We present results of a detailed x-ray scattering study on the rotator phases of normal alkanes: CH3–(CH2)n−2–CH3 (20≤n≤33). We have characterized a new tilted rotator phase and determined the temperature and chain length dependence of the distortion, tilt, and azimuthal order parameters which characterize the time-space averaged structures of the five rotator phases. We have shown that there is no strong even–odd chain length effect on the phase diagram within the rotator phases and have shown the continuity of that phase diagram in the 26-27 carbon vicinity.

We present the first calorimetric study of the normal alkanes CH3–(CH2)n−2–CH3 (21≤n≤30) covering the temperature range of the five rotator phases (whose structures were previously identified using x-ray scattering) with sufficient resolution to observe the various rotator to rotator transitions. We find first-order hexagonal–orthorhombic distortion transitions; second-order azimuthal tilt-angle rotation transitions, and two types of second-order tilting transitions, one of which has the higher symmetry phase at low temperature. These transitions appear to be mean field in character, in that they are without significant pretransitional fluctuations. We discuss the calorimetric signatures for the transitions in terms of the order parameters obtained from x-ray scattering data. In addition to the transitions, we find strong temperature variation of the heat capacity in the rotator phases not associated with the transitions.

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The effects of high pressure gases (P≤400 bar) on the RII, RI, and RV rotator phases of 21, 23, and 25 carbon normal alkanes were studied via x-ray scattering. We have measured the pressure and temperature dependence of the rotator structures and present these results in terms of the essential structural parameters: layer spacing, area per molecule, lattice distortion, and tilt. The pressure was generated by one of three gases: helium, nitrogen, or argon. In the rotator phases, argon and nitrogen intercalate between the layers while helium acts mostly as a noninteracting pressurizing medium. The thermal expansion and compressibility are anomalously large in the rotator phases, and this implies that the heat capacity in the rotator phases is dominated by anharmonic effects.