STRUCTURAL EFFECTS OF HIGH-PRESSURE GAS ON THE ROTATOR PHASES OF NORMAL-ALKANES

The effects of high pressure gases (P less than or equal to 400 bar) o n the R(II), R(I), and R(V) rotator phases of 21, 23, and 25 carbon no rmal alkanes were studied via x-ray scattering. We have measured the p ressure and temperature dependence of the rotator structures and prese nt these results in terms of the essential structural parameters: laye r spacing, area per molecule, lattice distortion, and tilt. The pressu re 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 ro tator phases, and this implies that the heat capacity in the rotator p hases is dominated by anharmonic effects.