An X-ray Study of the Magnetic Character of Liquid Crystalline Para-Azoxyanisol and a Comparison with the Isotropic Liquid

Abstract

The effect of a magnetic field upon the orientation of the liquid crystalline groups in para-azoxyanisol has been studied by means of the angular distribution of x-ray relative diffraction intensities, the magnetic field being oriented perpendicular to the incident x-ray beam and the axis of the spectrometer. The relative number of the large molecular groups in the liquid crystalline condition, oriented perpendicular to the field, was obtained by the application of the Boltzmann distribution law in two theories; one assuming a permanent magnetic moment and the other anisotropic polarization. The latter theory of orientation agrees with the claim of Foex as to the cause of the decrease of the magnitude of the diamagnetic susceptibility in the milky state. Moreover, the experiments, together with general considerations, are in harmony with the second theory. Thus the experiments agree with the contention of Zocher, Jezewski and Foex and are opposed to the conclusions of Kast who favored the first theory. The magnetic effect was decreased in a marked manner by the slow mechanical rotation of the cylindrical vessel containing the liquid crystal and also by the increase of the number of cycles of an alternating magnetic field. In the liquid crystalline state (117.4° to 134°C) the x-ray diffraction intensity at the principal maximum is 10 percent greater than in the transparent liquid (at 143°C). This is explained by the existence of cybotactic liquid groups in the latter case and the grouping of a large number of these "companies" into the large "regiment" which is responsible for the liquid crystalline phenomena. "A theoretical justification is found in Ornstein's and Oseen's theory of the liquid crystalline condition." The marked optical and the small x-ray difference in the two conditions are thus easily explained. The difference between the liquid crystalline and the transparent liquid condition is thus found to be mechanically not so great as optical observations would indicate. In detail using distances small compared to an optical wave-length the liquid is never isotropic but consists of the cybotactic groups oriented in all positions. The effect of the magnetic field upon the transparent liquid is too small to measure. This is explained by the smallness of the cybotactic groups. The view of the author as to the existence of temporary approximate groups (cybotactic) in the transparent liquid is clearly strengthened by the straightforward interpretation of these experiments.