(289i) Development of An Optimized Intermolecular Potential for Sulfur Dioxide

Sulfur dioxide is an important industrial pollutant, formed primarily from the combustion of sulfur containing coal.  SO₂ reacts with water to form sulfuric acid, and with increasingly strict environmental regulations, there is significant interest in the sequestration of SO₂ from flue gas in addition to CO₂.  One promising technology for the sequestration of SO₂ is the use of ionic liquids.  Recent publications have shown significant amounts of SO₂ may be physically adsorbed in ionic liquids[1].  However, given the multitude of potential combinations of anion and cation, the design of an optimal solvent of SO₂ capture by trial and error is likely to be an arduous process.  Computational methods have recently been used to investigate the interactions of SO₂ with ionic liquids[2-4], however, the force fields used for SO₂ have not undergone rigorous validation.    

In this work, a new force field for sulfur dioxide, capable of predicting accurately the vapor_liquid equilibria, critical properties, vapor pressure, and heats of vaporization is presented. The new force field reproduces the saturated liquid densities, vapor pressures and heats of vaporization to within 0.5, 2, and 2% of experiment, respectively. The predicted critical properties and the normal boiling point are in excellent agreement with experimental results.  Pair distribution functions are calculated for the S_S, S_O, and O_O interactions are in close agreement with neutron and X-ray scattering experiments. In addition to the new force field, similar calculations are performed for four SO­₂ intermolecular potentials proposed by Sokolic et al. (Sokolic, F.; Guissani, Y.; Guillot, B. J. Phys. Chem. 1985, 89, 3023], which show that these models work reasonably well near the state point where they were originally parametrized, but large errors in the predicted coexistence properties are displayed at higher and lower temperatures. Errors in the vapor-pressure suggest that the Sokolic force fields underestimate electrostatic contributions to the intermolecular potential.  Comparison of the radial distribution functions show the local structure is only weakly affected by the different force field parameters.

1.             Anderson, J.L., et al., Measurement of SO₂ solubility in ionic liquids. Journal of Physical Chemistry B, 2006. 110(31): p. 15059-15062.

2.             Baer, M., et al., Interpreting Vibrational Sum-Frequency Spectra of Sulfur Dioxide at the Air/Water Interface: A Comprehensive Molecular Dynamics Study. Journal of Physical Chemistry B, 2010. 114(21): p. 7245-7249.

3.             Wick, C.D., T.M. Chang, and L.X. Dang, Molecular mechanism of CO₂ and SO₂ molecules binding to the air/liquid interface of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid: a molecular dynamics study with polarizable potential models. J Phys Chem B, 2010. 114(46): p. 14965-71.

4.             Siqueira, L.J., et al., Shielding of ionic interactions by sulfur dioxide in an ionic liquid. J Phys Chem B, 2008. 112(20): p. 6430-5.