Transport Properties of Systems with Long Range Interactions
The aim of this project is to develop algorithms for equilibrium and non-equilibrium molecular dynamics simulation of molecular systems with electrostatic interactions, in order to apply them to three different problems. The first is related to the intrinsic time-periodicity of phase variables occurring in the simulation of shear flow using the standard Sllod algorithm. Periodic time dependence of the response is of practical interest because it can cause wrong estimates of viscosity in sheared fluids, and of fundamental interest because it is not clear whether its origin is in the change of structure with lattice strain in equilibrium, or if it is purely dynamical. The second aims to relate the structural changes in associated liquids and electrolytes to anomalous behaviour of their transport properties. In particular, the aim is to find the structural reasons for the sign reversal in the Soret coefficient of salt solution in water with increase in salt concentaration. The third concerns dynamical properties of a solution of ions in water confined between charged walls and forming an electrical double layer, a problem which has applications in biological systems.
Principal Investigator Janka Petravic |
Project x31 Facilities Used SC |
RFCD Codes 250602, 250699 |
Significant Achievements, Anticipated Outcomes and Future Work
The first year of this project has been devoted to development and testing of programs. The basic equilibrium programs for rigid and (semi-) flexible molecules with partial charges were tested on models for xylene isomers and ethanol. Xylene isomers are aromatics occuring in crude oil and the possibility of estimating their transport properties using molecular simulation is of interest to oil industry. It was shown that the available force field gives a sufficiently good description of intermolecular interactions to reproduce not only densities and viscosities for a wide range of temperatures and pressures, but also their peculiar counter-intuitive order. The equilibrium simulation of ethanol verified that the code was able to reproduce published simulation results for the dependence of diffusion coefficient on temperature, and was subsequently used to determine the influence of pressure on static structure and stability of hydrogen bonds and consequently on transport. The way in which different internal degrees of freedom (such as torsion and bending) influence diffusion was also explored.
The Sllod program for molecular systems with electrostatic interactions applied to liquid ethanol gave excellent agreement when extrapolated to equilibrium Green-Kubo results. It showed that in ethanol under strong shear, hydrogen bonded chains would become longer, more aligned with the x-axis and have less branching, and hydrogen bonds would become less stable. This results in faster decrease of viscosity with shear rate in ethanol than in say, butane.
Another test system was molten sodium chloride. After verifying that shear viscosity from Sllod calculation extrapolates to Green-Kubo zero-shear result, we determined maximum shear rate for which the standard kinetic temperature control is applicable. For higher shear rates, if linear streaming velocity profile is assumed with kinetic thermostat, one obtains wrong structural characterisics of the mixture. This can be avoided by controlling configurational temperature, which has particularly simple form for ionic systems. This system will be used to show that, when infinitely-ranged interactions are present, the amplitude of Sllod oscillations of shear viscosity is much larger than for Lennard-Jones systems and is present even for systems with very large number of particles.
Thermal conductivity of water using was calculated using a newly developed expression for heat flux in systems with electrostatic interactions, and was found to give good agreement with experimental data. This algorithm will be used in the future to find if the dependence of thermal conductivity on concentration of dissolved sodium chloride ions is well represented, and to calculate thermal diffusion coefficient of this mixture depending on concentration. It will also be applied to calculation of diffusion coefficients in the electrical double layer problem.
Computational Techniques Used
In all cases, molecular dynamics programs with Ewald summation were used, with special adaptations made for the case of Sllod algorithm and its peculiar periodic boundary conditions.
Publications, Awards and External Funding
There are 3 publications submitted, but they have not yet been accepted.