Molecular Simulation of the Phase Behaviour of Fluids

                 

Principal Investigator

Richard J. Sadus

School of Information Technology

Swinburne University of Technology

Co-Investigators

Gianluca Marcelli

Ya Song Wei

School of Information Technology

Swinburne University of Technology

 

Projects

g36 - VPP300

The aim of this work is to use molecular simulation
techniques to investigate the effect of
intermolecular interactions on the phase transitions of pure fluids and fluid mixtures.

The phase behaviour of fluids is influenced profoundly by molecular interactions. This is evident qualitatively by the different types of phase transitions exhibited by molecules of different physical and chemical properties. Molecular simulation techniques provide powerful tools for relating this link exactly to intermolecular interactions. In contrast to conventional theoretical methods such as equation of state calculations, the phase transitions predicted by molecular simulation are solely the outcome of the choice of intermolecular potential and the nature of multi-body interactions of the fluid. The Gibbs ensemble method and recent improvements to histogram reweighing methods allow us to investigate efficiently phase transitions via molecular simulation. Many applications of the Gibbs ensemble have been reported assuming pairwise additivity, however the affect of three- or more-body interactions has not been investigated widely.

 
           
                 

   

 

What are the results to date and the future work?

The Role of Three-Body Interactions on the Phase behaviour of Pure fluids

The Gibbs ensemble algorithm has been implemented to determine the vapour-liquid equilibria of both pure fluids and binary mixtures. Earlier simulation work on the phase behaviour of fluids has been confined almost exclusively to pair potentials. Typically, a very simple intermolecular potential is used such as the Lennard-Jones (6-12) potential is used. Previous attempts to investigate multibody interactions have been confined exclusively to the triple-dipole dispersion term as represented by the Axilrod-Teller potential. We have employed accurate two-body potentials to investigate the phase behaviour of argon, krypton and xenon. In addition, the contribution of three body interactions arising from triple-dipole, dipole-dipole-quadrupole, dipole-dipole-quadrupole, triple-quadrupole and fourth order triple-dipole interactions. The results indicated that the addition of three-body interactions have a profound influence on the phase behaviour of pure fluids. When three-body interactions are included, the agreement of theory with experiment is within experimental error.

                 
Appendix B -

                 

     

Intermolecular Interactions Between Dissimilar Molecules and the Phase Behaviour of Ternary Mixtures

Most simulation studies of fluids, have concentrated almost exclusively on either pure fluids or binary mixtures. We have used molecular simulation to examine the effect of the addition of a third component on both the vapour-liquid and liquid-liquid equilibria exhibited by ternary mixtures. The results indicate that the addition of a small amount of a third component has a substantial effect on the observed phase behaviour. The simulations are particularly useful in quantifying the relative role of competing pair interactions. In contrast to binary mixtures in which there is only one unlike interaction, ternary mixtures have three competing unlike interactions resulting from interactions between molecules of component 1 and component 2, component 1 and component 3 and component 2 and component 3. The role of these interactions was investigated.

Future work will concentrate on using molecular simulation techniques to investigate the properties of large molecules. Algorithms have been developed to handle the generate of chain molecules interacting via reliable pair potentials. These will be implemented to characterise the phase behaviour of polymers and dendrimers.

What computational techniques are used?

The NPT and NVT-Gibbs ensemble techniques were used to simulate the coexistence of two liquid phases. A total of 500 molecules were partitioned between two boxes to simulate the two coexisting liquid phases. The temperature of the entire system was held constant and surface effects were avoided by placing each box at the centre of a periodic array of identical boxes. Equilibrium was achieved by attempting molecular displacements (for internal equilibrium), volume fluctuations (for mechanical equilibrium) and particle interchanges between the boxes (for material equilibrium).

The simulations were performed in cycles with each cycle consisting of 500 attempted displacements, a single volume fluctuation, and 500 interchange attempts. The maximum molecular displacement and volume changes were adjusted to obtain, where possible, a 50% acceptance rate for the attempted move. Ensemble averages were accumulated only after the system had reached equilibrium. The equilibration period was typically 2500 cycles and a further 2500 cycles was used to accumulate the averages. The calculations were truncated at intermolecular separations greater than half the box length, and appropriate long-range corrections were used to obtain the full contribution of pair interactions to energy and pressure.

Publications

Sadus, R. J, Exact calculation of the effect of three-body Axilrod-Teller interactions on vapour-liquid phase coexistence. Fluid Phase Equilibria, 144, 351-360 (1998).

Sadus, R. J, Effect of three-body interactions between dissimilar molecules on the phase behaviour of binary mixtures: the transition from vapor-liquid equilibria to type III behaviour. Industrial & Engineering Chemistry Research, 37, 2977-2982 (1998).

     
- Appendix B

 
     

       
Sadus, R.J., The effect of three-body interactions on the liquid-liquid phase coexistence of binary fluid mixtures. Fluid Phase Equilibria, 150-151, 63-72 (1998).
Sadus, R.J., Molecular simulation of the phase behaviour of ternary fluid mixtures: the effect of a third component on vapour-liquid and liquid-liquid coexistence. Fluid Phase Equilibria, 1999, accepted.
Marcelli, G. and Sadus, R.J., Molecular simulation of the phase behaviour of noble gases using accurate two-body and three-body intermolecular potentials. The Journal of Chemical Physics, 1999, submitted.
       
Appendix B -