Research Project


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Research Project

  1. 1. Using Monte Carlo simulations to study wetting behavior of ionic liquids on solid surfaces<br />Kaustubh Sunil Rane<br />Department of Chemical and Biological Engineering<br />School of Engineering and Applied Sciences<br />State University of New York<br />University at Buffalo<br />12/15/2010<br />
  2. 2. Goals of my project<br />To study<br />Wetting behavior of molten alkali halides<br />Wetting behavior of room temperature ionic liquids (RTILs)<br />Electro-wetting on dielectric<br />Using Monte Carlo simulations<br />2<br />
  3. 3. Why study wetting behavior of ionic liquids ?<br /> Micro-reactors<br />Reactors of future<br /> RTILs<br />Solvents of future<br />Wetting behavior important<br />Green technology<br />How they will operate?<br />Electro-wetting!<br />3<br />
  4. 4. Why use Monte Carlo simulations ?<br />They give accurate molecular level understanding.<br />Help explain phenomena not understood by experimental and theoretical methods.<br />Contact angle saturation (electro-wetting)<br />Edge over Molecular Dynamics in many respects.<br />Good tool to relate properties to the chemical structure.<br />Important to predict properties of yet to be discovered substances: Great for RTILs<br />4<br />
  5. 5. Did anyone study this before?<br />Wetting properties: yes<br />RTILs studied as contact angle probe fluids<br />Electro-wetting studied due to numerous applications<br />Molecular Dynamics used to study wetting behavior of molten alkali halides.<br />Using Monte Carlo simulations: No<br />Used to obtain density profile of ions near the charged surface.<br />5<br />
  6. 6. What are the techniques I will use?<br />Grand canonical transition matrix Monte Carlo<br />Free energy based method to get contact angles<br />Expanded ensemble techniques<br />Fine lattice strategy<br />Distance biased additions and deletions<br />6<br />
  7. 7. Wetting thermodynamics: Part1<br />Partial wetting<br />Complete wetting<br /><ul><li> Transition between states with different thickness of liquid films.
  8. 8. The edge of the drop is an open system, so grand potential is minimized to obtain equilibrium thickness. </li></ul>7<br />
  9. 9. Wetting thermodynamics: Part2<br />The effective interfacial potential depends on thickness of liquid film:<br />Minimizing with respect to l gives the equilibrium thickness of film.<br />At the wetting transition l has two minima:<br /> 1. At some finite value<br /> 2. At infinity<br />Saam W. F., J Low Temp Phys (2009) 157: 77-100<br />8<br />
  10. 10. Contact angle from Monte Carlo simulations<br />V(l) can be obtained from surface density probability distribution in GCMC<br />Grzelak et al., J. Chem Phys., 128, 014710/1(2008)<br />9<br />
  11. 11. Fine lattice strategy<br />Interaction energy calculations in ionic systems are very expensive.<br />Energy calculations required after each move in the simulation.<br />To avoid repetition of such calculations, fine lattice technique is used.<br />The space is divided into very fine lattice. Atom centers can occupy lattice sites only.<br />Energies between lattice sites separated by all possible distances are calculated and stored in an array.<br />During simulations, the values are just called from array.<br />Faster than continuum approach.<br />10<br />
  12. 12. Distance biased additions and deletions<br />Cations and anions added simultaneously for charge neutrality.<br />Acceptance of random additions is very low due to presence of neutral clusters.<br />Counterions are preferentially added to and deleted from sites near each other.<br />The probability of addition and deletion of the second ion decreases with the distance from the first ion.<br />Acceptance of additions and deletions greatly increased.<br />11<br />
  13. 13. My system<br />Spherical cations and anions of same size with following potential:<br />αgives the strength of ionic interaction with respect to LJ interaction:<br />The interaction with structure-less wall given as follows:<br />12<br />
  14. 14. Why use α ?<br />For realistic ionic systems, numerous sampling difficulties in GCMC.<br />It is necessary to go gradually from LJ fluids to molten salts.<br />α makes that possible.<br />Values of α to be tested: 0, 1, 10, 100, 1000<br />Started with α = 100 <br />Sufficiently ionic and <br />Sufficiently LJ <br />13<br />
  15. 15. Liquid-vapor co-existence curves<br />Helps selecting temperature and saturation potential for interfacial simulations and validates the interaction potential.<br />For α = 100, Tc ~ 6.2<br />14<br />
  16. 16. Free energy profile from interfacial simulations<br />15<br />
  17. 17. Next step<br />To see how contact angles change with temperature and surface strength.<br />Repeat the exercise for other values of α .<br />For α = 1000, see the effect of size asymmetry.<br />How anion to cation size ratio affects the contact angles<br />16<br />
  18. 18. Wetting by RTILs<br />Wetting behavior of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) will be studied.<br />United atom model developed by Maginn group will be <br /> used.<br />Experimental works done to understand their applicability as contact angle probe fluids and in electro-wetting.<br />No molecular simulation study done to understand their wetting behavior.<br />Shah, J.K. and E.J. Maginn, Fluid Phase Equilib., 2004. 222-223:p. 195-203.<br />17<br />
  19. 19. Electro-wetting on dielectric<br />Contact angle changes on applying potential difference across conducting liquid and electrode.<br />The main problem: Contact angle saturation: <br />Theories and experiments unable to explain this.<br />Monte Carlo simulations can give molecular level<br /> understanding.<br />Shamai R, Andelman D, Berge B, & Hayes R (2008) Soft Matter 4(1):38-45 <br />Quinn, R. Sedev and J. Ralston, J. Phys. Chem. B, 2005, 109, 6268.<br />18<br />
  20. 20. Acknowledgements<br />Dr. Jeffrey Errington (Thesis advisor)<br />Research funded by: Computation aided by:<br />19<br />