Molecular conformations and orientations of fatty acid and

Molecular conformations and orientations of fatty acid and phospholipid monolayers at the air/water interface: A vibrational sum frequency spectroscopic study Kandice L. Harper, Laura F. Voss, Gang Ma, and Heather C. Allen Department of Chemistry The Ohio State University Columbus, Ohio Funding NSF-ATM

Fat-coated Aerosols Dobson et al. , PNAS. , 97, 11864, 2000 • IPCC estimates ~ 3. 3 billion metric tons of salt spray enters the atmosphere yearly. • Biological oil slick provides fatty acids that can coat particles with organic surfactants (200 Tg/year).

Fat-coated Aerosols C 16 C 18 Tervahattu et al. , J. Geophys. Res. , 107, D 7, 4053, 2002. • Palmitic acid (C 16) is most abundant fatty acid. • Oleic acid (C 18) is most abundant unsaturated fatty acid.

Broadband Vibrational Sum Frequency Generation Spectroscopy ωsum = ω800 nm + ωIR 2 ps, 800 nm 100 fs, IR 300 cm- 1 bandwidth spectrum from every laser pulse ωVIS ωIR Media must lack inversion symmetry! Signal is a function of number density and order! ωSFG

surface pressure (m. N/m) SFG + Langmuir Trough mean molecular area (Å2/molecule) • Simultaneous acquisition of thermodynamic and spectroscopic data

Compression of d 31 -Palmitic Acid: surface pressure (m. N/m) Simultaneous Langmuir Isotherm and SFG time (sec)

Compression of d 31 -Palmitic Acid: surface pressure (m. N/m) Simultaneous Langmuir Isotherm and SFG time (sec)

Snapshots of d 31 -Palmitic Acid Compression 0 sec gas-liquid coexistence 990 sec solid 1020 sec collapsed 600 sec liquid solid 870 sec liquid collapsed gas-liquid

ωVIS ωIR ωSFG d 31 -Palmitic Acid Monolayer CD 3 SS 2 minute acquisition ssp polarization combination CD 3 FR CD 3 AS • CD 2 symmetric and asymmetric stretches are absent. • Monolayer is well-ordered in all-trans configuration.

ωVIS ωIR ωSFG Oleic Acid Monolayer CH 3 SS 2 minute acquisition ssp polarization combination CH 2 SS CH 3 FR CH 2 FR =CH olefinic stretch • CH 2 symmetric stretch is present. • Oleic acid monolayer exhibits gauche defects.

surface pressure (m. N/m) Compression Isotherms of Fatty Acids d 31 -palmitic acid 1 d 31 -pa: 1 oleic acid mean molecular area (Å2/molecule) • Isotherm of mixed monolayer resembles isotherm of oleic acid (crystalline phase not evident). • Oleic acid fluidizes d 31 -palmitic acid. • d 31 -palmitic acid improves the ability of the oleic acid molecules to pack.

Compression of Mixed Monolayer: surface pressure (m. N/m) Simultaneous Langmuir Isotherm and SFG mean molecular area (Å2/molecule)

Compression of Mixed Monolayer: surface pressure (m. N/m) Simultaneous Langmuir Isotherm and SFG mean molecular area (Å2/molecule)

Snapshots of Mixed Monolayer Compression 1140 sec collapsed 0 sec gas 810 sec liquid collapsed gas

Pure and Mixed Fatty Acid Monolayers Pure Monolayer: d 31 -palmitic acid Mixed Monolayers: 1 d 31 -pa: 1 pa 1 d 31 -pa: 1 oa 5 minute acquisition ssp polarization combination CD 3 SS CD 2 SS CD 3 FR CD 2 AS CD 3 AS • CD 2 symmetric and asymmetric stretches of d 31 palmitic acid are present in spectrum of mixed monolayer. • Oleic acid molecules disrupt order of d 31 -palmitic acid molecules in mixed monolayer.

Conclusion • Oleic acid molecules disrupt the order of palmitic acid molecules in a mixed monolayer. - Conformational change of d 31 -palmitic acid molecules is evident in SFG spectra. - Fluidization of d 31 palmitic acid molecules is evident in isotherm. • Palmitic acid increases the ability of the oleic acid molecules to pack in a mixed monolayer-as evident in isotherm.