Ivo Nezbeda 1,2 , Ariel A. Chialvo 3,2 , and Peter T. Cummings 2,3

1. Effect of the Range of Interactionson the Properties of FluidsEquilibria of CO2, Acetone, Methanol and Water Ivo Nezbeda 1,2, Ariel A. Chialvo 3,2, and Peter T. Cummings 2,3 1 Institute of Chemical Process Fundamentals. Academic of Sciences, 16502 Prague 6 - Suchdol, Czech Republic 2Departments of Chemical Engineering. University of Tennessee, Knoxville, TN 37996-2200, U.S.A. 3 Chemical Sciences Division. Oak Ridge National Laboratory, Oak Ridge, TN 37881-6110, U.S.A.

2. Rationale • Generally long-range forces have negligible effect on the microstructure of fluids • the structure of realistic model fluids and their short-range counterpart are (for all practical purposes) identical • Thermodynamic properties of fluids are accurately estimated by those of the short-range model counterparts • e.g., configurational energy of the short-range models account for 95% of the total property • Long-range forces affect only details of the orientational correlations • however, the dielectric constant remains unaffected • These findings support the development of fast converging perturbation expansions about the short-range reference • i.e., long-range Coulombic interactions treated as a perturbation

3. Goals • Determine the effect of the long-range Coulombic interactions on the vapor-liquid equilibria properties of polar and associating fluids • most realistic intermolecular potential models available • carbon dioxide, acetone, methanol, and water • Interpret simulation results and develop simple perturbation approaches for rigorous modeling • modeling of aqueous solutions without resorting to long-range interactions • e.g., I. Nezbeda, Mol. Phys., 99, 1631-1639 (2001) • truly molecular-based equation of state for engineering calculations • e.g., recently proposed equation for water (Nezbeda & Weingerl, Mol. Phys., 99, 1595-1606 (2001))

4. Range of Intermolecular Interactions • Basic definitions • Separation between short- and long-range potential interaction

5. Range of Intermolecular Interactions • Basic definitions • Switching function for the range transition (e.g., rss=rOO for the case of water and methanol)

6. Range of Intermolecular Interactions • Simulation details • VLE simulations by NVT-GEMC • Isochoric simulations by NVT-MD • 516<N<700 for GEMC • N=500 for MD • cutoff distance ~ 3.6-5.0 sss (i.e., ~12-19Å) • electrostatics via reaction field • Nosé thermostat for MD • quaternion dynamics • [rL, rU] chosen according to the location of the first peak of the RDF for the reference sites

7. Vapor-Liquid Equilibrium of Model Carbon Dioxide • Harris-Yung’s EPM2 model (*) • Estimated critical conditions from Wegner expansion • Short-range potential: Tc=310.8K, rc=458.6kg/m3 • Full potential: Tc=310.9K, rc=455.1kg/m3 (*) Harris and Yung, JCP, 99 (1995)

8. Vapor-Liquid Equilibrium of Model Acetone • Jedlovszky-Pálinkás model (*) • Estimated critical conditions from Wegner expansion • Short-range potential: Tc=505.5K, rc=275.0kg/m3 • Full potential: Tc=499.3K, rc=273.3kg/m3 (*) Jedlovszky and Pálinkás, Mol. Phys., 84 (1995)

9. Vapor-Liquid Equilibrium of Model Methanol • OPLS model (*) • Estimated critical conditions from Wegner expansion • Short-range potential: Tc=483.4K, rc=250.2kg/m3 • Full potential: Tc=484.6K, rc=258.2kg/m3 (*)Jorgensen et al., JACS, 106 (1984)

10. Vapor-Liquid Equilibrium of Model Water • TIP4P model (*) • Estimated critical conditions from Wegner expansion • Short-range potential: Tc=564.9K, rc=339.4kg/m3 • Full potential: Tc=566.1K, rc=321.8kg/m3 (*) Jorgensen, JCP, 77 (1982)

11. Vapor-Liquid Equilibrium • Effect of range on vapor pressure (*) (*) Nezbeda et al., (2001)

12. Structure and Thermodynamics of Model Methanol • Effect of range on properties along isochore r=0.76g/cc (*) (*) Nezbeda et al. (2001)

13. Structure and Thermodynamics of Model Methanol • Effect of range on structure along isochore r=0.76g/cc (*) T=298K (*) Nezbeda et al. (2001)

14. Structure and Thermodynamics of Model Methanol • Effect of range on structure along isochore r=0.76g/cc (*) T=548K (*) Nezbeda et al. (2001)

15. Interpretation of Simulation Results • Gibbs-Duhem equations including force-field variables (*) • Define coupling parameter l, i.e., • Apply equilibrium conditions • Derive Clapeyron equation (*) Nezbeda et al. (2001)

16. Interpretation of Simulation Results • Gibbs-Duhem equations including force-field variables (*) • Particular cases can be derived depending on relative sizes of the involved properties in each phase • typical case (water and acetone) • additional cases apply to carbon dioxide and methanol (*) Nezbeda et al. (2001)

17. Summary and Final Remarks • Spatial and orientational distributions of molecules are marginally affected by long-range forces • Long-range forces affect details of the orientational ordering at short-range distances. • orientational correlations in the short- and full-range systems are qualitatively similar • integrals over these correlations, e.g., dielectric constant, do not differ significantly • Similar behavior is found in the dependence of thermodynamic properties, i.e., energy and pressure, on the range of the potential

18. Summary and Final Remarks • Critical conditions appear to be unaffected by the long-range forces • These findings lend support to the use of perturbation expansion in the development of truly molecular-based equations of state

19. Acknowledgements • Research Support • Grant Agency of the Czech Republic (Grant No 203/99/0134) • Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract number DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC • For more info visit our web_sites • http://www.icpf.cas.cz/theory/IvoNez.html • http://www.ornl.gov/divisions/casd • http://flory.engr.utk.edu/~aac

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