P. Damay

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.

Structure of powder deuteroammonia between 2 and 180 K revisited: A refinement of the neutron diffraction pattern taking into account molecular reorientations: analysis of the diffuse intensity

The standard Rietveld profile analysis is a powerful tool for refining the structure of powder crystals. Nevertheless, in case of molecular crystals, where rigid groups undergo reorientations of large amplitude in well‐defined directions, thermal ellipsoids, even with anisotropic terms, may not be sufficient to account for the motion. That is the case for deuteroammonia for which strong librations of the ND3 groups are observed even at 2 K. In this paper we present a refinement of the structure of deuteroammonia using a model allowing the ND3 group to perform reorientations about the easy C3 axis of the cubic lattice; the adjustment of diffraction patterns measured at 2, 78, and 180 K shows the temperature dependence of the librational amplitude. The geometry of the ammonia molecule in the crystal is found to be the same as in the gas phase [rN–D=1.008(4) A] and is in excellent agreement with that determined by the analysis of the intramolecular structure factor for large momentum transfers; furthermore t...

On the structure of solid Ca(ND3)x; with x≈6

The bcc structure of Ca(ND3)5.7 is found to be stable down to He temperatures. The crystal is characterized by frozen‐in orientational disorder and also positional disorder of the ND3 molecules. The latter may be explained by a distribution of octahedrally coordinated and tetrahedrally coordinated Ca ions. The relation of occupation numbers with stoichiometry cannot be made quantitatively, however. There are no indications of a significant departure from the geometry of a free ammonia molecule in the solid.

Analysis of the transverse and the longitudinal pseudodiffusion of CO2 in sub- and supercritical states: a molecular-dynamics analysis.

We have performed molecular-dynamics simulations of CO(2) system along the gas-liquid coexistence curve and on the isochore 94.22 cm(3) mol(-1) (which corresponds to the critical isochore). The calculation has been carried out in order to analyze the diffusion of CO(2) and particularly to figure out how the diffusion coefficient may be decomposed along the molecular axes. This makes it possible to analyze the anisotropy of the diffusion along these axes and to shed light on the microscopic changes which accompany such behavior. This anisotropy is traced back to the effect of the translation-rotation coupling (TRC) along the molecular axes. Along the liquid-gas coexistence curve, the pseudolongitudinal diffusion is found to be more rapid than the transverse one. The opposite trend is found along the isochore 94.22 cm(3) mol(-1). The role of the local structure was explored by calculating intermediate scattering function and the autocorrelation functions for the forces acting along the molecular axes. It is shown that the strength of the TRC effect is correlated to the difference between the relaxation times of the local structure, that of the reorientation along the molecular axes, and that of the translational motion. The analysis of the correlation time and the average mean square force along the longitudinal and transverse directions confirms the anisotropy of the local environment that determines the translational dynamics of a molecule.

Low-frequency Raman spectra of sub- and supercritical CO2 : Qualitative analysis of the diffusion coefficient behavior

We report the results of the low-frequency Raman experiments on CO(2) which were carried out in a wide density range, along the liquid-gas coexistence curve in a temperature range of 293-303 K, and on the critical isochore of 94.4 cm(3) mol(-1) in a temperature range of 304-315 K. In our approach, the qualitative behavior of the diffusion coefficient D is predicted, assuming the following: first, that the low-frequency Raman spectra can be interpreted in terms of the translation rotation motions; second, that the random force could be replaced by the total force to calculate the friction coefficient; and finally, that the Einstein frequency is associated with the position of the maximum of the low-frequency Raman spectrum. The results show that the diffusion coefficient increases along the coexistence curve, and its values are almost constant on the critical isochore. The predicted values reproduce qualitatively those obtained by other techniques. The values of D were also calculated by molecular-dynamics simulation and they qualitatively reproduce the behavior of D.

The critical scattering universal function measured through sans: An experimental test of the scaling laws

Abstract Recent measurements of the critical scattering from a simple-liquid system (Xenon) by means of small-angle neutron scattering (SANS) are reported. A study of the critical scattering function g ( qξ ) was performed, leading to a determination of the critical indices and a test of the validity of the scaling hypothesis. Here we report the results of the preliminary data analysis.

A precise determination of the critical index η by Small Angle Neutron Scattering (SANS)

Abstract A precise determination of the critical index η has been obtained by SANS on the sodium-ND 3 mixture. The values η = 0.0317 ± 0.0013 8 and 0.0302 ± 0.0015 7 have been found for two experiments, using the Fisher/Langer scattering formalism. These results reconciliate SANS and light scattering experiments and are in favor of renormalization group theory predictions.

Quantum Rotation of Ammonia Molecules in Disordered Ca(NH3)6

Calcium hexaammine is a low electron density, metallic compound. In this expanded metal, the ammonia molecules show important rotational and reorientational motions down to 20 K as deduced from the study of the quasielastic neutron scattering ( LECLERCQ et al./1/ ). A complete understanding of the inelastic scattering needs a simultaneous determination of the structure in the whole temperature range. Since the calcium compound crystallizes in a high symmetry group (bcc) and since no phase transition occurs as the temperature is decreased, the ammonia molecules appear to be in a frozen-in orientationally disordered state at low temperature. For temperature lower than 10 K, the Inelastic Neutron Scattering spectrum shows a very asymmetric line shape with a maximum near the energy of uniaxial rotation of the NH3 / ND3 groups. This is interpreted by quantum rotation in a (random) distribution of low hindering potentials. It is shown that these properties are very sensitive to non-stoichiometry effects.

THERMOELECTRIC POWER AND THOMSON COEFFICIENT IN THE INTERMEDIATE RANGE OF CONCENTRATIONS

The thermoelectric power of the K-NH3 solutions in the concentration range x2 = 1 to 8.8 per cent has been measured as a function of temperature between — 65° and — 20°. The method used gives fast readings and might be applied to less stable solutions. The negative temperature coefficient and large anomaly of the thermoelectric power in the 1.5 to 2.7 per cent region have been related to formation of small aggregates of solvated cations over which are delocalized the electrons. The anomalies in compressibility, and temperature and pressure effects on electrical conductivity, at about two per cent, have been interpreted in the same way.

Critical Slowing Down in a Simple Fluid System as Measured by Neutron Spin Echo Technique. Crossover Between Hydrodynamics and Critical Dynamics

Both the mesoscopic structure (as seen by small angle neutron scattering) and the dynamics (as seen by neutron spin echo technique) of the critical fluctuations were measured in CO2 close to the liquid-gas phase transition. The static aspects of the critical behavior of CO2 were first measured on the critical isochore for reduced temperature from 6 10−5 to 6 10−2 (t = (T-Tc)/Tc, Tc the critical temperature) allowing to characterize precisely the critical behaviour of the correlation length ξ of the density fluctuations. Then, the dynamics was probed over the Q-interval 0.025–0.21 A−1 for correlation lengths ξ ranging between 10 A and 400 A. Neutrons are well suited for studying the critical regime (Qξ ≫ 1) and the crossover region between the hydrodynamic and critical regimes (Qξ∼1). The measured widths of the Rayleigh line for values of Qξ ranging from 0.3 to 32 are in very good agreement with the Kawasaki function proposed to describe the crossover.