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

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 velocity^2/3. 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

Effects of Fluid Confinement and Temperature in Supported Acetophenone Films

Solid–liquid phase transitions are thought to be well understood in bulk phases of matter, but in thin films or interfacial volumes, melting and freezing transitions can exhibit significant departures from expected behaviors. Here, we show multiple solid–liquid phase transitions in thin films (50–500 nm) of the molecular fluid acetophenone. Transitions are driven by both geometric confinement and temperature, as characterized by spectroscopy. Fluid film confinement is controlled by systematic variation of the supported film thicknesses, and the same films are passed through cooling–heating cycles to generate amorphous or crystalline films with distinctly different molecular environments. Specifically, multiple temperature cycles reveal a distinct conditioning dependence, wherein phase transitions may or may not exhibit significant changes in the infrared absorption profile over the temperature cycle, indicating distinct crystalline and liquid-like phases. Significant effects of supercooling are also observed as a result of the highly confined nature of the thin-film sampling geometry. Interestingly, the spectral profiles recorded as a function of film temperature show clear evidence of molecular reordering phase transitions, which is similar to observations in variable thickness films held at constant temperature. The changes in spectral absorption profiles confirm the confinement-induced crystalline ordering and provide evidence that molecular confinement effects can extend beyond 100 nm from a surface, which is much larger than conventionally accepted “interfacial” volumes. Ultimately, the extended crystalline ordering within liquid films could offer important new avenues to tune the physical properties of designer interfaces

Structure of Aqueous Water Films on Textured −OH-Terminated Self-Assembled Monolayers

We report the thickness and interfacial molecular structure of thin (1–3 nm) aqueous films supported on hydroxyl-terminated self-assembled monolayers over a silver substrate. The water film structure is studied as a function of varying the monolayer’s methylene chain lengths. Analysis techniques include ellipsometry, contact angle, and polarization modulation reflection adsorption infrared spectroscopy. The aqueous film thicknesses follow 4-mercaptobutanol (4-MBU) > 11-mercaptoundecanol (11-MUD) > 6-mercaptohexanol (6-MHE) > 9-mercaptononanol (9-MNO). Water contact angle measurements across the same surfaces are very similar; however, vibrational spectroscopic analysis of the films shows that intermolecular bonding patterns of D₂O are significantly different from those of bulk D₂O. This evokes unique interfacial molecular architectures for each of these films. The structural differences depend on the nature of the SAM structure and resulting water–SAM interactions, which are evident from PM-IRRAS data. Spectroscopic peak intensity ratios of ν­(O–D) modes suggest more asymmetric hydrogen-bonded D₂O character near 9-MNO surfaces, whereas 4-MDU, 6-MHE, and 11-MUD surfaces exhibit increasingly symmetric hydrogen-bonded D₂O character. From this, we propose a model for film structure