Ions at the Surface of a Room-Temperature Ionic Liquid Cherry S. Santos Advisor: Dr. Steve Baldelli Chemistry Department University of Houston, Houston, TX 21 June 2007

Room-Temperature Ionic Liquids Common cations and anions [CF3COO]- [MS]- Pyridinium ion Ammonium ion [PF6]- Phosphonium ion Imidazolium ion [BF4]- [CH3SO3]-

Room-Temperature Ionic Liquids Its Properties and Future Hexane Water [BMIM][PF6] Extraction Catalysis (Ref: Mehnert, C. P. Chem. Eur. J. 2005, 11, 50-56) Electrochemistry Properties High thermal stability Negligible vapor pressure Good ionic conductivity Wide window of electrochemical stability Applications/Uses Solvents synthesis, catalysis Electrolytes electrochemistry, batteries, fuel cells, solar cells Gas-separation processes

Room-Temperature Ionic Liquids Sample Preparation 1-Butyl-3-methylimidazolium Methyl Sulfate [BMIM][MS]a Step 1b Step 2c 1-Butyl-3-methylimidazolium Methyl Sulfonate [BMIM][CH3SO3] a Holbrey, J. D. et. al. Green Chem. 2002, 4, 407-413. b Armstrong, D. W. et. al. Anal. Chem. 1999, 71, 3873-3876. c Ren, R. X. et. al. PCT Patent, 2003, WO/03/051894.

Sum-Frequency Generation (SFG) Vibrational Spectroscopy Nd:YAG Laser OPG/OPA 532 nm 1064 nm SFG Monochromator/ PMT SFG Cell IR (Tunable) = Sum-frequency intensity = Damping constant = Hyperpolarizability = Number of modes = 2nd order nonlinear susceptibility = Electric field energy Only sensitive to molecules in a non-centrosymmetric environment Different polarization combinations provide information on the orientation of surface molecules

SFG Spectra: C-H Stretch Modes [BMIM][MS] [BMIM][CH3SO3] ~3020 cm-1 ~2850 cm-1 ~2880 cm-1 ~2970 cm-1 ~3175 cm-1 1 5 3 4 2 Structure and numbering Scheme for [BMIM]+

Orientation Analysis: [BMIM][MS] Surface Normal θ C3 Polarization Selection Rules: CH3 and CH2 Methyl, CH3: ss mode: Issp >> Ippp > Isps ~ Ipss as mode: Ippp > Issp Small tilt angle, θ: Isps and Ipss are the largest for the as mode, and the smallest for the ss mode. Methylene, CH2 Small tilt angle, θ: as mode peaks are present in the sps and pss spectra, larger than in ppp. Ref. Wang, H.F. et. al. J. Phys. Chem B 2005, 109, 14118-14129 Wang, H. F. at. al. J. Phys. Chem B 2004, 108, 7297-7306 CH3 group orientation with respect to the surface normal Polarization null angle for CH3 of methyl sulfate (■) and CH3 of butyl chain (▲)

Orientation Analysis: [BMIM][MS] Theoretical plots of polarization null angle CH3 of methyl sulfate, red→black θ=45°→70°, ∆5° CH3 of butyl chain, red→blue θ=50°→70°, ∆5° ~62° ~53°

Phase Analysis: [BMIM][MS] Experimental SFG spectrum of [BMIM][MS] Simulation with c(2)MS same sign as c(2)CH3 Simulation with c(2)MS opposite sign as c(2)CH3

Phase Analysis: [BMIM][CH3SO3] Experimental SFG spectrum of [BMIM][CH3SO3] Simulation with c(2)CH3-CH3SO3 same sign as c(2)CH3 Simulation with c(2)CH3-CH3SO3 different sign as c(2)CH3

Orientation: Gas-Liquid Interface [BMIM][X] Butyl chain is projecting towards the gas phase The imidazolium ring is nearly parallel to the surface plane [BMIM]+ exhibits similar orientation regardless of the anion

X-Ray Crystallography [BMIM][MS] [BMIM][CH3SO3] Molecular Structure Packing along the a axis

Conclusions Surface orientation of the ions: [BMIM]+, [MS]-, [CH3SO3]-, has been shown using SFG along with the PNA measurements of the tilt angle of the methyl group. The CH3 groups are pointing upwards based on the phase analysis, implying that the butyl chain is also projecting away from the liquid phase. The imidazolium ring seems to be lying parallel to the surface plane. Information on the solid state structure of the ionic liquid as well as the orientation and location of ions at the gas-liquid interface is obtained using SFG and X-Ray crystallography. [BMIM][CH3SO3] [BMIM][MS]

Acknowledgments Robert A. Welch Foundation Petroleum Research Fund Dr. Sergey Dibrov Dr. J. Kochi, University of Houston, Houston, Texas Baldelli Group Dr. Steve Baldelli