Sum frequency generation studies on the structure and dynamics of Langmuir Blodgett Films

Organic monolayers play an important role in materials chemistry, atmospheric science and biological processes. This thesis addresses the structure and dynamics of organic monolayers in an attempt to understand the forces and interactions between their head groups and alkyl chains and therefore provide insight into the microscopic characteristics and their relationships to the macroscopic physical properties. Sum frequency generation (SFG) has been used to characterize the molecular structure and orientation in Langmuir Blodgett (LB) films. The quantitative interpretation of SFG spectra is dependant upon the quality of spectral fitting as well as the model chosen for describing the molecular system. Several analysis methods were used which yielded a self consistent interpretation of the spectra. Mixed LB films of stearonitrile and ferric stearate were shown to deposit either Z-type or Y-type, respectively, due to differences in the macroscopic wettability. SFG,AFM, FTIR and X-ray diffraction were used to confirm the structure and crystallinity of these films. Unfavourable headgroup-headgroup interactions in stearonitrile resulted in large unit cells and account for the differences in wettability. The phase of a material can have a direct bearing on its macroscopic properties. In order to elucidate on phase changes in LB films, the SFG spectra were monitored as a function of temperature. A phase transition was observed at 155 K which coincided with the appearance of ice-like peaks in the hydrogen bonded region of the spectrum of water. The water was found to be associated with the hydrophilic headgroup of the LB film. This phase behaviour suggests that the ordering of the underlying water can directly affect the structure of the LB film. Time resolved SFG spectroscopy was used to examine the interfacial order and dynamics of LB films. This technique provides a measure of the inhomogeneity of the system which reflects the nature of the chemical environment. This technique was applied to both the CaF2/air and CaF2/D2O interfaces. Results indicated that the solvent monolayer interactions do not contribute to the observed inhomogeneity of these systems.