Interfacial Studies of Fatty Acid Monolayers:Structure, Organization, and Solvation by Sum Frequency Generation Vibrational Spectroscopy

Marine aerosols have direct effects on the physics and chemistry of marine atmosphere. In a global dimension, marine aerosols are a key factor in controlling the global climate change by scattering and absorbing solar radiations. Because of limited understanding of interfacial molecular structure and heterogeneous chemistry, model studies of fatty acid monolayers at the air-liquid interface are capable of providing new insight into the aerosol chemistry. In this dissertation, a broad bandwidth sum frequency generation (BBSFG) vibrational technique was used to investigate surface structure, organization, and solvation of monolayer systems on aqueous surfaces. The first molecule of interest is palmitic acid (PA, C₁₆). One of the key findings is that deprotonation can be initiated by ionic binding to the fatty acid headgroups, even at neutral pH. The binding affinity increases in the order that Na⁺ ~ Mg²⁺ < K⁺ < Ca²⁺. However, the binding of these four cations has little effect on the order and the orientation of the acyl chain in PA with respect to pure water. In addition, the interfacial water structures underneath the PA monolayers also reveal considerable spectral transformations when exposed to Mg²⁺ and Ca²⁺. At low concentration (0.1M), three bands were observed in the hydrogen bonding region: ~3600 cm^-1 (hydrogen-bonded fatty acid headgroups), ~3400 cm^-1 (weakly hydrogen-bonded water molecules), and ~3200 cm^-1 (strongly hydrogen-bonded water molecules). At 0.3 M, the intensities of these three bands start to decrease for Mg2+ and Ca2+. However, in concentrated Mg²⁺ and Ca²⁺ solutions (~2.0 M), the ~3400 cm^-1 band and the ~3200 cm^-1 band start to converge and to peak at 3300 cm^-1 with enhanced intensity. This may suggest that there is significant water restructuring in the course of increasing concentration due to charge neutralization effects at the surface. More importantly, at concentrated conditions, the already disrupted hydrogen-bonding network reorganizes and reverts to its original hydrogen-bonding network as appeared at the neat solution interface. Finally, the observed spectral intensity trends are consistent among the probed regions from 1300 cm^-1 to 3800 cm^-1 that encompasses the stretching vibrational modes of COO-, C=O, C-H, and O-H.

In the structural studies of monounsaturated isomers of oleic acid (OA) and elaidic acid (EA) at the air/liquid interface, we determined that the methyl-sided alkyl chain in OA and EA is responsible for the initial molecular interactions among neighboring molecules; on the other hand, the carboxyl-sided alkyl chain is accountable for the tighter packing as it adopts a near all-trans conformation and positions closer to the surface normal. More importantly, considerable degrees of conformational ordering already start to emerge at 3 mN/m in both OA and EA alkyl chains at the carboxyl side; moreover, an EA monolayer is capable of being tightly packed with more enhanced conformational order than OA at the same physical conditions.