Study of liquid crystalline state of p-n-alkyloxybenzoic acids according to their IR absorption spectra
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For the three acids investigated (p-n-propyl-, p-n-heptyl-, and p-n-nonyloxybenzoic acids), the existence of a high-temperature solid phase is observed, which is probably the “rotational” α-phase. The identity of the spectra of these acids, taken in the mesophase and in the high-temperature solid phase, indicate a rod-shaped, cylindrical form of the n-alkyl groups of the molecules of these substances in the liquid crystalline state on account of rotation of the molecules around their long axes; the steric properties of such molecules are one of the prerequisities for the formation of a parallel orientation of the molecules in the mesophase.
The IR spectra of p-n-cetyloxybenzoic acid, obtained in the liquid crystalline phase, are identical with the spectra obtained in the Isotropic liquid phase. The identity of the spectra indicates the formation of a large number of rotational isomer s of the n-alkyl chain of the molecules in the mesophase, which leads to a disturbanace of the rod-shaped form of the molecule. This lowers the degree of ordering of the molecules in the smectic phase of p-n-cetyloxybenzoic acid in comparison with the ordering of the molecules in the same phase of heptyl- and nonyloxybenzoic acids.
In the smectic and nematic phases of p-n-heptyl-and p-n-nonyloxybenzoic acids, the n-alkyl groups of the molecules exist in the same rotational isomeric forms.
KeywordsAbsorption Spectrum Liquid Phase Crystalline Phase Nematic Phasis Liquid Crystalline Phase
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