ORIENTATIONAL MOLECULAR DYNAMICS AT THE AIR/WATER INTERFACE STUDIED WITH SFG SPECTROSCOPY Igor Stiopkin, Achani Yatawara, Himali Jayathilake, Champika Weeraman, and Alexander Benderskii Department of Chemistry Wayne State University Detroit, MI

Orientational Dynamics in Bulk and at Interfaces How orientation dynamics in bulk is different from dynamics at interfaces? Water Air

 Static orientation is often assumed in SFG data analysis  SFG signal strengths were analyzed for fast and slow (static) orientational dynamics limits X. Wei, Y. R. Shen Phys. Rev. Lett. 2001, 86, 4799 J. Fourkas, R. Walker, S. Can, E. Gershgoren J. Phys. Chem. C 2007, 111, 8902 Orientational Dynamics at Surfaces Studied with SFG  Reorientation time scale can be extracted  Molecular reorientation dynamics affects VSFG line shapes Our Work:

Model System: Propiolic Acid at Air/Water Interface  Rigid linear molecule  C≡C bond is probed, IR & Raman active  Carboxylate group is H-bonded z x y Out of Plane In Plane

SFG Spectroscopy SFG is sensitive to molecular orientations & light polarizations z x y |v=0  |v=1  IR vis SFG 

Polarization Combinations in SFG PPPSSPSPSPPPSSPSPS vis IRSFG 3 (out of 8) SFG polarization combinations are nonvanishing z y x vis IR P S S P S P SFG

Out of Plane Dynamics - PPP PPPPPP visIRSFG |v=0  |v=1  IR vis SFG t z y x IR P vis P P SFG SFG-FIDFrequency-domain SFG τ FID  1/Γ = Γ vibr + Γ out-of-plane Time Frequency Γ

Out of Plane Dynamics - SSP SFG-FIDFrequency-domain SFG SSPSSP visIRSFG Γ τ  1/Γ z y x vis IR P S S SFG |v=0  |v=1  IR vis SFG t

In Plane Dynamics - SPS z y x vis IR S P S SFG SPSSPS visIRSFG SFG-FIDFrequency-domain SFG Γ τ  1/Γ = Γ vibr + Γ in-plane |v=0  |v=1  IR vis SFG t

Experimental SSP SFG - Out of Plane Dynamics SSPSSP visIRSFG Time-domain SFG-FID Frequency-domain SFG Γ=10.7±1 cm -1 Γ=10.8±1 cm -1 z P S S ‘Spectroscopic Line Shapes Of Broad Band Sum Frequency Generation’, Himali Jayathilake, Igor Stiopkin, Champika Weeraman, Achani Yatawara, Alexander Benderskii

Experimental PPP SFG - Out of Plane Dynamics PPPPPP visIRSFG Time-domain SFG-FID Frequency-domain SFG Γ=11±2 cm -1 z P P P

Experimental SPS SFG - In Plane Dynamics SPSSPS visIRSFG Time-domain SFG-FID Γ=15±1 cm -1 Frequency-domain SFG z y x S P S S

Orientational Dynamics in SFG Line Shapes Time-domain SFG-FID Frequency-domain SFG Γ SSP = 10.7±1 cm -1 Γ SPS = 15±1 cm -1 Γ rot ~ 4±1 cm -1 τ rot ~ 1.1±0.3 ps In-plane rotation:

Alms, Bauer, Brauman, Pecora J. Chem. Phys. 1973, 59, Orientational Dynamics in Bulk Liquids

R = hydrodynamic radius of rotating molecule rotating molecule η = viscosity Stokes-Einstein relation: Reorientation time τ or Stokes-Einstein Relation

C2C2 C3C3 C1C1 -CO 2 - PA R H (exp)= 1.9 Å air/water interface R H (calc)= 3.4 Å - bulk Stokes-Einstein relation: air water Hydrodynamic Diameters vs. Calculated Diameters for Formic (C 1 ), Acetic (C 2 ) and Propionic (C 3 ) Acid Anions

R = hydrodynamic radius of rotating molecule rotating molecule η = viscosity Stokes-Einstein relation: Reorientation time τ or Orientational Dynamics – Interface vs. Bulk Faster reorientation at the interface than bulk water: Faster reorientation at the interface than bulk water: 1.1±0.3 ps vs ps 1.1±0.3 ps vs psEither - Hydrodynamic radius R (i.e. hydration shell) of the molecule is smaller at the interface than in the bulk - Hydrodynamic radius R (i.e. hydration shell) of the molecule is smaller at the interface than in the bulkOr - Viscosity η around the molecule is smaller at the interface than in the bulk - Viscosity η around the molecule is smaller at the interface than in the bulkOr - Stokes-Einstein breaks down near the interface - Stokes-Einstein breaks down near the interface

Conclusions  SFG line shape analysis recovers time scales of molecular orientational dynamics at interfaces  In-plane reorientation time scale for small H-bonding molecule at interface is very different from the bulk-phase time scale  We are currently modeling time-evolving molecular orientational distributions at interfaces for calculation of SFG signals

Acknowledgments Prof. Alexander Benderskii Achani Yatawara Himali Jayathilake Champika Weeraman Fadel Shalhout Adib Samin WSU IMR WSU GRA NSF CAREER Grant No Funding