Abdenacer Idrissi

A molecular dynamics study of the urea/water mixture

The short time dynamics of the water–urea mixture is studied herein by molecular dynamics simulations. The local anisotropy of the dynamical properties arising from the highly directional hydrogen bonding interactions and their modification under the influence of increasing the urea concentration is the major focus of the present report. Study of the direction dependant translation and rotation self-diffusion coefficients of both species, as well as that of velocity autocorrelation functions and corresponding power spectra, indicate that the addition of urea leads to an overall isotropy and stiffening of the short time dynamics of both species. It is speculated that some of the well-known properties of the urea/water mixture, such as the denaturation of proteins for example, could find an explanation in these features.

Concentrated aqueous urea solutions : A molecular dynamics study of different models

Molecular dynamics studies of concentrated aqueous urea solutions, with urea mole fraction ranging from 0.04 to 0.25, have been performed, essentially focussed on the study of the tendency of urea to self-aggregate. Four of the existing urea models have been studied herein, with number of particles of N = 256 and N = 864. It was generally found that unusually long MD runs were required in order to reach proper thermodynamic equilibrium, and particularly so for the N = 256 systems. This slow convergence of the trajectories is related here to the urea self-aggregation tendency that appears in our calculation not to be limited to pairing processes only. Moreover, no evidence for urea dimerization has been found in the present calculations. Evidence for urea self-aggregation is the most visible in the structural properties, particularly in the way density–density correlations decay to their unit asymptotic limit. Densities, energies, diffusion constants, and structural data have been calculated for all models, and compared with corresponding experimental data. This comparison allows a classification of the different models in terms of their relative agreements with experimental data. Urea aggregation appears to be quite different for different models. ©2002 American Institute of Physics.

Translational Diffusion in Mixtures of Imidazolium ILs with Polar Aprotic Molecular Solvents

Self-diffusion coefficients of cations and solvent molecules were determined with (1)H NMR in mixtures of 1-n-butyl-3-methylimidazolium (Bmim(+)) tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethylsulfonyl)imide (TFSI(-)) with acetonitrile (AN), γ-butyrolactone (γ-BL), and propylene carbonate (PC) over the entire composition range at 300 K. The relative diffusivities of solvent molecules to cations as a function of concentration were found to depend on the solvent but not on the anion (i.e., IL). In all cases the values exhibit a plateau at low IL content (x(IL) < 0.2) and then increase steeply (AN), moderately (γ-BL), or negligibly (PC) at higher IL concentrations. This behavior was related to the different solvation patterns in the employed solvents. In BmimPF6-based systems, anionic diffusivities were followed via (31)P nuclei and found to be higher than the corresponding cation values in IL-poor systems and lower in the IL-rich region. The inversion point of relative ionic diffusivities was found around equimolar composition and does not depend on the solvent. At this point, a distinct change in the ion-diffusion mechanism appears to take place.

Self-association of urea in aqueous solutions: a Voronoi polyhedron analysis study.

Molecular dynamics simulation of the aqueous solutions of urea of seven different concentrations (including neat water as a reference system) has been performed on the isothermal-isobaric (N,p,T) ensemble. The ability of the urea molecules of self-association is investigated by means of the method of Voronoi polyhedra. For this purpose, all the analyses are repeated by removing one of the two components from the sample configurations and considering only the other one. In this way, the analysis of self-aggregation is reduced to the analysis of voids, a problem that can routinely be addressed by means of Voronoi analysis. The obtained results show that the urea molecules exhibit self-association behavior, which is found to be the strongest at the urea mole fraction of 0.23. However, the size of these urea aggregates is found to be rather limited; on average, they are built up by 3-4 molecules, and never exceed the size of 20-25 molecules.

Can existing models qualitatively describe the mixing behavior of acetone with water

The Helmholtz free energy of neat water, neat acetone, and acetone-water mixtures of various compositions covering the acetone mole fraction range of 0.02-0.26 is calculated at 300 K by computer simulation using the method of thermodynamic integration. In the calculations the mixtures of Kirkwood-Buff force field (KBFF) acetone with both TIP4P and SPC/E water are considered. The Helmholtz free energy of mixing calculated from the free energy difference of the mixture and of the two neat phases is found to be positive at each composition considered, indicating that the studied systems are thermodynamically unstable. The range of immiscibility is estimated to extend from the acetone mole fraction value below 0.01 to about 0.28 for both model pairs. Since a previous investigation [A. Perera and F. Sokolic, J. Chem. Phys. 121, 11272 (2004)] showed that, with the exception of SPC/E water and KBFF acetone, acetone-water model pairs exhibit demixing behavior, the present result points out that currently no existing acetone model can qualitatively reproduce the phase behavior of acetone-water mixtures, i.e., the well known experimental fact that acetone is miscible with water in any proportion.

