X-Ray Scattering Study of Stuctures and Phase Transitions of Normal Alkanes

A comprehensive study of the structures and phase transitions for normal alkanes is presented. Three different aspects are investigated: rotator phases in pure and mixed chain length normal alkanes; anomalous melting in alkane thin films; and nucleation in alkane emulsions. The structures of the five rotator phases are characterized with distortion, tilt and azimuthal order parameters. A new tilted rotator phase, R_{V}, is fully identified. The entropically favored lower packing density phases, rm R_{II}, R_{III}, and R_ {IV}, dominate at the high temperature, while increased packing density and distortion phases, R_{I} and R _{V}, are favored at the low temperature, as a result of the increasing importance of intermolecular interactions. Chain end effects modify this behavior by favoring tilted phases, rm R_{III}, R_{IV}, and R_{V}. The main effect of mixing is the increased stability of the rotator phases. Several universal trends are discovered in comparing the mixed-chain and single-chain materials: the normalized distortion in the R_{I} phase is reduced; the hexagonal R_{II} phase is favored over the R_{I} phase; and the tilted R_{V} phase is suppressed. A weakened coupling between layers caused by chain mixing is the origin of the modification of the rotator phase diagrams. A significant modification on melting of the n-tricosane (rm C_ {23}H_{48}) thin films is discovered. The melting of these thin films differs from that of bulk alkanes in two important ways. The melting temperature is depressed as the film thickness is reduced. The structures of solid rotator phases prior to melting change depending on the film thickness. The depression in the melting can be described by a simple mean field model. The nucleation kinetics of monodispersed emulsions of n- rm C_{15}H_{32}, n-rm C_{16}H_ {34}, and the C16-C17 mixtures, with sizes from ~0.1 to 10mu m, were characterized. Significant supercooling was discovered. A quench-depth dependent nucleation time lag is observed. A size dependence of melting is found among n-rm C_{16}H_ {34}, and the C16-C17 mixture emulsions, but not in n-C_{15}H_ {32} emulsions. The nucleation frequencies determined by time resolved x-ray scattering and differential scanning calorimetry are in good agreement with each other, which show significant deviation from the known theory. The liquid-to-crystal interfacial free energy of n- rm C_{16}H_{34} emulsions shows a slight size dependence. This is attributed to step nucleation, plausibly caused by the presence of surface and bulk nucleation, thus nucleation is a continuous process.