42 research outputs found
Molecular structural order and anomalies in liquid silica
The present investigation examines the relationship between structural order,
diffusivity anomalies, and density anomalies in liquid silica by means of
molecular dynamics simulations. We use previously defined orientational and
translational order parameters to quantify local structural order in atomic
configurations. Extensive simulations are performed at different state points
to measure structural order, diffusivity, and thermodynamic properties. It is
found that silica shares many trends recently reported for water [J. R.
Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate
densities, the distribution of local orientational order is bimodal. At fixed
temperature, order parameter extrema occur upon compression: a maximum in
orientational order followed by a minimum in translational order. Unlike water,
however, silica's translational order parameter minimum is broad, and there is
no range of thermodynamic conditions where both parameters are strictly
coupled. Furthermore, the temperature-density regime where both structural
order parameters decrease upon isothermal compression (the structurally
anomalous regime) does not encompass the region of diffusivity anomalies, as
was the case for water.Comment: 30 pages, 8 figure
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
Ion association in concentrated NaCI brines from ambient to supercritical conditions: results from classical molecular dynamics simulations
Highly concentrated NaCl brines are important geothermal fluids; chloride complexation of metals in such brines increases the solubility of minerals and plays a fundamental role in the genesis of hydrothermal ore deposits. There is experimental evidence that the molecular nature of the NaClâwater system changes over the pressureâtemperature range of the Earth's crust. A transition of concentrated NaClâH(2)O brines to a "hydrous molten salt" at high P and T has been argued to stabilize an aqueous fluid phase in the deep crust. In this work, we have done molecular dynamic simulations using classical potentials to determine the nature of concentrated (0.5â16 m) NaClâwater mixtures under ambient (25°C, 1 bar), hydrothermal (325°C, 1 kbar) and deep crustal (625°C, 15 kbar) conditions. We used the well-established SPCE model for water together with the Smith and Dang Lennard-Jones potentials for the ions (J. Chem. Phys., 1994, 100, 3757). With increasing temperature at 1 kbar, the dielectric constant of water decreases to give extensive ion-association and the formation of polyatomic (Na(n)Cl(m))(n-m )clusters in addition to simple NaCl ion pairs. Large polyatomic (Na(n)Cl(m))(n-m )clusters resemble what would be expected in a hydrous NaCl melt in which water and NaCl were completely miscible. Although ion association decreases with pressure, temperatures of 625°C are not enough to overcome pressures of 15 kbar; consequently, there is still enhanced NaâCl association in brines under deep crustal conditions
Ătude par monte carlo de la solvatation en phase gazeuse des ions halogĂ©nures par des molĂ©cules protiques (H
Les agrégats ioniques [math] avec X = Cl, Br, I et n = 1,..., 8, ont été étudiés par simulation de Monte-Carlo. En utilisant des potentiels semi-empiriques incluant les effets de polarisation, il a été possible de retrouver, avec une bonne précision, les enthalpies d'agrégation expérimentales disponibles dans la littérature. Les résultats des simulations montrent que la propension des solvants protiques (eau et methanol) à stabiliser plus fortement les agrégats ioniques que ne le font les solvants
aprotiques (acétonitri1e et acétone) résulte, chez les premiers, de
l'instauration d'un réseau de liaisons hydrogÚne entre les molécules de solvant, coopérativité qui est absente chez les solvants aprotiques. En outre, le nombre de coordination, la géométrie de la premiÚre couche ainsi que l'effet dû à la taille de l'ion sont discutés en détail
Composés nitroaromatiques cytotoxiques et radiosensibilisateurs : études cinétiques de la dismutation des radicaux par radiolyse pulsée
Les composĂ©s nitroaromatiques ArNO2 possĂšdent des propriĂ©tĂ©s radiosensibilisatrices in vitro. Ces composĂ©s sont aussi des antibiotiques envers des cellules hypoxiques ou anoxiques. Ces propriĂ©tĂ©s pharmacologiques sont reliĂ©es Ă la rĂ©duction des ArNO2 par des rĂ©actions catalysĂ©es ou non par une nitro-rĂ©ductase, pour former d'abord le radical [math]- ou [math].L'Ă©tude, par radiolyse pulsĂ©e, de l'interaction du radical nitro avec divers extraits cellulaires ou enzymes pures, ayant une activitĂ© de nitro-rĂ©ductase, d'oxygĂšne-rĂ©ductase, de superoxyde dismutase ou de nitrite-rĂ©ductase, conduit Ă conclure Ă l'absence d'une activitĂ© de nitro-radical dismutase. Dans les cellules hypoxiques, le radical nitro disparaĂźt probablement par la rĂ©action de dismutation. Cette Ă©tape non catalysĂ©e pourrait donc ĂȘtre une des Ă©tapes cinĂ©tiques limitantes du processus cellulaire de rĂ©duction de ArNO2
Radicaux libres de composés nitroaromatiques cytotoxiques et radiosensibilisateurs : paramÚtres cinétiques et interactions avec des oxydases à cuivre
Les interactions du radical nitro de composĂ©s ArNO2 avec deux oxydases Ă cuivre, la laccase et la cĂ©ruloplasmine, ont Ă©tĂ© Ă©tudiĂ©es par radiolyse pulsĂ©e en observant les variations d'absorbance vers 410 nm, maximum du radical, et vers 610 nm, maximum du site Cu2+ de type 1. On observe un transfert d'Ă©lectron rapide (1x106 Ă 4x107 M-1s-1) entre le radical et Cu2+de type 1. Le site n'est pas rĂ©oxyde par ArNO2, mais l'est par O2. Cette rĂ©action de transfert d'Ă©lectron ne correspond pas Ă une rĂ©action enzymatique. Ces deux oxydases n'ont d'activitĂ© ni de nitroreductase ni de nitro-radical dismutase.La concentration plasmatique normale de la cĂ©ruloplasmine, voisine de 2 ”M, peut atteindre 10 ”M dans le fluide interstitiel de tumeurs ou dans des cas d'inflammation. La rĂ©action observĂ©e entre radical nitro et cĂ©ruloplasmine peut correspondre Ă un cycle "futile" au mĂȘme titre que celui observĂ© avec O2, et donc avoir de l'importance dans la pharmacocinĂ©tique des composĂ©s nitroaromatiques
The partial pair correlation functions of dense supercritical water
Neutron diffraction measurements of heavy water and of two isotopic
mixtures at supercritical state ( and ) are presented. In combining the set of neutron diffraction
data with previous X-rays measurements of Yamanaka et al. (J. Chem. Phys.,
101 (1994) 9830), it has been possible by using a Monte Carlo method to reach the
partial pair correlation functions , and .
The results are compared with molecular-dynamics simulations using the SPCE pair
potential for water. These new results confirm that hydrogen bonding is still present in
dense supercritical water