Free Energy of Mixing of Acetone and Methanol: A Computer Simulation Investigation

The change of the Helmholtz free energy, internal energy, and entropy accompanying the mixing of acetone and methanol is calculated in the entire composition range by the method of thermodynamic integration using three different potential model combinations of the two compounds. In the first system, both molecules are described by the OPLS, and in the second system, both molecules are described by the original TraPPE force field, whereas in the third system a modified version of the TraPPE potential is used for acetone in combination with the original TraPPE model of methanol. The results reveal that, in contrast with the acetone-water system, all of these three model combinations are able to reproduce the full miscibility of acetone and methanol, although the thermodynamic driving force of this mixing is very small. It is also seen, in accordance with the finding of former structural analyses, that the mixing of the two components is driven by the entropy term corresponding to the ideal mixing, which is large enough to overcompensate the effect of the energy increase and entropy loss due to the interaction of the unlike components in the mixtures. Among the three model combinations, the use of the original TraPPE model of methanol and modified TraPPE model of acetone turns out to be clearly the best in this respect, as it is able to reproduce the experimental free energy, internal energy, and entropy of mixing values within 0.15 kJ/mol, 0.2 kJ/mol, and 1 J/(mol K), respectively, in the entire composition range. The success of this model combination originates from the fact that the use of the modified TraPPE model of acetone instead of the original one in these mixtures improves the reproduction of the entropy of mixing, while it retains the ability of the original model of excellently reproducing the internal energy of mixing.

Hydration free energy difference of acetone, acetamide, and urea

The hydration free energy and hydration entropy difference of urea and acetone, and of acetamide and acetone have been calculated both by free energy perturbation and by the method of thermodynamic integration. The obtained results show a striking asymmetry between the thermodynamic changes accompanying the replacement of the first and second CH(3) group of acetone by NH(2). Thus, the first CH(3)NH(2) exchange is found to lead to an about 10 kJmol decrease in the energy, 8 kJmol decrease in the Helmholtz free energy, and 5-10 Jmol K decrease in the entropy of hydration, while similar values accompanying the second CH(3)NH(2) exchange are found to be about -65 kJmol, -35 kJmol and -100 Jmol K, respectively. These results indicate that the two NH(2) groups of the urea molecule have a strong synergetic effect on the thermodynamics of the hydration of urea. The fact that the replacement of the two CH(3) groups of acetone by NH(2) leads to a strong decrease in the hydration entropy indicates that urea clearly has an ordering effect on nearby water.

The study of aqueous isopropanol solutions at various concentrations: low frequency Raman spectroscopy and molecular dynamics simulations

The low-frequency Raman spectra of aqueous isopropanol mixtures at different proportions have been recorded. The influence of the isopropanol concentration on the position of the peak at n2 ¼ 190 cm 21 associated to hydrogen bond between water molecules has been analyzed. The small shift to lower frequencies of the n2 band indicates that isopropanol has a small effect of a structure breaker of hydrogen bond between water molecules. This result is correlated to the degree of self aggregation at a microscopic level of isopropanol. q 2003 Elsevier Science B.V. All rights reserved.

Detailed insight into the hydrogen bonding interactions in acetone–methanol mixtures. A molecular dynamics simulation and Voronoi polyhedra analysis study

Voronoi polyhedra (VP) analysis of mixtures of acetone and methanol is reported on the basis of molecular dynamics computer simulations, performed at 300 K and 1 bar. The composition of the systems investigated covers the entire range from neat acetone to neat methanol. Distribution of the volume, reciprocal volume and asphericity parameter of the VP as well as that of the area of the individual VP faces and of the radius of the empty voids located between the molecules are calculated. To investigate the tendency of the like molecules to self-associate the analyses are repeated by disregarding one of the two components. The self-aggregates of the disregarded component thus turn into large empty voids, which are easily detectable in VP analysis. The obtained results reveal that both molecules show self-association, but this behavior is considerably stronger among the acetone than among the methanol molecules. The strongest self-association of the acetone and methanol molecules is found in their mole fraction ranges of 02-0.5 and 0.5-0.6, respectively. The caging effect around the methanol molecules is found to be stronger than around acetones. Finally, the local environment of the acetone molecules turns out to be more spherical than that of the methanols, not only in the respective neat liquids, but also in their mixtures.

Investigation of the Local Structure in Sub and Supercritical Ammonia Using the Nearest Neighbor Approach: A Molecular Dynamics Analysis

Molecular dynamics simulations of ammonia were performed in the (N,P,T) ensemble along the isobar 135 bar and in the temperature range between 250 and 500 K that encompasses the sub and supercritical states of ammonia. Six simple interaction potential models (Lennard-Jones pair potential between the atomic sites, plus a Coulomb interaction between atomic partial charges) of ammonia reported in the literature were analyzed. Liquid-gas coexistence curve, critical temperature, and structural data (radial distribution functions) have been calculated for all models and compared with the corresponding experimental data. After choosing the appropriate potential model, we have investigated the local structure by analyzing the nearest neighbor radial, mutual orientation, and interaction energy distributions. The change in the local structure was traced back to the change of the nonlinear behavior (which is more pronounced at low temperatures) of the average distance between a reference ammonia molecule and its subsequent nearest neighbor. Our results suggest to use the position of the maximum in the fluctuation of the average distance to define the border of the first solvation shell (particularly at high temperature when the minimum of the radial distribution is not well-defined). Indeed, the effect of the temperature on the position of this maximum shows clearly that the spatial extent of the solvation shell increases with a concomitant decrease of the involved number of ammonia molecules. Furthermore, our results show that the signature of the hydrogen bonding is mainly observed for temperature below 300 K. This signature is quantified by a short distance contribution to the closest radial nearest neighbor distribution, by a strong mutual orientation (defined by the angles between the axis joining the nitrogen atoms and the molecular axes) and by a strong attractive character of the total interaction energy.