SO2 (g) SO2(g) + H2O

Oxidation

HSO3-

H+/HSO4- /SO4

2-

V.O.C. S.O.A. : Function of Interfacial Species

Mineral Core

SO42-

(ads)

H2SO4

Oxidation

Bisulfate Dehydration at the Vapor/Solution Interface Probed by Vibrational Sum Frequency Generation Spectroscopy

Aaron M. Jubb and Heather C. Allen67th meeting of the International Symposium on Molecular Spectroscopy

June 19, 2012 – Columbus, OHajubb@chemistry.ohio-state.edu

Raman

• Change in polarizability with

vibration

vis

Stokes

Vibrational Spectroscopies Utilized

IR

vis

IR

sum

v = 0

v = 1

Virtual excited state

vis

Virtual excited state

Anti-

Stokes

Infrared

• Change in dipole moment with

vibration

Vibrational Sum Frequency Generation

• Lack of inversion symmetry

• Vibration must be both IR and Raman active

What role does cation (H+, Na+, Mg2+) identity play for HSO4

- behavior?

• Ion-pair formation?

• Hydration disruption?

How does interfacial behavior compare to bulk behavior?• VSFG versus Raman

Target νSS – SO3 of HSO4-

50

0

x104 1300120011001000900800

Wavenumber (cm-1

)

1

0

0.4 x H2SO4 - Infrared 0.4 x H2SO4 - Raman Component Peaks

Ram

an C

oun

ts (

x104 )

Ab

sorb

ance

A - Infrared

B - Raman

AS - SO3 & AS - SO42-

SS - SO3

SS - SO42-

SS - S-(OH)

Dehydration versus Ion-pairing

Walrafen, G.E.; Yan, W.-H.; Chu, Y.C. J. Phys. Chem. A 2002, 106, 10162-10173. Fung, K.H.; Tang, I.N. Appl. Spectrosc. 1992, 46(6), 1189-1193.

Dehydration• Breaks H-bonds

• Shortens S-O bonds• Blue shift for νSS – SO3

Ion-Pairing• Induces polarization • Electron withdrawing• Lengthens S-O bonds• Red shift for νSS – SO3

2-2+ 2-

2+2-

2+

Solvent-Separated Ion Pair (SSIP) Solvent-Shared Ion Pair (SIP) Contact Ion Pair (CIP)

H2SO4 results correspond well with literature

Blue shifted from bulk frequency of 1035 cm-1

pH 1 Na2SO4 solutions exhibit bimodal spectra

Indicates two distinct populations of HSO4-

molecules at interface

MgSO4 results feature only blue-shifted peak

Entire HSO4- interfacial population disrupted

VSFG: HSO4- Dehydration

0.15

0.00

0.11

0.00

0.15

0.00

1150110010501000950

Wavenumber (cm-1

)

VS

FG

In

ten

sity

(a.

u.)

C - 0.035 x MgSO4 pH 1

B - 0.033 x Na2SO4 pH 1

A - 0.4 x H2SO4

Ion-Pairing Dehydration

A.M. Jubb and H.C. Allen, J. Phys. Chem. C, 2012, DOI: 10.1021/jp302585h

Cations Induce Dehydration Within Interface

Walrafen, G.E.; Yan, W.-H.; Chu, Y.C. J. Phys. Chem. A 2002, 106, 10162-10173. Fung, K.H.; Tang, I.N. Appl. Spectrosc. 1992, 46(6), 1189-1193.

Dehydration• Breaks H-bonds

• Shortens S-O bonds• Blue shift for νSS – SO3

Ion-Pairing• Induces polarization • Electron withdrawing• Lengthens S-O bonds• Red shift for νSS – SO3

2-2+ 2-

2+2-

2+

Solvent-Separated Ion Pair (SSIP) Solvent-Shared Ion Pair (SIP) Contact Ion Pair (CIP)

12001150110010501000

Wavenumber (cm-1

)

20

0

x102

25

0

x102

AS - SO42-

SS - SO3

A - Na2SO4 pH 1

B - MgSO4 pH 1

~ 0.034 x 0.017 x 0.009 x

Ram

an C

ount

s (x

102 )

Bulk solution HSO4- νSS – SO3

frequency increases with:

