Multipole Model of the Molecular Asymmetry in Thermotropic and Lyotropic Liquid Crystals. Volume and Surface Effects

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Abstract
The molecular asymmetry of mesogenic molecules is defined as a deviation of the corresponding molecular property from spherical and axial symmetry. Three types of asymmetry-electrical, steric and biphilic, are considered using a multipole representation. Two basic molecular shapes are included-rod-like and plate-like. The first three multipoles-monopole, dipole, and quadrupole, are involved. In the steric case a monopole does not exist. The effective shape of a molecule includes not only steric, but also electrical intermolecular repulsive interactions. The biphilic asymmetry is determined by the distribution of the hydrophilic and hydrophobic groups along the molecule. Three types of generalized fields—electrical, deformational, and biphilic, are also defined and their volume and surface sources are discussed. The effects of the molecular asymmetry on the intermolecular interactions in thermotropic (rod-like and disc-like) and lyotropic liquid crystals are qualitatively discussed in an analogy to the electrical case. The possible pure and mixed asymmetry effects under the action of volume and surface induced generalized fields are also revealed. Some of these effects (like flexoelectricity) may have an equivalent macroscopic demonstration, but their molecular origin can be very different. Besides well known effects, there are several new effects predicted and classified. Their experimental registration may prove the effectiveness of the proposed classification. This classification is also very useful for the common description of thermotropic and lyotropic liquid crystals.

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