Vibrational sum frequency spectroscopy of thin film interfaces.

We describe a basic theoretical treatment of how film-substrate and substrate-environment (air, water, and solution) interfaces can be selectively probed by controlling the film thickness and beam angles in a visible-infrared sum frequency generation experiment. In this model, we also account for the unique interfacial environment that may have optical properties that differ from the adjacent bulk phases. We see that this affects components of the electric field that are perpendicular to the surface such as when p-polarized light is used. We then provide an example using the glass-polydimethylsiloxane-air system and model the fields at both surfaces of the polymer. This is followed by some practical considerations for setting up such experiments and some typical experimental results.

Determination of Surface Preference Using Heterospectral Surface-Bulk Correlation Spectroscopy.

We outline a method by which the surface preference of a species in a multicomponent mixture may be obtained using surface-specific visible-infrared sum frequency generation (SFG) spectroscopy combined with bulk infrared absorption and/or Raman data. In general, the problem is complicated by the fact that the SFG signal is a function of both the surface coverage and the structure of the molecules. Two-dimensional correlation analysis can be used to reveal which spectral features are changing synchronously, that is, in phase with each other, and which ones are evolving in a manner that is phase-shifted by 90° (asynchronous correlation) as a function of the bulk composition. We provide a framework for determining the surface preference from the correlations between the vibrational modes in the SFG spectra and between the modes from SFG and bulk infrared and/or Raman spectra. When compared to the equivalent analysis performed using the SFG spectra alone, this method can be used with the data obtained using a single-beam polarization and in congested spectral regions where fitting to isolate the behavior of individual vibrational modes is not robust.

Silica Surface Charge Enhancement at Elevated Temperatures Revealed by Interfacial Water Signals.

The structure of water adjacent to silica is sensitive to the degree of deprotonation of surface silanol groups. As a result, close inspection of signals originating from these water molecules can be used to reveal the surface charge density. We have used nonlinear vibrational spectroscopy of the water O-H stretching band over a temperature range of 10-75 °C to account for the increase in surface potential from deprotonation. We demonstrate that the behavior at the silica surface is a balance between increasing surface charge and a decreasing contribution of water molecules aligned by the surface charge. Together with a model that accounts for two different types of silanol sites, we use our data to report the changes in enthalpy and entropy for deprotonation at each site. This is the first experimental determination of these thermodynamic parameters for hydrated silanol groups at the silica surface, critical to a wide range of geochemical and technological applications.