2)O)=0.95).

The present study analyses the large structural differences, first observed using X-ray diffraction, between 1-alkyl-3-methylimidazolium-based ionic liquids, [Cnmim][Ntf2] (n = 3, 6, 9), and their counterparts with ether-substituted alkyl side chains, [(C1OC1)(n/3)mim][Ntf2] (n = 3, 6, 9). The MD simulations-obtained using a non-polarizable atomistic force-field to model the ionic liquids under discussion-demonstrate that the suppression of the nanostructured nature in the ionic liquids with ether chains is persistent along the entire series and it is not due to any modification of the polar network of the ionic liquid but rather due to the different morphologies of the non-polar regions that surround it. The modification of the non-polar regions-shift from bulky segregated domains in [Cnmim][Ntf2] to thin enveloping ones in [(C1OC1)(n/3)mim][Ntf2]-are caused by the inability of the oxygen-substituted alkyl side chains to pack effectively side by side, the existence of kinks along the chain that lead eventually to intra-molecular, scorpion-like interactions between the chains and the imidazolium ring, and by their stronger interactions with the cations of the polar network via the lone electron pairs of the ether oxygen atoms.

A new polymorph of 4′-hydroxyacetophenone (form I, monoclinic, P21/c, Z′ = 1) was isolated and characterized. The structural differences between this phase and the previously known one (form II, orthorhombic, P212121, Z′ = 2) were investigated by X-ray diffraction. The two polymorphs exhibit distinct packing features and, at the molecular level, they seem to differ by the relative conformations of the OH and C(O)CH3 groups. The stability domains of the two phases from 298.15 K to the fusion temperature were also studied by a variety of thermodynamic methods and by density functional theory calculations. On the basis of the obtained results, p−T and Δf G m o−T phase diagrams for 4′-hydroxyacetophenone were defined. Differential scanning calorimetry experiments indicated that the system is enantiotropic, with form II first transforming into form I at 351.2 ± 2.7 K, followed by fusion of form I at 381.9 ± 0.1 K. Solution calorimetry demonstrated that form II is more stable than form I at 298.15 K, with Δtrs H m o(II→I) = 0.49 ± 0.13 kJ mol−1. Despite this small enthalpy difference compared to the thermal energy at 298.15 K (RT = 2.5 kJ mol−1), a sample of form I could be stored at ambient temperature, for at least 1 year, without change. Results of B3LYP/6–31G(d,p) calculations indicated that the most stable conformation of the isolated molecule is also that corresponding to the most stable polymorph of 4′-hydroxyacetophenone at ambient temperature (form II). The computations further suggest that the occurrence of the II → I transition through a simple rotation of the OH group is unlikely. Finally, the fact that the more stable form II has a greater Z′ than the less stable form I contrasts with the recent proposal that high Z′ polymorphs are metastable precursors of lower Z′ forms along the crystallization pathway.

This work explores the possibility of increasing the ionicity of ionic liquids via the solubilization of inorganic salts in their midst. The resulting purely ionic media-distinct ionic liquid plus inorganic salt mixtures-are liquid in an extensive concentration range and can be aptly denominated High Ionicity Ionic Liquids (HIILs).

Motivated by the need for reliable experimental data for the assessment of theoretical predictions, this work proposes a data set of enthalpies of sublimation determined for specific crystalline structures, for the validation of molecular force fields (FF). The selected data were used to explore the ability of the OPLS-AA parametrization to investigate the properties of solid materials in molecular dynamics simulations. Furthermore, several approaches to improve this parametrization were also considered. These modifications consisted in replacing the original FF atomic point charges (APC), by values calculated using quantum chemical methods, and by the implementation of a polarizable FF. The obtained results indicated that, in general, the best agreement between theoretical and experimental data is found when the OPLS-AA force field is used with the original APC or when these are replaced by ChelpG charges, computed at the MP2/aug-cc-pVDZ level of theory, for isolated molecules in the gaseous phase. If a good description of the energetic relations between the polymorphs of a compound is required then either the use of polarizable FF or the use of charges determined taking into account the vicinity of the molecules in the crystal (combining the ChelpG and MP2/cc-pVDZ methods) is recommended. Finally, it was concluded that density functional theory methods, like B3LYP or B3PW91, are not advisable for the evaluation of APC of organic compounds for molecular dynamic simulations. Instead, the MP2 method should be considered.

A systematic molecular dynamics study using large simulation boxes has been performed in order to extend the analysis of the mesoscopic segregation behavior observed in ionic liquids of the 1,3-dialkyl-imidazolium bis(trifluoromethylsulfonyl)imide homologous series, [C(n)C(mim)][Ntf2] (2 ≤ n ≤ 10, 2 ≤ m ≤ n). The analyses include the discussion of the 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 relation to the so-called intermediate peaks; and the characterization of the polar network and the nonpolar regions that are formed along the series using aggregate analyses by means of five different statistical tools. The analyses confirmed that the percolation of the nonpolar regions into a continuous domain occurs when the total number of carbon atoms in the alkyl chains exceeds six but that this is not a sufficient condition for the emergence of a distinct and intense prepeak. The existence of such a peak also requires that the longer alkyl chain contains more than a critical alkyl length (CAL) of five carbon atoms.

Despite many previous important contributions to the characterization of the liquid-liquid phase behavior of ionic liquids (ILs) plus water systems, a gap still exists as far as the effect of isomers (of ILs) is concerned. Therefore, in this work, a comprehensive study of the liquid-liquid equilibria between water and isomeric pyridinium-based ionic liquids has been performed. Atmospheric pressure mutual solubilities between water and pyridinium-based ionic liquids combined with the common anion bis[(trifluoromethyl)sulfonyl]imide were experimentally determined between (288.15 and 318.15) K. The main goal of this work is to study the isomeric effects on the pyridinium-based cation, namely, the structural and positional isomerism, as well as the alkyl side chain length. To the best of our knowledge, the influence of both structural and positional isomerism on the liquid-liquid behavior in ionic-liquid-water-containing systems is an unexplored field and is here assessed for the first time. Moreover, from the experimental solubility data, several infinite dilution molar thermodynamic functions of solution, namely, the Gibbs energy, the enthalpy, and the entropy, were estimated and discussed. In addition, aiming at gathering a broader picture of the underlying thermodynamic solvation phenomenon, molecular dynamics simulations were also carried out for the same experimental systems.