60 research outputs found
Thermodynamics of Mixing Water with Dimethyl Sulfoxide, as Seen from Computer Simulations
The Helmholtz free energy, energy, and entropy of mixing of eight different models of dimethyl sulfoxide (DMSO) with four widely used water models are calculated at 298 K over the entire composition range by means of thermodynamic integration along a suitably chosen thermodynamic path, and compared with experimental data. All 32 model combinations considered are able to reproduce the experimental values rather well, within RT (free energy and energy) and R (entropy) at any composition, and quite often the deviation from the experimental data is even smaller, being in the order of the uncertainty of the calculated free energy or energy, and entropy values of 0.1 kJ/mol and 0.1 J/(mol K), respectively. On the other hand, none of the model combinations considered can accurately reproduce all three experimental functions simultaneously. Furthermore, the fact that the entropy of mixing changes sign with increasing DMSO mole fraction is only reproduced by a handful of model pairs. Model combinations that (i) give the best reproduction of the experimental free energy, while still reasonably well reproducing the experimental energy and entropy of mixing, and (ii) that give the best reproduction of the experimental energy and entropy, while still reasonably well reproducing the experimental free energy of mixing, are identified
Local structure of dilute aqueous DMSO solutions, as seen from molecular dynamics simulations
The information about the structure of dimethyl sulfoxide (DMSO)-water mixtures at relatively low DMSO mole fractions is an important step in order to understand their cryoprotective properties as well as the solvation process of proteins and amino acids. Classical MD simulations, using the potential model combination that best reproduces the free energy of mixing of these compounds, are used to analyze the local structure of DMSO-water mixtures at DMSO mole fractions below 0.2. Significant changes in the local structure of DMSO are observed around the DMSO mole fraction of 0.1. The array of evidence, based on the cluster and the metric and topological parameters of the Voronoi polyhedra distributions, indicates that these changes are associated with the simultaneous increase of the number of DMSO-water and decrease of water-water hydrogen bonds with increasing DMSO concentration. The inversion between the dominance of these two types of H-bonds occurs around X-DMSO = 0.1, above which the DMSO-DMSO interactions also start playing an important role. In other words, below the DMSO mole fraction of 0.1, DMSO molecules are mainly solvated by water molecules, while above it, their solvation shell consists of a mixture of water and DMSO. The trigonal, tetrahedral, and trigonal bipyramidal distributions of water shift to lower corresponding order parameter values indicating the loosening of these orientations. Adding DMSO does not affect the hydrogen bonding between a reference water molecule and its first neighbor hydrogen bonded water molecules, while it increases the bent hydrogen bond geometry involving the second ones. The close-packed local structure of the third, fourth, and fifth water neighbors also is reinforced. In accordance with previous theoretical and experimental data, the hydrogen bonding between water and the first, the second, and the third DMSO neighbors is stronger than that with its corresponding water neighbors. At a given DMSO mole fraction, the behavior of the intensity of the high orientational order parameter values indicates that water molecules are more ordered in the vicinity of the hydrophilic group while their structure is close-packed near the hydrophobic group of DMSO. Published by AIP Publishing
Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations
Description of the local microscopic
structure in ionic liquids
(ILs) is a prerequisite to obtain a comprehensive understanding of
the influence of the nature of ions on the properties of ILs. The
local structure is mainly determined by the spatial arrangement of
the nearest neighboring ions. Therefore, the main interaction patterns
in ILs, such as cation–anion H-bond-like motifs, cation–cation
alkyl tail aggregation, and ring stacking, were considered within
the framework of the nearest-neighbor approach with respect to each
particular interaction site. We employed classical molecular dynamics
(MD) simulations to study in detail the spatial, radial, and orientational
relative distribution of ions in a set of imidazolium-based ILs, in
which the 1-butyl-3-methylimidazolium (C<sub>4</sub>mim<sup>+</sup>) cation is coupled with the acetate (OAc<sup>–</sup>), chloride
(Cl<sup>–</sup>), tetrafluoroborate (BF<sub>4</sub><sup>–</sup>), hexafluorophosphate (PF<sub>6</sub><sup>–</sup>), trifluoromethanesulfonate
(TfO<sup>–</sup>), or bis(trifluoromethanesulfonyl)amide (TFSA<sup>–</sup>) anion. It was established that several structural
properties are strongly anion-specific, while some can be treated
as universally applicable to ILs, regardless of the nature of the
anion. Namely, strongly basic anions, such as OAc<sup>–</sup> and Cl<sup>–</sup>, prefer to be located in the imidazolium
ring plane next to the C–H<sup>2/4–5</sup> sites. By
contrast, the other four bulky and weakly coordinating anions tend
to occupy positions above/below the plane. Similarly, the H-bond-like
interactions involving the H<sup>2</sup> site are found to be particularly
enhanced in comparison with the ones at H<sup>4–5</sup> in
the case of asymmetric and/or more basic anions (C<sub>4</sub>mimOAc,
C<sub>4</sub>mimCl, C<sub>4</sub>mimTfO, and C<sub>4</sub>mimTFSA),
in accordance with recent spectroscopic and theoretical findings.
Other IL-specific details related to the multiple H-bond-like binding
and cation stacking issues are also discussed in this paper. The secondary
H-bonding of anions with the alkyl hydrogen atoms of cations as well
as the cation–cation alkyl chain aggregation turned out to
be poorly sensitive to the nature of the anion
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