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