Order in the rotator phase of n-alkanes

The "rotator" phase is stable over a temperature interval of as much as 19°C in a 79:21 solid solution of n-heneicosane (C₂₁) and n-tricosane (C₂₃). Two rotator modifications appear: a lower-temperature face-centered orthorhombic (FCO) and a higher-temperature rhombohedral (hexagonal) phase. Both are identical with those found in pure odd alkanes. Their temperature behavior was examined by monitoring the correlation splitting of IR- and Raman-active vibration modes, crystal lattice parameters, and specific entropy. The ratio of unit cell parameters a/b was found to change with temperature almost continuously in the rotator phase, from 1.5 (equal to the value for the ordered low-temperature phase) to 3^1/2 (hexagonal lattice value). Correlation splitting (Δν) of the IR-active CH₂ rocking mode reaches 8.5 cm^-1 in the rotator phase at the lowest temperature, closely approaching the value for the ordered phase. With increasing temperature the splitting gradually disappears. The Raman-active CH₂ bending mode behaves similarly. Application of a simple model of finite arrays of coupled oscillators gives an average correlated domain as consisting of 20-30 molecules within a layer in the FCO phase at the lowest temperature. The domains decrease in size as hexagonal symmetry is approached. From symmetry and entropy consideration it was deduced that four equivalent orientations are available to a molecule in the FCO form and six or possibly more in the hexagonal form. The almost continuous nature of the orthorhombic-to-hexagonal transition in the rotator phase is discussed.