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

Ions at the Surface of a Ions at the Surface of a Room-Temperature Ionic LiquidRoom-Temperature Ionic Liquid

Cherry S. SantosAdvisor: Dr. Steve Baldelli

Chemistry DepartmentUniversity of Houston, Houston, TX

21 June 2007

Room-Temperature Ionic Liquids

Common cations and anions

Pyridinium ion Ammonium ion

Imidazolium ionPhosphonium ion[BF4]-

[PF6]-

[CF3COO]- [MS]-

[CH3SO3]-

Room-Temperature Ionic LiquidsIts Properties and Future

PropertiesHigh thermal stabilityNegligible vapor pressureGood ionic conductivityWide window of electrochemical stability

Applications/UsesSolvents

synthesis, catalysis

Electrolytes

electrochemistry, batteries,

fuel cells, solar cells

Gas-separation processes

Hexane

Water

[BMIM][PF6]

Extraction

Catalysis (Ref: Mehnert, C. P. Chem. Eur. J. 2005, 11, 50-56)

Electrochemistry

Room-Temperature Ionic Liquids Sample Preparation

NN(H2C)3 CH3

S

O

O-O

O

CH3H3C

[BMIM][MS]

NN(H2C)3

H3C

(CH3)2OSO3

1-BIM

Toluene< 40oC

1-Butyl-3-methylimidazolium Methyl Sulfate [BMIM][MS]a

1-Butyl-3-methylimidazolium Methyl Sulfonate [BMIM][CH3SO3]

NN

Cl

Reflux (72 h) ~70oC

NN(H2C)3 CH3

H3C Cl-

[BMIM][Cl]MIM

Step 1b

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.

Step 2c

NN(H2C)3 CH3

SH3C

[BMIM][CH3SO3]

CH3SO3H

[BMIM][Cl]

OO

O

CH3

NN(H2C)3 CH3

H3C Cl-+

OH Cl

Distill100oC

Sum-Frequency Generation (SFG) Vibrational Spectroscopy

OPG/OPANd:YAG Laser

1064 nm

SFG Cell

Monochromator/PMT

I P E E

N

i

SF Vis IR

IR n n

( ) :( ) ( )

( )

( )

2 2 2

2

2

I

E

N

SF

n

( )

( )

2

2

= 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

532 nm

IR(Tunable)

SFG

SFG Spectra: C-H Stretch Modes

Structure and numbering Scheme for [BMIM]+

~3020 cm-1

~2850 cm-1

~2880 cm-1

~2970 cm-1

~3175 cm-1

15

34

2

[BMIM][CH3SO3]

2700 2850 3000 3150 3300

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

SF

G I

nten

sity

(a.

u.)

Wavenumber(cm-1)

ssp

[BMIM][MS]

2700 2850 3000 3150 3300

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

SF

G In

tens

ity (

a.u.

)

Wavenumber (cm-1)

ssp

Orientation Analysis: [BMIM][MS]

CH3 group orientation with respect to the surface normal

-50 0 50 100 150 200 250 300 350 400

0.0

0.1

0.2

0.3

0.4

SF

G In

ten

sity

(a

.u.)

Polarization angle (degrees)

CH3SO

4

CH3 (butyl)

Polarization null angle for CH3 of methyl sulfate (■) and CH3 of

butyl chain (▲)

Surface

Surface Normal

θ

C3

Orientation Analysis: [BMIM][MS]

1

1.5

22.5

33.5

4

Analyzer Angle (D egrees)5 ° 10 ° 15 ° 20°

SF

G I

nten

sity

(a.

u.)

0.5

45°55°70°

5 ° 10 ° 15 ° 20 ° 25 ° 30 °

0.51

1.5

22.5

33.5

4

Analyzer Ang le (D egrees)

SF

G I

nten

sity

(a.

u.) 55° 50°60°

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]

2800 2900 3000 3100 3200

0.2

0.4

0.6

0.8

1

SF

G I

nt e

nsi ty

(a

. u. )

Wavenumber (cm-1)

2800 2900 3000 3100 3200 3300

0.2

0.4

0.6

0.8

1

SF

G I n

t en

sity

(a

. u. )

Wavenumber (cm-1)

Experimental SFG spectrum of [BMIM][MS]

Simulation with MS opposite sign as

CH3

Simulation with S same sign as

CH3

2700 2850 3000 3150 3300

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

SF

G In

tens

ity (

a.u.

)

Wavenumber (cm-1)

ssp

ISF CH CH FR M S CH M S FR

3 3 3

2 2 2 2( ) ( ) ( ) ( )

Phase Analysis: [BMIM][CH3SO3]

2800 2900 3000 3100 3200 3300

0.2

0.4

0.6

0.8

1

SF

G I n

t ens

i ty (

a. u

. )

Wavenumber (cm-1)

2800 2900 3000 3100 3200 3300

0.2

0.4

0.6

0.8

1

SF

G I n

t ens

i ty (

a. u

. )

Wavenumber (cm-1)

Experimental SFG spectrum of [BMIM][CH3SO3]

Simulation with CH3-CH3SO3 different sign

as CH3

Simulation with CH3-CH3SO3 same sign

as CH3

2700 2850 3000 3150 3300

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

SF

G I

nten

sity

(a.

u.)

Wavenumber(cm-1)

ssp

ISF CH CH FR CH CH SO

3 3 3 3 3

2 2 2( ) ( ) ( )

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

2700 2850 3000 3150 3300

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

SF

G In

tens

ity (

a.u.

)

Wavenumber (cm-1)

[BMIM][BF4]

X-Ray Crystallography

[BMIM][MS] [BMIM][CH3SO3]

Packing along the a axis

Molecular Structure

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.

[BMIM][MS][BMIM][CH3SO3]

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.

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.

Acknowledgments

Robert A. Welch Foundation

Petroleum Research Fund

Dr. Sergey Dibrov

Dr. J. Kochi, University of Houston, Houston, Texas

Baldelli Group

Dr. Steve Baldelli