Interfacial Inversion, Interference, and IR Absorption in Vibrational Sum Frequency Scattering Experiments

Molecular interfacial structure greatly determines thepropertiesof nano- and microscale systems. Vibrational sum frequency scattering(SFS) spectroscopy is a unique interface-selective tool to measurethe interfacial vibrational spectrum of sub-micron to micron-scaleobjects dispersed in liquid and solid media. The interfacial structureis extracted from the interfacial susceptibility, a physical propertyderived from the intensity. Here, we describe the effect of infraredabsorption that occurs in a bulk medium that is spectroscopicallycomplex and use the results to investigate the effects of interfacialinversion, interfacial interference, and interfacial interferencecombined with absorption. We use the same three chemicals to do so,hexadecane oil, water, and a neutral Span80 surfactant. For all cases,the effective surface susceptibility can be retrieved from the intensity.We further find that inverting the phases results in different interfacialstructures, even though they are composed of the same three chemicals,and explain this in terms of the different interactions that are necessaryto stabilize the drops: steric stabilization for water drops in oilvs. charge stabilization for oil drops in water. Interfacial interferencecan be used to estimate the surface density of different compounds.