Research project for PhD.

1. Using Monte Carlo simulations to study wetting behavior of ionic liquids on solid surfaces Kaustubh Sunil Rane Department of Chemical and Biological Engineering School of Engineering and Applied Sciences State University of New York University at Buffalo 12/15/2010

2. Goals of my project To study Wetting behavior of molten alkali halides Wetting behavior of room temperature ionic liquids (RTILs) Electro-wetting on dielectric Using Monte Carlo simulations 2

3. Why study wetting behavior of ionic liquids ? Micro-reactors Reactors of future RTILs Solvents of future Wetting behavior important Green technology How they will operate? Electro-wetting! 3

4. Why use Monte Carlo simulations ? They give accurate molecular level understanding. Help explain phenomena not understood by experimental and theoretical methods. Contact angle saturation (electro-wetting) Edge over Molecular Dynamics in many respects. Good tool to relate properties to the chemical structure. Important to predict properties of yet to be discovered substances: Great for RTILs 4

5. Did anyone study this before? Wetting properties: yes RTILs studied as contact angle probe fluids Electro-wetting studied due to numerous applications Molecular Dynamics used to study wetting behavior of molten alkali halides. Using Monte Carlo simulations: No Used to obtain density profile of ions near the charged surface. 5

6. What are the techniques I will use? Grand canonical transition matrix Monte Carlo Free energy based method to get contact angles Expanded ensemble techniques Fine lattice strategy Distance biased additions and deletions 6

8. The edge of the drop is an open system, so grand potential is minimized to obtain equilibrium thickness. 7

9. Wetting thermodynamics: Part2 The effective interfacial potential depends on thickness of liquid film: Minimizing with respect to l gives the equilibrium thickness of film. At the wetting transition l has two minima: 1. At some finite value 2. At infinity Saam W. F., J Low Temp Phys (2009) 157: 77-100 8

10. Contact angle from Monte Carlo simulations V(l) can be obtained from surface density probability distribution in GCMC Grzelak et al., J. Chem Phys., 128, 014710/1(2008) 9

11. Fine lattice strategy Interaction energy calculations in ionic systems are very expensive. Energy calculations required after each move in the simulation. To avoid repetition of such calculations, fine lattice technique is used. The space is divided into very fine lattice. Atom centers can occupy lattice sites only. Energies between lattice sites separated by all possible distances are calculated and stored in an array. During simulations, the values are just called from array. Faster than continuum approach. 10

12. Distance biased additions and deletions Cations and anions added simultaneously for charge neutrality. Acceptance of random additions is very low due to presence of neutral clusters. Counterions are preferentially added to and deleted from sites near each other. The probability of addition and deletion of the second ion decreases with the distance from the first ion. Acceptance of additions and deletions greatly increased. 11

13. My system Spherical cations and anions of same size with following potential: αgives the strength of ionic interaction with respect to LJ interaction: The interaction with structure-less wall given as follows: 12

14. Why use α ? For realistic ionic systems, numerous sampling difficulties in GCMC. It is necessary to go gradually from LJ fluids to molten salts. α makes that possible. Values of α to be tested: 0, 1, 10, 100, 1000 Started with α = 100 Sufficiently ionic and Sufficiently LJ 13

15. Liquid-vapor co-existence curves Helps selecting temperature and saturation potential for interfacial simulations and validates the interaction potential. For α = 100, Tc ~ 6.2 14

16. Free energy profile from interfacial simulations 15

17. Next step To see how contact angles change with temperature and surface strength. Repeat the exercise for other values of α . For α = 1000, see the effect of size asymmetry. How anion to cation size ratio affects the contact angles 16

18. Wetting by RTILs Wetting behavior of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) will be studied. United atom model developed by Maginn group will be used. Experimental works done to understand their applicability as contact angle probe fluids and in electro-wetting. No molecular simulation study done to understand their wetting behavior. Shah, J.K. and E.J. Maginn, Fluid Phase Equilib., 2004. 222-223:p. 195-203. 17

19. Electro-wetting on dielectric Contact angle changes on applying potential difference across conducting liquid and electrode. The main problem: Contact angle saturation: Theories and experiments unable to explain this. Monte Carlo simulations can give molecular level understanding. Shamai R, Andelman D, Berge B, & Hayes R (2008) Soft Matter 4(1):38-45 Quinn, R. Sedev and J. Ralston, J. Phys. Chem. B, 2005, 109, 6268. 18

20. Acknowledgements Dr. Jeffrey Errington (Thesis advisor) Research funded by: Computation aided by: 19