• Concentration

• H+ < Na+ < Mg2+

Na2SO4 exhibits higher νSS – SO3 intensity than MgSO4

• CIP formation between SO42- and Mg2+

inhibiting HSO4- formation – Kester et

al. 1972

Raman: HSO4- Bulk Dehydration

2-2+ 2-

2+2-

2+

Solvent-Separated Ion Pair (SSIP) Solvent-Shared Ion Pair (SIP) Contact Ion Pair (CIP)

A.M. Jubb and H.C. Allen, J. Phys. Chem. C, 2012, DOI: 10.1021/jp302585h

HSO4- νSS – SO3 frequency increases:

• At vapor/solution interfaces

• With concentration

• H+ < Na+ < Mg2+

νSS – SO3 frequency blue-shifts indicate HSO4- dehydration

First time VSFG has been used to directly interrogate cation effects on HSO4

- at vapor/solution interfaces

Implies acid catalyzed aerosol reactions may be inhibited by interfacial cations

Summary

Acknowledgements

50

0

x104 1300120011001000900800

Wavenumber (cm-1

)

1

0

0.4 x H2SO4 - Infrared 0.4 x H2SO4 - Raman Component Peaks

Ram

an C

ount

s (x

104 )

Ab

sorb

ance

A - Infrared

B - Raman

AS - SO3 & AS - SO42-

SS - SO3

SS - SO42-

SS - S-(OH)

25

0

x103

1300120011001000900800

Wavenumber (cm-1

)

0.4

0.0

0.033 x Na2SO4 pH 1 0.017 x Na2SO4 pH 1 0.009 x Na2SO4 pH 1

A - Infrared

B - Raman

Ram

an C

oun

ts SS - SO42-

AS - SO42-

SS - SO3

Ram

an C

ount

s (x

103 )

AS - SO3

Abs

orb

ance

28

0

x103

1300120011001000900800

Wavenumber (cm-1

)

0.45

0.00

0.035 x MgSO4 pH 1 0.017 x MgSO4 pH 1 0.009 x MgSO4 pH 1

SS - SO42-

SS - SO3

AS - SO42-

Ram

an C

oun

ts (

x103 )

A - Infrared

B - Raman

Ab

sorb

ance

AS - SO3

22

0

x103

120011001000900800

Raman Shift (cm-1

)

-1000

0

1000

0

1 2

0.033 x Na2SO4 pH 1 Fit Lorentzian Peak Components Fit Residual

A

B

C

Ram

an C

oun

ts

0.14

0.00

VS

FG

In

ten

sity

(a.

u.)

1150110010501000

Wavenumber (cm-1

)

0.033 x Na2SO4 pH 1

0.033 x Na2SO4 pH 7

0.14

0.00

VS

FG

In

ten

sity

(a.

u.)

1150110010501000

Wavenumber (cm-1

)

0.035 x MgSO4 pH 1

0.035 x MgSO4 pH 7

Table 1. Curve-Fitting Results for VSFG Spectra

Non-Resonant Terms

Peak 1

Peak 2

Concentration B ϕ ων (cm-1) Aν Γ (cm-1) ων (cm-1) Aν Γ (cm-1)

H2SO4 0.4 x 0.03 0.01 1050 6.9 26.0 N/A N/A N/A

Na2SO4 0.033 x 0.05 0.03 1048 1.7 8.6 1067 3.8 19.7

MgSO4 0.035 x 0.02 0.00 N/A N/A N/A 1069 7.0 22.4

Solution Concentration

(x)

ω

(cm-1)

I

(counts)

FWHM

(cm-1)

Area

H2SO4 0.4 1036 ± 0 3.7 (± 0.05)

x 105

51 ± 1 3.0 (± 0.07)

x 107

Na2SO4 0.009 1053 ± 1 711 ± 47 27 ± 3 29906 ± 2643

0.017 1053 ± 2 880 ± 137 19 ± 5 26152 ± 6697

0.033 1054 ± 2 1020 ± 215 20 ± 8 32705 ± 11947

MgSO4 0.009 1052 ± 2 475 ± 79 18 ± 5 13181 ± 3741

0.017 1055 ± 3 427 ± 127 19 ± 10 12806 ± 6307

0.035 1059 ± 6 402 ± 250 19 ± 22 12175 ± 12768