Structural Changes in Acetophenone Fluid Films as a Function of Nanoscale Thickness

Abstract

We report experimental observations of a developing fluid/solid interface by examining acetophenone films of varying thicknesses, supported on solid silver substrates. A dynamic wetting technique provides experimental control of fluid film thickness, as a function of rotational velocity. Ellipsometry and infrared reflection absorption spectroscopy data are analyzed to provide absolute film thickness and details of the changing chemical environment for varying film thickness. These data are compared to theoretical models that predict fluid film thicknesses, based on physical-chemical properties of the acetophenone/silver pair. As the velocity of the substrate is varied from 0.003 cm s-1 to 1.872 cm s-1, the fluid film's thickness changes from a ca. 200 nm to 2 μm. This increase in film thickness with increasing velocity follows a Landau trend, which is linear with respect to velocity2/3. Our data also show clear evidence of molecular orientation changes, as a function of film thickness, which occur as the thinner films are increasingly comprised of acetophenone molecules within a confined, interfacial environment. The spectral changes for the thinnest fluid films (<100 nm) are shown to exhibit features similar to transmission Fourier transform infrared (FTIR) data of frozen acetophenone, suggesting that these films are highly ordered, as a result of their nanometer-scale confinement.