X-ray Photgraphic Studies of Liquid Crystals: IV. The Isotropic, Nematic, and Smectic A Phases of Some 4-Alkoxybenzal-4′-Ethylanilines

From X-ray photographs of some 4-alkoxybenzal-4′-ethylanilines (CnH2n+1O—C6H4—CH=N—C6H4—C2H5; n = 5, 6, 7) we have calculated the average intermolecular distance (D) between neighboring parallel molecules in the isotropic, nematic and smectic A phases, the apparent length (l) of the molecules in the isotropic phase, and the thickness (d) of the smectic layers. All D values for the smectic and nematic phases are between 4.94 and 5.03 Å. In the isotropic phase, the slopes of the D versus t plots for C6 and C7 (0.0024 and 0.0028 Å/°C, respectively) are approximately equal to those reported earlier, but for C5 the slope is somewhat larger (0.0038). The slopes of the D versus t plots for the nematic phase are of the order of 0.01 Å/°C, and for the smectic phase of C7 the slope is about half of this; for the nematic phases the slope appears to increase with decreasing length of the stability range. All l versus t plots were definitely concave. Extrapolation of the curves to the N-I point yielded l values very close (within 0.4 Å) to lengths calculated from molecular models. For C7, d is apparently constant, and about 1.5 Å less than the molecular length.