Patrice Bordat

Does the interaction potential determine both the fragility of a liquid and the vibrational properties of its glassy state

By performing molecular dynamics simulations of binary Lennard-Jones systems with three different potentials, we show that the increase of anharmonicity and capacity for intermolecular coupling of the potential is the cause of (i) the increase of kinetic fragility and nonexponentiality in the liquid state, and (ii) the T(g)-scaled temperature dependence of the nonergodicity parameter determined by the vibrations at low temperatures in the glassy state. Naturally, these parameters correlate with each other, as observed experimentally by T. Scopigno et al. [Science 302, 849 (2003)]

The shear viscosity of molecular fluids: A calculation by reverse nonequilibrium molecular dynamics

The reverse nonequilibrium molecular dynamics [F. Muller-Plathe, Phys. Rev. E 49, 359 (1999)] presented for the calculation of the shear viscosity of Lennard-Jones liquids has been extended to atomistic models of molecular liquids. The method is improved to overcome the problems due to the detailed molecular models. The new technique is besides a test with a Lennard-Jones fluid, applied on different realistic systems: liquid nitrogen, water, and hexane, in order to cover a large range of interactions and systems/architectures. We show that all the advantages of the method itemized previously are still valid, and that it has a very good efficiency and accuracy making it very competitive.

Watching the Photo-Oxidation of a Single Aromatic Hydrocarbon Molecule

The photooxidation of single dye molecules can be followed by confocal fluorescence microscopy. The self-sensitized reaction with singlet oxygen leads to a suite of products, which may be differentiated spectrally. Tentative structures for certain photoproducts have been obtained from quantum-chemical calculations.

The influence of interaction details on the thermal diffusion in binary Lennard-Jones liquids

There exists a disturbing controversy in the literature about the sign of the Soret effect in binary mixtures of model fluids (Lennard-Jones atoms), whose components differ only in their molecular diameter. For such mixtures, the dependence of the Soret coefficient on the state (liquid versus supercritical), on the system size and on details of handling the range and the cutoff of the Lennard-Jones potential is examined by molecular-dynamics simulations. We establish unambiguously the direction of the Soret effect: Under all circumstances investigated, large particles are driven to the hot region. At supercritical densities, the Soret effect is considerably smaller than in the dense liquid and, furthermore, details of the attractive tail of the Lennard-Jones potential become much more important.

The breakdown of the Stokes-Einstein relation in supercooled binary liquids

Using reverse non-equilibrium molecular dynamics simulations, we report the calculation of the shear viscosity and the tracer diffusion coefficient of a binary Lennard-Jones mixture that is known as a model glass-former. Several remarkable temperatures are well reproduced in our calculations, i.e.?TS (the onset of slow dynamics), Tc (the critical temperature predicted by the mode-coupling theory) and TK (the Kauzmann temperature). A breakdown of the Stokes?Einstein relation is found at temperature TS. We propose that, at low temperatures below TS, the size of single-particle positional fluctuations between particle-hopping events corresponds to the length measured by the Stokes?Einstein relation, which is equated to the hydrodynamic radius of particles at high temperatures.

An improved dimethyl sulfoxide force field for molecular dynamics simulations

A united-atom molecular simulation force field for liquid dimethyl sulfoxide has been found to produce unacceptably inaccurate densities when used with a reaction-field or Ewald treatment of the electrostatic interactions. The force field is mildly reparameterized leading to a smaller dipole moment and slightly larger methyl groups. In addition to being compatible with the more sophisticated treatment of Coulombic interactions, the new force field also results in a significantly better description of the diffusion coefficient, the shear viscosity and the dielectric constant. Other liquid properties remain at the satisfactory quality of the Liu et al. model.

Breakdown of the Stokes–Einstein relation in Lennard-Jones glassforming mixtures with different interaction potential

The breakdown of the Stokes–Einstein relation was investigated for three glass-forming models composed of mixtures of Lennard-Jones A-B particles, which have been constructed by modifying the shape of the interaction potential between A particles. By performing molecular dynamics simulations, we show that these mixtures intrinsically possess different organizations. The breakdown of the Stokes–Einstein relation particularly occurs at different temperatures for each type of particles and it is directly related to the dynamical decoupling between A and B particles and the formation or not of paths where fast particles show jumplike motions. The effective size of each particles and the fraction of slow and fast particles were also determined. Similarity with silicate glasses including mixed alkali effect is discussed.

Molecular mechanisms of photo-induced spectral diffusion of single terrylene molecules in p terphenyl

An empirical molecular model of crystals of p-terphenyl doped with terrylene is developed and compared with recent experiments on single guest molecules. The model provides an assignment of the experimental electronic origins to the substitution sites and an interpretation of the photo-induced frequency jumps of single terrylene molecules. The Stark shifts of terrylene in the different sites are estimated by a coupled Hartree–Fock semi-empirical calculation. The topology of the many-dimensional energy surface of the X1 electronic origin is explored with simulated annealing.

Investigation of molecular dimers by ensemble and single molecule spectroscopy

We have investigated molecular dimers with different electronic coupling strengths by bulk and single molecule spectroscopy. In one of the dimers the two monomers (perylene-monoimide) are directly connected via a single bond while in the other one they are separated by the benzil motif. The close proximity of the monomers in the first case gives rise to excitonic band splitting which is clearly observable in the bulk absorption spectra. For the benzil structure the electronic interactions are governed by Forster-type energy hopping between the monomers. Fluorescence intensity trajectories at the single molecule level show one-step and two-step bleaching behaviour which appears to be very similar for both dimers. However, emission spectra recorded simultaneously with the trajectories indicate spectral changes which allow to distinguish between weakly and strongly coupled dimers. In the latter case the spectral shape changes significantly when excitonic coupling has been lifted because of photochemical transformation of one of the monomers.

Association of indigo with zeolites for improved color stabilization.

The durability of an organic color and its resistance against external chemical agents and exposure to light can be significantly enhanced by hybridizing the natural dye with a mineral. In search for stable natural pigments, the present work focuses on the association of indigo blue with several zeolitic matrices (LTA zeolite, mordenite, MFI zeolite). The manufacturing of the hybrid pigment is tested under varying oxidizing conditions, using Raman and ultraviolet–visible (UV-Vis) spectrometric techniques. Blending indigo with MFI is shown to yield the most stable composite in all of our artificial indigo pigments. In the absence of defects and substituted cations such as aluminum in the framework of the MFI zeolite matrix, we show that matching the pore size with the dimensions of the guest indigo molecule is the key factor. The evidence for the high color stability of indigo@MFI opens a new path for modeling the stability of indigo in various alumino-silicate substrates such as in the historical Maya Blue pigment.