Transport Properties of Systems with Long Range Interactions

Significant Achievements, Anticipated Outcomes and Future Work

We investigated transport properties of molten salts, such as shear viscosity and electrical conductivity, in dc and ac fields using non-equilibrium molecular dynamics. In particular, we studied the influence of the thermostat used in simulation on nonlinear response. Molten salts were shown to have a larger Newtonian region of shear viscosity than simple liquids, and the choice of thermostatting method had little influence on the results for the investigated range of shear rates. For large fields, quantitative differences of unexpected size can be seen in the melt. In the supercritical fluid, different thermostats predict qualitatively different behaviour and structure. While the kinetic-type thermostats predict increased association of ions in the field, configurational thermostat predicts enhanced dissociation. In an alternating field, the response is linear for high frequencies, whereas for low frequencies it contains higher harmonics of the field. The time-dependent shape of the response can be deduced from the knowledge of currents in dc fields. Only the first harmonic of this current contributes to the dissipation. We plan to expand our investigation to structure and transport of polyvalent molten salts and room temperature ionic liquids. We studied the change of properties of the Lennard-Jones system under strain for a number of temperatures along a high-density isochore crossing the liquid-solid coexistence region. Larger systems (>500 atoms) exhibit the known properties of solids and liquids. While a solid can store the deformation energy and sustain shear stress proportional to strain, in a liquid the deformation energy and stress can be stored only for a short period of time, after which the energy dissipates and the stress relaxes. Relaxation is related to the propagation and dissipation of the transverse momentum current. In a smaller liquid system in the “strained” periodic boundary conditions shear stress does not relax completely – the transverse momentum current does not dissipate over the wavelengths of the order of the simulation box length. The system size necessary for the shear stress to vanish in the strained boundary conditions is related to the size of cooperative effects in stress relaxation. It decreases with the increase in temperature and is smaller for softer potentials. Its origin is in the possibility of configurational rearrangement of a group of atoms independently of its environment, like in the “cooperatively rearranging regions” in the Adam-Gibbs theory. Future plans include studies of rearrangement and phase transitions in ionic systems.

Computational Techniques Used

In the molten sodium chloride part of the project, we used nonequilibrium molecular dynamics methods coupled with a kinetic profile-biased and configurational profile-unbiased thermostat. The algorithm for electric conductivity simply involves direct substitution of electric field in the Newton’s equations of motion and adding a thermostat. For simulations of shear flow and determination of viscosity we used the so-called “Sllod” algorithm with “sliding brick” periodic boundary conditions. Ewald sum was used to calculate electrostatic interactions, and in the case for shear flow it was adapted to the periodically deforming geometry present in the shearing periodic boundary conditions. Shear stress relaxation was studied using equilibrium methods and transient response nonequilibrium simulations.

Publications, Awards and External Funding

External Funding and Awards

None.

Publications

1. J. Petravic and J. Delhommelle, “Influence of temperature, pressure and internal degrees of freedom on hydrogen bonding and diffusion in liquid ethanol”, Chem. Phys. 286, 303-314 (2003)
2. J. Delhommelle and J. Petravic, “Shear viscosity of molten sodium chloride”, J. Chem. Phys. 118, 2783-2791 (2003).
3. J. Petravic and J. Delhommelle, “Conductivity of molten sodium chloride and its supercritical vapor in strong dc electric fields”, J. Chem. Phys 118,. 7477-7485 (2003).
4. J. Petravic, “Some properties of isolated systems in external fields”, Phys. Rev. E 68, 011104/1-9, (2003).
5. J. Petravic and J. Delhommelle, “Conductivity of molten sodium chloride in an alternating electric field”, J. Chem. Phys. 119, 8511-8518 (2003).
6. J. Petravic and J. Delhommelle, “Non-equilibrium molecular dynamics simulations of molten sodium chloride”, Proceedings of the 15th Symposium on Thermophysical Properties, Boulder, Colorado, USA, June 22-27, 2003.