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Final Report Summary - FILMMOLECSTRUCT (Molecular Structure in Thin Wetting Films)

The aim of this project was to obtain molecular information on the structure of thin liquid films confined between a gas and solid interface, as an effort to relate macroscopic measurable parameters, such us disjoining pressures, to structural properties at the molecular level (Figure 1). The approach consisted in combining an apparatus capable of accurately measuring interfacial forces in wetting films (air/liquid/solid) with state of the art surface specific vibrational spectroscopic techniques.

Wetting films play a crucial role in the understanding of many practical applications, including mineral flotation, surface cleaning processes, and the coagulation of colloidal dispersions. The stability of these films depends on the molecular interactions between the two interfaces which macroscopically manifest themselves as surface forces. The measurement of these forces, in particular using the thin film pressure balance (TFPB) apparatus, has provided valuable insight in the identification of the different types of interactions, which could be of electrostatic, dispersive, steric and/or structural nature, just to mention a few. Nonetheless, the link between the measured forces and the molecular origin of these interactions remains elusive, as force measurements provide no direct chemical, structural or conformation information, which are expected to change significantly in confined geometries. In this sense vibrational spectroscopic techniques, in particular those sensitive to molecules at interfaces can provide an important part of the puzzle: chemical and orientation molecular information.

Recent advances in vibrational spectroscopic techniques allow collection of unprecedented information from molecules at interfaces. In this category Vibrational Sum Frequency Spectroscopy (VSFS) has emerged as an invaluable tool. The intrinsic surface specificity of VSFS distinguishes molecules at the interface from a vast excess of the same molecules present in the bulk, even at a submonolayer coverage. Moreover, current developments in conventional Raman scattering, in particular when using a Total Internal Reflection (TIR) geometry, have demonstrated that submonolayer sensitivity is achievable and that it can also be used to determine the conformation of surface molecules.

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