1. 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

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

3. 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

4. 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,
b Armstrong, D. W. et. al. Anal. Chem. 1999, 71,
c Ren, R. X. et. al. PCT Patent, 2003, WO/03/

5. 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

6. 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]+

7. 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,
Wang, H. F. at. al. J. Phys. Chem B 2004, 108,
CH3 group orientation with respect
to the surface normal
Polarization null angle for CH3 of
methyl sulfate (■) and CH3 of
butyl chain (▲)

8. 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°

9. 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

10. 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

11. 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

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

13. 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]

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