A Spectroscopic Study of Interfacial Films: Internal Structuring, Stability, and Hydration

Abstract: Monolayers of molecules are capable of completely changing the nature of surfaces and their interactions with the surroundings despite their almost negligible thickness. In the research presented in this doctoral thesis the surface specific technique Vibrational Sum Frequency Spectroscopy (VSFS) was combined with the Langmuir trough to investigate various manifestations of monolayers, such as biomimetic membranes, mono methyl branched long chain fatty acids in contact with air, and hydrophobic silane monolayers in contact with water.VSFS was applied to in situ studies of the oxidative degradation of Langmuir monolayers of 1,2-diacylphosphocholines with identical C-18 chains featuring various degrees of unsaturation. The monolayer area and vinyl CH signal intensity were probed at constant surface pressure to monitor the degradation. The rapid degradation of the unsaturated lipids in contact with the ambient air is attributed to oxidation of the C=C bonds mediated by reactive species in the air and can be inhibited by purging the compartment surrounding the monolayer with nitrogen.The molecular structure and order of Langmuir monolayers of 1,2-distearoyl-phosphocholine (18:0 PC) and their hydrating water were investigated at different surface pressures using VSFS. The monolayers are conformationally well ordered at all surface pressures and the signal intensity increases due to larger molecular number density at increasing surface pressures. Also, water signals with different vibrational frequencies are observed in different polarization combinations.Additionally, a selection of common phospholipids (18:0 PC, 18:0 PC-D83 and 18:0 PS) were Langmuir-Blodgett (LB) deposited on CaF2 substrates and the CH- and OH-stretching regions as well as lower wavenumber regions were probed using VSFS. The orientation of the conformationally well ordered aliphatic chains was determined to be approximately perpendicular to the sample surface.Monolayers of eicosanoic acid, its iso (19-MEA), and anteiso (18-MEA) analogues were investigated with VSFS, AFM imaging, and the Langmuir trough. The EA forms smooth, featureless monolayers when deposited on silica, while 19-MEA and 18-MEA form 10 -50 nm large domains with homogeneous size distribution. It was not possible to discriminate between the monolayers of racemic and chiral 18-MEA using any of the techniques employedThe influence of the experimental geometry on the SF spectral shape and en-hancement at and near total internal reflection conditions (TIR) were systematically investigated by comparing simulations with recorded data from a hydrophobized silica / water interface. The data agree qualitatively, but not quantitatively, with the simulations, and the reasons were critically discussed.The water structure next to ordered and disordered hydrophobic silane monolayers on silica was investigated using VSFS. The results indicate that the structure of water next to a well ordered hydrophobic monolayer, with the exception of the first layer of water molecules, is not much different from that in the isotropic bulk. This is in contrast to the previous notion where such monolayers were assumed to induce a more ordered interfacial water structure.